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Electric Drives and Electromechanical Systems 2nd Edition 2019

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Electric Drives and
Electromechanical Systems
Applications and Control
Second Edition
Richard Crowder
School of Electronics and Computer Science
University of Southampton
Southampton, United Kingdom

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List of principal symbols
The following are the principal symbols used in this book. In general, they are also defined
within the text. Lower-case symbols normally refer to instantaneous values, and upper-case
symbols to constants. In practice, a symbol may refer to more than one quantity; however,
within a particular context, the meaning should be clear.
B
Bg
Crr
d
ep
ess
Ea , Eb , Ec
Eg
Ek
Em
Er
fe
fp
fs
F
Ff
FL
I
Ieff
IL
Im
Itot
Ia
If
Im
Ir
Ir’
IR
IS
K
Ke
Kg
Kp
Ks
Kt
KT
KT0

Damping constant
Air-gap flux
Coefficient of rolling resistance
Duty cycle
Per-phase r.m.s. e.m.f.
Steady-state error
Direct-current (d.c.) brushless motor back e.m.f.
Tachogenerator voltage
Kinetic energy
Direct Current (d.c.) brushed-motor back e.m.f.
Regenerative energy
Supply frequency
Sampling frequency
PWM switching frequency
Force
Frictional force
External linear force
Moment of inertia; Current
Inertia referred to the output of the motor
Load inertia
Motor inertia
Total inertia of a rotating system
Armature current
Field current
Induction motor magnetisation current
Induction motor rotor current referred to the stator
Induction motor rotor current
Regenerative current
Induction motor stator current
Shaft stiffness
Voltage constant for a direct current (d.c). brushed motor
Tachogenerator voltage constant
Proportional gain
Effective stiffness of a joint
Direct current brushed motor torque constant
Induction-motor torque constant
d.c. brushless-motor torque constant

xi

xii

List of principal symbols

La
Ls
n
n
N
NL
Np
NT
p
ppr
P
Pi
Po
Ra
s
Rr0
Rs
RT
s
td
tm
tz
to
T
Tcm
Te
Tf
Ti
TL
TM
To
Tpeak
Ts
Tw
T0
vas , vbs , vcs
vd , vs
vds , vqs
Va , Vb , Vc
Vc
Vcpk
VL
Vm
Vr
Vs
a
b
D
ε

Armature inductance
Induction motor stator inductance
Gear ratio
Optimum gear ratio
Number of teeth on a gear wheel
Lead-screw input speed
Turns per pole
Number of teeth on belt-drive pulley
Number of pole pairs in a motor
Pulses per revolution
Power
Input power
Output power
Armature resistance
Induction motor slip
Referred motor resistance
Stator resistance
Number of teeth on a stepper-motor stack
Induction motor slip
Dwell time between two moves
Time to complete a move
Time to reach standstill under regeneration
Time to reach zero terminal voltage under regeneration
Torque
Continuous torque requirement
Electrical torque output
Friction torque
Input torque
Load torque
Motor torque
Output torque
Peak torque
Torque stiffness
Windage torque
Stall torque
Voltages in a stationary three-axis frame
Voltages in a rotating two-axis frame
Voltages in a stationary two-axis frame
Brushless-motor terminal voltages
PWM control voltage; Cutting speed
Peak control voltage in a PWM generator
Linear speed
Direct current (d.c.) brushed motor terminal voltage
Induction motor induced rotor voltage
Supply voltage
Acceleration
Torque angle
Peak current deviation in a pulse width modulated drive
Efficiency

List of principal symbols

z
q
qe
m
r
f
u
ue
ui
uint
um
un
uo
ur
us
u0

Damping
Rotary position
Static-position error
Coefficient of friction
PWM amplifier load factor
Flux linkage
Rotational speed
Synchronous speed
Input speed
Initial speed prior to application of regeneration
Mechanical speed
Natural frequency
Output speed
Rotor electrical speed
Slip frequency
No-load speed

xiii

Preface
Since the completion of the first edition of this book in 2004, the landscape associated with
drive systems has significantly evolved, with the widespread introduction of networking,
including connections to the Internet to realise the Industry 4.0 concept. This revised version
reflects these changes, while maintaining a focus on the underlying electromechanical
system, as the incorrect selection of the motor can never be overcome by the application of
modern control theory, advanced artificial intelligence or data analytics.
Electrical drives act as the electromechanical energy converter in a wide range of
applications, for example machine tools in manufacturing industries, photocopies, electric
automotive applications, prosthetic hands and other medical devices; some are obvious other
not so, until the they fail. It is critically important that the correct drive is matched to the
application with due regard to its requirements. With the recent developments in power
semiconductors and microprocessors with signal processing capabilities, the technology of
the modern drive system has changed dramatically in recent years. However, the selection of
a drive system relies on a systems approach - without which, it is highly probable that either
the mechanical, electrical electronic or computational will not perform to the required
standard or fail if not be fully considered.
A complete drive system consists of many different components; hence this book has
been structured to present a logical discussion, on a wide range of topics relating to selection
of the complete motor-drive system. It does not, however, extend to a detailed consideration
of control and electromagnetic theory; if the reader wishes to pursue this path many excellent
books or academic papers are available, some of which are noted in the bibliography.
The structure of the book is as follows. Chapter 1 gives a brief overview of the problems
that need to be solved, with emphasis on a wide range of electromechanical applications,
including machine tools, robotics and related high-performance applications. Chapters 2 and
3 concentrate on the problem of motor-drive selection and give an insight into the decisions
required during this procedure. It is hoped that this will lift the veil on what is thought
by many to be a black art, or on what more commonly falls into the gap between the
responsibilities of electrical, electronic, and mechanical engineers. Chapter 3 concludes with
suitable algorithms to size a wide range of applications. Chapter 4 considers the types,
selection and installation of velocity and position transducers, the correct selection of which
will have a significant impact on the overall performance of the system. To illustrate the
various points in the chapters, use has been made of a range of numerical examples, and
hopefully these will show how the theory can be applied.
While the initial chapters concentrate on the mechanical aspects of a drive application,
the second part of the book concentrates on the main classes of drives, which are available,
and are used, to drive the applications discussed in Chapter 1. The technologies considered
include: the brushed d.c. motor (Chapter 5), brushless motors (Chapter 6), vector-controlled
induction motors (Chapter 7), and the stepper motor (Chapter 8). In addition, several types of
actuators fall outside this rather arbitrary classification system and are considered in Chapter
9. It should be recognised that some of the larger drive systems have been omitted, due to the
application domain being restricted to small or medium sized applications. Within each of
these chapters there is an overview of the relevant theory, and an examination of the specific
drive and control requirements.

ix

x Preface

Chapter 10 briefly reviews the theory and architecture of current digital controllers,
including the programmable logic controller (PLC). Due to the increasing reliance on
decentralised control within many application domains, a review of network technologies, the
Internet of Things and the related challenges including cyber security are highlighted in
Chapter 11.
Finally the production of a book such as this is not a solitary affair although it tends to
be at times. I must acknowledge the help and assistance given by my colleagues in industry
and academia; finally, I would particularly like to acknowledge my wife Lucy and my daughter
Emma for their continued support throughout the writing period.

d Dr. Richard Michael Crowder,
Electronics and Computer Science,
University of Southampton

Contents
Preface

ix

List of principal symbols

xi

1. Electromechanical systems

1

1.1 Principles of automation

2

1.2 Machine tools

4

1.3 Robotics

12

1.4 Automotive applications

25

1.5 Aerospace applications

27

1.6 Motion-control systems

30

1.7 Summary

33

References

2. Analysing a drive system

33

37

2.1 Rotary systems

38

2.2 Linear systems

46

2.3 Wheeled systems

46

2.4 Force based systems

49

2.5 Friction

50

2.6 Motion trajectories

52

2.7 Assessment of a motor-drive system

56

2.8 Summary

71

References

71

v

vi

Contents

3. Power transmission and sizing

73

3.1 Gearboxes

74

3.2 Lead and ball screws

82

3.3 Belt drives

85

3.4 Bearings

88

3.5 Couplings

93

3.6 Shafts

95

3.7 Linear drive considerations

97

3.8 Review of motor-drive sizing

98

3.9 Summary

106

Reference

106

4. Velocity and position transducers

107

4.1 The performance of measurement systems

108

4.2 Rotating velocity transducers

116

4.3 Position transducers

119

4.4 Installation considerations for position and velocity transducers

130

4.5 Summary

133

References

134

5. Brushed direct-current motors

135

5.1 Review of motor theory

136

5.2 Direct-current motors

137

5.3 Drives for d.c. brushed motors

143

5.4 Regeneration

157

5.5 Summary

163

Reference

163

6. Brushless motors
6.1 The brushless d.c. motor

165
168

Contents

vii

6.2 Sinewave-wound brushless motors

177

6.3 Linear motors

182

6.4 Summary

184

References

7. Induction motors

184

187

7.1 Induction motor characteristics

188

7.2 Scalar control

193

7.3 Vector control

196

7.4 Matrix converter

205

7.5 Summary

206

References

8. Stepper motors

207

209

8.1 Principles of stepper-motor operation

210

8.2 Static-position accuracy

216

8.3 Torque-speed characteristics

217

8.4 Control of stepper motors

220

8.5 Summary

226

References

9. Related motors and actuators

226

227

9.1 Voice coils

228

9.2 Limited-angle torque motors

229

9.3 Piezoelectric motors

232

9.4 Shape-memory alloy

233

9.5 Switched reluctance motors

235

9.6 Summary

238

References

238

viii

Contents

10. Controllers for automation

241

10.1 Servo control

242

10.2 Simulation of drives and controllers

252

10.3 Motion controllers

258

10.4 Programmable logic controllers

260

10.5 Summary

270

References

270

11. Cyber Physical systems and security

271

11.1 Conventional networks

272

11.2 Supervisory control and data acquisition

280

11.3 Industry 4.0

281

11.4 Risks due to the convergence IT and IACS systems

285

11.5 Cybersecurity

286

11.6 Concluding comments

288

References

Appendix 1: Units and conversion factors
Index

295

289

291

1
Electromechanical systems
Chapter outline
1.1 Principles of automation ................................................................................................................ 2
1.2 Machine tools .................................................................................................................................. 4
1.2.1 Conventional subtractive machining processes.................................................................. 5
1.2.2 Non-conventional subtractive machining processes.......................................................... 7
1.2.3 Additive manufacturing processes .................................................................................... 10
1.2.4 Machining centres............................................................................................................... 12
1.3 Robotics.......................................................................................................................................... 12
1.3.1 Industrial robotics ............................................................................................................... 13
1.3.2 Robotic end effectors ......................................................................................................... 18
1.3.3 Mobile and swarm robotics ............................................................................................... 22
1.3.4 Walking robots.................................................................................................................... 23
1.4 Automotive applications .............................................................................................................. 25
1.4.1 Conventional vehicles ......................................................................................................... 25
1.4.2 Electric vehicles.................................................................................................................... 26
1.5 Aerospace applications................................................................................................................. 27
1.6 Motion-control systems................................................................................................................ 30
1.7 Summary ........................................................................................................................................ 33
References............................................................................................................................................. 33

In the design of any complex system, all the relevant design details must be considered
to ensure the development of a successful product. In the development of motion
systems, problems in the design process are most likely to occur in the actuator or
motor-drive system. When designing any actuation system, mechanical designers work
with electrical and electronic systems engineers, and if care is not taken, confusion will
result. The objective of this book is to discuss some of the electric motor-drive systems
in common use, and to identify the issues that arise in the selection of the correct
components and systems for specific applications.
The initial step in the selection of any element of a motor-drive system is to determine a clear understanding of its requirements both mechanically (e.g. torque, force
speed, fixings) and electrical (e.g. power supply, sensing requirements, network interface). In this chapter, a range of applications is considered, ranging from industrial
automation through to the aerospace and automotive sectors.
Electric Drives and Electromechanical Systems. https://doi.org/10.1016/B978-0-08-102884-1.00001-7
Copyright © 2020 Elsevier Ltd. All rights reserved.

1

2

Electric Drives and Electromechanical Systems

1.1 Principles of automation
Automation is defined as the technology which is concerned with the application of
mechanical, electrical, and computer systems in the operation and control of a processes. In general, an automated production process can be classified into one of three
groups: fixed, programmable, or flexible.
 Fixed automation is typically employed for products with a very high production
rate; the high initial cost of fixed-automation plant can therefore be spread over a
very large number of units. Fixed-automation systems are used to manufacture
products as diverse as cigarettes and steel nails. The significant feature of fixed
automation is that the sequence of the manufacturing operations is fixed by the
design of the production machinery, and therefore the sequence cannot easily be
modified at a later stage of a product’s life cycle.
 Programmable automation can be considered to exist where the production
equipment is designed to allow a range of similar products to be produced. The
production sequence is controlled by a stored program, but to achieve a product change-over, considerable reprogramming and tooling changes will be
required. In any case, the process machine is a stand-alone item, operating
independently of any other machine in the factory; this principle of automation
can be found in most manufacturing processes and it leads to islands of automation. The concept of programmable automation has its roots in the Jacquard
looms of the nineteenth century, where weaving patterns were stored on a
punched-card system.
 Flexible automation is an enhancement of programmable automation in which a
computer-based manufacturing system has the capability to change the
manufacturing program and the physical configuration of the machine tool or cell
with a minimal loss in production time. In many systems the machining programs
are prepared at a location remote from the machine, and they are then transmitted
as required over a computer-based local-area communication network.
The basic design of machine tools and other systems used in manufacturing processes changed little from the eighteenth century to the late 1940s. There was a gradual
improvement during this period as the metal cutting changed from an art to a science as
there was an increased understanding of the materials used in cutting tools. However,
the first significant change to machine-tool technology was the introduction of
numerical-control (NC) and computer-numerical-control (CNC) systems.
To an operator, the differences between these two technologies are small, both
operate from a stored program, which was originally on punched tape, then computer
media such as magnetic tapes and discs, and currently stored centrally and distributed
over a network. The stored program in a NC machine is directly read and used to control
the machine; the logic within the controller is dedicated to that task. A CNC machine tool
incorporates a dedicated computer to execute the program. The use of the computer

Chapter 1  Electromechanical systems

3

gives a considerable number of other features, including data collection and communication with other machine tools or computers over a computer network. In addition to
the possibility of changing the operating program of a CNC system, the executive software of the computer can be changed, which allows the performance of the system to be
modified at minimum cost. The application of NC and CNC technology permitted a
complete revolution of the machine tool industry and the manufacturing industries it
supported. The introduction of electronic systems into conventional machine tools was
initially undertaken in the late 1940s by the United States Air Force to increase the
quality and productivity of machined aircraft parts. The rapid advances of electronics
and computing systems during the 1960s and 1970s permitted the complete automation
of machine tools and the parallel development of industrial robots. This was followed
during the 1980s by the integration of robots, machine tools, and material handling
systems into computer-controlled factory environments. The logical conclusion of this
trend is that individual product quality is no longer controlled by direct intervention of
an operator. Since the machining parameters are stored either within the machine or at a
remote location for direct downloading via a network (see Chapter 11) a capability exists
for the complete repeatability of a product, both by mass production and in limited
batches (which can be as small as single components). Until the 1990’s machining
normally involved the removal of material from the workpiece to form the final object e
this is subtractive machine. Between 1980 and the early 1990’s several processes were
developed that allowed an object to be built up from layers of material, this is termed
additive manufacturing. Additive manufacturing is considered to be a disruptive technology, due to its flexibility, speed and ability to produce single units at a cost that is very
similar those produced during a long production runs.
A typical CNC machine tool, robot or multi-axis system, whatever its function, consists of several common elements (see Fig. 1.1). The axis position, or the speed controllers, and the machining-process controller are configured to form a hierarchical
control architecture. In this approach, controlled motion (position and speed) of the axes
is necessary; this requires the provision of actuators, either linear or rotary, associated
power controllers to produce motion, and appropriate sensors to measure the variables.
The overall control of the system is vested in the system computer, which, apart from
sequencing the operation of the overall system, handles the communication between the
operator and the user’s network. It should be noted that industrial robots, which are an
important element of any automated factory, can be considered to be a specific type of
machine tool.
The development of technologies including artificial intelligence and the Internet of
Things has led to the development on the Industry 4.0 concept, which is widely
considered to be the fourth industry revolution, based on the development of smart
factories. Within a smart factory, cyber-physical systems monitor physical processes,
which can then be communicated to allow the decision to be made on a decentralized
basis. This approach to manufacturing is further discussed in Chapter 11, together with
the cyber security issued raised with this approach.

4

Electric Drives and Electromechanical Systems

FIG. 1.1 Outline of a control architecture for a typical CNC machine tool, robot or similar multi-axis system. The
number of individual motion axes, and the interface to the process are determined by the system’s functionality.
A direct connection to the organisation’s network is provided to allow the implementation of Industry 4.0 and
related concept.

1.2 Machine tools
Until recently, the basic stages in manufacturing have not changed over the centuries:
material must be moved, machined, and processed. When considering the history of
manufacturing facilities, it should be remembered that they are but the latest step in a
continuing process that started during the Industrial Revolution in the second half of
the eighteenth century. The machine-tool industry developed during the Industrial
Revolution in response to the demands of the manufacturers of steam engines for
industrial, marine, and railway applications. During this period, the basic principles of
accurate manufacturing and quality were developed by, amongst others, James Nasmyth
and Joseph Whitworth. These engineers developed machine tools to make good the
deficiencies of the rural workers and others drawn into the manufacturing towns of
Victorian England, and to solve production problems which could not be solved by the
existing techniques. Increased accuracy led to advantages from the interchangeability of
parts in complex assemblies. This led, in turn, to mass production, which was first realised
in North America with products (such as sewing machines and typewriters) whose
commercial viability could only be realised by high-volume manufacturing (Rolt, 1986).

Chapter 1  Electromechanical systems

5

The demands of the market place for cost reductions and the requirement for
increased product quality has led to dramatic changes in all aspects of manufacturing
industry, on an international scale, since 1970. These changes, together with the introduction of new management techniques in manufacturing, have necessitated a
considerable improvement in performance and costs at all stages of the manufacturing
process. The response has been a considerable investment in automated systems by
manufacturing and process industries. To fully appreciate the complexities found in
modern machine tools, we will consider a number of the processes in detail, firstly
subtractive machining processes and then additive manufacturing.

1.2.1

Conventional subtractive machining processes

Subtractive machining is the global term for a range of processes in which the geometry
of a workpiece is modified by the controlled removal of material. This approach to
machining is highly versatile since it can produce a wide variety of shapes and surface
finishes. To fully understand the requirements for controlling a machine tool, the individual machining process must be considered in some detail. Machining can be
classified as either conventional subtractive machining, where material is removed by
direct physical contact between the tool and the workpiece, or non-conventional subtractive machining, where there is no physical contact between the tool and the
workpiece.
In a conventional subtractive machining operation, material is removed by the
relative motion between the tool and the workpiece in one of five basic processes:
turning, milling, drilling, shaping, or grinding. In all machining operations, several
process parameters must be controlled, particularly those determining the rate of material removal; and the more accurately these parameters are controlled the higher is the
quality of the finished product (Waters, 1996). In sizing the drives of the axes in any
machine tool, the torques and speed drives that are required in the machining process
must be considered in detail.
Fig. 1.2 illustrates the turning operation, found in a lathe, where the tool is moved
relative to the workplace. The power required by the turning operation is of most

FIG. 1.2 The turning process, where a workpiece of an initial diameter D is being reduced to d; Fc is the
tangential cutting force, Ff the feed force, N is the spindle speed, and f the feed rate. In the diagram the depth
of the cut is exaggerated.

6

Electric Drives and Electromechanical Systems

concern during the roughing cut (that is, when the cutting depth is at its maximum),
when it is essential to ensure that the drive system will produce sufficient power for the
operation. The main parameters are the tangential cutting force, Fc, and the cutting
speed, Vc. The cutting speed is defined as the relative velocity between the tool and the
surface of the workpiece (m min1). In the turning operation, the cutting speed is
directly related to the spindle speed, N (rev min1), by;
Vc ¼ dpN

(1.1)

where, d, is the diameter of the cut being made. The tangential force experienced by the
cutter can be determined from knowledge of the process, in particular the specific
cutting force, K, is determined by the manufacturer of the cutting tool, which is a
function of the materials involved, and other parameters, including the cutting angles
and the tools design. Based on the tangential forces, the power requirement of the
spindle drive can be estimated using;
Power ¼

Vc Fc
60

(1.2)

In modern CNC lathes, the feed rate and the depth of the cut will be individually
controlled using separate motion-control systems. While the forces will be considerably
smaller than those experienced by the spindle, they still must be quantified during any
design process. The locations of the forces at the point of cutting also shown in Fig. 1.2;
their magnitudes are, in practice, a function of the approach and cutting angles of the
tool. Their determination of these magnitudes is outside the scope of this book, but it
can be found in texts relating to machining processes or manufacturers’ data sheets.
In a face-milling operation, the workpiece is moved relative to the cutting tool, as
shown in Fig. 1.3. The power required by the cutter, for a cut of depth, W, can be estimated using;
Power ¼

d f W
Rp

(1.3)

where Rp is the quantity of material removed in m3min1 kW1and is a function of the
cutter speed and tooling provided by the tool manufacturer.

FIG. 1.3 The face-milling process, where the workpiece is being reduced in height by d, f is the feed speed of the
cutter relative to the workpiece, W is the depth of the cut, and N the rotational speed of the milling cutter.

Chapter 1  Electromechanical systems

7

The determination of the cutting forces is outside the scope of this book, because the
resolution of the forces along the primary axes is a function of the cutting angle and of
the path of the cutter relative to the material being milled, and hence reference should be
made to the data provided by the tool manufacturer.
The forces and powers required by the other three subtractive processes, drilling,
planing, and grinding, can be determined in a similar manner. The sizes of the drives for
the controlled axes in all types of conventional machine tools must be carefully determined to ensure that the required accuracy is maintained under all load conditions. In
addition, a spindle or axis drive unable to provide the required speed or torque will cause
a reduction in the surface quality, or, in extreme cases, damage to the machine tool or to
the workpiece.

1.2.2

Non-conventional subtractive machining processes

Non-conventional processes are widely used to produce products whose materials
cannot be machined by conventional processes, for example, because of the workpiece’s
extreme hardness or the required operation cannot be achieved by normal machine
processes (for example, if there are exceptionally small holes or complex profiles).
A range of non-conventional processes are now available, including;





Laser cutting and electron beam machining
Electrochemical machining (ECM)
Electrodischarge machining (EDM)
Water jet machining

In the laser cutting process Fig. 1.4A a focused high-energy laser beam is moved over
the material to be cut. With suitable optical and laser systems, a spot size with a diameter
of 250 mm and a power level of 107 W mm2 can be achieved. As in conventional
machining the feed speed must be accurately controlled to achieve the required quality
of finish. In addition, the laser will not penetrate the material if the feed is too fast, or it
will remove too much material if it is too slow. Laser cutting has a low efficiency, but it
has a wide range of applications, from the production of cooling holes in aerospace
components to the cutting of cloth in garment manufacture. It is normal practice,
because of the size and delicate nature of laser optics, the laser is fixed, and the workpiece is positioned using a multi-axis table. The rigidity of the structure is critical to the
quality of the spot, since any vibration will cause the spot to change to an ellipse, with an
increase in the cutting time and a reduction in the accuracy. It is common practice to
build small-hole laser drills or similar systems on artificial granite bed-plates since the
high density of the structure dampens any vibration.
In electron beam machining, a focused beam of electrons is used in a similar fashion
to a laser, however the beam is generated and accelerated by a cathode-anode
arrangement. As the beam consists of electrons it can be steered by the application of
a magnetic field. The beam can be focused to spot of 10e200 mm in diameter giving a

8

Electric Drives and Electromechanical Systems

FIG. 1.4 The operating principles of a number of unconventional, subtractive, manufacturing processes. (A) Laser
Cutting. (B) Electrochemical machining (ECM). (C) Electrodischarge Machining (EDM).

typical power density of 6500 GW mm2 At this power a 125 mm diameter hole in a steel
sheet 1.25 mm thick can be cut almost instantly. As in the case of a laser, the beam
source is stationary, and the workpiece is moved on an X-Y table. The process is
complicated by the fact that it is undertaken in a vacuum due to the nature of the
electron beam. This requires the use of drives and tables that can operate in a vacuum,
and do not contaminate the environment.

Chapter 1  Electromechanical systems

9

Electrochemical machining is effectively the reverse of electroplating. Metal is
removed from the workpiece, which takes up the exact shape of the tool. This technique
has the advantage of producing very accurate copies of the tool, with no tool wear, and it
is widely used in the manufacture of moulds for the plastics industry and aerospace
components. The principal features of the process are shown in Fig. 1.4B. A voltage is
applied between the tool and the workpiece, and material is removed from the workpiece in the presence of an electrolyte. With a high level of electrolyte flow, which is
normally supplied via small holes in the tooling, the waste product is flushed from the
gap and held in solution prior to being filtered out in the electrolyte-supply plant. While
the voltage between the tool and the workpiece is in the range 8e20 V, the currents will
be considerable, as the metal removal rate is typically 1600 mm3 min1 per 1000 A. To
achieve satisfactory machining, the gap between the tool and the workpiece must be
kept in the range 0.1e0.2 mm. While no direct machining force is required, the feed drive
must overcome the forces due to the high electrolyte pressure in the gap. Due to the high
currents involved, considerable damage would occur if the feed-rate was higher than the
metal removal rate, allowing the die and the blank tool collided. To ensure this does not
occur, the voltage across the gap is closely monitored, and is used to modify the predefined feed rates, and, in the event of a collision, to remove the machining power.
In electrodischarge machining Fig. 1.4C a controlled spark is generated using a
special-purpose power supply between the workpiece and the electrode. Because of the
high temperature (10 000 C) small pieces of the workpiece and the tool are vaporised;
the blast caused by the spark removes the waste so that it can be flushed away by the
electrolyte. The choice of the electrode (for example, copper, carbon) and the dielectric
(for example, mineral oil, paraffin, or deionised water) is determined by the material
being machined and the quality of the finish required. As material from the workpiece is
removed, the electrode is advanced to achieve a constant discharge voltage.
Due to the nature of the process, the electrode position oscilates at the pulse frequency, and this requires a drive with a high dynamic response; in many cases a hydraulic drive is used, though these are now being superseded by electrical systems.
Several different configurations can be used, including wire machining, small-hole
drilling, and die sinking. In electrodischarge wire machining, the electrode is a moving
wire, which can be moved relative to the workpiece in up to five axes; this allows the
production of complex shapes that could not be easily produced by any other means.
Water jet machining involves the use of a very-high pressure of water directed at the
material being cut. The water is typically pressurised to between 1300 and 4000 bar and
with a nozzle diameter of 0.18e0.4 mm, a water velocity of over 800 m s1 results. With a
suitable feed rate, the water will cleanly cut through a wide range of materials, including
paper, wood and fibreglass. If an abrasive powder, such as silicon carbide, is added to the
water a substantial increase in performance is possible though at a cost of increased
nozzle ware. With the addition of an abrasive powder, steel plate over 50 mm thick can
easily be cut. The key advantages of this process include very low side forces, which
allows the user to machine a part with walls as thin as 0.5 mm without damage, allowing

10 Electric Drives and Electromechanical Systems

for close nesting of parts and maximum material usage. In addition, the process does not
generate heat hence it is possible to machine without hardening the material, generating
poisonous fumes, recasting, or distortion.

1.2.3

Additive manufacturing processes

Additive manufacturing is defined as the process of joining material to make objects
from a 3D data model, usually layer upon layer. While the term 3D printing is widely
used, a range of Additive Manufacturing (AM) processes, the most familiar are summarised in Table 1.1 and are formally defined in ISO/ASTM52900-15, Standard
Terminology for Additive Manufacturing e General Principles e Terminology (2015). In
all the processes detailed the underlying concept of the process is similar, a machining
head “prints” a single layer of the object on a pervious layer, after to which the object is
moved away from the machining heads by the thickness of the layer (Ford et al. 2019,
Frazer. 2014) . The printing operation can either be achieved by the addition of material
as found in material jetting or fused filament processing, Fig. 1.5A or the solidification of
a liquid polymer or metal powder by a laser, Fig. 1.5B.
In all cases, the manufacturing process is similar, and consists of three distinct steps.
The additive manufacturing process requires a series of closed 2D contours that are filled
with solidified material as the layers are fused together. To achieve this, a 3D model of
the object to be printed is created. This model can be generated using computer-aided
design (CAD) software or through reverse engineering techniques by for example by
scanning the object. It is widely recognized that the latter approach can have copyright
and other quality control implications (Fadhel et al., 2013). The CAD file is then

Table 1.1 Summary of available 3D printing processes, where the object is built up
from layers of raw material that are fused together.
Process

Methodology

Polymerisation

An ultraviolet laser is used to cure and harden a liquid photopolymer resin. The object being
printed is formed on the build platform, which is lowered by the thickness of the layer after
each layer is printed.
A polymer (e.g. polypropylene, PMMA, or ABS) is extruded onto a build platform using either
a continuous or Drop on Demand approach
Uses a powder-based material including metals, polymers or ceramics and a liquid binder.
After the components has been printed it is subjected to additional curing in an oven.
A polymer (e.g. ABS or Nylon) is drawn through a nozzle, heated and extruded on to the
build platform.
A laser or electron beam is used to melt and fuse a powder-based material together. The
processes involve the spreading of the powder material over previous layers prior to it being
fused.
Involves feeding powder or wire into an energy source (usually a laser or electron beam) to
form a melted or sintered layer on a substrate, widely used as a coating or repair process.

Material Jetting
Binder Jetting
Fused Filament
Fabrication
Powder Bed Fusion

Directed Energy
Deposition

Chapter 1  Electromechanical systems

11

FIG. 1.5 The implementation of the Fused Filament Fabrication and Polymerisation additive manufacturing
processes. (A) Fused Filament Fabrication: the nozzle mechanism draws material from a reel, and after melting,
deposits it on the built table. The build table is lowered to allow the next slice to be deposited. (B)
Polymerisation: the object is printed using a photopolymer resin. The laser is used to solidify the resin to form the
printed layer, after a layer is printed, the build platform is lowered to allow the next layer to be printed.

converted to a standard additive manufacturing file format - usually an STL (stereolithography) file, the file defines series of closed polygons that correspond to the
different layers that are to be printed. Once the required manufacturing data has been
generated it is down loaded to the process machine. Finally, the additive manufacturing
machine builds the model layer by layer. The layer thickness dictates the final quality
and depends on the machine and process. For Fused Deposition Modelling a layer
thickness of 0.254 mm is typical, Polymerisation can generate layers of thickness in the
range of 0.05e0.1 mm. After printing, the object may require additional cooling and
curing periods prior to cleaning or machining to finalise the production process.
The advantages of the additive manufacturing process give the designer significant
design flexibility. Additive manufacturing allows objects to be printed in one single
process. As the constraints of subtractive machining are removed, together with the
production of specialist tooling, very complex shapes to be produced. Additive
manufacturing will bring significant benefits to many areas, for example in medicine
where prosthetic parts can be fully customised to the patient and their individual
requirements, at substantially reduced cost (Bose et al, 2013).
Additive manufacturing has significant cost benefits, including reduction of material
used, the additive manufacturing process use the same amount on material as in the object
produced, with zero scrap. As the design is in electronic format, it can be transmitted to a
remote machine, this reducing transport costs and allowing, in particular, when spare parts
for ships or aircraft are required at a remote location (Chekurov et al., 2018).

12 Electric Drives and Electromechanical Systems

1.2.4

Machining centres

The introduction of CNC systems has had a significant effect on the design of machine
tools. The increased cost of machine tools requires higher utilisation; for example,
instead of a manual machine running for a single shift, a CNC machine may be required
to run continually for an extended period. The penalty for this is that the machine must
be designed to withstand the extra wear and tear. It is possible for CNC machines to
reduce the non-productive time in the operating cycle by the application of automation,
such as the loading and unloading of parts and tool changing. Under automatic tool
changing several tools are stored in a magazine; the tools are selected, when they are
required, by a program and they are loaded into the machining head, and as this occurs
the system will be updated with changes in the cutting parameters and tool offsets.
Inspection probes can also be stored, allowing in-machine inspection. In a machining
centre fitted with automatic part changing, parts can be presented to the machine on
pallets, allowing for work to be removed from an area of the machine without stopping
the machining cycle. This will give a far better usage of the machine, including unmanned operation. It has been estimated that seventy per cent of all manufacturing is
carried out in batches of fifty or less. With manual operation (or even with programmable automation) batches of these sizes were uneconomical; however, with the
introduction of advanced machining centres together with additive manufacturing, the
economic-batch size is equal to or approaching one.

1.3 Robotics
The development of the industrial robot can be traced to the work undertaken in the
United States at the Oak Ridge and Algonne National Laboratories into mechanical
teleoperated manipulators for handling nuclear material during the 1940’s. It was soon
realised that, by the addition of powered actuators and a stored program system, a
manipulator could perform the autonomous and repetitive tasks. Even with the
considerable advances in sensing systems, control strategies, and artificial intelligence,
the current range of industrial robots are not significantly different from the initial
concept. Industrial robots can be considered to be general-purpose reprogrammable
machine tools moving an end effector, which holds either components or a tool. The
functions of a robot are best summarised by considering the following definition of an
industrial robot as used by the Robotic Industries Association (Shell and Hall, 2000);
An industrial robot is a reprogrammable device designed to both manipulate
and transport parts, tools, or specialised manufacturing implements through
programmed motions for the performance of specific manufacturing tasks.
Since the early industrial robots, technology has advanced considerable with the
development of both static and mobile robots that include sensors (e.g. vision or tactile)

Chapter 1  Electromechanical systems

13

together with controllers based on artificial intelligence to undertake applications as
diverse as planetary exploration to acting as health care assistances. The definition given
by Arkin (1998), proposes a far more general definition, namely:
An intelligent robot is a machine able to extract information from its environment
and use knowledge about its world to move safely in a meaningful and purposive
manner.
The definition should be considered to be at the intersection between biological science
and robotic engineering. Advanced robotic systems and animals both are capable or
mobility, behavioural aspects; incorporate sensors and actuators and require an
autonomous control system that enables them to successfully carry out various tasks in a
complex, dynamic world. This allowed the following conclusion to be made, “the study
of autonomous robots was analogous to the study of animal behaviour” (Webb, 2001).
This allows the following objectives to be achieved:
 Robots to be used to model aspects of animal behaviour or functionality to expand
the understanding of biological systems operate in an real world environment.
 Incorprate biologically inspired attributes or systems in to robots to improve their
operational capability, for example improve mobility by using legs as opposed to
wheels.
In the following section an overview of robotic applications will be undertaking
considering both industrial robots, and the application of biological inspiration in both
manipulative and legged applications.

1.3.1

Industrial robotics

The mechanical structure of a conventional industrial robot can be divided into two
parts, the main manipulator and a wrist assembly. The manipulator will position the end
effector while the wrist will control its orientation. The structure of the robot consists of
links and joints; a joint allows relative motion between two links. A link and is associated
joint is considered as a joint-link pair for the purpose of analysis. Two types of joints are
used: a revolute joint to produce rotation, and a linear or prismatic joint to provide linear
motion. A minimum of six joints are required to achieve complete control of the end
effector’s position and orientation. Even though many robot configurations are possible,
only five configurations are commonly used within the industrial environment:
 Polar. This configuration has a linear extending arm (Joint 3) which is capable of
being rotated around the horizontal (Joint 2) and vertical axes (Joint 1). This
configuration is widely used in the automotive industry due to its good reach
capability, Fig. 1.6A.
 Cylindrical. This comprises a linear extending arm (Joint 2) which can be moved
vertically up and down (Joint 3) around a rotating column (Joint 1). This is a

14 Electric Drives and Electromechanical Systems

FIG. 1.6 The standard configurations of industrial manipulators using only three joints are shown in (A) to (E).
A typical three jointed robot wrist is shown in (f). (A) Polar. (B) Cylindrical. (C) Cartesian and Gantry. (D) Jointed
Arm. (E) SCARA. (F) Wrist.

simple configuration to control, but it has limited reach and obstacle-avoidance
capabilities, Fig. 1.6B.
 Cartesian and Gantry. This robot comprises three orthogonal linear joints (Joints
1e3). Gantry robots are far more rigid than the basic Cartesian configuration; they
have considerable reach capabilities, and they require a minimum floor area for
the robot itself, Fig. 1.6C.
 Jointed Arm. These robots consist of three joints (Joints 1e3) arranged in an
anthropomorphic configuration. This is the most widely used configuration in general manufacturing applications, Fig. 1.6D.
 Selective-compliance-assembly robotic arm. A SCARA robot consists of two rotary
axes (Joints 1e2) and a linear joint (Joint 3). The arm is very rigid in the vertical
direction but is compliant in the horizontal direction. These attributes make it
suitable for certain assembly tasks, in particular printed circuit boards, Fig. 1.6E.
A conventional robotic manipulator has three joints; this allows the tool at the end of
the arm to be positioned anywhere in the robot’s working envelope. To orientate the
tools, three additional joints are required; these are normally mounted at the end of the
arm in a wrist assembly. One design approach to a wrist is shown in Fig. 1.6F, it must be
noted that the design of the wrist can have a significant impact on the manipulator’s

Chapter 1  Electromechanical systems

15

performance, for example if the wrist has a significant mass, it will reduce the overall
capabilities of the robot. The arm and the wrist give the robot the required six degrees of
freedom which permit the tool to be positioned and orientated without restrictions in
three-dimension space as required by the task.
The selection of a robot can be a significant problem for a design engineer, and the
choice depends on a rage of factors, including the task to be performed. One of the
earliest applications of robotics was within a foundry; such environments are hazardous
to human operators due to noise, heat, and fumes from the process. This is a classic
application of a robot being used to replace workers because of environmental hazards.
Other tasks which suggest the use of robots include repetitive work cycles, the moving of
difficult or hazardous materials, and requirements for multishift operation. Robots that
have been installed in manufacturing industry are normally employed in one of four
application groups: materials handling, process operations, assembly, or inspection. The
control of a robot in the performance of a task necessitates that all the joints can be
accurately controlled. A basic robot controller is configured as a hierarchical structure,
similar to that of a CNC machine tool; each joint actuator has a local motion controller,
with a main supervisory controller which coordinates the motion of each joint to achieve
the end effector trajectory that is required by the task. As robot control theory has
developed so the sophistication of the controller and the algorithms has increased.
Controllers can be broadly classified into one of four groups:
 Limited sequence control. This was used on low-cost robots which are typically
designed for pick-and-place operation. Control was usually achieved by the use of
mechanical stops on the robot’s joint which control the end positions of each
movement. A step-by-step sequential controller is used to sequence the joints and
hence to produce the correct cycle.
 Stored program with point-to-point control. Instead of the mechanical stops of the
limited-sequence robot, the locations are stored in memory and played back as
required. However, the end effector’s path is not controlled; only the joint end
points are verified before the program moves to the next step.
 Stored program with continuous-path control. The path control is similar to a CNC
contouring controller. During the robot’s motion the joint’s position and speed are
continually measured and are controlled against the values stored in the program.
 Intelligent-robot control. Using sensors, the robot is capable of interacting with its
environment for example, by following a welding seam or undertaking a detailed
component inspection. As the intelligence increases so the complexity of the control hardware and its software also increase.
In order for the robot to completes at task, it is required to move the end effector from
its initial position to the final position. To achieve this, the robot’s control system must
plan and execute a motion trajectory; this trajectory is a sequence of individual joint
positions, velocities, and accelerations that will ensure that the robot’s end effector
moves through the correct sequence of positions. It should be recognised that even

16 Electric Drives and Electromechanical Systems

though robotic manipulators are being considered, there is no difference between their
control and the control of the positioning axes of a CNC machine tool. The use of
polynomials to describe a trajectory is discussed in Section 2.5.
The trajectory that the end effector, and hence each joint, follows can be generated
from a knowledge of the robot’s kinematics, which defines the relationships between the
individual joints and the end effector’s position in Cartesian space. The solution of
the end effector position from the joint variables is termed forward kinematics, while the
determination of the joint variables from the robot’s position is termed inverse kinematics. To move the joints to the required position the actuators need to be driven under
closed loop control to a required position, within actuator space. The mapping between
the joint, actuator and Cartesian space is shown in Fig. 1.7.
The trajectory that the end effector, and hence each joint, is generated from a knowledge
of the robot’s kinematics, which defines the relationships between the individual joints.
Robotic kinematics is based on the use of homogeneous transformations (Paul, 1984). A
transformation of a space H is represented by a 4  4 matrix which defines rotation and
translation; given a point u, its transform V can be represented by the matrix product,
V ¼ Hu

(1.4)

Following an identical argument, the end of a robot arm can be directly related to another
point on the robot or anywhere else in space. Since a robot consists of a number of links and
joints, it is convenient to use an homogeneous matrix, based on the DenaviteHartenberg
approach wher four parameters associated with each joint-link pair (Denavit and
Hartenberg, 1955; Paul, 1984). For a robot with n joints, the transformation,0Tn, specifies the
location of a tool interface coordinate frame with respect to the base coordinate system and
is the chain product of successive coordinate transformation matrices for each individual
joint-link pair,i1Ai, which can be expressed as,
0

Tn ¼ 0 Ai 1 A2 2 A3 ..:n1 An

(1.5)

In order to determine the change of joint position required to change the end
effectors’ position, use is made on inverse kinematics. Consider the case of a six-axis
manipulator that is required to move an object, where the manipulator is positioned

FIG. 1.7 The mapping between the actuators, joint and work space found in a robotic application. The number of
variables in the cartesian work space is six (three position, three orientation), while the number of variables in
joint and actuator space are determined by the manipulator’s design.

Chapter 1  Electromechanical systems

17

FIG. 1.8 The transformations that need to be considered when controlling the position and trajectory of a six-axis
robot. In this application the robot is required to place its end effector on the box, i.e. reducing the distance
represented by the transformation, B, to zero.

with respect to the base frame by a transform O (see Fig. 1.8). The position and orientation of the tool interface of the six-axis manipulator is described by 0T6, and the
position of the end effector relative to the tool interface is given by E. The object to be
moved is positioned at G, relative to the origin, and the location of the end effector
relative to the object is B. Hence, it is possible to equate the position of the end effector
by two routes, firstly via the manipulator and secondly via the object, giving,
O0 T6 E ¼ BG

(1.6)

Hence,
0

T6 ¼ O1 BGE 1

(1.7)

As T6, is limited to six variables in Cartesian space, the six individual joint positions
are determined by solving the resultant six simultaneous equations. However, problems
will occur when the robot or manipulator is considered to be kinematically redundant.
A kinematically redundant manipulator has more than six joints, hence a unique solution
is not possible and are widely found in specialist applications, for example snake-like
robots used to inspect the internal structures of nuclear reactors.
For a smooth path to be followed, the value of B determined as a function of time. The
robot’s positional information is used to generate the required joint position by the
inverse kinematic solution of the 0T6 matrix. In practice, the algorithms required to
obtain these solutions are complicated by the occurrence of multiple solutions and
singularities, which are resolved by defining the trajectory, and is solution prior to
moving the robot. Usually, it is desirable that the motion of the robot is smooth; hence,
the first derivative (that is, the speed) must be continuous. As the robot moves, the
dynamics of the robot changes, as the forces and inertias seen by individual joints
change constantly. If the position loops are individually closed, a poor end-effector
response results, with a slow speed and unnecessary vibration. To improve the robot’s
performance, and increasingly that of CNC machine tools, considerable use of is made
0

18 Electric Drives and Electromechanical Systems

of the real-time solution of dynamic equations and adaptive control algorithms, as
discussed in Chapter 5, Brushed direct-current motors.

1.3.2

Robotic end effectors

Dextrous manipulation is an area of robotics where an end effector with co-operating
multiple fingers is capable of grasping and manipulating an object. The development
of such hands is a significant electromechanical design challenge, as the inclusion of
multiple fingers requires a significant number of actuators to be fitted into a confined
space. A dexterous end effector can manipulate an object so that it can be arbitrarily
relocated to complete a task. One of the main characteristics of the dextrous manipulation is that it is object and not task centred. It should be noted that dexterity and
dextrous are being used to define attributes to an end effector: a dexterous end effector
may not have the ability to undertake a task that a human considers as dexterous. As
dexterous manipulation is quintessentially a human activity, a majority of the dexterous
robotic end effectors developed to date have significant anthropomorphic characteristics. In view of the importance of this research area a considerable body of research
literature on the analysis of the grasp quality and its control is currently available; the
reviews by Okamura et al. (2000) and Biagiotti et al. (2008) provide an excellent introduction to the field.
As a dextrous end effector needs to replicate some or all the functionality of the human
hand, an understanding of human hand functionality is required in the design process. It
is recognised that there are five functions attributed to the hand: manipulation, sensation
and touch, stabilisation as a means of support, protection, and expression and communication, in robotic systems only the first three need to be considered. The hand can
function either dynamically or statically, its function is the sum of many sub-movements;
these movements may be used to explore an object and be involved in actions such as
grasping and carrying as well as provide dexterity and maintaining stability.
The hand may be used in a multitude of postures and movements, which in most
cases involve both the thumb and other digits. There are two basic postures of the human hand: the power grasp and the precision grasp. The power grasp, Fig. 1.9A, is used

FIG. 1.9 The power grasp (A) and precision grasp (B) of the human hand.

Chapter 1  Electromechanical systems

19

where full strength is needed, is where the object is held in a clamp formed by the partly
flexed fingers and often a wide area of the palm. The hand conforms to the size and
shape of the object. All four fingers flex, with each finger accommodating a position so
that force can be applied, and the force applied to the object to perform a task or resist
motion. In a precision grasp, Fig. 1.9B, there is a greater control of the finger and thumb
position than in the power grasp. The precision grasp is carried out between the tip of
the thumb and that of one or more of the fingers. The object is held relatively lightly and
manipulated between the thumb and related finger or fingers.
The human hand consists of a palm, four fingers and a thumb. The internal structure
consists of nineteen major bones, twenty muscles within the hand, tendons from forearm muscles, and a considerable number of ligaments. The muscles in the body of the
hand are smaller and less powerful than the forearm muscles and are used more for the
precise movements rather than the power grasps. A hand is covered with skin that
contains a wide range of sensors (e.g. temperature, tactile, vibration) and provides the
protective compliant covering.
The classification of movements of the hand in which work is involved can be placed
in two main areas: prehensile and non-prehensile. A prehensile movement is a
controlled action in which an object is held in a grasp or pinching action partly or wholly
in the working envelope of the hand, while a non-prehensile movement is one, which
may involve the whole hand, fingers, or a finger but in which no object is grasped or held.
The movement may be a pushing one such as pushing an object, or a finger-lifting action
such as playing the piano.
The dynamic specification of the human hand can be summarised as:





Typical forces in the range 285e534 N during a power grasp.
Typical forces in the range 55e133 N during a precision grasp.
Maximum joint velocity 600os1.
Maximum repetitive motion frequency, 5 Hz.

The development of dextrous hands or end effectors has been of considerable
importance to the academic robotic research community for many years, and while the
following examples are in no way exhaustive they do however present some of the
thinking that has gone into dextrous robotic systems.
 A significant robotic end effector was the University of Southampton’s Whole Arm
Manipulator (Crowder, 1991). This manipulator was developed at for insertion into
a human sized rubber glove, for use in a conventional glove box. Due to this
design requirement, the manipulator has an anthropomorphic end effector with
four adaptive fingers and a prehensile thumb, Fig. 1.10. Due to size constraints the
degrees of freedom within the hand were limited to three.
 The Stanford/JPL hand (sometimes termed the Salisbury hand) was designed as a
research tool in the control and design of articulated hands. In order to minimise
the weight and volume of the hand the motors are located on the forearm of the

20 Electric Drives and Electromechanical Systems

FIG. 1.10 The Whole Arm Manipulator’s anthropomorphic hand. The manipulator was designed to access a
glovebox through the same rubber glove as used by human operators.

serving manipulator and use Teflon-coated cables in flexible sleeves to transmit
forces to the finger joints. To reduce coupling and to make the finger systems
modular, the designers used four cables for each three degree of freedom finger
making each finger identical and independently controllable (Salisbury, 1985).
 The Utah-MIT Dexterous hand (Jacobsen et al., 1986), is an example of an
advanced dexterous system. The hand comprises three fingers and an opposed
thumb. Each finger consists of a base roll joint, and three consecutive pitch joints.
The thumb and fingers have the same basic arrangement, except the thumb has a
lower yaw joint in place of the roll joint. The hand is tendon driven from external
actuators.
 The Robonaut Hand (Ambrose et al., 2000), is one of the first systems being specifically developed for use in outer space: its size and capability is close to that of a
suited astronaut’s hand. Each Robonaut Hand has a total of fourteen degrees of
freedom, consisting of a forearm which houses the motors and drive electronics, a
two degree of freedom wrist, and a five finger, twelve degree of freedom hand.
The design of the fingers and their operation is the key to the satisfactory operation of
a dexterous hand. It is clear that two constraints exist. The work by Salisbury (1985)
indicated that the individual fingers should be multi-jointed, with a minimum of three
joints and segments per finger. In addition, a power grasp takes place in the lower part of
the finger, while during a precision grasp it is the position and forces applied at the
fingertip that is of the prime importance. It is normal practice for the precision and
power grasp not to occur at the same time.
In the design of robotic dexterous end effectors, the main limitation is the actuation
technology: it is recognised that an under-actuated approach may be required, where the
number of actuators used is less than the actual number of degrees of freedom in the
hand. Under-actuation is achieved by linking one or more finger segments or fingers
together: this approach was used in Southampton’s Whole Arm Manipulator, Fig. 1.10.
As discussed by Birglen et al. (2008) approximately 50% of all robotic hands are of

Chapter 1  Electromechanical systems

21

underactuated design, in an attempt to replicate the complexities of the human hand
with its 20 degrees of freedom.
The location and method of transmission of power is crucial to the successful
operation of any end effector, in particular the end effector size should be compact and
consistent with the size of the manipulator. Both fully and under-actuated dexterous
artificial hands have been developed using electric, pneumatic or hydraulic actuators.
The use of electrically powered actuators has, however, been the most widely used, due
to both its convenience and its simplicity compared to the other approaches. The use of
electrically powered actuator systems ensures that the joint has good stiffness and
bandwidth. One drawback with this approach is the relatively low power to weight/
volume ratio which can lead to a bulky solution: however, the developments in magnetic
materials and advanced motor design have (and will continue to) reduced this problem.
In many designs the actuators are mounted outside the hand with power transmission
being achieved by tendons. Pneumatic actuators exhibit relatively low actuation bandwidth and stiffness and consequently, continuous control is complex. Actuation solutions developed using pneumatics (if the pump and distribution system are ignored)
offer low weight and compact actuators that provide considerable force. Hydraulic actuators can be classified somewhere in between pneumatics and electrically powered
actuators. With hydraulics the system stiffness is good due to the low compressibility of
the fluid. While pneumatic actuators can be used with gas pressures up to 5e10 MPa,
hydraulic actuators will work with up to 300 MPa. One approach that is being considered
at present is the development of artificial muscles; Klute et al. (2002) provide a detailed
overview of the biomechanics approach to aspects of muscles and joint actuation. In
addition, the paper presents details of a range of muscle designs, including those based
on pneumatic design which can provide 2000 N of force. This force equates to that
provided by the human’s triceps. The design consists of an inflatable bladder sheathed
double helical weave so that the actuator contracts lengthwise when it expands radially.
Other approaches to the design for artificial muscles have been based on technologies
including shape-memory alloy, pneumatics, electro-resistive gels and dielectric elastomers which are discussed in Chapter 9, Related motors and actuators.
When considering conventional technologies, the resultant design may be bulky and
therefore the actuators must be placed somewhere behind the wrist to reduce system
inertia. In these systems power is always transmitted to the fingers by using tendons or
cables. Tendon transmission systems provide a low inertia and low friction approach for
low power systems. As the force transmitted increases considerable problems can be
experienced with cable wear, friction and side loads in the pulleys. One of the main
difficulties in controlling tendon systems is the that force is unidirectional - a tendon
cannot work in compression. The alternative approach to joint actuation is to use a solid
link which has a bi-directional force characteristic, thus it can both push and pull a
finger segment. The use of a solid link reduced the number of connections to an individual finger segment. The disadvantage of this approach is a slower non-linear dynamic
response, and that ball screw or crank arrangement is required close to the point of

22 Electric Drives and Electromechanical Systems

actuation. Irrespective of the detailed design of the individual fingers, they are required
to be mounted on a supporting structure, this is more fully discussed in academic papers
such as Pons et al. (1999).

1.3.3

Mobile and swarm robotics

In recent years there has been a considerable increase in the types and capabilities of
mobile robots, and in general three classes can be identified: UAV (unmanned aerial
vehicles), UGV (unmanned ground vehicles) and UUV (unmanned underwater vehicles).
In certain cases, the design and control theory for a mobile robot has drawn heavily on
biological systems, leading to a further class, biologically inspired robotics and swarm
robots. An early example of this type of robot was the Machina Speculatrix developed by
W. Grey Walter (Holland, 2003), which captured several principles including simplicity,
exploration, attraction, aversion and discernment. Since this original work a considerable number of robots have been developed including both wheeled and legged. The
applications for mobile robots are wide-ranging and include:
 Manufacturing systems. Mobile robots are widely used to move material around
factories. The mobile robot is guided through the factory using underfloor wiring
or visual guidelines. In most systems the robots follow a fixed path under the control of the plant’s controller, hence they can move product as required by the
manufacturing process.
 Security systems. The use of a mobile robot is considered to be a cost-effective
approach to patrolling large warehouses or other buildings. Equipped with sensors
they are able to detect intruders and fires.
 Ordinance and explosive disposal. Large numbers of mobile robots have been
developed to assist with searching and disposal of explosives, one example being
the British Army’s Wheelbarrow robots. The goal of these robots is to allow the inspection and destruction of a suspect device from a distance without risking the
life of a bomb disposal officer.
 Planetary exploration. Fig. 1.11 shows an artist’s impression of one of the two Mars
rovers that were landed during January 2004. Spirit and Opportunity have considerably exceeded their primary objective of exploring Mars for 90 days, with Spirit
operated for over 2000 martian days and driven over 7.7 km and Opportunity
operated for over 5000 martian days before being engulfed in a global martian
dust storm. In that time Opportunity has driven over 45 km and returned over
220,000 images to earth. It should be noted that each rover incorporated 39 d.c.
brushed ironless rotor motors (see Section 5.2), used to drive the wheels, robotic
arm and camera positioning. These motors were of standard designs with a number of minor variations, particularly as the motors must endure extreme conditions, such as variations in temperature which can range from -120 C to þ25 C.

Chapter 1  Electromechanical systems

23

FIG. 1.11 An artist’s impression of the rover Spirit on the surface of Mars. The robotic arm used to position scientific
instruments and the moveable camera mast are clearly visible. Credit: Image reproduced courtesy of NASA.

While many mobile robots operate independently, either autonomously or as a
teleoperated system, swarm robotics is an approach that involves coordination of large
numbers of robots and are used to study how a large number of relatively simple
physically embodied agents can be designed such that a desired collective behaviour
emerges from the local interactions among agents and between the agents and the
environment (Bayindir and Sahin, 2007). Swarm robotics has its origins in the study of
social insects where they work collaboratively, to achieve tasks that could not be achieved individually, for example, a colony of ants can cross wide void or pull leaf edges
together to form a nest; bees and wasps build nests and termites build nests with
remarkable heights and complexity (Bonabeau et al., 1999). With these characteristics
(Beni, 2005), swarm robotic systems can be differentiated from other multi-robot systems by having the following key aspects: autonomous mobility, limited sensing and
communication abilities, simplicity, decentralised control or coordination mechanism,
homogeneity and scalability, Fig. 1.12. They also have to aim the following advantages
(Doriigo et al., 2013): robustness where the system should be able to continue to operate
when there are failures in the individuals or disturbances in the environment, flexibility
where the system should be able to adapt to the dynamics of the number of robots and
the change of tasks, and scalability where the system should be able to operate with
different numbers of robots.

1.3.4

Walking robots

While many mobile robots are wheeled, there is increasing interest in legged systems,
partly due to increased research activity in the field of biologically inspired robotics and
biomechanical systems. A walking robot are more versatile than wheeled robots and can
traverse many different terrains, however these advantages require increased complexity
and power consumption. Many legged designs have been realised, ranging from military
logistic carriers to small replicas of insects. These robots, termed biometric robots,

24 Electric Drives and Electromechanical Systems

FIG. 1.12 A swarm robot measuring 100 mm  100 mm based on a single PCB board design, using an ARM CortexM0 as the main controller. The robot has configurable analogue and digital subsystem which together with the
capability for I2C communication provides facilities for reconfiguration and expansion. Eight infra red (IR) LEDs
and four IR photodiodes are used detect objects to avoid as well as providing low level communication to other
swarm robots. In addition the robot is fitted with a ground facing colour sensor, accelerometer and
magnetometer.

mimic the structure and movement of humans and animals. Of particular interest to the
research community is the construction and control of dynamically stable legged robots.
In the design of these systems the following constraints exist (Robinson et al., 1999).
Firstly, the robot must be self-supporting, which puts severe limits on the force/mass
and power/mass ratio of the actuators. Secondly, the actuators of the robot must not be
damaged during impact steps or falls and must maintain stability following an impact.
Finally, the actuators may need to be force controllable because the in some approaches
the algorithms used for robot locomotion are force based.
A large number of walking robots have been developed with two, four or six legs, as
discussed by Zhou and Bi (2012) for a variety of applications ranging from pure research
through to load carrying for military or humanitarian logistics requirements in areas
where wheeled vehicles could not be used. Fig. 1.13 shows two possible approaches to
the design of a leg. One of two possible approaches are possible dependant of the design
of the joints that replicate the motion of the hip. The placement of the two coincident
hip joints has the advantage of placing the respective actuators on or close to the vehicle
body, so that their mass is not carried by the leg during motion. In addition, the design of
the hip joint provides optimum workspace and simplifies the kinematics. There are two
variations of this approach to leg design leg: mammal or insect inspiration. In the leg
based on a mammal, the knee joint is placed under the hip, Fig. 1.13A, this approach is
used in either biped robots or four legged running robots. In the insect-based design, the
knee joint is located laterally or at a position higher than the hip, Fig. 1.13B, this design to
typically used in six-legged insect-based robots.

Chapter 1  Electromechanical systems

25

FIG. 1.13 Kinematic design of three jointed legs used in robotic applications. (A) Robotic leg that can be used in
walking robots inspired by mammals. (B) Robotic leg that can be used in walking robots inspired by insects.

A related application is the use using powered exoskeletons as part of the treatment of
patients during rehabilitation following a stroke or spinal cord injury. The exoskeletons
are effectively walking robots that are designed to support the patient and aim to
enhance the rehabilitation process.

1.4 Automotive applications
The current automotive market is in a state of considerable flux, with rapid advances in
autonomous vehicle technology, as well as the goal of reducing our dependence on
petrol and diesel. In both cases considerable use is made of electromechanical actuators
to operate a range of function. In the electric car the prime mover is an electric motor.

1.4.1

Conventional vehicles

In the majority of cars electric motors undertake functions that were either formerly
considered the domain of mechanical linkages or to provide additional features that
increase driver comfort or safety. The conventional brushed d.c. motors, can be found
in body and convenience areas, for example windscreen wipers and electric windows.
Increasingly brushless motors are also being used in open loop pump drives and air
conditioning applications. It is estimated that on average a modern car has over 40
individual motors, Fig. 1.14. With the rapid introduction of autonomous systems in to
vehicles systems such as intelligent brake-control, throttle-by-wire and steer-by-wire
that require a sensor, a control unit and an electric motor. It has been estimated that
the electrical load in a car will increase from to around 2.5 kW, with a peak value of over
12 kW. This implies that the electrical system will have to be redesigned from the
current 12 V d.c. technology to use at higher voltages. One of the possible options is a

26 Electric Drives and Electromechanical Systems

FIG. 1.14 Overview of the electrically operated functions supplied in a current petrol powered car.

multi-voltage system with some functions remaining at 12 V, and others operating from
voltages as high as 48 V, (Kassakian et al., 1996), this will allow for the reduction in
cabling volume as a higher voltage results lower current draw.

1.4.2

Electric vehicles

Electric cars were first developed in the early 1900’s and were widely used, until the
introduction of low cost, mass produced petrol engine cars by Ford. It is only recently with
increasing concerns about the environment that there has been a determined move to
replace petrol and diesel vehicles with all electric versions. Currently vehicle can be
supplied in two main versions, a hybrid, where a small petrol engine supplements the
energy stored in the battery to allow increased range and less dependence on charging
points, or the pure electric vehicle that depends solely on the stored energy in the battery.
Electric machines drives are a key part of the three enabling technologies for electric
vehicles and hybrid electric vehicles, the other being the batteries and overall control
system. The basic characteristics which are required of the electrical motor and its
associated drive system include:
 A high torque density and power density
 High torque at low-speed with the capability for starting and driving up hill, in
addition the availability of high power at high-speed is required for safe driving.
 High efficiency over the required torque and speed ranges, particularly at low
torque operation
 High reliability and robustness for given the environment
 Acceptable costs, particulary those assocated with the battery.
The actual drive motor used is a company specific decision as while the induction
motor, permanent-magnet synchronous motor or switched reluctance motor can provide the required characteristics.

Chapter 1  Electromechanical systems

27

1.5 Aerospace applications
As with automotive applications, the aerospace industry is faced with challenge of
reducing operating costs, particularly fuel, and reducing environmental impact. In
addressing this challenge, designers are increasingly turning to the concept of the moreelectric-aircraft and the all-electric-aircraft. In this approach systems are replaced with
an electrically powered equivalent. While this will increase the aircraft’s electrical power
requirements, there is an overall saving in weight and increase in efficiency (Jones, 2002).
Fig. 1.15 shows the key components and load centres for the all-electric-aircraft. Two
main areas are being addressed, cabin air supply and actuators.
In the majority of passenger aircraft, the cabin air supply is bled from the engines, this
has a detrimental impact on engine performance as in most conditions, conventional
pneumatic systems withdraw more power than needed, causing excess energy to be
dumped. In bleed-less technology, no HP air is extracted from the engines, allowing
more efficient thrust production and engine operations. It is estimated at a bleed-less
configuration will give a 2%e3% increase in efficiency. However, this comes at a cost
of increasing the on-board generating capability an increased complexity to the
electrical power distribution system. It should be noted that the electrical power requirements of a modern aircraft are considerable, for example the Airbus A380 has a
generating capability of 600kVA. One of the challenges in modern aircraft is the approach
to the generation and power management, particularly as the speed of the engine is not
constant, two approaches can be considered:
 A constant frequency supply, typically 400Hz, however this required the generator to
be fitted with constant speed drive as part of the integrated drive generator. A constant
supply frequency allows easy conversion of power to other voltages as required.

FIG. 1.15 The all-electric-aircraft concept. The possible location of electrically powered actuators, drives and
related functions within a civil aircraft are shown. The location of power generation is highlighted in bold italics,
the APU (auxiliary power unit) is normally only run on the ground. The actuators for the flying surfaces can either
be powered by hydraulics or electrical power. The hydraulic pumps can be either directly coupled to the engines
or driven by an electric motor.

28 Electric Drives and Electromechanical Systems

 Suppling the aircraft with a variable frequency supply (normally in the range
300e800Hz) and convert a proportion to either a fixed frequency supply or d.c. as
required. In any aircraft a substantial proportion of the electrical supply goes on
heating (galley ovens, de-icing), which does not require a constant frequency
supply.
Flying surfaces actuators of civil aircraft are conventionally powered through three
independent and segregated hydraulic systems. In general, these systems are heavy,
complex to install and costly to maintain. Electrically powered flight systems are not
new: several aircraft developed in the 1950’s and 60’s incorporated electrically actuated
control functions, however they were exceptions to the general design philosophy of the
time.
In the more- or all-electric-aircraft the distribution of power for flight actuation will
be through the electrical system, as opposed to the currently used bulk hydraulic system.
It has been estimated that the all-electric-aircraft could have a weight reduction of over
5000 kg over existing designs, which could be converted into an increase in range or a
reduction in fuel costs. To implement power-by-wire, high-performance electrically
powered actuators and related systems are required, (Howse, 2003). Electrically powered
flight actuators can take one of two principal configurations, the electromechanical
actuator with mechanical gearing, and the electrohydrostatic actuator, or EHA, with
fluidic gearing, between the motor and the actuated surface.
In an EHA, hydraulic fluid is used to move a conventional hydraulic actuator, the
speed and direction of which are controlled by the fluid flow from an electric motor
driven hydraulic pump, (Crowder and Maxwell, 2002). If a displacement pump (see
Fig. 1.16) is used, where the piston’s diameter is dp and the pitch diameter is dpp, the flow
rate Q(t) as a function of the pump speed, up(t) can be determined to be,
QðtÞ ¼ Dup ðtÞ

(1.8)

where the pump constant, D, is given by,
D¼

pdp2 dpp tana
4

(1.9)

Hence the flow rate in a variable-displacement pump unit can controlled by
adjustment of the swash plate angle, a, and hence piston displacement. In this approach
two motor-drives are required, a fixed speed drive for the pump, and a small variablespeed drive for positioning the swash plate. A different approach is just to control the
rotational speed of a displacement pump, up, where a is fixed this design only requires
the use of a single variable-speed motor drive.
Fig. 1.17 shows a possible concept for an electrohydrostatic actuator suitable for medium
power surfaces, such as the ailerons. In most future designs the fixed pump option will be
considered for the rudder, which requires a far higher power output. In the actuator the basic
hydraulic system consists of the pump, actuator, and accumulator. Valve A ensures that the
low-pressure sides of the pump and actuator are maintained at the accumulator’s pressure,

Chapter 1  Electromechanical systems

29

FIG. 1.16 The displacement hydraulic pump used in an EHA. Driving the valve cylinder causes the pistons to
operate, the amount of stroke is determined either by the rotation speed, or the swash plate angle, a.
The clearances between the cylinder block and, the valve block and casing have been exaggerated.

therefore ensuring that cavitation does not occur in the system. In practice, the motor can
have a flooded air gap, allowing the motor to be cooled by the hydraulic fluid, cooling oil is
taken from the high-pressure side of the pump and returned to the accumulator. The
accumulator has a number of functions: maintaining the low pressure in the system to an
acceptable value, acting as the hydraulic fluid’s thermal radiator, and making up any fluid
loss. It is envisaged that the unit is sealed at manufacture, and the complete actuator
considered to be a line replacement unit.
The flow of hydraulic fluid, and hence the actuator’s displacement, is determined by
the pump’s velocity. To obtain the specified required slew rate, the required motor speed
of approximately 10,000 rpm will be required, depend on the pump and actuators size. It
should be noted that when the actuator is stationary, the motor will still rotate at a low
speed (typically less than 100 rpm), because of the small leakage flows that occur within
the actuator and pump. In an actuator of this type, the peak pressure differential within
such a system is typically 20 MPa.
The motor used in this application can be a sinusoidally wound permanent magnet
synchronous motor, the speed controller with vector control to achieve good low speed
performance. An outer digital servo loop maintains the demanded actuator position,
with a LVDT measuring position. The controller determines the motor, and hence pump
velocity. Power conversion is undertaken using a conventional three phase IGBT bridge.

30 Electric Drives and Electromechanical Systems

FIG. 1.17 Outline of an Electrohydrostatic Actuator for use in an aircraft. Valve A is a bypass valve that allows the
ram to move under external forces in the case of a drive failure, valve B ensures that the pressure of the input
side of the pump does not go below that of the accumulator.

In an aircraft application the power will be directly supplied from the aircraft’s bus, in
the all-electric-aircraft this is expected to be at 270 V dc, as opposed to the current 110 V,
400 Hz ac systems. To prevent excessive bus voltages when the motor drive is regenerating under certain aerodynamic conditions, particularly when the aerodynamic loading
back drives the actuator, a bus voltage regulator to required dissipate excess power is
required, as discussed in Section 5.4.

1.6 Motion-control systems
In this brief review of the motion requirements of machine tools, robotics and related
systems, the satisfactory control of the axes, either individually or as a coordinate group,
is paramount to the implementation of a successful system. In order to achieve this
control, the relationship between the mechanical aspects of the complete system and its
actuators needs to be fully understood. Even with the best control system and algorithms
available, it will not perform to specification if the load cannot be accelerated or
decelerated to the correct speed within the required time and if that speed cannot be
held to the required accuracy.
A motion-control system consists of several elements (see Fig. 1.18) whose characteristics must be carefully determined in order to optimise the performance of the
complete system. A motion control system consists of five elements:
 The controller that implements the main control algorithms (normally either speed
or position control) and provides the interface between the motion-control system
and the main control system and/or the user.

Chapter 1  Electromechanical systems

31

FIG. 1.18 Block diagram of a motion-control system typically used in robots and machine tools.

 The encoders and transducers required to provide feedback of the load’s position
and speed to the controller.
 The motor controller, and motor. In most cases, these can be considered to be an
integral package, as the operation and characteristics of the motor being totally
dependent on its control package. Depending on the motor type, position and
current feedback may be required.
 The transmission system. This takes the motor output and undertakes the required
speed changes and, if required, a rotary-to-linear translation.
 The load. The driven elements greatly influence the operation of the complete system. It should be noted that a number of parameters, including inertia, external
loads, and friction, may vary as a function of time, and need to be fully determined
at the start of the design process.
The key to successful implementation of a drive system is full identification of the
applications needs and hence its requirements; these are summarised in Table 1.2. In
order to select the correct system elements for an application, a number of activities,
ranging over all aspects of the application, have to be undertaken. The key stages of the
process can be identified as follows:
 Collection of the data. The key to satisfactory selection and commissioning of a
motor drive system is the collection of all the relevant data before starting the
sizing and selection process. The information obtained will mostly relate to the
system’s operation but may also include commercial considerations.
 Sizing of the system. The size of the various drive components can be determined
from the data collected earlier.
 Identification of the system to be used. Once the size of the various elements and
the application requirements are known, the identity of the various elements can
be indicated. At this stage, the types of the motor, feedback transducer, and
controller required can be finalised.
 Selection of the components. Using the acquired knowledge, the selection process
can be started. If the items cannot be supplied and the selection revised, or the

32 Electric Drives and Electromechanical Systems

Table 1.2 Requirements to be considered during the selection of the components of
a motor-drive system.
Load requirements

System integration

Life-cycle costs

Environmental factors

Maximum speed
Acceleration and deceleration
Motion profile
Dynamic response
External forces
Mechanical fittings
Bearing and couplings
Cooling
Compatibility with existing systems
Electrical supply specifications and compatibility
Provision of cyber security measures if required
Initial costs
Operational costs
Maintenance costs
Disposal costs
Safety and risk management
Electromagnetic compatibility
Climatic and humidity ranges

specification of a component is changed, the effect on the complete system must
be considered.
 Verification. Prior to procuring the components, a complete check must be made
to ensure that the system fits together in the space allocated by the other members
of the design team.
 Testing. Theoretically, if all the above steps have been correctly followed, there
should be no problems. But this is not always the case in the real world, commissioning modification may be required. If this is required care must be taken to
ensure that the performance of the system is not degraded.
One of the main design decisions that is required is the selection of the correct motor
technology. With the rapid development in this field, number of options are available;
each option will have benefits and disadvantages. In the consideration of the complete
system the motor determines the characteristic of the drive, and hence determines the
power converter and control requirements. A wide range of possibilities exist, however
only a limited number of combinations will have the broad characteristics which are
necessary for machine-tool and robotic applications, namely:






A high speed-to-torque ratio.
Four-quadrant capability.
The ability to produce torque at standstill.
A high power-to-weight ratio.
High system reliability.

Chapter 1  Electromechanical systems

33

The following motor-drive systems satisfy these criteria, and are widely used in
machine tool, robotic and other high-performance applications:
 Brushed, permanent-magnet, d.c. motors with a pulse width modulated or linear
drive systems (see Chapter 5);
 Brushless, d.c., permanent-magnet motors, either with trapezoidal or sinusoidal
windings (see Chapter 6);
 Vector, or flux-controlled induction motors (see Chapter 7)
 Stepper motors (see Chapter 8)
Except for brushed, permanent-magnet d.c. motors, all the other machines are totally
dependent on their power controller and will be treated as integrated drives. The list
above covers most widely used motors, however recent development has allowed the
introduction of other motors, ranging from the piezoelectric motor to the switched
reluctance motor; these and other motors are briefly discussed in Chapter 9.

1.7 Summary
This chapter has briefly reviewed a number of application areas where high performance
servo drives are required. It has been clearly demonstrated that the satisfactory performance of the overall system is dependent on all the components in the motor-drive
system and its associated controllers; in particular, it is dependent on its ability to
provide the required speed and torque performance. The determination of the characteristics that are required is a crucial step in the specification of such systems, and this
will be discussed in subsequent chapters.

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