chapter 9 related motors and actuators

8/2/2019 Chapter 9 Related Motors and Actuators

1/12

Chapter 9Related motors and actuatorsThe previous chapters have considering the motors and drives that are normallyused within the range of applications that have been identified in Chapter 1. However, there are a number of specialist or unconventional motors that can and arebeing used in an increasing number of applications. These motors may be selectedfor a wide verity of reasons, both technical and comm ercial. This chapter considersa number of theses motors and their associated controllers, therefore allowing thedesign engineer to have an overview of all available technologies. In this chapterthe following motors are considered:

voice coil actuators limited-angle torque motors piezoelectric motors switched reluctance motors shape memory alloy, SMA.

While these motors currently have specialist niches in servo drive applications, arange of exciting motors are currently being developed based a wide range of technologies, including electrostatic and micro electromechanical (MEM)technologies,and these will no doubt find their way into more general use over time (Hameyerand Belmans, 1999). Currently this technology is still the research stage, but theappUcations currently being explored are significant and challenging, and for example include the manipulation of a single DNA molecule (Chiou and Lee, 2005).

9.1 Voice coilsVoice coils or solenoids are ideally suited for short linear (typically less than 50mm) closed-loop servo applications and both operate on similar principles. In avoice coil, the actual coil m oves, while in a solenoid, the iron core moves. Typical

235


2/12

236 9.1. VOICE COILSSoft iron core for fluxreturn

Magnet Tubular coil

Output

Figure 9.1. The cross section of a voice coil; the dimensions of the air gap hasbeen exaggerated.positioning appHcations include direct drives on pick and place equipment, medicalequipment, and mirror tilt and focusing actuators. In addition voice coils can alsobe used in appHcations where precise force control is required, due to the linearforce versus current characteristics.

A voice coil is wound in such a way that no commutation is required, hence asimple linear amplifier can be used to control the actuator's position. The result isa much simpler and more reliable system. Voice coils have a number of significantadvantages including small size, very low electrical and mechanical time constants,and low moving mass that allows allows for high accelerations, though this dependson the load being moved.Voice coil actuators are direct drive, limited motion devices that utilise a permanent magnet field and coil winding (conductor) to produce a force proportionalto the current applied to the coil. These non-commutated electromagnet devicesare used in Hnear (or rotary) motion applications requiring a linear force output,high acceleration, or high frequency actuation.The electromechanical conversion mechanism of a voice coil actuator is governed by the Lorentz force principle; which states that if current-carrying conductoris placed in a magnetic field, a force will result. The magnitude of the force is determined by the magnetic flux destiny, B, the current, z, hence for a winding of A^turns, the resultant force is given by

F = BLiN (9.1)In its simplest form, a linear voice coil actuator is a tubular coil of wire situatedwithin a radially oriented magnetic field, as shown in Figure 9.1. The field is produced by permanent magnets embedded on the inside of a ferromagnetic cylinder.


3/12

CHAPTER 9, RELATED MOTORS AND ACTUATORS 237The inner core of ferromagnetic material is aligned alon g the axial centreline of thecoil, and joined at one end to the permanent magnet assembly, is used to completethe magnetic circuit. The force generated axially upon the coil when current flowsthrough the coil will produce relative motion between the field assembly and thecoil, provided the force is large enough to overcome friction, inertia, and any o therforces from loads attached to the coil. Fo r a specific op erating displacem ent of theactuator, the axial lengths of the coil and the magn et assem blies can be chosen suchthat the force vs displacem ent curve can be optim ised, resulting in the reduction offorce at the mid-stroke force being limited to less than 5% of the maximum force.

The sizing and selection of a voice coil actuator is no different from any otherUnear application, the process defined in Section 3.8.4 can be followed.

9.2 Limited-angle torque motorsLim ited-angle torque m otors are a range of special-purpose m otors that are capableof giving controllable motion up to 90 from their rest position. While brushlessmotors, as discussed in Chapter 6, have many benefits, they have the penalty ofbeing relatively expensive and complex, if only a limited range of motion is required. The requirement for a limited range of movement can be found in manyapplications, including the operation of air or hydraulic servo-valves and oscillatingmirrors. In addition, their inherent reliability of operation makes a limited-angletorque motors an ideal solution for applications where limited actuation is critical, for exam ple in spacecraft latches, where the only previous solution w as to usepyrotechnics.

The basic construction of a limited-angle torque motor is shown in Figure 9.2.While they are broadly similar to brushless d.c. motors, the limited-angle torquemoto r is a single-phase dev ice, which eliminate the need for the com mu tation logicand the three-phase pow er bridge that are found in multiphase m achine s. Thetorque motor's winding can be wound in conventional slots or as a toroid overa slotless stator. The rotor in a limited-angle torquer incorpo rates one or moremagnets. The slot-wound limited-angle torque motor has a number of advantagesover toroidally wound motors; in particular they have better thermal dissipationand a higher torque constant. How ever, because of the presence of slots, the outputtorque ripple and hysteresis losses are greater. Th e torque ripple can be con sideredto be zero with toroidally w ound m otors due to the non-varying reluctanc e path andthe large air gap. In addition the slot-wound limited-angle torque motor exhibits ahigher motor constant. Km, than the corresponding toroidally wound motor, dueto the larger number of conductors that are exposed to the magnetic field.

Cogging is essentially zero in toroidally wound limited-angle torque motor,a result of a non-varying reluctance path and relatively large air gap . Toroidallywound armatures, moreover, are typically moulded onto the stator, which protectsthe windings from damage and holds them in place.

In the selection of a limited-angle torqu e mo tor for an application, a num ber of


4/12

238 92. LIMITED-ANGLE TORQUE MOTO RSgtator

Stator

(a) Slotted armature. (b) Toroid armature.Figure 9.2. Internal construction of limited angle torque motors.

Torque Torque

+45^ -45Rotor position(a) Slotted armature.

0Rotor position

(b) Toroid armature.+45

Figure 9.3. Torque-position characteristics for a limited angle torque motor.parameters shall be considered, including:

Peak torque. As in a conventional motor, this is the torque which is developed at the rated current.

Excursion angle. This is the maximum angle that the rotor can move fromthe peak-torque position, and it is normally expressed as a plus/ minus v alue.Figure 9.3 shows typical characteristics for a slot-wound and a toroidallywound motor. In the latter case, the constant-torque region should be noted.Lim ited-angle torque motors are currently available in ratings from 7 x lO""^to 0.142 N m, with excursion angles between 18 and 90.

As limited-angle torque motor are single-phase motors, they are easily controlled by single-phase bipolar PWM amplifiers which are identical to those usedwith brushed d.c. motors. In certain applications, a linear amplifier could be usedto increase the bandw idth and to reduce the electrical noise. The lim ited-angletorque motor produces torque through a rotation angle determined by the numberof motor poles. Current of one polarity produces clockwise torque, and vice versa.Manufacturers generally provide a theoretical torque versus shaft-position curve.Typically, the characteristic cu rve for a slotted arm ature limited-ang le torque motoris represented by a cosine function; that is

T = T cos-ON (9.2)


5/12

CHAPTER 9. RELATED MOTORS AND ACTUATORS 239Torque

Loadtorque

Usable displacement-Rotor position

Figure 9.4. The restriction in usable displacement of a limited-angle torque motoras a function of load torque.where 6 is angle of rotation, A^ is number of poles, and Tp is the peak torque. Thegeneral torque characteristic for toroidally wound motors can be represented by asimilar curve, but it may also have aflat op.

The selection of a limited-angle torque motor for an application follows anidentical route to that of any motor. The process starts with the determinationof the application's constraints and of the performance which is required. Oncethe torque, and the angle over which it is to be applied, has been determined, thesuppliers data must be referred to. As the torque-angle characteristic of limited-angle torque motor is sinusoidal, care must be taken to ensure that these devicescan produce the required torque throughout the proposed actuation angle, as shownin Figure 9.4.

9.3 Piezoelectric motorsMany specialist applications require motors of extremely high resolution, forexample, micropositioning stages, fibre-optic positioning, and medical catheterplacement. One motor that can meet these requirements is the piezoelectric motor. When compared to a conventional motors and its associated power train, thepiezoelectric motor has a faster response times, far higher precision, inherent brakecapability with no backlash, high power-to-weight ratio, and is of smaller size.

The operation of this motor is based on the use of piezoelectric materials wherea material is capable of being deformed by the application of a voltage. A rangeof materials such as quartz (Si02) or barium titanate (BaTiOa) exhibit the piezoelectric effect. However in motors normally mass-produced polycrystalline piezoelectric ceramic is used. To produce a suitable ceramic, a number of chemicalsare processed, pressed to shape,fired,and polarised. Polarisation is achieved usinghigh electricfields(2500 V/mm) to align material domains along a primary axis. InFigure 9.5(c), a voltage is applied to a piezoelectric crystal to produce a displacement. If the material has a displacement constant of 5(X) pm V~^ the application


6/12

240 9.4. SWITCHED RELUCTANCE MOTOR S

(a)

f f \ \ / , \ \\ / M ' \

(b)

(c)Figure 9.5. The characteristic of a piezoelectric material, (a) shows dom ains inthe the unpolarised m aterial, which align when po larised, as shown in (b). Theapplication of a voltage causes axial displacement, d.

of 200 V, will produces an axial displacemen t of 0.1 fim.Figure 9.6 shows the basic concepts of a piezoelectric motor. Two piezoelectric crystals are preloaded against a flat wear surface, by way of the motor shoe,

to produce a normal con tact force. The friction is important in the design of themotor, since the friction force is used to translate the motion of the piezoelectricceramic into the motor's output. As a positive sinusoidal voltage waveform is applied w hich increase its thickness, the axial motion imp arts a frictional force alongthe wear strip. When the drive voltage goes negative, the same crystal thicknesscontac ts. Th is action creates a separation between the motor shoe and the wearstrip, allowing the motor to return to its original position without dragging thewear strip backward. As the drive voltage swings positive again, the crystal strokecycle repeats and the wear strip moves another incremental step to the left.

9.4 Switched reluctance motorsWhile not originally designed for high-performance servo applications, theswitched reluctance motor is making inroads into this area, due to the availabilityof low-cost digital signal proce ssing. T he sw itched reluctance m otor is particularlysuitable to a wide range of applications due to the robustness of the mech anical andelectrical design.

In a reluctance m achine, the torque is produced by the moving comp onen t mo ving to a position such that the inductance of the excited winding is max imised. Themoving component is typically the machine's rotor - which can be either internalor external depending on the design - or a linear com ponen t in the case of a linear


7/12

CHAPTER 9. RELATED MOTORS AND ACTUATORS 241

^ ^

PreloadSpings

n(a) The motor at rest (Vs 0): the motor head ispreloaded against the wear surface.

E S ^PreloadSpings

D(b) On excitation of the piezoelectric actuator (V^ > 0),the head moves against the wear surface, moving thewear surface.

Gap

^ ^PreloadSpings

(c) Excitation of the piezoelectric material (Vs < 0),releases the actuator for the wear surface, allowing theactuator to return to its initial position.Figure 9.6. The operation of a piezoelectric motor.


8/12

242 9.4. SWITCHED RELUCTANCE MOTO RS

indings

Stator

Figure 9.7. The cross section of a switched reluctance motor.

reluctance motor.The switched reluctance motor is topologically and electromagnetically simi

lar in design to the variable-reluctance stepper motor discussed in Section 8.1.2.The key differences lie in the details of the engineering design, the approach tocontrol, and hence its performance characteristics. The switched reluctance motoris operated under closed loop control, with a shaft mounted encoder being usedto synchronise the phase currents with rotor position. In comparison the variable-reluctance stepper motor is operated open loop.

Th e operating principles of the switch reluctance mach ine can be considered byexamination of Figure 9.7. The number of cycles of torque production per motorrevolutions is given by

S = mNr (9.3)where m is the num ber of p hases, and A^ the num ber of ph ases. A m ore detailedanalysis of the mo tor can be found in Miller (2001). The vo ltage equation fora single phase can be calculated in a similar fashion to that used for a brushlessmotor

V ^ Ri-^ ~dt Ri + UJrd^p (9.4)

where v is the terminal voltage, i is the phase current, ip is the flux-linkage involt-seconds, R is the phase voltage, L is the inductance of the phase winding, 9


9/12

CHAPTER 9. RELATED MOTORS AND ACTUATORS 243is the rotor position and Um is the rotor's angular velocity. This equation can beexpanded to give

v = Ri + iUm~^ =Ri + L+ uJmi-jK (9-5)du dt duIn a similar fashion to a d.c. brushed mo tor it is useful to consider the terminalvoltage V as the sum of three components: the resistive voltage drop, the voltagedrop due to the inductance and rate of change of current, and the back e.m.f. term,e

e = u:J^ (9.6)From equation 9.5 it is possible to calculate the instantaneous electrical power, vi ,as,

^ 9 ^ di .9 dL ,_ _^vi = Ri^-\-Li+ i^rrT-TT: (9 .7 )di dBwhich allows the rate of change in magnetic energy to be calculated:

The electromagnetic torque generated by the motor can therefore be determinedfrom the instantaneous electrical power minus the resistive voltages drops due andthe rate of change of magnetic stored energy:

Te = ^^-^^ (9.9)

Th e rate of chang e of inductance as a function of rotor position is one of the designparameters of the switched reluctance machine. From equation 9.9 it is clear thatthe torque does not depend on the direction of current flow, however the voltagemust be reversed to reduce the flux-linkage to zero. A suitable power circuit fora single winding is shown in Figure 9.8. It is imm ediately clear that this circuitis far more robust that the conventional PWM bridge shown in Figure 6.5(a), as aUne-to-Une short circuit is not possible.

The circuit shown in Figure 9.8 is capable of operating the motor as either amotor or a generator, as vi can either be positive or negative, and the power flowis determined by the switching pattern of the power bridge relative to the rotor'sposition. A block diagram of a suitable controller for a basic switched reluctancemotor is shown in Figure 9.9. It is recognised that although this type of driveis simple, and gives adequate performance for speed control, it is incapable ofproviding instan taneous torque control as required by a servo or similar application.


10/12

244 9.4. SWITCHED RELUCTANCE MOTORS

1^

q"QlpHH : SRM ^- Phase

HrH Q2Figure 9.8. A single phaseleg as used in a switched reluctance motor. The currentdirection is determined by Ql and Q2, with the respectiveflywheeldiodes.

VelocityController

1 ,

PWMController

chapter 9 related motors and actuators

Documents