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SAE TECHNICAL

PAPER SERIES 2006-01-0296

The Potential of Switched Reluctance

Motor Technology for Electro-Mechanical

Brake Applications

H. Klode, A. M. Omekanda and B. Lequesne

Delphi Corporation

S. GopalakrishnanGM Research Labs

A. Khalil, S. Underwood and I. HusainThe University of Akron

Reprinted From: Simulation & Modeling Mechatronics(SP-2030)

2006 SAE World CongressDetroit, Michigan

April 3-6, 2006

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Copyright2006 SAE International

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE.

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1 This work was performed while Dr. Gopalakrishnan was with Delphi Corporation.

ABSTRACT

Electro-mechanical brakes (EMBs) are emerging as a

new approach to enhance brake system features as wellas braking performance. This paper takes a fresh lookat the switched reluctance (SR) drive as a possibleprime mover technology for EMB applications. Theswitched reluctance motor has attractive potential, inview of its robustness, dynamic bandwidth and faulttolerance. An overall assessment of the approach ismade based on bench performance of a prototype EMBcaliper with an SR drive executing typical brakingpatterns. It is shown that the SR motor can provide therequired overall brake actuator performance. Variousimplementation options are examined to lower cost, withparticular focus on electronic design, control algorithms

and motor position sensing.

INTRODUCTION

Electro-mechanical brake systems offer, whencompared against their hydraulic counterparts, thepotential for a more straightforward implementation ofvarious braking-related control functions (like Brake-by-Wire, Integrated Electric Parking Brake, ABS, TCS,

ACC, Electronic Stability Enhancement), the promise ofleaner operations during the vehicle assembly phase(e.g. no need for handling brake fluid and makinghydraulic line connections) and last but not least, apositive impact on the environment due to the absenceof hydraulic fluids in the life cycle of the system [1-5].Work is therefore underway on all-electric brakeactuators, which feature brake calipers that are directlydriven by an electric motor via a gear-ballscrew system.

This paper examines the potential of switchedreluctance motor technology for EMB systems. Thistechnology has so far found relatively few applications,mainly due to cost (small airgap, inverter switch count

and lay-out, and high-resolution sensor), acoustic noiseand development complexity [6-10]. Therefore, thepermanent-magnet (PM) DC motor, either in its

mechanically commutated form or as its electronicallycommutated counterpart, has been the prime mover ofchoice in many systems. Major reasons for choosingPM motor technology can be found in its high efficiencyrelative control simplicity (due to the inherently linearcurrent-to-torque relationship), its widespread use andacceptance and also a well developed and readilyavailable design and manufacturing infrastructure. Budespite those apparent advantages, the PM mototechnology presents its own inherent set of challengeswhich may justify revisiting the use of SR motors for thespecific EMB environment. The first issue that can beclearly identified is the temperature dependency of the

performance of any permanent-magnet motor. Due tothe motors physical proximity to the brake rotor in anEMB application, the overall change in motor operatingtemperature can approach 200C, which translates intoa 20% to 25% change of the motor torque constant in amotor with typical high-energy rare-earth magnets. Asecond issue one has to keep in mind if specifying a PMmotor for an EMB application, especially a brushless PMmotor with low winding resistance, is the uncontrollablegenerator effect during a winding short: EMB actuatorsare typically required to self-release to a very low applyforce under fault conditions when the electrical power tothe EMB controller is removed either inadvertently or

deliberately (fail silent requirement). A winding shorcircuit would delay or impede such a self-release.

The SR motor, by contrast, is known for its temperature-independent torque and rather fault tolerant (althoughnot fault free) performance. The phase independence inSR drives does in particular allow for motor limp homecapability even under a partial controller or motor faultwhich adds a highly desirable level of redundancy to theEMB system. Whether it is possible to benefit from

2006-01-0296

The Potential of Switched Reluctance Motor Technology forElectro-Mechanical Brake Applications

H. Klode, A. M. Omekanda and B. LequesneDelphi Corporation

S. GopalakrishnanGM Research Labs

A. Khalil, S. Underwood and I. HusainThe University of Akron

Copyright 2006 SAE International

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these qualities depends on how well the SRdisadvantages can be addressed or at least sufficientlymitigated in the particular context of EMBs. This study isa first step in answering this question.

A general description of the proposed SR motor-drivenEMB system is first presented, along with both static anddynamic performance results. Then, the SR-based EMBsystem is investigated from a value perspective byidentifying some of the general cost concerns inherent toSR drive technology. The issue of the number of powerswitches in the converter, typically two per phase[11,12], was addressed previously [13]. It is also typicalto see SR motor control schemes that rely on a high-resolution shaft encoder [6]. In this paper, various semi-sensorless and fully sensorless motor position-sensingschemes are developed for the highly demanding EMBactuator environment. A particular aspect that deservesconsideration is that previously proposed position-sensorless SR control schemes [13-20] do not cover theentire speed range of the motor from no-load speed totrue stall performance at zero rpm. Therefore, much ofthe work presented here addresses novel, EMB-specific

sensorless control approaches for both low and highspeed motor operation that are tailored to providesmooth transitions during speed ramp-ups andreversals.

SYSTEM DESCRIPTION

ELECTROMECHANICAL BRAKE DESCRIPTION

The basic configuration of the EMB system investigatedin this paper consists of a rotary motor coupled to aplanetary-gear and ball-screw assembly that convertsthe motor rotation into linear force and travel suitable for

operation as a brake (Figs. 1 and 2). In this system,mechanical motor power is generated via the highspeed/low torque approach, which yields a smallermotor size and mass and makes the motor compatiblewith an existing caliper. The motor controller used to runthe specific experiments is kept as a separate unit forthis specific bench set-up, while potential EMBproduction versions would most likely feature anintegrated controller unit to eliminate motor-controllerleads for reduced cost and increased reliability. In sucha system, the motor can be small, with a torquerequirement of 1 Nm or less. The motor brings thebrake pads into contact with the rotor and generates the

necessary clamp force, via the aforementionedplanetary-gear system and ball-screw assembly. Thispad/rotor contact then provides the vehicle wheel-braking torque necessary to stop the vehicle (typically1000 Nm - 2000 Nm maximum), as in hydraulic systems.The motor can be seen as simply an activator and itstorque as a control variable for the vehicle braking.

Ball screw assembly

Motor

Brake pads

Caliper

Planetary gear assembly

Fig. 1: Sketch of proposed experimental EMB system

Fig. 2: Electro-mechanical brake, with caliper bracketand brake pads removed. SR motor fits inside housing

SWITCHED RELUCTANCE MOTOR DESCRIPTION

SR machines are simple and rugged by constructionTheir operating principle is based on the tendency of arotor pole pair to align with an energized stator pole pairTheir construction is different from that of othermachines, since they require saliency for both stator androtor, and an independent winding for each phaseThe stator and rotor of the SR machine are both made o

stacked steel laminations, while the electrical windingsare wound concentrically around each stator tooth. Theconcentric winding approach avoids phase overlapsthus yielding a very reliable and compact machinedesign. SR machines do not require permanenmagnets to generate torque, they have a very widespeed range, and offer a very good dynamic responsedue to their small rotor inertia. Other advantagesinclude the possibility to operate at higher temperatures[7], while providing at the same time increased faulttolerance due to the independent excitation of themachine phases [6,8].

The machine designed for this application has 8 statorpoles and 6 rotor poles. This so-called 8/6 configurationhas the ability to produce a relative uniform torque as afunction of rotor position down to zero speed, which is ahighly beneficial feature in any actuator application. Ioperates with 4 independent phases. A photo of themachine components is presented as Fig. 3. The crosssection of the machine laminations is shown in Fig. 4along with finite-element computed magnetic flux lines.

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Fig. 3: Photo of SR machine components

Fig. 4: SR motor cross section and magnetic flux linesat aligned (left) and unaligned (right) positions

There are various compromises, however, that one hasto make when using an SR machine especially in aperformance-sensitive application like the EMB. Thefirst issue relates to the fact that SR machines need ingeneral a somewhat smaller and more consistentmechanical air gap between rotor and stator than a PMmotor of comparable torque density and size. Also, thesalient structure of the SR machine in conjunction withthe magnetic excitation can sometimes createobjectionable acoustical noise. However, due to theinternal location of the SR motor inside the EMB, theoverall external EMB noise appears to be comparable tothat of an EMB with a PM brushless motor. Finally, SRmachines require typically rotor position sensors with aresolution higher than that of a PM brushless motor withequivalent static and dynamic performance. Since thismay also add to the controller cost, the rotor positionsensor issue is specifically addressed later in the paper.

SR DRIVE AND CONTROL

The standard drive configuration for an 8/6 SR machineconsists of one leg per phase, resulting in an overallcount of 8 switches and 8 diodes, as shown in Fig. 5. Inthis low voltage/high current application (12V/45A),MOSFET switches are used to minimize parasiticlosses. Compared with a typical drive system for a PMbrushless motor that features 6 switches and noadditional diodes, the discussed SR machine driveappears to be less favorable. Alternative converterconfigurations have therefore been proposed [11,12],the most promising of which is one with a 6-switchdesign whereby two opposite phases share one switch.

This alternate configuration was studied in [13] for thisapplication. The results, while promising overallindicate however a slightly slower response time.

a b c dVdc

+

-

T5 T6 T7 T8

T1T2 T3 T4D1

D5

D6 D7 D8

D4D3

Fig. 5: Standard SR power converter configuration

Concerning control, every SR machine exhibits a highlynon-linear behavior between its electro-mechanical andelectro-magnetic parameters. Accordingly, in order tomaximize the performance of a given SR machine, thetimes (or angles when using a time-speed based scale)at which individual phases are turned on and turned offmust be controlled to the highest accuracy and as afunction of the machines operating points (current

position and speed). At high speed for instance, it takesa longer angular span to reach a given current level dueto the machines higher generated voltage, and thecurrent must therefore be turned on earlierFurthermore, any highly dynamic EMB response reques(like for instance during ABS operation) puts the SRmachine into an energy absorption mode, which requirestorque and direction reversals at the highest possiblerate. The controller must therefore be designed for bothpositive and negative torques and speeds, which is alsoreferred to as a full 4-quadrant control strategy. Detailsare provided in [5,21,22].

SYSTEM PERFORMANCE

STATIC BEHAVIOR

The static performance of an SR motor is characterizedas the torque developed by a single phase for variousangular rotor positions and phase current levels. This isshown in Fig. 6 with data as calculated from a magneticfinite element model, and as obtained from tests. Thetotal maximum available torque, then, is the sum overtime of the torque from the various phases. Thismaximum torque also depends on speed, because thevoltage generated by the motor at higher speedsreduces the amount of current that can be reached

within a cycle, and depends as well on the timing of thecurrent pulses (time at which the current is turned onand off), the latter being a matter of design. Turn-on andturn-off angles are generally chosen to maximizeefficiency, torque, or other parameters. In thisapplication, the available torque is of paramountimportance to provide the best dynamic EMB responsepossible from a given motor.

Winding coil(1 of 8)

Stator pole(1 of 8)

Rotor pole(1 of 6)

Shaft

Motor housing

Stator

Rotor

End cap

Stator core and windings

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Finite element results Test dataFig. 6: Static performance, SR motor phase torque

versus angular rotor position for various current levels

Fig. 7 shows the overall machine torque versus speed,as test data. Below base speed, the torque is limited bythe imposed current limit (45 A, phase current, here).

Above this speed, the torque is limited by the generatedvoltage, which limits the available current.

Fig. 7: Motor torque-speed characteristic

DYNAMIC BEHAVIOR

The dynamic performance of the system was evaluatedby having the EMB perform typical brake scenarios. Theexamples shown here are: a brake apply, a brakerelease, and a set of two sudden direction reversals. Inthe experimental EMB bench set-up, a dSpace systemwas used to perform higher level control functions andgeneral data acquisition tasks, while providing at thesame time the required links between the SR controllerand the various sensors for brake force (locatedbetween the brake pads), motor position and motorcurrents.

Brake apply

Fig. 8 shows the response of the brake system to amaximum clamping force (1 p.u.) command. Fig. 9shows the associated DC bus current profiles. Thecurrent is generally indicative of the motor torque level.One can observe: A large current (torque) pulse at start-up for the initial acceleration of the motor; lower currentlevels as the motor reaches higher speeds while thecaliper still provides a fairly low level of brakeengagement; and, increasing levels of current as thebrake is progressively more applied. The maximum DC-bus current level, 40 A (equivalent to 45 A phasecurrent), is approached when the caliper reaches the

targeted maximum force (short of the 200-ms mark inFig. 9), although less current is subsequently sufficientto hold this force level after 200 ms. The 200-msresponse time to maximum clamp force for thisprototype was chosen to put the electric brake on anapproximate par with a hydraulic system from a dynamicperformance point of view. The electrical brake isexpected to be superior in the areas of controllabilityease of vehicle assembly, and environmental impact.

Fig. 8: Brake apply response

Fig. 9: Current profile during brake apply

Brake release

A fast brake release is important under nomina

operating conditions (active release), but is also criticalas mentioned earlier, when the EMB enters fail silentmode (passive release). Fig. 10 shows such a faisilent release event in which the motor is let go withoutany electrical assist. Due to the low SR motor dragtorque and moment of inertia, full brake release from90% of maximum force is accomplished in well under100 msec.

Fig. 10: Brake release pattern

Brake reversals

Fast motor and actuator reversals are critical wheneverhighly dynamic brake performance is required, as

40

Time (ms)

20

0

Current (A)

Base speed

Response

Command

Force (p.u.)

Time (ms)

1.00

0.75

0.50

0.25

200100 30000

1000 200 300

Force response

Releasetime

Time (ms)

Force

(p.u.)

1.0

0.8

0.6

0.4

0.2

0100 2000 50 150

0.2

0.4

0.6

0.8

0

Torque(Nm)

0 1000 2000 3000 4000 5000

Speed (r/min)

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encountered for instance during an ABS event. Fig. 11shows an example where the brake is actively releasedfrom 75% of maximum clamp force, then reapplied whena lower (50%) force level is reached.

Fig. 11: Brake reversal and re-apply performance

The force overshoot (during the brake release) andundershoot (during the brake reapply) are 0.10 p.u. and0.23 p.u., respectively, with corresponding durations of6 msec and 10 msec. The force overshoot is smaller

than the undershoot because the counter-acting caliperforce always helps to slow the motor motion down whenthe motor rotates in apply direction, while the oppositeis true when it is commanded to reverse direction fromrelease to apply.

SENSORLESS MOTOR CONTROL

High performance SR drives require precise motor shaftposition information, which translates typically into asensor resolution of less than 0.5 mechanical degrees.

An investigation of algorithm-based sensorless schemesis thus logical in order to avoid the cost that isassociated with hardware-based position sensors.However, the current state of sensorless SR drivetechnology does not provide for a single algorithm thatcan cover both low and high motor speeds. Higherspeed algorithms proposed so far rely on voltagesinduced by flux variations [16-18]. These voltages arealso a function of speed, thus essentially unavailable atlow speed. For the opposite end of the operatingspectrum, current pulse injection has been proposed forlower speeds [14,15], but the motor inductance (whichthe algorithm requires as an input) can be read fromthese pulses only as long as the motor speed is lowenough so that the generated motor voltage can beneglected. For fully sensorless operation, therefore, two

separate algorithms must be used, one for lower speedsand the other one for higher speeds.

While high-resolution encoders are usually a cost issue,

low-resolution sensors, with a resolution of say 15, arefairly cost competitive in automotive applications. Thisopens the possibility of using such a low-resolutionsensor in stand-alone mode, or, for increased SR motorperformance, as a complement to a sensorlessalgorithm. Therefore, several possible strategiesemerge:

1) Strategy 1: No physical sensor; high-speedsensorless algorithm only, for simplicity [19].2) Strategy 2: Hybrid sensor system with a lowresolution sensor for low speed, and a high-speedsensorless algorithm [13].3) Strategy 3: Fully sensorless [20,23].

These various strategies were tested in both speed andposition control modes and compared with one anotherFirst, however, the two algorithms chosen for the fullysensorless strategy, one for lower and one for higherspeeds, are briefly described.

SENSORLESS ALGORITHMS

As explained previously, the fundamental operatingprinciple of SR machines requires that both stator androtor exhibit magnetic saliency as a function of angularrotor position. Although this magnetic saliency isprimarily used in the motors energy conversion processit can also provide indirect position information: As therotor changes its angular position, magnetic saliency ofthe rotor and stator create a phase inductance profile

that changes accordingly as a function of motor positionand phase current. This changing inductance profile canbe used to determine indirectly the rotor position throughmethods that involve current probing and/or othermodel-based techniques, which will be explained inmore detail later on. In other words, position estimationin an SR machine is based on inductance estimationfrom which position is derived either through a look-uptable or a suitable motor model. Inductance estimationin turn involves voltage and current measurementswhich are directly impacted by the speed of the motorTherefore, the relationship between the inductance andthose measurements is affected by motor speed in such

a way that different position sensing approaches areneeded depending on motor speed.

Lower-speed algorithm

As mentioned before, motor position estimation at lowspeeds does not work with schemes based on fluxestimation, but requires instead an active probing (a.k.apulse injection) technique: A series of short voltagepulses is applied to an idle phase of the machine tocreate a series of current responses. The currenresponses, in turn, are measured and used to estimatethe phase inductance and determine from it the

matching rotor position via a predefined look-up tableIn an 8/6 machine, the preferred idle phase for thesepulse injections is the one opposite to the active phaseIt is subjected to a series of voltage pulses having fixedwidth. The resulting current is then sampled at the endof each pulse. Fig. 12 shows exemplary injectionpulses, plotted against time, within one rotor step of 15mechanical degrees. The general voltage-currenrelationships in a phase are:

Brake reapply

Brake release

Time (ms)

0.50

1.00

0

0.25

0.75

Force (p.u.)

Command

Response

200100 3000

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dt

diLir

dt

d

dt

di

iirv

dt

idirv

)(

),(

(1)

where v is the voltage applied to the phase, i is the

current, is the flux linkage, L() is the incremental

inductance, ris the resistance (all phase quantities), is

the position, and is the speed. Neglecting first theresistive drop, and then considering that, at low speedsand currents, the generated motor voltage is negligible,yields:

t

iLv

peak

)( (2)

where ipeak is the current sampled at the end of the

pulse, and t is the pulse width, see Fig. 12. Theinductance can be calculated from Eq. 2. In turn, theposition is estimated from the inductance reading, via apreviously established look-up table.

Fig. 12: Injected phase voltage and current waveforms

Eq. 2 is an approximation that ceases to be valid whenthe generated voltage reaches a certain level. This fact,

as well as the limited time span between successivephase conductions at higher speeds, limits thepracticality of the method to about base speed.

Fig. 13: Estimated rotor position derived from pulseinjections during motor start-up

The ability to predict position with this method isdemonstrated in Fig. 13 with estimated rotor positionshown versus time during motor start-up. The rotorposition shown in this and subsequent figures is theposition in mechanical degrees, with the angle countreturning to zero at the end of every electrical machine

period (60) for the purpose of easier graphical depiction(wrapped plot). There are 4 phase commutations permachine period, and one can see some minor hesitation

during transitions from one phase to another. Theseimperfections do not affect the ability to ramp up themotor speed smoothly.

Higher-speed algorithm

For higher speeds, the previously discussed pulseinjection technique becomes ineffective, and positioninformation is now estimated via a completely differentapproach: A model based on the machines electricaand magnetic parameters is derived and an observerconstructed to compare the model results withmeasurements. The comparison yields a match whenthe correct position is converged upon.

The machine flux model selected for this studyexpresses the flux as an exponential function of phase

current and a function of position f() [24,25]:

)(1 fis e (3)where sis the saturated flux linkage of the machine and

f(

) is in general expressed as a strictly positive Fourierexpansion. The phase flux is calculated from theintegral form of Faradays law using current and voltagemeasurements:

tdrtitvtt

0 ))()(()( (4)Convergence of the observer with the measured data isobtained via a sliding-mode approach [25] and, morespecifically, by using a second order sliding modeobserver according to the following equations:

)sgn( ek

)sgn( ek (5)

where k and k are constants, and e is an errorfunction defined as:

4

1

)(n

nnnge (6)

where n and n are the calculated and estimated flux

in phase n, respectively, and )(ng is a trigonometric

function selected to ensure proper convergence of theerror function [25].

The ability of the algorithm to correctly estimate the rotorposition at higher motor speeds is shown in Fig. 14Since the algorithm runs continuously, there is notransition from one phase to another as was observedunder the low-speed algorithm. Therefore, theestimation is smoother overall and achieves an accuratematch when compared against the position signal from areference encoder. Note also that, as seen as inFig. 14, convergence is obtained at speeds down tobelow base speed (base speed is about 1,500 r/min forthis motor). Both algorithms, therefore, can overlap ovea speed range of about 50% to 100% of base speed, a

t

ipeak

Phasecurrent

Phasevoltage

10020 40 60 80 120

20

40

60

0

Time (msec)

Rotor position ()

0

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feature that is used for effective and smooth transitionsduring accelerations and decelerations.

Fig. 14: Position estimation via flux observer at highermotor speed (1,200 r/min)

As just shown, the pulse-injection based approach for

low speeds and the observer-based algorithm for highspeeds form the basic building blocks in sensorless SRmotor controls. We can now return to discussing thestrategic implementation of those algorithms for thisspecific EMB application.

STRATEGY 1: HIGH SPEED ALGORITHM ONLY

In order to simplify the complexity of the controls, it isconceivable to run the machine with only a high-speedalgorithm [19,26]. Starting from zero speed is achievedby first aligning the rotor with one particular stator phase,then energizing the other phases in stepper motorfashion according to a set excitation sequence. Thismay be possible in an EMB actuator because the motorstarts typically under very light mechanical load (brakepads are disengaged from the brake rotor), andtherefore in a known and consistent manner. For speedreversals, when the motor traverses the low speedregion, control is performed by estimating the rotorposition in an open loop manner. An example of thistype of operation is shown in Figs. 15 and 16 (motortested on a dynamometer).

These figures demonstrate the general ability of themotor to drive a load dynamically in forward and reversedirections with this approach. However, one can

observe some speed chatter after zero speed has beencrossed (Fig. 15), indicating a lower level of motorcontrol quality. There are also other drawbacks to thisstrategy: While the motor is generally at no-load when itstarts, this cannot be guaranteed all the time. In fact,one can envision the motor being stopped under someload for an extended period of time after which restartingmay not be as robust as required in a brake application.These disadvantages more than offset the advantagesof Strategy #1, which is based on low cost (no sensor)and simpler computation (only one sensorlessalgorithm). Strategy #1 was thus not developed beyond

a dynamometer setup, and was not tested in an EMBenvironment.

Fig. 15: Measured and estimated speed, Strategy # 1

Fig. 16: Measured and estimated positionduring rotation-direction change, Strategy # 1

STRATEGY 2: HYBRID POSITION ESTIMATIONWITH LOW-RESOLUTION POSITION SENSOR AND

HIGH SPEED ALGORITHM

With Strategy #2, motor starting and operation at lowspeed are performed by relying on low-resolutionposition feedback every 15 mechanical degrees, whichrequires, for instance, only 2 Hall effect sensors for thediscussed 8/6 SR motor with a 15rotor step angle [13]The high-speed algorithm takes over as soon as it hasconverged. In addition, the low-resolution encoder isused to help with the convergence process by providingan approximate initial value for the computation. Thesensor also provides bounds for the sensorlessalgorithm estimation, which is particularly useful during

the transition at medium speeds.

Figs. 17 and 18 demonstrate on a dynamometer smoothmotor operation in both directions, and at various levelsof speed, including the particularly critical rotation-direction change. The estimated speed is smoothbefore, during, and after crossing the zero speed pointThe trace Actual position in Fig. 18 was obtained withan incremental encoder running in parallel forcomparison.

Rotor position ()

Rotor position ()Encoder position

Estimated position

Rotor position ()

Time (sec)

Time (sec)

Measured

EstimatedZero-speed crossing

MotorSpeed(r/min)

Zero-speed crossing

Measured

0

30

60Rotor

position()

Estimated

60

30

0

0

30

60

0.69 0.70 0.71 0.72 0.73

0.69 0.70 0.71 0.72 0.73

Time (sec)

1.0 1.2 1.4 1.6 1.8 2.0

1.42 1.44 1.46 1.48 1.50

-3500

-3500

3500

-3500

3500

0

0

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Fig. 17: Measured and estimated speed, Strategy # 2

Fig. 18: Measured and estimated positionduring rotation-direction change, Strategy # 2

Figs. 19-21 show dynamic results on a caliper. Note thefaster motion (shorter time scale) during these tests.Fig. 19 shows the estimated speed over a caliper-position loop. Fig. 20 shows the corresponding speed-reversal region with an enlarged scale.

Fig. 19: Estimated motor speed versus time in position-controlled loop on a caliper, Strategy # 2.

Fig. 20: Measured and estimated positionduring speed reversal, Strategy # 2.

As seen in Fig. 20, the high-speed estimation algorithmdoes not converge during the speed reversal due to thedominating integrator time delay and the quick reversal

of the low-inertia SR motor. During the reversal, thehybrid method thus surrenders control to the low-resolution encoder and makes its output the dominantposition signal.

When the motor is holding a constant position (steadybrake apply), the SR motor dithers around zero speedThis is shown in Fig. 21. In that case, the high-speedalgorithm does not converge, and the low-resolutionsensor output is always active. Given the high gear ratioused in an EMB, this dither in motor position shouldhave minimal impact on the brake-force level. In fact, iwill facilitate restart by avoiding stiction.

Fig. 21: Measured and estimated positionduring position holding at zero speed, Strategy # 2

Compared with operation under high-resolution positionencoder feedback, overshoots during reversals areincreased by 20%, and apply times are also lengthenedby +30%, (not shown in the figures) mostly because of alonger dead time at the beginning of motion when thelow resolution encoder is actively used. This, perhapscould be remedied at least in part by increasing the

sensor resolution from 15to 7.5, or by pre-positioning

the rotor as was done for Strategy 1.

STRATEGY 3: FULLY SENSORLESS

A fully sensorless approach that does not rely on anyhardware-based rotor position sensors was alsoinvestigated [20,23], and the results are shown in thefollowing figures. The implementation required carefusoftware development in order to minimize the algorithm

execution time and limit it to less than 100 s, sincelonger loop times would have lead to an overaldeterioration of the control quality. First, rotor positionestimation is shown in Fig. 22 as derived from both the

low and high-speed algorithms during a change ofdirection under closed-loop speed control. During thetime span covered by the figure, the system transitionsfrom the higher-speed algorithm (left hand side), througha lower speed region including zero-speed crossing, andback to a region of higher, but reversed speed where thehigh-speed algorithm is again active. The high-speedalgorithm does not converge at lower speeds, andsimilarly the low speed algorithm does not converge ahigher speeds, demonstrating their ability to complemeneach other (discontinuities in the figure traces are a signof non-convergence). However, with an appropriate

Time (sec)

Motor speed (r/min)

Estimated

Command

Estimated

ActualZero-speed crossing

Time (msec)

0

2000

4000

-2000

Motorspeed(r/min)

100 120 140 160 180 20080

20

Rotorposition

()

0

40

60

Actual position

Estimated position

160

Time (msec)

156 164152150

Time (msec)

Estimatedposition

Actual position

0

10

20

30

Rotor osition

280 290 300

-1000

0

1000

500

-500

0 2 4 6 8 10 12 14 16

9.1 9.2 9.3 9.4 9.5

Time (sec)

20

0

40

60

Rotor position ()

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transition between the two algorithms, smooth positionestimation is obtained!

Fig. 22: Estimated motor position, Strategy # 3

Fig. 23: Current in one phase, Strategy # 3

The corresponding phase current, Fig. 23, shows thecurrent either as torque producing, larger pulses, orused for position sensing, as reflected in the center ofthe figure by the series of short pulses.

Fig. 24: EMB force and motor speed response underfully sensorless control, Strategy #3

Fig. 24 shows an example of brake caliper operationwith brake apply at first, and then a reversal, along withthe motor speed as estimated by the algorithm. Thisfigure demonstrates that smooth control of the EMBsystem can be achieved under fully sensorless control,although the undershoot (pointed to in Fig. 24, top) islarger than what had been observed in similar tests withan encoder. The overall motion was also slower (applyor release) without sensor by 30%.

SENSORLESS OPERATION: COMPARISON

Three sensorless or semi-sensorless strategies wereimplemented on the bench:

The first strategy, with an open loop approach forstarting and at low speeds, showed some limitation afterzero speed crossing, and for an eventual restarting othe caliper under load after a long period in one position.

The fully sensorless strategy has been demonstratedsuccessfully, although a small loss in performance isobserved (slower apply and larger overshoots). Theperformance loss, it is believed, could be compensatedfor with a motor optimized for torque and speed whenoperated under fully sensorless control, although thismay partially offset the cost saving resulting fromeliminating the sensor. Improvements in controllespeed (reduced loop time) would have a beneficiaimpact on EMB performance and should be consideredin future investigations.

The hybrid approach with an inexpensive sensor for the

low speed range and a sensorless algorithm forimproved control at high speeds comes across as astrong contender in the near term. Operation wassmooth from an overall viewpoint. In addition, havingboth a sensor and a sensorless algorithm provides awelcome degree of redundancy for this application.

CONCLUSION

The paper summarizes the results of an in-depthinvestigation of the application of switched reluctancemotor technology in an electro-mechanical brakeenvironment. The switched reluctance motor is wel

suited for the application, and could provide advantagesthat include operation at extended temperatures, highmechanical bandwidth, inherent fault tolerance and faisilence. The paper also discusses investigations carriedout to reduce system cost, with a particular emphasis onsensorless position control of the switched reluctancemachine. Fully sensorless operation is possible, but areduction in processing time may be needed to avoid theminor degradation in system performance that wasobserved. A hybrid approach, with both a low-resolutionsensor and a sensorless algorithm, may be preferable inthe meantime as a robust and cost-effective solution.

ACKNOWLEDGMENTS

The authors are grateful for funding made possible, inpart, by a grant from the Indiana Department ofCommerce and the US Department of Energy. Theyalso acknowledge the earlier work done in this area byDr. Syed Hossain, former PhD student at the Universityof Akron. Finally, they would also like to thank MessrsLonnie Frost, Eric Nedelcu, and Stanley Rawski atDelphi Research Labs, as well as their colleagues at theDelphi Energy & Chassis Innovation Center.

0

30

60

0.230 0.240 0.250 0.260 0.270 0.280

Time sec

Rotor position ()

Estimation from low speed alg.

Estimation from highspeed alg.

Lowspeed alg. active

0

5

10

Phase current (A)

0.23 0.24 0.26 0.28 0.30 0.32

Time sec

Torque-producing current

Position-sensingcurrent

0

0.4

0.6

0.8

Force(p.u.)

UndershootCommandResponse

0

2000

4000

-2000

-4000

Motorspeed(r/min)

0 0.2 0.4 0.6 0.8 1.0 1.2

0 0.2 0.4 0.6 0.8 1.0 1.2

Time sec

Time sec

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CONTACT

Mr. Klode is with Delphi Corp., Energy & ChassisInnovation Center, Dayton, OH. Drs. Omekanda andLequesne are, and Dr. Gopalakrishnan was at the time

of this work, with Delphi Corp., Delphi Research LabsShelby Twp, MI. Dr. Khalil, Mr. Underwood, andDr. Husain are with the University of Akron, Akron, OH