Ingenious Digital Controller for Switched Reluctance Motor Using Verilog (HDL)

7/27/2019 Ingenious Digital Controller for Switched Reluctance Motor Using Verilog (HDL)

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Ingenious Digital controller for Switched ReluctanceM otor using Verilog (HDL)S.Paramasivam, R.Arumugam, S.Vijayan, SBala muruga n,G.Venkatesan and B.UmamaheswariDepartment of Electrical and Electronics EngineeringAnna University-Chennai-600025-Indiaparamaskam sk3rediffmail.com an d atuniueainiiiannauniv.edu

Abstract- This paper presents an advanced method ofposition- estimation and P WM generation for a Switched ingenious digital controller of a Switched Reluctance Motorusing Verilog is developed by using verilogger pro 6.5 Ink.Reluctance Motor (SRM) using Verilog. SRM cannot be rundirectly from the mains and requires a power converter andamicable controller, which must be integrated with themotor. The conventional controllers occupy more space andconsume more energy. In this paper, Verilog has been usedfor the rotor position estimation and PWM generation tocontrol a 3 phase 614 pole Switched Reluctance Motor. Withthis method it is possible to save a considerable amount ofenergy as well as space in the controll er circuits since all thecontrol algorithms are implemented using a single chip.Both voltage and current control techniqu es are discussed.Keywords: Switched Reluctance Motor (SRM), ' PWM,Rotor position, Digital Controller and Verilog

1. INTRODUCTIONA Switched Reluctance Motor (SR M) is a doubly salientand singly excited motor [1,2]. t has a simple magneticstructure, which carries windings on its stator and no

windings on the rotor. Th e motor operates on the principlethat the rotor tends to occupy the position, where theinductance of the energized phase windings is maximized.The operation of the motor is controlled by suitablychoosing the instances of switching odoff of excitation.Thus the determination of the rotor position is veryimportant for the precise control of the motor. Manymethods are available for the determination of the rotorposition o f an SR M. .O f these m ethods, a n on-intrusivemethod using flux-current characteristics is enunciated inthis paper since this method gives accurate flux estimationsat higher speeds. A Look-up table relating flux-current-rotorposition is pre-computed from the static characteristics ofthe motor using Magnet 6 . The use of single chip designwith all control algorithms saves a considerable amount oftime and gives a better rotor position estimation. Once arotor position is computed the controller will he able togenerate the necessary PWM signals to switch o d o f f heconverter circuit, The proposed method incorporates thevoltage and current control, which has not been taken intoaccount in the earlier methods available in the'literature forSRM control. The key aspect of the research is to get thedesired rotor position and PWM generation as the controlleroutput. The hardware description for implementing the

ROTOR POSITION ESTIMATIONA number of methods are available in literature for theestimation of the rotor position of a SR motor [3,4,5].Among these methods, a non-intrusive method involvingfluxicurrent is adopted. By knowing the phase current andvoltage of an energized phase of the motor, the rotorposition can he estimated using following relationship ( I ) .The block diagram of the rotor position estimation is shownin Figure. ( I ) . The unique relationship between the flux,current and rotor position in an SRM at any instant of time isshown in Figure. (2).

Y =j (V - R)dt (1 )

Y =I(V-IR)dt

L r - 7ook Up Table+ eFigure 1. BlockD iagram o f Rotor Position Estimation

Once the flux linkage value is found from the aboveequation (I); then the position of the rotor can be foundfrom th e look-up table, which contain s the flux linkage (y ),current (i) and rotor position (8)f the motor.

&NECESSITY OFROTOR OSITION ESTIMATIONIt is necessary to estimate the rotor position to have a betteroperation of the motor. The flux-current characteristics areobtained by conducting magnetic field simulation on themotor under static conditions using Magnet 6 . I n ou rresearch the pre-computed v/ - i -0 characteristics arestored in a look-up table to estimate the rotor position fromthe measured phase voltages and phase currents. Positionestimation updates are carried out sequentially by measuringvoltage and current from the three phases. Most of theindirect rotor position estimation algorithms do not
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concentrate on the st arting condition. In this paper a softstaning algorithm is included.

Flux Linkage -Current Characte risticsI

ii 0 5 10 15Current in Amperes

Figure.2.Flux linkage- Current Char acte risti cs for differentrotor positions

4. ROT OR POSITION ESTIMATION ALGORITHMI. Energize all the phases with reduced voltage for a

short period less than 2 msec2. Measure all the phase currents and voltagcs3. Set the current comparison coun t = I . If the currentcomparison count is not equal to 1, go to step 54. Identify the phasc carrying maximum current furswitching (active phase)5. Calculate the Flux linkage using E qn ( I ) .6 . Estimate the Rotor Position From loop up table.7. Go to step 2

5. CONTROLLEREALIZATIONThe Generalized Controller Schem e for the complete digitalcontroller is shown in Figure .3. Six analog signals aresensed through current and voltage sensors and aremultiplexed through the 8-1 analog Multiplexer. TheMultiplexer output is passed to the %bit ADC that convertsanalog inputs to digital signals that are fed to the digitalcontroller (Verilog HDL). 'The design implementation wasdone using Verilogger Pro 6.5 with EDA tools. The blockdiagram of the Digital Controller using Verilog is shown inFigure. 45. CounterA Counter is used to generate the sequence of numbers,which is used as select line for Dem ultiplexe r. This countercan be operated at any desired fiequency. Hence it ispossible to synchronize this digital controlle r with the 8-bitADC and associated analog Multiplexer.5.2 DeniulfiplexerA Demultiplexer is capable of routin g a single input line toone of several data-output lines. Control signals are used t o

select the destination of input. Thcse outputs arc given loactive phase detector.5 .3 Aclive phase delecloiThe purpose of the Active phase detector is to detect thephasc to be cnergized at startup. Active Phase Detector is asimple current comparator, which compares the magnitudcof the current of thc threc phases. The phase, which iscarrying largest current is said to be active phase, since itwill lhave lower inductance d ue to higher air ga p reluctance.Three p hase currents are sensed and fed to the active phasedetector. It compares phase currents and outputs activephasc current value. This phase current value is passed toflux-linkage estimator. A simple program of the active phasedetector is given below.Program 15.61inodule active phasc detctor (vout,. -

iout, il , i2, i3,vl v2,v3);output [7:0] vout, iout; reg [7:0] vout,ioiit;input [7:0] il , i2, i3,vl,v2,v3;always @(il or i2 or i3 or V I or v2 or v3 )i f ( i l>iZ&&il>i3)beginiout=il ;v o o t=v l ;en delseif(i2>i l&&i2>i3)heginiout=i2;voutLv2;endelseif( i3>iZ&&i3>il)beginiout=i3;vout=v3;endendmodule

5 . 4 'lux Linkage eslimatorThe ilux linkage is calculated using recursive estimationtcchnique. i.e., the dh value of the flux linkagc is foundCram n th value o f the voltage , urrent and (n-l)Ih value ofthe flux linkage, which ensures the better estimation of theflux linkage.

y,, (V,-I , ,Wt+Y,,- , (2)V, . s the phase voltage at nl h instant in voltsI, . s the phase current at n"' instant in ampsv,,.,is the flux linkage at n-I"' instant in Weber turnsR- Resistance of the phase winding in ohms.At startup when thc phase is about to be energized, the valueof th e (t1-1)'~ instant flux linkage is assumed to be zero.


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Pig.3.Generalixcd Controller Scliemc for thc completc digital controllcr

Deinultipl Look-up .lIEt?,exer 'lsblc Prcdictor

c y - U4

Fig.4.Digital controller using Verilog5. 5 Look up Tuhl rLook up table is formed wherein the values of the fluxlinkage, current and rotor position are stored. The valucs ofthe flux linkage and current for different rotor positions of a614 pole SK M are obtained form thc electromagneticsimulation software Magnet 6. Thc vdlucs of the fluxlinkages obtained from Magnet 6 arc fractional numbers. I norder to represent the fractional num bers in HEX format, thenumbcrs havc to be scaled properly, thcreby it is possible torepresent flux linkage and current for different rotorpositions in HEX format.5 . 6Rotor position EsfimatorThe main purpose of the rotor position estimator is toestimate the rotor position accurately. Th e flux linkageohtaincd form the equation (2) and its correspon ding currentare used to estimate rotor position from the look up table.The rotor position information obtained form the look uptable can be effectively used to switch o d o f f the phasewindings of the SRM.5.7Speed estimatorThe speed at n'l' instant can he calculated using the followingequation (3 )

(3 )

The value of theta is taken from the theta predictor. Whereq, s actua l speed at n'll instan t radlsec, O,,s rotor positionvaluc at nlh insvmt and Q,,., is rotor position WIUC at n. ~~"instant.5 .8Speed compumtorThe actual spccd obtained form thc cquatioii ( 3 ) is comparedwith the reference spccd and the error is multiplied by K1 inorder to get thc duty cycle and currcnt reference (or voltagcand current control schem es rcspectively.5 .9Curren t comparatorThc actual current is compared with the reference currcnt. Ifthe reference current is higher than the actual current theoutput of the current comparator is one according lo he dutycycle otherwisc it will.be zero.6 . PWM GENERATIONOGIC ORVOLTAGE A N D

CURRENT ONTROLI n voltage control techniqucs, the voltage applicd to themotor phase s is kept constant during the complete samplingperiod of the spccd control loop. The commutation of thephases is linked to the position of thc rotor. A speedcontroller directly controls the voltage applied to thc phase.


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The speed comparator finds the dif'ercnce hetwecn thedesired speed and the actual speed and generates the dutycycle, which generates the desired phase voltage for theSRM convertcr. The phase voltage is kept constant during acomplete dwell angle of one phase. Thc dwell is inothing but,conduction period of the phase in motoring mode. Thismethod is also called single pulse operation or high-speedoperation. The Block diagram o f Voltage Control is shownin rigure 5 . I'igure. 7 show the single pulse operationwaveform and Figure.8. Shows the RTL model of it

Figure.5. Block diagram of the Voltage ControlIn current control techniques the voltage applicd to themotor phases is modulated to reach the desircd current at thepowered phase. For most applications, the desired current isconstant during the complete sampling pcriod ol' the speedcontrol loop. The commutation of the phases is linked to theposition of the rotor. A current controller controls thevoltage applied to thc phase with an external speed controlloop. The speed comparator processes the differencebetween the desired spccd and the actual speed andgenerates the desircd phase current. The current controllerevaluates the difference between actual and desired phasecurrent and calculates the appropriate PWM duty cyclc. Thephase voltage is defined by a PWM duty cyclc implementedat thc DC-Bus voltage of ' the converter. Thus, the phasevoltage is modulated at the rate of the current control loop.Therc are two Current control techniques, which are popularfor SR M current control.

Figure.6. Block diagram of the Hysteresis and DeltaModulation controllerThey are hysteresis and delta modulation control. The blockdiagrams of the both controllers are shown in Pigure.6. Inhysteresis control method, the reference current generatedby the speed comparator will have a bandwidth of +A [ ,which limits the fundamental switching frequency of thephases. In Delta modulation control mcthod, the referencecurrent gcnerated by the speed comparator will not have anybandwidth. Whene ver the actual curre111 greater than thcreference current the phase is turned off and actual currcntlcsser than the refercnce current thc phase is turned onwhich increases the fundamental switching li'equcncy of

the phases. The purpose of the D-latch in delta modulationcontrol is to maintain the previous state of th e PWM signaltill next input causcs the changc i n output, which limits thehndamental switching frcquency O S the convertcrconsiderably. I:igure.9 and 1 I show the hysteresis and d e bmodulation control PWM output waveform for various inputconditions and Figure .10 and 12 show the ICIL model 0 1them.

I:igure.7.PWM Waveforms Ibr Voltage Control

Figure.8. RTL for Voltage Control7 . R E G I S l E R TRANSFERE V E LThe Register-Transfcr-Levcl inodels of difrerent controlschemes are given below(a)Single Pulse Operation:

Number of ports -129Number of nets -679Number of instances -630Num ber orreferences to this view -0Numbcr of GND - 1Number of VCC - INu mb er o f g a m - 18 40(b)Hysteresis Cont ro l method;

Number of ports - I 8 6Number ol'nets -773Num ber of instances -667Numbcr of references to this view -0Number of GND - INumber of gates -1920


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Figure.9. PWM Waveforms fo r Hysteresis Current Control

Figure.10. RTL for Hysteresis Current Control

Figure. I1.Delta modulation P WM output waveform

(e) Delta Modulation Control method:Number of ports -153Number of nets -700Number ofinstanccs -643Number ofreferences to this view -0Number of GND -1Number ofgates -1878

Figurc. 12. RT L for Delta Modulation current ControlS.CONCLUSION

The design of a digital controller for an SR Motor has beendeveloped using Verilog(H1)L)which includes indirect rotnrposition esliniation techniques. An accurate rotor positiondetection technique is explained and simulation details aregiven, which provide an improved way of measuring thcrotor position In SR motor drives. In this rcsearch bothvoltage and current control arc included.The proposed scheme successfully predicts thc rotorposition of the SR M. Since there is a soft start algorithm, 110difficulty is expcrienced during starting and in zero speedoperation oft he motor. Th e position estimator implementedusing Verilog (HDL).does not impose restrictions on a11operating speeds and conditions. The next stage oft his workwill extended to implement the Digital controller usingFPGA. When it is implemented, the controller can befabricated as a single chip, which will occupy lrss spacecompared with other conventional controllers.

REFERENCES[ I ] . T.I.E.Miller,"Switched Rcluctancc Motors and theirControl" Magna Physics Publishing and Clarendon Press,Oxford, 1993.[2]. R.Krishnan, R.Arumugam and J.F.Lindsay, "DesignProcedure for Switched Reluctance Motors", IEEE Trans.On Industry Applications Society, Vo1.24, No.3, pp 456-461, MayIJune, 1988.[3]. lqhal Flusain, "Indirect rotor-position estimationtechniques for switched reluctance motors-a review".[4]. Dr.R.Arumugam, S.Paramasivam, RDhanasekaran andVth andr asek ar "Design of Digital Controller for SwitchedReluctance Motor Using VHDL" HDLCON-2002proceedings, PP 143-149[SI.LBhaskar, "Vcrilog H DL Primer"[ 6 ] Samir Palinkar,"Verilog HDL ,A Gdide to Digital DesignSynthesis".[7]. www.syncad.com.
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Ingenious Digital Controller for Switched Reluctance Motor Using Verilog (HDL)

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