630 IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 9, NO. 2, MAY 2013

Development of High-Reliability EV and HEV IMPropulsion Drive With Ultra-Low Latency HIL

EnvironmentEvgenije M. Adžić, Member, IEEE, Milan S. Adžić, Vladimir A. Katić, Senior Member, IEEE,

Darko P. Marčetić, Member, IEEE, and Nikola L. Čelanović, Member, IEEE

Abstract—This paper proposes an improved and robust methodof minimizing the error in propulsion-drive line-currents thatare reconstructed from a single dc-link current measurement.The proposed algorithm extends and then shortens the relevantphase pulse-widths in order to provide optimal sampling ofthe dc-link currents in two consecutive pulsewidth modulation(PWM) periods. The proposed PWM pattern control enables animproved sampling method which cancels offset jitter-like wave-form errors present in all three reconstructed line-currents, whichis due to a specific combination of nonsimultaneously sampleddc-link current and line-current PWM ripple. The improvementin induction motor drive accuracy using a single current-sensorand no shaft sensor (as proposed in this paper), over that ofconventional methods, is shown. Thanks to an ultra-low latencyhardware-in-the loop (HIL) emulator, the proposed algorithm,its implementation on a DSP processor, code optimization and“laboratory” testing were all merged into one development step.In order to perform final tests of the proposed current-reconstruc-tion algorithm and to verify the usefulness of the developed HILplatform by means of comparison, experimental results obtainedon a real hardware setup are provided.

Index Terms—Current measurement, electric vehicles (EV),hardware-in-the-loop (HIL), induction motor (IM) drives.

I. INTRODUCTION

W ITH a large number of microprocessors controllingthe majority of functions in a modern automobile,

it is impossible to overemphasize the importance of hard-ware-in-the-loop (HIL) testing infrastructures. In fact, togetherwith the microprocessor, software testing within a HIL frame-work is what has made all the increased safety and comfortfeatures of modern automobiles possible. More recently, withthe fast paced development of hybrid electric vehicles (HEVs)

Manuscript received December 06, 2011; revised March 07, 2012, May 13,2012, and July 17, 2012; accepted September 17, 2012. Date of publicationOctober 04, 2012; date of current version January 09, 2013. This work wassupported by the Ministry of Education and Science of Republic of Serbia underProject III 42004. Paper no. TII-11-0981.E. M. Adžić, V. A. Katić, D. P. Marčetić, and N. L. Čelanović are with

the Faculty of Technical Sciences, University of Novi Sad, Novi Sad 21000,Serbia (e-mail: evgenije@uns.ac.rs; katav@uns.ac.rs; damar@uns.ac.rs; niko-lace@uns.ac.rs).M. S. Adžić is with the Subotica Tech—College of Applied Sciences, Uni-

versity of Novi Sad, Novi Sad 21000, Serbia (e-mail: adzicm@vts.su.ac.rs).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TII.2012.2222649

and electric vehicles (EVs), power electronics and digitalcontrol technology has entered another safety-critical functionof the modern automobile; the propulsion.The propulsion system consists of one or more AC electric

motors controlled by power electronic converters and suppliedby energy stored in batteries or produced by fuel-cells or by anelectric generator. Real-time simulation (RTS) and HIL supporthave been recognized as crucial tools for assisting engineers inefficient power electronics (PEs) and motors controls develop-ment [1]–[3] and to this end, HIL emulators are increasinglyexploited [4]–[7]. This is known to be a very efficient methodbut some improvements are still needed. A recently proposedRTS/HIL platform based on a novel ultra-low latency (ULL)processor design, has greatly enhanced in-depth research intoimproving the control strategies of electric drives [8].This paper addresses two key aspects of drives technology,

which are not restricted to but are of particular interest for theautomotive industry: 1) sensorless control technology that holdsthe key to reliable and maintenance-free operation under all op-erating conditions, and 2) drives-specific ULL HIL that dramat-ically accelerates the pace of product prototyping.In order to reach the full potential of power electronics in

the field of motor control and satisfy the high expectations ofthe automotive market, reliability and control strategies mustbe significantly improved [1]. Large number of sensors, cableharnesses and signal-conditioning circuitry deteriorate systemreliability, increase maintenance needs and raise system costs.The reliability of a speed sensor attached to a motor shaft has

been recognized by academia and industry alike as a liability, re-sulting in the development of many shaft-sensorless techniques[9]–[14]. In a first step, speed information from a shaft sensor issubstituted by a speed-estimation using motor voltage and mea-sured motor phase-currents [9]–[12]. A second step in the effortto minimize the number of sensors—and also the focus of thiswork—is to substitute the two motor phase-current sensors bya single dc-link current sensor and a smart reconstruction algo-rithm [15].Most of the problematic issues in this area, including unob-

servable, low modulation-index operation, partially observablesector transition instances and also ever-present current mea-surement phase-shifts have already been resolved [15]–[24].However, the application of a single-current sensor scheme invector controlled drives with current regulation reveals moreoutstanding issues. The nature of the offset jitter-like waveformerror in reconstructed line-currents, due to nonsynchronized

1551-3203/$31.00 © 2012 IEEE

ADŽIĆ et al.: DEVELOPMENT OF HIGH-RELIABILITY EV AND HEV IM PROPULSION DRIVE WITH ULL HIL ENVIRONMENT 631

Fig. 1. ULL HIL experimental setup with controller and interface board.

sampling and motor line-current ripple at pulsewidth modu-lation (PWM) frequency, is explained in detail [25]. As thiskind of distortion is especially apparent during light loads andmachine-flux reduction, some kind of correction technique isneeded. Since on an urban driving cycle, a vehicle’s tractionmachine operates most frequently at light loads and a widespeed range of 3–4 times the nominal speed is required [26],[27], this issue becomes important.After presenting the problem with a conventional reconstruc-

tion method, this paper proposes a simple correction schemefor eliminating the offset jitter-like waveform errors in the re-constructed line-currents. The reliability improvement of thecomplete single current-sensor/shaft-sensorless drive is demon-strated using a HIL drives emulator and finally proved using areal hardware (HW) test-bed.

II. HIL DESCRIPTION

In the early stages of the control algorithm development andfurther, in order to validate the effectiveness and reliability ofthe proposed current-reconstruction method, the new ULL HILtest platform was used.The ULL HIL emulator comprised a programmable, appli-

cation-specific processing architecture based on a FPGA boardwith fast analog/digital input/output interfaces and with a sup-porting software tool-chain performing the function of powerelectronics circuit analyzer/compiler [8]. This approach pro-vided real-time execution with a 1 emulation time-step in-cluding the input/output interface latency.ULL allowed the use of a relatively high, e.g., 2 kHz,

switching frequency for the PWM, throughout all the exper-iments of this paper. A PWM frequency of 2 kHz does notrepresent the ULL HIL limit but is set by the used real HWlimitations (bandwidth of the dc-link current sensor) and for thesake of comparing ULL HIL responses with those obtained bya real HW system. The HIL emulator’s ULL properties provedto be particularly helpful during the control algorithm designbecause its time-interrupt handling nature requires precisePWM signal shifting and fine alignment of the sampling timesinside the short PWM periods of 500 . Fig. 1 shows the ULLHIL setup for evaluation of the sensorless induction motor (IM)drive with the proposed line-current reconstruction method.The same controller interfaced with the ULL HIL system, based

Fig. 2. IM drive with single dc-link current sensor (line-currents measurementused for comparing and testing purpose).

Fig. 3. Three-phase PWM pattern with two active voltage vectors and corre-sponding dc-link current waveform. Sampling moments included.

on the Texas Instruments DSP TMS320F2812, also controlsthe real HW system.

III. IM MOTOR DRIVE AND LINE CURRENT RECONSTRUCTIONTECHNIQUE

The most reliable three-phase motor line-current measure-ment is the use of only one current transducer placed in theinverter dc circuit, as shown in Fig. 2. Fig. 2 shows the ULLHIL software Schematic Editor/Circuit Compiler which allowsusers to build a variety of PE configurations such as the induc-tion motor drive, which is the object of this paper.The correlation between the dc-link current and the

motor line-currents depends on the states of the in-verter switches. There are six switch-state combinations whichyield the active voltage vectors and two which re-sult in nonactive or zero vectors ( and ). During the appli-cation of nonactive vectors, the dc-link current is always zero,while during the application of the active vectors, it is alwaysrelated to one of the line-currents [15].A typical 3-phase PWM period contains two active voltage

vectors that are on long enough for current reconstruction, asshown in Fig. 3. The figure shows three PWM signals for thevoltage vector in the first voltage sector, in which phase A hasthe highest and phase C the lowest, voltage command. Duringeach of the two active voltage vectors, and , one motor

632 IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 9, NO. 2, MAY 2013

Fig. 4. Areas in – frame in which current reconstruction is not possible.

line-current can be sampled, and , respectively. Since thesum of all three line-currents flowing to the motor must be zero,measuring the two line-currents allows a determination of theremaining third motor line-current, which stays unobservableduring the entire PWM period ( in Fig. 3).For the three-phase current reconstruction principle to work,

both dc-link current-sampling instants must be precisely syn-chronized with the PWM pulses, usually referred to the begin-ning or the middle of the PWM period (Trig signal in Fig. 3).According to Fig. 3, the optimal sampling instants should be cal-culated relative to the active vector transition moments. How-ever, for a reliable dc-link current reading, the signal samplingmust take place after an additional, precalculated delay,

(1)

The sample delays include total switching-device turn-ondelay-time , dc-link current measurement signal rise-time

and signal settling-time . The parameter in-cludes a dead-time that is automatically inserted by the DSP,IGBT driver response-time, PWM signal-processing time andworst-case switching-device (e.g. IGBT) on-time delay.The values of all the delays involved in the dc-link current

sampling process are of the order of a few and this indicatesthat only a low-latency real-time emulator can provide adequateaccuracy.

A. Modified PWM Pattern and Sampling Instants for ReliableLine-Current Reconstruction

Practical difficulties with dc-link current measurement canoccur when a PWM period includes active vectors that arepresent only for a short time, which happens in two commoncases (Fig. 4).The first case is a low modulation index, when both active

vectors are present for a short time. The second case occurs re-gardless of the modulation index, when the reference voltagevector passes near or falls on one of the six active vectors andproduces at least one short active vector. During these situa-tions, the method may be unable to reliably measure and calcu-late the motor line-currents.Therefore, it is desirable to use one of the suggested methods

which achieve effective dc current measurement by modifying

Fig. 5. Modified PWM pattern: the PWMB signal is shifted to the right to formthe first sampling-window wide enough for reliable current reading.

the three-phase PWM voltage patterns sufficiently, if longer ac-tive vectors are needed [15]–[21]. Otherwise, one can estimatemotor line-currents during nonmeasurable periods withoutmodifying the PWM pattern using mathematical machinemodels and the reference voltage vector information [22]–[24].In this paper such a PWM scheme is used [18]. Fig. 5 shows

the applied solution with a recorded example where the first ac-tive voltage vector on the lagging side of the PWM period isless than the predetermined shortest active vector time or min-imal dc current sampling-window

(2)

where is the minimum time during which the dc-linkcurrent signal has to be present at the DSP analog inputs afterthe sampling has been initiated. Generally, the PWM signal as-sociated with the phase having the middle voltage command

is shifted to the right in order to form a sufficientlylong first sampling window on the lagging side of the PWM pe-riod. Then, the duration of the succeeding voltage vector is cal-culated and if needed, the PWM signal associated with the phasehaving the highest voltage command is also shiftedto the right in order to form a second sampling window. Aftermodifying the PWM, a dc-link current measurement can be reli-ably taken at the first possible instant. The sampling close to thebeginning of the short active vector would result in its lowestpossible extension and would have less impact on the currentripple and result in less audible noise.An implementation of precise PWM signal-shifting method

and correct alignment of the sampling signals with beginning (ormiddle or end) of the active vectors can be accurately simulatedand rapidly verified only by using an ULL HIL emulator.

IV. RECONSTRUCTED MOTOR LINE-CURRENT WAVEFORMERROR DUE TO CURRENT RIPPLE

Regardless of the selected PWM shifting method for line-cur-rent reconstruction, the end results cannot be the line currentssampled in the middle of the PWM period. To make things evenworse, two observable line-currents cannot be sampled simulta-neously, each is sampled when available and thus with a certaintime-displacement between the two samples. As a result, thesetwo values are phase-shifted differently from the middle pointof the PWM period, in which the average PWM value of the

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TABLE IRIPPLE ERROR IN RECONSTRUCTED LINE CURRENT

Fig. 6. HIL experimental results—reconstructed line-current samplesin comparison to the line-current average values during PWM periodsfor no-load operation (25 Hz, 72 Vrms, 1.2 Arms).

line-current is located (see Fig. 3). A combination of time-dis-placement of the two line-current samples and current PWMripple produces a particularly shaped error of the reconstructedline-current, as uncovered in previous work [25] and now byextensive HIL emulations and experimental setup of which themain results are shown in this section.Table I and Figs. 6 and 7 give an overall view of the com-

plex, ripple-caused error in the reconstructed line-current (for example). Approximate errors are given for each sector, andespecially for the reference voltage vector position at the begin-ning, middle and end of the sector. In Table I, qualitativelyrepresents the error of the sampled (or calculated) current dueto the line-current PWM ripple and sample time-displacementfrom PWM middle point. It is clear that two abrupt excursionsin the current signal can be expected during one period of fun-damental output voltage. The ripple-induced error exhibits stepsof amplitude twice per period (transitions between sectors1 2 and 5 6 for line-current ) and it practically producesan offset section. For the other two line-currents, transition off-sets would appear at different positions (1 2 and 3 4 for, and 3 4 and 5 6 for current).Fig. 6 shows HIL results that demonstrate the erroneous

offset of the two sectors, with the measured and reconstructedmotor line-currents for the open-loop control and no-load sce-nario (where the magnitude of the line-current is comparableto the current ripple level). The experimental results given inFig. 7 and obtained on the matching real HW model show thesame behavior concerning the reconstructed current-waveformerror.

Fig. 7. Experimental results—reconstructed line-current samples incomparison to the line-current average values during PWM periodsfor no-load operation (25 Hz, 72 Vrms, 1.2 Arms).

TABLE IIFIRST ACTIVE VOLTAGE VECTOR COMPONENTS THROUGH EACH SECTOR

A. Quantitative Analysis of Stator Current Rate of Change

Qualitative analysis given in [25] revealed unusual recon-struction error in line-currents, which was confirmed in real-time emulated and real HW system. A possible way to quantita-tively describe the stator current rate of change (during the zerovoltage vector, and , and during the first activevector, and ), is to use the machine model ina stationary reference frame, which is native to the vector con-trolled drives. For induction motor drive, a current rateof change can be calculated using following equation:

(3)

where are: —equivalent leakage inductance of stator wind-ings ; and —appliedvoltage vector values during considered ripple; —equivalentstator resistance ;—electrical rotor angular frequency; and —rotor

fluxes; —rotor time constant; and —actual andprevious PWM period.Equation (3) can be used to calculate the rate of change of the

stator currents for a given voltage command. During the zerovector, components and are zero. During the first activevector (in lagging side of PWM period), voltage components

and are dependent on the sector and on stator windingsconnection, shown in Table II.

634 IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 9, NO. 2, MAY 2013

The obtained change of rates in original phase domain

(4)

together with the known durations of the voltage vectors (fromPWM modulator), can be used to predicted the difference, ,between the first and second sample from each line current meanvalue (see Fig. 3)

(5)

However, this paper proposes simple method (Section V)which does not need nor include prediction of stator currentrate of change.

V. IMPROVED SCHEME FOR THE CANCELATION OF OFFSETJITTER-LIKE ERROR IN THE RECONSTRUCTED LINE CURRENTS

The motor currents are conventionally reconstructed usingtwo dc-link current samples only, taken in a half (leading orlagging) of single PWM period at different time instants relativeto the PWM line-current ripple middle point. The result is anerroneous two sectors offset in the line and – currents and anincrease of the inherent 6th harmonic jitter in the -currents.Predicted line current sample error [(5)] can be used to alignsampled current values to the PWM middle point and to correctthe reconstructed line currents.However, the basic idea suggested in this paper represents an

improvement of the ingenious method proposed in [15], whichused the line-currents measured in both halves of the naturallysymmetrical PWM switching period. The method proposed in[15] samples the dc current in the center of the active voltagevectors four times during one PWM period and calculatesthe two available line-current values by averaging the sam-ples from two matching vector pairs. This approach providesconcurrent measurement of all three line-currents (referred tothe center of a PWM period). It effectively cancels a currentripple-caused waveform error in the reconstructed line-cur-rents and eliminates the current samples’ mutual phase-shift.Besides its simplicity, this method is completely insensitiveto machine parameter variances as opposed to more complexobserver-based methods. However, in [15] the critical casesof a reference voltage vector passing between the six possibleactive vectors or with a low modulation index are neglectedand not considered. The authors in [16] clearly emphasize thatduring these cases and with the PWM modified scheme used(where PWM signals are not symmetrical as assumed), thesimultaneously sampled line-currents cannot be acquired. Itclearly indicates there is a need to provide an improved pro-cedure for reliably and more accurately measuring the motorline-currents, while maintaining simplicity.

Fig. 8. Proposed dc-link current sampling and modified PWM pattern.

In view of the high PWM switching frequencies (up to 10kHz) and the motor power range (usually between 10 and 100kW) used for electric vehicle drives, one can conclude that thereis no need for the very highest current control-loop samplingrate at the PWM level. This fact allow us to record line-cur-rent information on the lagging side of one PWM period andthen on the leading side of the subsequent PWM period andcalculate the available line-currents by simple averaging of thecorresponding recorded values. In this way, all three estimatedline-currents would be referred to the same instant reflecting theaverage current value in two consecutive PWM periods. Theproposed method enables us to improve the PWM pattern con-trol in order to account for critical cases. It represents an ex-tension of the modified PWM pattern explained in Section II-Awhere under critical conditions, the lagging half-pulse width isshifted to the right and the leading half-pulse width in the sub-sequent PWM period is shifted to the left in order to create suf-ficient sampling windows for current measurement.Fig. 8 shows the dc-link current during two consecutive PWM

periods and details related to the proposed method. The line-currents at the time instant , representing average valuesin two consecutive PWM periods, can be obtained using simplecalculation

(6)

It remains to assign the resultant currents , , and tomotor line-currents , , and depending on the actualsector number. The first improvements in the reconstructedline-current waveform can be observed in Figs. 6 and 7 bycomparing and signals.

VI. EXPERIMENTAL RESULTS

An experimental setup which represents a typical electric ve-hicle drive-train with an induction machine, a two-level voltage

ADŽIĆ et al.: DEVELOPMENT OF HIGH-RELIABILITY EV AND HEV IM PROPULSION DRIVE WITH ULL HIL ENVIRONMENT 635

Fig. 9. IM drive used in tests.

TABLE IIIPARAMETERS OF EXPERIMENTAL SETUP (USED ALSO FOR HIL)

TABLE IVPER-PHASE IM EQUIVALENT CIRCUIT PARAMETERS (ALSO IN HIL MODEL)

source inverter and a battery, is shown in Fig. 9. Machine, in-verter and control parameters are given in Table III. The per-phase equivalent circuit parameters for this induction machineare given in Table IV. During all the reported experiments, themotor was lightly loaded (15% rated, 1.1 Nm) by another ma-chine controlled by an industrial frequency-converter in torquemode. Light motor load resulted in a relatively low rms currentvalue and in significant distortion of the reconstructed currentwaveforms, where usefulness of proposed method is more ob-vious. For experimental purposes, a direct rotor field-orientedcontrol (DFOC) was implemented [14] and used (Fig. 10).For a speed controller gains set according to module optimum

(Table III) and a ramp-change (0.4 s) in speed reference from0.05 to 0.375 p.u., Figs. 11 and 12 compare the dynamic speedresponses of the sensorless IM drive with both a conventionaland the proposed current-reconstruction method. Both, the HILmodel (b) and the real HW experimental results (a) are pre-sented, for the sake of comparison. Besides themeasured and es-timated speeds ( and ), Figs. 11 and 12 show rotating refer-

Fig. 10. Implemented sensorless DFOC speed control scheme.

Fig. 11. Real HW and HIL responses with change in speed reference from0.05 to 0.375 p.u., using conventionally reconstructed line-current feedback.(a) Dynamic speed response in real HW system. (b) Dynamic speed response inHIL model.

ence frame ( ) current references ( and ) and -com-ponents of the actual measured motor line-currents ( and ).Since current-reconstruction and current control loop dy-

namics are an order of magnitude faster than the speed controlloop, the reconstruction method does not affect the speeddynamic performance of the drive. The results from Figs. 11and 12 show an excellent match between hardware and HILresponses, except for slight differences in transition processand steady-state for the speed. Because the exact value of theconsidered system’s moment of inertia is unknown, there areslight differences in transition.In the control system with a conventionally reconstructed

line-current feedback, there are significant oscillations of-components of the actual line-currents compared to the case

where the proposed current-reconstruction is used. Even moreimportantly, there is noticeable offset especially in -currentcomponent (Fig. 11) which is mostly canceled using the pro-posed algorithm (Fig. 12).