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Karol Kyslan, Frantiek Durovsk

Dynamic Emulation of Mechanical Loads An Approach Basedon Industrial Drives Features

DOIUDKIFAC

10.7305/automatika.54-3.184681.532.017:621.313.5(621.8)2.1.4; 2.8.1

Original scientic paper

Dynamic emulation of mechanical loads presents a modern and interesting approach for testing and validatingperformance of electrical drives without a real mechanical load included in the test rig. The paper presents an ap-proach to dynamic emulation of mechanical loads when the load torque and inertia mass of emulated load can besignicantly greater than that of laboratory test rig. Closed-loop control of load torque and feedforward compensa-tion of inertia and friction torques are used in a test rig. The approach is focused on the use with standard industrialconverters. The described method can be used for design and validation of speed control algorithms in mechatronicapplications. Experimental results with the emulation of linear loads are presented in end of the paper.

Key words: Dynamic emulation, Test rig, Industrial drive, RT-LAB

Dinami cka simulacija mehani ckih optere cenja pristup zasnovan na svojstvima industrijskih elektromo-tornih pogona. Dinamicka simulacija mehanickih opterecenja predstavlja moderan i zanimljiv pristup testiranju ivalidaciji ponaanja elektromotornih pogona bez ukljucenog stvarnog mehanickog opterecenja u eksperimentalnipostav. U radu je predstavljen pristup s dinamickom simulacijom mehanickih opterecenja za slucaj kada momenttereta ili moment tromosti simuliranog tereta mogu biti daleko veci od onih dostupnih u eksperimentalnom postavu.U postavu se koristi upravljanje momentom tereta u zatvorenoj petlji uz unaprijednu petlju kompenzacije momentatromosti i momenata trenja. Pristup je usmjeren na upotrebu standardnih industrijskih pretvaraca. Opisana metodamoe se koristiti za sintezu i validaciju algoritama za upravljanje po brzini u mehatronickim primjenama. U radusu predstavljeni eksperimentalni rezultati za slucaj simulacije linearnih tereta.

Klju cne rije ci: dinamicka simulacija tereta, eksperimentalni postav, industrijski elektromotorni pogon, RT-LAB

1 INTRODUCTION

The term emulation was used for the rst time by theIBM Corporation [1] in the eld of computer science inorder to qualify a special kind of software. This softwarewas able to imitate certain platform behaviour by runningon another platform. In general, emulation is the imitationof the system or its part by another platform or technicaldevice so that the imitating system behaves in the sameway as the imitated system would do. For the same inputdata, theoutput data of imitated and imitatingsystems haveto be exactly the same [2]. The imitating system is calledemulator and the imitation technique is emulation .

In recent yearsemulation methods are beingused in de-sign and validation of control algorithms for variable posi-tion, speed and torque drives in mechatronic applications.It is very convenient to use emulation technique for a de-vice that is not available for the control algorithm testing

and experimentation. For example high power drive, multi-

motor drive, etc. are rarelyavailable in laboratory facilities.Furthermore, in economic and space terms, it is not practi-cal to keep different kinds of mechanical loads in the labo-ratory. A more convenient approach consists in emulatingthese mechanical loads electrically. The most protable so-lution is to use two electrical drives coupled by commonshaft as presented in Fig. 1. The drive for which the con-trol algorithm will be developed is called drive under test .The drive under test is loaded by a torque controlled loadmachine known as a dynamometer . The torque of the dy-namometer is controlled in a way that it creates the sametorque on the shaft of the drive under test as the desiredload would be both during steady and transient states. Thetechnique is widely known as load emulation .

The dynamometers have been mostly used to performstatic load tests of electrical machines [3], static emula-tion [4] or load emulation under open-loop conditions [5].However, the methods [3]-[5] are not sufcient for test-

ing of drives in the cases of dominant nonlinear and dy-

Online ISSN 1848-3380, Print ISSN 0005-1144ATKAFF 54(3), 356363(2013)

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Dynamic Emulation of Mechanical Loads An Approach Based on Industrial Drives Features K. Kyslan, F. Durovsk

eq. (12) is simplied to:

(s) = 1

sJ em[T e (s) T ext (s)] +

+ s

J T s2 + ( k1 + k2 )s + k3T F (s). (14)

From (11) and (14) it is obvious, that if (13) is ful-lled, the actual speed of drive under test is equal to theemulated speed and pole-zero structure of emulated loadis preserved. However, real speed of drive under test isaffected by presence of linear and nonlinear torques T F .They are assumed to be compensated widely by the fric-tion compensation function in the converter control, rep-resented by input T F comp in Fig. 3. In case of unsatisfac-

tory compensation, the inuence of this input to the sys-tem behaviour is presented in Fig. 4. It becomes importantwithin a certain frequency band, which can be altered bythe choice of controller parameters k1 ,k2 and k3 .

-100

-80

-60

-40

-20

0

M a g n

i t u

d e

( d B )

10-2

10-1

100

101

102

103

-90

0

90

P h a s e

( d e g

)

Frequency (rad/sec)

Fig. 4. Bode plot of r /T F

-2

-1

0

1

2

M a g n

i t u d e

( d B )

10-2

10-1

100

101

102

103

-10

-5

0

5

10

P h a s e

( d e g

)

Frequency (rad/sec)

Fig. 5. Bode plot of / em , estimation of J T

Controller parameters for the frequency responses inFig. 4 were chosen in the way, that even if friction com-pensation of converter is unsatisfactory, inuence of theunwanted friction is rejected through gain of -15dB by dy-namic emulation control.

4.2 Choice of parameters

Characteristic equation of the system, based on (14), is:

s2 + k1 + k2

J T s +

k3J T

= 0 . (15)

If (15) is compared to the general equation of the 2ndorder element:

s2

+ 2 d0 s + 2

0 = 0, (16)where d presents the damping and 0 the frequency of

the 2nd order element, we get:

20 = k3J T

, 2d0 = k1 + k2

J T . (17)

Thus values of the controller parameters can be calcu-lated as:

k1 = 2d0 J T k2 , (18)

k2 = 2d0 J T k1 , (19)

k3 = J T 2

0 . (20)4.3 Impact of the test-rig inertia estimation

Inuence of the test-rig inertia estimation on the speedtracking is presented in Fig. 5 ( J T = J T blue, J T =1.2 J T green, J T = 0.8 J T red). In the most cases,the test-rig inertia can be obtained with good accuracy. If not, its inuence has to be taken into consideration.

5 EXPERIMENTAL RESULTS

In the following text the emulation of linear loads willbe presented. The dynamics of linear load are presented in(11). Additional load input T ext is applied to the model of emulated load:

T ext = 0 .7 T nomDC . (21)

In (21) T nomDC is nominal torque value of drive un-der test. First, the case with the per-unit inertia mass of the emulated load equals to the per-unit estimated valueof the test rig according to (13) is presented. Referencespeed and load torque were chosen in such a way thatthe speed controller output does not reach the saturationlimit of 170%. The results are presented in Fig. 6. Speed

controller parameters were chosen in order to exemplify

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0 4 8 12 16

-100

-80

-60

-40

-20

0

20

40

60

80

100

time [s]

s p e e

d [ % ]

0 4 8 12 16-100

-50

0

50

100

150

time [s]

t o r q u e

[ % ]

0 4 8 12 16-10

-8

-6

-4

-2

0

2

4

6

8

10

time [s]

s p e e

d e r r o r

[ % ]

Fig. 6. Linear load, J em = J T , T ext = 70%Top: desired (blue), actual (green), emulated (red) speed Middle: speed error (emulated speed-actual speed) Bottom: speed controller output T e (blue), friction com- pensation T Fcomp (green), compensation torque T c (red),drive under tests actual torque T DU T (purple), load torque T L (grey)

the tracking performance and thus presented results do notshow high quality of control. It is shown in the top of Fig. 6at the time of t = 1 s and t = 17 s, respectively, whenstep change of T ext is applied and considerable speed de-crease can be observed. This is moreover an instant of thelargest speed error. In any other instant a good tracking of the speed can be observed and speed error is within thelimit of 2%. Therefore, the green line, representing themeasured actual speed on the top of Fig. 6 and Fig. 7 ishardly visible. The noise of the load torque actual value ispresent because of the backlash effect in the test-rig me-chanics.

In the second experiment, the inertia mass of per-unitemulated load was increased to the value:

J em = 3 J T , (22)and no additional torque was applied:

T ext = 0 N m. (23)

The results are presented in Fig. 7. Behaviour of loadtorque is as expected. Until the time t = 8 s load torqueequals zero. In the time instant t = 8 s, the inertia masswas increased according to (22) and speed controller re-mains unchanged. The higher torque value is required andthe overshoot of speed is increased. Likewise, the backlasheffect can be observed on the load torque value.

6 CONCLUSION

Novel speed control structure for dynamic emulation of mechanical loads is presented in this work. Using of per-unit inertia calculus enables emulating of inertia mass withmuch higher values than the value of the test rig inertia.

In Section 4 it has been shown, that the method pre-serves pole-zero structure of emulated load. The perfor-mance of the method was tested with the emulation of linear load and very good speed tracking was obtained.Method is mainly focused on the use with industrial con-

verters. Obviously, it can be used with any other type of converter,but friction compensationhas to be implementedin order to restrain unwanted friction torques in the testbench.

The reason for the research in this branch of electricalengineering is the possibility of control algorithms testingfor industrial applications in the laboratory conditions.

Stability analysis and emulation of nonlinear loads willbe the subjects of future publications. Likewise, the trans-fer functions of the torque circuits will be considered andadded into the control structure in order to improve the per-formance of the method. They are difcult to model accu-

rately, but their identication can be done.

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0 4 8 12 16-10

0

10

20

30

40

50

60

time [s]

s p e e

d [ % ]

0 4 8 12 16-3

-2

-1

0

1

2

3

time [s]

s p e e

d e r r o r

[ % ]

0 4 8 12 16-50

-40

-30

-20

-10

0

10

20

30

40

50

time [s]

t o r q u e

[ % ]

Fig. 7. Linear load, J em = 3J T Top: desired (blue), actual (green), emulated (red) speed Middle: speed error (emulated speed-actual speed) Bottom: speed controller output T e (blue), friction com- pensation T Fcomp (green), compensation torque T c (red),drive under tests actual torque T DU T (purple), load torque T L (grey)

APPENDIX A ELECTRICAL MACHINES ANDLOAD PARAMETERS

Machine under test:V sup = 400 V, f = 50 Hz, P = 4 .2 kW, n = 935 rpm,

I = 9.2 A, T N = 31 .7 Nm.Load machine:

V sup = 400 V, f = 50 Hz, P = 7 .5 kW, n =1450 rpm, I = 15.2 A, T N = 50 Nm.

Total real inertia mass (included both machines andcoupling):

J = 0 .0981 kg m2 .

ACKNOWLEDGMENT

This work is the partial result of the project implemen-tation Research of Modules for Intelligent Robotic Sys-tems , OPVaV-2009/2.2/05-SORO, ITMS 26220220141,supported by the Research&Development OperationalProgramme funded by European Regional DevelopmentFund (ERDF). This work was also supported by the Slo-vak Research and Development Agency under the contractNo. APVV-0185-10.

REFERENCES

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[5] Sandholdt, P.; Ritchie, E.; Pedersen, J.K.; Betz, R.E., "A dy-namometer performing dynamical emulation of loads withnonlinear friction," Industrial Electronics, 1996. ISIE 96,Proceedings of the IEEE International Symposium on , vol.2,pp.873-878 vol.2, 17-20 Jun 1996.

[6] Hakan Akpolat, Z.; Asher, G.M.; Clare, J.C., "Dynamic em-ulation of mechanical loads using a vector-controlled in-duction motor-generator set," Industrial Electronics, IEEE Transactions on , vol.46, no.2, pp.370-379, Apr 1999.

[7] Arellano-Padilla, J.; Asher, G.M.; Sumner, M., "Control of an AC Dynamometer for Dynamic Emulation of MechanicalLoads With Stiff and Flexible Shafts," Industrial Electron-ics, IEEE Transactions on , vol.53, no.4, pp.1250-1260, June2006.

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[8] alman, M,; Macko, R., Design and realization of pro-grammable emulator of mechanical loads,16th IFAC World Congress , Czech Republic, vol. 16, 2005. pp. 246-250.

[9] Rodic, M.; Jezernik, K.; Trlep, M., "Control Design inMechatronic Systems Using Dynamic Emulation of Mechan-ical Loads," Industrial Electronics, 2005. ISIE 2005. Pro-ceedings of the IEEE International Symposium on , vol.4, pp.1635- 1640, June 20-23, 2005.

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Karol Kyslan received M.Sc. and Ph.D. degreesfrom the Technical University of Koice, Koice,Slovak Republic, in 2009 and 2012, respectively.He is currently an Assistant Professor at the De-partment of Electrical Engineering and Mecha-tronics, Faculty of Electrical Engineering and In-formatics, Technical University of Koice. Hisresearch interests are the control and simulationof electrical drives and emulation of mechanicalloads.

Frantiek Durovsk received M.Sc. and Ph.D.degrees in electrical engineering from the Tech-nical University of Koice, Slovak Republic, in1983 and 1993, respectively. He is currently anAssociated Professor at the Department of Elec-trical Engineering and Mechatronics, Faculty of Electrical Engineering and Informatics, Techni-cal University of Koice. His elds of researchinterests are motion control, electrical drives inindustrial and automotive applications and con-trol and simulation of mechatronic systems.

AUTHORS ADDRESSESAsst. Prof. Karol Kyslan, Ph.D.Assoc. Prof. Frantiek Durovsk, Ph.D.

Department of Electrical Drives and Mechatronics,Faculty of Electrical Engineering and Informatics,Technical University of Koice,Letna 9, 04200, Koice, Slovakiaemail: karol.kyslan@tuke.sk, frantisek.durovsky@tuke.sk

Received: 2012-01-30Accepted: 2012-11-20

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