2004 351h Annual IEEE Power Electronics Specialisfs Conference
Aachen. Gemany, 2004
Hardware-in-the-loop simulation based design and experimental evaluation of DTC
stsategies
S.Vamsidhar
Indian Institute of Technology-Bombay
Email: [email protected]
B.G.Femandes
Indian Institute of Technology-Bombay
Email: [email protected]
Abslracl-In
this paper, a method for designing and
implementing direct torque control of induction motor based on
hardware-in-the-loop simulation (HIL simulation) is proposed.
Two different strategies ( I ) Conventional direct torque control
and (2) Space vector P W M based direct torque control are
tested and implemented. The entire simulation is performed in
real time using TMS3ZOFt40 digital signal processor. The
dynamic machine model with 5 state variables is solved in real
time in order to determine the state variables and hence various
output quantities. Using the same controller and software used
in HIL simulation and with additional signal conditioning
interface circuitry, the results obtained from real time
simulation are experimentally validated on a 1.5-kW induction
motor drive. A new tool known as “automatic code generation”
is introduced, which is capable of generating assembly language
code for real time simulation of electric drives. The results of
real time simulation and those obtained from a laboratory
prototype are presented.
1.
software, which processes the reference commands and
outputs generated by the dynamic model of the plant and
takes necessary action.
If the results obtained in the HIL simulation are
satisfactory, then the same control software can be used to
validate the control strategy experimentally on a real plant.
INPUTS (real analog Inputs)
TMSS20F240 OSP CONTROLLER
~
INTRODUCTION
Traditional software based simulation has the
disadvantage of being unable to exactly replicate real
operating conditions. It does not take into account the
limitations of the digital controller, like saturation of values
in fixed point DSP systems during the intermediate
calculations if they exceed unity. It also does not take into
account the finite resolution of DSP registers. One way to
bridge the gap between the simulation and real conditions is
Hardware-in-the-loop simulation.
Hardware-in-the-loop (HIL) simulation is a kind of
simulation where the input and output signals of the
simulator show the same time-dependent values as in a real
process. Such simulators allow testing of the real control
system under different loads and supply conditions. While
designing the digital control system for power electronic
converters and high performance drives, HIL testing is
increasingly being recognized as an effective approach
wherein the controller is designed and tested at the same
time with the application code.
A simple block diagram of an HIL simulator for testing a
power electronic motor drive application is shown in Fig. 1.
The inputs to the simulator are real analog inputs fed to the
digital signal processor through an ADC or a digital I/O
port. These inputs can be reference commands like speed or
torque command in case of closed loop electric drives. The
dynamic model of the plant represents the mathematical
model of the inverter fed motor in the form of state
equations. The controller block represents the actual control
0-7803-8399-0/04/$20.0002004 IEEE.
Fig. I, Block diagram of an HIL simulator
The advantages of real time simulation while designing
electric drives are:
Interaction with the simulated system and realistic
selection of sampling time and integration time
step.
Improvement in reliability, as the controller is
designed and tested at the same time.
Developing and testing of the controller in the
absence of a motor drive. (In the development stage
of high power electric drives, any bug in the control
software can lead to unnecessary losses and hence
it becomes difficult to debug it with the motor and
inverter at different loads.)
Ability to re-run the same tests under the exactly
same conditions.
~-
Initiallv digital real time simulation was used in the area
of large power system analysis. In [I-31 the methodology
followed for simulating electric drives in real time is
described. However, it is clearly mentioned in [ l ] and [2]
that the simulations are performed in a non-real time system.
Though [3] deals with real time virtual system for electric
drive testing, only the results of induction motor fed from
PWM inverter are given. In most cases, real time simulation
is implemented on a platform that is different from the final
target controller. Recently, HIL testing in power electronic
3615
Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on December 3, 2008 at 06:28 from IEEE Xplore. Restrictions apply.
~
Auchen. Germany, 2004
2004 35th Annual IEEE Power Electronics Speciulists Conference
control was described in [4] and [ 5 ] wherein a boost
converter with state space feedback was studied and
analyzed.
In the present work, real time models of induction motor
and DTC strategies are built and tested independently. They
are then integrated to complete a HIL environment for
testing the induction motor with DTC technique. In order to
optimize the time resources of DSP, all the real time models
are developed in assembly code and executed on
TMS320F240 DSP.
appropriate voltage vectors in order to increase or decrease
the torque and flux. The block diagram for a conventional
DTC drive is shown in Fig. 3. At every sampling period, the
switching vectors are selected so that the stator-flux linkage
error and torque error are controlled within the hystersis
band.
Section I1 and Ill deal with the principle and
implementation of conventional DTC drive and space vector
PWM based DTC drive respectively. Section IV explains
the steps to build the hardware in the loop simulation
environment for both the drives mentioned above. Section V
introduces a tool “automatic code generation”, which is
capable of generating assembly code for real time simulation
of electric drives. Section VI describes the modeling of the
electric drive suitable for real time simulations and
integration techniques used for solving state-equations. In
section VII, the results of HIL simulation and those obtained
from the experimental prototype are presented and
compared.
11.
ddXlS
Fig. 2. Movement ofrtator flux with respect to rotor flux.
DIRECTTORQUE CONTROL
The electromagnetic torque of the induction motor in the
stator reference frame is given by equation (I):
Where is= L,L, - L’,
Since rotor time constant of squirrel cage induction motor
is high, the rotor flux linkage changes slowly as compared to
stator flux linkage. Assuming both to be constant, it follows
from ( I ) that changing ‘y’ in the required direction can
rapidly change the torque.
- This implies
Neglecting stator ohmic drop, pv,
that the inverter voltage directly impresses the stator flux. A
forward switching
- active voltage vector causes quick
movement of y, and hence torque increases with ‘7’.
-
However, when the zero vectors are applied, Y,$ becomes
-
Fig. 3. Block diagram for DTC drive.
In most cases the inverter switching vectors are not able to
compensate for the torque and flux errors. This is because of
the following reason: voltage vector is applied based on the
sign of the torque and flux errors. These vectors are applied
for the entire switching cycle. If the flux (or torque) error is
small, it will exceed the upper bound early in the cycle and
will continue to increase thereby causing high ripple as
shown in Fig. 4. Moreover, switching frequency is not
constant. Therefore a control strategy, which takes into
account the magnitude of torque and flux errors to generate a
reference voltage vector with constant switching frequency,
will reduce the torque and flux ripples significantly.
stationary. Since y , continues to move forward, ‘y’ and
hence torque decreases (althoughslightly). Therefore it is
possible to change the speed of y , by changing the time
duration between zero and non-zero vectors. The movement
of stator flux vector relative to rotor flux vector is shown in
the Fig. 2.
-
Depending on the position of y , it is possible to switch
the appropriate vectors to control both flux and torque. An
optimum switching table is constructed for picking up
a,
Fig. 4. Flux (or Torque) ripple in Conventional DTC
3616
Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on December 3, 2008 at 06:28 from IEEE Xplore. Restrictions apply.
Aachen, Germany, 2004
2004 35th Annunl IEEE Power Electronics Specinlists Conference
111.
IV.
SPACE
VECTORPWM BASEDDTC DRIVE
Space vector PWM (SVPWM) based DTC generates the
switching vector of the inverter based on the magnitude of
the torque and flux errors. The block diagram of SVPWM
based DTC induction motor drive is shown in Fig. 5.
The d-q components of reference voltage vector in stator
flux reference frame (i.e. U, = wyr and SO ,I = ( [ / d )
are given by [7]:
V,; = (K,,, + S ) ( T : - T e ) + vV,w,
(3)
P
The equations for d-q components of machine stator voltage
in stator flux reference frame are:
Vd = R A , + PY,
(4)
v, = R.Aq + wVn.I."
(5)
and
the
electromagnetic
3P
T =--I
e
4
torque
is
given
HARDWARE-IN-THE -LOOP SIMULATION
In the present work, controllers for conventional DTC
drive and space vector PWM based DTC drive are designed
based on real time simulation. The following steps are
followed while developing HIL simulation environment for
DTC drive.
Development of application software, which
implements the DTC strategy in real time.
Modeling the inverter and induction machine in
per-unit system, which reliably handles the
intermediate calculations without saturation at any
point.
Solving the state equations to get the following
state variables:
d and q-axis stator fluxes.
d and q-axis rotor fluxes.
Speed of the motor.
Calculating the outputs which are fed back to the
controller (Torque and flux).
The block diagrams for HIL simulation of conventional
DTC and space vector PWM based DTC are shown in Figs.
6 and 7 respectively.
DSP
by:
i
TMS320F240 CONTROLLER
torque
command
I'
If the stator flux is constant, then the torque can be
controlled by the imaginary component, V,-the torque
component of the voltage vector.
flux
command
Flux control is accomplished by controlling the real
component, Vd- the flux component of the voltage vector.
For each sampling period Ts, one can approximate Vsd as
Fig. 6. Black diagram for HIL simulation of conventional DTC
drive.
DSP TMS320FZiO CONTROLLER
Fig. 1. Black diagram for HIL simulation of space wclor PWM bas
DTC drive.
FlUI
ImYrm
,~yEWDII""I~IuUTcm
0
Fig. 5. Space vector PWM based DTC drive
The input to the inverter model in both the cases is the
switching states of the inverter. Depending on the
conducting state of the switches, the output of the inverter
model (voltages) is fed to the motor model.
3617
Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on December 3, 2008 at 06:28 from IEEE Xplore. Restrictions apply.
2004 35rh Annual IEEE Power Elecrronics Specialisrs Conference
V.
Aocken, Germany, 2004
AUTOMATIC CODE GENERATION
The control software (assembly code) which is used for
simulating an electric drive in real time on a target DSP can
be automatically generated using an off-line software
developed in C++. This software generates the code for a
particular type of machine and control strategy. In the
present work, software for induction motor drive with DTC
strategy is developed.
Where 4 refers to the base speed, q. refers to the rotor
electrical speed, and J is the per-unit moment of inertia.
The stator and rotor currents are obtained by solving the flux
-current model. They are given by equations (14-17).
isdn =
L, v, - L, V
r h
"h
(14)
" h .L,v
appp"prl*e actlo"
by changing
bascvaucs
irqn =
W b.L,
T, = Y.& .& - Y sq" .is*
(18)
Euler's explicit integration method with a time step of
Sops is used for real time simulation and experimentation.
Euler's explicit method to calculate d-axis stator flux is
given by the following equation:
y.Tdn
= vsdno
+ 6T * dYv, (Where bT is the integration
time step)
The second term in the above equation involves a
multiplication by a very small quantity (m). The resulting
term will be a very small value. If Park's fifth order model is
used, then this term cannot be precisely represented on a 16bit DSP register. However in the modified flux linkage
model, this quantity is multiplied by the base speed. Hence,
the product can now be precisely represented on a 16-bit
DSP register.
VII.
VI.
MODELING ANDNUMERICALINTEGRNION
Induction machine is modeled using the concept of
modified flux linkage [6] with five first order differential
equations (9 -13) as follows:
"b
Where L: = L,L, - L: and zbis impedance base.
The normalized electromagnetic torque is obtained as
Fig. 8. Automatic code generator for real time simulation ofelectric driver
The software structure is shown in Fig. 8. Input to the
software is the machine parameters and control strategy.
Dynamic analysis of the drive-motor system is carried out
for a time frame of 5 secs (can be changed). During this
process, the software also checks for any saturation of the
values. Here, saturation refers to the value of any variable
exceeding unity. In case any variable saturates; corrective
action is taken by changing the appropriate base. This
process is iteratively repeated until none of the variables are
saturated. Per unit quantities thus obtained are used for real
time simulation on the target DSP.
The automatic code generation unit uses the "Files"
concept in C++ to extract the assembly code for plant model
and control strategy from a library. This library contains
assembly codes for various plants and control strategies,
which are extensively tested and optimized. Real time
simulation results are obtained by executing the generated
assembly code on the target DSP controller.
4 .wq. - L, V,"
RESULTS
The HIL simulation results and those obtained from
experimental prototype are presented in Figs. (9-24). Results
of conventional DTC are presented in Figs. (9-16) and those
for space vector PWM based DTC are presented in Figs.
(17-24). It can be observed that there is a good agreement
between the HIL simulation results and experimental results.
It can also be observed that there is a significant decrease in
torque and flux ripple by space vector PWM based DTC
drive compared to conventional DTC drive (Figs. 9 and 17).
The steady state current (Figs. 14 and 22) and flux
waveforms (Figs. 16 and 24) also show significant reduction
in harmonics by space vector PWM based DTC drive.
3618
Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on December 3, 2008 at 06:28 from IEEE Xplore. Restrictions apply.
Aachen, Germany, 2004
2004 35th Annual IEEE Power Electronics Specialists Conference
..
..
2 02
,,
.
.
..
..
,,
.
..
..
.,
,
..
..
,,
.
.
....+....;......L~
. ....!, ......:, ....................
. ,
I
Q O
"
0
0
i
06
'
:
'
08
1
1.2
1.4
16
1.8
02
0.4
05
08
t
1.2
I.4
16
1.8
.
,.
.
,.
.
..
.
,.
:
,.
. .. ; ~ . ~ . .
,. .....~
. ...
I
04
..
.
'
,
02
..
,
",*
,
...
'
.
..
.
,.
...... I ......t~.....,.....
...........................
.....,......(*......,......,......I
F .fir.:;/,
h3
..,
2
2
!
8
:n,
' - -
-.,a.,
I
,
o>
,
04
06
08
I
I,
1.
I 6
!B
>
Fig. IO. Staning transienfs,Con" DTC (Expt)
.....l .............
:
,
01
.
i..
.
~ , ........
j
.:
,
~
..
.
..... :. ..... j,. ...............
.
i i j
I
Fig.
Fig. 16. Stator flux components,C o w DTC (Expt)
Fig. 12. Speed revenal transients, Conv DTC (Expt)
3619
Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on December 3, 2008 at 06:28 from IEEE Xplore. Restrictions apply.
2004 35lh Annual IEEE Power Electronics Specialisls Conference
;o.
i
:.
.
...
:
..
...
oi2
014
:.
.
:. :. : .
.
.. . ... ... ...
.
I t l l t)' ,a
:.
:.
,
... ...
ois 0;
,
,
Aachen. Germany, 2004
:.
,
,
o 2 ...................
. :~ .. ..1...~I
. . I. .......;.......:....~.....
. ......:......
l
i
.
.. .
: L , , ,' ''1 ,
bi
00
................. I......L ......:......:...........
4 0 8
!
~
0
0
.
.
.
.
.
0.2
0,
116
OB
I
......i.......
,
.
12
11
2
In
..I.
.....:.....
. . ..
16
2
18
Fig. 18. Startingtransients, SVM- DTC (Expt)
... ~8.
.. ~;
. i
. ..... . ...... . .....,....
. ~........
~
,
j~
:
:
:
.
i 1
...........
i~
I
!
Fig 23 Stator flux components. SVM- DTC (HIL)
Fig. 19. Speed reversal transients, SVM- DTC (HII.)
.
v
.
.
.
.
.
..
.
,:
.
.
.
:
L
.
1
,
.
.
.
.
.
:
.
.
.
:
.
.
I
Fig. 20. Speed reversal transients, SVM-DTC (Expt)
Fig. 24. Stator flux components, SVM-DTC (Expt)
3620
Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on December 3, 2008 at 06:28 from IEEE Xplore. Restrictions apply.
2004 351h Annual IEEE Power Electronics Specialisls Conference
VHI.
CONCLUSIONS
In this paper, a reliable method for designing, developing
and testing of digital controllers for power electronic and
high performance electric drive applications is presented.
HIL simulation and experimental verification of direct
torque controlled induction motor using conventional and
space vector PWM strategies are carried out.
The integration step time and sampling frequency can be
optimally chosen. Real time simulation also reduces the risk
of discovering any error in the last stage of implementation.
A new tool “automatic code generator” is introduced and
its software structure is described. This tool is useful for
faster testing and development of digital controllers for high
performance electric drives.
From experimental results it can also be observed that
there is a significant improvement in the drive performance
(like reduction in torque, flux ripple and reduction in current
harmonics) by space vector PWM based DTC strategy as
compared to conventional DTC strategy.
Aochen. Germany, 2004
[21 N. Sureshbabu, S. Seshagiri, A. MWNI and B. K. Powell, “On real
time simulation of induction motors.” in Proc. of IEEE American
Contr?l Conference. 1999, pp.719-723.
[3] F. C. D e w L. Cristaldi, A. Ferrero and A. Monti, “Real time vinual
system for electric drive testing: basic concepts and implementation,”
in Proc. oflEEE Electrolechnical Conference. 1996, pp.513-516.
[41 S. Lentijo, A. Monti, E. Santhi, C.Welch and R. Dougal, “ A new
testing tool for power electronic digital CO~VOI,” in Proc of IEEE
Power Elecironies Speclolist Conference, 2003, pp. 107-1 I I .
[51 B. Lu, A. Monti and R. h u g a l , “Real-time hardware-in-the-loop
testing during design of power electronic M ~ O I S , ” in Proc. of IEEE
Industrial Electronics Conference- 2003, pp. 1840-1845.
[6] R. Krishnm,
Electric Motor Drives: Modeling Analysis and
Control,” Prentice hall press, 2001, pp.2 18-223.
[7] C. Lascu, 1. Boldea and F. Blasbjerg, “A modified direct torque
control for induction motor sensorless drive,” IEEE Tons. Industry
Applicarions, 2000, pp.122-130.
APPENDIX
A. Motor parameters
Rated power
Rated speed
1.5 kW
1460 rpm
Rs
8.49 R
rL
1.65
L,
L,
L,
J
0.5253H
0.5253H
0.5015 H
0.02 kg-m*
R
B. Nomenclature
Number of poles
Differential operator
per unit resistance
Impedance base
Motor inductance parameters
d and q-axis per-unit voltages
d and q-axis per-unit currents
Flux phasors
Torque angle
Speed of stator flux vector
REFERENCES
[I]
R. Champagne, L. A. Dessaint, G. Sybille and B. Khodabakhchian,
“An approach for real-time simulation of electric drives,” in Proc, of
IEEE Canadian Conference. 2000, pp.340-344.
362 I
Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on December 3, 2008 at 06:28 from IEEE Xplore. Restrictions apply.