RUN-TIME RECONFIGURATION OF AC DRIVE CONTROLLERS

Vásárhelyi József

University of MiskolcDepartment of Automation

E-mail: vajo@mazsola.iit.uni-miskolc.hu

Details about the author 1983 - Graduated in Electronics and

Telecommunication at Technical University of Cluj, Faculty of Electrical Engineering, Romania

Since 1992 lecturer at the University of Miskolc Department of Automation, Hungary

Presently PhD student at the Technical University of Cluj, Department of Electrical Drives and Robots, tutor: Prof.Dr. Mária Imecs

Research interests: configurable architectures and their application to control devices

Summary INTRODUCTION

– Short introduction to reconfigurable systems and why are they used or should be used in AC drive control?

BACKGROUND– Presents the background of AC motor control.

IMPELMENTATION STRUCTURE– Give an answer of (a) possible hardware structure(s)

RECONFIGURABLE CONTROLLER IMPLEMENTATION– Presents the idea of reconfigurable controller

TIME CONSTRAINTS OF THE RECONFIGURABLE CONTROLLER– Time constrains of the reconfiguration process are presented– Implementation issues and possible reconfigurable structures are

presented CONCLUSION

– Conclusions and future work are presented

Introduction

There are different approaches to define the reconfigurable systems [Brebner, Hauck, Luk, Maciejowski, Shirazi, Vuillemin].

Reconfigurable systems are usually considered those computing platforms whose architecture is modified by the software to suit the application at hand.

Most of Re-configurable Computing Systems are plug-in boards made for standard computers and they act as a Co-processor attached to the main micro-processing unit.

There was demonstrated significant potential for the acceleration of computing in general-purpose applications [Hauck, Smith, Villasenor, Vuillemin].

To treat the reconfiguration as a process one need a simple model for specifying and optimising designs, which contain elements that can be reconfigured at runtime.

•Comparing to the number of applications known in the reconfigurable filed just a few of them are concentrated in the study of vector control for AC drives.

•Vector control is a special field for digital signal processing.

•There are known dedicated DSP processors for digital motor control and successful implementations of vector control [Beierke] are referred. The DSP implementation of speed-sensorless induction motor drive using artificial intelligence is also known [Vas].

•Up to now the studied literature by the author, only the research of Monmasson and his group is reported as direct application of reconfigurable structures in vector control for AC Drives [Monmasson, Tazi]. The most significant result introduced in reconfigurable control was the parallel-machine control architecture.

The necessity of reconfiguration is based upon the practical observations that the performances of different types of vector controlled drives are different, depending primarily on the range of speed.

It is known that the rotor flux oriented vector control is simpler to implement and therefore, widely used. One drawback of this method is the low efficiency at low ranges of speed.

For lower speed range, the stator flux oriented vector control is preferred.

1. Background

Complex industrial systems and robotics make use of electrical drives.

Research efforts to find the optimal solution for AC motor control.

Since the reconfiguration idea appeared by the introduction of Field Programmable Gate Arrays [FPGA] there is an increasing interest to find other solutions then DSPs for AC motor Control.

Conclusion: Find a solution for reconfigurable control instead of using adaptive control

Speed

feedback

PWM

[is]

PowerConverter

-

+

motor

Current

feedback

-

+

imR

Set parameters

Reference

speed

Magnetising

Flux

SpeedController

FluxController

CurrentController

+Flux

Model

Vector control structure for AC drive

Source Texas Instruments

• Most of the motor control applications use asynchronous motors.

•The most often used method to control induction motors is the field oriented control method to achieve the best dynamic behaviour.

• Using the Park’s direct and reverse transformations the AC drive can be controlled like a separately exited DC machine, whereby the direct (d) path is representing the flux building component and the quadrate (q) path sets the electrical torque.

• Best results are obtained when the magnetising current imR is kept constant, which is direct proportional to the rotor flux r

under the assumption that the main inductance Lh is constant

0

i

dt

dL1

i

iL

i

i

dt

dL

i

iR

u

u mrs

sq

sdmrs

sq

sds

sq

sds

sq

sd

0

i

dt

di

1

i

i mrrmr

rmrsq

sd

sqmrSpe iiL1z2

3m

dt

d

z

Jmm

pre

(1)

(2)

(3) - (4)

Based on the mathematical model of the induction machine in field co-ordinates, given in equation (1-4), a controller was developed and a flux model was derived.

Orientation-fieldComputation

imrd

imrq

sinrcosr

Inductionmachine

Speed Controller

SystemTransformations

Three-PhaseAC Quantities

Field Oriented DC Quantities

Two Phase AC Quantities

Integrator

r

[is]*

Two-Level Current

Tracking Controller

r

*r

Position Controller

Co-ordinateTransformation

[(r)]

isd

isq

i*sd

i*sq

[is]*

VSI

CurrentSensors

TA[D(-r)]

TS[A]-1

TS[A]AF C1Imr

Lm

-

-

Flux Controller

-

imr

r

-

zp

i*sdr

i*sdr

Control Strategy

Vector control system for voltage-source inverter-fed induction machine

2. IMPELMENTATION STRUCTURES

The control system presents modularity as shown in the previous figure. The main modules are:

System transformations – direct and reverse Park’s transforms.

Orientation field computation Control Strategy Co-ordinate transformation There is need for an extra module, not presented on the

figure, which used for the external A/D conversion control. This modularity allows exploiting of all the parallelism of

the control algorithm.

Reconfiguration bus*

Orientation FieldComputation(CSL/FPGA)

ControlStrategy

(CSL/FPGA)

SystemTransformation 3to2

(CSL/FPGA)

ControllerSupervisor

Triscend’s CSoCCore

System Transformation 2to3PWM. Control signals

(CSL/FPGA)

Co-Ordinatetransformation(CSL/FPGA)

Host PC

Controller bus

AC Motor

PWM

A/D Control(CSL/FPGA)

Starting from the mentioned modularity a reconfigurable controller structure it is introduced.

The reconfigurable controller concept

STATE 2dx/dt=A2x+B2u

y=C2x

Power-on Init

STATE 1dx/dt=A1x+B1u

y=C1x

STATE ndx/dt=Anx+Bnu

y=Cnx

Implement different controllers for the same controlled process.

Each controller structure can be seen as a distinct state of a state machine.

Transition from one state to the other can be determined by the state parameters of the controlled system.

If a transition condition occurs, i.e. the motor speed reference transits a limit value, the need for reconfiguration is fulfilled and the controller generates the self re-configuration process.

The desired re-configurable controller can be implemented under the following conditions:

External memory is needed to store the several configurations (Configuration Store).

Either software or hardware has to be capable to start a reconfiguration on need. (Configuration Starter).

The evolution of the system must be predictable in order to pre-compute the possible configuration.

The system control states have to be quantified and finite; that is a condition imposed by the finite capacity of available external memory. (Configuration Memory)

The existence of ’high-fidelity’ models and effective approximation-identification algorithms for multivariable systems..

Programmable logic structures considered for the hardware support of the controller:

DLL

DLL

DLL

DLL

IOBs

IOBs

IOB

s

IOB

sVersaRing

VersaRing

Ver

saR

ing

VersaR

ingBR

AM

s BR

AM

sIOBs

Triscend’s CSoC:• Configurable System Logic• Incorporated processor core• External and internal memory• Ability to start self reconfiguration

Xilinx’s FPGA Virtex:• Abundant logic resources• Internal memory• Ability for partial reconfiguration• High computing speed• Relative high reconfiguration frequency

3. TIME CONSTRAINTS OF THE RECONFIGURABLE CONTROLLER

One can see that the controller structure is implemented in the configurable logic and the controller supervisor is a processor core. Depending on the implementation hardware the reconfiguration can be done as:

Partial reconfiguration – reconfiguring each module step by step conform to the method introduced by Hauck. The method is called pipeline morphing, intended to reduce the latency involved in reconfiguring from one pipeline to another. (This is the case if Xilinx FPGA it is used.)

Total reconfiguration – reconfiguring the controller as a whole. (This is the case of Triscend’s CSoC.)

The reconfiguration times computed for partial or total

reconfiguration methods are: For the partial reconfiguration (reconfiguration is done by

pipeline morphing) the maximum reconfiguration frequency is 66 MHz if the best existing hardware support is the Virtex FPGA. Reconfiguration of each module can be done if SelectMAP mode is used and the time needed for reconfiguration is under 1 ms.

The time needed for total reconfiguration of a CSoC by using the parallel mode initialisation, is 7.4 ms at 40MHz-reconfiguration frequency. This involves for implementation of the reconfigurable controller the use of two CSoC chips.

CQ

P

C’[is] [is]*Q and P represent the two control structures, C and C’ represent the reconfiguration control, [is] and [is]* are the observed current signal and the current control signal, respectively.

Testing CSoC chip CSL resources used in the implementation of the controller:

4. CONCLUSIONS

It was explained why reconfigurable structures are used in vector control.

The controller modularity help reconfiguration The controller states are quantified. Reconfiguration have to be done between two sampling event. Reconfiguration time have to be less or equal to sampling

period of the controller. Reconfiguration time may became critical and there is need for

reconfigurable structures with faster reconfiguration time.

Conclusion II.

Future work: Finalise the CSoC implementation and test with an

AC drive. Implementation of control structures in Xilinx

Virtex FPGA Create standalone module library for the

reconfigurable controller.