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Issue #217 August 2008
Subcategory Winner - Microchip 2007 Design Contest
INTELLIGENT ENERGY SOLUTIONS
Electric Vehicle Inverter Design
Build A System For Powering AC Induction Motors
by Dan Hall, Tristan Kasmer, Doug Krahn, Adam McIntyre, and Dena Ponech
Start | Power Inverter | Gate Drivers | Control Board | Space Vector Modulation | PID Tuning | Altering Motor Control Parameters | HMI | Protocol | Firmware/Software | Sources & PDF
PID TUNING
There are abundant sources of material available on the Internet and in libraries all around the world that discuss, in great detail, the methods for tuning proportional, integral, and derivative (PID) control loops. Therefore, it is not necessary to discuss such things here, but it may be helpful to present some details that are specific to this project.
Electric motors of all sizes and descriptions are built for specific applications. For our electric vehicle project, we decided to use a squirrel-cage AC induction motor. To use this motor for such an application, it is obvious that some tuning of the motor controller is required. To tune the motor, we developed a LabVIEW application that enables you to tune the PID variables. Then, after starting the motor, you can watch the motor response in real time.
Motor response is critical to the overall operation of the EV system. When a new motor is connected to the motor controller circuit board, the PID parameters must be properly aligned. If the motor does not respond well to input like stepping on the accelerator, there needs to be alterations to the PID control loops.
The proportional gain of the controller determines the maximum output level of the control loop. If the proportional gain is too low, the output of the control loop will never reach the set value of the input. If the proportional gain is too high, the output will oscillate and may become unstable. An ideal output from the proportional stage will closely follow the desired value without any oscillations or ringing when it reaches the steady state.
The integral stage is meant to reduce the steady state error, but integral gain can introduce ringing and overshoot. The derivative stage is meant to reduce the ringing and overshoot, but derivative gain can introduce steady state error. An ideal balance between integral and derivative gain can be achieved through trial and tuning to give an optimal response.
The spatial vector modulation software that we downloaded from Microchip contains three PID loops.[9] One loop controls the flux field of the stator, the second loop controls the torque current in the stator, and the third loop, which contains the two previously mentioned loops, controls the angular velocity of the rotor. When you input a velocity demand or input a setpoint (acceleration/deceleration on the potentiometer), the torque and flux loops are passed a set value from the velocity loop and execute the appropriate controls. The feedback loop signals for the torque and flux loop are generated by the Hall-effect current sensors on ia and ib and the velocity demand feedback is given by the encoder (see Figure 4).
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| Figure 4 —This is a block diagram of a PID control loop from Microchip Technology’s application note AN908. |
We elected not to use the velocity control loop for acceleration of the motor. The gas pedal of an internal combustion car sets acceleration but does not set the speed. Because the acceleration of a motor is directly proportional to the torque, we passed the input demand value from the gas pedal directly to the torque control loop. Luckily, SVM provided us with the isolated control over the torque that gives the end user a more realistic feeling gas pedal.
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