Final Year Project: September 2012

· PID

The PID acts as the controller of the system. The steady state (Ess) error gives response to PID to correct the error. A PID controller calculates an "error" value as the difference between a measured process variable (actual value) and a desired set point. The controller attempts to minimize the error by adjusting the process control inputs.

The PID controller calculation (algorithm) involves three separate constant parameters, and is accordingly sometimes called three term control: the proportional, integral and derivative values, denoted P, I, and D.

Proportional Gain kp produces an output value that is proportional to the current error value. This kp will adjusting the time response which reducing the rise time (Tr). However, high proportional gain kp will results the system become unstable which is increasing overshoot of the system.

Integral Gain ki produces a contribution to both the magnitude of the error and the duration of the error. The Integral Gain ki accelerates the movement of the process towards set point and eliminates the steady state error that occurs to the controller. However, it also can cause the present value to overshoot the setpoint value of the system.

Derivative Gain kd slows the rate of change of the controller output. kd used to reduce the magnitude of the overshoot produced by the proportional and integral component and improve the combined controller-process stability. However, the derivative term slows the transient response of the controller.

For the stability of this DC motor speed control system process, the gain of Kp, Ki and Kd is tuned by using manual tuning method. This method is first by setting set ki and kd values to zero. Increase the kp until the output of the loop oscillates, then kp should be set to approximately half of that value for a "quarter amplitude decay" type response. Then increase ki until any offset is corrected in sufficient time for the process. However, too much ki will cause instability. Finally, increase kd, if required, until the loop is acceptably quick to reach its reference after a load disturbance. However, too much kd will cause excessive response and overshoot. A fast PID loop tuning usually overshoots slightly to reach the setpoint more quickly.

In this process control, the output of the PID controller (algorithm) gives the range of output 0-100 duty cycle for the input of the PWM

· MOTOR

Refer to the graph above; the speed of the motor is proportional to the duty cycle. In this system, the range speed of the motor is 0-2600 RPM and the voltage supply to the motor is 0-5V depends on the duty cycle (0-100%) of the PWM.

The Motor Voltage (Vm) that supplied to the motor from the PWM and MOSFET give variable speed to the motor depending on the variable voltage supplied.

The desire speed that set at the setpoint input for example 2000 RPM needs at a range of 4.2V. Thus the PWM must inject at least 80% - 90% duty cycle to supply the motor input for producing 2000 RPM.

Final Year Project