ECT 464

1. ECT 464 Lecture 16 PID control

2. Today’s Quote: Patience is the ability to keep your motor idling when you feel like stripping your gears. Better a patient man than a warrior,        a man who controls his temper than one who takes a city. Proverbs 16:32

3. Proportional/Integral/DerivativeControl • The PID instruction is an output instruction that controls physical properties such as temperature, pressure, liquid level, or flow rate using process loops.

4. PID Concept • The PID instruction normally controls a closed loop using inputs from an analog input module and providing an output to an analog output module. For temperature control, you can convert the analog output to a time proportioning on/off output for driving a heater or cooling unit.

5. PID Concept • The PID instruction can be operated in the timed mode or the Selectable Time Interrupt (STI mode). In the timed mode, the instruction updates its output periodically at a user-selectable rate. In the STI mode, the instruction should be placed in an STI interrupt subroutine. It then updates its output every time the STI subroutine is scanned. The STI time interval and the PID loop update rate must be the same in order for the equation to execute properly.

7. PID Control • The PID equation controls the process by sending an output signal to the control valve. The greater the error between the setpoint and process variable input, the greater the output signal. Alternately, the smaller the error, the smaller the output signal. An additional value (feed forward or bias) can be added to the control output as an offset. The PID result (control variable) drives the process variable toward the set point.

8. PID Equation

9. PID Instruction

11. PID Tuning • PID tuning is a difficult process. However there are some simple algorithms to follow to get a system approximately tuned. We will be using one such algorithm in lab.