PLCs PID Control - Speed Control Loop (Part 2 of 2)
Speed Control Loop
A good example of a speed control loop is the action taken when adjusting the speed of the belt (or conveyor) to maintain the speed at a desired RPM (or Travel Time). This typically involves an encoder mounted on the belt/conveyor, connected to a PLC high-speed counter (HSC), used as a feedback to measure its speed.
Based on this feedback, PID performed a control action to adjust the analog output, or pulse with modulated output signal – PWM, until the process speed stabilizes at the desired set point (SP) value. The sensed encoder feedback is the process variable, or process value (PV). The desired speed is called the set point (SP). The output of the process, analog output, PWM, is called the manipulated variable (MV). The difference between the speed detected (HSC) and the set point, is the error, (e).
The error quantifies whether the speed is too high or too low, and by how many requirements there are for this application.
- PLC with transistor outputs for PWM speed control, or
- PLC with analog output expansion module
- Touch-screen as machine interface to make system adjustments
All Panasonic PLC's supports High Speed Counter input, but frequency varies based on PLC type and series. Measurement of HSC needs to be calculated before using in speed control PID Function Block. For example, raw data of HSC would be increasing in value, and needs to be processed in PLC interrupt and calculate to RPM or frequency. Demo programming software can be downloaded from the Panasonic Industrial Website.