The PIDA element performs a PID control function using the built in algorithms and yields an analog output.
The main purpose of this article is to document provide a working example of how a PIDA should be written.
The snippet below shows a typical direct acting PID controlled valve ladder logic.
- Rung 1 outlines PID functions
- Rung 2 logic is to send the output from the PIDA function block (Modbus register 41021) to the physical valve via analog output (Modbus register 40001)
Here are he settings for the PIDA function block.
And this is what the output SCAL function block setting looks like.
Direct Acting or Reverse Acting
Essentially, in a direct acting PID controlled valve, positive error increases the control output. Contrary to that, in the reverse acting PID controlled valve, positive error decreases control output.
The snippet below outlines direct acting (Rung 1) and reverse acting (Rung 5) logic for PID valve