Closed-loop control in ST: hysteresis first, then a practical PID

First: do you actually need PID?

Plenty of plant loops do not. If the final element is a pump that is either on or off, hysteresis control is the right answer, not the humble one:

IF PV <= SP - 3.0 THEN
  OnOff := TRUE;
ELSIF PV >= SP + 3.0 THEN
  OnOff := FALSE;
END_IF;

The dead band sets the starts per hour. A narrow band means tight control and a burnt motor. Large motors have a permitted number of starts, so the dead band is equipment life, not just precision. Use PID when the final element is modulating: a control valve, a VFD, a heater.

PID as arithmetic, line by line

IEC 61131-3 does not define a PID block, so every vendor ships their own. When you need yours:

Err   := SP - PV;
P     := Kp * Err;
Integ := Integ + Ki * Err * Ts;
D     := Kd * (Err - PrevErr) / Ts;
CV    := LIMIT(0.0, P + Integ + D, 100.0);
PrevErr := Err;

Ts is the scan time in seconds. On most process loops Kd stays at zero: derivative amplifies noise, and in practice it earns its place only on slow temperature loops.

Anti-windup

This is where hand-written controllers usually fail. If the tank is empty and the pump is at 100%, the error is large and persistent, so the integral climbs to an absurd number. When the value finally reaches setpoint the integral is still holding that number, so the process sails past the target and stays there for a minute.

The fix: do not integrate while the output is saturated and the error pushes it further into saturation.

CVraw := Kp * Err + Integ;
CV    := LIMIT(0.0, CVraw, 100.0);
Winding := (CVraw > 100.0 AND Err > 0.0) OR (CVraw < 0.0 AND Err < 0.0);
IF NOT Winding THEN
  Integ := Integ + Ki * Err * Ts;
END_IF;

Then clamp the integral anyway: Integ := LIMIT(-100.0, Integ, 100.0);. Belt and braces.

Bumpless transfer

The controller is in manual, the operator has it at 40%, and they switch to auto. If your integral is zero, the output jumps. The trick is simple: make the integral track the real output while in manual.

IF NOT Auto THEN
  Integ := CVmanual - Kp * Err;
END_IF;

On the first auto scan the computed output equals exactly what manual was holding, and the line feels nothing. The same principle applies after a fault reset, a range change, or a power-up.

Tuning in plain words

A method that always works: set Ki = 0 and Kd = 0, raise Kp until the process oscillates gently and continuously, then halve it. Now raise Ki until the offset closes in a time you accept. And the golden rule: a calm loop that arrives in twenty seconds beats a nervous one that arrives in five and hunts.

Time proportioning

A PID output is 0-100%, and the pump here is on or off. The bridge is time proportioning: a ramp climbing 0 to 100 every two seconds, with the pump on while the demand is above the ramp. A 30% demand gives 0.6 s of run out of every 2 s. It is exactly what an industrial heater thermostat does.