Short Help For Finding The Control Parameters P, I And D - KPM PROCESS SENSORS METIS M311 Operating Manual

7.4.3.1 Short help for finding the control parameters P, I and D

Basically, in a thermal process, different control parameters will be determined for different temperatures.
▪ Proportional band Xp:
Xp is a dimensionless number representing the reciprocal of the gain of the controller.
At Xp = 100% the controller gain is 1.
at Xp = 0.1% the gain is 1000, at Xp = 1000% the gain is only one tenth.
This amplification simply increases the control output (manipulated variable) without delay, the further apart
the setpoint and actual value are. If you select Xp too small (=gain too high), it may be that the controller
tends to oscillate. If Xp is too large (= gain too small), the control output may not be modulated high enough
at all.
With proportional control alone, the actual value of the temperature always remains below the setpoint (if no
oscillation occurs), because if the control deviation is 0, the control output would also be set to 0 and no fur-
ther heating energy would be supplied. A permanent control deviation is thus obtained.
▪ Integral time Ki:
Ki is specified in units of time (e.g. in seconds). It represents a kind of smoothing or delay time and inte-
grates control deviations to correct the control output so that no remaining control deviation remains.
Basic rule:
Ki must be at least as large as the (thermal) time constant of the heating circuit.
For example, for a ceramic black body, Ki must typically be in the range of minutes, while fractions of a sec-
ond may be sufficient for a filament.
Increasing the time Ki makes the control process slower but also more stable.
Decreasing Ki can result in overshoot or even continuous oscillation.
▪ Derivative time Kd:
Kd is also given in units of time. It represents the look-ahead component of the controller. For example, if the
setpoint has almost reached the actual value but is still rising sharply, Kd "brakes" the rapid increase and
prevents overshoot.
Method:
Start with Kd = 0, then set Kd = Ki/4 and see if the transient effect goes better.
60 / 81
METIS M311 / M322 / H311 / H322 (12-pin device connector)