6.9 Controller tuning and control parameters

A controller has to match the heating zone. A massive manifold plate reacts sluggishly and forgives large control steps; a small nozzle reacts quickly and overshoots if treated the same way. The control parameters of a channel are therefore not a matter of taste but a measured quantity – and the controller tuning is the measurement.

Two kinds of numbers

Numbers	describe	Meaning
Ks, Tg, Tu	the heating zone	How strongly the zone reacts to power (Ks), how long it needs for that (Tg) and how long it does not react at all (Tu). Result of the measurement.
Kp, Tn, Tv	the controller	How vigorously it answers a deviation (Kp), how quickly it compensates a remaining offset (Tn) and how strongly it reacts to changes in anticipation (Tv). Result of the calculation.
Cid, Cnb	the damping	They reduce the I and the D component while the deviation or its rate of change is large. They suppress overshoot after a setpoint step. Only assigned with the tuning rule “Modified”.

NOTE

Ks, Tg and Tu are properties of the tool. Changing them does not make the control better – it changes the assumption on which the control parameters are based. If you want to adjust the control, change Kp, Tn and Tv; if you want to tell the device how the zone behaves, run the controller tuning.

How the tuning measures

The tuning runs in two phases. First the channel heats at constant power, and the Thermonom looks for the steepest point of the temperature rise. Then it switches the power off and observes the cooling. From the gradient of the two curves and the time offset between switching on and the first reaction, Ks, Tg and Tu result; the tuning rule turns these into Kp, Tn and Tv. The procedure with the applicable times is described under “Adaptation”.

Figure 23: The tuning heats at constant power until the rise has passed its maximum, and afterwards observes the cooling. The model of the heating zone arises from both gradients.

For the measurement to be worth anything, the zone has to be cold enough: a zone that already sits close to its set temperature provides no usable rise. In that case the tuning aborts with a message. It also detects three sensor faults that are hard to distinguish in normal operation: a swapped polarity (the temperature falls while heating), a short circuit (no signal) and a sensor that does measure but does not sit at this heating zone (the signal does not react to the power). The last case is the most valuable one – it finds swapped sensor leads in the tool.

Tuning rule

From the same model, controllers of differing tameness can be calculated:

Standard yields a restrained controller: slower at settling, but without a tendency to overshoot. The right choice for zones where an overtemperature damages the material.
Modified yields a more vigorous controller and additionally sets the dampings Cid and Cnb. It holds the temperature more precisely and rejects disturbances faster; the damping prevents the stronger intervention from overshooting after a setpoint step.

NOTE

The tuning rule is chosen before the tuning run: it determines how the controller values arise from the model. If you change it afterwards, you have to repeat the tuning – otherwise the old controller values remain in place.

If the control is not satisfactory

The first thing to reach for is not the parameter list but another tuning run on a cold zone. If something remains to be desired afterwards:

The channel oscillates around the setpoint: reduce the controller gain Kp or increase the reset time Tn.
The channel takes too long until a deviation disappears: reduce Tn.
The channel overshoots after heating up: use the tuning rule “Modified”, or limit the heat-up via the heat-up rate, see Section 6.3, “Setpoint guidance: ramp and synchronization”.

Make changes one at a time and in small steps, and watch the channel afterwards. With the recording (“Trace”) two settings can be compared cleanly instead of judging them from memory. The controller values are listed in Section 11.3, “Notifications” and can be exported there. Recording several channels at the same time is resource-intensive and can slow down the response of the user interface.