Operating instructions

The viscosity of the heat transfer liquid changes very strongly with
the temperature. The liquids have higher viscosity at low tempera-
tures. Therefore, the control quality is generally worse at low tem-
peratures. For this reason, the controller should be set at the lower
end of the temperature range to be covered. If the control is stable
at low temperatures, then it is generally also stable at high temper-
atures. On the other hand, if a system is just still stable at high
temperatures, it is highly probable it will be unstable at low temper-
atures, i.e. it oscillates.
For example, if the operating temperature range of a
system is -20 ... 80 °C, the controller setting should be
made for approx. -10 ... 20 °C.
The picture on the left shows optimum setting of the control param-
eters.
If the Xp parameter is selected too large, the actual value reaches
the proportional range early and the P part becomes smaller than
100 % of the control value. The approach to the setpoint slows
down. Thus, the simultaneously integrating I part has more time to
build up its control value portion. If the setpoint is reached, the I
part summed too much results in overshooting beyond the set-
point. If the proportional range Xp is reduced, the P part remains at
100 % longer. Therefore the actual value approaches the setpoint
more quickly and the I part has less time to integrate the control
difference. The overshooting is reduced.
Other precautions
Information about possible incor-
rect settings
Fig. 33: Optimum setting
Fig. 34: Control parameter Xp too
large
Operation
Variocool 50