Temperature Control - Theben KNX Series Handbook

Appendix

6.5 Temperature control

6.5.1 Introduction
If the device is not configured as a switching controller, it can alternatively be configured as a P
or as a PI controller, whereby PI control is preferable.
With the proportional controller (P controller), the actuating value is statically adjusted to the
control deviation.
The proportional integral controller (PI controller) is far more flexible, i.e. it controls dynamically,
i.e. more quickly and more accurately.
To explain the function of both temperature controls, the following example compares the room
to be heated with a vessel
The filling level of the vessel denotes the room temperature.
The water feed stands for the radiator output.
The heat losses of the room are shown by a discharge.
In our example, the maximum feed is assumed at 4 litres per minute and at the same time is the
maximum heat output of the radiator.
This maximum output is achieved with an actuating value of 100%.
Accordingly, with an actuating value of 50% only half of the water volume, i.e. 2 litres per
minute, would flow into our vessel.
The bandwidth is 4 l.
This means, the controller will control at 100%, as long as the actual value will be smaller or
equal (21 l – 4 l) = 17 l.
Task:
Desired filling volume:
21 litres (= set point)
When should the feed be reduced, in order to prevent an overflow? :
4l below the desired filling volume, i.e. at 21l – 4l = 17l (= bandwidth)
Original filling volume
15 l (=actual value)
The losses are 1 l/minute
RAMSES 718 P / RAMSES 718 S 92