Explanation Of Controller Function; 2-Point Controller - ABB i-bus KNX Product Manual

®
ABB i-bus KNX
Appendix
12.4

Explanation of controller function

The various control types possible with this device are described below.
12.4.1

2-point controller

A 2-point controller possesses two output states (On/Off) that change depending on the actual value.
If the actual value is higher than the parametrized setpoint, the associated control value is 0.
If the actual value is less than the parametrized setpoint, the associated control value is 1.
Application for a 2-point controller:
•
As the control value oscillates only between the two On and Off states, the controller can control an
electrothermal valve connected to a switch actuator or a valve drive actuator.
•
Control of an electric heater via a relay output, as a distinction is also made only between On and Off
with this heater.
CAUTION –
The number of relay operating cycles must be observed. The relay switches each time the control value
changes. The maximum number of relay operating cycles, particularly under load, is limited. This can
quickly exceed the relay's service life, especially when controlling an electric heater (which often
represents high loads).
Example:
If the control value changes only ten times per day, this already amounts to 3,650 operating cycles per
year.
If the control value changes 50 times per day, this already amounts to 18,250 operating cycles per year.
A 2-point controller can quickly compensate for control deviations in case of large changes in the
command variable (setpoint temperature). However, it never settles and it tends to lead to system
overshoot (exceeding the setpoint temperature) in this process. To avoid overshooting the initial states,
2-point controllers always feature integrated hysteresis that fluctuates around the setpoint.
The function of hysteresis is to allow the control value to switch only when the control value is
exceeded/fallen below by a certain value. This reduces number of control value changes and therefore
leads to smoother control.
234 2CDC508220D0211 Rev. A | VC/S 4.x.1