Issue
What are CZT, CXT, CXR Controllers?
Why are CZT, CXT, CXR Controllers used?
What is Proportional, Integral Action and Derivative control.
Why does a controller deliver different pulse lengths to the valve and not drive it fully open or closed?
What is Pb, IA, PID?
Environment
- CZT
- CXT
- CXR
- DC1100
- DC1400
- Proportional, Integral Action, Derivative, Pb, IA, PID
Cause
What do these controllers do and what are their applications?
What is Proportional, Integral Action and Derivative control.
Why does a controller deliver different pulse lengths to the valve and not drive it fully open or closed?
What is Pb, IA, PID?
Why is a 0-10V Signal used?
Why is a 3 point control / Open Closed actuator used?
Resolution
Proportional Controllers are used in applications where there are long time lags e.g. room control, return air control, storage calorifier (HWS). Where the controlling sensor is a medium to long distance from the heat/cooling source.
An example of a proportional controller is the Satchwell CZT.
Integral action controllers are used in applications where there are short time lags e.g. supply air control, frost coil control, non-storage calorifiers. Where the controlling sensor is a short distance from the heat/cooling source.
An example of a integral controller is the Satchwell CXT or the CXR. The CXR is a controller where the supply air is controlled, but the setpoint is reset by the return air or space temperature, therefore the time lag is short. Other controllers that fall into this category are the DC1100, DC1400 again the controlling sensor (mixed flow temp) is located a short distance from the heat source.
The term proportional band (Pb) is where the valve is moved to a position proportional to the deviation from setpoint. 0-10V modulating actuators are generally used on these controllers.
The term integral action (Ia) is where the valve is moved at a rate according to the deviation from setpoint i.e. the valve is given longer pulses the further away from setpoint, shorter pulses as it gets closer. 3 point control/pulsed pair/open-close actuators are generally used on these controllers.
Further to proportional controllers, inherent offset (the setpoint is never quite reached) can be an issue. Integral action can be applied to remove this offset. These controllers are primarily proportional, but integral action can be found on them and is applied. Hence the term PI or PI controller.
Derivative control is generally only found on BMS controllers and is used occasionally with PI control, hence the term PID. Here the derivative action moves the valve proportionally to the change from setpoint. It is complex and is rarely used.
Further detail about CSC controllers can be found here.
Further detail about CXT controllers can be found here.
Further detail about CXR controllers can be found here.
Further detail about CZT controllers can be found here.
Further detail about DC1100 and DC1400 controllers can be found here.
