Why does the Y-output of a DDC Suite controller change if the setpoint is changed?

FAQ #101424

 

Symptom
The output value of a controller of the DDC Suite is "jumping" if the setpoint is changed. This kind of output value jumps can be observed even if the actual value is still higher than the setpoint (e.g. if the room temperatur is 20°C and the setpoint is changed from 24.0°C to 22.0°C).


Reason
This behaviour is a standard behaviour of the PI controller used by the DDC Suite by default (the output value is the sum of the proportional part and the integral part which is responsible to eliminate a persistent controller deviation which can be caused by the pure P-controller). In case the setpoint is changed, the proportional part is changing immediately while the integral part will be reduced over time (resulting in the same valve postion of 0%). This behaviour is identical to the one applied by the PI controllers from the Heavac FBox library.

Solution PG5 2.0
In case this behaviour is not wished (in order to keep the valve closed on changes of the setpoint if the actual value is still smaller than the setpoint) the controller algorithm used by the DDC Suite can be changed:
You have to create a Global Symbol File ".sy5" and create a Constant "K" public symbol with the path "DDC_Suite.Controller.PID_Mode" inside this File.
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During the next build, the message window from PG5 will indicate that the "Modified PID algorithm" is used (for all the FBoxes in this Fupla). This algorithm will disable the I-portion of the output in case the actual value is higher than the setpoint.
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Solution PG5 1.4
In PG5 1.4, add a K-constant with the name "DDC_Suite.Controller.PID_Mode" to your public symbols:

During the next build, the message window from PG5 will indicate that the "Modified PID algorithm" is used (for all the FBoxes in this Fupla). This algorithm will disable the I-portion of the output in case the actual value is higher than the setpoint.

The "modified PID algorithm" will no longer lead to jumps of the controller output as long as the valve is closed:


By default (if the above mentioned symbol does not exist), the standard PID algorithm will be used and the message in the message window is "Standard PID algorithm enabled"

Remarks

• The "Modified PID algorithm" is implemented in the DDC Suite library version
  - 2.5.150 and later (for PG5 1.4.300)
  - 2.6.150 and later (for PG5 2.0) 
• In DDC Suite V3 this option will be integrated to the Init FBox of the DDC Suite.
• When working with the Heavac FBox library, the P-PI controller can be used in order to avoid the output jumps of PI controllers (the P-PI controller is only adding the I-portion in case the setpoint is near the actual value).