Hello,

Is there any easy way to implement a deadband on the PI controller, as there doesn't seem to be an in-built option?

Thanks.

Hi, thanks for your reply.

I am controlling a constant-pressure variable-volume extract ventilation system, controlling the static pressure measured within a low-velocity plenum near to the fan.

I am using P+I. I have carried out a tuning process, and the loop is stable in the sense that there are no large variations in static pressure occurring. However, any small variation (e.g. a few Pa) in the static pressure (e.g. wind gusts at roof level where the system discharges, or small changes in air volume through one of the air terminals) is resulting in a correspondingly small change in the manipulated variable that doesn't need to happen. And wouldn't, if a deadband was available.

I am not concerned about this from the point of view of the process, as +/- 10Pa is acceptable. But I am concerned that the fan speed is effectively nearly always changing by a small amount, and so over time this will result in undue wear of the fan.

I have tried applying filtering to my PV input; two Analog Filter blocks in series, both set to 256 samples. This has helped a bit.

I could always apply some filtering to the PI controller output too, but it just seems odd to not have a deadband available.

I guess I have been spoilt all these years by the PID blocks in the S7-300 PLCs 

Thanks very much for this.

I did try and experiment a bit while on site last week to see if I could implement a deadband, by switching the PI controller to manual control when within the deadband, and using the previous output value as the manual value. It was tricky to implement it without using circular references, and when I did work around that issue, it didn't behave as expected.

Being able to use a max/min block as a memory should allow me to take manual control.

Thanks, I'll let you know how I get on.

Yeah that works now with the max/min block as a memory.

In the attached file:

Input I1 enables the PI controller to run. B003 defines the setpoint, this is passed to the PI controller and the two comparators.

When the PV enters the deadband (+/-5 in this case) the max/min is triggered to store a value that is passed into the manual paramater of the PI controller. After a short delay, the PI controller is put into manual control. It stays in that state until the PV moves outside of the deadband.