[TRNSYS-users] PID in cooling mode

[David Bradley [SEL]]

Krishna,
  My apologies for the delay in getting back to you about this question. 
I needed to check with the master of PID controllers in TRNSYS (Michael 
Kummert) before replying coherently. There are two ways that you can use 
Type23 in a cooling mode application. If you have a cooling case in 
which (for example) you are using a PID as a flow controller to keep a 
temperature below some desired set point then you can set Type23's 
proportional gain constant to a negative value in which case the PID 
will increase flow as the monitored temperature gets higher and higher. 
In this case minimum control signal (parameter 4) should be zero and the 
maximum control signal should be a positive value.

In another case, you might want your PID to provide both heating and 
cooling. In this situation, the control signal's magnitide should 
increase as you get farther and farther from the setpoint but the sign 
of the control signal should change depending on whether you are above 
or below the setpoint. In this case, you set the proportional gain to a 
positive value but you allow the Type23 calculated control signal to 
vary between a negative and a positive number. I will send you an 
example that Michael was kind enough to generate. I do not want to clog 
up everyone's InBox so I will send it in a seperate email.
Kind regards,
  David

Krishna wrote:

> I am having to reverse the control signal (1-control signal) for 
> controlling the chilled water flow with a PID controller.
> Could any one please let me know if this is the right way to do it.
> thankyou,
> Krishna
>
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-- 
**
David BRADLEY
Solar Energy Laboratory
University of Wisconsin - Madison
1500 Engineering Drive
Madison, WI  53706
United States

Ph: +1 608 263 1589
Fax: +1 608 262 8464

Email: dbradley at engr.wisc.edu

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