At this point the loop runs, but not very well. I would like the loop to heat, only when below the set point and cool, only when above the set point. Right now, if the set point is 150degF above the PV, the loop will heat then cool, heat then cool, over and over, and slowly heat up. The same is true for when the SP is below the PV.

Thanks in advance

 

Just in order to make sure it is a tuning problem and not something else...

What kind of output do you have? Is it an analog (4-20mA or similar) output or a digital (on/off) pulse width modulated output?


/Johan Bengtsson

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You need to control the heating and cooling with different PIDs. The Setpoint for each PID should be offset from the desired setpoint by enough to allow some overshoot of the controllers. e.g. the heating loop setpoint might be a few degrees below the desired setpoint, while the setpoint for the cooling loop may be a few degrees above the desired setpoint. This builds in a deadband to prevent unnecessary cycling.

Then you need to tune each loop with enough Integral gain to prevent excessive overshoot.

Dick Caro
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A normal PID loop, properly tuned, will not do what you describe, unless the control algorithm has been specifically designed to exclude the possibility of no action - i.e. it must be always heating or cooling. If the control output is basically digital (on or off) then you are probably using duty-cycling to simulate variable control. Just program the duty-cycling to utilize the all-off state.

-Robert Scott
Real-Time Specialties
Embedded Systems Consulting

 

Can you give some more details.. Is it heating and overshooting the SP then cooling and to go under the SP again? Is this repeating but slowly settling about the SP? If this is the situation then it's just a matter of adjusting your P & I parameters of the loop. If this is not what you are meaning please clarify what you are doing.

 

I have abandoned the heat/cool loop with dual outputs. I am going back to two pid loops with built in offsets to simulate deadband.

 

your first solution should be better - properly tuned and set up.

/Johan Bengtsson

Do you need education in the area of automation?
> ----------------------------------------
> P&L, Innovation in training
> Box 252, S-281 23 Hässleholm SWEDEN
> Tel: +46 451 74 44 00, Fax: +46 451 898 33
> E-mail: [email protected]
> Internet: http://www.pol.se/
> ----------------------------------------