Proportional band help

[frantisikas]

Hello,

I need help with PID control. Is it possible to make proportional band different around Set Point? For example Set Point is 70°C, I want lower limit to be 63°C
 (start of proportional band) and upper limit 74°C (end of proportional band).

 

[Joe Tauser]

The proportional band is symmetrical around the setpoint by definition - you don't get to have one side bigger than the other.  Sorry.

Joe T.

[Isakovic]

Maybe if you set up logic to change P parameter when temperature rises over setpoint. Although that would probably be unstable.

 

[frantisikas]

What if I set up two PID loops? Both works at same time but I take output value from one and when temperature reaches set point I take output value from another? Will this make system unstable too? I can take output value from first PID loop until temperature reaches set point -1°C, then use last output vale until temperature reaches set point+1°C and then take output value from another PID loop. 

[Isakovic]

I guess that too would be unstable. Why do you need unsymmetrical Proportional band? Is overshoot too big if you want it to be more aggressive when temperature gets too high? Maybe try parameters that give softer approach to setpoint.

I don't know what kind of process you control, but with temperature it helps to turn off integral part when output reaches 100%, there's a function block for that.