hi,

i have a crucial situation.

i am trying to tune a control loop. this loop demands error minimization along with restricted control valve movement. that is the control valve opening should be within a fixed range, say 50% to 60%. basically too much control valve movement is not allowed. actual percentage doesn't matter, it should be within limits anytime. what should i do?

 

<< this loop demands error minimization along with restricted control valve movement. that is the control valve opening should be within a fixed range, say 50% to 60%. basically too much control valve movement is not allowed. actual percentage doesn't matter, it should be within limits>>

Don't use a PID controller. You can create a rule-based control something like a Fuzzy Logic controller that is subject to your rules on control valve movement. It may even work, just don't expect it to bring your process back under control if the deviation from setpoint is large.

Dick Caro
===============================================================
Richard H. Caro, CEO, CMC Associates
Certified Automation Professional (ISA)
Buy my books at the ISA Bookstore:
Wireless Networks for Industrial Automation
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===============================================================

 

Compromise - you can't have both.

Also, who says your valve can't move outside the 50-60% opening range and why do they say it? Are they really correct? Can you change the valve arrangement?

With the requirement for very tight control to setpoint, "tune by feel" probably isn't going to work for you. If you don't have access to a software tuning tool then I'd suggest Lambada or Tau Ratio tuning as simple methods that can easily be done by hand (even if it doesn't always give the "best" results). But seriously, a good PID tuning tool could solve this problem for you in minutes. Start here .... http://www.expertune.com/standard.html

Rob
www[.]lymac.co.nz

 

> along with restricted control valve movement. that is the control valve
> opening should be within a fixed range, say 50% to 60%. basically too much
> control valve movement is not allowed. Don't use a PID controller. You can
> create a rule-based control something like a Fuzzy Logic controller that is

You set the O/p hi limit and O/p low limit of the controller. its called clamping. it can be done while configuring the parameters of the controller in DCS or while viewing the controller properties. You can try it. it has worked for me though not for exactly similar situation.

><< this loop demands error minimization actual percentage doesn't matter, it
> should be within limits>>

I don't think its possible. if error minimization is your priority then you will have to tune the loop. But if you seriously can get the desired result if the controller command is 50-60% and doesn't matter....than i will prefer you keep the controller in 'Manual' mode rather than 'Auto' mode with a command of 50% or 60% whichever suits you.

If you do have to keep controller in Auto, type of controller and tuning....P, PI, PID....it depends on your application. How fast do you want the response, overshoots.......personally i will start with a Proportional controller.

 

thank you Caro and Rob for reaffirming my belief.
world is a good place to live.

actually the system is interconnected. the output of the first loop becomes the input for the second loop. and that's why..

as far as i understand now, the solution is compromise.

a very happy new year to all
long live control forum