You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
Thread Tools Display Modes
Unread October 4th, 2007, 07:55 PM   #1
Doug_Adam
Member
Australia

Doug_Adam is offline
 
Doug_Adam's Avatar
 
Join Date: Sep 2002
Location: Perth
Posts: 949
Feed Forward control - checking my solution

I had a bit of a problem implementing feed forward control on some flow control valves. During steady state, the control with feed forward was much better than the straight PID control. However, on startup and during setpoint changes, the time taken for the flow rate to change generally resulted in some reset windup with a small but unacceptable overshoot and a long settling time.

I overcame this on startup by disabling the integral control for the first 10 seconds, and so far the small adjustments to the setpoint by the operators haven't caused much of a problem. However, I was looking at another possible solution where for every setpoint change, I will set the integral hold function for 5 to 10 seconds.

Now the question I would like to ask is for those who use feed forward control, is this a common problem? And do you normally disable the PID control during setpoint changes? Is there another way of doing this?

Thanks in advance.

Doug
  Reply With Quote
Unread October 4th, 2007, 11:22 PM   #2
Peter Nachtwey
Member
United States

Peter Nachtwey is offline
 
Peter Nachtwey's Avatar
 
Join Date: Apr 2002
Location: Vancouver, WA, US
Posts: 6,974
Quote:
Originally Posted by Doug_Adam
Now the question I would like to ask is for those who use feed forward control, is this a common problem?
No.

Quote:
And do you normally disable the PID control during setpoint changes?
No.

Quote:
Is there another way of doing this?
Probably but what are you really doing? Is the set point a flow or a level? If the SP is a flow, then is the flow linear with the control output?
  Reply With Quote
Unread October 5th, 2007, 12:50 AM   #3
kamenges
Member
United States

kamenges is offline
 
kamenges's Avatar
 
Join Date: Nov 2002
Location: Brillion, WI
Posts: 3,886
You are asking your system to do something it can't do. You are making a change in setpoint faster than the system is capable of changing the actual value.

This really isn't so strange in process control system, where the setpoint is often changed in steps. However, in properly tuned first order systems without feed forward, the PI integral time is typically matched to the process time constant. Effectively what happens is the PI integral sum grows at a rate that matches the rate of change of the process. Ultimately you end up with an integral value that provides the stady state output required to keep the process where you want it.

Now you add feed forward. Lets assume you can calculate a perfect feed forward term. In steady state you have zero PI loop output. You make a step change in the setpoint. The new feed forward value immediately jumps to the correct value that will ultimately give you the desired steady state value. However, it takes time for your process to reach the new setpoint. So you have a certain amount of time where you have error. This error will cause the PI integrator to move in one direction or the other to correct for this error.

As I said above, ultimately the integrator would provide all the output needed to keep the process at the steady state value. And the PI integrator will continue integrating until the setpoint is reached and passed. However, you are already providing the required output as part of the feed forward calculation. So all that integrator value that was added to help your process get to setpoint is actually output you ultimately don't need. And there is your overshoot.

That is the why. I'm not sure how I would fix it exactly. I know the first thing I would do is ramp my setpoint changes at a rate that the process can keep up with. Your actuator takes some time to achieve the new flow rate setting. you absolutely can't change the setpoint faster than that.

Keith
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
PID - what is it anyway? Ron Beaufort LIVE PLC Questions And Answers 21 November 25th, 2014 07:36 PM
Hydraulic Winch Control shelnian LIVE PLC Questions And Answers 10 May 3rd, 2006 10:51 AM
Dcs, Scada hasnoclue LIVE PLC Questions And Answers 15 April 11th, 2005 07:12 AM
how often should I trigger the PID? Ron Beaufort LIVE PLC Questions And Answers 11 February 22nd, 2003 11:57 AM
grounding power sources drewcrew6 LIVE PLC Questions And Answers 18 December 12th, 2002 06:16 PM


All times are GMT -5. The time now is 12:39 AM.


.