×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Feedforward Feedback Control Standard

Feedforward Feedback Control Standard

Feedforward Feedback Control Standard

(OP)
I am a civil engineering PE working on a Construction project that includes a control system. The control system is intended to regulate the position of a control valve via feedforward-feedback control scheme in response to 4-20 mA signals from two sensors in comparison to a known setpoint. Is there a recognized industry standard that prescribes how this type of control strategy is normally mathmatically implemented? I would be grateful for any feedback. Thank you.

RE: Feedforward Feedback Control Standard

There is no standard for feed forwards. There are two main ways of implementing feed forward. One is to have a feed forward gain each of the derivatives of the SP. The other method is to have an exaggerated SP. This is kind of like holding a carrot out in front of a donkey. You make an exaggerate motion with the carrot so the donkey walks exactly where you want the donkey to step.

If you only care about regulating the position of a control valve then you shouldn't need feed forwards UNLESS you need to precisely control the rate of how that valve changes position.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

RE: Feedforward Feedback Control Standard

Peter's response is for feedforward that is used for setpoint tracking, not feedforward that is used for disturbance rejection. That follows from his knowledge of motion control systems that are trying to track a changing setpoint.

For disturbance rejection in process control, i.e., regulating a variable to a constant setpoint, feedforward is used differently. Given a known output disturbance transfer function Gd(s) that relates the disturbance input to the controlled output variable, and a known plant transfer function Gp(s), the feedforward controller transfer function CFF(s) for disturbance rejection would be:

CFF(s) = -Gd(s)/Gp(s)

The measured disturbance is passed through this transfer function and the resulting signal is added to the output of the feedback controller to make the total control signal sent to the plant.

There are two methods typically used to find the CFF, both deriving from the above equation. If first-order transfer function approximations (without dead time) are known for the disturbance and the plant, the resulting controller is a lead-lag controller:

CFF(s) = [-Kdps+1)]/[-Kpds+1)]

The lead-lag function is available in most commercial controllers like PLCs or DCSs. However, for many people it's too complicated to find both transfer function models, so an approximation is used that just requires the steady-state gains to be known for the disturbance and plant functions:

CFF = -Kd/-Kp

This works fine for most applications, as the feedback controller will "trim" any remaining errors that the feedforward controller doesn't remove from disturbances.


xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources