John Egnew, a training consultant and instructor in Emerson’s Educational Services has posted another tip in his series of looptips. John’s looptip #12 is entitled Don’t Throw Away a Good Thing.
In it, he references how a positioner used on a control valve in a fast-acting loop may actually make the loop more unstable or difficult to control. The likely culprit may be too high of a loop gain. An example of this type of loop might be a fast fluid flow application.
If this is the case, the solution is having the travel feedback signal from the positioner be the inner loop of a cascaded loop. The inner loop of cascade control must be faster than the outer loop.
He also offers specific recommendations whether your loop is running under electronic or pneumatic control.
I hope these tips along with some of the wisdom conveyed by Terry Blevins and Greg McMillan over on the ModelingAndControl.com blog help you tackle some of these situations which can impact the performance of your process.
