Feedback Control Roundup Part 2

How do controllers go out of tune? How can I recognize them? What do I do?

How do controllers go out of tune? One of three reasons: –It was not set up properly in the first place –Something in the physical world has changed Worn parts - valve, motor, dampers, etc. Change in operating conditions affected the transfer function –Flow rate affecting time constant –Pressure affecting gain –Some Mech E installed a new piece of equipment –Someone changed it because they didn’t like it Usually decreasing Kc or increasing  I to make the controller less responsive.

How can I recognize them? Controller in manual (turned off) Sustained oscillations Complaints from operators (users) Occasionally, equipment damage, usually from too-aggressive a controller.

What do I do? 1.Understand the control objective Understand the current control configuration and why it is not meeting the objective 2.Figure out the process: inputs, outputs Rough idea of relationship(s) between input(s) and output(s) Transfer function model(s), time permitting 3.Design a controller for the process to meet the objective IMC tuning 4.Simulate the controller in closed loop to make sure you got it right, and to be sure it will work if the process changes 5.Implement the controller If you understand the effect of Kc and  I, skip steps 3 and 4 and change them directly, but carefully.

Physical Examples Vehicle control Thickness/width control in extrusion Engine control: fuel injection under load Antenna position on moving vehicle