7.2.1 Loop Response

If we ignore the situation of loop oscillation, there are three categories of loop performance:

 

Under Damped - In this situation the terms are set to prevent oscillation but generally lead to an overshoot of the Process Value followed by decaying oscillation to finally settle at the Setpoint. This type of response can give a minimum time to Setpoint but overshoot may cause problems in certain situations and the loop may be sensitive to sudden changes in Process Value. This will result in further decaying oscillations before settling once again.

 

Critically Damped - This represents an ideal situation where overshoot to small step changes does not occur and the process responds to changes in a controlled, non-oscillatory manner.

 

Over Damped - In this situation the loop responds in a controlled but sluggish manner which will result in a loop performance which is not ideal and unnecessarily slow.

 

The balancing of the P, I and D terms depends totally upon the nature of the process to be controlled.

 

In a plastics extruder, for example, a barrel zone will have a different response to a die, casting roll, drive loop, thickness control loop or pressure loop. In order to achieve the best performance from an extrusion line all loop tuning parameters must be set to their optimum values.