A unified approach to tuning has been found that enables a common and simplified method for setting PID tuning parameters. Key features can be used to eliminate the need for retuning to deal with different dynamics and objectives. Here are steps 6-11 in a methodology that integrates a unified tuning approach and key features to minimize implementation and maintenance efforts.
6. Turn on external reset feedback. Make sure the external reset feedback signal is correctly propagated back to the PID (e.g. BKCAL signal) especially if there are split range, signal characterizer, or signal selector blocks on the PID output.
7. For final control elements that are slow or that have deadband or resolution limit, use a fast readback of the valve position or variable frequency drive speed as the external reset feedback to prevent a burst of oscillations from the PID output changing faster than the final control element can respond.
8. For final control elements that create limit cycles from resolution limits and deadband, use a fast readback of the valve position or variable frequency drive speed to stop the limit cycles.
9. For cascade control, use the PV of the secondary loop as the external reset feedback to prevent a burst of oscillations from violation of the cascade rule where the secondary loop must be significantly faster than the primary loop.
10. For setpoint filters of secondary loops for coordination of flow loops, use the PV of the secondary loop as the external reset feedback to prevent the need to retune the PID.
11. For setpoint rate limits use the PV of the analog output block or secondary loop as the external reset feedback to prevent the need to retune the PID. Add setpoint rate limits to minimize the interaction between loops and to provide directional move suppression to enable a fast getaway for abnormal conditions and a slow approach to optimum particularly useful for pH, surge, and valve position control (VPC). For valve position control, use an enhanced PID developed for wireless with a threshold sensitivity limit to ignore insignificant changes in the valve position to be optimized.
For details on the above steps and more than you ever expected you should know about PID control see the post by Jim Cahill that has the AIChE 2013 Spring Meeting in San Antonio paper and presentation “A Unified PID Control Methodology”. Jim did a great job as usual in highlighting the most significant aspects.
