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 12-16 in a methodology that integrates a unified tuning approach and key features to minimize implementation and maintenance efforts.
12. Add output tracking for a full throttle (bang-bang control) strategy for the fastest possible time to reach setpoint on startup and for batch operations with minimal overshoot. The PID output tracks an output limit until the PV value one dead time into the future reaches set point. At this time the output is set and held at the final resting value for one dead time and then released from output tracking for PID control.
13. Add output tracking logic to momentarily track an output that insures equipment and environmental protection. For compressor surge protection track a sufficiently large opening of the surge valves. Hold the protective output long enough to stabilize loop before returning the surge control PID to auto. This strategy is also known as an open loop backup. To prevent a RCRA pH violation, track a rapidly incrementing reagent valve position to prevent an effluent excursion below 2 pH or above 12 pH. When the pH recovers, return the pH PID to auto. This strategy is also known as a kicker.
14. Add feedforward control for large and fast measured disturbances. For flow feedforward, use a ratio and bias station so the operator can enter a desired flow ratio and see the actual flow ratio. Setup the PID to provide a bias correction to the manipulated flow. Add dynamic compensation (dead time and lead-lag blocks) to the feedforward so the manipulated flow arrives at the same point in the process at the same time as the measured disturbance.
15. For wireless devices or analyzers that introduce a large discontinuous PV update delay use an enhanced PID to eliminate the need to retune the controller to prevent oscillations. If the update delay is much larger than the 63% process response time, the PID gain can be set as large as the inverse of the open loop gain for self-regulating processes.
16. For valve position control (VPC), make sure the key PID features are used. For a description of these feature and examples of VPC for process optimization, see the Nov 2011 Control article “Don’t Over Look PID for APC.” The use of an enhanced PID enables the use of full PID action in the VPC and simplifies the tuning of the VPC. While model predictive control can provide a more sophisticated optimization, the VPC can be implemented by a simple configuration change that involves setting up a PID as the VPC. The minimal implementation cost and time is attractive for small optimizations.
The use of key PID features noted in steps 6-16 enable the achievement of a wide spectrum of plant objectives without the need to use special tuning that might adversely affect loop performance for unmeasured disturbances.
For details on the above step 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.
