(6b) Deep Understanding and Great Achievements of PID Control

Shinskey’s articles and books were the best source of what was truly important and achievable by PID control. His extensive analysis of process relationships and dynamics and the PID controller capability are unmatched and is the key to process control systems that can increase process capacity and efficiency. He has been the biggest influence in my career. Many of my accomplishments can be traced back to knowledge I gained from Shinskey. Here I provide a summary of what is foremost for me with benefits that extend beyond distillation into all types of unit operations.

Process metrics, such as integrated error and peak error can readily be estimated for disturbances from the controller tuning settings showing the impact of poor tuning. The disturbances are not on the process output as what is commonly shown in publications but are on the process input. The implications in terms of focusing on what is truly important and eliminating distractions from special algorithms supposedly better than PID depend upon this fundamental understanding. PID feedback and feedforward tuning settings and tests should be based on recognition of the location and dynamics of the disturbance. Any desired setpoint response can be achieved by the simple addition of a setpoint lead-lag. The tuning and capability for level control can be the key to the best choice and performance of distillation column control strategies. The use of external-reset feedback can automatically prevent problems with cascade control, poor control valve response and the use of analyzers with variable sample or cycle times. Furthermore, dead time compensation can be better achieved and its limits recognized. The benefits gained from the insights can continually grow. It is our job to make sure this is the case.