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Nov
08

Biggest Opportunities for Process Control Improvement – Basic Control

One of the best kept secrets is that a good portion of the benefits from the application of advanced process control (APC) originates from the improvements made to the basic control system as part of the APC project. While Model Predictive Control (MPC) and Neural Networks (NN), and Real Time Optimization (RTO) are glamorous and powerful, a much more extensive aspect of process control improvement involves the control valves, measurements, and PID controllers that form the foundation supporting the performance of any APC technology. A good APC team will use a person experienced in basic control to take a good look at the location and types of sensors and valves and the tuning and configuration of the PID controllers at the get go. To help locate and verify improvements, the set points of PID controllers used by the APC system are stepped in both directions and the dynamics and tuning of the PID loops are identified. The basic improvements on then put in place before process testing and data are used to build and verify MPC, NN, or RTO models. Instead of waiting for an APC project, why not get a head start on the benefits of better basic control now by checking the following? The values listed below are for a 2% required change in controller output, a 1% allowable control error, a process dead time of 10 seconds, a process time constant of 10 seconds, process gain of 1, and a disturbance time constant of 1 minute.

(1) Does the control valve quickly respond to the required changes in the controller output (e.g. half deadband and resolution < 0.2% and response time < 5 seconds)?

(2) Is the sensor noise and repeatability error less than 1/10 the allowable control error (e.g. noise and repeatability error < 0.1%)?

(3) Is the delay and lag of the measurement less than 1/10 the process dead time and process time constant (e.g. measurement delay and lag < 1 second)?

(4) Is the execution time of the controller less than 1/5 the process dead time and process time constant (e.g. control module execution time < 2 seconds)?

(5) Is the controller tuned to provide a standard deviation of the controlled variable less than 1/5 the allowable error (e.g. std dev < 0.2%)?

The additional dead time introduced by the automation system can be approximated here as the summation of the delay and lags introduced by the valve, sensor, and controller. The delay from the valve is the pre-stroke dead time and the delay from the dead band and resolution limit per equation 2-50 in Advanced Control Unleashed. The valve lag is roughly 1/4 of its response time minus the delay. The dead time from the controller is on the average 1/2 of the control module execution time. Adhering to the check list above should provide a peak error less than allowable control error for the case cited but results must be confirmed by loop performance monitoring in the plant.