PID World Tour--The Final Performance

The dynamic reset limit can open opportunities important for sustainable manufacturing and, in particular, abnormal situation management and optimization. If a setpoint velocity limit is set in the analog output block, the dynamic reset limit prevents the PID from going faster than the velocity limit. The PID can achieve a slow approach to an optimum and a fast recovery upon encroachment of a constraint, such as encountered in the prevention of compressor surge, exothermic reactor runaway, Resource Conservation & Recovery Act (RCRA) pH violations and bioreactor biomass starvation.

In the past, an open-loop backup (kicker) was used for these applications because the tuning of the controller for drastically different speeds of actuation is problematic. The dynamic reset limit option eliminates the need to tune the controller based on direction and the concern about the exact value of the velocity limit. The tuning is set for the fastest recovery. The velocity limit is adjusted for the slowest approach to the optimum.

Measurements or final control elements can fail to update because there is a loss of communication or a component failure to last value. Communication failures can occur in bus systems due to link failures and in wireless systems due to low battery power. Sensor failures to last value occur in the presence of cracked or coated pH glass electrodes and plugged DP impulse lines. Final element failure to the last value occurs when a control valve does not move because of excessive stiction from high temperature, binding or occlusion of the internal control element from the build-up of solids and coatings, or the last position air failure of piston actuators.

The enhanced PID developed for wireless operation suspends integral action when there is no update. As a result, the PID output stays at the last value before the failure, which is the least disruptive action to the process. When the measurement or valve responds, or the communication is restored, the enhanced PID makes a small proportional and derivative mode and gradual integral mode correction based on the difference between the current and the last known value. The enhanced PID can ride out update failures without overreaction.

Believe it or Don't

• An automation control engineer became CEO because his creative designs made the company's manufacturing plants more profitable and maintainable.
• A split range point was computed based process gains to help linearize the loop.
• A split range loop did not oscillate across the split range point.
• A bioreactor batch worth over a million dollars was used to justify a third pH electrode.
• A company's purchasing department requested the packaged equipment supplier use the most accurate and reliable instrumentation for the application.
• A thermowell and sensing element was designed for fastest response.
• An academic paper studied tuning and performance of PID for disturbances at different locations and with different lags.
• The better threshold sensitivity of a diaphragm actuator designed for throttling valves compared to a piston actuator designed for on-off valves was recognized.
• A digital positioner was tuned by a user for maximum performance in an application.
• A maintenance department tested instrumentation for performance and maintainability to develop plant standards.