How to Use External-Reset Feedback for Cascade Control

The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #95.

The value of the external-reset feedback feature in PID controllers is becoming increasingly apparent to me. I got a preview of its importance when I found that 1980s vintage DCS required a fix for override control that was addressed in the next generation of DCS by this feature. This wakeup call was followed by the realization that this feature offered improved analyzer, blend, cascade, decoupling, feedforward, surge, valve position, and wireless Brewery contol displaycontrol with a side benefit of simplified deadtime compensation.

Override controllers in a 1980s vintage DCS could walk off to an output limit when not selected despite the use of proper mode and configuration. The fix was to insert a filter in the path of each integral track signal where the filter time was equal to the reset time of the respective overridden PID. The external-reset feedback feature of the next generation of DCS provided this filter inherently in the external-reset signal by virtue of the positive feedback implementation of the integral mode. Besides solving the override problem, this key feature created many opportunities and solved the problems from a controller output changing too fast.

If the process controller output changes faster than the manipulated variable can respond, integral action will keep pushing the primary controller output beyond what is needed. When the effect of the overcompensation is seen in the primary loop, the integral action repeats the overcompensation in the opposite direction. The result is cycling.

The manipulated variable can be the setpoint of a secondary loop (e.g., a flow loop) or a signal to a final control element (e.g., a control valve, damper, or variable speed drive). The slowness of the manipulated variable response appears mostly as a rate limit due to a setpoint rate limit in the secondary loop or analog output block, integral action, or slewing rate limit. Consequently, for small errors the rate of change of the process controller output is rather slow and within the rate limit of the manipulated variable. Tuning tests and operation with small setpoint changes or small output changes show everything is fine. For large upsets and major setpoint changes, the loop bursts into oscillations that in most cases eventually decay. This occasional instability is insidious. Creative explanations are offered.

Fortunately, the solution is simple: just turn on the external-reset feedback (e.g., dynamic reset limit). The process variable (PV) of the manipulated variable must be set up as the back-calculate output signal and connected as the back-calculate input for the external reset feedback. No retuning is required. The user is free to use directional setpoint rate limits to reduce interactions and achieve equipment and environmental protection, coordination, and optimization objectives (Tip #72) and more effective valve position control strategies (Tip #97).

Concept: External-reset feedback prevents the burst of oscillations caused by a primary controller outrunning a secondary loop or final control element. This key feature in modern PID controllers eliminates the need to retune primary controllers because directional setpoint rate limits are added to analog output blocks and secondary controllers to achieve process objectives.

101 Tips for a Successful Automation CareerDetails: Turn on external-reset feedback in any controller whenever a manipulated variable may not respond as fast as the controller output. Use external-reset feedback for cascade control, setpoint rate limits in analog output blocks, and slow final control elements. Setup the PV of the secondary loop, analog output, and final control element as the back-calculate input for the external-reset feedback. Makes sure the PV value used for the back-calculate input is responding as fast as the actual PV is changing.

Watch-outs: The second, third, and fourth HART variable for position readback and speed feedback are too slow. A variable with an update rate at least twice as fast as the PID execution rate is needed. For split ranged loops, the back-calculate signals must be correctly connected to reflect which secondary flow loop, control valve, or damper is actually being manipulated.

Exceptions: If you do not have the positive feedback implementation of the integral mode, the external-reset feedback is not an option.

Insight: The positive feedback implementation of the integral mode enables the use of an external reset signal that automatically accounts for slowness in anything being driven by the PID output.

Rule of Thumb: Use external-reset feedback to prevent oscillations from a slow secondary loop or final control element and to open up opportunities to meet process objectives by setting setpoint rate limits.

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