The post, Many Objectives, Many Worlds of Process Control first appeared on ControlGlobal.com's Control Talk blog.
In many publications on process control, the common metric you see is integrated absolute error for a step disturbance on the process output. In many tests for tuning, setpoint changes are made and the most important criteria becomes overshoot of setpoint. Increasingly, oscillations of any type are looked at as inherently bad. What is really important varies because of the different loops and types of processes. Here we seek to open minds and develop a better understanding of what is important.
– Compressor surge, SIS activation, relief activation, undesirable reactions, poor cell health
– total amount of off-spec product to enable closer operation to optimum setpoint
– Interaction with heat integration and recycle loops in hydrocarbon gas unit operations
– Batch cycle time, startup time, transition time to new products and operating rates
– Wasted energy-reactants-reagents, poor cell health (high osmotic pressure)
– Passing of changes in input flows to output flows upsetting downstream unit ops
– Resonance, interaction and propagation of disturbances to other loops
* FRV is the Final Resting Value of PID output. Overshoot of FRV is necessary for setpoint and load response for integrating and runaway processes. However for self-regulating processes not involving highly mixed vessels (e.g., heat exchangers and plug flow reactors), aggressive action in terms of PID output can upset other loops and unit operations that are affected by the flow manipulated by the PID. Not recognized in the literature is that external-reset feedback of the manipulated flow enables setpoint rate limits to smooth out changes in manipulated flows without affecting the PID tuning.
– Fast self-regulating responses, interactions and complex secondary responses with sensitivity to SP and FRV overshoot, split range crossings and utility interactions.
– Important loops tend to have slow near or true integrating and runaway responses with minimizing peak and integrated errors and rise time as key objectives.
– Important loops tend to have fast near or true integrating responses with minimizing peak and integrated errors and interactions as key objectives.
– Fast self-regulating responses and interactions with propagation of variability into product (little to no attenuation of oscillations by back mixed volumes) with extreme sensitive to variability and resonance. Loops (particularly for sheets) can be dead time dominant due to transportation delays unless there are heat transfer lags.
– Most important loops tend have slow near or true integrating responses with extreme sensitivity to SP and FRV overshoot, split range crossings and utility interactions. Load disturbances originating from cells are incredibly slow and therefore not an issue.
A critical insight is that most disturbances are on the process input not the process output and are not step changes. The fastest disturbances are generally flow or liquid pressure but even these have an 86% response time of at least several seconds because of the 86% response time of valves and the tuning of PID controllers. The fastest and most disruptive disturbances are often manual actions by an operator or setpoint changes by a batch sequence. Setpoint rate limits and a 2 Degrees of Freedom (2DOF) PID structure with Beta and Gamma approaching zero can eliminate much of the disruption from setpoint changes by slowing down changes in the PID output from proportional and derivative action. A disturbance to a loop can be considered to be fast if it has a 86% response time less than the loop deadtime.
If you would like to hear more on this, checkout the ISA Mentor Program Webinar Recording: PID Options and Solutions Part1
If you want to be able to explain this to young engineers, check out the dictionary for translation of slang terms in the Control Talk Column “Hands-on Labs build real skills.”
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