Cascade, Scan Time, PID Tuning

When Should You "Slow Down" a PID Loop by Making the Loop Update Time Such That the Loop Executes Less Often?

By Bela Liptak

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There is one case where matching scan period to process dead time is beneficial: for a pure dead-time process using an integral-only controller. But if the scan period is too short, the loop will cycle uniformly. Also, the control action needs to be synchronized with any upset for maximum performance—if not, the result is no better than continuous PI control.

In cascade control, the inner loop needs to be at least three times faster than the outer loop from process dynamics alone. Detuning the master controller or slowing its sampling will not improve control. I remember once trying to control compressor discharge pressure by manipulating flow in cascade, but pressure and flow are equally fast, so the controllers fought each other. The single pressure loop worked better.

Exothermic reactor outlet temperature needs to be controlled in cascade, manipulating coolant exit temperature for a stirred-tank reactor, or coolant inlet temperature for a once-through reactor. These were tested extensively in the field.

The reason for filtering in a control loop is to reduce valve wear, and my advice from a performance standpoint is that, the less filtering, the better performance in response to load changes.

There is no benefit for executing a controller faster than its process variable (PV) is updated because the loop is open during the PV scan interval, but it should be synchronized with the PV scan. Some parameters change together with flow, in which case robustness should be measured against fractional flow changes.

There is no definition of "detuning" except as the opposite of tuning. It refers to reducing loop gain below the optimum to add stability and increase robustness. Robustness is defined as the fractional change in any process parameter such as gain or dead time that will cause a loop to oscillate uniformly. Detuning is used to reduce loop interaction and to increase robustness. Integrated error increases directly with the product of proportional band and integral time, and detuning raises both. Interaction is best minimized by proper loop configuration, using relative gain analysis (RGA) as a guide, and partial decoupling where necessary. Robustness is achieved without sacrificing performance with proper valve characterization, and gain scheduling where necessary.

I'm not sure what "analog sample rate" means, except in digital filtering. A digital filter may average data over one scan period, or up to five for more smoothing. Just remember the delay associated with the average value it is reporting.

Greg Shinskey
shinskey@metrocast.net

A: Increasing the scan time will add dead time (1/2 of scan time) and increase the integrated error for unmeasured disturbances. The increase in scan time will not normally change the integral contribution over a period of many scans. For fast loops where the existing loop dead time is rather small, increasing the dead time via an increase in scan time could make the oscillations significantly worse if they are due to disturbances.

Be careful about the use of scan rate versus scan time. Scan rate can be mistaken to be frequency of updates rather than time between updates.

There is a slim chance the oscillations are due to noise or analyzer cycle time. Increasing the scan time will improve the signal-to-noise ratio, helping the loop deal with disturbances. If there is a large analyzer cycle time, synchronizing the scan time with the cycle time will improve control and perhaps eliminate oscillations from the analyzer cycle time.

The simple solution is to increase the reset time. For integrating process, the product of the controller gain and reset time (sec) must be larger than twice the inverse of the integrating process gain (%PV/sec/%Out) to prevent slow rolling oscillations. Adding a setpoint rate limit in the analog output block and using external-reset feedback with the positive feedback implementation of integral can do the same thing (slow down the integral mode) without retuning.  However, your PID controller may not have this option, and you need to be careful about the connection of the back-calculate signals associated with external-reset feedback.

If the oscillations are due to a limit cycle from valve backlash or stick-slip, increasing the reset time will increase the period (slow the oscillations down). If you have an integral dead band option in your PID, setting the integral dead band equal to the amplitude of the oscillations should stop integral action, stopping the limit cycle.  If integral action is used in the positioner, integral dead band must be set there as well. Alternately, if you turn on external-reset feedback with the positive feedback implementation of integral action and for the back calculate signal use a fast read back of actual valve position for direct manipulation of a control valve or use the secondary loop PV for cascade control, you can stop the oscillations from backlash, stick-slip or a slow valve or slow secondary loop.

Greg McMillan
gkmcmi@austin.rr.com

A: Dead time, transportation delay, sampling time and/or transmission delay are different words for the same phenomenon. When dead time is part of a feedback control loop, it makes the performance poorer. Dead time, by whatever name, is significant in relation to the principal time constant or period of the process or instrumentalities being controlled. For good performance it should be less than 2% of the time-constant or period. Greater delays will force reduced response performance.

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