Checklist for PID Migration Tips

Older Distributed Control Systems (DCS) and analog controllers tended to have different tuning setting units and methods of implementing integral and derivative action. A lack of understanding of the difference between the old and new PID features and tuning settings can lead to poor and even unstable control when migrating to a new DCS. Here is a checklist to help avoid these problems and get a greater value of the benefits offered by a new DCS.

Many analog controllers and older DCS used an integrator as depicted in control theory text books to implement integral action. The newer DCS uses a positive feedback implementation of integral action as seen in slide 15 of the presentation "Effective Use of PID Features for Loop Performance and Optimization." This presentation and the 4/26/2012 Control Talk blog "What is the Key PID Feature for Basic and Advanced Control" show how this implementation enables external reset feedback that can prevent oscillations from violation of the cascade rule, reduce oscillations from interaction, stop limit cycles, provide directional move suppression, make override control and valve position simpler and more effective, and enable an enhanced PID for wireless and analyzer control. If you don't understand this new capability you are missing out on numerous opportunities for process control improvement.

The PID tuning setting units vary quite a bit. The very first thing to do is to convert tuning settings to the units used in the new DCS. The proportional mode setting can be a proportional band (%) or a gain (dimensionless). Some PLC used engineering units internally in the PID algorithm making the gain setting effectively in engineering units. Hopefully you do not have this bizarre case.  The reset setting can be repeats per minute, repeats per second, minutes per repeat, and seconds per repeat. Be cognizant of the fact that reset setting in time units may have dropped the "per repeat" term. A reset time setting in "seconds" is really a setting in "seconds per repeat" meaning that the contribution of the proportional mode is repeated by the integral mode in the specified number of seconds.

Many analog and older DCS used the Series Form whereas most new DCS have the ISA Standard Form as the default. The Series form that puts the derivative mode first and in series with the proportional and integral modes, inherently prevented adverse consequences from the user setting the rate time equal to or larger than the reset time. Many tuning methods developed prior to the 1990s had the Series Form in mind and even advocated equal rate time and reset time settings. A rate time setting approaching a reset time setting will cause excessive oscillations in the ISA Standard Form. If the rate time is set greater than the reset time, instability can result.

In the Series Form, an interaction factor increases the reset time and decreases the rate time to prevent instability. The effective settings are the ISA Standard Form in terms actual PID action. You can see in the conversion of the Series settings to the ISA Standard Form the interaction factor at work as detailed in Appendix K for the long overdue 4th edition of my book Tuning and Control Loop Performance being published by Momentum Press in June 2013. You can see the adverse consequences of not getting this right for a pH loop in the post "Getting the Converted PID Values Right". If the rate time is zero, the interaction factor is one and there is no need to convert settings when going from the Series to the ISA Standard Form. You still need to convert settings based on the difference in tuning setting units.

Analog controllers and older DCS also tended to have the output scale in percent whereas the output scale in newer DCS can be in engineering units. An output scale in engineering units is particularly useful for the primary PID of a cascade control system whose output is the setpoint of a secondary PID.

Here is a checklist for PID settings when migrating to a new DCS (e.g. DeltaV) from older DCS systems (e.g. PRoVOX) or analog controllers. There are many features and parameters that vary with the DCS supplier. It is imperative the DCS documentation and supplier expertise be fully utilized and all migrations tested by a real time simulation for stability and effectiveness. Once you get past the conversion process see the presentation and the 6/21/2012 Control Talk Blog "Checklist for Best PID Performance" to help you get the most out of your PID. 

•1.       For cascade control set the output scale of the primary PID in engineering units of the PV scale of the secondary loop.

•2.       For cascade control set the low and high output limits in engineering units (DeltaV default is 0-100%).

•3.       Set the anti-reset windup (ARW) limits to match the output limits using same units as output limits unless there is some special need for ARW limits to be set otherwise due to pneumatic positioner and excessive stick-slip. The best movement off of the output limit for saturation is achieved when the ARW limits match the output limits (DeltaV default is 0-100%). See the 11/20/2012 Control Talk Blog "How do you Effectively Use Anti-Reset Options Tips"   

•4.       Convert controller gain setting units (be especially aware of the inverse relationship between proportional band and gain).

•5.       Convert reset units setting (be especially aware of the inverse relationship between repeats per minute and seconds per repeat).

•6.       Convert rate units setting and make the alpha setting the same for the rate filter.

•7.       If rate time is not zero and ISA Standard Form is used, convert Series Form gain setting to ISA Standard Form gain setting.

•8.       If rate time is not zero and ISA Standard Form is used, convert Series Form reset time setting to ISA Standard Form reset time setting.

•9.       If rate time is not zero and ISA Standard Form is used, convert Series Form rate time setting to ISA Standard Form rate time setting.

•10.   For override control if the positive feedback implementation of integral mode is used, remove the filter on external reset signal used to prevent walk-off since this filter is already there.

•11.   For cascade control, turn on external reset feedback (dynamic reset limit) and use PV of secondary loop for external reset feedback to automatically prevent burst of oscillations from violation of cascade rule that secondary loop must be 5x faster than primary loop.

•12.   For slow or sticky valve, turn on external reset feedback (dynamic reset limit) and use a fast PV readback for external reset feedback to automatically prevent burst of oscillations from violation of cascade rule that positioner feedback loop must be 5x faster than primary loop and to prevent limit cycles from stick-slip. Note that the PV readback must normally be faster than a secondary HART variable update time.

•13.   For wireless control and at-line or on-line analyzer, use an enhanced PID developed for wireless that suspends integral action between updates (PIDPlus option) and uses elapsed time in the derivative action. The external-reset option should automatically be turned on. Follow checklist items 11 and 12. See the July/August 2010 article "Wireless - Overcoming the challenges of PID control and analyzer applications" for more details.

•14.   Make sure the BKCAL signals are connected properly paying particular attention to the propagation of the BKCAL settings for intervening blocks for split range, signal characterization, and override control 

I can't emphasize enough the headaches caused to users not realizing the output scale can be in engineering units and not realizing the output limits and ARW limits stay at 0-100% despite a change in the output scale. I troubleshot a DeltaV configuration because the PID controllers were behaving badly. The contract firm that did the migration of the PRoVOX configuration set up an elaborate series of math blocks to do engineering units conversion for cascade loops. The BKCAL signals were lost in the math blocks. The user was incredibly grateful when I told him to get rid of the math blocks, set the output scale and limits correctly in engineering units, and connect the secondary loop PV as the BKCAL signal.