To support the check list in part 8 to get the most out of your PID, we offer some of the considerations in the use of PID features. The number of PID options and parameters can seem overwhelming at first. Hopefully the knowledge gained here will foster an appreciation of the flexibility and capability leading to a positive attitude toward the possibilities.
Most PID controllers use the ISA “Standard” form. Analog controllers used the “Series” form where the derivative calculation was done on the rate of change of the process variable (PV) in series with proportional and integral calculations. This form was principally the result of analog circuitry limitations. An advantage of the “Series” form is the inherent prevention of the effective rate time from exceeding the effective reset time preventing an instability from excessive rate action. The inherent protection is the result of an interaction factor that reduces the effective rate time and controller gain and increases the controller reset time as shown in Equations 3-6 thru 3-9 in the chapter Basic-Control from the ISA book Advanced Temperature Measurement and Control. The interaction factor becomes 1.0 if the rate time is zero. These equations and interaction factors can be used for the conversion of an analog controller’s “Series” PID tuning to a DCS “Standard” PID tuning. This chapter also provides figures and a discussion of the various forms and structures commonly offered in a modern DCS.
The PID structure mode commonly used is “PI on error and D on PV.” This structure will provide a step change in the PID output from the proportional mode for a step change in the PID setpoint. Operator or sequence initiated changes in operating point are step changes in the setpoint. This step change in the output from proportional action helps get the PV to setpoint faster, which is important for reducing startup, transition, and batch times. However, this initial kick in the output is more likely to cause overshoot of the setpoint. The selection of the structure “I on error and P and D on PV” can eliminate this overshoot but the time to reach setpoint (rise time) may be painfully slow.
There is a unified approach where tuning for maximum disturbance rejection and the structure “PI on error and D on PV” can be used to minimize rise time with no overshoot. The approach is to add a lead-lag to the setpoint change. The lag time is set equal to the reset time and the lead time is set less than or equal to ¼ of the lag time. The lead time is reduced to provide a slower approach to setpoint. Recent tests show a lead 1/10 of lag most useful. For the latest developments see the InTech Feb 2012 article “PID tuning rules”
We need to be careful about always demanding simple solutions and just clicking on buttons. While this may seem to offer a reduced configuration effort, what it really leads to is a discredit and subsequent loss of expertise and employment of engineers and technicians responsible for the configuration and an eventual loss of automation system performance. Parameters and features are hidden from the customer to make the product seem more attractive shielding the user from complexity.
There are exceptions to every rule due to the nonlinearity and diversity in the process industries. There is nothing more frustrating to a user than a feature that cannot be enabled or disabled or a parameter that cannot be set. The long term effects are often not even recognized because no one is left who understands what could have been and should have been creating a positive feedback of cost cutting simplistic views. I encourage users to ask for more rather than less lists of adjustable parameters and selectable options. A multi-view approach may be best. There could be an executive “overview” of the key parameters, a “flexible” view for the normal user, and an “advanced” view to expand the horizons and break paradigms. The “flexible” view should help develop field expertise. The “advanced” view should encourage creativity. These views increase our profession’s value and visibility so we don’t reach a “Point of No Return” as noted in the site entry “How to Succeed – Part 2“.
This does not mean every application is a special case. We benefit enormously from conceptual thinking and seeing the commonality (see site entry “Top Ten Limitations – Conceptual Thinking“). My goal, possibly as a result of my education as a physicist, is a unified theory or approach that cuts to essence of the solution. The virtual plant enhancement of field experience with an advanced PID offers a conceptual understanding with far reaching benefits.
Next week we continue our discussion of PID features.