"Ask the Experts" is moderated by Béla Lipták, process control consultant and editor of the Instrument Engineer's Handbook (IEH). The 4th edition of Volume 3, Process Software and Networks, is in progress. If you are qualified to contribute to this volume, or if you are qualified to answer questions in this column or want to ask a question, write to [email protected].
Q: I am running an experiment with a level control loop, and I found a problem with the performance of the controller when I change the control mode, because it considers that as a disturbance.
For example, I start the experiment using a PI controller, and the level goes to steady state and equal to the setpoint. Then I want to change to proportional-only control, so I delete the integral term, but at this instant the level starts changing, reaching another steady state. I always thought that disturbances come from changes in the setpoint or loads, but not from control mode changes. Could you please explain this to me?
I tried two things:
- Switching to manual before changing controller settings, but we can change them only in automatic mode.
- Deleting the integral term gradually (0.01 per second), but it didn't work either.
In both cases, when the integral is deleted, the controller output suddenly goes to zero. Then it goes to other values, causing the process variable to change.
There are two basic types of digital algorithms, positional and velocity. With positional the full output is recalculated every cycle: m = Kc (e + ∑edt/Ti + Td ∆e/∆t) + b.
When velocity algorithm is used, the value of the previous output signal (m) is held in memory, and every cycle only the required change in that output signal is calculated: ∆m = Kc (∆e + e∆t/ Ti + Td ∆(∆e/∆t).
It seems that you have a badly designed positional algorithm. A PID algorithm shouldn't change its output because of a change in the control modes. This is a bad algorithm. The programmer who put it into the PLC should be fired.
Actually, it would be even more useful if the association of the instrument manufacturers provided some kind of certification for all control and measurement instruments, so that the users could check if the supplier's claims (including accuracy, rangeability, etc.) were correct.
A: When you reach a steady state at setpoint by means of integral action, the controller output and its internal bias will match the current process load. The controller you are using apparently switches to proportional, using a fixed bias, when you delete the integral mode. If that fixed bias is different from the existing output, it will bump the output to that bias value, thereby upsetting the loop. Setting the proportional controller's bias equal to the output before deleting the integral mode would eliminate this effect.
The disturbance you see is not the result of tuning, but of switching controller modes. If you were to increase integral time to its maximum value instead of deleting it, no bump should result. (However, some PID controller designs do cause a change in output when the proportional gain is changed, and that is undesirable.)
Your controller is either defective or designed incorrectly—get another one. The behavior you describe will not happen with a well-designed controller. The transition will be bumpless.
A: This is most likely a problem with the PID aglorithm. A well-designed PID controller would not immediately bump the output on changing the integral term. Changing proportional gain would change the output right away.
A: It appears as though the calculation of the PID formula is a very primitive structure like this:
e = (Ysp-Y)
I = I + e*dt
m= Kc*e + Kc*I/Π)
in which the integral term is the bias. Certainly, when the process is at setpoint, and e = 0, if the integral term is removed, then the bias is removed, and the output calculation is based on m = Kc*e, which is zero.
There are many ways that software developers can solve this problem, and it is a curious situation to think that a PLC manufacturer would not have provided a better algorithm. Perhaps Ghanima could choose another PI variant from the ones offered on the PLC.
R. Russell Rhinehart, PhD
A: Given the constraints that seem to be in place, the only possibility I can see is to solve this problem is for the questioner to set up multiple controllers with their PV and SP inputs paralleled and select the output he wants at the time he wants.
Obviously, the outputs would only be relatively matching (for bumpless transfer) when process conditions, controller mode and tuning dictated.
In fact, how often they matched, or their convergence, might give him some input on the feasibility of what he is trying to do.
I use the term "bumpless" because of the modern controller feature called "bumpless transfer," which reduces such output changes after a controller auto-manual or mode switch.
When going from PI to P-only control, the P gain would have to change to compensate for the I contribution, or a bias added or some combination of these made for the output to stay at what it was prior to the switch and keep the process steady.
Al Pawlowski, PE
A: A very slow integral results in an equally long lag in the control loop. Most likely there will be a very long period oscillation. When the process changes, the operator will need to offset the setpoint to move the process to the desired value, and allow the controller to react to short term changes. The operator then has to watch the output drift slowly as the reset drifts. And we wonder why the loop gets set to manual?
Once upon a time, we had "manual reset," also called zero adjustment, but it would stay put and not drift. However, that was with the good old pneumatic controllers we knew and loved.
A: It very much depends on who wrote the PID computer algorithm. The Fieldbus Foundation specifies a standard PID algorithm by its parameters.
The implementation of the control algorithm is in the hands of the vendor.
I assume you are talking about software because you "delete" the integral term. That would be the only solution, given the constraints. However, the time constant should be very long—small repeats/min, large time constant in seconds or minutes.
A: This sounds like an oddity in the algorithm. Perhaps removing the integral has dropped you back to proportional, plus manual reset, with a default value for manual reset, rather than the value stored as the integral term.
Ian H. Gibson
A: A well-designed PID should not bump/change output while any of PID parameters (K, Ti, Td) are changing. This is the basis for implementing in PID adaptive control, and adjusting parameters without affecting controller output.
A: When using P + I in the steady state, the output of the controller is wholly contributed to by the integral term; i.e., the integration of prior error signal over time. In the steady state, the P (proportional) section of that controller is subject to a zero-error signal and, hence, contributes nothing to the output.
When eliminating the integral (I) function while in automatic, you eliminate its output contribution and cause the process, via the valve, to be bumped. The level will change and no longer match the setpoint. The error signal internal to the controller will rise from zero to a new value that causes the level to rise toward the setpoint. The level will not restore to the setpoint because, in proportional control, an error or offset is necessary to have a controller output.
Some controllers have a manual reset provision that can be used to eliminate the offset, but only for one load condition. Most processes have more than one load condition, and thus, P + I is the better solution. The control settings for typical controllers, since modern times, can be changed when in balance and while in the auto mode without causing a process upset other than that dictated by the function value change. But, eliminating or adding functions must be done while in manual.
I am not familiar with the detailed construction of the controllers you mentioned. The fact that you did not have the problem using other controllers suggests that the controller you are using is either malfunctioning or unsuitable for your level application.
In a level application, integral or reset action is a handicap during start-up or sudden, large load changes since, in its raw form, it causes the process variable to overshoot the setpoint. Competent controller or control algorithm manufacturers have devised a variety of techniques to overcome this problem. Perhaps the controller or control algorithm you are using is unsuitable in an application such as yours. I would make inquiries to the manufacturer to satisfy yourself on this point.
Otto Muller-Girard, PE