PID Tuning Step by Sep (Part2)

In this part we will continue with some PID basics before getting into exact Tuning procedure.

Quick refresh for the 1st set of questions on this link, and we could continue ..

Q10. What are the main features in a robust PID controller?

Beside implementing PID algorithms, the controller should include the following features:

1. Manual/Auto mode selection: (It is a basic controller feature)
2. Alarming feature.
3. Security feature.
4. Setpoint/Output Limitation: Some applications should have a limmited Min/Max. values for controller SP or its O/P. Exceeding these values could harm the process. Only operator could go beyond these limits by putting the controller into manual mode and intervene personally for such manipulation.
5. Setpoint Tracking: (Will be discussed)
6. Output Tracking: (Will be discussed)

Most of Microprocessoer based or soft controllers (Blocks within PLC/DCS) includes all of the above features and more. But some (Cheap) stand-alone controllers could miss some of them. Finally it is a budgetry decision from the end user.

Manual/Auto feature and switching between them created potential issues for the PID controller. Such Issues and solutions are to be discussed in the following questions.

Q11. What is "Integral Wind-Up"?

It is an issue existing only with controllers that are containing "I" part.

Let's take an example for illustration: Imagine an operator switched the controller to manual and raise the O/P value going to the valve, allowing PV to approach SP. When nearly PV = SP, operator switches back to auto, expecting PID controller will start working again from this new starting point. Operater will be confused when he has an immediate jumping (for O/P going to valve) to another value!! - We could sum up what happened in the following:

1. Controller (while in manual) will keep applying its own algorithms. It never stops implementing and evaluating its PID equations.
2. All the time spent in manual while PV away from SP is a chance for a long term accumulated error, which will cause the controller’s integral term "I" to “wind up” the O/P.
3. When controller is back to Auto .. All the "Wound-up" O/P quantity which was precalculated (in manual) will be applied to the final control element (valve/pump) .. Causing the process to be disturbed.

It would be fair to say that "Integral Wind-Up" issue (sometimes called "Reset Wind-Up" or just "Wind-Up") .. is not only because of Manual/Auto transition, but also could happen while the controller is in auto and PV can't achieve SP for some reasons.

For example: There is a sticky valve in the loop that is not responding correctly to the controller signal .. So the "I" control part will continually increase the O/P signal (winding it up) in an effort to make PV achieving SP and correct this error.

Another example with the Override control: In this control strategy .. simply we have 2 controllers one of them is overriding the other and taking the ownership of controlling the loop (for a while). So the overriden controller will not be able to achieve its own SP during this phase .. Consequently, it will wind up.

Indicating the "Wind-UP" with graph - We could note the response of each control part (P, I and D) easily in the following graph, and we can see that since PV never equals SP again, so "integral response" never stops ramping upward and winding up.

When combining into single PID output, the three actions produce the following response:

In the next part we will demonstrate the rest of Manual/Auto transition issues and how to handle them ..