Do I need a feedback loop while testing PID controller?

Question

This is a homework for my university course control theory.

The task is simple: Test the P, the I and the D controllers separately to see the effect. We don't have to connect the controllers to a system it means IN -> controller - out, but the instruction doesn't say if we have to use a unity gain feedback loop.

Does it make sense ( practical use ) to test a PID controller without a feedback loop?

The inputs are step function for P and I controllers and a ramp function for the D controller.

And I do everything in Simulink.

Answer 1

I am having trouble imaging a PID controller without a feedback loop. The reason is that P, I and D controllers each calculate their response based on the error between the target and the current state. If there is no feedback to provide the current state, then I can't imagine how they would work in principle.

---

UPDATE: After reading Abel's post, I realized (because I had to do some development of PID controllers) that in the early development stage of a controller, where you need to have a first feel of the behavior of the controller the most basic technique is to inject an artificial signal (e.g. a step function in the system). This is considered open loop (because there are no feedback from the output).

However, in the later stages of the development where you need to actually test the controller with simulations in a real environment, there are the following two options:

• Software in the loop (SIL) where the entire plant and the interaction between the controller and the plant is software (in Simulink)

• Hardware in the loop (HIL) where you can actually connect a Real time Matlab/Simulink PC to the plant and monitor/adjust the controller

Figure: Difference between SIL and HIL. Source: The CDIO model in architectural education and research by design

In both those cases, although you might not actually have feedback loop on both of them, there is still a loop connecting the plant's output to the input of the controller.

Answer 2

Testing physical controller hardware in "open loop" isn't uncommon, but it would just be part of a test, and the feedback line can be split at a point and treated as both input and output of a test setup.

For example you can use the feedback sensor to measure what the system is doing and inject a fake sensor signal along with a desired input to observe whether the system is behaving as you would expect it to. Whether such a system is considered closed or open loop depends on what part of the system you consider. A simple decision to abort the test based on behavior can be considered feedback. Naturally, when you have the fake sensor match the real, that becomes something close to the normal operation closed loop system.