In a standard feedback control system, you have a set point, a controller, a plant, and a sensor to enable feedback. The sensor measures the process variable (or in other words, the argument you are actually trying to control).
Now, I want to add a feedforward component. Referring to this MATLAB example, when implementing feedforward component, you require an additional disturbance model and a feedforward controller.
This is where I need some clarification. When adding a feedforward component, do I have to measure the disturbance separately from the process variable? Or is disturbance equal to my process variable itself? If the prior, in my system, I only have a sensor to measure to the process variable and I have no way of measuring the disturbance. Is it possible to implement a feedforward component into my control system?
Best Answer
Yes - I support the answer from Marko Bursic; more than that, I would not use the term "feedforward loop" (because it is, in fact no loop at all). For my opinion, it is better to say "feedforward path".
Such a path does exist sometimes - depending on the actual system. Sometimes it contains a disturbance signal but it also can be responsible for feeding through a paprt of the input signal.
Simple example: A resistor Rb between the base and the collector of a common-emitter gain stage. The resistor Rb has the task to provide DC or/and AC negative feedback - however, at the same time it allows a small feedthrough signal from the input directly to the output (collector node). At this node it is added (with 180deg phase shift) to the "normal" output voltage (caused by the collector current variations).
Therefore, there are two open questions: (a) Why do you "want" to add a feedforward path and (b) why do you think we would in this case "require an additional disturbance model"?