When is the error signal in a feedback control system not input minus output?

State one condition under which the error signal of a feedback control system is not the difference between the input and the output.

Help.

Greetings Xyrra, the only thing or condition that comes to mind is that the feedback signal is positive and not negative. Negative feedback would produce an Error signal (usually denoted as E(t) or E(s)) that is the difference of the input (or Reference signal) and the output (or Feedback signal). If the fedback signal is put into the summation point as a positive quantity, then the resulting E(t) or Error signal is a sum. Hope this helps somehow.

Thank you so much Mr. Serrano. Can I ask what book this answer came from? Thank you once again.

You are most welcome Xyrra. I have a set of personal notes that I took that from. The notes are based on the text book Automatic Control Systems by Benjamin C. Kuo, 7th Edition (Amazon has some available, used, starting at $20 USD). There is a section in this book, section 3-3-1 (which starts on page 84), that discusses block diagrams of control systems. The following page, page 85, has four diagrams of which one (diagram b) shows the possibility of having a positive feedback signal, and also shows the resulting error signal as a sum. It is from here where that information originates.

I have never directly delt with positive feedback to know its effects or how it can be used to one's advantage, except to say you have to be careful with it because it can cause instability. I have delt only with negative feedback, and therefore know how beneficial it can be.

Thank you so much Mr. Serrano! Sorry for the very late response. Got me busy with some stuffs.