control engineering query

catiny

I am doing a coursework assignment, in control engineering, and need to know the difference between an open loop transfer function and a closed loop transfer function?
I'm aware that the answer is probably very obvious, but I'm just looking for clarification!

Many thanks!

Red Alert

Open loop controller is one where the controller solely operates on the input signal:

The input r(t) feeds to the controller, which drives the plant.

Closed loop controller is one where the output is fed back to the input in some way:

So the input signal, say r(t) is summed with minus the output y(t) of the plant. The result of that is fed to the controller, driving the plant.

A simple-to-understand closed-loop system might be when you've cruise control engaged on your car: the cruise controller determines the required engine power based on your road speed.

bnt

The term "open loop" is a bit confusing, because it's basically the opposite of "closed loop". It doesn't always mean "no control at all", just that the input control is not "formally" determined by the output.

An example is a biker who opens the throttle and speeds off, without controlling for a particular speed. The actual speed is then determined by the external conditions (e.g. open road vs. a brick wall). If at any point the rider decides "too fast!" and throttles back to a "safe" speed, he's closing the control loop.

PS: there's a third form of control called "feed-forward", which is when the control system responds not to a change in the output variable, but to another form of disturbance. Using the biker example: he sees a hill starting and throttles up in response to that, rather than to anything happening to his speed.

Laurenzo

Hey guys I'm a final year mechanical engineering student in DIT with one exam left before I can graduate. The exam is Control Systems and it's on Thurs 27th August. Wondering if anyone could help me at this short notice?
I am willing to do/pay whatever it takes. Dublin area