Cruise Control in your car. You set the cruise speed, the closed loop comes from a speed sensor off of the speedometer. If the speed drops, i.e. you start going up hill, the sensor knows it, tells the closed loop controller that, and the controller automatically increases the throttle to give the engine more gas.
It's an apposite example of closed loop and one that I've used before but I'd have called that feedback rather than feed forward.
You have an input demand signal that asks for 70mph output. And you have a sensor that compares actual output and feeds that
back to the input of the controller where it is compared with the input demand. Any error acts to correct the difference.
My lecturer for that subject had a nice succinct definition for a closed loop system.
"Error actuated and power amplifying."
That was four decades ago then some. But still valid.
We do a fair bit of work in the variable speed drives area. Some are retrofits using existing motors where closed loop control is required. Those can be a bit messy. The old speed feedback unit might be an analogue tachogenerator, subsequently out of production and no easy means of fitting an encoder. We have manufactured toothed steel wheels to fit and used an inductive proximity sensor to count pulses. We have a pretty good machine shop so we can custom manufacture them as needed.
We have a retrofit project going through at the moment where we need to detect speed. It's an existing speed controlled application with eight 200kW vertical shaft wound rotor motors. Speed feedback is currently computed from slip frequency and rotor voltage. That's fine. The system has been in operation for over twenty years. Recently we have been experiencing problems with dirty contacts causing the rotor contactor to drop out. The machine stalls with the stator still energised. The shaft mounted fan no longer provides cooling and the motor cooks.
A shaft mounted speed sensor would seem like an easy solution. But there's no way to mount that. Nor our toothed wheels. I think we can fit proximity sensors to detect the cooling fan blades. We are already undertaking the work. All the cabling is in place. We'll complete one to prove the system works. Access to do that has been an issue. It's a pumping station for London and production takes priority.
Their engineering wants it done. We want it done.
Frustrating doesn't have enough syllables...