Quick questions on Machine control systems for QCE Engineering Unit 4

Every controlled machine can be described as a flow from input to output:

In open-loop control the output is not measured or fed back. The system carries out its instruction regardless of the result. A toaster runs its heater for a set time whether or not the bread is actually done; a basic sprinkler runs for a fixed period regardless of soil moisture. Open-loop control is simple and cheap, but it cannot correct for disturbances or errors, so it suits tasks where the result is predictable.

In closed-loop control a sensor measures the actual output and feeds it back to be compared with the desired value, the set point. The difference is the error signal. The controller acts on the error to drive the output toward the target, then keeps checking. A thermostat-controlled heater measures the room temperature, compares it with the set point, and switches the heater on or off to hold it.

The float sensor measures the actual level as $760\,\text{mm}$.

The controller switches the pump on. Water flows in and the level rises.

The sensor keeps reading the level. When it reaches $800\,\text{mm}$ the error is zero, the controller switches the pump off, and the system holds. If the level later drops $20\,\text{mm}$ again, the cycle repeats automatically.