Why feedback?
A controller compares what you wanted with what happened, then acts on the difference.
First, picture it
Cruise control without feedback would use one fixed throttle setting. A hill changes the speed, but the throttle never reacts. With feedback, measured speed changes the command until the error shrinks.
What the model says
In a negative-feedback loop, the reference r and measured output y form the error e=r−y. The controller C turns e into an input u; the plant P turns u into y. Disturbances enter the plant separately. Feedback can reject disturbances, but sensor noise, delays and limited actuator authority can make aggressive control harmful.
e(t)=r(t)-y(t), u(t)=C[e](t)The minus sign is the feedback direction; reversing it changes the loop profoundly.
Make it concrete
Raise the motor's load torque. Open-loop voltage stays fixed and speed falls; a speed controller sees a positive error and increases voltage, until its limit is reached.
Try the motor load pulseFeedback does not erase physical limits. A saturated actuator cannot provide the extra effort the controller asks for.
Check your understanding+
If a load suddenly slows a motor, what changes first in a speed-feedback loop?
The measured output falls; the error grows, then the controller increases its command.