Open loop and closed loop: what feedback does

Feedback means measuring the result again and feeding it back into the input. The difference between open and closed loop, using motor speed as the example.

Open loop

Open loop. It outputs a fixed value without looking at the result.

You only set the input ("60% PWM gives roughly 3000 rpm") and never check the result. It is simple and cheap, but when a load is applied or the battery voltage drops, the speed drops with it. Stepper-motor positioning and a toaster timer are open loop.

Closed loop

Closed loop. The actual value is measured and the input adjusted by the difference from the target.

A sensor measures the actual speed, the difference from the target (the error) is calculated, and the input is changed in the direction that reduces the error.

target → (+) → controller → plant (motor) → output
          ↑(-)                            │
          └──────── sensor ←──────────────┘

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What feedback gives and what it costs

You gain	The cost
Robustness to disturbances (load changes)	You need a sensor
The target is held even if the plant differs a little	Poor tuning causes oscillation or divergence
The response can be made faster	Sensor noise gets into the output

In embedded systems

A control loop is, in the end, code that runs at a fixed period. Read the sensor → calculate the error → controller → update the output, every period. If the period jitters, so does control performance, so it runs from a timer interrupt or a high-priority task.