Feedback control (closed loop)

An open-loop system sends a command and hopes for the best: a toaster timer, a fan set to a fixed speed. A closed-loop (feedback) system measures the result (temperature, speed, position, pressure), compares it with the target (the setpoint), and adjusts the actuator based on the difference (the error). Thermostats, cruise control and most motor controllers are feedback systems.

Feedback is what lets a product hold its spec despite things you can't control: a door left open, a battery sagging, a heavier load, part-to-part variation from the factory. It also brings new risks. A loop tuned too aggressively oscillates or goes unstable, and a slow or noisy sensor limits how good the control can ever be.

Where it bites: the sensor becomes part of the product's critical path. Its accuracy, response time, placement and failure modes set the ceiling on performance, and a feedback loop with a failed sensor can drive an actuator hard against its limit. Plan for sensor fault detection and a defined safe fallback behavior.