control//feedback control//control loop//disturbance

A disturbance is anything that moves the controlled quantity without asking the controller: a door opened in a cold room, a hill under a car on cruise control, a gust on a drone, a colder feed entering a heat exchanger. Rejecting disturbances is the main reason feedback exists at all, since a plant that never met one could be driven from a model with a fixed command.

A disturbance is anything that moves the controlled quantity without asking the controller: a door opened in a cold room, a hill under a car on cruise control, a gust on a drone, a colder feed entering a heat exchanger. Rejecting disturbances is the main reason feedback exists at all, since a plant that never met one could be driven from a model with a fixed command.

Disturbances are told apart by where they enter, because that decides how soon the loop can see them. A load disturbance acts on the plant itself, at its input or inside it, and the controller learns about it only after it has travelled through the plant's lag to the sensor. An output disturbance shows up directly in the measured output. Sensor noise is not a disturbance of the process at all, only of the measurement, and a loop that answers it moves the actuator for nothing.

Steady disturbances need integral action. A constant load (the slope, a heavier part on a conveyor) leaves a proportional controller with a permanent offset, because some error has to remain to produce the extra push; the integral term builds that push up by itself and brings the error back to zero (integral action, steady-state error).

A disturbance that is measured can be cancelled before it shows: feedforward reads the cold feed's temperature and adds heat at once, and the feedback cleans up what the cancellation gets wrong.

Disturbance rejection and setpoint tracking pull tuning in different directions. Process plants mostly reject loads at constant setpoints and servo systems mostly track; the classic Ziegler and Nichols rules favour load rejection and give large overshoot on setpoint steps (PID tuning).