control//feedback control//control loop

A control loop keeps a physical quantity where someone wants it (the temperature of an oven, the speed of a conveyor, the level in a tank) by measuring it all the time and correcting the input that moves it. Every loop, from a thermostat to a drone's altitude hold, has the same parts in the same order, and naming them is the first step of any design and of any fault-finding: when a loop misbehaves, the cause sits in one of these parts or in the time a signal takes to go round.

A control loop keeps a physical quantity where someone wants it (the temperature of an oven, the speed of a conveyor, the level in a tank) by measuring it all the time and correcting the input that moves it. Every loop, from a thermostat to a drone's altitude hold, has the same parts in the same order, and naming them is the first step of any design and of any fault-finding: when a loop misbehaves, the cause sits in one of these parts or in the time a signal takes to go round.

1The setpoint says what is wanted2The sensor measures what there isthe process variable3The controller compares the twothe error4The actuator applies the controller's commandthe manipulated variable5The plant responds, pushed also by disturbances

The measured quantity is the process variable yyy, the input the controller is allowed to move is the manipulated variable uuu, and the gap between setpoint and measurement is the error signal e=r−ye=r-ye=r−y. The loop is closed because each action changes the measurement that decides the next action. Cut the sensor cable and it becomes open-loop control: a command sent out with no way of knowing whether it worked.

A cruise control shows every part. The setpoint is the speed the driver chose, the sensor reads wheel speed, the controller is a few lines of code in the engine computer, the actuator is the throttle, the plant is the car with its mass and drag, and a hill is a disturbance.

A loop is only as good as its weakest part. A controller cannot correct what the sensor does not see (noise, a slow or biased reading), nor demand more than the actuator can give (saturation), and every part adds delay. Many loops that oscillate in the field are not badly tuned so much as loops with too much lag between action and measurement (delay and lag).

Feedback corrects after the fact. Feedforward acts before, from a measured disturbance or a known change of setpoint, and most real loops combine the two.

How the controller decides is, in most of industry, a PID controller; how a loop is judged is its step response; whether it is stable, and with what margin, is read in its frequency response.