Glossary term
Open-Loop Control
Control action that applies commands without using measured output feedback to correct error or reject disturbances.
Definition
conceptOpen-loop control is control action that does not use measured output feedback to correct error between actual behaviour and a reference.
Open-loop control computes an actuator command from a reference, schedule, or model without measuring the controlled output to correct error. It can be simple, fast, and stable by construction, but it cannot automatically reject disturbances, drift, wear, or modelling error.
Open-loop control applies a command without using measured output feedback to correct the result. A timer that runs a heater for a fixed duration, a stepper motor commanded by pulses without position verification, or a valve opened to a calibrated percentage based on a flow schedule are open-loop examples.
A simplified representation is:
where u(t) is the command, r(t) is the desired input or schedule, and \theta represents calibration parameters. The measured output y(t) is not used in the command law.
When it is appropriate
Open-loop control is useful when the process is repeatable, disturbances are small, sensors are unavailable or unnecessary, the cost of feedback is not justified, or the command is used as a feedforward term inside a larger control strategy. It is common in sequencing, startup routines, dosing by time, calibrated motion, waveform generation, actuator pre-positioning, and systems where feedback would be too slow or noisy.
The weakness is lack of correction. Supply voltage, friction, backlash, load variation, temperature, component ageing, hydraulic pressure, clogging, and manufacturing tolerance can all change the actual output while the command remains unchanged. If the output must meet a tight tolerance under variable conditions, closed-loop control or periodic verification is usually required.
Engineering checks
Open-loop designs depend heavily on calibration, margins, interlocks, and fault detection. A conveyor may run open-loop but still need jam detection. A burner pre-purge may be timed but still need permissive checks. A motor move may be pulse-commanded but still need a home switch or limit switch to recover lost position.
Common mistakes
A common mistake is to call a system open-loop simply because the controller is simple. If measured output is used to correct error, it is closed-loop. Another mistake is assuming repeatability from a clean commissioning test while ignoring wear, drift, supply variation, or environmental changes. A good review states the disturbance envelope, calibration method, expected error, required margin, and independent checks used when the command fails to produce the intended output.