Open-Loop and Closed-Loop Control

Two methods are distinguished for achieving the goal controlled variable = reference variable: open-loop control and closed-loop control. The difference between the approaches lies in the quantity that is measured. In open-loop control, all disturbance variables are measured and the input quantity is changed on the basis of these measurements. In closed-loop control, the controlled variable (actual value) is measured.

When an animal grows a winter coat or a hedgehog buries itself in leaves, the animals are not reacting directly to the outside temperature. The hedgehog does not dig itself in deeper when it gets even colder. Nor does the coat get longer the colder it gets. The intensity of the cold has less to do with it than the season. So the animals do not react because their core temperature briefly deviates from the setpoint. They react to the fact that it is getting colder and that winter will soon come.

This behaviour is “open-loop controlled”. The animal “measures” a disturbance: the outside temperature or the season. The reaction does not take place because of the deviation of the actual value from the setpoint. With open-loop control, you can generally only bring the actual value close to the setpoint.

To become more precise, we need an intervention that acts depending on the deviation between core temperature (actual value) and setpoint. If your body is a little too warm, you sweat slightly. In an 85 °C sauna you sweat more. This intervention depends directly on the size of the deviation of the actual value (core temperature) from the setpoint (37 °C). For this, the body measures its core temperature and “controls” sweat production in fine steps so that actual value = setpoint always applies.

A closed-loop controlled system measures the actual value (controlled variable) and drives the actuator on the basis of the deviation between setpoint and actual value. An open-loop controlled system measures the disturbance and, on the basis of this information, drives the actuator roughly in the right direction.

In both cases we have a system with an actual value that should match a setpoint. In both cases we need an actuator to intervene. We always try to use closed-loop control for all systems. This always brings us closer to the goal actual value = setpoint than an open-loop approach. We sometimes add an open-loop approach (winter coat) so that the closed-loop system gets close to the setpoint.

In both methods there is a central block that drives the system on the basis of the measurements. In the open-loop system it is called the “open-loop controller” (Steuerung). In the closed-loop system we call it the “controller” (Regler). Why is not everything closed-loop controlled, why are there also open-loop systems? Because the actual value cannot always be measured.