In many machines and industrial plants, physical variables must be maintained at a predetermined value independent of external influences. To achieve this three functions must take place. Measurement, comparison and adjustment. The required cycle of operation takes place in a so-called control loop. Control loops can be classed as open or closed.
Open loop control
An example of an open control loop is a radiator in a central heating system. The supply of warm water and, thus, the temperature in the room is “controlled” or adjusted with a manually operated valve. As soon as the room temperature rises, the valve must be closed by hand. As soon as the room temperature drops, the valve must be opened by hand, i.e. the rise and fall of the temperature is not controlled automatically.
This is an open-loop control system with no automatic feedback loop.
Adding a thermostatically controlled radiator valve that opens and closes in accordance with room temperature converts this open loop into a closed loop.
Closed loop control
DIN standard 19226 defines the terms “Control and Adjustment” as follows: “Control and adjustment is an operation in which a physical variable (e.g. temperature, pressure etc.) is continuously measured and compared to a previously specified value of the variable with the aim of matching the two.
The resulting closed sequence of actions occurs in a closed loop, the closed-control loop.
”At the example of temperature regulation in a heating radiator, the actual temperature is measured with a temperature sensor and compared to the predetermined value. As soon as there is a difference between the desired value and the measured value, a signal is transmitted to the valve to open or to close. This means that the temperature is held at the predetermined value irrespective of any outside conditions.
Discontinous action control
A process which takes place step-by-step is called discontinuous.
A discontinuous-action controller influences the process with short switching actions at a constant level of energy. Discontinuous-action controllers are therefore also called switching controllers.
Controllers assure the actuating function by triggering a sequence of energy pulses. These pulses have influencing times at fixed energy levels but limited influencing periods.
Continuous action control
Continuous-action controllers assure the actuating function by having a continuous influence on the process.
Controlling takes place permanently. The controlled variable can have any value within the defined control range.
Non-intermittent, random control signals between 0 and100 % are triggered.