Rievtech xLogic ELC Series User Manual page 174

The control device is formed from the actuator and the control.
The control and controller together form the regulating device.
The following picture gives an abstract portrayal of the situation described above.
The comparing element uses the sensor to compare the command variable with the process value. If the
command variables and process value deviate from one another, this results in a positive or negative loop
error that in turn changes the process value.
Control loop
The process value x influences the manipulated variable M by means of the regulating device. This
creates a closed circuit that is also known as a control loop.
If, in the example above, the window is opened, the temperature in the room drops. The person must
increase the heat output of the heater. If the heat output is increased too much, it will get too hot. The
person must then reduce the heat output.
If the heat output is increased or reduced too quickly, then the control loop starts to sway. The room
temperature fluctuates. It is either too hot or too cold. To prevent this, the person must carefully and
slowly reduce or increase the heat output.
Loop error
The loop error is the difference between the command variable and the process value. In other words:
the deviation of a process value from a set value.
174