Siemens SIMATIC S7-1200 Function Manual page 17

Actuators are distinguished as follows:
● Proportional actuators with constant actuating signal
These elements set degrees of opening, angular positions or positions in proportion to
the output value. The output value has an analog effect on the process within the control
range.
Actuators in this group include spring-loaded pneumatic drives, as well as motorized
drives with position feedback for which a position control system is formed.
An continuous controller, such as PID_Compact, generates the output value.
● Proportional actuators with pulse-width modulated signal
These actuators are used to generate the output of pulses with a length proportional to
the output value within the sampling time intervals. The actuator - e.g. a heating resistor
or cooling apparatus - is switched on in isochronous mode for durations that differ
depending on the output value.
The actuating signal can assume unipolar "On" or "Off" states, or represent bipolar states
such as "open/close", "forward/backward", "accelerate/brake".
The output value is generated by a two-step controller such as PID_Compact with pulse-
width modulation.
● Actuators with integral action and three-step actuating signal
Actuators are frequently operated by motors with an on period that is proportional to the
actuator travel of the choke element. This includes elements such as valves, shutters,
and gate valves. In spite of their different design, all of these actuators follow the effect of
an integral action at the input of the controlled system.
A step controller, such as PID_3Step. generates the output value.
PID control
Function Manual, 03/2017, A5E35300227-AC
Principles for control
2.1 Controlled system and actuators
17