Siemens SIMATIC S7-1200 Function Manual page 25

Two-step controllers with feedback
The behavior of two-step controllers in the case of controlled systems with larger delay
times, such as furnaces where the functional space is separated from the heating, can be
improved by the use of electronic feedback.
The feedback is used to increase the switching frequency of the controller, which reduces
the amplitude of the process value. In addition, the control-action results can be improved
substantially in dynamic operation. The limit for the switching frequency is set by the output
level. It should not exceed 1 to 5 switches per minute at mechanical actuators, such as
relays and contactors. In the case of voltage and current outputs with downstream thyristor
or Triac controllers high switching frequencies can be selected that exceed the limit
frequency of the controlled system by far.
Since the switching pulses can no longer be determined at the output of the controlled
system, results comparable with those of continuous controllers are obtained.
The output value is generated by pulse-width modulation of the output value of a continuous
controller.
Two-step controllers with feedback are used for temperature control in furnaces, at
processing machines in the plastics, textile, paper, rubber and foodstuff industries as well as
for heating and cooling devices.
PID control
Function Manual, 03/2017, A5E35300227-AC
Principles for control
2.4 Pulse controller
25