Siemens SIMATIC S5 Manual page 192

Notes on Controller Settings
2-step controller with feedback
The action of 2-step controllers on controlled systems with long delay times, e.g. furnaces in which
the chamber is separate from the heating, can be improved by electronic feedback.
With the aid of the feedback, the operating frequency of the controller is increased reducing the
amplitude of the controlled variable. In addition to this, the control results in dynamic operation can
be greatly improved. The limit for the operating frequency is set by the output stage. Mechanical
actuations such as relays and contactors should not be switched more than 1 to 5 times per minute.
For binary voltage and current outputs connected to thyristor or triac actuators, operating
frequencies can be selected far higher than the limit frequency of the controlled system.
Since the switching pulses at the output of the controlled system can no longer be detected, the
results obtained are similar to those with continuous controllers.
In contrast to a continuous controller in which the amplitude of the output signal represents the
manipulated variable, the output variable of a 2-step controller with feedback is formed by pulse
duration modulation.
2-step controllers with feedback are used, for example, for temperature control in furnaces and
ovens in the plastic, textile, paper, rubber and foodstuff industries and for heating and cooling
devices.
3-step controller
3-step controllers are used for heating/cooling. These controllers have two switching points. By
means of electronic feedback structures, the controller results are optimized. Areas of application
for such controllers are heating, refrigeration and climatic chambers and tool heating systems for
plastic processing machines.
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IP244
4–126
C79000–B8576–C860–02