PID control
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PID control (speed control)
The PID control can be used to change the delivered quantity – i.e. the volume flow – of usual com-
mercial circulating pumps. That allows the system to maintain temperatures (differences). It can be
used not only for the speed control but also for the control of the burner performance and others.
The following simple solar diagram illustrates the possibilities of this process:
Absolute value
control = Stabilizing a sensor
T.coll
can be stabilized very well at one temperature (such as 60°C) via the speed control. If solar
radiation decreases,
T.coll
becomes cooler. The control unit reduces the speed and hence the flow
rate. That leads to a long dwell time for the heat transfer medium in the collector, which in turn in-
creases
T.coll
.
As an alternative, a constant return (
T.ref
.) can make sense in various systems (such as cylinder
load). That requires an inverse control characteristic. If
T.ref
increases, the heat exchanger adds too
little energy to the cylinder. Thus the flow rate is reduced. Greater dwell time in the exchanger cools
the heat transfer medium more, thus reducing
T.ref
.
Stabilizing
T.cyl
would not make sense because the varying flow rate would not affect
T.cyl
directly
and thus not lead to a functioning regulating circuit.
Differential control
= Keeping the temperature between two sensors stable.
Keeping the temperature difference constant between, for example,
T.coll
and
T.ref
leads to a “slid-
ing“ operation of the collector. If
T.coll
drops as a result of reduced irradiation, the difference between
T.coll
and
T.ref
also falls. The control unit reduces the speed leading to a greater dwell time of the
medium in the collector and hence to a greater difference
T.coll
–
T.ref
.
Event control
= If a set temperature event occurs, the speed control starts, thus keeping a sensor
constant.
If, for instance,
T.cyl
reaches 60°C (activation threshold), the collector is to be kept stable at a certain
temperature. The respective sensor is stabilized just as in the absolute value control.
Note:
If the absolute value control (stabilization of a sensor) and the differential control (stabilizing the
difference between the two sensors) are both active at the same time, the slower speed of the two
methods “wins”. The event control “overwrites” the speed results from other regulation methods. This
means that a defined event can block the absolute value or differential control.
