Operating Manual CMGZ432
7
5 Controller theory
5.1 Tension control loops
When manufacturing and processing foils, wires, ropes, paper and fabric sheets, it is
important that the product is under constant tension when guided across the rollers.
Tension may change when humidity, temperature, winding or unwinding diameters vary
or when the sheets are being printed, coated, glued or pressed. Tension is measured
constantly and maintained at the correct value with the FMS force measuring and control
system.
5.2 PID controller
The function of any control loop is to
maintain the feedback value exactly at the
level of the reference and to minimize the
influence of any interference on the
control loop. In addition, the control loop
must be stable under all operating
conditions.
These aims can only be achieved if the
dynamic behaviour of the control loop is
adapted to the machine.
The PID controller used in the CMGZ432
calculates an output signal that
corresponds to the addition of „P“, „I“
and „D“ component. The „D“ component can be skipped alternatively. Due to the digital
design, the controller has an exactly reproducible behaviour, because every parameter is
known as an exact number which doesn’t drift away. Due to that, it has high long-time and
temperature stability. This feature also allows to exchange an electronic unit without
readjusting.
„P“ component
A controller with only a proportional component emits an output signal that is
proportional to the error.If the error is zero, the output signal also will be zero. A small
error only can create a small output signal which is not high enough to compensate the
complete error. That means, that a controller with only a proportional component will have a
steady error. The characteristic value of a „P“ controller is the proportional factor X
p
.
„I“ component
A controller with an integral component adds the error to the output signal continuously
and emits this output signal. Due to that, the output signal will be enlarged or reduced
until the error is zero. This output signal is maintained until a new error occurs. The
integral component therefore allows zero error in steady state. The characteristic value of
an „I“ controller is the settling time T
n
.
„D“ component
A controller with a differential component has an output signal proportional to the
changing speed of the error. If the error changes in a step, the output will show the
characteristic peak impulse. Therefore, a „D“ controller reacts even if only a small
controller error occurs. The characteristic value of a „D“ controller is the derivative
action time T
V
.
fig 3: Step response of a PID controller
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