Operating Manual CMGZ 622
10
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 FMS tension
control system 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 gives 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 characteristics of a. P-controller are 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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