Zehnder Option Box V2 Manual página 30

15 Proportional Integral Differential (PID) controller
Setpoint
Most controllers of the ventilation unit are of
the PID type (Proportional Integral Differential).
Controllers are trying to reduce an error to
zero.
Controller circuits – unlike steering circuits – are
closed loop, meaning there is a feedback of the
result of the control to the process. Steering
circuits are unable to reduce an error to zero,
simply because there is no error, since there is
no setpoint. There is a direct relation between
the steering input and the steering output.
The output of the proportional controller is
depending on the error divided by a proportional
band. A proportional band is a setting,
determining the sensitivity of the output to the
error. Proportional control is not depending on
time. By itself, it is not capable of reducing the
error to zero. It leaves a steady state error. The
output of the integral controller is depending on
the sum of errors in time, and the integral time.
The integral time is the time required for full
output, at an error the size of the proportional
band. The longer the integral time, the longer it
30 - EN
Proportional controller
error
+
-
Differential time, change of error
measured value
Proportional band, error
Integral controller
Integral time, sum of errors
Differential controller
takes to reduce the steady state error to zero. If
the integral time is too low however, the process
will not be able to keep up with the control, and
instability will occur.
The output of the differential action is
depending on the rate of change, which is the
difference between the current error and the
previous error. The differential output is the
differential time times the rate of change. The
longer the differential time, the more aggressive
the differential action. If a response to the
rate of change is not required, the differential
time is set to zero and the controller is called
PI-controller (Proportional Integral controller).
Differential action is required only, if the rate of
change of a process matters, for example for
humidity control. Anticipating on fast humidity
change reduces the problem of high humidity.
process
measurement