Proportional Integral Differential (Pid) Controller - Zehnder Option Box Manuel De L'installateur

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 proportional band. 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.
28 - EN
Proportional controller
error
-
Differential controller
Differential time, change of error
measured value
Proportional band, error
Integral controller
Integral time, sum of errors
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 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 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