Fridgewatch 2100 Controller Mk4
Publication 3-58
Section 3
Page 24 of 152
Issue 1.1 : 01/08
The size of the proportional band is defined by operator setting S5. A
large proportional band achieves stable control, however, little corrective
action is taken for relatively large errors, hence the control response is
slow. A large proportional band also permits a large steady state error to
exist with no corrective action taking place. Reducing the proportional
band reduces steady state error, but control can become unstable as
relatively small deviations from the set point result in a large corrective
action being taken.
5.2.2.
Integral Action Time
Steady state errors can practically be eliminated whilst achieving stable
control by introducing an integral term into the control action.
Integral control integrates error with respect to time, that is, a small
fraction of the error is added to a running total at each control
assessment. The size of the fraction is determined by the chosen
integral action time, defined by operator setting S6. The greater the
integral action time the smaller the fraction. For example, halving the
integral action time from the default setting of 300 seconds to 150
seconds would halve the time taken to generate a load pulse from a small
steady state error.
In practical terms, integral control causes control action to be taken when
small errors exist over periods of time, even though these errors may be
too small for significant action to be taken by proportional control alone.
If integral action time is too short, hunting around the set point occurs.
Setting integral action time to its maximum value reduces integral action
to such an extent that it has virtually no influence on control. Integral
control may be also disabled by setting integral action time S6 to zero.
5.2.3.
Derivative Action Time
Derivative control looks at the rate of change of error. If the measured
variable is moving away from the set point the derivative term increases
the amount of corrective control action. If the measured variable is
moving towards the set point the derivative term decreases the amount of
corrective action or even applies the opposite control action. Derivative
control therefore acts to return the measured variable as quickly as
possible towards the set point in cases of rapidly applied or fluctuating
loads, and also aids stability of the control system by opposing overshoot.
Increasing derivative action time, defined by operator setting S7,
increases the derivative action effect. Decreasing derivative action time
reduces the effect; a zero value for S7 disables derivative control
completely although this is not normally recommended.
5.2.4. PID
Control
Set-up
As we have seen, by varying the size of the proportional band, integral
action time and derivative action time, it is possible to tune the
Fridgewatch 2100 Controller to produce different control characteristics,
say for a fast response to load changes, or for very close set point
control. The value for each variable can only be arrived at by subjecting
Fridgewatch 2100 to system fluctuations under actual site conditions and
modifying each term until the desired response is achieved.
