24
7
Setting of P, I and D Control Terms
The PROPORTIONAL, INTEGRAL and DERIVATIVE control terms may be displayed and set
by means of the recessed P, I and D buttons. Use a pointed object to press these buttons.
The system is already set with optimised values of these parameters: it should
not
be
necessary to change them, or to read the rest of this section. The default values are P=20,
I=2.5, D=0.5
P indicates the PROPORTIONAL BAND in Kelvin to a resolution of 0.001K
I indicates the INTEGRAL ACTION TIME in minutes, covering a range of 0 to 140 minutes in
steps of 0.1 minute.
D indicates the DERIVATIVE ACTION TIME in minutes, covering a range of 0 to 273 minutes
(Though values beyond 70 minutes are unlikely to be required in practice.).
In North America, a different terminology exists for 3-term control.
PROPORTIONAL BAND is replaced by its reciprocal, GAIN.
INTEGRAL ACTION is replaced by RESET. This may either be specified as a time (as for
integral action) or as its reciprocal, "REPEATS PER MINUTE".
DERIVATIVE ACTION is replaced by RATE. Again this may be specified as a time or as
repeats per minute.
RAISE and LOWER may be used to vary the control terms whilst in LOCAL control. When
you have changed the values, carry out a STORE command as follows: Press and hold the
recessed button CAL/STORE, and press SET. This will store the new values in non-volatile
memory. (NB This will interrupt the flow momentarily. If a STORE command is executed
when the controller is in Automatic mode, then this will also set the “Auto-Start” mode -
see 4.3.)
The main purpose of DERIVATIVE action is to reduce overshoot, when approaching a new
set temperature. For most systems derivative action will not be required and may be left at
zero. (Hold LOWER pressed for a second after 000.0 is displayed to ensure that there is not
a small residual setting of less than 0.05 mins which will show as zero).
The P and I controls should not normally be set to zero, since this would correspond to
ON/OFF control.
The following procedure gives a good rule-of-thumb for setting the controls to a value
that is close to optimum.
a)
Set I for a time much longer than the expected response time of the system.
b)
Set D to zero.
c)
Select AUTO and reduce P until the temperature starts to oscillate above and below
some mean value (not necessarily the set point).
