Despatch P1400+ Temperature Controller User Manual
Page 59
53
P
Input Span
T
6
SECTION 9: MANUALLY TUNING CONTROLLERS
9.1 Single Control Tuning (PID with Primary Output only)
This simple technique balances the need to reach setpoint quickly, with the wish to limit
setpoint
overshoot at start-up or during process changes. It determines values for the
Primary Proportional
Band
(
Pb_P
),
Integral Time Constant
(
ArSt
) and
Derivative Time Constant
(
RAte
) that allow the
PID control
algorithm to give acceptable results in most applications that use a
single control
device.
CAUTION:
This technique is suitable only for processes that are not harmed by large fluctuations in
the process variable.
1. Check that the
Setpoint Upper Limit
(
SPuL
) and
Setpoint Lower Limit
(
SPLL
) are set to
safe levels for your process. Adjust if required.
2. Set the
Setpoint
to the normal operating value for the process (or to a lower value if
overshoots beyond this value might cause damage).
3. Select
On-Off control
(i.e. set
Pb_P
= 0).
4. Switch on the process. The
process variable
will oscillate about the setpoint. Record the
Peak-to-Peak variation (
P
) of the first cycle (i.e. the difference between the highest value of
the first overshoot and the lowest value of the first undershoot), and the time period of the
oscillation (
T
) in minutes. See the example diagram below
- Manually Tuning
PID.
5. Calculate the
PID control
parameters using the formula below.
Input Span
is the difference
between
Scale Range Lower Limit
and
Scale Range Upper Limit
:
Pb_P
= x 100
ArSt
= T min
rATE
= minutes
Figure 34. Manually Tuning PID
Time
P
ro
c
e
s
s
V
a
ri
a
b
le