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...APPENDIX B
B3 Setting Up Three Term (PID) Control Parameters
To enable a process to be controlled satisfactorily, the following
conditions must apply:
a) The process must be capable of reaching a natural balance
with a steady load.
b) It must be possible to introduce small changes into the
system without destroying either the process or the product.
The
determines the gain of the system. (the
Proportional Band
gain is the reciprocal of the proportional band setting, e.g. a
setting of 20% is equivalent to a gain of 5). If the proportional
band is too narrow, the control loop may become unstable and
cause the system to oscillate. With proportional band control
only, the system normally stabilizes eventually but at a value
which is offset from the set point.
The addition of
Integral Action Time
too short, can cause the system to go into oscillation. The
introduction of
Derivative Action Time
by the process to stabilize.
B4 Manual Tuning
Before starting up a new process or changing an existing one:
a) Select the
Config. Control
set to
– see Section 5.7.
PID
b) Select the
PID Controller
–
Proportional Band
100%
–
Integral Time
0
–
Derivative Time
0
Notes.
• If the system goes into oscillation with increasing
amplitude (Fig. B4 Mode B), reset the proportional
band to 200%. If oscillation continues as in Mode B,
increase the proportional band further until the
system ceases to oscillate.
• If the system oscillates as in Fig. B4 Mode A, or does
not oscillate, refer to step c).
c) Reduce the
Proportional Band
observe the response. Continue until the process cycles
continuously without reaching a stable condition (i.e. a
sustained oscillation with constant amplitude as shown in
Mode C). This is the critical point.
d) Note the cycle time 't' (Fig. B4 Mode C) and the
(critical value) setting.
Band
e) Set
to:
Proportional Band
1.6 times the critical value (for P+D or P+I+D control)
2.2 times the critical value (for P+I control)
2.0 times the critical value (for P only control)
f)
Set
to:
Integral Time
t
(for P+I+D control)
2
t
(for P+D control)
1.2
80
removes the offset but, if set
reduces the time required
page and ensure that
Controller
page and set the following:
(off)
– see Section 5.7.1
(off)
by 20% increments and
Proportional
g) Set
Derivative Time
t
(for P+I+D control)
8
t
(for P+D control)
12
The analyzer is now ready for fine tuning by small adjustments to
the P, I and D terms, after the introduction of a small disturbance
of the set point.
is
Fig. B4 Control Conditions
to:
Response Time
Time
Mode A
Response Time
Time
Mode B
Response Time
Cycle Time t
Time
Mode C
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