38
TROUBLESHOOTING PID TUNING
After carrying out Self-Tuning or Manual Tuning procedure, if the process control loop is slightly unstable and oscillates, it may
be due to the fact that the loop was not properly tuned or the procedures described above are not quite appropriate for this
loop. In such cases, instead of repeating the extensive Tuning procedures, several things can be tried out to repair the Tuning
of the control loop as described below:
Ÿ
Compare the period of oscillation (T) (Refer Figure 10.2) to the ‘Integral Time’ constant setting. If the ‘Integral Time’ is very
small, slightly increase it. The ‘Integral Time’ constant can be increased atleast as long as the period of oscillation.
Ÿ
If the control loop continues to oscillate, slightly increase ‘Proportional Band’ value to eliminate the oscillations
Ÿ
Some control loops have certain physical characteristics which prevent them from being tuned with derivative action. For
example; in a process where air, gas, liquid or pressure is to be controlled. Upon adjusting the ‘Integral Time’ constant and
‘Proportional Band’ as stated above if the loop does not stabilize, try reducing or removing the ‘Derivative Time’ constant.
Ÿ
If the ‘Minimum ON Time’ to drive the motor for initiating valve movement is set too large, it may impare the effectiveness of
the Tuning and control and increase the risk of oscillations. Set the ‘Minimum ON Time’ value appropriately so as it cannot
affect the valve movements as well as eliminates oscillations.
User Manual
neuro 104
Type Of Control
Proportion
Band
Integral
Time
Derivative
Time
Proportional Only
Propor Integral
(P + I)
Propor Integral
+ Derivative (P + I + D)
2 X
P
2.22 X
P
1.67 X
P
OFF
0.8 X
T
0.5 X
T
OFF
OFF
0.13 X
T
Table 10.1
