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Controller Tuning
If the controller is withdrawn from the control panel face,
further adjustments are available which are used to tune
the controller to the process.
When a control loop is commissioned, the controller
settings are adjusted to correspond to those, which have
been specified during the design of the control system. If
a large section of process is to be commissioned; possibly
a mathematical model of the process will have been
developed from which the optimum controller settings can be
calculated for efficient and stable operation. It is these
values which are set into the controller before start-up
and, if calculated correctly, no further adjustment will
be required.
In some cases it will be necessary to tune a controller
without having the benefit of knowing what the settings
should be. It must always be remembered that the
adjustments cover a very wide range of sensitivity and
response. If adjusted haphazardly, the process may shut
down and damage to equipment and lost production may occur.
The task of controller tuning is usually left to an
instrument technician with experience in the cause and
effect of process reaction and controller adjustments.
There are many trial and error methods of controller tuning
which do not involve mathematical analysis and should be
demonstrated by an experienced person, otherwise shutdowns
may occur.
The first adjustment, which would normally be made, would
be to set forward or reverse action as required. A forward
acting controller has increasing output in response to an
increasing measured variable. A reverse acting controller
has decreasing output in response to an increasing measured
variable.
Empirical Tuning Method
Proportional only controller
- With transfer switch at manua1, set PB at maximum
or at safe high value, usually 200% PB.
- Move transfer switch to auto and make changes in
set point. The time required for the disturbance to settle
may then be noted.
- Continue reducing band-width to half its previous
value until the oscillation do not die away, But continue
to be perceptible.
- Now increase the band-width to twice its value.
This gives the required stability, that is, the minimum
stabilising time and minimum offset.
Proportional plus integral action
- Set the Integral Action Time (IAT) to maximum.
- Adjust the proportional band as for a proportional
controller.
- Decrease the IAT in steps, each step being such
that line IAT 1s halved at each adjustment. Below some
critical value, depending upon the lag characteristics of
the process, hunting will occur. This hunting Indicates
that the IAT has been reduced too far.
- Now increase the time to approximately twice this
value to restore the desired stability.
Proportional plus derivative action
- Adjust the Derivative Action Time (DAT) to its
minimum value.
- Adjust the proportional band as described for
proportional controller, but do not increase the band when
hunting occurs.
- Increase the DAT (that is, double each setting) so
that; the hunting caused by the narrow band is eliminated.
- Continue to narrow the band and again increase the
DAT until the hunting is eliminated.
- Repeat previous step until further increase of the
derivative action time fails to eliminate the hunting
introduced by the reduction of the proportional band, or
tends to increase it. This establishes the optimum value
of the DAT and the hunting should be eliminated by
increasing the width of the proportional band slightly.
Proportional plus integral plus derivative action
- Set IAT to a maximum.
- Set DAT to a minimum.
- Adjust the proportional band as for a P + D
controller.
- Adjust derivative using same procedure as for
above, P + D.
- Adjust integral to a related value of the final
derivative setting.
A three-term controller is therefore adjusted as for a P +
D controller and the integral value simply related to the
derivative value.
In many cases, the setting procedure may be shortened by
omitting settings, which are outside the probable range.
The process should then respond to set point or load
changes, where integral action removes offset and the
second overshoot of set point is approximately 1/4 the
amplitude of the first. This is commonly referred to as
the 1/4 decay method and is generally agreed to be the
optimum controller setting for a P + I controller.
The above method is only used when no other controller
setting data is available and must be practised with care.
Optimum Settings (Ultimate Method)
The closed loop or ultimate method involves finding the
point where the system becomes unstable and using this as a
basis to calculate the optimum settings.
The following steps may be used to determine ultimate PB
and period:
1. Switch the controller to automatic.
2. Turn off all integral and derivative action.
3. Set the proportional band to high value and reduce
this value to the point where the system becomes unstable
and oscillates with constant amplitude. Sometimes a small
step change is required to force the system into its
unstable mode. The below figure showing typical response
obtained when determining ultimate proportional band and
ultimate period time.
4. The proportional band that required causing
continuous oscillation is the ultimate value Bu.
5. The ultimate periodic time is Pu.
6. From these two values the optimum setting can be
calculated.
• For proportional action only
PB% = 2 Bu %
• Proportional + Integral
PB% = 2.2 Bu %
Integral action time = Pu / 1.2 minutes/repeat
• Proportional + Integral + Derivative
PB%=1.67Bu
Integral action time = Pu / 2 minutes/repeat
Derivative action = Pu / 8 minutes
Example 1
When a step change was applied to a closed loop system with
a PB% of 14% sustained oscillation of the output was
observed. The time between two adjacent peaks was measured
as 1.2 minutes. Calculate the optimum setting for a P only
& P+I+D system.
[(28%) & (23.38%, 0.6 min/rep, 0.15 min)].
Example 2
A closed loop control system is found to oscillate when the
proportional band is reduced to 23%. A trace on a chart
gives a measurement of 6 mm between adjacent peaks. If the
chart speed of the recorder is 10mm per minute, calculate
the optimum controller settings for P+I+D control.
(38.41%, 0.3 min/rep, 0.08 min)
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