3200 Series
Part No HA028651 Issue 7.0 Apr-09 53
11.2.2
How To Tune
1.
Set the setpoint to the value at which you will
normally operate the process.
2.
In the ‘CTRL’ list, select ‘ATUNE’ and set it to
‘On’.
3.
Press the Page and Scroll buttons together to
return to the Home display. The display will
flash ‘
Tune
’ to indicate that tuning is in
progress.
4.
The controller induces an oscillation in the
temperature by first turning the heating on,
and then off. The first cycle is not complete
until the measured value has reached the
required setpoint.
5.
After two cycles of oscillation the tuning is
completed and the tuner switches itself off.
6.
The controller then calculates the tuning
parameters and resumes normal control
action.
If you want ‘Proportional only’, ‘PD’, or ‘PI’ control, you
should set the ‘TI’ or ‘TD’ parameters to off before
commencing the tuning cycle. The tuner will leave
them off and will not calculate a value for them.
Typical automatic tuning cycle
11.2.3
Calculation of the cutback values
Low cutback
and
High cutback
are values that restrict
the amount of overshoot, or undershoot, that occurs
during large step changes in PV (for example, under
start-up conditions).
If either low cutback, or high cutback, is set to ‘Auto’
the values are fixed at three times the proportional
band, and are not changed during automatic tuning.
To tune the cutback values, first set them to values
other than Auto, then perform a tune as usual.
11.2.4
Manual Tuning
If for any reason automatic tuning gives unsatisfactory
results, you can tune the controller manually. There are
a number of standard methods for manual tuning. The
one described here is the Ziegler-Nichols method.
With the process at its normal running conditions:
Set the Integral Time and the Derivative Time to OFF.
Set High Cutback and Low Cutback to ‘Auto’.
Ignore the fact that the PV may not settle precisely at
the setpoint.
If the PV is stable, reduce the proportional band so that
the PV just starts to oscillate. If PV is already oscillating,
increase the proportional band until it just stops
oscillating. Allow enough time between each
adjustment for the loop to stabilise. Make a note of the
proportional band value ‘P’ and the period of oscillation
‘T’.
Set the proportional band, integral time and derivative
time parameter values according to the calculations
given in the table below:-
Type of
control
Proportional
band (P)
Integral time
(I) seconds
Derivative
time (D)
seconds
Proportional
only
2xB OFF OFF
P + I
2.2xB
0.8xT
OFF
P + I + D
1.7xB
0.5xT
0.12xT
11.2.5
Setting the Cutback Values
The above procedure sets up the parameters for
optimum steady state control. If unacceptable levels of
overshoot or undershoot occur during start-up, or for
large step changes in PV, then manually set the cutback
parameters.
Proceed as follows:
Set the low and high cutback values to three
proportional bandwidths (that is to say,
C B . H I
=
C B . LO
= 3 x
P B
).
Note the level of overshoot, or undershoot, that occurs
for large PV changes (see the diagrams below).
In example (a) increase Low Cutback by the undershoot
value. In example (b) reduce Low Cutback by the
overshoot value.
Example
(a)
Example
(b)
Where the PV approaches setpoint from above, you can
set High Cutback in a similar manner.
Time
Setpoint
PV
Auto-tune starts 1 minute after
being turned on to determine
steady state conditions.
Tuning normally takes place at a
PV which has a value of setpoint
x 0.7.
The power is automatically
turned on and off to cause
oscillations.
From the results the values
shown in the table are
calculated
U
S
Time
PV
U
S
Time
PV
