77
During pre-tune, the controller outputs full primary
power until the process reaches the specified test
point. Power is then removed (full secondary power ap-
plied for dual control), causing an oscillation which the
pre-tune algorithm uses to calculate the proportional
band(s), integral and derivative time. The pre-tune pro-
cess is shown below.
Oscillation Peak
Test Point specified, or
for “standard” pre-
tune
Control Power
Pre-Tune
engaged
here
+100% Power (HEAT output)
Initial PV
Process Variable (PV)
Pre-Tune Value set or
for Std Pre-Tune =
SP – Initial PV
2
Setpoint (SP)
-100% Power (Cool output)
Figure 58. Pre-Tune Operation
Pre-tune is selected from the automatic tuning menu. It
will not engage if either primary or secondary outputs
on a controller are set for On-Off control, during set-
point/profile ramping or if the process variable is less
than 5% of the input span from the setpoint.
Note:
To pre-tune a cascade, first select “Cascade-
Open” to tune the PID set(s) on the slave. After the
slave has successfully tuned, remember to pre-tune
the master/slave combination (this time select “Cas-
cade-Closed”). The cascade remains open until you
do this.
Auto Pre-Tune
As a single-shot operation, pre-tune will automatically
disengage once complete, but can be configured to
run at every power up using the auto pre-tune function.
If auto pre-tune is selected, a Standard Pre-tune will
attempt to run at every power up, applying new tun-
ing terms to the current Active PID set. Auto pre-tune
will not be able to test the process if at the time the
controller is powered up, either primary or secondary
outputs are set for On-Off control, during setpoint/pro-
file ramping or if the process variable is less than 5% of
the input span from the setpoint. Auto pre-tune is not
possible with cascade control mode.
Self-Tune
If engaged, self-tune uses a pattern recognition algo-
rithm to continuously monitor and adjust for control
deviation. It optimizes the tuning by applying new PID
terms to the current Active PID set while the control-
ler is operating. In VMD control mode, derivative is not
applied by self-tune, and the controller is optimized for
PI control.
Temperature
Setpoint 1
Setpoint Change
Load Disturbance
Time
Setpoint 2
Start-up
Figure 59. Self-Tune Operation
Summary of Contents for Chromalox 4081
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