Lake Shore Model 331 Temperature Controller User’s Manual
4.9
AUTO TUNE (Closed-Loop PID Control)
The Model 331 automates the tuning process of typical cryogenic systems with the AutoTune feature.
For additional information about the algorithm refer to Paragraph 2.8.
Before initiating AutoTune the cooling system must be set up properly with control sensor and heater
making it capable of closed-loop control. AutoTune works only with one control loop at a time and does
not set the manual heater power output or heater range. The control sensor must have a valid
temperature response curve assigned to it. An appropriate heater range must also be determined as
described in Paragraph 2.7.1. Choosing good initial control parameters by experimenting with Manual
PID tuning can speed up the AutoTune process. If no initial parameters are known start with the default
values of P = 50 and I = 20. It is better to set an initial P value that causes the system to be more active
than desired. Starting with a low P value can increase the time and number of attempts required to
tune.
There are three AutoTune modes available. They result in slightly different system characteristics.
Auto PI is recommended for most applications.
Auto P
– Sets only the P parameter value. I and D are set to 0 no matter what the initial values are.
This mode is recommended for systems that have very long lag times or nonlinearity that prevents
stable PI control. Expect some overshoot or undershoot of the setpoint and stable temperature
control below the setpoint value.
Auto PI
– Sets values for both P and I parameters. D is set to zero. This mode is recommended for
stable control at a constant temperature. It may take slightly longer to stabilize after setpoint change
than Auto PID. Expect some overshoot or undershoot of the setpoint and stable temperature control
at the setpoint value.
Auto PID
– Sets values for P, I and D parameters. D is always set to 100%. This mode is
recommended when setpoint changes are frequent but temperature is allowed to stabilize between
changes. Stability at setpoint may be worse than Auto PI in noisy systems. Expect slightly less
overshoot or undershoot than the other modes and control at the setpoint value.
Once AutoTune mode is selected, the Tune annunciator turns on steady to indicate that AutoTune is
on. No activity takes place until the setpoint is changed at least 0.5 K. At that time, the Tune
annunciator blinks to indicate the instrument is gathering data. This process takes from 1 to 17 minutes
depending on the system reaction time. The tune annunciator stops blinking when calculations are
complete and new parameter values have been stored. The annunciator will also stop blinking if the
algorithm is unable to complete. Possible reasons include: setpoint change too small, manual control
parameter changed during tuning, heater not turned on, or control sensor curve not selected.
If the controller is not tuned satisfactorily on the first attempt, make several small (2 degree) setpoint
changes to see if better parameter values are calculated.
To select an AutoTune mode press the
AutoTune
key, and press either the
s
,
t
, or
AutoTune
key to
cycle the display to AutoTune PID. You will see the following display.
Select for Loop 1 °®
Tune Mode Auto PID
Use the
s
or
t
key to cycle between Auto PID, Auto PI, and Auto P. Press the
Enter
key. The
controller is now in Autotuning mode. When the AutoTune feature is on, the front panel
Tune
LED will
be on steady. When there is a setpoint change and the Model 331 is actively gathering data, the
Tune
LED will blink.
Operation
4-19
