5 . 1 G e n e r a l 73
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Chapter 5: Advanced Operation
5.1 General
This chapter provides information on the advanced operation of the Model 336 tem-
perature controller.
5.2 Autotune
The Model 336 can automate the tuning process of typical cryogenic systems with
the Autotune feature. For additional information about the algorithm refer
to section 2.9.
Before initiating the Autotune process, properly configure the cooling system with
control input sensor and heater output to make it capable of closed-loop control.
Assign the control sensor with a valid temperature response curve. Also determine an
appropriate heater range as described in section 2.8.1. The system must be coarsely
maintaining temperature within 5 K of the setpoint where new tuning parameters
are desired in order for the Autotuning process to initiate. Autotune works only with
one control loop at a time and does not set the manual output or heater range. For
autotuning to work properly on a control loop with a scanner input assigned (Model
3062 4-channel scanner option), only that scanner input channel can be enabled; all
other scanner input channels must be disabled.
To initiate the Autotune process, press Autotune, then select an Autotune mode.
There are three Autotune modes available. They result in slightly different system
characteristics. Autotune PI is recommended for most applications.
D Autotune 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.
D Autotune PI: sets values for both P and I parameters. D is set to 0. 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.
D Autotune 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
Autotune PI in noisy systems. Expect slightly less overshoot or undershoot than
the other modes and control at the setpoint value.
When the Autotune process is initiated, the P, I, D, and Manual Output parameters are
removed from the display and the “Autotuning” message appears in the lower right
corner. Below the Autotuning message, the current status of the process is displayed.
The status message blinks to indicate that the algorithm is still processing. If an error
occurs, the status message stops blinking and displays an error message containing
the stage in which Autotune failed. See TABLE 5-1 for a description of the Autotune
stages, reasons for failure, and possible solutions. When the process completes suc-
cessfully, the previous P, I, and D parameters are replaced by the newly acquired val-
ues. To cancel the Autotune process, press Autotune, and choos Yes to the “cancel
Autotune” prompt.
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