5.1 Ge n e r a l 67
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Chapter 5: Advanced Operation
5.1 General
This chapter provides information on the advanced operation of the Model 335 tem-
perature controller.
5.2 Autotune
The Model 335 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, the cooling system must be configured prop-
erly with control input sensor and heater output making it capable of closed-loop
control. The control sensor must have a valid temperature response curve assigned to
it. An appropriate heater range must also be determined 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 initi
-
ate. Autotune works only with one control loop at a time and does not set the manual
output or heater range.
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 Autotune PID. Expect some over
-
shoot or undershoot of the setpoint and stable temperature control at the set-
point 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, a status message will blink over the top of the
setpoint and heater output information in the Two Input One Loop display mode and
the Input
Display mode associated with the control loop being tuned (FIGURE 5-1).
This message indicates the status of the Autotune process. For example, if Autotune P
is the selected Autotune mode, “Atune Stage: 1 of 7” is displayed when the Autotune
process first begins. The status message blinks to indicate that the algorithm is still
processing, which also allows the setpoint and heater output information to be mon
-
itored during the Autotune process. If an error occurs, the status message stops blink-
ing and displays an error message containing the stage in which Autotune failed.
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