Cryo-con Model 24C Appendix C: Troubleshooting Guide
Symptom Condition
Autotune indicates a status
of ‘Abort’ or ‘Fail’.
Autotune will only abort if the control loops are not engaged or there is
an invalid temperature reading on the control input channel. If it cannot
generate a solution because of issues in the system dynamics, it will
indicate a status of ‘Fail’.
Autotune times out and
does not generate effective
PID parameters.
Extend the Display Filter time constant to reduce system level noise
and try autotune again. The display filter is described in the System
Functions Menu section. Systems using diode type sensors above 50K
will usually require a 4 or 8 second time constant. This setting may be
returned to any desired value once tuning is complete.
Switch to the lowest possible heater range that will control at the target
setpoint.
Try autotuning in the PI- mode instead of PID. Most cryogenic systems
do not benefit from the D term.
If a Cryo-cooler is being used, set the controller’s cryocooler filter to
Input mode. This may be returned to Off or Cancel mode once tuning is
complete.
Experiment with the DeltaP parameter. Increasing it often improves
autotune success.
Temperature Measurement Errors
Symptom Condition
Noise on temperature
measurements.
Possible causes:
1. Excessive noise pickup, especially AC power line noise.
Check your wiring and shielding. Sensors must be floating,
so check that there is no continuity between the sensor
connection and ground. Review the System Shielding and
Grounding Issues section.
Note: Cryo-con controllers use a shielding scheme that is
slightly different than some other controllers. If you are using
cable sets made for use with other controllers, some shield
connections may need to change. If pin 3 of the input
connector is connected to the cable shield, disconnect it and
either re-connect the shield to the backshell contact or leave
the shield floating. No connection should ever be made to pin
3 of the input connector.
Check for shielding problems by temporarily removing the
input connector’s backshell. If the noise changes
significantly, current is being carried by the shields and is
being coupled into the controller.
2. Use a longer display filter time constant to reduce displayed
noise.
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