Induced voltage giving "out-of-range measurement readings.

[jollyrobbie]

Hello N.I.

I've got a good one for you....

Our company (AMI) are using PXI 8184RT's in a PXI 1031 4-Slot Chassis, with a PXI 6251M DAQ Card. 

Also, SCXI 1001 12 Slot Chassis, with corresonding (Thermocouples) SCXI 1102-TBX1303, (Analog Inputs) SCXI 1102C-TBX 96, (Voltage Inputs) SCXI 1125-SCXI 1327, (Analog Outputs) SCXI 1125-SCXI 1327.

 Okay, here's the problem:

 We've written software to control various instrument devices, E-load, Mass Flow Controllers, Eurotherm Temp Controllers. All ranges are at a setpoint of zero, until a value is set to the desired amount. Software is not the issue. After 9 months in operation, we are starting to see "induced voltages" changing the setpoint of zero to a higher value. Now this increases the setpoint above the desired, thus triggering the instrument  "out-of-range".  There are 10 units with the same Hardware/Software configuration. This problem started primarily with one unit, after a re-boot of the PXI, the values will return to zero, then after a day of testing, the "induced voltage" would float back, causing the setpoint to an out-of-range value. Now it seems, some of the other units are being affected by the same sympton.

 If one of the module cards were slowly degrading, would this affect the rest of the chassis?

                                           Any help would be greatly appreciated,

                                              Rob Carleton - AMI Tech.

[Travis G.]

Hello Rob,

Both the PXI-6251 M Series DAQ device and SCXI modules require calibration in order to keep the measurements within spec.  The Specifications document for these devices list absolute accuracies based the amount of time since last calibration.  As time passes, the ADC and other components on the analog input circuitry begin to stray from their exact specification.  It may just be time for the hardware to be recalibrated.  One big cause of inaccuracies is a change in the temperature of the measurement hardware.  You've mentioned that your measurements stray from their desired values over working day.  The DAQ device may be heating up within the PXI chassis throughout the day and throwing your measurements off.  One thing you can try to account for this temperature change is to periodically self-calibrate your DAQ device within Measurement & Automation Explorer (MAX).  This self-calibration compares the values being read from a high-precision voltage source onboard the device and adjusts the calibration coefficients accordingly.  This requires no external connections an you can even perform this calibration programmatically with the DAQmx Self Calibrate.vi.

If this doesn't solve the problem, you may need to have your device externally recalibrated.  Even the high-precision voltage source onboard the device used for self-calibration can stray over time, and needs to be periodically calibrated using an external high-precision voltage source.  You can have your hardware externally calibrated by NI or an approved metrology lab.  Information about getting your device externally calibrated can be found at www.ni.com/calibration.

I hope this helps and let us know if this solves the problem.

Regards,
Travis G.
Applications Engineering
National Instruments
www.ni.com/support