Open Circuit strain gauge fault

[Paul_Tyler]

I have set up a data logger to record 25 strain gauges for a long term fatigue test. I am using SCC SG02 signal conditioning modules with a PCI-6224 M-series Multifunction DAQ.

If I lose a strain gauge (e.g. on channel 10) I will lose all subsequent channels as well (e.g. channels 11-24). All these channels go full-scale negative.

Is there any way of fixing this? Is it related to scan rate or some other software setting?

Thanks

Paul Tyler

RR – Derby, England

[SachaE]

Hi Paul,

Could you please let me know what wiring you have at the moment - it might be easiest if you could point me to the appropriate page in the manual (link below)

http://www.ni.com/pdf/manuals/371073b.pdf

This could be due to charge pickup and having no leak path for the amplifier on the DAQ card to get rid of it.

If you're using DAQmx, then can you setup a task in Measurement and Automation explorer, and see if that suffers in the same way?

If you setup a task (or configuration if you're using traditional DAQ) to acquire all the channels around the faulty one instead, does that work?

What version of DAQ are you using, and what's your programming environment and version?

Thanks

Sacha Emery

National Instruments (UK)

[Paul_Tyler]

Hello Sacha,

My wiring is based on Application Note 078 figures 12 and 13. It is similar to user guide page 20, but the shunt cal module is across R3 instead of R4.

My programming environment is Microsoft Visual C++ version 6.0. I am using NI-DAQmx drivers and C API.

If in my application I bypass the open-circuit channel it appears to collect data from the others OK. However this will not do as a long term solution. The main problems are configuring the system prior to delivery of the strain-gauged test piece; and loss of data during the fatigue test.

I have not tried using NI-MAX, as the system is with the customer.

Thanks

Paul Tyler
RR - Derby, England

[SachaE]

Hi Paul,

  Scanning open channels is outside the recommended use for SCXI modules with a multiplexer. If  you need to scan open channels in a SCXI module that uses a multiplexer the best thing to do is provide an actual input for the open channels. Even grounding open channels is a reasonable solution.

With SCXI, during normal testing procedures you'll get open channels to occur due to a strain gauge breaking, it is necessary to detect channels that move outside or reasonable levels, due to the voltage railing from an open circuit and then force the channel to ground using software and prevent the increased settling time of the open channel from affecting the sampling rate for all the other multiplexed channels.

Since you're using the per channel scc, you can't acheive this in software directly. If you don't scan that channel though (by stopping the task, and reconfiguring with the channel removed from the task list) that should resolve the problem.

Sorry it's not better news, but it's a possible way forward.

Sacha Emery

National Instruments (UK)

[Paul_Tyler]

Thanks Sacha.

Is there any way that the effect could be minimised? How about slowing down the scan rate, introducing a leak path, scanning each channel several times with different gains?

In my software I create the channels using DAQmxCreateAIVoltageChan, with the terminalConfig parameter set to DAQmx_Val_RSE. Is this the correct one to use?

Regards

Paul Tyler

RR - Derby, England

[Logan_K]

Hello Paul,

With SCC, you should always use non-referenced singled ended (NRSE), so in your function you should use "DAQmx_Val_NRSE". This could be a large part of why you are getting significant cross-talk between your signals. Try this and let me know what happens, because M-series board should be fairly immune to cross-talk from saturation on the input amplifiers.

Also, if you are still seeing significant cross-talk, then the only practical way to reduce it is to reduce your convert clock rate using the function "DAQmxSetAIConvRate()". The slowest rate you can pick will be tied to the sample rate that you choose.

Regards,

Logan Kunitz

National Instruments - Austin, TX