cDAQ 9219 No AI Strain Gauge Amp

[Steve289]

Hello again all,

I am trying to measure a voltage output from a Sensotech Strain Amp.  The voltage is a typical +-5Vdc.  When I disconnect my two wires from pin4 (+) and pin5 (-) and measure the voltage with a DMM there is no problem, +-cal gives me 5volts.  However when I reconnect the wires to AI0 on the 9219 I get no reading and no response from my +-cal.  I have the module in high speed mode running at 100hz.  I have tried other speed settings and that had no effect.  I'm thinking its has something to do with the diff. measurement type for the module and the output type for the amp.  I think the output from the amp is a floating output and I read in an NI whitepaper that when trying to measure a floating signal with a diff. connection each leg of the signal needs to be brought back to ai ground via high ohm (10K-100K) resistors.  Well I don't think I have an AI ground on this module so I brought it back to the grounding screw on the side of the chassis (9172), just to see if it would do anything, and it did nothing.  I have no idea on this gremlin.  Like always your help is greatly appreciated and thank you very much in advance.

Steve.

[PBear]

Hi Steve,
You are correct in believing the 9219 reads in differential signals. As far as connecting the signal, I too am somewhat confused as to why you are not getting the signals you are expecting. Make sure your positive and negative signal wires from your strain device are wired into pins 4 and 5 respectively on the 9219. This is documented in the product’s user manual found here.

While connected to the 9219, can you confirm that the voltage difference between the two terminals is 5V with your DMM? I assume that you are trying to set this up in Measurement and Automation Explorer using the test panels. Make sure your range is set properly and if need be, find a power supply and measure a known voltage. Consult with the documentation of your strain sensor to understand the output signal’s characteristics. With this information, try reading this document (Field Wiring and Noise Considerations for Analog Signals) detailing wiring configurations for a successful reading. Also, it is unclear what +-cal represents.

I hope this advise helps.

[Steve289]

Hello,

I verified that the (+) output from the amp is connected to pin 4 and the (-) output is connect to pin 5, still no output reading.  I even switched the wires just for the fun of it, still no output reading.  The +-Cal is a switch on the amp that when pressed in the positive direction will cause my output signal to go to +5Vdc and pressed neg causes the output to go -5Vdc.  If I connect my DMM to the output wires while it they are plugged into the 9219 module I get a zero reading even when using the +-cal button.  If I remove the wires from the module and connect directly to the DMM then my +-cal gives me +-5Vdc on my DMM.  I've tried viewing this data in MAX and also with a simple vi with a DAQ READ in a WHILE LOOP.  I went to a simple vi just so I could force the module out of its 2Hz high accuracy mode and into the high speed (100hz) mode.  Either way I get the same behavior in MAX or in my VI.  I've used this module to collect data from a clamp on current probe, (+) output to pin4 (-) output to pin5 and it worked fine.  Thanks for the ideas, if you think of anyhting else please pass it on.

Thanks

Steve

[Brad_K]

Hi Steve,

You probably already did this, but if your strain gage amp is externally powered, make sure you disable the NI 9219's built-in excitation by using the "voltage" version of DAQmx Create Channel.vi, not the "strain" or "custom voltage with excitation" versions.

Also, using external signal conditioning with the NI 9219 requires you to scale your data from voltage to strain in your application or using a DAQmx custom scale. The NI 9219 driver in DAQmx only handles scaling for the NI 9219's built-in signal conditioning.

Brad