Use USB-9162 to measure full bridge, output is non-linear

[JayWang]

I use a NI USB-9162 to measure pressure (a full bridge) with LabView.  The output is not linear relative to input (with USB-9126's internal 2.5V excitation or with external 5V excitation).  If use the pressure transducer (full bridge) with a Omega signal conditioner, the output is linear, which indicated the transducer is fine.  Please help.  Thanks.

[R Sandoval]

Hello Jay,

Just to further clarify, are you using a USB-9162 sleeve with a Bridge C Series Module?  If so, could you specify the C Series Module?  Moreover, could you provide more specifications about the Omega signal conditioner (eg. input and output voltage range)?  Thanks.

[JayWang]

Hi Roman:

Happy New Year.

Thanks for your response.  I am using a USB-9219 with USB-9162 sleeve.

The 500 psi pressure transducer is a ASHCROFT K8 series, a full  bridge, 3-10 VDC excitation, 1.4 mA supply current, 6-18 mV/V output. 

If treat the pressure transducer as a full bridge (connection on USB-9219: 3 Ex+, 4 Hi, 5 Ex-, 6 Lo), using internal 2.5 VDC excitation and read with Labview, the out put is non-linear (as shown in the attached plot). Different channel gave same result.  Based on conversation with ASHCROFT tech support, 2.5VDC excitation is lower, but will not cause the non-linear issue.

To check if the pressure transducer itself is linear, I connected it to a Omega OM5-IMV-100A signal conditioner (isolated milivot input, -100 mV to 100 mV, output -5V to 5V) with 5VDC excitation, the measured output (voltage) is linear with respect to pressure.

If I provide 5VDC excitation to the transducer, and read as voltage (not bridge) with USB-9219, the output is perfectly linear (as shwon in the attached plot). 

Please advise.

Thanks.

Jay

[JayWang]

Since I can not open the above attached word file, here I re-submit the pdf file.

Thanks.

Jay

[R Sandoval]

Hello Jay,

What happens when you supply an excitation voltage of 5V without the signal conditioner?  According to the online data sheet, the ASHCROFT K8 has a working supply voltage range from 3-10V.  You may also want to characterize the Analog-to-Digital converter by sending a linear voltage sweep across the analog input and identify if the values correspond correctly.  Doing so may help to find non-linear characteristics of the module which typically should be minimal.  Furthermore, what kind of additional filtering does the Omega signal conditioner provide?  I was able to reference the specifications sheet but was wondering if the conditioner provided a linear gain or a different filtered function.

[JayWang]

Thanks for the reply.

The second figure in the pdf file in the post was obtained with 5 VDC excitation, without signal condintioner, with the output voltage fed into NI USB-9219 as a voltage input.  The results is linear with respect to pressure. 

I tried to provide external 5VDC excitation to the transducer and with output fed into the NI USB-9219 as full bridge (USB-9219 only allow 2.5V internal excitation, so in the device set up still was 2.5V internal excitation).  The output response (read from Labview) is non-linear. 

I do not have other NI device with bridge input which has higher than 2.5V excitation.

Any suggestion.

Thanks.

Jay

[Ali M]

Hello,

Have you tried testing your readings by creating a Custom Voltage with Excitation in Measurement & Automation Explorer (MAX)? This measurement type allows you to use sensors that require voltage excitation, such as bridge sensors. Please implement this and provide an update of your findings

Best regards,

Ali M

Applications Engineer

National Instruments

www.ni.com/support