Issue with fixed point number output from 9219 module for full bridge measurement (cRIO/FPGA)

Question

Hi,
I have a question regarding the fixed point output acquired from a 9219 module (in FPGA on a cRIO) when setup to acquire a strain measurement (full bridge).
Software: Labview 2009
Hardware: cRIO-9012 (NI-RIO 3.2.1), NI-9219 module
The 9219 module is configured in the project as follow:
Ch0: Full-Bridge +/- 7.8mV/V
Ch1: Voltage +/- 4V
Ch2: Voltage +/- 15V
Ch3: Voltage +/- 60V
The calibration mode is 'Calibrated' so the FPGA outputs fixed point numbers. 
My issue is that the precision of the fixed point number for Ch0 (strain) is "(fixed point <+/-32,-1>[-2.5e1,2.5e1]:1.16e-10)", which indicates that the fixed point number is a value between +/- 0.25 and not the +7.8mV/V as I expected?
The fixed point number does not change in precision whether the range for the strain measurement is set at +/- 7.8mV/V or +/-64mV/V (the two available options).
As the fixed point number doesn't change precision I'm assuming that changing the range of the strain measurement changes the resolution of the acquired number? And as such I will need to perform additional scaling on the fixed point number to convert it to the expected range?
There is no mention of scaling of the voltage or strain measurements mentioned in any documentation or examples, with the only scaling example provided for the thermocouple measurements.
Any help/clarification is much appreciated.
Regards,
Mike

Answer 1

Hello Mike,
Hopefully I can help clarify some of the behavior you are seeing.
My issue is that the precision of the fixed point number for Ch0 (strain) is "(fixed point <+/-32,-1>[-2.5e1,2.5e1]:1.16e-10)", which indicates that the fixed point number is a value between +/- 0.25 and not the +7.8mV/V as I expected?
For calibrated values on the FPGA VI, the returned data is a Voltage measurement, not a directly calculated strain value. Based on the specified ranges for the 9219 in a Full bridge configuration, 250 mV will encompass all possible input values, at the module provided excitation (2-2.7V dependent on the sensor gage resistance).
The fixed point number does not change in precision whether the range for the strain measurement is set at +/- 7.8mV/V or +/-64mV/V (the two available options).
As you may notice from the fix-point definition, the fixed point data contains 32-bit precision which is larger than the acquired precision of 24-bits provided by the 9219. The fixed point data-type is coded to accept input encompassing both the range and precision of the instrument; such that, no additional coercion of the input data values is required based on user-defined software settings, i.e. the bridge sensitivities +/- 7.8mV/V or +/-64mV/V. 
As the fixed point number doesn't change precision I'm assuming that changing the range of the strain measurement changes the resolution of the acquired number? And as such I will need to perform additional scaling on the fixed point number to convert it to the expected range?
The documentation does not clearly define that by varying the discrete levels of strain input (+/- 7.8mV/V or +/-64mV/V) the range of the ADC on the module is also adjusted. I am working to follow up further on this topic, to provide a clarification on the module documentation. As for scaling, the voltage values acquired regardless of the ADC resolution will still be related strain via the bridge sensitivity. The resolution of the ADC will simply define the smallest measurable change in the strain value. 
There is no mention of scaling of the voltage or strain measurements mentioned in any documentation or examples, with the only scaling example provided for the thermocouple measurements.
For converting the acquired voltage values to a strain measurement, I would recommend the documentation linked here for a detailed explanation on strain calculation. Often, users will forward the acquired voltage data as fixed point values through a DMA FIFO to the RT controller on the Compact RIO; such that, they may handle conversion from voltage to strain using floating point math in real-time. 
I hopes these responses provided a bit of clarity. I will continue to work to provide additional information on the 9219 specification information. Please post back any further questions.
Cheers!
 Edit: Forgot to add the link.
Message Edited by Pcorcs on 04-14-2010 04:55 PM
Patrick Corcoran
Application Engineering Specialist | Control
National Instruments