Flexlogger vs Labview

[wade75]

I'm seeing a phenomenon when using Flexlogger and monitoring data on a NI 9239 cDaq module.  Flexlogger will let you configure an accleration channel and read data.  However when I try to configure in Labview and do the same thing I get an error stating that the measurement type doesn't match the types available on the device?  How can this be?  Flexlogger I'm assuming is using DAQmx drivers under the hood.  Does flexlogger just read everything as an analog voltage +- 10V signal and the applies the sensitivity scale to the data as opposed to reading the data from the driver as scaled units such as Labview would?  It seems weird that Flexlogger and access certain measurement types on devices that aren't really capable of measuring?

[Devin_K]

Hi wade75,

In general, yes, FlexLogger will allow for many different measurement types for modules that at first seem like it should not be supported.  However, FlexLogger has a very different approach to measurements compared to LabVIEW.  In LabVIEW you're directly setting up the type of measurement task you want, while FlexLogger is abstracting the details away and just asking you what real-world "Physical Measurement" your channel will be reading (which could be pretty-much anything, even "Custom"), and then picking a supported "Sensor Class" (which more closely aligns with the LabVIEW task type).

As you mention, if you select "Acceleration" for a NI 9239 module, "Voltage" will automatically be selected as the sensor type, since that is the only measurement type that the NI 9239 supports.  However, if you selected "Acceleration" on a NI 9234 module (a sound & vibration module), you will be able to select between "Voltage" and "IEPE" for the sensor class, since that modules supports both measurement types.

Why does FlexLogger handle it this way?  Well there are many different types of sensors out there, and in many cases the physical signal that is actually read by the DAQ card is actually just a scaled representation of the actual physical measurement.  For example, consider a sensor like this Infineon current sensor (it's not for acceleration, but the same concept applies). It uses Hall probes to measure current values up to +/- 120A.  However, it outputs a scaled voltage representative of that current value, which you can see on Table 4 on page 8 of the datasheet (ex. at full range there is a 10 mV/A sensitivity).  So in FlexLogger you could use a card which supports general voltage measurements such as the NI 9201 (or even NI 9239) and select "Current" as your physical and "Voltage" as your sensor class, then set up a Sensitivity scale with the correct value from the datasheet.

You can actually accomplish something similar in LabVIEW by setting up a Voltage task but wiring in a Custom Scale, which allows you to define the scaling parameters as well as pre-scaled and scaled units (which is just a string), effectively allowing you to measure any physical measurement with any measurement type.  Although FlexLogger gives a bunch of common physical measurements, it also allows for "Custom" which allows you to provide your own unit string, similar to how custom LabVIEW scales work.

Sorry if that was long, but hopefully it explains why FlexLogger shows all these different measurement options for your channels.