Load Cell & Strain Gage issue

[Ycano]

I'm using Omega LC202 (load cell) with NI SCXI system.

I've used the load cell with NI DAQCard-6042e (PCI DAQ System).

However, I just changed the PCI DAQCard with SCXI-1600 (USB DAQ system) because of a defective issue.

Signal setting in data neighborhood of MAX was "voltage" and it worked fine.

But now, signal only works on "Strain" setting in MAX, since I changed PCI DAQcard to usb DAQ system.

When I test the load cell as "voltage" in Data Neighborhood, signal is flat and no response.

Only works as "Strain" in the MAX.

If I use "Strain" for load cells, how can I scale the signal? (microstrain > lbs.) 

Any comments or suggestions are greatly appreciated.

Thank you.

Message Edited by Ycano on 05-05-2008 12:12 PM

[BrowningG]

Hello Ycano,

There are a few options when using a load cell with DAQmx.  First, what SCXI device is your load cell connected to?  You have specified the controller, but not the signal conditioning module that the load cell is actually attached to.  You can continue to use a strain task for you measurement.  The output of this task will be strain (a unit less) quantity, but you can affect the scaling of the measurement by changing the strain gauge information for the task.  Most importantly, the input called gauge factor which specifies the sensitivity of the strain gages and relates the change in electrical resistance to the change in strain. Refer to the sensor documentation to determine this value.  This tutorial shows each of the equations used by DAQmx to convert the read voltage into strain based on the strain gauge parameters provided when you setup the task.  By changing the gauge factor you can see how DAQmx will change the change the scaling of the load cell.  You can also use a custom scale with your strain gauge task to change the scaling of the output as well. 

A better task to use would be the custom voltage with excitation task type.   This type of task will apply an excitation to a sensor and read voltage from the output.  From this you can use a custom scale to convert your voltage reading into force by using the scaling factor provided with the specifications sheet. 

[Ycano]

Thanks Browning.

My system: SCXI-1000 (SCXI-1520+1314 & SCXI-1600) with load cells.

As you mentioned, I agree that a better task to use would be the custom voltage with excitation task type.
However, I don't know why there is no signal response on the custom voltage excitation task type.
Only strain task type works in the MAX.
Why I want to use the custom voltage excitation task type is the convenience of scaling (volt to lbs).
Actually, I already finished the scaling work.
But, in terms of strain task type,
I have to scale by manual (need to make a calibration table).
However, Strain task is fine for me, though.


Many thanks,

[Ycano]

Thanks Browning.

My system: SCXI-1000 (SCXI-1520+1314 & SCXI-1600) with load cells.

As you mentioned, I agree that a better task to use would be the custom voltage with excitation task type.
However, I don't know why there is no signal response on the custom voltage excitation task type.
Only strain task type works in the MAX.
Why I want to use the custom voltage excitation task type is the convenience of scaling (volt to lbs).
The range of scale (min & max) was derived from +-0.2mv (Output) & spec. of load cell (Min & Max lbs).
So, I could see the signal graph as force value scaled from voltage value.

However, I have no idea about scaling of strain task type.
How can I input Min & Max of Strain value?
Do I have to scale by manual (need to make a calibration table)?


Many thanks,

[BrowningG]

Hello Ycano,

What voltage range do you expect from your load cell?  It could be that the output voltage is small and you are not seeing the proper scaling on your output.  If you use a custom voltage with excitation task, you can scale the voltage read by your device by using a scale.   The screenshot below shows how to start making a custom scale in MAX.  Follow the steps in the wizard to create a scale.  One of the scale types is a linear scale where you can multiply the reading by the module by a constant and then add an offset.  Check out this knowledgebase or more information on a custom scale.  Also, the DAQmx Help (if you are using windows in Start -> Programs->National Instruments -> NI-DAQ -> NI-DAQmx Help) has more information on scales as well.  Search for custom scales on the Index tab to bring up the list of topics involving custom scales. 

A full bridge type I strain gauge converts a specific voltage ratio into strain by using only the gauge factor.  If you open the DAQmx Help and use the index tab to search for strain a list of topics regarding strain will come up.  The first subsection is bridge configurations and goes over the ways that DAQmx calculates strain based on your particular configuration.  The Full Bridge Type I uses only the gauge factor your specify to convert the voltage ratio (specified at the top of the help section) to strain (the output of the measurement).  By specifying the gauge factor you can scale your voltage reading by multiplying it by a constant.  You can also use a custom scale with this type of task to convert the strain reading into pounds. 

When you create a new task of any type, there are two inputs where you can specify the maximum and minimum input limits.  These input limits are always specified in the units of the scale you are using.  For instance, if you have a custom voltage with excitation task with no scale specified, the units are in volts.  If you are using a custom voltage with excitation task with a scale that converts the volts to pounds, the units are in pounds.  The input limits for a strain task are specified in strain.