DAQ USB 6008 and Measuring mV Output from a Load Cell

[redtop91]

Hello NI. I am attempting to measure mV output from Tension load cells and I was wondering what settings needed to be adjusted in the DAQmx Assistant to give the type of resolution I need. I read the thread here: http://forums.ni.com/t5/Multifunction-DAQ/USB-6008-mV-data-acquisition/td-p/483514 but I was unable to understand what the OP's solution ended up being. Did he simply adjust the Max and Min voltages from DAQmx Assist (Pictured in attached JPG. No Flash on this PC and no Admin rights to install for Image Insert. Sorry)? My VI is setup to both display and output voltage readings to an Excel Spreadsheet in CSV format. The graphical representation isn't really important to me but I need the data to be accurate. Will Labview Log this accurately in my CSV file without any adjustment? Thanks in advance. I'm a Mechanical Engineering intern with limited knowledge of electronics outside of a basic Electrical Engineering Intro course so laymans terms are appreciated. 

[Eric_L]

Hello,

The USB-6008 has 12bits of resolution. This means that there are 2^12 divisions in the voltage range. When using a larger input range such as -10 to 10, the voltage resolution would be 20/((2^12)-1). Using a -1 to 1 voltage range would mean having 2/((2^12)-1) voltage resolution. You would still have to account for noise in your signal which makes reading small signals difficult. You may want to average multiple points to help reduce the noise in your measurement.

Eric

[SnowMule]

Or amplify/condition the signal before feeding it into the DAQ. 

Op-amps are pretty easy to work with and it overcomes a lot of the loading issues with the low impedance of the 6008's inputs.  An Instrumentation Amplifier circuit will also work for your application.  Off-the-shelf load cell amplifiers are also available.

[redtop91]

---

@Eric_L wrote:

When using a larger input range such as -10 to 10, the voltage resolution would be 20/((2^12)-1). Using a -1 to 1 voltage range would mean having 2/((2^12)-1) voltage resolution.

---

I'm sorry I don't follow those numbers can you clarify them? I'm sorry I'm fairly new to Labview.

---

@Eric_L wrote:

You may want to average multiple points to help reduce the noise in your measurement.

Eric

---

I took the output data from the DAQ and input it into an amplitude and level measurements express VI then output the cycle average to my CSV file and it provided a LOT more clarity but now I'm uncertain of what cycle the express VI is using for this calculation. Also this cut down on the data points output to my CSV file which worries me a little.

[Eric_L]

Hello Redtop,

I was just illustrating how a smaller voltage range provides smaller voltage steps and better voltage resolution. The following article does a good job in explaining it if you would like to learn more about it.

http://en.wikipedia.org/wiki/Analog-to-digital_converter

You will want to set your voltage range to -1 to 1 which is the minimum voltage range for your DAQ device. This will give you better resolution of small voltage signals.

The Express VI (Amplitude and Level Measurements) that you are using will average the data for one cycle of a periodic signal. I would recommend using a point by point average (mean pt by pt) with a set sample length since you signal is not really periodic.

Eric

[redtop91]

Ok that's moving me very close to a solution. One last matter is that the voltage I'm logging is being logged simulateously on 3 channels from 3 different load cells. The Point by Point Mean seems to have averaged all 3 voltages and made them into one stream of CSV values. How do I keep these separated?

[Dennis_Knutson]

The point-by-point function has a scalar for an input and you would obviously need to have three separate ones for each channel so I don't see how you could be combining them. Attach your actual VI.

[redtop91]

Here's my VI

[Dennis_Knutson]

There is no mean function there, The point-by-point would be inapprpriate anyway. You would want to index eah column and use the regular mean function on each.

[Eric_L]

Hello Redtop,

If performance is not a major issue, you can use a for loop to index the array and use the point by point average VI. This will allow you to have a moving average from a fix number of data points.

Eric