Load Cell NI 6259

[libindaniel2000]

I am dumbfounded with this problem.

I have a load cell attached to NI 6259 via 'Custom Voltage Using Excitation Voltage"

Settings: Full Bridge, External excitation of 10 volts.

The setup is: +- excitation goes to an external power supply and +- signal goes to DAQ (channels, say 20 and 21). It is grounded with the power supply ground as shown in this figure: http://www.omega.com/techref/das/loadcell.html

When the signal was checked to scale with Ex, I get no change in signal when load is applied on the load cell.

When I deselected the scale with Ex, it worked fine. The rated output is 1.4mV/V so I gave the maximum and minimum value of 15 and -15 mV. I calibrated it and I got a Standard Deviation of less than 0.2%.

Now, I added another load cell to the DAQ (at a different channel) along with a pressure transducer (RSE: ground and signal) and a RPM signal (RSE: signal and ground). All of a sudden, the load cell stopped giving me signal variation with load. It stopped working! I played around with the settings shown above but nothing changed.

Am I missing something? Do I need a singal conditioner. Do I need a 'strain gage excitation module' kind equipment?

[Basil-B]

Hi Daniel,

can you post your code so we can have a better idea of the way you have it set up in LabVIEW? Also, is your second load cell connected to an external power supply with proper bridge set up as well? What do you mean by scale with EX? When you remove the second load cell does the first one start giving you readings again? (assuming you are not in scale by ex). 

Regards,
Basil Beirouti

Applications Engineering

National Instruments

[libindaniel2000]

By 'Scale with EX', I meant scale by ex.

Here is the attached code.

When only one load cell is attached, it still does not work.

The second load cell is connected to the same power supply. It is a regulated power supply unit that is putting out 10 V. The excitation voltage wires (both + and -) from the two load cells are connected to the power supply's +ve and -ve wire terminals. Both the load cells are also grounded to this power supply.

Both are configured in the same manner: full bridge, 10V external, differential, scale by ex. Both the load cells are LSP series from Transducer Techniques, one is a 10 kg load cell, the other is a 2 kg load cell, for thrust and torque, respectively.

[Basil-B]

Hi Daniel,

I took a look at your code and there is a lot going on there. It is not recommended to have nested while loops, it makes it hard to determine what will happen and whether the outermost one will run more than once. Second of all, I see two different DAQ read VIs, one of which has an array output that turns into dynamic data and there are 4 channels coming out. I thought you were only using two. At this point anything could be going wrong in your code, I'm surprised it's not erroring out or anything like that. 

Regards,

Basil Beirouti

Applications Engineering

National Instruments

[libindaniel2000]

I had a feeling that the nested while loops might not be the way to go. I will fix that. I am using 4 analog inputs: 2 load cells, 1 pressure transducer and 1 RPM sensor.

So, there is no way to hone down the possible reason for the load cells not giving accurate voltage readings?

[Basil-B]

Hi Daniel,

Can you try to make a simpler version of the code which just has one while loop and a multichannel continuous read VI. Verify that both load cells give accurate readings there and then we'll build on that. If we're troubleshooting reading in the signal then it's better to do so in a VI that does only that, rather than also doing a bunch of post processing and analysis. Let's get something that gives you the readings you expect first and then we'll work on building up the analysis functionality you need.