Load Cell Configuration Issues

[mfitzsimons]

I have an issue configuring a Lebow load cell.  I have read as much NI documentation that I can find but it is not operating properly.  I have an SCC-SG24 installed in a SCC 68 box.  I have configured MAX to use the SG24 Strain Gauge Module.  My primary problem is that when there is no load on the Lebow Load Cell the measurement will float constantly upward and will not level off.  The documentation seems to contradict itself by saying it operates in differential mode but only RSE is available.  Also, I am getting a negative deflection even though I have everything wired correctly.  I am using the load cell with NI’s example program Aqu Wheatstone Bridge Samples (With Calibration).vi

I am new to the world of load cells so some help would be appreciated!  See attached document for images of problem.

[Matthew_Kelton]

Most load cells like this I have used are already conditioned, so you don't need the SG24.  The ones I have used in the past will have 4 or 5 wires.  Power +/-, Output +/- and possibly a ground.  You power the unit on the Power +/- with 10V (whatever the manf. spec), and you meas on the output +/-.  You may need to do some conditioning for the small signals, you may not.  There was no wiring notes in the spec sheet you posted.  Do you have that info?

[mfitzsimons]

Matthew,

Thanks for the response!  Wow, I thought I needed signal conditioning.  It has 4 wires on the load cell schematic (physically on the load cell).  Standard Wheatstone bridge configuration.  Two for excitation and two for signal marked with both marked as +/-.  So does this imply that that I just power the excitation lines with 10 volts and run the signal lines directly back to an analog I/O?  That was actually working well but the singal level was very low in the order of millivolts.

[mfitzsimons]

Matthew,

Thanks for the response!  Wow, I thought I needed signal conditioning.  It has 4 wires on the load cell schematic (physically on the load cell).  Standard Wheatstone bridge configuration.  Two for excitation and two for signal marked with both marked as +/-.  So does this imply that that I just power the excitation lines with 10 volts and run the signal lines directly back to an analog I/O?  That was actually working well but the singal level was very low in the order of millivolts.

[Matthew_Kelton]

That's what you can do.  That was the signal conditioning I mentioned you may need to handle mV.  Since you have the SCC cage, you could use the SCC-AI06 or SCC-AI07 which have 100mV and 50mV inputs.

[mfitzsimons]

Thanks again!  I will talk to NI about an exchange.  Seems I have the wrong tool for the job.

[Logan_K]

I don't have time to give you a detailed response right now, but I wanted to jump in and say that I do believe you have the right module for measuring your load cell, and that this issue needs to be debugged further before deciding on new equipment. Could you reply with your specific wiring configuration?

Thx,
Logan K.
National Instruments - R&D

[CAS_MattF]

Logan,

Agree, I think it is the right one because it is the only one that supports 10 V excitation.  Everything is attached in the original post (look at max configuration).  Load cell is four wire labeled + Exc, -Exc, +Sig, and - Sig. Excitation measures 10 VDC.

[RavensFan]

Double check to see that the load cell wires are labelled properly and don't have a problem such as a broken lead.  Since it's a 350 ohm bridge, you will see 350 ohms if you measure across the Exc + and - leads, and likewise across the Sense + and - leads.  Any other combination of leads should show up as about 263 ohms.

Also, once you have it working, but if you find you get a negative load when you want positive (normally tension would be positive and compression would be negative, but it's really up to you how you want it for your application) you can swap either the sense leads with each other, or the Excitation leads with each other, and that will swap the sign of the incoming signal.

[CAS_MattF]

Thanks!  As stated I am new to load cells so tips are appreciated.

I measured the resistance and it is 350 ohms across +/- excitation and 350 ohms across +/- signal.  I physically opened the connector on the load cell to confirm the wires to the pins.  I am confident it is connected correctly.

The physical connections I have to the SCC SG24 are:

Exc – to SCC Pin 1 V(ex)-

Exc + to SCC Pin 2 V(ex)+

Sig - to SCC Pin 5 ACH(x)-

Sig + to SCC Pin 6 ACH(x)+

I am using a NI supplied external power to the SCC68 since I have four modules plugged in.  I have verified that the SG24 V(ex) is putting out a 10 VDC voltage.  My expectation was to select SCC Mod1/ai0 and I would read back the voltage at 100x gain that could be converted to a load.

I made several measurements using MAX and this is what I know.

• SCC Mod1/ai0 will only allow NRSE mode.  This is interesting because the manual states it should work in differential mode.  The measurement starts at 0.0835 volts and floats constantly upwards.
• Dev1/ai0 in Diff mode has a no load range of -0.54 to -0.63 volts bouncing between the range
• Dev1/ai0 in RSR mode has a no load range of 0.007 to 0.0095 volts bouncing between the range
• Dev1/ai0 in NRSE mode has a no load range of 4 volts and floats constantly upwards

So, what is the right mode to use with the SCC SG24?