Load Cell Configuration Issues

[Matthew_Kelton]

You stated earlier that when you didn't use the SG24 and powered the unit seperately, that the unit appeared to work, just the signal level was in the mV range.  If you saw that voltage went up when you applied a load and held steady, then the load cell is working and the leads are correct.

Looking at the SG24 manual, the manual indicates the module doesn't work in differential mode in the quasi-schematics.  They appear to use a diff op-amp circuit to get it to SE.

When you tested the 10V output, was that with the load attached?

The SCC-68 manual is a bit confusing in the NRSE section, but states that you need to short AISENSE to AIGND on p25 (after stating that you shouldn't do it, but page 16 says it must be done - maybe a typo on p25 - should say "if you DO NOT have SCC modules installed"?).  Has that been done?

Since you have a terminal block for the AI as well, you can probe to make sure the signal is making it through the SG24 as well (i.e. it is not damaged).

Matthew

[CAS_MattF]

Matthew, thanks for you additional comments.  To clarify my early statements, the load cell seems to work fine in mV range was using the SG24 in Reference Single Ended (RSE) mode.  This produces a usable voltage in mVolt range that I can use to make a measurement.  All measurements were made using the SCC SG24.  In my opinion the load cell is fine and operating correctly.

Measured 10 VDC with the load cell attached.  No, I have not tried to short AI sense to ground and didn’t see that in the manual.  May explain why it is a float condition.  Also, my NI FE suggested a pull down resistor (1 meg ohm) to ground.  Need someone from NI to give direction on this.  I can’t afford to blow out any equipment.

The issue to me seems to be two fold:

• In MAX using the SCC SG24 the only mode available is NRSE mode that causes the signal to float
• I can use just dev1/ai0 in RSE but don’t get the amplification that was the reason for buying the SG24 module

[Matthew_Kelton]

According to this manual, which I think would be the same one that came with your SCC-68, on p 16, it states that if you are using SCC modules you must do this.  This would explain why it works in RSE and not in NRSE.

I'm not sure the 1MOhm resitor on the input signal will do you any good, as the diff op amp circuit will still give you a NRSE output.

[CAS_MattF]

Good catch!  I was looking for SG24 / Load Cell specific information.  Now I have a signal that is essentially 0 with no load and no drift.  Thanks!

Last question, in MAX shouldn't I see the singal level at 100x?  I am applying about 150 LB to a 500 LB load cell but still in millivolts.  I understand that I need to multiple the voltage to get is converted into pounds.  Just expected the voltage to be in the order of volts and not millivolts.  See attached images.

Thanks again!

.

[johnsold]

Matt,

That looks about right for an unamplified load cell output. The transducer scale is usually a few mV/V at full scale load. The mV refer to the signal connection and the V to the excitation.

Lynn

[CAS_MattF]

Thanks ALL, I am just trying to do it right and appreciate everyone's wisdom.