NI 9237 excitation voltage and remote sense

[MGodin]

What are the limitations of the NI 9237 Bridge AI module with regards to excitation voltage level and remote sense?

My configuration:  Two (2) 350-ohm full-bridge xducers, which I want to excite with 5 VDC (I don't want to drop excitation to 2.5 or 3.3).  I calculate that the power requirements of the xducers will total ~143 mW.  I note that NI recommends that the total power is kept below 150 mW for the '9237.

The xducers will be located at a distance of 50 feet from the NI 9237, connected via 24 AWG lead wires.  The Remote Sense (RS) leads will be connected at the input terminals of the xducers.

My understanding is that remote sense is typically used to force the power supply to counteract the lead wire voltage drop by raising the voltage at the load input terminals. If successful, it will exactly cancel the drop along the leads, yielding the correct voltage on the input terminals of the load.

However, I assume there must be a limit to how much the '9237 can offset the voltage drop.  RS will probably work as advertised at 10 feet, but fail at 1000 ft.  Right?  So how do I determine if my proposed configuration will work as I intend?

[Wilfried_71]

Where did you see your specifications of length ? (10 feet vs 1000 ft ?) Because I don't find this in the product manual of the 9237 ...

In my opinion, the RS permit us to correct the resistance of the wires and the unbalanced of the bridge due to the difference of resistance between wires and strain gages. So I think you can plug without problem in this configuration. In you want to calculate the dissipation power in the wires : R x i2 ... Or you can plug a power supply in the external terminal block.

Bye

[MGodin]

My apologies for misleading comment...

I did not see any specification of length (10 vs 1000) in the product manual, I simply took a guess by way of example.

Yes, I suppose I could calculate power dissipation and adhere to the 150mW limit, but I was hoping to receive a more definitive spec from NI, rather than assuming.  In my experience, power supply manufacturers often list RS spec as, e.g., "compensates up to 0.5 V drop per output line..."

Yes, I agree I could use external excitation, but my goal is for absolute minimal equipment to support field investigations (less to carry around).

Thanks for the reply. 

[Wilfried_71]

Ok, I think the RS are designed for such problem ... and for me 50 feet is not so long !

[Hani R]

Hi MGodin,

Thanks for contacting National Instruments.  The remote sensing on the NI 9237 does not result in an adjustment of the excitation voltage.  Remote sense will simply be used to compensate for the lead resistance. The resulting measurement will be compensated to remove the gain error caused by the lead resistance.  You would still need to adhere to the 150mW limit of the device to ensure that you receive accurate data.  As already mentioned, the other alternative would be to use an external power supply. 

[qNorman]

Hello,

What about the maximum distance from NI 9237 and transducer? I have to measure deformation by a 350 Ohm full-bridge with available 6 wires + shield cable.

regards / s.q. 

[enderWiggin10]

Hi qNorman,

I am not sure that I can give you a concrete answer on the maximum distance of the strain gauge from your module.  However, I can recommend half or full-bridge configuration that way you can use remote sense on the 9237 module.  This will allow for you to account for lead resistance in your connection, and the voltage drop that is encountered in the wire length.  Excitation voltage degredation is the main enemy in the setup, which remote sense accounts for. 

Best,