Constant current excitation for strain gauges

[Peter W Brown]

Ideally I would like to log 46 strain gauges using a scxi-1000. At the moment we have two 1102B modules, one 1100 module, and a 1122 module.

The 1122 module supports quarter bridge strain gauges but is very limited in scan rates when logging 16 strain gauges all at once.

Stuart from ni gave me an idea of constant current excitation for the strain gauges. The 1581 would be very suitable seem it delivers 32 channels of costant current and would suit the 32 channels of the 1102B we already have. Only problem is whether the 100 micro amps will be sufficient current to excite the strain gauges with reasonable accuracy.

The remaining strain gauges that wouldn't fit into the 1581 would run on a 6 channel strain gauge amplifier and then run through the other 1102 module we have and use a datataker for any left overs.

Any other suggestions? I have looked into buying either the 1521B for 24 channel quarter bridge or two 1520's but am unable to purchase these at the current time.

Thanks for your time.

Message Edited by Peter W Brown on 01-11-2006 09:55 PM

[Logan_K]

The full range of a strain gage in most applications is going to be less than 1 ohm, which means that a typically strain application needs to be able to measure the output on the order of fractions of an ohm to be even remotely close to accurate.

The reason strain gages are typically put in a bridge configuration is so that their output can be centered around 0V which enables the signal conditioning module to apply very large amounts of gain to the measurement (up to 2000 on the SCXI-1520). Without this gain, the sensitivity of the strain gage is so small that you will be very hard-pressed to get remotely accurate readings.

For this reason alone, I don't think there is a current-excitation solution available that will enable you to get sufficient resolution from the strain gage reading.

The SCXI-1521 is probably your best solution for measuring a large number of quarter bridge strain gages. If you also need to measure other bridge configurations (i.e. half or full) then the SCXI-1520 is your other option.

-Logan K