Strain Gauges to measure torque

[beerco]

I'm having an argument with someone on another forum regarding the accuracy of some torque measuring devices (namely two power meters used in bicycling - the power tap vs. the SRM.)

The question I have is when measuring torque on a tube, why would you use the set up here which uses 4 sets of two gauges :

http://biketechreview.com/archive/powermeter_files/image015.jpg

as opposed to the torque setup described here:

http://zone.ni.com/devzone/conceptd.nsf/webmain/E4D21D414901984486256CB7007115AC

when the latter uses 4 fewer gauges?

Is it something to do with the way the measured area is machined flat?

Secondly, would this setup be more or less accurate than a single (4 gauge) full bridge setup measuring bending torque?

I'm an EE, but don't know too much about strain gauges and how to correctly apply them.

Thanks,

-Andy

 

[itsmoked]

You pose an interesting question....

I would expect that the Powertap maker is striving for more accuracy and higher sensitivity especially if this cylinder is supporting a load. Four would allow nulling of the bending moment.

It looks to me like your second link prefers 45 degree full bridges crossed on opposite sides(last example) whereas the photo shows half bridges in four places.

It has to do with the physical size the guages have to be verse the size of the cylinder they are monitoring.

Picture this: The cylinder has a bending moment across it because the axle is holding up something heavy. The bottom of the tube would be in tension the top would be in compression. There would theoretically be spot on the sides that have neither. But on a small axle you would need a reeellly small guage to be there. Since these guages can't be that small they will be using some of their range seeing bending moment. Hence they are probably trying to get more resolution and reduce the cyclic amplitude by using four half bridges instead of two full bridges.