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Measuring Force on Class Project

by mcnaryxc
Tags: class, force, measuring, project
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Feb17-12, 04:10 AM
P: 5
On my senior project, i've figured out everything I need to measure all the parameters I needed EXCEPT force. I know that I need to use strain gauges, and I've used them in my instruments class with Labview and a DAQ. While I'm still using Labview, i'm now using an Arduino so things are a little different. But, I think the problem is coming down to how I'm trying to measure the strain to convert into force.

The best way to think about what force I need to measure is a standard horizontal cantilever beam. This beam is made out of 6061 Aluminum and is 11" long by 1" wide by 1" thick, so it's pretty thick. One end is fixed, other has a single force pushing up. Because the beam needs to handle 75lbs of force, a 1/2" steel bolt is used to hold the beam fixed, drilled into the beam about 2".

So here's my problem. My class never taught me how to attach strain gauges. The class also used a beam that was a lot thinner than this. I bought some strain gauges from Omega that are 8mm by 2mm, single-axis designed for aluminum. I researched how to attach the gauges, so I glued two down using superglue (cyanoacrylate) and soldered wires to them. The two are right in the middle of the length of the beam, one on top and one on the bottom, so I could make a Wheatstone half-bridge. I glued them by aligning the gauge as best as I could, used clear tape as a hinge, and cleaned the surface to be glued on with rubbing alcohol. After soldering, I measured 350 Ohms on both gauges, like they are designed to have. I wired it all up like a half-bridge, put the ends I need to measure through an instrument amplifier (AD623), and then zero'ed out the voltage reading. When I tried to put force into the beam, the voltage reading stayed at zero (with a little noise from the amp). I know I have everything hooked up correctly since I suspected that was the problem first. I have the amp maxed out with a gain of 1000, but lower gains don't cause any change. It's still not working. And in case your thinking it, I can't use a load cell outside the beam.

I'm stuck with a bunch of questions which leave me buying more gauges. Before I go buy more gauges, I have some questions. Am I measuring in the wrong spot? Should I be measuring as close to the fixed end of the beam instead of the middle? Am I using the wrong adhesive? Do I need bigger strain gauges? Should I use a full-bridge instead of a half? Thanks for any help and suggestions.
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Feb21-12, 10:14 AM
P: 100
For a cantilever beam, deflection is always maximum at point farthest from attachment while stress is maximum at a point near attachment. But I wonder if you are to measure deflection alone, why dont you consider dial gauge for this purpose. Use it along the length of beam and get deflections

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