Monday, February 25, 2013

V3 Upgrade–New Right Leg Sensors

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So V2 had some superior measurement features. If you recall here I discussed about how applying a torque through my foot caused the readings to skew. So installing a couple of strain gauges like V2 on the top and the bottom create a very good and more isolated sensor. This is ACTUALLY how Rotor Power works, but the gauges are on the INSIDE. I can’t do that, not unless I drilled holes… this would cause stress risers and likely lead to fatigue failure.

As an idiot, I used two gauges from different batches. This caused a mismatch in the wheatstone bridge causing a huge voltage offset. I therefore needed to drop the gain, but I didn’t have a high quality resistor to use, so I had to sub in a 1% through hole. I’m keen to move on to the next prototype with the ADS1247 as this type of issue becomes irrelevant.

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I’m getting much more consistent readings now. The only problem is it won’t read 100% right, about 96% at best. I’m not quite sure why. However the strain gauges are sensitive enough to pick up the weight of the crank arms causing a torque due to gravity. I suspect that this is what is happening, but can’t be sure. I wish I had debug serial access while it was on the bike.

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You can see the old shear gauge still inside. It’s a bit of a rats nest of wires down there.

 

As an aside:

I’ve been sick the last week which has delayed testing and updating. However in my downtime I finally figured out the strain gauge arrangement used by Look Keo Power Pedals! It’s a differential sensor such that it can remove bending moments, that is why it requires 8 strain gauges per pedal. This setup has a decrease in sensitivity but it’s the only way they can make it work. I am thinking about making a video explaining how to do this and why, but it might be a while before I get around to it.

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