Monday, February 25, 2013

V3 Upgrade–New Right Leg Sensors


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.


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.




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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