Saturday, September 21, 2013

Enclosure and Carbon Fibre Cranks


The enclosure is getting closer to what I want. I’ve ditched the wrap-around plastic on the drive side arm and I’m contemplating using a wire from the BB Axle (plastic cap) to the electronics enclosure. This is similar to Pioneer’s design. It’s not great but it’s easier and may help with speeding up the process to beta and compatibility initially with other cranksets.


As for calibration of the carbon fibre crankset, it looks very linear at least initially. Way to go SRAM for QA. This’ll make the comparison of the Quarq unit to Accuity so much easier. It’s expected to be linear as a new crank but my background research shows that at higher usage factors it’ll go non-linear. I can’t get the specific answer out of any manufacturer I tired so far. If it goes non-linear it can be calibrated out, however a much more sophisticated calibration is needed – unlike the simple hanging weight test.

A load cell rated to 1000lbs is tested at multiple points including 1000lbs and quotes linearity. If anyone ever gets a power meter from me they will be getting documentation on the calibration values and it won’t just be a single weight test. I want it tested to at least about 170N-M of torque which correlates to a 100kg person’s entire weight.


The calibration values all come out to be nearly identical. The weights that were used were measured on a calibrated scale at work and confirmed (to the 0.1 gram) with my household scale to the gram. The linear regressions are almost perfect


I plotted the R^2 values. Looks nice and linear to me. Also, keep in mind that the number of counts is only on a 16 bit scale. I’m capturing at 24 bits, but  the last few are noisy. At 16 bits it’s noise free (it’s actually noise free to 18 bits, but it’s hard to do 18 bit math with a 32 bit microcontroller without pointless programming).

Back to the carbon crank, until there are some fatigue cycles on the crank, a new carbon fibre crankset is giving reliable and accurate readings. Time will tell once the carbon starts to fatigue.

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