Sunday, October 21, 2012

V3–A little too sensitive?

Coming soon, current consumption measurements!


Above is the raw data from the calibration test. I used the setup below with a few weights from my dumbbell that weight approximately 10lbs. Here’s is how it breaks down. As you may realize that the sensors max out at a relatively low load – It’d probably be fine for testing or a 140lbs climber, but not an expected maximum. I have another resistor to set a lower gain, so I might be able to adjust this easily.

What should the maximum per pedal load be. Let’s take a worse case scenario of a 1800 watt sprinter. We’ll assume 172.5mm crank arms and 90rpm.

P = T * w = 1800 = T* (90/60 *2*pi) => T = 191 N-m

T = Lever arm * force => force = T/L = 191/0.1725 = 1107 N = 112kg

Surprisingly my max load with margin is approximately this! However we’d have to account for the downstroke being stronger than the upstroke, so the best way to do this is assume one leg is doing all the work. So in short, I need to reduce my gain to about 125 from 250 approximately. Although I might use different gains as swapping the differential pins for the Right should give me back some sensitivity.

  Left Right
Zero (bit) -13273.2 -5097.73
Loaded (bit) -17315.5 -7141.56
Unloaded (bit) -13286.1 -5104.38
Difference (bit) -4042.31 -2043.82
Load(N) 44.59091 44.59091
Torque (172.5mm) 7.691932 7.691932
Bits/N-m -525.526 -265.71
Max Torque (with margin) 83.80372 96.60148
Load (kg) 49.5228 57.08548

So I realized a couple of things. Switching the polarity of the supply and ground on the strain gauge also flips the direction it changes when loaded, so I need to swap the signal outputs for the right sensor as it goes negative when positively loaded. The problem is I was hoping to use more of the ADC range by using a lower ref voltage (e.g: –10000 to +32765, instead of 0 to 32765 which allows me to use a higher gain and thus add accuracy). I don’t know why I didn’t realize this in the last post. I’ll look at this fix tomorrow or Tuesday.

I’ve left it running for approximately an hour and it’s holding steady at –13273 +/-4 and –5097 +/3.


A little two sided tape holds the board on nicely two the back of the crank arm. Should nicely clear the crank ring too!


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