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Pedal load sensor/ power meter for auto shifter.
I’ve started another thread for this one but its really part of the automatic shifter project. I’m hoping to make it capable of being a power meter but I think if I can use it as a pedal load sensor for the auto shifter I’ll be happy. It’s an experiment to see what’s possible with the cheapest parts I can get hold of. If something doesn’t work or isn’t accurate enough I’ll get a more expensive version.

It’s based on this design by stoppi71 I’ve focused to start with on a robust, metal to reduce interference and hopefully water resistant casing to house the electronics. It’s definitely going to be one of the heaviest power meters. Hopefully it will succeed where other homemade power meters are likely to fail in that they won’t last because everything is stuck on the outside of the crank.

I’m hoping to place to amplifier circuit close to the strain gauges on the crank itself in its own metal box. This will then need only 4 wires to link with the rest of the electronics in a housing attached like a third chainring.

Power will be provided, as l know it won’t be very efficient, by 2 well used Li-ion 1.5Ah cells from my old Windows phone. In one of the other compartments will be room for an Arduino Nano. The forth is a spare in case I need a larger amp circuit for example or a gyro module to measure cadence. The NRF24L01 wireless module will obviously have to be mounted externally probably just sealed in a plastic bag to start with.

[Image: MSx6iH.jpg]

The crank that I’m using has a well protected area in a corner where the strain should be quite high. To attach the strain gauges I have just used basic super glue, will probably turn out to be a bad idea. They are then sealed with a popular 2 part glue called Alaldite. On top if this I placed a layer of aluminium foil, might block interference from RF maybe, never seen anyone else do it.

[Image: TWZFer.jpg]

I choose to use 4 strain gauges in a full bridge and use 2 as temperature compensation. Partly due to limited space but also other people seemed to be doing it that way and having success. I’m wondering now if it would have been better to use all the gauges to measure strain, would get double the voltage change.

The next decision is how to wire up the wheatstone bridge. The first circuit diagram was one I found on the internet however I calculated that the voltage change across the bridge would be very small.
[Image: vewYLK.jpg]
I started with a temperature compensation strain gauge on each side of the bridge. Initial signs with a multimeter were very good, I could measure a +- 0.1mV difference just bending the crank by hand. Further investigation revealed as much drift from temperature changes as the voltage variation due to bending.

Next I rewired the bridge like this. [Image: TR7Cqb.jpg]
The no load voltage is more stable although not great but I still get the same voltage change under load. To get the correct no load voltage I have connected a resistor across one of the gauges. The amplifier circuit based on a LF353 has been set up with a gain of 3000. This is giving me a voltage output of between about  0.25V - 2.5V.
- Oran
With everything wired together temporarily and a Bluetooth module connected (also temporary while I get the NRF24L01s to work) I uploaded a basic code to the Arduino. It just sends the voltage reading via serial every 50 milliseconds. The Bluetooth graphics app on my phone means I can monitor live graphs and data log if needed.

[Image: pccy9h.jpg]

This is my basic way of checking the response to load. I just roll the bike backwards until the desired load is on the spring balance and then lock the wheel. I was able to get a nice linear graph of voltage against load which I’m happy with. To balance the Wheatstone bridge and get a positive voltage offset I’ve connected a high value resister across one of the strain gauges. Might change it to a potentiometer.

[Image: 1IYKCS.png]

This is a screenshot of a graph on my phone with me standing on the pedal.
[Image: 8MlCid.png]

[Image: rFYgYm.jpg]

All wired ready to be assembled unfortunately the amp circuit is not working since soldering new wires. I also think there is an intermittent fault with a strain gauge or a wire in the Wheatstone bridge. Sometimes the offset voltage increases by a few milliamps and so the amplifier gives it’s maximum output. So close to working but not quite there yet.
- Oran
The amp circuit fault turned out to be my mistake, cut the veroboard track in the wrong place.  With that working I assembled everything for a test ride. The results were a little disappointing as it only worked for a few minutes before the intermittent short came back. I did record a little data which I’m very impressed by, it’s so close to working. Once I get to the bottom of the short it will be mostly code writing.

 [Image: wfwVwr.png]

As I had some data I thought I’d have a go at creating a polar chart of my pedal stroke. I averaged 5 pedal rotations and estimated the angle. Will be interesting to compare my pedal stroke with a NuVinci hub and derailleur gears to see whether there is any difference.

[Image: Zrob1D.jpg]
- Oran
Wow... Missed this one!

Using load cells on the pedals... This bike is soooo connected!
You are about to be the least significant mass on this bike Tongue

I did not have the time to fully read all of the text up there, but I will tomorrow (going to sleep)

Keep up the good work!
Si ça a déjà été fait, je peux le faire
Si ça n'a jamais été fait, donnez-moi juste le temps de trouver comment !


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