I’m involved in a project to build a pedal powered aircraft (no, seriously!) and we want to add a power meter.
Commercial cycle power meters are available, but are all very, very expensive and most of them still seem to be in the development stage. Power meters also seem to be popular in the hacking community, but it seems they’re a lot more difficult than they first appear (there are plenty of blogs/website that describe work-in-progress, but few that describe a finished product).
There seem to be several approaches to this problem including..... Measure the pressure on the pedals Measure the strain in the crank. Measure the tension in the chain.
I want to try something different. I want to place a pulse-counter of some sort on the crank by the pedals, and a second similar pulse counter on the drive shaft by the propeller. At low power I’d measure the time between the pulses produced by each device, as the power increases, the tension in the drive mechanism will increase, the drive shaft will twist, the chain will stretch and the time difference between the pulses should increase.
The time increase should be proportional to the torque, which should in turn be proportional to the power.
The problem with this method is that it would rely on very accurate timing information, I had planned to use a magnet and hall probe to count each pulse, but I understand from looking at some technical data sheets that hall probes aren’t suitable for this because the magnetic flux that triggers them can vary with environmental fluctuations.
In summary, what can I use to accurately and repeatably measure shaft rotation?
Comments on what I’m planning and/or alternative methods of doing it are also welcome.