Strain Gage Placement II

A project log for Human Power Output Measurement Device For Cycling

A training tool that provides real-time human power output and form feedback

valgusvalgus 08/13/2014 at 16:500 Comments

Aforementioned napkin calculations suggest that the 2.5% strain afforded by the purchased strain gages is an order of magnitude more than the peak strain induced in a crank given a 2224N pedal load (331Nm equivalent at bottom bracket). I'll still proceed with creating a FEA to provide a more accurate representation of strain three dimensionally. This will take into account the axial torsion due to off-axis loading (pedals aren't in-line with cranks). The lesson learned is that the strain gages should likely be placed as close to the bottom bracket as possible to improve sensitivity.

Purchased 3 crank arms that will arrive this Friday, and will subsequently be instrumented once the strain gages arrive.

Currently designing a test rig that will rigidly hold a crank arm so that the device may be calibrated using an Instron servohydraulic load frame.

While doing some research, Keith Wakeham's previous Hackaday project surfaced. Tremendous amount of work (quality and quantity), yet his blog suggests that work has stopped. It seems a true open source power meter has yet to be created. I think getting away from the usual ANT+ devices (e.g. Garmin) and using smart phones will provide greater flexibility in software design, and allow anyone to produce novel training programs utilizing the power meter. Also I want to do everything possible to avoid using an intermediary microcontroller between the instrumentation amplifier and bluetooth device. This is mainly to reduce power requirements, but also to simplify the circuit, minimize size, and keep component/manufacturing costs low.