FEA Results- No Strain Gages Yet...

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 08/31/2014 at 23:120 Comments

Still waiting on the strain gages to swim over from China. As I was waiting, I did some simple FEA on a left crank arm to support my initial strain gage placement. I plan on using 4 gages to create a complete wheatstone bridge that will [hopefully] measure applied bending moments, and yet be insensitive to pure tensile force (when crank is bottom-dead-center and the cyclist is not actively pedaling). The results corroborate the napkin calculations, that strain magnitudes will be far from the gage's 2.5% limit, and therefore ideal placement should be closest to the bottom bracket spindle to increase sensitivity. The FEA results presented were calculated using a 200lbf shear load applied to the end of the "pedal" perpendicular to the long axis of the crank (first and second image), and also with the 200lbf shear load applied parallel to the long axis of the crank (third image, "pure tensile force"). The purpose of the simulation where the force is parallel to the long axis of the crank was to determine how precise/accurate strain gage placement needs to be in order to guarantee that the gages are only sensitive to bending moment, and not a tensile force. Given the region of very low strain along the midline of the crank (third image), it should provide the desired result. Fingers crossed.

Hopefully the strain gages arrive soon, and the trial-and-error process of instrumenting the crank arms can begin. In preparation I'll purchase some hobby-quality 2-part epoxy (Home Depot, Lowes, Ace, etc), and cyanoacrylate + hardener glues. CA glue will be used to adhere the strain gages to the crank arm, while the epoxy will form a protective shield in the inevitable event that I drop it. Note that I'm not purchasing from Omega or the like. Nothing against them, but as cost effectiveness one of my main deliverables, I've decided to take the risk in hopes of finding and demonstrating lower cost alternatives that perform equal to or better than traditional suppliers (Omega, Vishay, etc).