This project started internally at Fotonik when Henrik C. Pedersen participated in his first bicycle race with 1500 contestants. Despite the many high-end bicycles, none had power meters. It turned out that the cheapest power meter you could get at the time would cost 2000 EUR, far too expensive for even the most serious amateur. An internal project aimed at developing an inexpensive optical power sensor was started and soon after the external partners Rolf Østergaard and Leif Andersson joined the team. Eventually the spin-off company Sensitivus was formed.
PROBLEM
The cycle power meters in 2014 were all based on electrical strain gauges placed either in the spider or in the rear hub. The electrical signal from up to 16 strain gauges had to be balanced and processed to fully compensate for temperature changes and mechanical twisting not contributing to forward movement of the bike. Therefore the systems were very complicated and too expensive.
SOLUTION
The basic power sensor comprised - in its essence - a thin metal tube with a VCSEL laser placed at one end, a bi-cell photo detector placed at the opposite end and a lens placed in between, which images the VCSEL on the bi-cell. The sensor is fixedly attached to a crank arm, so that when a force is exerted on the pedal (and the crank arm slightly bends), the light spot will accordingly start to move across the bi-cell. By subtracting the signals from the bi-cell the force – and thereby the torque - can be directly measured. The power is then simply found by multiplying the torque with the cadence of the crank arm.
