Electric bikes can be operated just like normal bicycles, but they can also be power-driven by a throttle or pedaled with the help of 'pedal-assist' (or 'PAS').
A torque sensor is a type of throttle control that determines how much power to apply to the motor depending on how hard the rider is pedaling. Electric bikes are not efficient when accelerating up to speed. A torque sensor allows the rider to help the ebike when it needs it the most: when it is accelerating and when climbing.
An ebike with a torque sensor also allows the rider to utilize the bike’s shifter gear train to find just the right gear where you can maintain the desired speed for the desired pedal input, just the same as riding a regular bike. Riding in pedal-assist mode, and not using a throttle only, increases the ebike’s range before recharging the battery becomes necessary.
A cadence sensor measures simply how fast you are pedaling. Simple cadence sensors are found on most pedal assist system (PAS) bikes on the market today. There is a safety consideration with a cadence sensor. Imagine you are pedaling hard on a cadence pedal assist system and your chain slips off, which results in you doing some fast rotations on the pedal crank. This would tell the bike to give you full throttle, and possibly propel the ebike into an accident.
If I was going to design an ebike using an Arduino Uno R3 then I would first look at existing ebikes already designed, built, and marketed so as to know what products and features already exist. We are free to use the good ideas/features of others without infringing on patent rights. Here are two ebike makers that offer a wide variety of automatic controls:
http://www.currietech.com/help-and-support/electric-bike-e-facts/
http://ezeebike.com/bikes-and-options/
For those not familiar with ebikes, this Wikipedia page gives an introduction, definition of terms, and a general overview:
http://en.wikipedia.org/wiki/Electric_bicycle
