Overview

Design Objective

The objective of this design project was to integrate automatic continuously variable transmission, (CVT), for bicycles along with automatic gear shifting. Continuously variable transmission supports a continuous range of gear ratios, as opposed to a fixed set of gear ratios, and adding automatic transmission alleviates the user from the responsibility of deciding on when to switch gears. The objective of the automatic transmission is to find the best gear ratio to allow the user to pedal at the desired cadence, as specified by them.

Our solution

Mechanical

Mechanical Design

The key to this design is that the radius of the sliders can be expanded or contracted. The outer side of the sliders will be where the teeth will be placed and where the belt would rest. The sliders would acts as a gear or pulley. The rotation of the outer plate would be finely controlled by a motor and therefore be able to adjust the diameter of the gear with infinite amount of steps.

Electrical

As mentioned previously, the target is to keep user cadence as constant as possible. In order to know this value, a reed switch will be used. The reed switch is an electromagnetic switch that closes when it comes into contact with a magnetic force. Magnets will be placed on the near outer diameter of the bicycle wheel and this will allow the Arduino controller to receive signal inputs per bike wheel revolution, which can be converted to a rotational velocity. A filter and schmidt trigger circuit will be used in order to eliminate noise and to debounce the input signal to further increase accuracy.

Software

The basic motivation behind the algorithm that will be implemented is to help the user maintain their current linear bike speed, while adjust the gear ratio to maintain their desired cadence. The algorithm will multiply the current gear ratio by the the ratio between the desired cadence and the current cadence to get the new gear ratio (shown in the figure below).

A software simulation can be shown below, where the bicycle speed changes, but the biker’s cadence is kept constant (from 10 to 30 seconds). From 0-10 seconds and 25-38 seconds, the biker leaves the supported output speed range for a desired cadence of 80 rpm, and the gear mechanism is unable to keep the bike cadence constant.