Torque Sensorless Power Assisted Bicycle System Design

The power assisted bicycle integrates the economy and environmentallyfriendliness, which are an ideal exercises machines as well as a vehicle. The maindifference between a power assisted bicycle and a traditional electric bicycle lies in theway of the power management. The latter is an open-loop speed control system, whilethe former is a closed-loop system. It includes the battery, in-wheel motor, controller,indicator, torque sensor, the frame and so on. The controller receives the signal sent bythe torque sensor and senses the vehicle condition, and then adjusts the power output. Apower assisted bicycle can make you feel bionic and it allows for a more intuitive ridingexperience.The torque sensor is an important part of the power assisted bicycle, but theproblems with it are as follow. Its cost is very high, and the consistency is poor andvulnerable to environmental impacts. In addition, the assembly accuracy of the torquesensor is high and every vehicle needs to be tested individually. So we hope to find asolution to estimate the crank torque without using the torque sensor. The main contentsof the paper are as follows:This paper analyzed the relationship between the crank torque and its position whenriding a bicycle under different conditions. The crank revolution period is divided intofour power out sector and the crank torque at0°,45°,90°,135°and180°issampled. The torque-position curve is depicted by interpolation. By the analysis of thedata the function relationship between the crank torque and the crank position isformulated. So the crank torque can be estimated by sensing the crank speed. Then themathematical model of power assisted bicycle system was established. The powerassisted bicycle is driven by the crank torque applied by the rider and the motor torqueand resisted by the wind resistance, the friction between the tire and the road and thegravity component along the road surface. Based on the calculation of the environmentresistance, the speed control, the torque control and the power assistance ratio wereexamined. Baded on that the transfer function model of the power assisted bicycle wasestablished and a PI-type controller was designed. The command of the controller is thecrank torque divided by the torque constant. The difference between the command andthe current feedback is feed to the PI-type controller whose output is the PWM dutycycle. At last the Matlab/Simulink model of the overall system was established and thesimulation was performed. The simulation results indicate that the motor output tractsthe crank toque and the motor produces the assistance torque.