Abstract

Actuators that interact with the environment and/or humans require a variable reduction ratio to maintain high motor efficiency over a wide range of output loads. Actively controlled continuous variable transmission (CVT) allows the motor to operate with high efficiency. Most CVT designs have a complex shifting controller design and need high power for gear shifting. Spherical CVTs have simplified the shifting controller design, desirable for mobile robot applications. However, this design lacks torque capacity and is inefficient beyond a certain gear ratio. This study presents the modeling, design, and characterization of a transmission capable of actively controlling the reduction ratio. Power transmission through the proposed compression mechanism using a compression spring without an external energy source enabled a compact design. A wide range of transmission ratio (~ 4.2:1) based on kinematic analysis of the toroidal drive design. The reduction ratio shifting module enabled a low power consumption during shifting (< 3.5 W) and static condition (~ 0.2 W) through the non-backdrivability and self-locking features of the worm gear. The characterization results showed good power transmission efficiency of over 75% across the entire operating range. The system also exhibited a torque capacity of 2.4 Nm. It is equipped with overload protection features for slip between the disk and roller. Finally, we compare the performance of our proposed variable transmission with that of the existing system to demonstrate the applicability of our design.