A Continuously Variable Transmission System Designed for Human–Robot Interfaces

Abstract

Continuously Variable Transmission (CVT) systems are being used for many applications such as automotive transmissions, robotics, aerospace. In an ideal condition, these systems have the potential to provide continuously varying power transmission within a predefined limit. This transmission is accomplished with the help of friction, belt or gear systems. CVT can find application in a human-robot interface if design criteria such as backdrivability, independent output position and impedance variation, shock absorbing and low mass and inertia can be satisfied. Even if there are various CVT designs in the literature for human-robot interfaces, the primary limitation of the two-cone drive CVT designs is that the output torque and the output position cannot be altered independently. The reason for this problem is that the friction wheel, which is designed to transmit the torque from the input cone to the output cone, gives rise to remarkable longitudinal friction force along the linear way. In order to overcome this problem, a sphere is used in this work for the CVT design as the transmission element. In addition, it is stated in the literature that common CVT drive systems do not have the capability to be used in cyclic bidirectional motion. In the presented CVT design, a second sphere is added to the system with two springs from the lower part of the cones for pre-tension in order to solve the bidirectional transmission problem. In this paper, the working principle and conceptual design details of the novel two-cone CVT drive are presented. Experimental results showed that the novel CVT has the capacity to transmit bidirectional power with some accuracy.

Geometrical analysis of a continuously variable transmission system designed for human-robot interfaces

“…In order to validate the working principle, an initial prototype is manufactured and proof of design experiment is conducted to observe if the same amount of torque can be transmitted in both CW and CCW directions. This initial experiment validated the performance of the new CVT design, and it was shown that bidirectional power transmission and independent transmission ratio and position variation was accomplished (Mobedi and Dede, 2018). (Mobedi and Dede, 2018) In this initial prototype, the spheres are housed inside a carriage system.…”

“…This initial experiment validated the performance of the new CVT design, and it was shown that bidirectional power transmission and independent transmission ratio and position variation was accomplished (Mobedi and Dede, 2018). (Mobedi and Dede, 2018) In this initial prototype, the spheres are housed inside a carriage system. This carriage system is moved along a linear path to change the transmission ratio.…”

“…To overcome these problems, a significant modification is issued in the two-cone friction-drive CVT design by changing the transmission element to a double sphere. A previously built and reported (Mobedi and Dede, 2018) first prototype, which is used in the proof of concept tests, showed that this new CVT can perform the bidirectional torque transmission.…”

“…This situation results in slippage at the point of contact at lower ranges of transmission torque value. (Mobedi and Dede, 2018) In order to propose a solution to this problem, a second sphere below the cones is added to the system. Hence, when one of the cones is in Figure 5 situation, the other will be in Figure 4 situation.…”

“…The cones are denoted as (10) and the bedding pieces for the cones and the carriage's linear guides are denoted as ( 12) and ( 13) respectively in Figure 9. (Mobedi and Dede, 2018) By adjusting the normal force between the cones and sphere through varying the compression on springs denoted by (6), the transmission torque limits can be set. As an example, if it is required to set the torque transmission limit to lower values, the normal force is reduced by releasing the compression on the spring.…”