Twisted string-based passively variable transmission: Concept, model, and evaluation

• A novel passively variable transmission (PVT) based on twisted strings is proposed.
• The developed device is light, bidirectional, compliant, and mechanically simple.
• The PVT changes its operation range depending on the external load force.
• Accurate (<6% error) mathematical models are presented for PVT position and torque.

This paper presents a study on a novel passively variable transmission (PVT) which is capable of operating in a wide range of transmission ratios. The developed device is light, bidirectional, and compliant, which is achieved by incorporating a twisted string actuator (TSA) in it. TSA is a cheap and mechanically simple actuator, in which twisted strings contract as a result of twisting and therefore act as a linear transmission. If one introduces a physical offset between the twisted strings, this changes the ratio between the output speed and force provided by the actuator. By introducing a passive element that changes the offset between the strings depending on the magnitude of external load force, we developed a PVT which inherited the advantages of conventional TSAs. This paper describes the basic operation principle and presents mathematical model and experimental evaluation of the proposed variable transmission mechanism. Advantages such as light weight, compliance, and passive bidirectional adjustment of transmission ratio make the proposed device attractive for implementation in various areas of haptics, teleoperation, and wearable and lightweight robotics.

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