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The PVT, an Elastic Conservative Transmission

The Royal Institute of Technology (KTH), Department of Machine Design Mechatronics Lab, S-100 44 Stockholm, Sweden, ingvast{at}md.kth.se

Jan Wikander

The Royal Institute of Technology (KTH), Department of Machine Design Mechatronics Lab, S-100 44 Stockholm, Sweden, jan{at}md.kth.se

Christian Ridderström

The Royal Institute of Technology (KTH), Department of Machine Design Mechatronics Lab, S-100 44 Stockholm, Sweden, chr{at}md.kth.se

This paper presents an innovative transmission called the passively variable transmission (PVT) that has a high torque ratio for large loads and a low velocity ratio for small loads. The change in these ratios depends passively on the load, in contrast to the continuous variable transmission (CVT), where the transmission ratio is controlled explicitly. Another difference from the CVT is that the PVT is elastic and the term transmission ratio is therefore not applicable. A theory section formulates alternative ways of describing the torque and velocity relations for elastic conservative transmisions as well as other important properties.

This theory is used to analyze and illustrate the characteristics of a PVT. The theory is also used to compare the PVT with another novel elastic conservative transmission, called load sensitive CVT.

The nonlinearities and elasticity of the PVT make it difficult to control using linear control theory. Feedback linearization was therefore used to design a torque controller, and experimental results show low impedance at small loads. Further, the controller tracks a reference torque well as long as the reference rate does not cause motor saturation. The abilities of the PVT are also illustrated by comparing it with an actuator having a traditional transmission. The load case is recorded joint torques and angles from the carpus joint of a walking horse. Simulation show that the required peak power is reduced by more than 20% and the product of the maximum torque and the maximum angular velocity is reduced by approximately 30%.

Key Words: passively variable transmission • PVT • elastic conservative transmission • velocity ratio • torque ratio

The International Journal of Robotics Research, Vol. 25, No. 10, 1013-1032 (2006)
DOI: 10.1177/0278364906069188


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