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Design of energy efficient hopper by using parallel elasticity and wrapping cam mechanisms

Candan, Sinan Şahin
Improving energy efficiency and control accuracy in legged robot design are important due to demand for complex tasks and prolonged operation time. Solving such a problem can be tackled by the determination of mathematical models, selecting appropriate controllers, integrating actuators and mechanical leg design. Model and control method is selected as Spring Loaded Inverted Pendulum (SLIP) with constant forcing and virtual tunable damping. Parallel elastic and series elastic architectures are considered for actuator integration. Detailed models including realistic effects of drive train for series and parallel elastic actuation are provided along with relevant control methods. Energy loss, transport efficiency, and apex accuracy are compared for those models by forming grids of motor and gearbox selection where optimum voltage is selected for each point. Also, results for different natural frequencies are compared. Gearbox and motor parameters are obtained as continuous functions of single parameters by using catalog data to obtain a continuous grid. Parallel elasticity provides 72% improvement in energy efficiency, 36% improvement in transport efficiency and more than 10 times accuracy compared with series elasticity if the designs are optimum. Mechanical design is based on ATRIAS, which includes a four link leg design that is modified including a wrapping cam mechanism with a tensile spring. Wrapping cam mechanism provides a radial force changing linearly with height. Tensile spring eliminates the problems of buckling and increased leg inertia occuring in compression and leaf springs. A special cam surface is synthesized to match potential energies of SLIP and proposed design. A physical prototype is constructed and tested to show that wrapping cam provides the desired potential energy.