Spring Loaded Inverted Pendulum (SLIP) is a simple, descriptive and accurate model to study dynamic legged locomotion. Critical design decisions to realize SLIP based legged robots with high energy efficiency and control accuracy are actuation topology and controller. Recent studies converge on series elastic actuation (SEA) and parallel elastic actuation (PEA) regarding actuation whereas, a recently introduced control method, virtual tuning of damping (VTD) have proven to be superior for SEA over other control techniques. However, actuation topology is still under discussion and it is a highly coupled problem with the control approach. In this study, vertical hoppers with PEA and SEA configurations are compared under VTD controller to determine the one results in better energy efficiency and accuracy. PEA and SEA models are extended with drive-train details to provide more realistic results. Models are simulated with various gearboxes and motors to understand their effects. Comparisons among optimum topologies showed that VTD-PEA achieves 0.02% percent apex error where VTD-SEA achieves 0.5% apex-to-apex accuracy (25 times). VTD-PEA also achieved 40% better energy efficiency and 38% higher cost of transport than VTD-SEA.

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