Coupled dynamics of legged locomotion with suspended loads
Carrying loads with a rigid structure during legged locomotion is difficult, inefficient, and can lead to injury. A compliant suspension could be used to reduce the peak forces acting on the body and the energetic cost of locomotion when carrying a load. However, limited experimental data exists, and there is a need to develop a theoretical framework to predict the effect of a load suspension on legged locomotion and to determine whether a load suspension may present tradeoffs that are not yet well-understood. In particular, there are conflicting reports on the energetic cost of locomotion with different suspended load parameters, little is known about how the coupled body-load dynamics could affect the interaction forces and vertical body displacement, and it is unknown how the stability of locomotion may be affected by a suspended load. Here I develop an empirically validated theoretical framework for studying the energetics, forces, and stability of legged locomotion with a suspended load. My work shows that suspended loads can reduce the energetic cost and the peak forces acting on the body during walking and running compared to a stiffly or rigidly attached load if the damped natural frequency of the load suspension is tuned to be less than half of the primary locomotion frequency. I also show that a suspended load reduces the vertical displacement of the body and can resonate with multiple locomotion frequencies that must be considered in the suspension design. Further, I show that the suspended load is significantly less stable than a rigidly attached load, but that the stability of the body could improve during locomotion. The suspension damping ratio should be minimized to reduce the peak forces and energetic cost, but this presents a design tradeoff because higher damping is necessary to increase the load stability and reduce the settling time after a perturbation. This thesis expands our understanding of human, animal, and robot locomotion by establishing a theoretical framework that resolves previous discrepancies, makes correct energetic and dynamic predictions, and reveals design and biomechanical tradeoffs that could enable a more optimal design of load suspensions for everyday use.^
Justin E. Seipel, Purdue University.
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