CAREER: Mechanically Adaptive, Energetically Passive Robotics

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This Faculty Early Career Development Program (CAREER) project will create robot exoskeletons that provide the wearer with capabilities beyond those of an unassisted human, to an extent previously thought to require externally powered actuators, such as electric motors or pneumatic cylinders. Unpowered mechanical devices are already used for performance enhancement -- a person can travel much farther and faster on a bicycle than they can on foot. However, the exoskeletons considered in this project go beyond such familiar passive devices by integrating actively controlled energy storage, in the form of springs of controllable stiffness. The scientific contribution of this research is a theoretical framework for mechanically adaptive energetically passive robotics where the amount of energy stored in a spring of controllable stiffness can be decoupled from the deformation of the spring. This new capability could enable unprecedented human and robot mobility. As an example, a person jumping on a conventional spring trampoline can build up energy for a desired high leap by making a series of jumps of successively increasing height. In contrast, a controllable-spring robot limb could allow the necessary energy for the target leap to be accumulated over a series of fixed-depth squats, instead of jumps, which can be safer and more desirable. The project will have significant societal impacts by facilitating the creation of novel assistive devices for the elderly, performance-augmenting exoskeletons for emergency responders, and legged alternatives to bicycles for efficient unpowered transportation. The project will promote a new engineering curriculum emphasizing controlled energy storage and release in unpowered mechanical systems. Outreach activities will include public exhibits of prototypes and knowledge-based journalism about novel devices used for human augmentation and transportation.The focus of this research is a new modeling and design framework for the analysis and creation of mechanically adaptive, energetically passive robots. A key component of these robots is the integration of mechanically adaptive compliant structures, referred to as programmable springs. Programmable springs are a new class of mechanical elements that combine the benefit of levers and springs, with the key feature that enables near-zero energy cost mechanical adaptation, irrespective of the energy stored in the spring. The specific project goals are to (i) formulate a comprehensive model of robot limbs using programmable springs; (ii) define a resonance-based control method robots and humans could use to leverage the benefits of programmable springs, for example, optimally accumulate energy through multiple squats and steps despite limited limb force and limited limb motion; and (iii) establish the physical limits of legged transportation by consolidating the trade-off between performance and user-comfort similar to wheeled transportation. The results will enable levels of human mobility enhancement unprecedented in unpowered devices. The results will also lead to increased energy efficiency in mobile robotic systems for extended missions and improved autonomy.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.