Preliminary Experiments with an Actively Tuned Passive Dynamic Running Robot

This paper describes experiments with an electrically actuated one legged hopping robot the ARL Monopod While a spring mass system comprised of the leg spring and the body mass has previously provided the basic passive dynamic vertical motion we have now added a spring in se ries with the hip actuator to use the same passive dynamic principle for the leg swing motion This paper validates our previously proposed hip actua tion controller which actively tunes the leg swing motion during running to remain close to its passive dynamic response and at the same time main tains balance and controls the robot s desired forward speed Experimental data of stable running at m s show a reduction in hip actuation en ergy during ight and a mechanical energy expenditure for the complete robot of only W The resulting speci c resistance of the robot reduces to at m s which is about half the previously reported result for the robot without hip spring Introduction Energy autonomy is a necessity for virtually any practical mobile robot Achieving the required energy e cient running or walking in legged robots is particularly challenging since much energy can be expended for internal motions like vertical body motion or swinging the legs Since these motions do not contribute directly to mobility the energy used to produce them should be minimized Just like a pendulum produces periodic motion e ciently by cycling ki netic and gravitational potential energy the periodic internal motions needed in running can be produced e ciently as well by cycling kinetic and spring po tential energies Many animals are superbly designed to exploit this principle and manage to reduce the metabolic cost of running considerably by utilizing the elastic properties of their muscles tendons and bones In the area of robotic systems it was Raibert who pioneered the use of a system s unforced response its passive dynamics to generate the gross motion for locomotion To date virtually all dynamically stable running robots utilize an actively tuned spring mass system to provide the vertical robot mo tion A beautiful example of just how far the passive dynamics ap proach can be carried is provided by McGeer who built a two legged walking robot with knees which was able to walk down shallow inclines driven entirely by gravity without any actuation In we show that at m s the ARL Monopod uses about half of its total energy expenditure for swinging the leg Much of this energy can be saved since passive dynamics can provide the leg swing motion via a spring connecting the leg and the body at the hip joint as proposed in In our previous work we presented a new control strategy to stabilize passive dynamic running of a one legged robot An alternative approach for controlling the same robot system with compliant leg and hip was proposed by Fran cois and Samson The controller is derived based on a simpli ed and linearized dynamic model of the robot and was shown to be both stable and robust in simulations The current paper is devoted to the experimental validation of a controller proposed earlier In Sec the robot model and the design modi cations to the ARLMonopod are described which give rise to the ARLMonopod II The hip and leg controllers are described in Sec together with the experimental results Finally Sec computes and discusses the energy expenditure as well as the speci c resistance