Tutorial on Embodiment Passive dynamic walkers*

The passive dynamic walker, which goes back to McGeer (1990), is capable of walking down an incline without any actuation and without control. In other words, there are no motors, no sensors, and there is no microprocessor on the robot; it is brainless, so to speak. Its loco­motion is an outcome of the slope of the incline (gravity is the only power source), and the mechanical parameters of the walker (mainly leg segment lengths, mass distribution, and foot shape). The original walker had four legs to provide stability in the lateral direction; Collins et al. (2001) have construc­ted a two-legged version which balances by using a counter-swing of the arms that are attached rigidly to their opposing legs (see Fig. and Video

As the passive dynamic walkers demonstrate, locomotion can be realized through pure, but carefully tuned mechanics only. However, the ‘ecological niche' (i.e. the environment in which the robot is capable of operating) is extremely narrow: it only consists of inclines of certain angles. Therefore, the next objective is to extend this concept to machines with some practical capa­bility - that can actively walk on level ground (or even uphill) and that can cope with rough terrain.

Fig. The Cornell passive dynamic walker. It walks completely passively down an incline (Collins et al., 2001).


Video The Cornell passive dynamic walker. It walks completely passively down an incline (Collins et al., 2001).



*This case study has been adapted from Hoffmann and Pfeifer (2011).


Hoffmann, M. & Pfeifer, R. (2011), The implications of embodiment for behavior and cognition: animal and robotic case studies, in W. Tschacher & C. Bergomi, ed., The Implications of Embodiment: Cognition and Communication, Exeter: Imprint Academic, pp. 31-58.
McGeer, T. (1990), 'Passive Dynamic Walking', The International Journal of Robotics Research 9(2), 62-82.
Collins, S.; Wisse, M. & Ruina, A. (2001), 'A three-dimensional passive dynamic walking robot with two legs and knees', International Journal of Robotics Research 20, 607-15.