Passive dynamics

Passive dynamics is an approach to robotic movement control (especially walking), based on utilising the momentum of swinging limbs for greater efficiency. This method is based on using the morphology of a mechanical system as a basis for necessary controls. Passive dynamics are used to create robotic and prosthetic limbs that move more efficiently by conserving momentum and reducing the number of actuators required for motion.

History

The term and its principles were developed by Tad McGeer in the late 1980s. While at Simon Fraser University in Burnaby, British Columbia, McGeer showed that a human-like frame can walk itself down a slope without requiring muscles or motors. Unlike traditional robots, which guzzle energy by using motors to control every motion, McGeer's early passive-dynamic machines relied only on gravity and the natural swinging of their limbs to move forward down a slope.

Models

The original model for passive dynamics is based on human and animal leg motions. Completely actuated systems, such as the legs of the Honda Asimo robot, are not very efficient because each joint has a motor and control assembly. Human-like gaits are far more efficient because movement is sustained by the natural swing of the legs instead of motors placed at each joint.

Tad McGeer's 1990 paper "Passive Walking with Knees" provides an excellent overview on the advantages of knees for walking legs. This sounds like a silly topic for a paper, but McGeer clearly demonstrates that knees have many practical advantages for walking systems. Knees, according to McGeer, solve the problem of feet colliding with the ground when the leg swings forward, and also offers more stability in some settings. This paper and more of McGeer's can be found at the Cornell locomotion and robotics website.

Passive dynamics is a valuable addition to the field of controls because it approaches the control of a system as a combination of mechanical and electrical elements. While control methods have always been based on the mechanical actions (physics) of a system, passive dynamics utilizes the discovery of morphological computationfn|4. Morphological computation is the ability of the mechanical system to accomplish control functions.

Applying Passive Dynamics

Adding actuation to passive dynamic walkers result in highly efficient robotic walkers. Such walkers can be implemented at lower mass and use less energy because they walk effectively with only a couple motors. This combination results in a superior "specific cost of transport".

Energy efficiency in level-ground transport is quantified in terms of the dimensionless "specific cost of transport", which is the amount of energy required to carry a unit weight a unit distancefn|(1). Passive dynamic walkers such as the Cornell Efficient Bipedfn|(2) have the same specific cost of transport as humans, 0.20. Not incidentally, passive dynamic walkers have human-like gaits. By comparison, Honda's biped ASIMO, which does not utilize the passive dynamics of its own limbs, has a specific cost of transport of 3.23fn|(3).

The current distance record for walking robots, 9.07 km, is held by the passive dynamics based Cornell Ranger. [cite web | url = http://ruina.tam.cornell.edu/research/topics/locomotion_and_robotics/papers/CornellRanger/index.html |title =Cornell Ranger | publisher = Cornell University | accessdate = 2008-06-13]

The most exciting application for passive dynamics is its use in prosthetics. Since passive dynamics provides the mathematical models of efficient motion, it's an appropriate avenue to develop efficient limbs that require less energy for the people that need them. Andrew Hansen, Steven Gard and others have done extensive research in developing better foot prosthetics by utilizing passive dynamics. A link to a page describing part of their work is provided below.

http://www.medschool.northwestern.edu/depts/repoc/sections/research/projects/ambulate/foot_rocker_radius.html

References

*cite journal | author=Tad McGeer | title=Passive dynamic walking | journal=International Journal of Robotics Research | month=April | year=1990
*fnb|(1) cite journal | author=V. A. Tucker | title=The energetic cost of moving about | journal=American Scientist | year=1975 | volume=63 | pages=413–419
*fnb|(2) cite journal | author=Steve H Collins; Martijn Wisse; Andy Ruina | title=A 3-D Passive Dynamic Walking Robot with Two Legs and Knees | journal=International Journal of Robotics Research | year=2001 | volume=20 | pages=607–615 | doi=10.1177/02783640122067561
*fnb|(3) cite journal | author=Steve H Collins; Martijn Wisse; Andy Ruina; Russ Tedrake | title=Efficient bipedal robots based on passive-dynamic Walkers | journal=Science | year=2005 | volume=307 | pages=1082–1085 | doi=10.1126/science.1107799 | pmid=15718465 and cite journal | author=Steve H Collins; Andy Ruina | title=A bipedal walking robot with efficient and human-like gait | journal=Proc. IEEE International Conference on Robotics and Automation | year=2005
*fnb|(4)cite journal | author=Chandana Paul | title = Morphology and Computation | journal = Proceedings of the International Conference on the Simulation of Adaptive Behaviour | year = 2004 | pages = 33–38

External links

* [http://ruina.tam.cornell.edu/research/topics/locomotion_and_robotics/papers.htm Cornell Biorobotics and Locomotion Lab] — videos and papers on passive dynamic walkers, including McGeer's originals, the Cornell Efficient Walker, and the Cornell Ranger
* [http://www.droidlogic.com Droid Logic] — simulations of passive dynamic walkers and runners created using evolutionary robotics
* [http://www.ai.mit.edu/projects/leglab/robots/robots.html MIT Leg Lab] — walking and running robots that utilize natural dynamics
* [http://www-personal.umich.edu/~shc/robots.html Steve Collins' Robots page] — the Cornell Efficient Walker, its passive predecessor, and additional references