Passive dynamics
Encyclopedia
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 actuator
s required for motion.
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 expend 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.
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 computation. Morphological computation is the ability of the mechanical system to accomplish control functions.
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 distance. Passive dynamic walkers such as the Cornell
Efficient Biped 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 3.23.
The current distance record for walking robots, 65.17 km, is held by the passive dynamics based Cornell Ranger.
The most exciting application for passive dynamics is its use in prosthetics. Since passive dynamics provides the mathematical models of efficient motion, it is 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.
Robotics
Robotics is the branch of technology that deals with the design, construction, operation, structural disposition, manufacture and application of robots...
movement control (especially walking
Walking
Walking is one of the main gaits of locomotion among legged animals, and is typically slower than running and other gaits. Walking is defined by an 'inverted pendulum' gait in which the body vaults over the stiff limb or limbs with each step...
), based on utilising the momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
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 actuator
Actuator
An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
s required for motion.
History
The term and its principles were developed by Tad McGeer in the late 1980s. While at Simon Fraser UniversitySimon Fraser University
Simon Fraser University is a Canadian public research university in British Columbia with its main campus on Burnaby Mountain in Burnaby, and satellite campuses in Vancouver and Surrey. The main campus in Burnaby, located from downtown Vancouver, was established in 1965 and has more than 34,000...
in Burnaby, British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, McGeer showed that a human-like frame can walk itself down a slope without requiring muscles or motors. Unlike traditional robots, which expend 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 computation. 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 distance. Passive dynamic walkers such as the Cornell
Cornell University
Cornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...
Efficient Biped 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
ASIMO
is a humanoid robot created by Honda. Introduced in 2000, ASIMO, which is an acronym for "Advanced Step in Innovative MObility", was created to be a helper to people. With aspirations of helping people who lack full mobility, ASIMO is used to encourage young people to study science and mathematics...
, which does not utilize the passive dynamics of its own limbs, has a specific cost of transport of 3.23.
The current distance record for walking robots, 65.17 km, is held by the passive dynamics based Cornell Ranger.
The most exciting application for passive dynamics is its use in prosthetics. Since passive dynamics provides the mathematical models of efficient motion, it is 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.
External links
- Cornell Biorobotics and Locomotion Lab — videos and papers on passive dynamic walkers, including McGeer's originals, the Cornell Efficient Walker, and the Cornell Ranger
- Droid Logic — simulations of passive dynamic walkers and runners created using evolutionary roboticsEvolutionary roboticsEvolutionary robotics is a methodology that uses evolutionary computation to develop controllers for autonomous robots. Algorithms in ER frequently operate on populations of candidate controllers, initially selected from some distribution. This population is then repeatedly modified according to...
- MIT Leg Lab — walking and running robots that utilize natural dynamics
- Steve Collins' Robots page — the Cornell Efficient Walker, its passive predecessor, and additional references