Passive dynamics refers to the dynamical behavior of actuators, robots, or organisms when not drawing energy from a supply (e.g., batteries, fuel, ATP). Depending on the application, considering or altering the passive dynamics of a powered system can have drastic effects on performance, particularly energy economy, stability, and task bandwidth. Devices using no power source are considered “passive,” and their behavior is fully described by their passive dynamics.
In some fields of robotics (legged robotics in particular), design and more relaxed control of passive dynamics has become a complementary (or even alternative) approach to joint-positioning control methods developed through the 20th century. Additionally, the passive dynamics of animals have been of interest to biomechanists and integrative biologists, as these dynamics often underlie biological motions and couple with neuromechanical control.
Particularly relevant fields for investigating and engineering passive dynamics include legged locomotion and manipulation.
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. He 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.
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.
<Research of Passive Dynamic Walking , Tad McGeer>
There exists a class of two-legged machines for which walking is a natural dynamic mode. Once started on a shallow slope,
a machine of this class will settle into a steady gait quite comparable to human walking, without active control or energy input.
Interpretation and analysis of the physics are straightforward; the walking cycle, its stability, and its sensitivity
to parameter variations are easily calculated. Experiments with a test machine verify that the passive walking
effect can be readily exploited in practice. The dynamics are most clearly demonstrated by a machine powered only by
gravity, but they can be combined easily with active energy input to produce efficient and dextrous walking over a broad range of terrain.
More information, you can read the paper below: