The construction of a simple walking biped presents an interesting
in controls, as well as
in engineering design. From the engineering design point of view, the bipedal motion should be
simple enough for the project to be feasible within an academic semester. Still, it should be more
challenging than that of a toy robot.
Beginning with such a loose objective ("let's make something that
on two legs"), one quickly
gains appreciation of the connections between feedback control theory and the natural, instinctive,
action of walking. In particular, the classical inverted pendulum system appears as a core component
in bipedal walking. Imitating the human bipedal walk, however, is fairly complicated; it requires
ankle, knee and, possibly, waist joints, 2-dimensional stabilization.
(Here, one also learns a new respect for back problems).
The Hydrapod performs its awkward bipedal motion on a straight line
and uses only 1-DOF
ankle and waist joints. Each joint motion is controlled by a servomotor. Two more servomotors
are used to move balancing weights on each leg. The walking motion is a sequence of three basic
phases (vertical stabilization, step, exchange of planted foot; see the following schematic for details).
A study of the hydrapod's motion was conducted by implementing a
nonlinear model in
SIMULINK. The simulation of the motion provided insight on the stability/performance
requirements of the controllers, the effect of the various nonlinearities on the feedback loops,
the servomotor power requirements and the achievable angular velocities.
(The following picture shows the outer layer of the simulator.)
Two video clips are provided in the links below, showing the
of one step. The reference
angle commands to execute the step are issued manually, leaving plenty of room for improvement,
automation via computer control etc. Autonomy of motion is also left to the future. In fact the
Hydrapod uses banks of AA batteries as balancing weights, in anticipation of meeting the power
needs of autonomous motion. Finally, as a first video production, it suffers from the usual lack of
sufficient lighting, background, and all the not-so-special effects. Still, George Lucas beware!