An Investigation of Bipedal Walking Peter Sand, William Kunz, Oren Laskin
Introduction
Humanoid bipedal walking has the prospect of being a very flexible
framework for creating moving machines. Most robots use wheels, treads,
or large sets of legs to traverse their environments. These methods of
locomotion are often very well suited to particular terrains. Human-like
walking does not have the specialized performance of these other forms of
motion, but it is very general. A good biped provides a compromise
between the complex terrain handling of a four or six-legged robot and the
high speed of a wheeled vehicle on flat ground.
The primary challenge in achieving this generality in bipedal walking is
dynamics. We call an object statically stable if it can stop without
falling over (like a car), while we call an object dynamically stable if
it cannot stop without falling over (like a bicycle). Statically stable
bipedal walking is relatively easy: a cheap wind-up toy can walk forward
on two legs. To move quickly and to deal with complex terrain, it becomes
important to work with dynamic stability. In this case, the robot must
drive its motion such that it is continually falling towards its next
step, but never actually loses control.
Dynamically stable walking has been a challenging robotics problem for
many years. In the past, bipedal research has focused on dynamically
stable machines that are only capable of falling in certain directions due
to external supports. Recently, some groups have started to have success
with free-standing dynamic walking. The most well known example is Honda's
biped, a robot that took hundreds of scientists working with a very large
budget.
The Carnegie Mellon Robotics Club, despite its limited experience and
resources, has been experimenting with bipedal walking for several years.
As part of a SURG project last year, our biped was able to perform
free-standing dynamic walking on a flat surface. The mobility and
robustness of this biped was very limited, but it did give us significant
experience with the issues involved in bipedal walking.
This year, based on our previous experience, we have decided to take a
different approach. A prototype biped has been constructed that is much
simpler and lighter than our previous biped. The old biped had many
actuated joints; using it we were able to determine which joints were
essential and which were not. By eliminating several superfluous degrees
of freedom, the mechanical structure of the biped becomes much simpler and
therefore much lighter. The simplicity makes physical analysis much more
tractable, and the lightness significantly improves the speed and strength
of actuation.
Proposal
The current prototype biped consists of five model-airplane servos and a
several structural elements. One servo is attached to a vertical beam
that can function as an actuated torso. To work with different walking
algorithms, this beam can be augmented with weights or completely removed.
Each hip has two servos: one for side-to-side actuation and one for
forward actuation. By synchronized movement of both side-to-side servos,
the feet of the biped can be raised and lowered. The ankles are currently
unactuated. In dealing with uneven terrain, we will probably need to add
another servo or a passive ankle spring.
We will start by augmenting our current prototype biped with a sensor
system. In particular we will use two cameras, one facing forward and one
facing to the side. The previous biped used an accelerometer-based tilt
meter. This tilt meter was susceptible to deviations caused by the
acceleration that necessarily occurs when the biped moves. A vision based
system will completely eliminate that error because it always (with a
certain latency) gives the actual orientation of the robot with respect to
the world. We have already worked out the details of an efficient
algorithm to determine orientation using the assumption that human
environments generally have vertical lines. We will use two cheap CCD
cameras and a PC-104-Plus frame grabber. For now the computing will be
performed off-board, but the size of the frame grabber allows on-board
computing in the future.
Using the completed prototype, we then intend to investigate walking
algorithms. First we will examine the oscillatory side-to-side motion
that was the primary focus of last year's investigations. Once we have
adequate control over the height of the feet (something which we were
unable to attain last year due to the strength-to-weight ratios of the old
biped), we should be able to move the biped forward. Once we have reached
this point, we hope to concentrate on improved robustness, especially
in response to obstacles.
Applicants
Peter Sand is a senior in Computer Science who has perviously worked on
the Robotics Club biped projects. Oren Laskin is a senior in
Electrical and Computer Engineering. William Kunz is a sophomore in
Computer Science with a strong physics background.
Budget
Since the biped is structurally simpler than the previous biped, many
mechanical costs have been reduced. However, the vision system is a new
and rather non-trivial cost.