Tomorrow’s Super Robots May Owe Their Mobility To A
Cockroach’s Legs Today
The marriage of machine and biology
requires adopting the pliability and strength from the legs of this despised
insect
San Diego, CA – The cockroach is an insect
despised for its ubiquitousness, among other reasons. Yet, it may hold a key
to the next evolutionary step in the “life” of robots.
Background
For years, serious futurists could only imagine that
robots, such as the television model, would always be stiff, clumsy, and
prone to breakdown. This was before the advent of “Biomimetics,” a research
aimed at developing a new class of biologically inspired robots that exhibit
much greater robustness in performance in unstructured environments than
today's robots.
This new class of robots will be substantially more
compliant and stable than current robots, and will take advantage of new
developments in materials, fabrication technologies, sensors and actuators.
Materials found in nature differ significantly from those found in
human-made devices. Nature appears to design for “bending without breaking”
and employs tissues that are compliant and viscoelastic rather than stiff,
homogeneous, and isotropic. In addition, local variations in biological
materials, tailored to meet local variations in loading, are common. The
nonlinear, compliant, and inhomogeneous materials found in even the simplest
animals provide them with a sophistication and robustness that today’s
robots cannot match. And it is hard to find an animal as simple as the
cockroach.
Actually, the deathhead cockroach possesses legs with
compliant muscles and skeletal components that increase dynamic stability
and disturbance rejection. As the ability to analyze and fabricate
mechanisms with compliant and functionally-graded materials improves, the
opportunity exists to develop robots whose structures draw inspiration from
simple animals such as insects and crustaceans. One fertile area for
biomimetic design is the leg of walking or hopping robots, where leg
compliance is especially important.
One method for manufacturing such robots is Shape
Deposition Manufacturing (SDM), a rapid prototyping technology. SDM
addresses many limitations of traditional manufacturing and assembly by
enabling the in situ fabrication of mechanisms with complex geometry
and heterogeneous materials. Design and fabrication of layered and
heterogeneous materials (also called Functionally Graded Materials - FGMs)
has recently been a focus of research. FGMs enable control of local
variations of biomimetic components by selectively depositing soft and hard
materials. To produce biologically inspired components of biomimetic/mechanical
properties, a bridge between biological findings and SDM design
specifications was required.
The first demand for SDM is to characterize biological
structures and translate the characteristics into quantitative
specifications for mobile robots. The second requirement is to model SDM
material behavior to facilitate component design to meet these
specifications. To address these requirements experiments were conducted on
a hind leg of Blaberus discoidalis and described its response to both
step displacement inputs and sinusoidal displacement excitations. Next, a
test was carried out on one of the materials used in SDM, a soft
polyurethane polymer largely used as joint material in manufacture, and fit
the results to standard viscoelastic (pliable yet sturdy) materials and
models. Comparison and understanding of the mapping between these two
studies enable us to begin to design and manufacture legs similar to those
found in biology.
The Study
The authors of “Material Modeling for Shape Deposition
Manufacturing of Biomimetic Components,” are Xiaorong Xu, Wendy Cheng, Mark
R. Cutkosky and Motohide Hatanaka from Stanford University, and Daniel Dudek
and Robert J. Full at the University of California at Berkley, Department of
Integrative Biology, Berkeley, CA. The authors are presenting their work
at “The Power of Comparative Physiology: Evolution, Integration and
Application” meeting, sponsored by the American Physiological Society (APS)
August 24-28, 2002 at the Town & Country Hotel, San Diego, CA.
Methodology
Relaxation and dynamic experiments were carried out on
the hind leg of Blaberus discoidalis to aid in the selection of a
material behavior model and to quantify measures of roach leg response.
During testing, the coxa of the ablated metathoracic limb (hind limb) of the
cockroach was epoxied to 3/8” acrylic such that the coxa-femur and
femur-tibia joints were free to rotate. Cyanoacrylate was used to attach one
end of a stainless steel pin to the distal tip of the tibia; dental
impression compound was used to adhere the other end of the pin to the arm
of a servo-motor system. The leg was then displaced with the Aurora system,
which is based upon a high performance rotary moving coil motor supported by
precision ball bearings. The results are that the total error in the
force-displacement measurements to be less than four percent that of a
viscoelastic solid.
Results
The results indicate that a cockroach leg excited in a
direction orthogonal to the joint direction behaves similarly to a
viscoelastic material. The exponential nature of the force relaxation curves
suggests viscoelasticity. The hysteretic nature of the force-displacement
curves indicates that there is energy loss due to the internal friction,
which is a common characteristic for viscoelastic materials. The cockroach
leg is subject to a combination of bending and torsion in the experiment.
The overall effect can be modeled as a torsion spring with a moment arm.
Additional assumptions for the model include: (1) the axis of rotation for
the leg is constant during torsion and (2) the joint material can be
approximated using a lumped-parameter element with uniformly distributed
linear viscoelastic properties.
The SDM process allowed an integration of a range of
desired impedance into the structure of robot legs for improved robustness
and simpler control. SDM-compatible materials span a wide range of material
properties and the SDM process enables researchers to control local
variations through Functionally Graded Materials (FGM). With information
regarding the mechanical behavior of animal legs and the material
characteristics of SDM materials, the researchers developed guidelines for
biomimetic leg design.
Conclusions
Some polymer materials that can be used in SDM are
similar to the biological materials found in insect legs that exhibit
viscoelasticity. This inspires us to develop material models and design
methodologies that can be used to guide biomimetic robot leg design and
material selection. In this paper, we have discussed a simple linear, lumped
parameter model used to characterize cockroach leg behavior in relaxation
experiments and in response to sinusoidal excitations. We have also
developed a dynamic test machine and begun characterizing a polyurethane
material used for SDM fabrication of robot joints.
The current models of leg response assume linear
viscoelasticity. The correlation between these models and the results of the
experiments is relatively good at low frequencies and small displacements,
but deteriorates at higher frequencies and displacements as nonlinear
effects grow pronounced.
In addition, at very low frequencies, dynamic tests on
cockroach legs indicate a higher loss modulus than that predicted by a
standard linear model. Should these nonlinear aspects of leg behavior prove
important for locomotion, the researchers believed that better models had to
be developed better models to simulate the viscoelastic behavior of the leg
in a wide frequency range.
Additionally, to produce legs with mechanical response
similar to that of the real cockroach leg, enhanced characterization of
additional SDM materials is required. Knowledge of SDM material behavior,
along with information about the aspects of leg behavior important to
locomotion, will enable the issuance of general design guidelines for
designing biomimetic legs.
(It is worth noting that these legs have been used
to produce a remarkable successful robot from Stanford named SPRAWL. SPRAWL
can negotiate rough terrain without a brain or any reflexes because the
control is built into the smart or tuned legs described above.)
-end-
The
American Physiological Society (APS) is one of the world’s most prestigious
organizations for physiological scientists. These researchers specialize in
understanding the processes and functions by which animals live, and thus
ultimately underlie human health and disease. Founded in 1887 the Bethesda,
MD-based Society has more than 10,000 members and publishes 3,800 articles
in its 14 peer-reviewed journals each year.
***
EDITOR’S NOTE: For further
information or to schedule an interview, contact Donna Krupa at 703.967.2751
(cell), or by email at djkrupa1@aol.com.
