Cockroaches serve as model for more natural robots

Cockroaches serve as model for more natural robots
SAN DIEGO — The new field of biomimetics is attempting to replace the cogs and wheels of today's robots with more realistic muscles, tendons and nerves fashioned from conducting shape-changing materials. A recent symposium at the American Physiological Society's annual meeting on comparative physiology featured new ideas for robot construction gleaned from comparing the ways living organisms solve the same problems now facing robots. The new designs promise robots that are more natural and robust in unstructured environments.

Contemporary robots seem stiff, clumsy and error-prone compared with living systems, but the study of physiology could yield biomimetic robotics that are more effective at everyday tasks, said Robert Full, a professor at the University of California at Berkeley who moderated a session titled "Toward the Design of Artificial Muscle and Robots."

Several papers concentrated on materials that react like living muscle tissue. For instance, Stanford Research International's Roy Kornbluh discussed his work on electro-active elastomers as artificial muscles. Kornbluh envisions artificial muscles someday being used for everything from smart prostheses to airplanes that flap their wings.

Electro-activated elastomers sandwich a layer of polymer between two layers of compliant conductive materials that serve as electrodes, such as colloidal carbon in a polymer binder. When voltage is applied to the electrodes, the electrostatic force generated alternately squeezes or stretches the sandwiched polymer in a manner similar to actual muscles. Based on what engineers call Maxwell stress, this force was originally considered a nuisance by designers, but thanks to the discovery of soft-polymer thin films, it may someday power robotic arms.

Several other papers analyzed the "components" and "control" mechanisms of real living muscles, and the "intelligent mechanics" that simplify control problems for animals, always with the aim of building robotic components that mimic arms and legs. Not only are these engineered robotic components activated with artificial muscles, but they also follow the design principles and control mechanisms of living organisms. Several prosthetic designs using components modeled on living organisms were detailed at the meeting, as were robot prototypes.

Direct transfers

The most ambitious approach to biomimetics ditched traditional design engineering in favor of direct physical measurements of biological systems. Instead of selecting from among existing components such as stepper motors, hinges and ball joints, a biomimetic approach picks similar materials so that measurements of the biological system can be directly transferred to the artificial materials.

For example, a cockroach's leg was tethered to a bench and exercised using a highly precise measurement system, and from those measurements a robotic version could be automatically constructed from "graded" polymers by merely transferring the measurements. One research group Full works with reported actually doing just that — that is, designing a robotic version of a cockroach leg constructed from polymers with similar physical characteristics, creating a direct analog of the biological system.

According to Full, the materials engineers ordinarily work with are very unlike the materials found in nature. Natural materials are designed to bend without breaking; living tissues, for example, are pliable and viscoelastic — flexible yet sturdy — whereas today's robotic components are stiff, homogeneous and isotropic, said Full.

One of the biggest differences between human-designed robots and the biological components they are trying to mimic, he said, is that local variations in tissues are tailored to accommodate variations in loading during normal use. These nonlinear, compliant and heterogeneous materials account for the robustness and flexibility of biological systems.

Cockroach legs

For instance, a cockroach's leg is laced with compliant muscles and skeletal components that account for its dynamic stability and "disturbance rejection." In a natural environment, that lets them perform maneuvers such as crouching down to keep from being blown away. Full and his colleagues propose a general design method called shape-deposition manufacturing (SDM) that could imprint the physiological characteristics of any living limb onto compatible polymers.

SDM solves the problems of constructing heterogeneous materials by fabricating complex geometries in situ with automated deposition-and-removal equipment. Conventional computer numerical control milling machines that can remove material automatically are retrofitted with deposition tools so that the same machine can also deposit graded polymers as liquids that are cured with ultraviolet light. Using a list of measurements, SDM can manufacture a high-quality, functional replica of an internally complex structure like a cockroach leg that could not be practically fabricated with conventional processes.

Building robotic devices that could serve as direct replacements for their biological analogs is being widely researched with SDM and functionally graded materials. These new FGMs are being studied as components for biomimetic systems, which need a wide range of soft-to-hard materials to reproduce the measured variations found inside real biological systems.

The researchers were able to characterize the mechanical characteristics of the cockroach leg's biological structures, allowing them to translate those measurements into quantitative specifications for SDM. To do so, they had to model the range of SDM materials available and decide whether they were sufficient to reproduce the required performance characteristics. The work was performed on the hind leg of a Blaberus discoidalis cockroach, which was measured dynamically with both step displacement and sinusoidal displacement. The group is still characterizing polyurethane materials with which to fabricate a robotic version of the leg using SDM.