Insects and Inspired Artificial Robots
A
The creation of artificial devices with life-like characteristics has been pursued for over 2,000 years, beginning, as did so many things in our modern world, in Ancient Greece. For example, among the inventions of Hero of Alexandria were a windmill-operated pipe organ and a mechanical theatrical play.
B
With the raise of cybernetic approaches in the late 1940s and early 1950s. A wide variety of electromechanical machines designed to mimic biological processes and systems were constructed. Perhaps the best-known and most directly relevant to biorobotics is W. Gray Walters’ robotic “tortoises” Elsie and Elmer. Walters was a physiologist who made important early contributions to electroencephalography and clinical neurophysiology. His tortoises were small mobile robots covered by a hard shell. The robots were driven by steerable motorized wheels and possessed a headlight, a light sensor, and a touch sensor that responded when the shell was hit. Their behavior was controlled by electronic circuit analogues of neural circuits. The behavioral repertoire of the tortoises included exploration, both positive and negative phototropism, and obstacle avoidance. The activation of these different behaviors in interaction with the robots’ environment could produce a variety of behavioral sequences. Although originally designed to explore Walters’ theories of brain function, the tortoises became objects of popular fascination in much the same way that ancient automata did.
C
The seeds of the modern renaissance of biorobotics were sown from the mid 1980s to mid 1990s. A key event in this resurgence was Rodney Brooks’ work on behavior-based robots. Although not as directly based on biology as later work would be, Brooks argues that nontrivial and flexible behavior in a robot could be generated by the interaction between simple control machinery and its environment, demonstrating his point with robots accomplishing such tasks as insect-like walking. Another important milestone was Raibert’s work on hopping and legged robots, which emphasized the central role of energetics in the dynamic balance and locomotion of animals. Based on studies of serpentine motion, Hirose developed a number of snake-like locomotors and manipulators. In the early 1990s, Beer, Quinn, Chiel & Ritzmann developed a series of hexapod robots based directly on cockroach and stick insect body morphology and neural control. Early biorobotic work on the sensory side includes Franceshini’s robotic compound eye based on studies of insect eyes and motion-sensitive neurons in the fly, Webb’s robotic model of cricket phonotaxis and Grasso et al’s robotic model of lobster chemical orientation strategies. An early example of robots whose control was based on theories of human brain function is given by the work of Edelman et al.
D
There has been an explosion of work in biorobotics in recent years, with robotic vocal tracts, jaws, retinas, expressive faces, hands, arms, legs, etc. deployed on robotic worms, snakes, ants, flies, crickets, cockroaches, walking stick insects, dinosaurs, bats, lobsters, tuna, pickerel, turkeys, apes and humanoids. Thus, no brief survey could possibly do justice to the range of work being undertaken.
E
A recent example of biologically-inspired robotics is Spenko et al’s work on a hexapedal robotic climber called RiSE. In order to grip a vertical surface, this robot combines both bonding mechanisms inspired by the structure of gecko feet and interlocking mechanisms inspired by the structure of insect spines and claws. In addition, its design is based on a set of principles that have been found to be common to many climbing animals: a sprawled posture keeps the body close to the surface so as to reduce the pitch-back moment; front limbs pull inward and rear limbs push outward so as to counteract the pitch-back moment; a long body reduces the pull-in force required of the front limbs; lateral forces act inward toward the central axis of the body; complaint legs, ankles and toes so as to distribute contact forces. Each of the six legs of RiSE have two degrees of freedom and the robot also possesses s static tail that presses against the surface to reduce the pull-in forces required of the front legs. The robot uses a wave gait in which only one leg at a time is lifted from the surface. In addition to an open-loop gait generator, RiSE utilizes a variety of feedback controllers, including traction force control, normal force control and gait regulation. In addition, the robot has a pawing behavior that allows a foot that fails to grasp on initial contact to reestablish a grip on the climbing surface. Spenko et al have demonstrated that RiSE is able to traverse a variety of horizontal and vertical surfaces, including climbing trees and brick or cinder block walls.
F
A powerful example of biorobotic modeling is provided by the aerodynamics of insect flight. Although quasi-steady-state aerodynamical analyses of the sort used to understand aircraft have been successfully applied to larger animals, they have not been very successful for explaining the generation of lift in small flying insects due to the tiny wingspans, relatively slow flight speeds and extremely fast wing movements involved. However, a recent biorobotic model by Dickinson and colleagues has begun to shed considerable light on the unsteady aerodynamics insect flight. Because of the delicate size and high speed of insect wings, direct measurement of the forces involved is extremely difficult. For this reason, a robotic model with a 60 cm wingspan was used to explore the non-steady-state airflow during hovering by the fruit fly Drosophila melanogaster. In order to reproduce the Reynolds number relevant to small insects flying in air, their model was submerged in mineral oil and scaled both in space and time. Force sensors at the base of one wing allowed direct measurement of the forces produced and illumination of air bubbles in the tank allowed direct observation of the fluid flow around the robotic wings. Dickinson and colleagues found that three major mechanisms contributed to lift generation in the model. First, vortices formed at the leading edge of the wing produce lift during much of the power stroke. Second, additional lift is produced by circulation of air around the wings due to rapid rotation at the beginning and end of each stroke. Third, further forces are produces at the start of each upstroke and downstroke due to collisions of the wings with the swirling wake produced by the previous stroke, a mechanism termed wake capture. Due to the sensitivity of these latter two mechanisms to the timing of wing rotation, the model suggests that the control of small details of wing motion can be used in steering flight.
