But getting the robots to look and move realistically posed a challenge for their creators at Ohio State University's Art and Technology program.
The artists' key focus was devising a system that would give the Spider Bots fluid leg motion. Laser Reproductions, a rapid-prototyping service bureau in Gahanna, Ohio, helped build the parts and recommended which rapidprototyping process to use. "With 3D modeling and rapid prototyping I felt I had finally found an artistic engineering medium," says Ken Rinaldo, an instructor at Ohio State University.
Rinaldo's daddy-long-legslike robots have only six legs instead of the eight on real spiders. With six legs, the robots have a tripodic gait (three legs on the ground at all times) similar to the way cockroaches move. They can walk forward at multiple speeds and turn in either direction. The common method for effecting hexapod motion is to put a series of servomotors at each joint. But this approach would have required up to four motors on each Spider Bot joint to get the necessary motion. The resulting 24 motors would have been heavy and costly.
Instead, Rinaldo's design combines motors and pull strings with an intelligent servocontrol system to actuate the legs. Each leg only needs two motors incorporated in a tension-compression structure with pull-string mechanics. Dave Evans, lab coordinator at Laser Reproductions, says stereolithography edged out urethane castings for the Bots because it would have been difficult to build leg-joint molds for casting the urethane. Also, the SL resins, Somos 9120 and Somos 8110, were strong and flexible enough to give the joints and legs a lifelike look.
The legs are made from Somos 9120 SL resin from DSM Somos, Elgin, Ill. It has structural properties similar to that of polypropylene.
Each set of two legs acts like a flexible arch that is held in compression by additional parts built from Somos 8110 SL resin (which mimics polyethylene) and monofilament or fishing line attached to the servomotors.
The Spider Bot bodies were also made using rapid-prototyping plastics. This let the team quickly fine-tune the plastic's stiffness, flexibility, and translucence. The colored bodies are fabricated from semiclear polyurethane plastic, impregnated with Pantone colors to individualize each robot. Urethane is cast in a rapid-prototype mold made of RTV (room temperature vulcanization) silicone rubber using a master pattern made of the Somos 11120 material. This tough, rigid material from Somos is well suited for producing molds because of its dimensional stability.
The bodies were outfitted with microprocessors and sensors from Parallax Inc., Rocklin, Calif. Advanced features include a left and right-hemisphere microprocessor approach to parallel processing, Bluetooth for communicating between robots, infrared eyes for sight, ultrasonic ears for sound, and LEDs for visual feedback.