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Steerable monotread simplifies robots
Captive belt, flexing track/guide, and articulating drive enable single-tread steering
by John Lewis, Northeast Technical Editor -- Design News, March 11, 2002
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AURORA uses a frame system that consists of a continuous belt, the drive spine attached to it, and the guide spines, which deflect to bow the belt. The guide spines retain the bent shape to enact a turning motion. |
Pittsburgh, PA — Pittsburgh, PA—Most tracked mobile machines and robots use dual-treads, and change the speed of one track relative to the other for steering. In contrast, the AURORA (Advanced Urban RObot for Reconnaissance and Assessment) from Automatika uses a steerable, single-tread locomotor system that relies on three enabling innovations:
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a flexible elastomeric belt for the tread
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a guide spine that holds the belt captive, and flexes to shape the tread for steering
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and an articulating central drive spine
Departing from the theory that tracked vehicles need at least two treads to steer provides a smaller, lighter, less expensive design, according to Hagen Schempf, chairman and chief scientist. “Most compact dual tread units weigh 50 lbs,” he explains. “The AURORA prototype weighs only 23 lb, including the tread, front and rear posture hubs, and a central enclosure.
Schempf notes two key design challenges: develop a controllable flex structure to shape, guide, and retain a new type of laterally compliant, yet longitudinally rigid flex belt; and integrate driving and steering (articulation) in a compact package.
For the belt, a continuously cast urethane webbed-grouser belt design beat out competing technologies such as slatted conveyor sections, and plastic grousers with intermediate webbing because it was the most rugged, reliable, and straightforward to manufacture, according to Schempf.
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A single tred can steer |
A steerable track and guide system, called the drive spine, is glued to the belt’s inside-kerf. Made of medium durometer, urethane with embedded Kevlar backbone fibers, the custom-molded drive spine uses a set of drive pins with low friction ends to engage drive sprockets. “The biggest enemy is friction,” explains Schempf. “Right now we use sliding friction, but if we scale up in size we can use rolling elements instead.”
A pancake-style brushless motor drives a coaxial-mounted, two-stage, planetary gearbox at the end of the assembly to move the continuous tread. Internal clutches and brakes allow gear-ratio changes for low speed (climbing) and high-speed (escaping) operation. Two stepping motors are used, one at each end offset longitudinally from the cylindrical end sections, to articulate the drive spine. Each is geared through a helical gear set, and moves each cylindrical end-section ±30° for a net 60° steering curvature angle.
Test results indicate that the system is capable of rough terrain driving and steering, while also climbing stairs and achieving sufficient traction and floatation in sandy, wet, and soft soils. “It climbs 60% grade slopes, and crawls through vegetative stands much taller than itself. A paddle deploys for self righting and climbing onto obstacles taller than half its height,” Schempf notes.
“The guide spines worked surprisingly well,” Schempf says, “even becoming self cleaning due to the drive pins in the drive spine sweeping debris from the groove. The continuous belt system’s lack of pretension afforded by the complete capture of the belt, means you would have to destroy the belt in order to throw it.” Future guide spine development will include careful material selection to maximize wear and life without excessively impacting power draw.
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