At the Robotics Institute on the campus of Carnegie Mellon, grad student Jonathan Hurst is following the age-old engineering principle, "Keep it simple," as he explores two-legged motion systems. His initial prototype, for example, is so simple it only has a single leg and is constrained to two dimensions: up and down, and forward and back. But he is confident his prototype will let him determine the role compliance or muscle and tendon springiness (the opposite of stiffness) play in establishing walking and running gaits. Eventually, he would like to see his work applied to two-legged walking robots. But that day may still be a long way off, he says.

Researcher Jonathan Hurst pulls the cables controlling the shin portion of his single-leg walker, BiMasc.

His prototype, dubbed BiMasc (Biped with Mechanically Adjustable Series Compliance), consists of a hip, thigh, and shin segment, three motors that wind and unwind cables, five cable differentials made of pulleys, and a pair of springs. One motor controls leg length, one controls leg angle, and the third adjusts leg stiffness, which can take place on the fly. The robot is tethered, with power and control coming through an umbilical.

The leg uses off-board computers, a Compact PCI (from One Stop Systems), and a Kontron SBC with a Pentium M 2.0-GHz processor, and 512 Mbyte of RAM. "This is probably overkill," says Hurst. "But we don't want to be limited by processor speed."

The two springs (from Gordon Composites) are made of the same composites found in compound archer y bows. The springs are rectangles measuring 3 X 24 X0.25 in., and can store up to 300 J. The springs, like human muscle pairs, are set up in antagonism, pulling against one another and always in tension. This pretension, which simulates leg stiffness, is controlled by the third motor. The springs are clamped at one end and cables run from their free end around spiral pulleys. The pulleys let Hurst give the leg almost any linear or nonlinear spring function.

Many engineers, especially those with backgrounds in industrial robots, use electric motors or some other actuator that can be made to act like a spring through software. (In such a set up, the motor or actuator responds to displacement by applying a force equal to the spring constant multiplied times displacement, or f = kx, the classic spring equation.) "But these systems run into problems with impacts, as when the leg hits the ground. The motor's inertia dominates the behavior of the system and the simulated spring no longer follows the spring function programmed into the software," says Hurst.

Motors on BiMasc are custom wound (from Emotech) and were designed for high torque and small size. Horsepower is not as important as torque because when the leg is on the ground, motor speeds are low. "But they still need to generate enough torque to hold back the springs," says Hurst. "And when the leg is in the air, the leg segments must move quickly though there is little if any force on them."

The leg length and angle motors generate 30 N-m of peak torque, draw up to 30 A, and have a top speed of 1,300 rpm. The pretension motor has roughly half the torque but the same speed.

Cables and pulley-based differentials are used for their light weight, strength, zero backlash, and low cost. The differentials are limited in that they can't rotate indefinitely like gears, but that's not an issue with BiMasc. The cables (Saba Industries) are uncoated steel with high fiber counts for flexibility.