The Arms

The Arms::

Arms: Each arm is loosely based on the dimensions of a human arm with 6degrees-of-freedom, each powered by a DC electric motor through a series spring(a series elastic actuator, see (Pratt & Williamson 1995)). The spring providesaccurate torque feedback at each joint, and protects the motor gearbox fromshock loads. A low gain position control loop is implemented so that each jointacts as if it were a virtual spring with variable sti_ness, damping and equilibriumposition. These spring parameters can be changed, both to move the arm andto alter its dynamic behavior. Motion of the arm is achieved by changing theequilibrium positions of the joints, not by commanding the joint angles directly.There is considerable biological evidence for this spring-like property of arms(Zajac 1989, Cannon & Zahalak 1982, MacKay, Crammond, Kwan & Murphy1986).The spring-like property gives the arm a sensible \natural" behavior: if it isdisturbed, or hits an obstacle, the arm simply de ects out of the way. The dis-turbance is absorbed by the compliant characteristics of the system, and needsno explicit sensing or computation. The system also has a low frequency char-acteristic (large masses and soft springs) which allows for smooth arm motionat a slower command rate. This allows more time for computation, and makespossible the use of control systems with substantial delay (a condition akin to biological systems). The spring-like behavior also guarantees a stable system ifthe joint set-points are fed-forward to the arm.

Representatives of the press who are interested in acquiring further information about the Cog project should contact Elizabeth Thomson, thomson@mit.edu, from the MIT News Office,  http://web.mit.edu/newsoffice/www/ .