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Volume 25, Number 50 -- December 11, 1996

PEOPLE PUSH WHILE MACHINE DIRECTS. A collaborative robot -- a cobot -- being developed at Northwestern University gives a new kind of human-machine interface: You do the work and it determines, through its computer program, where the thing you are pushing or pulling can and cannot go. On an assembly line an ordinary industrial robot can do a lot of damage if it is not thinking the same way as the people around it. The robots are heavy and have powerful motors. A cobot cannot move on its own at all but helps human workers move it by providing constraints, a virtual surface to restrict motion.

In automotive assembly, workers must maneuver and attach bulky components such as seats, windshields, and doors without damaging the auto body. Hoists can reduce the weight burden. but add bulk that increases collisions and slows the process. The cobot would offer support against gravity, and give guidance. The worker would be able to push the component to where it goes without fear of collisions.

It's all done with computer-controlled wheels. Rollerblade wheels, which can change the direction of motion of a 200-lb skater in a fraction of a second, are ideal. Three wheels at the corners of a triangle are independently steered by a computer. Push it any way you want until you approach a virtual surface, where the computer steers the cobot's wheels so that only motion parallel to the virtual surface is possible.

Servocontrol selectively adds apparent degrees of freedom by steering the wheels so


that in caster mode the device appears to be unconstrained. Conventional robots have multiple mechanical degrees of freedom which may be selectively reduced through servocontrol to create apparent constraints. The cobot's low number of mechanical degrees of freedom (zero or one) allows it to passively redirect a user's motion.

The researchers speculate about the possibility of armlike cobots with revolute joints. The function of the rolling wheel in the tricycle cobot is to couple a pair of translational velocities, constraining them mechanically to a particular ratio under computer control. In a revolute cobot arm, the device analogous to a rolling wheel would have to couple two angular velocities rather than two translational velocities, constraining them mechanically to a similar ratio. The device would couple two consecutive joints of a serial arm, replacing a conventional robot's actuators. In one such device, linking two joints, the rotation of two shafts is constrained to be in a proportion dictated by the angle set on steering rollers.

The researchers expect to have a cobot working in a mock-up by summer and in an actual plant within two years. Eventually, basic cobots will be manufactured to be individually adapted to manufacturing stations. General Motors has been working with the Northwestern lab in developing cobots (partially funded by National Science Foundation), but has shown a willingness to share -- in fact it has been promoting the idea throughout the automotive industry. Patents have been applied for.


J. Edward Colgate. Assoc. Professor, Mechanical Eng., Northwestern Univ., Evanston, IL 60208.
Phone: 847-491-4264. Fax: 847-491-3915. Internet: