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Parallel Robots : Stronger, Faster, More Accurate

H4: a parallel robot with four degrees of freedom.

Most people when they hear the word 'robot' see mechanical arms and legs sticking out of a human-shaped metal body. But anthropomorphic robots should stand aside, for a new type of robot –faster, stronger, and more precise– is taking center stage. Behold the parallel robot!

Accelerating up to 50 g's, lifting several-ton loads in seconds, moving with nanometric precision, a new type of robot is making older robots feel their age. A distant cousin of anthropomorphic robots, parallel robots have been a research subject since the early 1980's and a object of EU research funding since 1995, when Germany pushed European institutions to act. The current European programme (Ahead) is led by a Spanish industrial concern and French participation is spearheaded by the CNRS within the framework of its programme Robea (see "To know more").
In order to understand how this type of robot works, it's best to take a "parallel" example. Think of holding a pencil at arm's length; the object weighs only a few grams but your shoulder muscles must hold all the weight of your arm as well. The shoulder works hard for a minimal load. The anthropomorphic robot with its multiple motors and joints runs up against the same limitations.
Parallel robots get around this significant difficulty by putting several simple, lightweight arms between the fixed part of the robot and the moveable part (a claw, a surgical instrument, a driving simulator). With each arm controlled by its own motor, placed as close as possible to the robot base, a computer makes sure the moveable part performs the desired action by coordinating the individual movements of the arms.

Load, speed, and accuracy gains
Taking the example of a flight simulator, it quickly becomes clear that given the considerable weight of the cabin, the arms of a traditional robot needed to shake it (to simulate turbulence) would be immense, and the strain on the "shoulders" would require huge motors. A parallel robot, however, with six actuators, would cut the load requirement of each motor by six.
In Switzerland robots run into the challenge of putting chocolates in boxes. Not that the chocolates are so heavy, but the work must be done quickly. Parallel robots with perfectly coordinated arms are capable of generating 10 g's of acceleration (compared to a conventional robot's 1 g).
Same results when parallel robots are asked to perform precision tasks. Whereas the margin of error of a traditional robot motor must be added to that of all the other motors, with a parallel robot, the same error can be averaged over the number of actuators.
This is not to say, however, that all of the problems a parallel robot can experience have been ironed out; powerful computers are needed to run the robot, the machinery is cumbersome, and the volume of work still inferior to that accomplished by the old-fashioned, articulated-arm robots. But the near future may see the association of anthropomorphic and parallel robots as a way to draw on the best of each. Progress of this sort would see robots taking on whole new tasks, such as surgery.




François Pierrot
Laboratoire d'informatique, de robotique et de microélectronique de Montpellier (LIRMM)
CNRS-Université Montpellier II

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