An M-840 PI hexapod from PI (Physik Instrumente) is used in an ultra-precise 6D-measuring system for optical surfaces.

Courtesy Fraunhofer Institute for Production Technology (IPT)

Articulate has two meanings — formulated with clarity and effectiveness — or a design with segments and sockets. Articulating robots are both — jointed arms used to precisely express motion tasks by moving products, assembling parts, and operating tools.

These robots are most often moved in Cartesian arrangements, because engineers are comfortable thinking of space in terms of X, Y, and Z coordinates. However, Cartesian axes are often carried perpendicularly, which draws more power. They also maintain less stiffness than parallel robots. Finally, most articulating robots have reduced payload when arms are fully extended, and can only reach a limited area.

Adept Technology Inc., Pleasanton, Calif. makes intelligent parallel robots loaded with vision and packaging-management software. Here, parallel has the same connotation that it does in circuits: The arms are not physically parallel, but all link to a common base and then come together at another point.

Adept's parallel robots are the only units with a four-arm rotational platform. In this design, four hung arms are free to swing with the platform and extend in length. These motions (usually concurrent) demand highly coordinated control, but push speed and acceleration limits: Certain models can sweep along and then back through a U-shaped path of 40+700+40 mm in less than half a second — moving a 6-kg payload as fast as 10 m/sec. (Just for comparison, one VS-Series articulating unit from DENSO Robotics, Long Beach, Calif., reaches to 850 mm, with full-circle 25+300+25-mm cycle times from 0.49 sec and repeatability from ±0.020 mm.)

“In industry, parallel kinematics is a general term for multi-axis motion in which each actuator affects each axis — in all degrees of freedom,” explains Stefan Vorndran of PI (Physik Instrumente) L.P., Auburn, Mass. In contrast, serial kinematics is more traditional and simple, in which axes are independent. “The market share of parallel robots, such as our hexapod, is still small, but steadily growing,” continues Vorndran. Another common parallel-kinematic arrangement is tripods. Camera tripods are one example: Adjusting one leg affects tip, tilt, and height.

Hexapods were first used in commercial flight simulators, because of their ability to move in all six degrees of freedom very precisely and quickly, while also controlling the center of rotation — the pivot point — to any position inside and outside the hexapod structure. “The versatility and ability to produce motion similar to that of the human hand is also spreading hexapod use in the medical field,” notes Vorndran.

Sensors for smarts

No matter the mechanical linkages used, the latest frontier in robotics is upfront programming — often with teaching by sensors. One such system gaining momentum is based on 3D vision. Not just for blockbuster movies, 3D mapping can speed installation of robotic workspaces and increase accuracy. However, some of these systems average $50,000 per setup, which can be prohibitively expensive, and require calibration.

Spatial Vision software for robots automatically identifi es dynamic points in space and calculates distances with inexpensive web cams.

One new 3D software package allows engineers to use lower-cost cameras — for example, those sold for webcams — to make robotics automatically tailor movements to an application: Spatial Vision 3D vision software developed by Universal Robotics from a machine-learning program called Neocortex, developed at NASA and Vanderbilt University. “Spatial Vision software will allow us to set a new price-performance point,” says Roger Christian of Motoman Inc., Dayton, Ohio.

Until now, only open-loop gain scheduling was reliable enough for robotic control; earlier adaptive-control types earned a bad reputation for inconsistent motion. Spatial Vision software captures data four to five times a second, giving robots the realtime input necessary to react to physical environments.

The sensor-software design has up to millimeter accuracy — suitable for discrete conveyor-based packaging and handing. How does it work? “Point two cameras at one workspace, and the software aligns their images,” explains Hob Wubbena of Universal Robotics. “First, two photos are taken of the active work area. Then, the software reconciles them for a 3D workspace representation.” From there, objects can be recognized using common algorithms, or in the case of the robot, a red diode is placed on the robot arm to identify a specific XYZ location for tracking.