Multi-head magic. (linear motor robots)

Though not yet in the manufacturing mainstream, linear motor robots are gathering momentum for assembly applications.

Like gravity-defying pucks on an inverted air hockey table, multiple, small robot heads glide about on a cushion of air while suspended from an overhead, horizontal platform. The heads move simultaneously in choreographed fashion, each carrying manipulators that reach downward, grip, rotate, release, and retract as required to pick and place parts and perform other assembly tasks on a work surface below.

To some, it might seem like magic. But in reality, it is powerful permanent magnets within the heads that hold them suspended from the overhead steel plate, called a platen. And it is electronically-switched, linear X-Y stepper motors that propel each head, by indexing over a precision grid etched into the platen. Power, control signals, and compressed air to provide the air cushion standoff from the platen are supplied to the heads via an umbilical cable attachment.

The technology is called linear motor robotics, or platen motor robotics. And since the first prototype systems were developed in the mid-1980s, platen-based cells have offered an intriguing alternative to conventional robots for use in light assembly applications. The approach promises a number of advantages, including reduced floor space requirements, improved safety, and lower maintenance costs. Well-known assemblers including Ford, General Motors, Hewlett-Packard, IBM, Northern Telecom, Rockwell, and others, in fact, are already using multi-head, linear motor robots in production.

But like most new technologies, linear motor robotics has not developed without growing pains. And despite the potential benefits, the approach has so far failed to achieve broad market acceptance. Among other things, early platen-based systems suffered from too much control complexity and incomplete product features, sources say. What's more, given the large installed base of conventional SCARA and articulated robot arms, many users have been reluctant to consider a change.

Now, however, a combination of new developments could help push linear motor robots more toward the industrial mainstream. New, more powerful and less complex control architectures are emerging, for example, which should make the technology more attractive, and easier to use. And work is underway by one Big Three auto maker to extend platen-based robotics beyond traditional light assembly jobs into heavier mechanical assembly applications.

Small Stuff

Platens and linear motor heads can be made to varying sizes. But for cost and other reasons, the technology has traditionally been limited to a relatively small scale suitable for electronics and small mechanical parts assembly. Motors measuring 5 1/2 in. sq working on 37- by 52-in. platens are typical. Current commercial systems allow a maximum of four heads to work simultaneously on a platen. With Z-axis travel at 2 to 4 in., payload capacity is typically pegged from about 1 to 10 lb. Accuracy and repeatability are specified down to [+ or -] 0.002 in. and [+ or -] 0.0005 in. respectively. Vision can be added to improve on those numbers.

"The appeal of this technology to us was that the modules lent themselves well to the size of the stuff that we wanted to move around, and that you could have multiples of them, moving in an overlapping work space," says Sidney Liebes? manager of assembly …