Mechanical Engineering-CIME, Nov 1993 v115 n11 p64(3)

Abstract: GE Fanuc Automation has developed a high-speed cutting system that cuts fabric at speeds up to 100 inches per second, 10 times faster than conventional methods for automated airbags. Its laser beam is 20 inches in diameter with focal length of 200 inches to better accomodate diverse cutting requirements. The laser beam fuses the edges of the nylon, preparing the bags for sewing. Fanuc's system will be adapted by clothing manufacturers and other textile industry firms.

Full Text: COPYRIGHT 1993 American Society of Mechanical Engineers

A high-speed laser system that employs advanced motion control techniques can cut automobile airbag fabric and similar materials at speeds up to 100 inches per second at 10 times the surface acceleration of traditional systems.

TO IMPROVE PRODUCTIVITY in the automotive airbag industry, a maker of motion control systems has developed a high-speed laser scanning system to cut fabric. The laser system, developed and patented in the United States by GE Fanuc Automation of Charlottesville, Va., cuts at speeds up to 100 inches per second and at surface accelerations up to 30 gs.

The key to the speed and acceleration abilities of the system lies in the laser beam's high mechanical advantage: the ratio of mirror motion to focal-point motion. A tilting and projecting mirror directs the movements of the fabric-cutting laser beam by means of a system of motors and linear actuators controlled by a RISC-based microprocessor. For every thousandth of an inch the mirror moves, the laser focal point moves 30 thousandths of an inch at the work surface. And the laser light is intense enough (about 10,000 watts per square millimeter) at the focal point to cut through the fabric at the high speed the process requires.

The high-speed cutting system is now being tested by manufacturers of airbags, apparel, and other sewn products. It would replace traditional fabric-manufacturing equipment. Computer-numerical-control knife machines and die-cutting systems also shape the woven nylon fabric used in most automotive airbags. In contrast to the 30-g laser sanning system, traditional airbag-cutting systems cut fabric at fewer than 3 gs.

To speed up production using conventional laser machines, airbag makers stack fabric layers on top of one another, separating each layer with a sheet of paper or plastic so that the layers don't weld together. After they are cut, the layers are sewn together to make the airbags. However, such a stacking and unstacking procedure is time-consuming and labor-intensive.

Because the laser scanning system cuts single layers of fabric at high speeds, there is no need to stack the fabric. Just as important, the laser system also fuses the edge of an airbag. In doing this, the system prevents fraying, a problem that sometimes arises from the use of automated knife systems. When a laser finishes cutting, the airbag is ready for sewing.

To develop such a system, Stan Ream, the manager of laser products at GE Fanuc, joined other engineers at the company to develop laser scanning equipment, such as the mirrors and lenses, required to generate a laser beam 20 inches in diameter; a precise system of linear actuators, which controls mirror motion; and a software program written to direct the movements of the mirror-control mechanism.

A BEAM'S JOURNEY

First, engineers gathered all details regarding the fabric-cutting process. They knew that the laser focal point would have to scan within a 6-foot by 6-foot plane and that the beam should be intense enough to cut easily through woven nylon.

To meet process requirements, the GE Fanuc team decided on a cutting system that would use a laser beam that reached 20 inches in diameter at its widest point. Further, the beam's focal length would be around 200 inches. Also necessary was a mirror 20 inches in diameter to reflect the beam when it is that size. An average conventional laser cutting system generates a beam that is smaller than 1 inch in diameter and has a focal length shorter than 10 inches.

GE Fanuc's system consists of a carbon dioxide laser-beam-generating unit, which sends a beam of light into a properitary optical unput unit. This unit polarizes the beam and expands its width.

From the optical input unit, the beam travels to a flat mirror. Then the light beam continues to expand as it reflects from the flat mirror and travels to a concave mirror. As it reaches the concave mirror, which is mounted about 200 inches from where the focal point contacts the work surface, the laser beam is 20 inches in diameter.

To this point the beam has been expanding, but from here it starts to converge. Reflecting from the mirror, the beam of light travels as it converges toward a scanner-head unit.

The scanner head tilts, projects, and retracts a third mirror--one that directs the movements of the laser beam on the work surface. The converging beam arrives from the concave mirror, reflects from the moving mirror, and continues to narrow as the light travels to the work sufrace.

The scanner-head mirror tilts so that the beam can move back and forth or side to side on the surface. Simultaneously, the mirror advances from the scanner head or retracts toward it.

Without the ability to project and retract, the laser focal point, which is also the cutting point, would swing an arc path in the vertical plane. The focal point would follow a path that contacts the work surface only once.

However, to achieve cutting accuracy, a focal point of virtually constant diameter (around 20 thousandths of an inch) must always contact the work surface. The distance traveled by the beam from the concave mirror to the work surface, the focal length, must remain virtually constant.

Orchestrating the motion of the moving mirror required extensive development of software and the building of extremely precise scanner-head components by GE Fanuc. For every x,y coordinate traced by the laser beam along the work surface, there is a moving-mirror position separate from all other mirror positions. The software algorithm calculates these mirror positions and links with electronic circuits to direct the scanner head and mirror, as required by the cutting process.

MIRROR MOTION

In the scanner head, three linear actuators connect to the back of the moving mirror at three points. It is the back and forth movements of actuator arms, in parallel, that tilt, project, and retract the mirror. Driving each actuator, at acceleration rates to 30,000 radians per second, is a low-inertia servo motor, which uses closed-loop encoder feedback to control speeds and torque. Each motor drives a linear actuator by way of a leadscrew. Working together, the actuators execute a new mirror position within 12 milliseconds; thus, for every computer command, the beam's focal point moves 1.2 inches, at 100 inches per second, on the work surface.

Such cutting speeds interest manufacturers of sewn products. Many prefer delivering products on demand (or on a just-in-time basis) rather than accumulating batches of products using stacking process methods.

Apart from airbags, the laser system shapes other fabrics, including those used inside automobiles and aircraft, for personal attire, and on office furniture. However, the system's uses are limited to cutting relatively thin materials, and it cannot be used to cut metal, although GE Fanuc is studying the potential of developing the laser system to weld the sheet metal used to make automobile shells.