Pinpoint positioning comes to motion control

Many innovative designs must wait years for manufacturing advances before achieving success. For example, active matrix liquid crystal displays (AMLCDs), commonly known as flat panels, are replacing CRTs in many applications such as industrial computers. However, low manufacturing yields, ranging from 10 to 30%, stood in the way of their mass production just five years ago. This is no longer the case, thanks to a new wave of precision inspection devices.

The key to improved manufacturing yields for AMLCDs: in-process inspection and repair. Industry experts report that recent breakthroughs have increased manufacturing yields as high as 50 to 75%. Here's a look at approaches engineers might use to optimize positioning resolution in their designs.

Air bearing pre-load. Engineers at Westborough, MA-based Dover Instrument Corp. developed a large-travel positioning stage, with repeatable positioning resolutions of plus or minus 0.25 micron. Recently installed at a major manufacturer of flat-panel inspection equipment, the stage has just over 28 inches of x-y travel.

The design offers performance, precision, and reliability that only a servo-controlled, linear-motor-driven, air-bearing system can achieve, according to Phil Greene, Dover's V.P. of engineering.

Dover's stage weighs about 3,500 lbs, of which about 3,000 lbs consists of granite furnished by Rock of Ages, Barre, VT. Granite, the material of choice for high precision machines, not only offers stability, but can be lapped flat to within 0.0001 inch, according to Greene. The stage's total area measures about 50 inches deep by 60 inches wide.

Permanent-magnet, brushless, dc linear motors supplied by Trilogy Systems Corp., Webster, TX, drive the x- and y-axes. LS-405 linear encoders from Shaumburg, IL-based Heidenhain Corp. provide position feedback at 0.1 micron resolution to a Delta Tau Data Systems' (Northridge, CA) PMAC controller.

Magnetically pre-loaded air bearings, supporting the x-axis, employ magnets bonded into the air bearing's surface. Magnetic forces reduce the air gap from about 0.005 to 0.0005 inch, improving the bearing stiffness. "The smaller the air gap the stiffer the bearing," explains Greene.

The magnetically pre-loaded design reduces the number of parts, saves space, and reduces machining time. Similar advantages are achieved in the vacuum pre-loaded granite puck that supports the load on the stage. Vacuum pre-loading reduces the air gap to about 0.0002 inch.

Simplified control. Dover's initial attempts to control the dual motor axis with slav-ing proved unsatisfactory, according to Greene. To overcome this problem Dover developed a new servo control algorithm, called "R-Theta" control. The technique, named for the familiar polar coordinates, serves as an alternative to traditional Cartesian techniques.

Dover's design has the x- and y-axes in the same plane with the granite surface plate supporting the load. Using co-planar axes eliminates structural deformations, as well as control complexities that compromise accuracy in stacked-stage designs.

Delta Tau's PMAC controller commands the dual-loop scheme. Each servo loop can be tuned individually due to the decoupled dynamics of the stage's linear and rotational degrees of freedom, Greene reports. The result: a high-bandwidth servo with no cross-coupling between the parallel motors.

Different twist to control. While Dover's efficient design strategy reduces the number of components and emphasizes control simplification, engineers at Santa Barbara, CA-based Teletrac Inc., took a different approach to precise positioning of flat-panel displays. Instead of moving the flat panel under a stationary probe, Teletrac's LA-9 edge and area prober employs four independent probe heads moving in the x-, y-, z-, and theta-directions to inspect a stationary panel.

The LA-9, recently installed at Xerox's Palo Alto, CA, research facility, has 26 independent axes of motion. An eight-axis Delta Tau PMAC controls each independent probe stage. These stages can be configured in a tight square measuring 2.5 cm on a side, or positioned independently anywhere over the flat-panel substrate.

"As the substrate gets larger, overall space requirements to move the larger panel make it more attractive to move the probe," Dave O'Connor, program manager of advanced systems at Teletrac, explains. The trade-off: a more complex control scheme.

Engineers set out to reduce the mass of the machine, to permit its installation on an upper floor at the customer site. Specifying an active-air isolation table (Newport Corp., Irvine, CA) instead of a granite base, accomplished this goal. "The table reduces weight, while damping very-low-frequency vibrations that the previous granite design couldn't," says O'Connor.

The x-stage, supported by Bedford, MA-based Schneeberger slides, carries the panels from the load/unload position to the center of the probing area, where it is locked into position. A Cognex 5218 vision system obtained from the Natick, MA-based supplier provides fiducial alignment. Linear encoders from Rennishaw Inc., Schaumburg, IL, provide position feedback from the x-stage to the controller. An Intel 486PX2/66 computer provides the operator interface.

Teletrac engineers designed an analog drive to work in conjunction with Trilogy's 3-phase, sine-commutated, linear motors. "Our commutation scheme allows us to run the motors very smoothly," according to O'Connor. Anorad Corp.'s (Hauppage, NY) piezoelectric motors drive the fine positioning z- and theta-axes giving plus or minus 10 micron accuracy.

Laser logic. To improve assembly precision, Simi Valley, CA-based View Engineering's Model 8100 relies on a precisely positioned laser for in-process inspection of surface-mount printed circuit boards (PCBs). The device measures solder pad volume prior to surface component assembly. The result: improved yields in the heating step of the surface mounting process.

"Performing re-work after expensive components are soldered wastes time and money," notes View engineer Chris Oberg. By maintaining consistent solder pad volume, manufacturers improve product quality and reliability, while reducing waste and expensive rework, he adds.

A precisely controlled laser scanner forms the heart of View's Model 8100. The 3-axis motion system uses recirculating leadscrews and linear bearings supplied by NSK Corp., Bloomingdale, IL. A Glentek dc servo motor from the El Segundo, CA-based supplier, drives each axis. Heidenhain LS-403 scales provide position feedback to a PCX/DSP controller produced by Motion Engineering, Inc., (MEI), Santa Barbara, CA. Repeatable positioning resolutions for the laser device are plus or minus 24.2 microns.

Just as the 8100 conserves the resources of PCB assemblers, MEI's PCX/DSP does the same for View's. In one application, Oberg reports, fork-lifts moving just outside the inspection area generated vibrations in the 8100's overhead positioning gantry: "MEI had solved a similar problem at another customer site, and shared the software with us to compensate for the vibrations."