Renishaw encoders

By virtually every measure of comparison, the new double-deck Airbus A380 Superjumbo is the largest commercial aircraft in the world. Able to accommodate up to 840 passengers, and with a wingspan in excess of 90m, building the giant double-decker calls for some highly innovative assembly techniques. One such technique is the use of several huge wing assembly robots from Electroimpact, Inc. (Mukilteo, WA) that depend on Renishaw RG4 linear encoder systems for positioning accuracy.

The four machines incorporate more than 1km of Renishaw 40µm metal scale tape to determine the precise position of traveling ‘yokes'. These massive four-story-high, 80 ton structures straddle the wings in order to drill, tap and rivet stringers to the skin panels along the entire span.

The huge twin-towered yokes—designated low voltage electromagnetic riveters (LVERs)—run on beds capable of supporting the entire wingspan. Each head must drill thousands of holes and apply all the rivets and bolts as it traverses the stringer/skin combinations, with their support columns. The heads must always be perpendicular to the skin surface, and therefore have to adjust to match geometry changes for camber, airfoil contour, dihedral, taper and sweepback, anywhere along the wing.

FOR DEMANDING MANUFACTURING APPLICATIONS, SUCH AS THE WING ROBOTS, RENISHAW OFFERS THE INDUSTRIAL-STRENGTH RGH41 READHEAD.

Fundamental to the process is location of the complex CNC riveting/drilling head. This in turn depends upon the linear accuracy, span wise, of the whole yoke structure as it picks up signals from the Renishaw RG4 linear encoder system. The positioning precision achieved with the Renishaw encoders is "absolutely amazing," says Peter Zieve, Electroimpact president and founder. "It gives us a highly repeatable process."

Electroimpact specializes in turnkey assembly systems for aerospace manufacturing. Its capabilities include CNC controlled drilling, drill-rivet, lock bolt and Hi-Lok installation machines. "We are now designing all our machines with Renishaw linear encoder systems in mind," says Zieve.

Electroimpact has played a significant role in providing Europe's Airbus Industry with ground-breaking equipment for wing assemblies. These range from the early Airbus A320 family to the four-engine A340, and now the huge A380 wings currently under build.

The Airbus automated fastening cells represent a vast improvement on earlier riveting systems, stresses Zieve. They offer 40% space reduction and save enormous amounts of time and labor by eliminating the need to first manually tack components, then split them for deburring, before re-tacking.

Measuring more than 45m along the lead edge, the A380 wings are built in Broughton, North Wales, UK, in a specially constructed, 900,000ft2 facility. Wing panels are machined from aluminum billets or castings up to 35m long, with up to 70% metal removal.

The four Electroimpact lines in the initial ‘Stage Zero' wing-build stations each have one LVER already installed, with second units on each machine in the course of construction, bringing the total number up to eight.

PRECISION WITH TAPE SIMPLICITY

Electroimpact sourced Renishaw's encoder systems for use with its original A320 equipment and has been happy to stay with Renishaw at each succeeding stage of growth, says Zieve. Key factors in that commitment, he says, have been Renishaw's proven global support and the application ease of the metal tape scale for the RG4 linear encoder system, which is available in continuous roll form. Supplied on a reel, the self-adhesive, flexible, 40-pitch scale was simply cut to the length required and fitted in-situ along the full traversing distance of the 160 meter-long wing assembly machine.

An open, non-contact, optical linear encoder, the RG4 system has a range of compact readheads offering resolutions from 10.0µm to 0.1µm and finer, speeds up to 15m/s, and industry standard output.

The RGS40 scale is made from thin flexible steel strip, gold plated for high reflectivity and corrosion resistance. A tough lacquer coating provides handling and contamination resistance and allows easy cleaning. Offered in pre-cut lengths or on continuous rolls, the scale features a self-adhesive backing for mounting to most engineering materials including metals, ceramics and composites. Scale installation is quick and easy at any stage of machine build, without the need for drilling.

By mounting directly to the machine, the scale matches the thermal expansion of the base material. Differential movement between the scale and substrate is close to zero, even over significant temperature changes.

The Electroimpact system also features Renishaw's patented integral set-up LED indicator, unique to the RG range. This simplifies installation with a green "go" signal when optimum installation conditions are met. The tri-color LED — green, orange and red — gives continuous indication of signal strength, eliminating need for specialized setup or for a test and service kit. Tri-state error indication on the incremental output channels warns of low signal amplitude during operation, eliminating the danger of encoder count loss.

For demanding manufacturing applications, such as the wing robots, Renishaw offers the industrial-strength RGH41 readhead. Ultra-compact design — 1.7 in. x 1.06 in. x 0.67 in. (44mm x 27mm x 17mm) — and shop-hardened, full metal jacket construction allows great application flexibility and plug-and-play convenience. Applications range from miniaturized electronics and board production to scientific stages, metrology and testing, robots, laser cutting, linearmotor drives, and automated assembly.

The non-contact optical encoder runs on the 40µm pitch scale with 0.8mm ride height. Proven in industrial applications on a wide range of Renishaw linear encoders, the non-contact readhead design eliminates wear, friction and hysteresis as reliability problems, while EMI/RFI shielding avoids "noise" effects. Patented filtering optics average reflections from multiple scale facets, ensuring excellent signal stability and accuracy, even with contamination or minor damage to the scale.