Compressed symbology is a product-identification method that was pioneered by NASA for tracking space-shuttle parts and is now being used to mark everything from groceries to automobile parts. Based on a system of two-dimensional marks applied to parts, compressed symbology was developed at NASA's Marshall Space Flight Center in response to the inherent need in the aerospace industry to track parts for configuration management.
The focus at Marshall is on ensuring the quality and safety of products. To be certain of the integrity of a product, each part must be tracked every step of the way — where it was made, who touched it along the way, and when and where it is installed. For NASA, that means tracking millions of parts — even tiny electrical parts no larger than a dime. Since bar codes were implemented in the mid-1980s, they have been used extensively and have saved NASA millions of dollars annually through automatic entry of data from manufacturing work orders and other paper media. However, bar-code labels have not worked well on some parts, especially small ones. Even in cases in which adhesive bar-code labels have been small enough, labels have come off, contaminating processes with glue and paper, and thereby giving rise to additional cleaning processes specifically designed to remove contaminants like glue from space-shuttle parts. In addition, spaceflight is also hostile to bar-code labels.
Another problem that has been encountered is that of compiling information about parts. At the beginning of the space-shuttle program, parts were tracked manually. As the flight rate increased, the amount of data collected was roughly equivalent to the amount of data collected in several large grocery stores each day. This situation created a backlog of paperwork that took as long as three months to catch up with the finished product.
Around this time, a Department of Defense study found that people made errors on one out of every 200 characters entered. For NASA, that meant, theoretically, that one out of every 10 part numbers entered into a data base was affected by a data-entry error. The direction was clear: To ensure the timely flight-worthiness of the space shuttle, it was becoming necessary to develop new techniques for marking parts without damaging them, and to develop an identification system, based on marks, as efficient as that of bar codes.
Because of the inherent limitations of bar codes for direct marking of parts in the aerospace industry, NASA sought a more suitable method of automated identification tracking. In September 1991, Marshall Space Flight Center established the Compressed Symbology Laboratory to investigate marks, roughly equivalent to bar codes, that could be applied directly to parts by use of permanent marking methods. The chosen general form for such marks was that of a two-dimensional matrix symbol that stores up to 100 times as much information as does a one-dimensional, linear bar code in the same area. The matrix symbol is a small square that resembles a checkerboard. The symbol is read by use of a charge-coupled-device (CCD) video camera.
Thirty marking methods were evaluated on more than 60 materials. Included were methods of computer-controlled direct marking by laser irradiation, dot peening, micro-sandblasting, and machine engraving; these methods worked well on metals. For marking such other materials as ceramics, mica, and graphite it was found necessary to apply permanent inks, precious metals, and ceramic-based coatings, sometimes in conjunction with engraving to penetrate coated surfaces.
Marking of textile products (including clothing, parachutes, tent materials, and other items) was upgraded by use of an automated-embroidery marking method. Gold was marked by depositing a thin liquid film of platinum. To facilitate the non-automated marking, new techniques of stenciling were tried. Some of these techniques were derived from a photographic transfer process, and others from computer-driven cutter/plotters. Finally, a process that closely resembles today's direct hot ink transfer was used in some applications to transfer patterns from film sheets to surfaces of parts.