Have you ever seen a piece of spaceflight hardware? When you do, you will notice some letters and numbers etched or inscribed on it. All NASA parts have identification, usually expressed in terms of part number, serial number, and the like. In most cases, this identification is permanently marked directly on the part for tracking throughout its life cycle. The recently approved NASA Technical Standard 6002 and Handbook 6003 (found at standards.nasa.gov ) added the matrix symbol to the identification scheme as shown in Figure 1. This put a checkerboard bar code on the part so that an optical scanner could read it. The intent was to make tracking parts as easy as checking out at the grocery store. The system works well as long as the matrix symbol is visible.
But what if the matrix symbol identification gets covered with paint or a similar coating? NASA has developed a method for reading the matrix symbol through up to 15 mils (25 μm) of paint (5 or 6 layers). This method of part identification involves coating selected patches on the objects with magnetic materials in matrix symbol patterns and reading the patterns by use of magneto- optical imaging equipment. The hand-held magnetic scanner, shown in Figure 2, is easy to use and is commercially available through a NASA licensee. It decodes the matrix symbol just like any other scanner. The magnetic marks can be read under conditions that would render optical methods useless. For example, the magnetic scanner can read magnetic marks in the dark or under bright ambient light that might interfere with optical reading of visible marks, symbols that are obscured by discoloration or contamination, in addition to symbols that are covered by paint. Furthermore, inasmuch as magnetic marks can be hidden from unaided view, they are less likely to be deliberately damaged or destroyed. They can even be hidden deliberately for security reasons.
Magnetic material can be applied as viscous ink or paste and even can be material should be one of high retentivity and high coercivity. The matrix symbol pattern can be defined by use of a stencil, or recesses to hold the magnetic material in the matrix symbol pattern can be formed by laser engraving, machine engraving, micro-abrasive blasting, laser etching, or any other suitable marking method. If the magnetic material as applied is not magnetized strongly enough to enable reliable detection over time, it can be magnetized again by use of a permanent magnet or electromagnet.
Bar codes were seldom seen before 1975 but are now common in every commercial outlet. They are on tags and labels of virtually every product. Likewise, direct part marking is now being popularized for tracking things that cannot be labeled. NASA tracks parts using direct part marking. The Department of Defense revised MIL STD 130 to include matrix symbols for direct part marking, and the automotive industry now complies with its B-17 specification for application of matrix symbols on many automobile parts. Now all those little marks that get covered with paint, whether they are on your automobile, jet fighter, weapon, or space shuttle, can be read with ease.
This work was done by Harry F. Schramm and Clyde S. Jones of Marshall Space Flight Center; Donald L Roxby and James D. Teed of Rockwell International Corp.; and William C. L. Shih, Gerald L. Fitzpatrick, and Craig Knisely of PRI Research and Development Corp.