Have null, will fly [data tagging]

This article is concerned exclusively with data tags which are machine readable and which do not require contact between the tag and the reader. Such devices are of considerable commercial interest, either because they speed up a data collection process, or because they provide a new function, or both. The goal of very low cost, vibration-free tagging was finally realised with SGL's 1995 invention of flying null (FN) technology. An FN data tag is the magnetic equivalent of an optical bar code. In the case of FN, however, the bars in the code are made from a very soft (low coercivity), high permeability, magnetic alloy, and the tag is read using a special magnetic reader. Very little material is required and there are no special packaging requirements, so FN tags can be made for a fraction of the cost of other tags. The reader is the key to the technology. It creates a narrow region of zero field (a null) in space, surrounded by regions where the field strength is sufficient to saturate the magnetic material used in the tag. In a typical implementation, it also applies a low-amplitude alternating magnetic field to the interrogation region, so that a soft magnetic element in the null region is driven into and out of saturation, thereby radiating harmonics of the interrogation frequency. These harmonics can be detected, and their time of occurrence related to the position of the element with respect to the null. In typical systems, a spatial resolution of better than 50 /spl mu/m can be achieved for a reader-to-tag separation of many millimetres.