This invention relates to devices for the unique marking and identification of various products, e.g., credit cards, machine parts, computer chips, bullets, etc., and to the fabrication and encoding of such devices, and particularly to the secure encoding of devices. By "secure encoding" is meant encodings which are difficult to change and/or eradicate, and which survive usage of the encoded product.
A wide variety of systems have been recently developed for mounting or implanting devices in various products as a means for marking, e.g., identifying the products. Possibly the best known product example is the "smart card", comprising a plastic credit card having embedded therein a microelectronic semiconductor device including a programmable memory. Using appropriate writing means, data about the owner of the card, including identification and financial data, can be written into the memory and later read out by insertion of the card into an appropriate read out machine located on a merchant's premises or the like.
Numerous other examples of systems and devices for uniquely encoding products are known and described, for example, in the following U.S. patents, the subject matter of which are incorporated herein by reference: U.S. Pat. Nos. 5,142,128 (Perkin et al), 4,010,355(Opicella, Jr. et al), 4,839,875 (Kuriyama et al), 4,827,110 (Rossi et al), 5,182,543 (Siegel et al), 5,166,676 (Milheiser), 4,959,515 (Zavrachy et al), 4,685,515 (Anderson et al), 5,175,424 (Lisimaque) and 4,752,776 (Katzenstein). The various devices and systems disclosed in these patents are likely useful, but all contain limitations and disadvantages relating, primarily, to large size, ability to operate in difficult media, strength or ability to withstand acceleration, the need for electrical or other connections, and/or the ability to be embedded in a hidden location in a product.
For example, and with reference to certain of the patents in the foregoing list having similarities to the present invention, Zavrachy et al (U.S. Pat. No. 4,959,515) disclose a structure comprising a plurality of cantilevered beams, the free ends of which overlie electrodes on a substrate. Upon the application of electrical charges to the beam ends and electrodes, the beams are deflected, thereby changing the spacing between the beam ends and the electrodes, and thereby providing an electrically detectable event, e.g., a variation in a parameter of an electrical circuit. By selectively disconnecting various ones of the beams from the associated electrical circuits, only selected beams remain to generate electrical signals upon the application of the electrical charges. The pattern of signals thus produced corresponds to a unique code for the device.
Disadvantages of the device are that it includes numerous electrical circuits all of which have to be individually addressed. The device is thus relatively complex, cannot be hidden in a product, and cannot withstand high temperatures.
Anderson (U.S. Pat. No. 4,686,515) discloses the use of a strip of magnetostrictive, amorphous metal adapted to be magnetically biased for mechanical resonance at a known frequency. Although not disclosed in the patent, the metal strip or strips are somehow positioned on a marker applied to the product to be identified so that each strip is free to vibrate. By applying to the markers both a dc bias field and an ac interrogation field from a reading mechanism, energy is alternately stored and released with the frequency of the ac field. The quantities of energy storage and release are greatest at the mechanical resonance frequency of the strip, hence an electrical signal is generated having a unique pattern characteristic of the particular resonant frequency of the strip. While the patent refers to the use of "strips", the patent is entirely silent as to how such plural strips would be fabricated, encoded, mounted and decoded. Also, the system disclosed is complex and expensive.
Not included in the foregoing list is U.S. Pat. No. 5,001,933 (Brand), which relates not to an identification system but to a vibration sensor. This patent discloses structure related to the present invention in that cantilevered beams are used, but, as with U.S. Pat. No. 4,959,515, the beams are individually electrically addressed, hence the device and associated systems are comparatively complex and expensive.
As explained hereinafter, the present invention provides significant advantages over the prior art.