WO1997041531A1

WO1997041531A1 - Multicredit inductive debit cell configuration

Info

Publication number: WO1997041531A1
Application number: PCT/BR1997/000013
Authority: WO
Inventors: Rubens Ramos Fernandes
Original assignee: TELECOMUNICAÇõES BRASILEIRAS S/A - TELEBRÁS
Priority date: 1996-04-25
Filing date: 1997-04-24
Publication date: 1997-11-06
Also published as: BR9601594A

Abstract

A debit card having a conducting metal foil (33) attached to an insulating substrate (11), in which data cells are formed by removing parts of said foil to form a round voided area (35, 36) from which a slit (41, 42) extends to the edge of said strip. The slit is bridged by at least one fusible fillet (37, 38). Two or more stacked mutually insulated similar subcells whose bodies and slits are aligned are used to increase the number of bits stored by each cell. Stacked subcells differ from each other in the conductivity by unit of area of the metal films, or in the dimensions or positioning of the fusible fillets, and therefore in the current that must be applied to the sensing coil for melting the fillet.

Description

MULTICRED1T INDUCTIVE DEBIT CELL CONFIGURATION

SPECIFICATION

The present invention refers to the technology of inductive debit cards, such as those described in the patent documents PI 7804885, DE 2558917 and US 4029945 and, more specifically, to increasing each cell's capacity to store information.

Disposable debit cards are already well known and widely used. In these cards each bit of information is stored in an inductive cell, of the type illustrated in Figure 1. A typical card comprises an insulating sheet 11 (usually a thermoplastic) with at least one strip of a conducting metal film 12 attached to its surface, patterned in such a way as to constitute a plurality of cells 13. In the best known version, said cells consist of a central void 14 in the metal film, substantially circular in form, with a narrow gap that extends from this circle to the edge of the metal strip, thus dividing the cell in two symmetrical halves. In order to provide the cell with electric continuity, a conducting fillet 15 is provided in a determined position along said gap, forming a "bridge" between its edges, in order to provide a closed ring-sahped conducting path around the cell.

Still according to figure 1, the readout of the cell's information is performed by means of the inductive sensor 20, comprising a high permeability core 21 surrounded by a winding 22, driven by an alternating current, said core being aligned with the center of the circular void 14. The magnetic field generated by the sensing coil induces a voltage around the perimeter of the circle; if the cell is intact, as shown in figure 1, a current 23 will circulate along the ring, and the cell will behave as the short-circuited secondary of a transformer whose primary is said sensing coil. Due to the mutual coupling, the load corresponding to the cell will be reflected to the primary winding, thus lowering the AC voltage in a detectable way and showing that the cell is intact.

If the ring is interrupted at any place along its perimeter, current 23 will not circulate and, therefore, no load will be reflected to the primary (winding 22). Therefore, the AC voltage across this coil will be grater, allowing a discriminating circuit to detect the condition of the interrupted ("burnt") cell. In the known systems, it is possible to change the condition of the cell from intact to burnt by driving the sensing coil with a stronger current than the one used for readout of the cell's condition. The current induced in the cell will also be larger, as will be the heating ofthe metal film produced by the Joule effect. Due to the fact that in the fillet 15 the current density is greater, said fillet will fuse, therefore changing the cell's condition.

With the improvement in manufacturing techniques, it is possible to reduce the area occupied by each cell and, therefore, to increase the number of cells on the card. On the other hand, this requires a correspondingly increased number of inductive sensors and a consequent reduction in their size, as well as a reduction in the distances between them. These factors increase the problems associated with the card's positioning inside the reading head, since the tolerances become more critical. Moreover, it is harder to reduce the dimensions of the inductive sensors than the size of the cells, because the latter are manufactured through photo-chemical processes which lend themselves to manufacturing automation, and the former are manufactured manually.

A possible solution to this problem is presented in the patent document PI 8805894, which describes a configuration in which each cell is fashioned with two or more fillets or "fusible bridges" placed in parallel. Such a configuration is shown in figure 2. By varying the characteristics of each one of these bridges, such as their resistance or mass, or even their distance from the central void 14, each fillet will fuse at a different current intensity. Moreover, it is supposed that the reflected loads corresponding to the various conditions are considerably different, i.e., all fillets intact, one interrupted bridge and the remaining ones intact, and so on. This feature allows each cell to contain two or more bits instead of a single bit. In figure 2, fillets 24, 25 and 26 have different lengths and, therefore, different resistances, which allows melting them selectively by choosing the value of the current applied to the sensing coil.

The aforementioned technique has serious limitations, the first one being the need for a greater definition in the photo-engraving process, resulting in higher costs and/or reduced reliability of the manufacturing process. Furthermore, the bulk resistivity of all fusible bridges is the same, since the cells are produced from a metal layer having a uniform composition. This requires to size these bridges with substantially different lengths, to yield discernible differences in the corresponding results. Even so, the loads reflected in the several conditions of the cell have values that are close to one another, since most of the current's path lies in the cell body, the fusible fillets being a very small portion of this path.

In view of the above, the present invention aims to provide a debit card in which it is possible to store a greater number of credits than in conventional cards, without the need to increase the photographic resolution used in the conventional process. A further object is to provide a substantial difference between reflected loads corresponding to the various logical conditions of cell, thus allowing a reliable detection of its state.

The foregoing aims are achieved by the invention through the provision of cells comprising two or more layers of conducting metal film patterned as similar superposed inductive subcells, said films being galvanically insulated from one another.

According to another feature of the invention, said cells have different resistances per unit area of the metal film, or dimensional differences in their bridging fillets, so that the current values needed to melt the various fillets are different.

According to another feature of the invention, the metal layers are applied to both surfaces ofthe insulating substrate.

According to yet another feature of the invention, the card is formed by the juxtaposition of many plates of insulating substrate, each of them having at least one of its surfaces provided with inductive metal subcells.

The details, further features and advantages of the present invention will be better understood from the following detailed description of a preferred embodiment, given as an example and not in a sense of limitation, when considered with the accompanying drawings wherein:

Figure 1 illustrates the shape of conventional cells, each of them carrying one bit of information, according to known techniques.

Figure 2 illustrates the shape of cells with more than one bit of information, according to known techniques. Figure 3 illustrates the configuration of a cell carrying two bits of information, built in accordance with the present invention.

Figure 4 illustrates the configuration of a multiple cell, obtained by superposing several plates of insulating substrate, metalized at least on one of their faces, according to the present invention. Referring more specifically to Figure 3, the present invention comprises a laminar insulating substrate 11, shaped lika a card, bearing on its upper surface 31 a first metal strip 33, and on its lower surface 32 a second metal strip 34, thicker than the first one. Both layers have inductive subcells fabricated by photo-etching. The body of each subcell is a circular void 35, 36, i.e. an area where the conducting film was removed by etching, from which a slit 41, 42 extends radially to the edge of the metal strip. Each slit is bridged by a metal fillet 37, 38, respectively, which form paths that provide electrical continuity for the circulation of the induced current in each of the subcells. Figure 3-b shows the card as seen from its upper side. It is understood that the position of the subcell on the lower side (figure 3-c) is coincident with the one on the upper side. If both subcells are intact, currents will circulate in the metal films on both sides, a condition that is translated by a greater load reflected on the sensing coil (not shown in figure 3). The melting of either one of the fusible fillets 37, 38 reduces this load considerably, since now current circulates in only one of the subcells. Because of the difference in thickness of layers 33 and 34, these have different unit area resistivities . Therefore, when the bunting current is applied to the sensing coil, a stronger current will be induced in the lower subcell, due to its smaller resistance. Both fillets have the same resistance, since fillet 38 is longer to compensate the greater thickness of the film. However, the influence of fillet resistances on the reflected load can be neglected, since practically all the path of the current lies in the subcell proper around the central voids 36, 37.

Consequently, the lower fillet 38 will be the first to melt since it absorbs a larger power which, as is known, is proportional to the square of the current.

This melting will cause a measurable change in the AC voltage across the sensing coil, since the reflected load corresponds only to the intact subcell, i.e., the upper one. The burning of this subcell is conducted similarly, by applying an alternate current to the sensing coil with enough intensity and/or duration to produce fusion, by Joule effect, of fillet 37.

In veiew of the foregoing, it can be seen that one single cell of the type shown in Figure 3 is able to store three information states, as follows: 1) both fillets intect; 2) one fused fillet and another intact; 3) both fillets fused.

It should be noted that thickness is not the only feature that can be modified. For different subcells, different fillet widths or lengths can be adopted, or even, different positions along the gap, i.e., different values of the relation a/b. Besides the dimensional variations, different alloys can be used for the metal films in each subcell, in order to get different unit area conductivities, fusion temperature, or specific heat of the materials.

The increased range of possibilities provided by the combination of the aforementioned resources allows manufacturing cells with more than three states. To this end, the card may be formed by two or more layers of metal- plated substract, using techniques that are similar to those widely adopted in manufacturing multilayer printed boards. As an example, figure 4 shows a structure in which four metal layers 43, 53, 63 and 68 are superposed. Said layers are applied to three substrate sheets 41, 51 and 61, making up four subcells 44, 54, 64 and 69 (the latter not visible in the figure), in order to allow storage of five discrete conditions. It can be seen that the fusible fillets are positioned differently along the gap (i.e., proportions a/b are different for each fillet) on the three subcells 44, 54 and 64. Different values of current applied to the sensing coil will melt different fillets, in case the three metal layers have the same unit area resistance. In fact, the length of the current path will be the shortest in subcell 44 and longest in subcell 64; the current circulating in the former will be stronger, since the short-circuited-turn resistance is lower. Subcell 67, not seen in the picture because it is located in the lower side of substrate 16, can be made of a metal alloy with a different composition from the others, so as to require a burning current that is substantially different from those of the other subcells. This difference can be either in unit area resistance as well as in the fusion temperature of the material, or still, in the specific heat of the alloy.

Claims

1. MULTICREDIT INDUCTIVE DEBIT CELL CONFIGURATION formed by removing selected portions of a metallic film in the form of a strip (33, 34, 43, 53, 63) attached to an insulating laminar substrate (11, 41, 51, 61), comprising a round voided body (35, 36, 44, 54, 64) and a slit (41, 42, 46, 56, 66) extending from the edge of said body to the edge of said strip and a fusible fillet (37, 38, 45, 55, 65) bridging the edges of said slit in order to form a closed substantially ring-shaped current path, characterized by the fact that each cell comprises two or more similar, concentric and superposed subcells, galvanically insulated from each other, and by the fact that current intensity required for fusion of the subcell fillets is different for each ofthe subcells.

2. MULTICREDIT INDUCTIVE DEBIT CELL CONFIGURATION as claimed in claim 1, characterized by the fact that the length of the fusible fillet ( 37, 38) in each subcell is different from the length in the other subcells that comprise a cell.

3. MULTICREDIT INDUCTIVE DEBIT CELL CONFIGURATION as claimed in claim 1, characterized by the fact that the cross section of the fusible fillet ( 37, 38) in each subcell is different from the cross section in the other subcells that comprise a cell.

4. MULTICREDIT INDUCTIVE DEBIT CELL CONFIGURATION as claimed in claim 1, characterized by the fact that the position (a/b) of the fusible fillet (45, 55, 65) along the slit ( 46, 56, 66) is different in each of the subcells that comprise a cell.

5. MULTICREDIT INDUCTIVE DEBIT CELL CONFIGURATION as claimed in claim 1, characterized by the fact that the unit area conductivities of the metal film (33, 34, 43, 53, 63, 68) are different for each one of the subcells that comprise a cell.

6. MULTICREDIT INDUCTIVE DEBIT CELL CONFIGURATION as claimed in claim 5, characterized by the fact that the differences in unit area conductivities of the metal film are provided by the differences in the composition of the alloys that make up said metal films.