FIELD OF THE INVENTION
This invention relates to surface pressure input panels generally and more specifically to a magnetic surface pressure input panel suitable for detecting complicated surface contours and irregularities such as a fingerprint pattern.
BACKGROUND OF THE INVENTION
The inside surface of the fingertip has intricate contours that are unique to each individual. This has made the fingerprint pattern an important vehicle for identification, particularly for law enforcement purposes. To facilitate identification via fingerprints, efforts have been made to develop fingerprint detection devices capable of electronically mapping an individual fingerprint pattern. Accurately mapping the contour of each person's fingertip, however, has proven to be a difficult and expensive task. This is because each fingerprint pattern is made up of a large number of convex portions, or ridges, separated by concave portions or grooves that generally extend in random directions around the fingertip. These ridges and grooves have minute dimensions, on the order of 100 um, making them difficult to detect.
Conventional fingerprint detection devices generally employ a surface pressure input panel that includes a contact sheet which changes in some manner when it is engaged by the various ridges and grooves of the fingertip. One such device comprises a conductive rubber sheet that undergoes local variations in conductivity when engaged by a surface with a pressure differential. Similar devices include a resistive film that varies in resistance with changes in surface pressure. These devices employ a means for detecting the changes in the contact sheet or film in order to electronically map the contour of the fingertip. The detection means, however, typically has difficulty detecting small pressure changes, particularly when a non-uniform pressing force is applied or the fingertip is contaminated with grease, sweat or the like. Therefore, these devices are often incapable of accurately detecting the small surface variations necessary for mapping the contour of a fingertip.
Other fingerprint detection devices have attempted to overcome this problem by employing MOS field-effect transistors or piezoelectric thin films to detect small surface variations (e.g., see Japanese Patent Application No. Hei 5-61966). These devices, however, generally require complicated manufacturing techniques and relatively expensive materials, such as silicon semiconductor substrates. Although these devices can produce high definition, they are difficult to manufacture and, therefore, are very expensive. Accordingly, manufacturing these devices to map a large detection area, such as a fingerprint pattern (which is very large compared to the size of the individual surface changes or irregularities) is not presently feasible.
SUMMARY OF THE INVENTION
The present invention is directed to an inexpensive surface pressure panel capable of detecting complicated surface contours such as a fingerprint pattern. To accomplish this, the invention comprises a surface pressure input panel having a flexible magnetic sheet coupled to a circuit plate. The magnetic sheet is adapted to conform to surface contours (e.g., deflect or sink) and the circuit plate comprises an
insulating base plate with appropriate electrode lines formed thereon. The circuit plate further includes a multiplicity of detection elements, each including a magnetic sensitive element, distributed over the area of the board, facing the magnetic sheet and coupled to the electrode lines. Each detection element is configured to generate an electric signal corresponding to the distance between the magnetic sensitive elements and respective overlying portions of the magnetic sheet. The electrode lines are preferably arranged in an X-Y grid so that the generated signal can be detected and the location of each detection element can be determined.
In a preferred configuration, the surface pressure input panel is an active matrix type array having first and second (e.g. X and Y) groups of electrode lines intersecting each other at intersection points to form a matrix pattern on the circuit plate. A detection element at each intersection point includes a magnetic sensitive element configured so that an electrical signal can be generated that is preferably proportional to the strength of a magnetic field received by the element. The magnetic sheet generates a magnetic field that increases in magnitude (with respect to the magnetic elements) as the sheet deflects or sinks toward the circuit plate so that the generated electrical signals are proportional to the distance between the magnetic sensitive elements and the respective overlying portions of the magnetic sheet. In this manner, the input panel can generate a plurality of different electric signals that are representative of the varying degrees of pressure applied by the object to the magnetic sheet, e.g. due to surface variations (ridges and grooves) of a fingerprint.
When installed, the pressure input panel of the invention is coupled to an X register and a Y direction switch which cycle through the groups of electrode lines so that a detector can correlate the generated electrical signals to the location of the detection units from which they emanate. The electrical signals are then appropriately processed so that the contour of the fingertip can be reproduced. With this configuration, the invention provides a relatively inexpensive surface pressure input panel that exhibits high definition over a relatively large detection area so that extremely complicated surfaces, such as fingerprints, can be accurately detected and reproduced.
In one embodiment, the magnetic sensitive elements each have a resistance to electric current generally proportional to the magnitude of the magnetic field to which the element is subjected. Thus, as the magnetic sheet is deflected towards the magnetic element, its resistance increases. The magnetic elements are coupled to electrode lines associated with each intersection point such that, when an electric potential is applied to the associated electrode lines, a current flows through the elements. This current will be generally proportional to the resistance of the magnetic element and, therefore, will reflect to the distance between the element and the respective overlying portion of the magnetic sheet (i.e., the pressure applied to the sheet by the object).
In another embodiment, the magnetic sensitive elements are coupled to the first and second groups of electrode lines by switches, preferably thin film transistors (TFT's) of the type widely used for driving liquid crystal displays. One end of the magnetic element is coupled to the transistor and the other end is coupled to a detection line. When an electric potential is applied to the electrode lines, a current, which is proportional to the resistance of the magnetic element, flows through the magnetic element and along the detection line, where it can be sensed by the detector.
In a further embodiment, the magnetic sensitive elements comprise conductive elements that preferably exhibit the