Signature verification apparatus and method utilizing relative angle measurements
BACKGROUND--FIELD OF THE INVENTION 
     The present invention is concerned with an apparatus and a method for comparing a sample signature with reference signature characteristics, and more particularly a low-cost stylus for use with algorithms that accurately and rapidly identify individuals. 
     BACKGROUND--DESCRIPTION OF PRIOR ART 
     For many years significant efforts have been made to develop systems that reliably establish the identity of a person. Such systems are deemed desirable from the point of view of both the identifier and the identified in a variety of settings. Some uses for identify verification systems include: credit card transactions, access to computer programs and information, electronic funds transfer, and the electronic transmission of legal documents and signatures. In these types of situations identity verification must be inexpensive, reliable, and non-intrusive. 
     Many proposed systems measure some biological trait unique to the individual. Reference measurements are taken and stored for comparison later at the time of verification. For example, there exist systems using voice recognition, palm print recognition, and laser retinal scans. These systems have achieved some success in terms of reliability and utility in restricted situations. Retinal scanners are used in some prisons and high security areas in government and industry. However, the public perception of intrusiveness has kept such systems from widespread applications in everyday transactions. Complexity of the systems and costs have also slowed development and implementation. 
     Identity verification by means of written signatures, electronically recorded, has long been known, in spite of significant shortcomings. One advantage of systems using the familiar signature and pen is that intrusiveness is not considered a problem. The signature is commonplace and expected in everyday financial transactions and as a means of general identification. Also, a major advantage of signatures as identification is the unique role that the signature plays in human affairs. The signature of ink on paper has been, and continues to be, the quintessential mark of business. No substitute for the signature is likely to be developed. Contracts, credit card slips, and checks become legal, once signed. The requirement of legal signatures on documents increasingly causes a bottleneck when the rest of the information and documentation is transmitted electronically. 
     Dynamic signature verification typically records the various pressures, velocities, accelerations, and directions of writing by means of transducers or accelerometers housed in a special pen or writing plenum. Reference signatures are acquired during an enrollment process. The electronic signals from the reference signatures are analyzed and stored for comparison to offered signatures during a verification transaction. 
     Dynamic signature verification systems should be distinguished from systems designed just to capture a signature and replicate it later without verification. Also, plenum (or pad-based) systems which house electronic devices in the writing pad rather than the pen itself should be distinguished. 
     The system described herein consists of a stylus (pen) containing pressure sensing transducers, a unit for amplifying, digitizing, sampling, storing, and transmitting these signals via phone. Finally, algorithms are developed for the analysis of the specific and unique type of signals sent by the data input stylus. Special algorithms are developed for the processing of signal trains from signatures for enrollment of reference signatures and the verification of sample signatures. 
     PRIOR ART 
     Known in the art of dynamic signature verification are a variety of pens (styli) containing electronic sensing means designed to measure some combination of dynamics such as pressure, acceleration, velocity, or direction produced while signing a signature. 
     U.S. Pat. No. 3,528,295 issued Sep. 15, 1970 to Johnson et al., is an early version of a data input stylus. Disclosed is a pen containing pressure responsive transducers mounted therein. In the first, relatively simple embodiment of the stylus, a single transducer is provided for measuring downward (z-axis) pressure. 
     In a more sophisticated embodiment of U.S. Pat. No. 3,528,295 four additional transducers are provided which are oriented in an orthogonal relationship to provide information in two additional axis (x and y directions). During writing signals would then be generated corresponding to the pressures exerted in the left to right directions on the plane, or paper (x-axis), far to near (y-axis), and downward (z-axis, sometimes called &#34;p&#34; for pressure downward). 
     Subsequent work continued to rely on the three axis model using varieties of dynamics along one, two, or all three axis. Obtaining unambiguous readings on the x-y plane (that is, distinguishing x signals from y signals on the plane) was problematic since it required that the pen be held in the same way and not rotated in the hand between signatures. 
     U.S. Pat. No. 3,906,444 issued Sep. 16, 1975 to Crane et al., demonstrates a pen shaped in such a way as to be held in only one correct way when signing. The output signals represent the direction that the pen is moved, which is converted into direction per unit of time. 
     U.S. Pat. No. 3,986,403 issued Oct. 19, 1976 to Hurd et al., measures pressures along the x, y, and z (also called &#34;p&#34; for pressure) axis using a number of strain gages. This pen is also designed to be held in a particular way. 
     U.S. Pat. No. 4,078,226 issued Mar. 7, 1978 to EerNisse et al., measures pressure forces proportional to the acceleration of the writing tip (muscle movements) distinguishing x and y forces. In one embodiment a pressure sensitive writing surface utilizing piezoelectric transducers is provided to produce an output representing the z (pressure downward) force. 
     U.S. Pat. No. 4,513,437 issued Apr. 23, 1985 to Chainer et al., is designed to measure acceleration along the x,y axis. That is, the orthogonal forces lying essentially in a plane perpendicular to the pen axis. The remaining axis, z (or pressure) is measured with a pressure transducer. 
     U.S. Pat. No. 4,896,543 issued Jan. 30, 1990 to Gullman discloses a three-axis force measurement stylus designed to generate an electrical signal representing the force applied in the direction of motion taken in writing. Strain gages are used to sense force in three dimensions (along three axis). 
     U.S. Pat. No. 5,111,004 issued May 5, 1992 to Gullman contains a series of sensor portions in a PZT annulus which is designed measure and distinguish pressure applied along the three distinct axis, x, y, and z. 
     Also known in the art are a number of methods of analysis or algorithms designed to analyze the incoming signals of a data input stylus to determine if the sample signature under consideration matches the reference signature data on file. 
     U.S. Pat. No. 3,983,535 issued Sep. 28, 1976 to Herbst et al., also has as a preferred embodiment input device a tablet, or plenum, which allows unambiguous x and y signals on a plane. This method is based on the theory that accelerations of the stylus, which are proportional to the muscle forces exerted by the signer, are of predetermined consistent durations when forming particular strokes. The nature of the process gives rise to various distortions in the time axis: e.g. pauses between sections of the name, skipped strokes, decorative rubrics, etc. Thus the signal is marked by regions of high correlation of unknown duration separated by variable regions of low correlation. Accordingly, the invention deals with a method of regional correlation which attempts to register these regions, at first based on stylus contact (segmentation), shift them individually to find the maximal of a correlation function. 
     U.S. Pat. No. 4,128,829 issued Dec. 5, 1978 also to Herbst et al. is an improvement of U.S. Pat. No. 3,983,535. While U.S. Pat. No. 3,983,535 was based on a single acceleration parameter of the signature dynamic, U.S. Pat. No. 4,128,829 uses two orthogonally disclosed (x and y axis) acceleration components along with pressure patterns (z axis). 
     The x, y, and z components must be processed in complex format and combined to form a complex correlation wherein the correlation magnitude is utilized for subsequent decision purposes. 
     The pen required would have x and y accelerometers along with the ability to measure the pressure signal with some sort of axially mounted pressure sensitive transducer mounted in the pen. The preferred pen would need to be un-oriented (not shaped to it was required to be held in a particular way to orient the accelerometers). Thus the analysis system is required to mathematically rotate the signals produced in order to obtain unambiguous x and y data. 
     U.S. Pat. No. 4,128,829 by Herbst requires a large amount of highly specialized and complex hardware. The operation of U.S. Pat. No. 4,128,829 must utilize an overall segmentation and relative shifting scheme as taught in Herbst U.S. Pat. No. 3,983,535. The system also requires pens having consistent characteristics for measurement. Finally, the criteria cut-off levels for accept or reject are based on empirical statistical results partially based on large population testing bases. 
     U.S. Pat. No. 4,553,258 issued Nov. 12, 1985 to Chainer et al., uses acceleration and pressure signals but is reliant on segmentation of signal trains based on pen-up and pen-down times. 
     U.S. Pat. No. 4,738,445 issued Apr. 5, 1988 by Gunderson et al., is a signature verification system utilizing a pen input device described in U.S. Pat. No. 4,813,437 which uses bimorph piezoelectric transducers which produce electrical signals in response to the rate of change of axial pressure on the pen and acceleration of the pen. This patent retains work done in U.S. Pat. No. 4,128,829, mentioned above, regarding the necessity of segmentation based on pen lifts and correlations calculations. 
     The similarity measure developed to accept or reject signatures uses a set of parameters that depend upon the normalized distributions of the input measures for a general population of users. And peior methods have relied upon existing data input styli or devices, limiting innovation. 
     Problems can be summarized as follows: 
     1. Signature verification systems begin at a data pickup point, either a stylus or a pressure pad. The requirement of distinguishing the &#34;x&#34; direction on the plane from the &#34;y&#34; direction causes a number of problems. One solution is to employ a pad as a pickup device so the &#34;x&#34; and &#34;y&#34; coordinates are more clearly separated. 
     In the development of data input pens an early requirement was that the pen be held in a particular way so as not to confuse the &#34;x&#34; and &#34;y&#34; measurements. Later, &#34;mathematical rotation&#34; of the data was substituted for the rotation of the pen, but the problems of complexity remained. 
     In order to make the precise differentiations required, the pens developed tended to be composed of delicate and sensitive parts, not easily calibrated, that were quite expensive. Making the pen durable and sensitive has been a problem. 
     2. The data produced by prior art data input pens (styli) is complex and has no clear analogy to a measured phenomenon. Often, population parameters are included in an attempt to bring logic to complex data resulting from the variation of signals coming in from three dimensions (x, y, and z). The complexity is compounded when additional calculations are used to convert speed into acceleration measures, or when absolute measures are employed. 
     3. Prior art signature verification analysis relies on the segmentation of the signature into signal parts based on pen lifts and pen-down times on the paper. Such segments are natural, but their variation from signature to signature of the same signer causes extreme complexity in allowing for, and correcting, such segment variation. 
     4. Finally, the accuracy of prior art systems is a problem. False rejections, based on a true signer being rejected for variations are not only most common, but provide most of the problems in practice. The algorithms developed are not easily tuned to the individual registrant and not easily changed to meet changing conditions. 
     OBJECTS AND ADVANTAGES 
     The disadvantages and obstacles in the prior art are overcome in the invention disclosed herein, by taking advantage of a fundamentally different stylus (data input pen) device. The data collection device allows the verification data to be collected and treated in fundamentally new ways in signature verification enrollment and verification algorithms. 
     Accordingly, several objects and advantages of my invention are: 
     First, that a fundamentally different stylus is disclosed that measures a new signature dynamic called the relative angle. Measurements are made of the variations around the relative angle of the signer. To measure the relative angle and the resulting variations around the angle, a new stylus design is incorporated with the following advantages, 
     a. The orientation of the pen on the writers hand (the rotation of the pen) is irrelevant to the measurements taken. Thus, the pen need not be shaped in a particular way nor the measurements manipulated or mathematically rotated later. 
     b. The measurements taken are simpler than those measured on a three axis model since the x and y axis are not differentiated, and only a ratio is needed to calculate the relative angle. 
     c. The resulting pen is more durable and less sensitive than those utilizing delicate electronic equipment. The measurements derived are relative to one another and absolute accuracy is not required to as great a degree. 
     d. The resulting stylus is less expensive and more durable since just two simple transducers are used to arrive at a ratio measurement. 
     e. Calibration is not necessary, nor any fine adjustments to components of the stylus. 
     A further fundamental difference and advantage is that the resulting measurements taken allow a simplified and more useful enrollment and verification method (algorithm). The disclosed algorithms have the following advantages: 
     a. The measurements taken are based on a concrete analogy to the physical reality of dynamic signature measurement. The relative angle is &#34;like&#34; the actual angle of the pen in relation to the surface of the paper. The simple ratio of direct pressure to lateral pressure on the stylus is the measurement definition of relative angle. This analogy allows for data reduction which is necessary for simplicity. Also, complex correlations without analogy are avoided as a method of verification. 
     b. The disclosed enrollment algorithm is more easily fine-tuned to the parameters of the individual signature. This allows more accuracy and the reduction of Type I (false rejection) and Type II errors (false acceptance) in verification. Also, there is no reliance on population parameters of signatures so probability plays a lesser role in verification. 
     c. The analysis of the data in the algorithms does not use segments based on pen-up and pen-down times. Segments used are simple divisions of the signal train which relativizes the duration of the signals and provides simple measurement points. 
     d. The increasingly higher calculation speeds of computers is taken advantage of to measure large numbers of measurement points based on the relative angle of a signature. The selection of just the most consistent from these large numbers tailors the enrollment to the true signer, and allows continual simple adjustments to be made to the signature discriminate measures. 
     e. Verification time is greatly reduced once enrollment is established since the algorithm examines only at assigned measurement points from the flood of incoming data. 
     f. The signature discriminates resulting are not extremely complex correlations, but rather highly individualized simple measures derived from the relative angle measures. 
     g. The verification criterion are easily updated to reflect changes in the way a signature is signed over time. Also, the signature stringency scores, the score of matched points that must be attained for verification, can be easily adjusted to meet a variety of changing conditions. 
    
    
     DRAWING FIGURES 
     In the drawings, closely related figures have the same number but different alphabetic suffixes. 
     FIG. 1 shows vector forces measured in prior art. 
     FIG. 2 shows the direct and lateral pressures and their relationship as a relative angle in the present invention. 
     FIG. 3 shows the pressure sensitive data input stylus. 
     FIG. 4 shows a signal processing unit diagram. 
     FIGS. 5A and 5B show a flow-chart indicating a computer algorithm for the signature enrollment process. 
     FIG. 6 is a flow-chart indicating a computer algorithm for the signature verification process. 
     FIG. 7 is a hypothetical chart of a signature using relative angle as the measurement method. 
     REFERENCE NUMERALS IN DRAWINGS 
     30 Stylus vector diagram 
     32 Downward force of handwriting 
     34 Pressure relay tip 
     36 Direct pressure vector 
     38 Lateral pressure vector 
     40 Relative angle 
     42 Electrical lead wires 
     50 Data input stylus 
     52 Stylus writing tip 
     54 Stylus tip opening 
     56 Stylus nose piece 
     58 Lateral pressure transducer 
     60 Stylus cartridge 
     62 Retainer ring 
     63 Plunger stop ring 
     64 Direct pressure plunger piece 
     66 Direct pressure transducer 
     68 Retainer plug 
     70 Stylus housing 
     72 Empty section of stylus 
     74 Transducer transmission wires 
     76 Stylus transmission tether wires 
     78 Vector force diagram 
     90 Direct pressure signal train input 
     92 Lateral pressure signal train input 
     94 Direct pressure amplification 
     96 Lateral pressure amplification 
     98 Power pack 
     100 Analog/Digital converter (sampler) 
     102 Computer memory 
     104 Microprocessor/sequencer 
     105 Computer storage 
     106 Modem 
     108 Microcomputer 
     110 Phone transmission link 
     130 Signal processing for enrollment 
     132 Begin enrollment (signal from central) 
     134 Signal trains received at central computer 
     136 &#34;D&#34; and &#34;L&#34; amplitude recorded 
     138 Ratio of &#34;D&#34; and &#34;L&#34; at each sample point 
     140 Define signal train segments 
     142 Calculate relative angle of segments and whole 
     144 Compare relative angle of segments (measurement points) 
     146 Calculate difference in relative angle of segments 
     148 Repeat above procedures for all signal trains 
     150 List measurement point values from all signal trains 
     152 Calculate range of values at each measurement point 
     154 Select least-range measurement points 
     156 Tentative measurement point discriminate selection 
     158 Test tentative measurement points 
     160 Adjust discriminates if necessary 
     162 Assign discriminates 
     164 End enrollment process 
     180 Verification signal processing 
     182 Extract measurement point values from signal train 
     184 Compare reference measurement point values to sample signature measurement point values. 
     186 Add total matches 
     188 Determine if score is above stringency cutoff 
     190 Determine if score is below stringency cutoff 
     192 Send accept message (if above stringency) 
     194 Send reject message (if below stringency) 
     210 Relative angle 
     212 Lateral pressure percentage 
     214 Direct pressure percentage 
     216 Quadrant segments 
     218 Decimated segments 
     220 Percentage of duration 
     222 Sample points, relative angle 
     224 Relative angle average line 
     226 Sample point variation around average 
     228 Pen up sample points 
    
    
     DESCRIPTION--FIGS. 1 TO 7 
     Before proceeding with the detailed description of the operation of the actual hardware and software of the present invention there will follow a brief description of the theoretical basis for the invention and a description of the forces which the system senses and analyzes. 
     The method of data collection and method of analysis of the present system is based on my work with actual writing instruments and the forces and pressures present when writing. Unlike prior art conceptualizations of three axis (dimensions) of motion and force in dynamic signature analysis, the present system is based on two basic pressures exerted on the stylus while writing. Prior art conceptualizations of the measurements possible with the stylus are indicated in FIG. 1. Prior theories were based on muscle pressures exerted through the stylus causing variations in downward pressure (z forces), and variations in directions, velocity, or acceleration along the x,y axis of the plane. 
     In contrast, FIG. 2 indicates the forces and resulting pressures measured in a fundamentally different way in the present invention. The downward force of handwriting 32 through the stylus 30, caused by hand muscle pressure is, at any given point in time, a simple downward force absorbed at pressure relay tip 34. Other writing dynamics may be present, such as the direction or acceleration of the stylus, but these dynamics are not measured in the present invention. At a given moment (a digitized sample point) during a signature the downward force of writing is absorbed by the pen starting at the pressure relay tip 34, in two fundamental and measurable ways. First, there is a direct pressure 36 which is the axial, plunger-like force produced in the stylus. Secondly, a lateral pressure 38 is produced because the pen is used at an angle to the writing surface, and because writing takes place from left to right moving the pen along in a lateral direction. 
     The two basic forces or pressures, added together, equal the total force. Importantly, the two forces present are not used as variables individually, but rather form a single measure based on their ratio, and the variation in that ratio. The two pressures, measured by transducers, produce electrical signals relayed for processing and analysis by electric lead wires 42. Measurements are more static than dynamic, as the ratio is measured at individual sample points based on the digitization of the data. These basic measures form the building blocks for the analysis of the variation of the ratio throughout the signature, and are manipulated to define unique signature discriminates for verification. 
     The ratio of the two basic pressures can be conceptualized and visualized as an angle 40 of the stylus to the writing surface. The ratio of lateral pressure 38 to direct pressure 36 indicates the angle of the pen to the surface which is known as the relative angle 40, or &#34;RA&#34;. As the angle of the pen becomes more perpendicular to the writing surface when pressure is applied, the relative direct pressure increases and the relative lateral pressure decreases. This relationship of direct and lateral pressure is reversed as the pen angle becomes more horizontal to the writing surface. The two pressures are measured in a stylus (FIG. 3) designed to measure the inverse correlation. As in any ratio relationship the actual total pressure and variations in gross pressure is held as a constant. The measurements are indicators of angle, not necessarily precise measurement of angle, since angle measurement, in itself, is not a goal of the invention. 
     This theory of analysis, based on lateral and direct pressures and the variations of their ratios during a signature, has an analogy which is helpful in visualizing the process. The analogy is that of a motion picture film taken of the hand and pen during the signing of a signature. If a camera took twenty frames per second and the signature lasted five seconds, the film could be made into one hundred still photographs of the signing. 
     These hundred photographs represent the digitization of the signal coming from the pen. They are static snapshots in time, like a digital sample of electronic signals. If the photos were laid out in order, some information about the signature would be available and some would not. The photographs would reveal the angle of the pen to the writing surface at each point in time, and would reveal the total time as the sum of the photos. Exact measurement of time and amplitude are only used for conversion to ratio relationships, however. Duration and amplitude are only finally measured in a relative way for analysis. 
     The photos provide limited information about the signature. But in this case using less information is not a hinderance, but rather, a desired advantage. Advanced artificial intelligence computer algorithms make use of the concept of data reduction by analogy. Maximal data collection is not attempted in favor of useful data collection based on a concrete analogy. This technique is aided by the increasing speed of computers which allow many calculations using the specifically limited data. 
     Continuing the analogy of the hundred photographs, the stylus angle in each photo could be measured with a protractor and the angle to the writing surface noted. When all the photos were so measured the average angle for the entire signature could be determined. This measure, called herein the relative angle average, is itself a variable since the same person could write at different angles depending on the desk height or other environmental conditions. The critical measurement is not the overall average relative angle, but the specific and unique variations around the relative angle which take place while signing a signature. The particular variations around the overall average relative angle are quantified by examining ratio relationships within in the signature, specifically between segments of the signature signal train. These variations I call measurement point values, which are also relative measures, not absolute. 
     The analysis performed here makes use of a technique known in the art as segmentation. That is, dividing the signature signal trains into parts for analysis. But segmentation is used here in a fundamentally different way. Much prior art segmentation has to do with the detection and definition of segments occurring as the stylus is lifted off the paper when signing, thus creating a series of natural segments of pen-down times. This method has caused great amounts of increasing complexity since signers naturally vary their signatures in ways that could change the segment count. For example, not dotting an &#34;i&#34; could change the entire segment profile. 
     Segmentation of signature signal trains is useful for analysis, but it can be more easily accomplished in the present system by simply dividing the total signal into ten equal parts. Similarly, additional segments can be created by dividing the signal into any number of equal parts. Signal trains can be divided into four equal parts, one hundred equal parts, or twenty five equal parts, and so forth. It will be obvious that this method standardizes the duration of the signal, making actual durations irrelevant. Also, since segment comparisons are used for analysis, it is apparent that only a small number of segmentations are necessary to provide large numbers of mathematical combinations of segments for analysis. 
     To perform an analysis, the first signature segment would be comprised of the first ten percent of the signal if segmentation were done into ten parts. This decimation of a signal 218 is indicated in FIG. 7. Quadratic segments 216 are also indicated in FIG. 7, each segment being one quarter of the signal. The average relative angle 224 is the average for the entire signal and will not, in most instances, be identical to individual segments. The relative angle of digitized sample points (indicated by the dots in FIG. 7) for the first decimated segment 218 will be different than the relative angle of the first quadratic segment 216. This difference is caused in part by the high direct pressure digitized sample points 222 which resulted, in this example, by the dotting of an &#34;i&#34; in a signature. Thus, the first segment would then be examined to determine the average relative angle of that segment alone. Then a comparison is made between the relative angle of the first segment and the relative angle of other segments and the signature as a whole. In addition, many other comparisons are made based on the possible mathematical combinations. Each specific comparison made defines a measurement point. Large numbers of measurement points can be extracted by comparing various types of segments to the whole and to each other (mathematical combinations) and comparing the relative angle differences, or other statistics (like the range in a segment, or segment cluster measures) applied to the ratio of the direct to the lateral forces. 
     In one specification of measurement based on the relative angle ratio (RA), comparisons of one segment to another, and to the whole, are made. A difference measurement is calculated, representative of a variation in the signal, called a measurement point value. It is critical to note that the ratio of direct and lateral forces is the electronic indicator of the angle of the pen, not a precise measurement for the angle. Pen angle to paper is the useful analogy for data reduction and analysis. This is important because the overall average relative angle, while important for relative measurements, is not the final measure of analysis. Measurement points reflect segment variation around the relative angle irrespective if the relative angle varies from signature to signature. It is the patterned variations around the average that is the identifier of signers. 
     When the stylus is used in a way nearly perpendicular to the surface the &#34;D&#34; or direct pressure predominates. When the pen is slanted greatly approaching being parallel with the surface the &#34;L&#34; or lateral pressure predominates. The two forces taken together always equal the total force applied. The relation of these force vectors to the data input stylus of FIG. 3 is shown in vector force diagram 78. The absolute force downward is treated as a constant rather than as a variable, as in prior art. 
     If the pen is initially calibrated so that at a 45 degree angle to the surface a downward pressure on the stylus tip measures fifty percent lateral pressure and fifty percent direct pressure, it will be seen that the actual force, no matter how hard or variable, will not affect the essential ratio of the two basic forces. Thus force itself is held as a constant with only the ratio being measured and indicating the angle. This is a critical distinction between the present methodology and earlier conceptualizations of the &#34;z&#34; force as being a variable reflecting variations in downward pressure to be measured. In the same way the lateral pressure does not attempt to make distinctions in direction or acceleration, but is useful only in combination (ratio) with the corresponding direct pressure, combining to create the measurements of variation around the average relative angle which are the essential measurements of this system. 
     Thus the measures that describe individuals by their signature is not sought in population parameters or complex correlations as in the prior art. The unique signature characteristics are determined by the individual signer as that signer provides a variety of signatures. The enrollment signatures instruct the enrolls to sign in a variety of ways, such as faster or slower, larger or smaller, and harder and softer, in order to make the consistent measurement points and measurement point values stand out more clearly in the analysis. 
     Recent increases in computer speed allow very large numbers of segment comparisons to be made inexpensively and quickly. Of the large number of measurement points examined by the computer, a smaller number will be consistent and relatively unique for that signer. These areas of consistency provide the simple, relatively invariate, unique measures for signature verification. The resulting small number of specific measurement points (MPs) that identify an individual are called the signature verification discriminates. 
     Once the specific discriminates have been identified in the enrollment process, the verification procedure is greatly simplified by extracting only these values needed for comparison from the incoming signals for comparisons to the reference standards. Parameters are also easily adjusted as conditions dictate. 
     FIG. 3 shows the structure and internal components of data input stylus 50. Stylus housing 70 is substantially tubular, housing components which are substantially cylindrical. Stylus housing 70 has an opening at one end for the writing tip 52 of stylus cartridge 60, and at the other end for transducer transmission wires 74 which attach to stylus transmission tether wires 76. Stylus tether wires 76 and transducer wires 74 supply electricity to direct pressure transducer 66 and transmit electric signals out of data input stylus 50. Stylus tether wires 76 and transducer wires 74 also supply electricity to lateral pressure transducer 58 and transmit electric signals out of stylus 50. Stylus housing 70 is composed of metal or durable plastic. 
     The pressures sensed while writing with data input stylus 50 originate in writing tip 52 as a signature is signed on a writing surface. The downward pressure of writing is indicated in vector force diagram 78 where &#34;P&#34; is handwriting pressure downward on the writing surface with data input stylus 50. In response to handwriting pressure two pressures are measured in the stylus, the lateral, or &#34;L&#34; pressure, and the direct, or &#34;D&#34; pressure, indicated in vector force diagram 78. 
     Writing tip 52 is part of the stylus cartridge 60 which is movable in response to handwriting pressures. In this embodiment stylus cartridge 60 is a ball point pen refill cartridge which is replaceable. Stylus cartridge 60 is movable, within predetermined limited range of motion, in two ways. First, direct pressure is registered through stylus cartridge 60, and transferred through direct pressure plunger piece 64, which is also movable axially within a predetermined range. Plunger piece 64 presses against direct pressure transducer 66 producing electrical signals proportional to variations in direct, axial pressure. 
     Excessive direct pressure against transducer 66 is prevented by plunger stop ring 63 attached to tubular portion of plunger piece 64. Stop ring 63 comes in contact with retainer ring 62 at maximum allowable pressure on transducer 66. Retainer plug 68 is attached to the interior surface of stylus housing 70 and holds direct pressure transducer 66 in place. Retainer ring 62 is secured to the interior of stylus housing 70. Plunger piece 64 is held in place by having the disc portion contiguous with transducer 66 on one side, while the other side is contiguous and held in place by retainer ring 62. Retainer ring 62 has a center hole through which passes the tubular portion of the direct pressure plunger piece 64. The tubular portion of direct pressure plunger piece 64 passes through the center hole of retainer ring 62 and secures stylus cartridge 60. The tubular portion of plunger piece 64 is of a larger diameter than stylus cartridge 60 so that limited lateral motion of stylus cartridge is permitted at the writing tip 52. Stylus cartridge 60 snaps into plunger piece 64 at the base so that the stylus cartridge will not fall out of stylus housing 70, yet limited lateral movement is permitted. 
     Writing tip 52 and stylus cartridge 60 are also movable laterally in response to handwriting pressures. Lateral pressure is transferred through stylus cartridge 60 as sideways pressure against lateral pressure transducer 58. Transducer 58 measures only one force, sideways, irrespective of direction. Transducer 58 is molded to surround stylus cartridge 60 but allows sufficient clearance so that the axial movement of stylus cartridge 60 is possible. Another embodiment uses a pressure sensitive pad, attached to the interior of stylus housing wrapped around interior of stylus housing 70, but not restricting the axial movement of stylus cartridge 60. 
     Transducer 58 is secured to the interior surface of stylus housing 70. The exterior surface of stylus cartridge 60, and the interior circumference of lateral pressure transducer 58, is composed of a low friction material to allow axial movement even when stylus cartridge 60 is in contact with lateral pressure transducer 58 when writing. 
     Nose piece 56 is removable allowing access to the interior of stylus housing 70. The hole in the tip of the nose piece, stylus tip opening 54, allows the stylus cartridge to pass through the stylus housing 70. Stylus tip opening 54 is of a predetermined diameter so as to prevent excessive lateral pressure against lateral pressure transducer 58 by limiting the lateral movement of stylus cartridge 60 to a predetermined lateral range of motion. 
     Lateral pressures against lateral pressure transducer 58 by stylus cartridge 60 during writing relay electrical signals proportional to pressure from lateral pressure transducer 58 through transducer transmission wires 74, through stylus transmission tether wires 76, to signal processing means. 
     In a like manner direct pressures against direct pressure transducer 66 relays electrical signals proportional to pressure from direct pressure transducer 66 through transducer wires 74, through stylus tether wires 76 to signal processing means. 
     Empty section 72 of the data input stylus provides room for electronic equipment, as miniaturization progresses, but such equipment is not part of this patent. Another embodiment allows larger transducer means to occupy the empty section 72 
     FIG. 4 diagrams the signal processing unit which receives signals from the pressure sensitive data input stylus 50 shown in FIG. 3. Signals produced in data input stylus 50 enter the signal processing unit from direct pressure signal train input 90, and lateral pressure signal train input 92. The inputs arrive by way of stylus tether wires 76 of data input stylus 50 shown in FIG. 3. 
     In FIG. 4 direct pressure transducer signal trains from stylus arrive at direct pressure signal input 90 and are amplified by the direct pressure amplifier 94. Lateral pressure transducer signal trains arriving at lateral pressure signal input 92 are amplified by the lateral pressure amplifier 96 
     At analog/digital converter 100 both arriving signals are sampled by the process of being digitized. Digitized signals are used in the system from this point forward. The digitized signal trains then are processed and operations sequenced by microprocessor/sequencer 104. In the preferred embodiment the computer memory 102, microprocessor 104, modem 106, and computer storage 105 are comprised of a microcomputer 108. In this embodiment signal inputs 90 and 92, analog/digital converter 100, and power pack 98 comprise a peripheral device for an existing microcomputer 108. In this embodiment the signal processing unit is linked to the central enrollment and verification computer at a distant location. The phone transmission link 110 allows sequencer to receive instructions from the central computer during enrollment algorithm procedures (FIG. 5 A-B) and during the verification algorithm procedures (FIG. 6). 
     In a second embodiment all of the components in FIG. 4 are contained in a signal processing unit. Power pack 98 supplies electricity to all components and is plugged into standard AC current. 
     Modem 106 sends digitized signal trains to a central verification and analysis center via phone transmission 110. But in a second embodiment parameters for verification are held in storage 105 while analysis of enrollment signatures still takes place at a central location via phone 110. 
     Modem 106 allows digitized signal trains to be sent to a central location and allows accept or reject messages to be sent back to the local verification station where the signal processing unit is located. The signal processing unit is also used to collect reference signatures for analysis and to transmit these signatures to a central analysis center where computer programs (FIG. 5 A-B) are used in the analysis of signal trains and the establishment of specific electronic signature discriminates for individual signers. 
     FIG. 5, A-B is a flow chart describing a computer algorithm used as a method for enrolling reference signatures in a signature verification system. The first step 130 provides signal trains to a central computer location originating when an enrolls uses the data input stylus shown in FIG. 3 to provide enrollment signature signals which are processed by machine method shown in FIG. 4. The data input pen shown in FIG. 3, and the signal processing device shown in FIG. 4, are used in the system to produce and transmit reference signatures as well as signatures to be verified at a later time. The enrollment process begins in the second step 132 as the process is initiated by the central computer and relayed to microprocessor 104 of the local signal processor. Digitized signals arrive via phone for analysis at a central computer 134. 
     The enrollment procedure requires an enrolle to use the data input stylus when signing twenty enrollment signatures. An enrollment form, used in the beginning of the enrollment process 132, indicates how the signatures are to be signed by the enrolle: faster vs slower, larger vs smaller, harder vs softer, and the resulting combinations of the above. These combinations result in requests for signatures signed in the following ways: fast/hard, fast/large, fast/small, fast/soft, slow/large, slow/small, slow/hard, slow/soft, large/hard, large/soft, small/hard, and small/soft. In addition to these specific signing instructions, the enrolle is asked on the form to provide eight additional normal signatures. 
     In the third step 134 the signals are received and stored at the central analysis location. Each signal train, representative of a signature, is treated separately during analysis. 
     The fourth step 136 records the direct pressure amplitude and the lateral pressure amplitude for each digitized sample point in the signal train. Digitization used for sampling involves a sampling rate in the range of 100 samples per second, so an average signature will consist of approximately 500-800 sample points and the accompanying values of direct and lateral pressure. There is, however, no need for a maximum allowable time to sign an enrollment reference signature. 
     The fifth step, 138 calculates the ratio of the direct pressure signal amplitude to the lateral pressure signal amplitude at each sample point. This standardizes amplitude as a ratio for further calculations. The resulting values are known as the relative angle ratios. 
     When the relative angle ratios have been calculated for all digitized sample points the analysis shifts to calculations involving the duration of the signal trains and the division of the signal train into parts, or segments. 
     The sixth step 140 divides the signal trains into equal part segments for analysis. The signal train is divided into quadrant segments, each segment being 25 percent. The same signal is then divided into decimated segments, each segment being 10 percent of the signal train. The signal train is then divided into twenty five segments, each being 4 percent of the signal train. And finally, the signal is divided into 50 parts, each segment being 2 percent of the signal train. This process is not limited to the segment divisions listed, and, if more segments are needed in the analysis, other kinds of segments can be defined and the signal train divided in other ways. The resulting segments are defined by the sample points and the values of the sample points falling within them. 
     The sixth step 140, defining segments, converts signal train durations to relative measures as each complete signal train is considered one hundred percent of the signal train, no matter how long, and then divided on that basis. 
     Having defined segments and assigned sample points and their ratio values to each segment, various calculations can be made for each defined segment. The seventh step 142 averages the amplitude ratios of sample points within each segment, and averages the ratio points for the entire signal train. This results in each segment having a relative angle measurement as well as the whole signal train having a relative angle measurement. The relative angle of the entire signal train might well vary between signal trains from the same signer, so the overall relative angle is not used as a discriminate. However, the relative angle of the entire signal is useful as the average around which the segment relative ratios fluctuate. The unique nature of some of these fluctuations for an individual signer form the basis of the signature discriminate measures. 
     The eighth step 144 compares all segment measurements to each other. Each segment is compared to each other segment in all possible mathematical combinations. When the signal train has been divided into four segments, ten segments, twenty five segments, and fifty segments, this results in eighty nine defined segments. There are 3,916 possible combinations of two segments, or comparisons, that can be made. Each comparison is called a measurement point. The number of measurement points needed is a variable depending on computer speed and comprehensiveness of analysis desired. By adding an additional segmentation to the above defined segments of seventy five segments, the total number of segments would then be the original eighty nine plus seventy five which equals one hundred and sixty four segments. The number of combinations thus increases to 13,366 combinations, or measurement points. 
     Additional measurement points can also be created by simply combining numbers of segments and treating the combination as one new segment. The number of possible combinations, and therefore the number of new measurement points, increases greatly when this method is used. There is no practical limit to segment definition and creation. 
     The ninth step 146, measures comparisons at each measurement point. Since each segment has a relative angle measurement, the difference in relative angle at a measurement point results in one measurement point value. If one hundred and sixty four defined segments were analyzed, as in the above paragraph, the 13,366 measurement points would have 13,366 measurement point values defining the signal train of the signature. 
     Step ten 148 repeats the above calculations for each of the twenty enrollment signature signal trains. Each signal train is analyzed individually, but each measurement point is assigned the measurement point values from all signals. 
     The second phase of the analysis of reference signatures is diagramed in FIG. 5-B. The first step 150 of phase two lists all of the measurement point values (MPV&#39;s) obtained from the reference signatures for each measurement point (MP). 
     The second step 152 calculates the range of values obtained at each measurement point from the twenty signatures. 
     The third step 154 selects a smaller number of measurement points having the lowest ranges, in this case 500 measurement points, to serve as tentative signature verification measurement points. 
     The fourth step 156 defines the selected low-range measurement points and their range of acceptable values. The range of acceptable values is simply the range of scores obtained at a given measurement point during enrollment. A tentative stringency score consists of the number of measurement point matches a sample signature must achieve to be verified. The tentative stringency score need only be an educated guess, based on other similar ranges of measurement points, prior to testing. 
     The fifth step 158 consists of testing the tentative measurement points and stringency score using actual signatures provided for the purpose by the enrolle. Here the phone link between the central analysis computer and the local signal processor is again used. A signature signal train is sent via phone lines to the central analysis center in order to test the tentative signature discriminates. 
     The sixth step 160 is to adjust the stringency score and the ranges of the measurement points if necessary, to balance the probability of false rejections of the true signer against the probability of false acceptance of a false signer. The enrolle can have some discretion in the decision about this balance based on the needs of the enrolle. In fact, a variety of stringency scores can be employed for one signer depending on the importance of the signature. For example, a lower stringency score for credit card purchases under one hundred dollars is possible. Thus the possibility of false rejection is decreased because the importance of the transaction is less serious. Extremely important signings might have an extremely high stringency score. 
     The seventh step in the second phase of the enrollment process 162, assigns the chosen measurement points, and their acceptable range of values to the individual enrolle. The chosen values are known as the signature verification discriminates. A second embodiment allows the signature verification discriminates now identified, to be relayed back to the storage capacity 105 of the signal processing unit. This allows the parameters for verification to be kept at the local level, if this is desirable in a particular application. 
     The eighth step 164, ends the enrollment process. 
     FIG. 6 is a flow chart describing a computer algorithm used as a method for signature verification, when enrollment signatures have been processed as shown in FIG. 5 A-B, and are on file. 
     The first step, 180 consists of a putative signer providing a signature with the data input stylus shown in FIG. 3, and the resulting signal trains amplified, digitized, and transmitted for analysis as shown in FIG.4 
     The second step 182 in signature verification involves the distant central computer extracting, from an incoming signal train of a signature, only the values of the measurement points previously chosen to be signature verification discriminates. 
     The third step 184 in signature verification compares the extracted measurement point values of the putative signature with the allowable range of values on file as the signature verification discriminates. 
     The fourth step 186 in signature verification consists of adding the number of matches achieved by the incoming signals. 
     If the number of matching points is at or above the predetermined stringency cutoff 188, an accept message 192 is sent. If the number of matching points is below the predetermined stringency cutoff 190, a reject message 194 is sent. 
     Not shown in FIG. 6 is the possible additional step of setting another score which, being close to the stringency cutoff, allows the putative signer to try again. This feature could help avoid false rejections of signers whose signature tended to change slightly over time. Consistent automatic fine tuning of signature discriminates is thus easily done as the system is used for verifications. 
     Another step not shown in FIG. 6 is the possibility of ongoing signature verification procedures indicating new measurement points which should become discriminates, and indicating current discriminates which are becoming less useful over time. 
     FIG. 7 is a graphic representation of a hypothetical signature plotted so as to indicate how the signal trains are related and analyzed. The vertical axis of the chart indicates the angle of the stylus relative to the writing surface, the relative angle 210. The relative angle is actually measured by the ratio of the direct and lateral pressures, also indicated along the vertical axis 212. This chart assumes a data input stylus of the type described herein calibrated so than downward pressure on the stylus tip when the stylus is at a 45 degree angle to the writing surface, registers 50 percent direct pressure 214 and 50 percent lateral pressure 212. 
     It is the ratio of the direct pressure to the lateral pressure which is actually measured and is correlated to an angle degree for clarity. Actual variations in pressure on transducers is eliminated as a variable by measuring only the relationship, that is the ratio, of the two pressures measured. 
     The horizontal axis of the chart in FIG. 7 indicates the duration of the signal train. But the final measure used is also standardized so that the whole signal train is 100 percent, no matter how long it actually is. Thus, actual duration is also eliminated from final calculations in favor of relative measures. This is important since variation in duration, as in amplitude occurs in different signatures of the true signer and is an inadequate discriminate measure. 
     The total signal train being 100 percent of itself, is then divided into equal parts called defined segments based on a percentage of the signal. The segments are defined as simple parts of the whole, the whole being one hundred percent 220. Quadrant segments 216 are made by dividing the signal train into equal quarter segments of twenty five percent each. Decimated segments 218 are defined by dividing the signal train into ten equal parts of ten percent each. The process of creating defined segments is not limited to the several indicated in FIG. 7 since the signal can be divided into 100, 20, 80, or any number of segments for analysis. In practical terms, only a few segment divisions are necessary for analysis since a large number of mathematical combinations result from a relatively small number of comparison units. 
     The plotted digitized sample points 226 in FIG. 7 represent the measurement of ratio between the direct force of the signal train and the lateral force of the signal train. The sum of the plotted points in FIG. 7 represent the digitized signals of a complete signature where the pen was lifted from the paper seven times, leaving eight natural segments of pen-down times. The pen-up times 228 are indicated in the chart as lying along the line where direct and lateral pressures are equal, which is the same as the 45 degree measurement. Pen-up times could also be represented as no signal at all, but this distinction is minor since these natural segments, which play a major role in much prior art, are eliminated from the analysis in the present system. Rather, segmentation, which is necessary, is achieved by simply dividing the signal trains into defined segments, being equal parts of the whole. Since the analysis of variation is problematic of any given digitized point in itself, comparisons are made of segments of varying lengths. For example, the first segment of the quadrant segments 216 could be compared to the first decimated segment 218 in terms of the average of the plotted points within the segments. It will be seen that the first quadratic segment will have more points of a higher relative angle than the first decimated segment, thus the average, not the total points, will be different. The comparison of two segments yields a measure of their difference, the measurement point value. The average of all plotted points in a signature is the relative angle 224 of the whole, which in this hypothetical chart, is 54 degrees, or a 40/60 ratio of direct to lateral force. The average of the whole is, however, a variable itself, since a signer might hold the stylus in different ways at different times owing to environmental conditions or other factors. Thus, this measure also is standardized, by examining the measurement point values as they vary around a given average. The actual measure of the average angle is not used as a discriminate. 
     SUMMARY, RAMIFICATIONS, AND SCOPE 
     Accordingly, the reader will see that the Signature Verification System disclosed makes possible less expensive, more durable, less complicated, and more reliable signature verification. This is possible because the data input device and the resulting data analysis is fundamentally different from previous signature verification systems. The simplified and more durable stylus along with the simplified and adaptable algorithm make use in a variety of settings practical. Use is not restricted to credit card point of sales since the stylus is easily adaptable to use as a computer peripheral at a remote location allowing access by identification in a safer way than code words or numbers. 
     The algorithm disclosed for use with the simplified and unique stylus also has a number of advantages allowing widespread applications: 
     1. The calculations required are simple and fast. 
     2. The enrollment takes advantage of individual variations, rather than treating this as a problem. 
     3. After enrollment, verification is exceptionally fast and accurate. 
     4. Stringency requirements are easily adjusted to conditions. 
     5. Resulting signals do not need manipulation based on how the user holds the pen. 
     6. Verification is not dependent on consistent signing. 
     Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, the pressure sensitive transducers contained in the stylus might consist of pressure sensitive film, pressure sensitive resistors, or any number of pressure sensitive devices small enough for the purpose. Similarly, the enrollment algorithm might make use of a variety of standard statistical measurements applied to the developed measures of relative angle in order to derive additional measurement points. Also, the device might also be useful in a variety of settings where personal identification is useful such as providing a legal signature transmitted electronically, or serving as the identification means for access to restricted areas. 
     Thus, the scope of this invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.