GB2199938A

GB2199938A - Force detection assembly

Info

Publication number: GB2199938A
Application number: GB08701136A
Authority: GB
Inventors: Raymond William Simpson-Davis
Original assignee: De la Rue Systems Ltd
Current assignee: De la Rue Systems Ltd
Priority date: 1987-01-20
Filing date: 1987-01-20
Publication date: 1988-07-20
Also published as: GB8701136D0

Abstract

A force detection assembly comprises a substantially rigid plate 1 mounted on a support (4) such that the plate is spaced from the support. An optical signal generator located centrally at 6 between the plate and the support injects optical signals into at least three and preferably four optical fibres 3 which extend beneath respective knife edges (2) or a mask coupled to the plate 1. The light transmission in the fibres changes in proportion to an applied load e.g. from a pen. Optical signal receivers (8) are positioned to receive light which has passed along the optical fibres 3 and to generate corresponding electrical signals. A force monitoring system to which the assembly is connected can determine the position and magnitude of an applied force on the plate from the signals generated by the optical signal receivers. The assembly may be used in a signature verification system for a cash dispenser. <IMAGE>

Description

FORCE DETECTION ASSEMBLY The invention relates to a force detection assembly of the type comprising a substantially rigid plate mounted on a support such that the plate is spaced from the support and defines a force application surface; and means for generating signals related to the load at at least three positions when a force is applied to the surface of the plate. Such assemblies are hereinafter referred to as of the kind described.

Force detection assemblies of the kind described are used in a number of different fields including the field of signature verification in which one or both of the position and magnitude of an applied force on the plate is determined at spaced intervals in time in order to determine certain characteristic features of the written signature. These characteristics of the written signature are then compared with previously generated reference characteristics in order to verify the written signature.

In accordance with one aspect of the present invention, a force detection assembly of the kind described is characterised in that the signal generating means comprises for each of at least three of the spaced positions an optical waveguide whose optical transmission characteristic varies in accordance with the load at that position; optical signal generating means for injecting optical signals into each waveguide; and optical signal receiving means for generating electrical signals related to the optical signals transmitted through each waveguide past the respective support positions.

The invention is based on the use of optical techniques to monitor loads at the support positions due to an applied force. In some examples, the optical transmission characteristic of the optical waveguides may be varied by inserting a mask into the optical path by an amount related to the applied load. Preferably, however, at the position associated with each waveguide, the waveguide is engaged by a member coupled with the plate.

This leads to a particularly simple arrangement which allows use to be made of the properties of certain optical fibres such as Herga fibre which attenuates optical signals transmitted along it in proportion to a load applied on the fibre. This allows the construction of a signature verification pad that has a minimal number of moving parts which gives a low cost reliable assembly.

In some examples, the optical signal generating means may comprise separate generators for each optical waveguide but preferably a single optical signal generator common to each of the waveguides is provided.

This reduces the power consumption of the device and simplifies techniques for compensating for variations in the optical signal generated by the optical signal generating means. Typically, the optical waveguides, such as optical fibres, extend from the support positions to the site of the common signal generator where they terminate.

As mentioned above, it is possible that the output signal from the optical signal generating means will vary with time and in order to compensate for this, the assembly may further comprise auxiliary optical detection means for monitoring the optical output of the optical signal generating means and for generating corresponding electrical signals.

In accordance with a second aspect of the present invention, a force monitoring system comprises a force detection assembly according to the first aspect of the invention; and processing means coupled to the optical signal receiving means for generating signals related to one or both of the magnitude of the applied force and the position of the applied force on the plate.

It is known that the position and magnitude of an applied force on a substantially rigid plate can be determined from the loads at at least three support positions of the plate. It is thus a simple matter to determine from the signals generated by the optical signal receiving means information relating to the position and magnitude of the applied force and this information can be sampled at regular intervals to obtain information relating to the variation of the force with time. It will be appreciated therefore that this invention is particularly suitable for use in a signature verification system associated, for example, with a cash dispenser and the like.

Preferably, the optical signal generating means comprises one or more light emitting diodes while the optical signal receiving means comprise photodetectors associated with each optical waveguide.

In this specification, the term optical is intended to refer to that part of the electro-magnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.

In order that the invention may be better understood, an example of a signature verification system incorporating an embodiment of a force detection assembly according to the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic plan of the force detection assembly; Figure 2 is a section taken on the line A-A in Figure 1; and, Figure 3 is a block diagram of the signature verification system.

The force detection assembly shown in the drawing comprises a pressure pad having a substantially rigid rectangular plate 1 with a depending knife edge 2 protruding beneath it at each corner. Each knife edge 2 rests on an optical fibre, such as a Herga fibre, 3. The plate 1 is located on a housing 4 by means of a diaphragm 5.

The optical fibres 3 extend from a central region 6 at which is sited a light emitting diode (LED) 7, to each of the knife edges 2. The optical fibres extend beneath the knife edges and terminate adjacent to respective photodetectors 8.

In use, the LED 7 is activated to cause optical signals to pass along all of the optical fibres 3 to be received by the photodetectors 8. If a force is applied on the upper surface 9 of the plate 1, this will cause certain loads to be applied via the knife edges 2 onto the optical fibres 3 which will cause corresponding attenuation of the transmitted light. The photodetectors 8 generate electrical signals which vary in accordance with the intensity of the received light and thus in accordance with the loads at each of the knife edges.

Figure 3 illustrates the pad, indicated by reference numeral 10, of Figures 1 and 2 connected into a signature verification system. The output signals from the photodetectors 8 are passed along lines 11 to analogue-to-digital converters (ADC) 12 which generate digital signals which are fed to a microprocessor 13.

The microprocessor 13 samples the information derived from the photodetectors 8 in order to determine the magnitude and position of the applied force at regularly spaced intervals in time. Where the force has been applied during the writing of a signature on the surface 9 or on a writing medium such as paper placed on the surface 9, the microprocessor 13 will determine certain characteristic features of that signature such as number of pen lifts, number of vertical reversals and the like which. it compares. with . reference characteristics previously stored in a random access memory 14. The result of the comparison (either successful or unsuccessful) is indicated by the CPU 13 by an appropriate output signal fed along a line 15 to a display or to equipment which is to be used by the operator.

The force applied on the plate 1 is determined by summing the loads detected at each of the four corners.

The position of the applied load (for example the position P) is determined in accordance with the formulae: y = H (RC + RD) P x = L (RA + RB) p where x, y, L, and H are as shown in Figure 1, RA, RB, RC, and RD are values related to the loads at the knife edges 2, and P is related to the applied force. In other words, P = RA + RB + RC + RD.

Since the output signal from:the LED 6 may vary in intensity, the assembly also includes an auxiliary detector 17 connected to an ADC 16 whose output is fed to the CPU 13 to tenable the CPU 13 to compensate for changes in the output light intensity.

Claims

1. A force detection assembly comprising a substantially visid plate mounted on a support-*ch that the plate is spaced from the support and defines a force application surface; and means for generating signals related to the load at at least three positions when a force is applied to the surface of the plate characterised in that the signal generating means comprises for each of at least three of the spaced positions an optical waveguide whose optical transmission characteristic varies in accordance with the load at that position; optical signal generating means for injecting optical signals into each waveguide; and optical signal receiving means for generating electrical signals related to the optical signals transmitted through each waveguide past the respective support positions.

2. An assembly according to claim 1, wherein at the position associated with each waveguide, the waveguide is engaged by a member coupled with the plate.

3. An assembly according to claim 1 or claim 2, wherein the optical signal generating means comprises an optical signal generator common to each of the waveguides.

4. An assembly according to any of the preceding claims, further comprising auxiliary optical detection means for monitoring the optical output of the optical signal generating means and for generating corresponding electrical signals.

5. A force detection assembly substantially as hereinbefore described with reference to the accompanying drawings.

6. A force monitoring system comprising a force detection assembly according to any of the preceding claims; and processing means coupled to the optical signal receiving means for generating signals related to one or both of the magnitude of the applied force and the position of the applied force on the plate.

7. A system according to claim 6, wherein the processing means comprises a microcomputer programmed to determine from the signals generated by the optical signal receiving means values related to one or both of the magnitude of the applied force and the position of the applied force on the plate.

8. A signature verification system including a force monitoring system according to claim 6 or claim 7, wherein the processing means is adapted to determine one or both of the position of an applied force on the plate and the magnitude of the applied force, at spaced intervals in time while a signature is written on the surface defined by the plate or on a writing medium placed on that surface thereby to determine characteristic features of the signature, and to compare the determined characteristic features with previously stored reference features in order to verify the written signature.