IMPROVEMENTS IN OR RELATING TO A METHOD AND APPARATUS FOR MEASURING SHEAR FORCE

Abstract: Method and apparatus for measuring the shear force acting over a first surface (4a, 27) by reason of its engagement with a second surface (3) presented by a sheet-form member (1). The second surface is formed with projections (4), the tips (6) of which deflect, in a direction parallel to the plane of the sheet, when subjected to the force. Recording means (7) record the displacement of the tips, and equipment (9) computes the magnitude and the direction of the shear force by reference to that displacement. The specification particularly describes the application of the invention to the measurement of shear forces acting upon a human foot, and to mechanical fingers used to grip components in the course of a robotic assembly process.

Description: IMPROVEMENTS IN OR RELATING TO A METHOD AND APPARATUS FOR MEASURING SHEAR FORCE This invention relates to the measurement of shear force. There are robotics and clinical applications, and many other industrial and medical situations, in which 1t would be advantageous to have a simple and robust apparatus and method to sense the distribution of shear force acting over a surface. As an example of the industrial case consider a robot using a gripper to hold a peg and attempting to insert the peg into a hole. This is a simple task typical of those found in industrial assembly. If the tolerance between the peg and the hole is small so that the peg is a tight fit in the hole, 1t will be necessary for the gripper to exert a force to permit its insertion process to take place. This is necessary to overcome the friction forces between the walls of the hole and the sides of the peg. If the gripper is holding the peg by its sides then these insertion forces must be in the form of shear forces between the surfaces of the gripper and the sides of the peg. If it were possible to sense these forces then it might be possible to guide the insertion process in some intelligent way so as to avoid jamming. A medical case might arise in measuring the force pattern between a sole of a foot and the surface on which 1t is standing. Such measurements might be useful in diagnosing pathological conditions such as those attending arthritis and like diseases, and may be used to indicate the success, or otherwise, of regimes of treatment. It is relatively common 1n clinical practice to measure the normal force pattern acting at the sole of the foot. It is not common, although it would be very desirable, to measure also the shear force pattern. To date however, methods for doing so have proved to be extremely limited and difficult to implement. As indicated in the last paragraph, prior proposals in this field - as summarised for example 1n IEEE Spectrum (New York, USA), vol. 22, No. 8, August 1985, at pages 46 - 52 - concentrate on normal and not shear forces, and most of them use piezoelectric or fibre-optic devices and so require extensive electrical connections and wiring. Patent Specification US-A-3987668 is an example of several prior proposals for measuring displacements, but not the forces that cause them. Patent Specifications FR-A-2266154 describes a method for measuring the normal - not shear - force to which a surface is exposed, and in addition to the surface itself and the measuring equipment requires, as separate components, both a photoelastic member and a pressure-transmitting device for interposition between the surface and that member. Specification FR-A-2294427 is an example of prior proposals which again concentrate upon normal and not shear forces, and in which imposes the requirement that a grid or like pattern must perform on the surface of the very article which is being tested. Specification US-A-2325490 is an example of prior proposals requiring resilient light-reflective material, which rely upon substantial changes in the quantity of light reflected when that resilient material is subjected to a normal force. The present invention arises from appreciating that variation in the shear force to which a surface 1s subjected may be evaluated quite simply by observing and comparing the deflections to which projections on a second and contacting surface are subjected. The invention is defined by the claims and will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which:- Figure 1 shows a sheet-form member, used in the practice of the present invention, in partly-sectioned elevation; Figure 2 is a plan view of the member of Figure 1; Figure 3 is similar to Figure 1, but shows the member when a shear force is being applied; Figure 4 is an axial section through apparatus to measure the shear force when male and female objects engage, and Figure 5 is a partly-sectioned elevation of apparatus to measure the shear force distribution under the sole of a human foot. Figure 1 shows a suitable sheet-form member. It comprises a thin sheet 1 of polyurethane-based material presenting opposite faces 2 and 3. The lower face 2 is flat but circular-section projections 4 are formed on the upper face 3. As Figure 2 shows best, these projections are located as if at the intersections of a regular grid. Typically, the ratio of the height of the projections 4 to the surface-to-surface depth of the base sheet will be in the range from 5:1 to 0.5:1. Typically also, the sheet will be made from a plastics material that is of course tough enough to withstand repeated deflection of the projections 4 by the shear forces to which they will be subjected, and transparent enough for those deflections to be observed and recorded by optical equipment located clear of the lower face 2 of the sheet. Suitable materials include many polymers and plastics, for instance compositions based on polyurethane or epoxy resins. On the top face 6 of each projection 4 is a mark 5. In Figure 2 the mark 5 is shown as a dark spot, but other marks capable of optical observation, for instance moulded marks, might be used. Suppose now that a surface 4a, in contact with the upper faces 6 of the projections 4, applies a shear force distribution F to them while the lower face 2 of the sheet 1 is held stationary. The result is as shown in Figure 3 where it is seen that the projections 4 are bent over sideways by the shear force. Provided the deflections are small enough, the deflection of an individual projection is proportional to the total shear force experienced at its upper face. The direction in which the deflection takes place indicates the direction of the force. According to the invention, the shear force distribution is measured by viewing the movement of the marks 5 from the other side of the sheet 1. A camera 7 (Figure 3) with associated optics 8 faces the flat unmoulded face 2 of the transparent material. An image is captured of the unstressed material before any shear force is applied. A second image is captured (as indicated schematically at 6a) after the application of the shear force F. The two images then undergo a subtraction process (in signal-processing equipment indicated at 9) whereby the grey-level values for each pixel in one image are subtracted from those for the same pixels in the other image. The result, after a number of enhancing procedures, is a picture consisting of pairs of marks. The distance between the marks in each pair indicates the magnitude of the shear force at that point. The direction from one mark to the other Indicates the direction of the shear force. Fig. 4 shows how the invention could be applied to a robotic gripper to detect the shear force distribution during automatic tasks such as assembly. The tip of each of the two fingers 10 of the gripper supports a piece of the moulded sheet 1. The projections 4 on face 3 of the sheet face outwards towards the object to be gripped (in this case the head 12 of a peg 13). Behind the sheet, the structure of the finger is interrupted by a window filled with a strong pane of transparent material 11. Behind each pane is a prism 14 which reflects the images of the marks 5 up a hollow portion 15 of the finger 10 and so via suitable focusing optics 16 to solid state camera devices 17 connected to processing equipment 9 as before. Such an arrangement allows the total shear force distribution on each side of the pet 13 to be measured as the peg is inserted into a corresponding socket 18 formed in a body 19 which is anchored to ground at 20, Fig. 5 shows how the invention can be used to measure the shear force distribution under the sole of a foot. The moulded polymer sheet 1 is placed with its projections 4 uppermost on top of a glass supporting plate 22 which is supported by a suitable base structure 23 above a camera 24 with suitable optics 25 and processing equipment 9. A patient, walking or running, places the sole 27 of his foot 26 on the moulded surface and the camera system records a series of images which, when analysed by methods as already outlined, provide a measurement of the time history of the shear force distribution. While the invention has been described with reference only to optical sensing and recording, it also includes systems in which the deflection of the projections is sensed in other ways - e.g. by electromagnetic or ultrasonic effects - by sensing devices located to the remote side of the sheet-form member so that the deflection of the projection is still sensed "through the sheet".

Claims: CLAIMS 1. A method of measuring the force acting upon a first surface (4a) when it engages with a sheet form member having a second surface (3, 4) which distorts in response to the engagement, by providing recording means (7) on the side of the sheet-form member (1) remote from the surface where the distortion occurs, by operating the recording means to record the distortion, and by providing processing equipment (9) to give an indication of the magnitude of the force by reference to the recorded distortion, characterised in that the measured force is a shear force acting upon the first surface, in that the second surface is formed with spaced-apart projections (4) which deflect when the two surfaces are in shearing motion relative to each other, and the recording means (7) operate to record the locations of the projection tips (6) under different shear force conditions. 2. A method of measuring, according to Claim 1, in which the projections are spaced-apart as at the intersections of a grid. 3. A method of measuring, according to Claim 1, in which the projections are circular in cross-section. 4. A method of measuring, according to Claim 1, in which the tips (6) of the projections carry marker means (5) on which the recording means (7) focus to produce their records. 5. A method of measuring, according to Claim 4, in which the sheet-form member is transparent and the recording means are optical. 6. A method of measuring, according to Claim 5, in which the recording means include prisms (14) to deflect an optical image of the locations of the tips of the projections. 7. Apparatus to measure shear force, using a method according to any of the preceding claims. 8. Apparatus according to Claim 7 to measure the shear force encountered when male (12, 13) and female (18, 19) objects engage, including means (20) to hold one of the objects steady and means (10) to grip the other object and move it so that engagement takes place, in which the sheet form member (2) is interposed between the gripping means and the gripped object. 9. A method of measuring shear force according to Claim 1, substantially as described with reference to the accompanying drawings. 10. Apparatus to measure shear force, according to Claim 7 and substantially as described with reference to the accompanying drawings.

Inventors: HEWIT JAMES ROBERT, HEWIT, JAMES, ROBERT
Applicants: NAT RES DEV, NATIONAL RESEARCH DEVELOPMENT CORPORATION
Application year: 1990
Main classification: G01B11
IPC codes: A61B5, G01B11, G01L1