FR2815242A1 - Equipment for evaluating support forces on feet, comprises a posture platform with three force sensors and means to find their center also a piezoelectric sensitive surface able to measure elemental forces - Google Patents

Abstract

The base of the equipment consists of a posture plate (4) which has three force sensors and is able to find the single equivalent force from their data. The posture plate has a top plate (3) and a piezoelectric sensitive surface (2) is glued to it. The piezoelectric sensitive surface is protected from scratching by a layer of PVC (5) or similar material and is divided into a large number of cells able to give a detailed picture of weight distribution on the feet.

Description

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ASSOCIATION OF AN ELECTRONIC PEDOMETER AND A PLATFORM
POSTURE FOR THE ACCURATE EVALUATION OF PODAL SUPPORTS.

 The present invention relates to a device intended to measure the excursions of the center of gravity and the distribution of the plantar supports of a subject standing on said device.

The technical sector of the invention is the field, but not only, of the production of evaluation and rehabilitation material for subjects suffering from postural and gait disorders as well as material for validation of orthopedic insoles and prostheses legs (hips, knees, etc.).

The method and device of the present invention can be used in any application, where a determination of the center of pressures, of a pressure imprint, or both, is sought.

 However, to simplify the presentation of the prior art, the following will essentially be cited as references to posturology and podiatry applications.

 In 1985, the French Posturology Association (A. F. P) founded by Doctor Pierre-Marie GAGEY published standards for the manufacture of a posturology platform (Ref. PF POSTURE).

For a more in-depth study of these standards it will be possible. usefully refer to the fairly complete publication on the subject, in the eight lessons of posturology published by A. F. P.

 The principle of determining the center of force of the platform to A.F.P standards, is based on the measurement of three reaction forces of the high stiffness support on which the subject is standing.

Said forces, according to standards A. F. P, are measured by force sensors arranged very precisely to form the three vertices of an equilateral triangle with 400 mm side.

According to this mode of determining the center of the pressures exerted by a standing subject on this type of platform, the coordinates of said center are calculated by a formula which takes into account the intensity of each of the three forces as well as the geometric location of their point of application. In order to facilitate understanding of this method of calculation, an illustration is given on plate 1, fig. 1, with weights: P / = 10kg, P-20kg and Pj = 40 kg attached respectively to the sensors of the vertices A, B, C of the equilateral triangle thus defined: the size of the arrows which symbolize the supports are in relation to the weights

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correspondents. It is shown that the coordinates X and Y of the center of the three forces exerted by said weights are given by the relations:

avec 1 =AB =BC =AC =400mmetPT=Pl +P2+P3 si l'on remplace dans les relations ci-dessus, les variables par les valeurs numériques données en exemple, on trouve : X = +5 14 mm et Y = -66 mm Les coordonnées de ce centre des forces sont rapportées à un repère orthonormé x 0 y dont l'origine 0 est situé au barycentre du triangle équilatéral. Mais il est aisé par simple calcul de transformation de repère de rapporter le centre des pressions à un référentiel x' 0'y'attaché au pieds du sujet. Pour ce faire, il faut connaître avec précision l'emplacement des pieds sur la plate-forme et surtout pouvoir dessiner le polygone de sustentation délimité par les bords externes des appuis plantaires. L'A. F. P. préconise de disposer les pieds du sujet sur la plate-forme en s'aidant de marques ou de cale-pieds amovibles selon une disposition convenue telle que les pieds forment un angle d'ouverture de 30 degrés. Connaissant la position de la cale par rapport au référentiel de la plate-forme ainsi que la pointure du sujet, le programme d'analyse est capable de calculer et de dessiner à l'écran de l'ordinateur le polygone de sustentation théorique DEFGHI, planche 1, figure 2 et rapporter les mesures dans un référentiel lié aux pieds X'0'Y' : 0'étant le centre dudit polygone. Dans ce cas précis, la transformation de repère se réduit à une translation selon l'axe Y. Cette procédure implique d'une part que les bords externes des pieds ainsi calés soient réellement les bords d'appuis au sol et que d'autre part la pointure indiquée par le sujet soit précise.

with 1 = AB = BC = AC = 400mmetPT = Pl + P2 + P3 if we replace in the relations above, the variables by the numerical values given in example, we find: X = +5 14 mm and Y = -66 mm The coordinates of this center of forces are related to an orthonormal reference frame x 0 y whose origin 0 is located at the barycenter of the equilateral triangle. But it is easy by simple calculation of transformation of reference frame to relate the center of pressures to a reference frame x '0' y 'attached to the feet of the subject. To do this, it is necessary to know precisely the location of the feet on the platform and above all to be able to draw the support polygon delimited by the external edges of the plantar supports. AFP recommends placing the subject's feet on the platform with the help of removable marks or toe clips according to an agreed arrangement such that the feet form an opening angle of 30 degrees. Knowing the position of the hold in relation to the reference frame of the platform as well as the size of the subject, the analysis program is capable of calculating and drawing on the computer screen the theoretical support polygon DEFGHI, plate 1, Figure 2 and report the measurements in a frame of reference linked to the feet X'0'Y ': 0' being the center of said polygon. In this specific case, the frame transformation is reduced to a translation along the Y axis. This procedure implies on the one hand that the external edges of the feet thus wedged are actually the edges of the supports on the ground and that on the other hand the size indicated by the subject is precise.

To better understand the subject of the invention, it is advisable to specify that according to the literature, the excursions of the center of pressures in a normal subject, are less than 10 millimeters around its average placement. For a measurement error of less than 1% - an error commonly tolerated in this area -, the precision and resolution of the measuring device of said pressure center must be less than one tenth of a millimeter. Certain commercial platforms are moreover capable of measuring the center of pressure with better precision. The use of the platform to AFP standards is therefore essential when it comes to determining with great precision the center of the forces exerted by a standing person. However, the use of this type of platform with a high stiffness support and resting on at least three load cells does not make it possible to answer a certain number of problems relating to the

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stratégie posturale du sujet et à la détermination précise du référentiel lié au polygone de sustentation.

postural strategy of the subject and the precise determination of the frame of reference linked to the support polygon.

 In the 75s, the C. N. R. S (L. A. A. S) automation laboratory in Toulouse developed artificial skin. This skin consists of a conductive polymer foam a few millimeters thick. The electrical resistance of this foam decreases in the direction of compression so that if this foam is placed on two neighboring flat electrodes, the electrical resistance measured between said electrodes will be inversely proportional to the compression force exerted between the foam and the plane containing the pairs of electrodes as illustrated in plate 2, figure 3. One of the applications offered by this material has been the production of electronic pedometers intended essentially for taking fingerprints for the static and dynamic analysis of the plantar supports. . Among the manufacturers who use this process, we can cite the company MIDI-CAPTEURS in Toulouse which has been manufacturing electronic pedometers for 15 years. This type of device is simple to make, inexpensive and has a surface resolution which only depends on the number of electrodes placed under the foam.

- l'inhomogénéité du matériau pose des problèmes de linéarisation et de calibrage. La détermination du centre des pressions avec ce type de dispositif ne peut se faire qu'approximativement, la mousse développe une certaine hystérésis mécanique ce qui limite la fréquence de scrutation des points de pression la mousse s'écrase sous la pression des pieds, ce qui modifie sensiblement la posture naturelle : la réaction du support est en effet élastique au lieu d'être rigide selon les recommandations de l'A. F. P car il donne l'impression que le sol se dérobe. However, the pedometers used in this field cannot be suitable for determining the center of pressure with great precision because they have the following drawbacks: the precision of the contact forces and their locations remain poor with regard to the requirements of stabilometry,

- the non-uniformity of the material poses problems of linearization and calibration. The determination of the center of pressures with this type of device can only be done approximately, the foam develops a certain mechanical hysteresis which limits the frequency of scanning of the pressure points the foam crashes under the pressure of the feet, which modifies the natural posture appreciably: the reaction of the support is indeed elastic instead of being rigid according to the recommendations of AF P because it gives the impression that the ground is slipping away.

In the 1990s, a new type of material appeared which corrects a certain number of faults in the foam pedometer: These are sensitive piezoresistive surfaces incorporating a large number of semiconductor cells (Ref. PODOMETER); They are offered in particular by IEE companies (surfaces

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F. S. RTM) au Luxembourg et par TESCAN aux U. S. A. Dans le même temps sont aussi apparus des polymères piézo-électriques proposés en film mince et qui développent une charge électrique lorsqu'ils sont soumis à une force de pincement. Ces films piézo- électriques sont notamment commercialisés par la société PIEZO-TECH à Saint Louis en France. Bien que ces deux technologies conviennent pour des réalisations selon l'invention, nous ne parlerons que des feuilles piézo-résistives qui présentent plus de facilité pour la mise en oeuvre et sont d'un moindre coût. Les capteurs piézo-résistifs du commerce selon l'invention sont constitués d'un semi conducteur souple se présentant sous forme d'une feuille plastique d'épaisseur inférieure à 0,3 millimètres et de dimension variable à la demande, intégrant une ou plusieurs cellules semi conductrices. La cellule FSR 1 par exemple dont la construction est illustrée figure 4, se compose de deux feuilles de polymères laminées ensemble. L'une des feuilles est recouverte d'un réseau d'électrodes à plages intercalées la, l'autre d'un matériau semi-conducteur lb.

FS RTM) in Luxembourg and by TESCAN in the USA At the same time, piezoelectric polymers, offered as thin films, also appeared, which developed an electric charge when subjected to a pinching force. These piezoelectric films are sold in particular by the company PIEZO-TECH in Saint Louis in France. Although these two technologies are suitable for embodiments according to the invention, we will only speak of piezoresistive sheets which are easier to use and are of lower cost. The commercial piezoresistive sensors according to the invention consist of a flexible semiconductor in the form of a plastic sheet of thickness less than 0.3 millimeters and of variable dimension on request, integrating one or more cells semi conductive. The FSR cell 1, for example, the construction of which is illustrated in FIG. 4, consists of two sheets of polymers laminated together. One of the sheets is covered with a network of electrodes with interleaved areas la, the other with a semiconductor material 1b.

When a force is applied to the FSR. the semiconductor material more or less shunts the electrodes with offset zones. The sensitive surface which comes under this technology is flexible, but does not crash, it can provide the intensity of a force according to a law of variation Resistance-Force if it is sandwiched between two planes subjected to a force of pinch. The sensitive surface which will be designated below: sensitive surface 2 (FIG. 5) can be composed of one or more cells 1. It should however be noted that such a surface can be delivered in various forms, dimensions and ranges of sensitivity. Although the law of variation of the resistance of the sensitive surface as a function of the force is not linear, it is possible to correctly determine the weight applied to said surface either by calculation if we know the law of variation, or at l using a correspondence table established during the construction of the device according to the invention by collecting the resistance as a function of the weight. Said correspondence table will be written in a non-volatile memory of the electronics module or in a configuration and calibration file which will be supplied with the operating software of the device according to the invention. According to a preferred manufacturing method, the piezo-sensitive surface will be composed of elementary cells organized in a matrix as shown diagrammatically in FIG. 6, so that if it is interposed between a rigid support and the feet, it is capable of providing without deform, the distribution and intensity of the plantar supports that can be visualized, for example, in a color scale or in gray levels (Figure 7). At least one commercial embodiment of an electronic pedometer is known with this type of surface offered by the Italian company BURATTO.

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 However, the linearity and the precision of the measurements remain insufficient for the precise determination of the center of the pressures.

 One solution to the problems posed above is a measurement method which combines, by superimposing them, a force platform as recommended for example by AFP, and a preferentially matrix piezo-sensitive surface of the FSR type, TEKSCAN or any other type of hard surface to crush. Indeed, the precise knowledge of the center of pressure as well as the distribution of the support forces is of great interest for the study of postural strategies. As illustrated in FIG. 8 and FIG. 9, the subject can achieve a center of pressure on the platform in an infinite number of ways. Only a surface comprising a large number of piezosensitive cells makes it possible to remove the ambiguities of the various postural strategies. The same result of the pressures (in the center of Figures 8 and 9) can be obtained for example with the heel supports of the left foot and the right forefoot (Figure 8) and vice versa with the supports of the right heel and the forefoot left (figure 9).

 Another advantage of such an association according to the invention is the possibility of using the accuracy of the platform to perform an automatic linearity correction and correction of the sensitive surface. For example, by exerting pressure on the platform according to the invention, using a press or a heavy object and comparing in position and intensity the centers of pressure provided by the two devices. The platform being the reference, it is then easy, by moving the point of application of the pressing force, to calculate the correction factors for each of the cells that make up the piezo-sensitive surface. These correction factors once stored in non-volatile memories for example will be used to perfect the measurement by correcting the gain and linearity errors.

 Another of the most obvious interests of such an association according to the invention is to get rid of the toe clips and the size of the subject since the imprint of the feet which is provided by the sensitive surface can serve to correctly delimit the support polygon and allow an unambiguous reference frame transformation in the subject's foot repository. This arrangement always makes it possible to report the measurements made by the platform in the frame of reference linked to the feet whatever their locations, while the use of the platform alone forces the experimenter to position the subject in a precise and determined manner.

 According to one of the embodiments, the sensitive surface 2 will be bonded to the entire surface of the top plate 3 of the platform 4. In order to protect the said surface from scratches

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 which could damage it, we could for example cover it with a sheet 5 of PVC or any other material fine enough not to alter the spatial resolution of the piezo-sensitive matrix and which would ensure good transmission of pressures.

 In order to reduce the manufacturing cost of this type of device, one or more sheets are available according to the invention which would only cover the useful support planes so as to save said surface. An example of an illustration with two piezosensitive sheets, one for the right foot and another for the left foot is given in Figure 11.

Said sheets are arranged at the supposed location of the feet of the subject and are of sufficient dimensions to be suitable for all types of sizes.

 According to another alternative embodiment which may have a commercial interest, the sensitive surface can be removable to be the subject of an option when purchasing a platform or to adapt to existing platforms. Said surface can be advantageously integrated into the cover of the platform. FIG. 12 shows such an arrangement: the surface 2 is bonded to the external face of the cover 6. In this embodiment according to the invention, the electrical connections of the cells of the matrix 2 lead to tracks deposited on the flexible support to a connector 7 which is fixed to one of the external sides of the cover 6. The connection is made by an external cord 9 via a connector 8 which is plugged into 7.

 In another embodiment of an electrical connection (in particular when the electronics are on board inside the platform), the connection is made automatically when the cover 6 is in place on the platform. To do this, the printed tracks 10 are folded at a right angle into a slot 11 adjacent and parallel to one of the sides of the cover. Said tracks are glued to the internal edge of the side of the cover so that the contacts 12 of the printed tracks come to bear on flexible blades 13 of the connector 14 which is fixed on the edge of one side of the plate 3. The connection of the matrix to the electronic interface (not shown) is finally carried out by wires welded to the pads 15 of the connector 14. A partial top view in FIG. 13 as well as a section in FIG. 14 along AA show this mechanism in detail. connection.

 In order to comply with the standards published by AFP, it is desirable to place the subject's feet in a particular arrangement such that said feet form an opening angle of 30 degrees. To do this, advantageously use a removable toe clip on the device according to the invention. FIG. 15 illustrates an exemplary embodiment of removable footrest 45 made of a PVC plastic material for example and of

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 dimension such as to be suitable for all sizes. As an indication, the part which abuts the internal edge of the foot will be 30 centimeters long and that which abuts the heel, 10 centimeters. The thickness of the shim will be at least 3 centimeters. In order to quickly set up and extract the footrest on the cover of the device according to the invention, there is an angle 46 embedded and screwed by two countersunk screws 47 on the internal face of the footrest 45 and comprising a crimped needle 48 at the other end. When the footrest is put in place, said needle is first introduced into a conical housing on the outer edge of the cover. Another angle 49 fixed on the heel side of the toe clip has a knurled needle screw which comes to penetrate the other side of the cover in a conical housing. The immobilization and the extraction of the toe clips are carried out simply by screwing and unscrewing the screw 50. FIG. 16 shows a section BB of the toe clips. One can advantageously use the holes 51 made in the mass of 45 to manipulate the toe clips.

 In an embodiment of an electronic interface for processing the signals coming from the force platform and from the electronic pedometer according to the invention, FIG. 17 illustrates here a nonlimiting example. One of the columns 16 and one of the lines 17 of the matrix are sequentially selected respectively by the electronic switches 18 and 19. At the intersection, the current which flows through the piezo-sensitive cell 20 supplied by the voltage source 21, is collected by resistance 22. Said current is inversely proportional to the resistance of cell 20, so that the voltage across terminals 22 is proportional to the pressure exerted on cell 20. Circuit 23 is an amplifier making it possible to adapt said voltage. The output of 23 drives an analog multiplexer 24 with at least four channels making it possible to select on request an acquisition coming from the sensitive surface 2 or from the force platform 4.

The signals from the force sensors 25 are presented to the multiplexer 24 after amplification by the amplification plate 26. The different analog signals are digitized by the device 27 which is an analog-digital converter. The device 28 is a microcontroller well known to electronics engineers, the role of which is to communicate with the computer (PC for example) on the one hand and to manage the devices 18, 19, 24, 27. Thus, according to an acquisition procedure, all the lines of the same column are switched in turn and the information contained at their intersections with said column is sequentially digitized and transmitted to the PC. This is done for all the columns of the matrix 2. At the end of the scanning of the whole matrix 2, the signals from the force sensors 25 are digitized sequentially.

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se trouve à l'intersection de la ligne et de la colonne sélectionneés par 32 et 33. A titre d'exemple, le circuit 33 est composé de 4 circuits intégrés CMOS de type CD 4067 , bien connu des électroniciens. Le courant circulant à travers la cellule 34 est mis en forme et amplifié par le circuit analogique 35 dont la sortie attaque un convertisseur Analogique Digital 36 de 8 bits de résolution par exemple. Le dispositif électronique 37 est une mémoire tampon de type FIFO pour First In, First Out il permet d'emmagasiner un grand nombre d'informations provenant du convertisseur à une cadence régulière et de les restituer à une cadence et une régularité compatibles avec les In another, but not the only, embodiment of the electronic interface for implementing and managing the device according to the invention, which combines a force platform and an electronic pedometer, the 2 piezo matrix surface sensitive of said pedometer, has a dimension of 50x50 centimeters and is composed of 4096 cells organized in a matrix of 64 rows and 64 columns. An elementary cell which can be compared to a pixel occupies an area of less than one square centimeter. The sequential scanning of the 4096 pixels constitutes a graphic image of the distribution of the podal forces. But it can be useful to analyze several images at a sufficient rate in order to faithfully reconstruct the excursions of the plantar supports. Good representation requires up to 100 images per second. In these conditions the frequency of the acquisition must be 4096x100, i. e of more than 400 kilohertz. To honor such a rate, we can either have an acquisition card embedded in a PC type computer with direct memory access (DMA) well known to computer scientists, or if we want to integrate all the electronics into the platform -form, have a special interface that can be schematized in Figure 18. In this operating mode, the pedometer part is framed at 29. The oscillator circuit 30, attacks a binary counter 31 of 4096 steps. The 6 least significant lines at the output of 31 attack a circuit 32, called a decoder 1 among 64. Said decoder makes it possible to select a line among the 64 lines of the sensitive matrix according to the binary code presented by the 6 input lines of 32. The line of the sensitive matrix thus selected is supplied by a stable voltage or current source . By way of example, circuit 32 is made up of 4 CMOS integrated circuits of type CD 4514, well known to electronics engineers. The 6 most significant lines at the output of 31 drive a circuit 33 called analog multiplexer / demultiplexer, 1 among 64. Said circuit 33 makes it possible to switch one column among 64 columns of the sensitive matrix. The column thus switched makes it possible to establish an ohmic continuity of the elementary cell 34 which

is located at the intersection of the row and the column selected by 32 and 33. For example, circuit 33 is made up of 4 CMOS integrated circuits of type CD 4067, well known to electronics engineers. The current flowing through the cell 34 is shaped and amplified by the analog circuit 35, the output of which attacks an Analog Digital Converter 36 of 8 bit resolution for example. The electronic device 37 is a FIFO type buffer memory for First In, First Out, it makes it possible to store a large amount of information coming from the converter at a regular rate and to restore them at a rate and a regularity compatible with the

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 recording possibilities of the computer connected downstream. In the worst case, the FIFO memory capacity must be sufficient to store the number of 4096 pixel 8-bit images required during a recording sequence. Thus for a recording duration of thirty seconds at the rate of ten images per second, it would be necessary to have at worst 30 × 10 × 4096 i. e nearly 1.3 Megabytes. The device 38 is a control logic circuit, it supplies the control signals necessary to control the converter 36, and the FIFO 37. The electronic interface of the platform framed at 38, preferably comprises three force sensors arranged at the top of '' an equilateral triangle with a side of 40 centimeters in accordance with AFP standards. For a more in-depth study of a variant of AFP standards platform, can we usefully refer to the publication of the patent? FR 99.04850 of April 14, 1999. Said interface is composed of an amplification plate 40 for 3 sensors with resistive gauge bridge 39. The amplified output signals of the three sensors are digitalized by the Analogical Digital converter 41 with three channels. The resolution of said converter is preferably 16 bits and its sampling rate is at least 40 hertz. The digital information from the converter 41 and the FIFO 37 is centralized by the device 42 called a programmable microcontroller well known to electronics engineers. Said device also ensures on the one hand by its control lines the management of circuits 37, 38, 41 and on the other hand by its communication lines the transfer of data to the computer 43 via one of the exchange ports which can be the serial port, the parallel port, the USB port or any other external port of a computer. With the system as just described schematically it is possible to use a portable computer to execute a recording sequence on the device according to the invention, to process it, and to display the results according to it. running of the posture analysis program provided by the manufacturer.

Claims (9)

 1-Device for measuring the position, the displacement of the pressure center and the distribution of the podal support forces of a person, characterized in that it consists of a posture platform (4) comprising at at least three force sensors capable of providing the center of said forces with precision and of a piezosensitive surface (2) arranged on the platform and made up of several piezzosensitive cells each occupying an elementary surface, the whole forming a matrix so that that by electronic means one can read therein information proportional to the force to which each of said elementary surfaces is subjected.  2-Device according to claim 1 characterized in that the piezosensitive surface consists of a flexible plastic film comprising piezoresistive semiconductor cells or piezoelectric cells 3-Device according to claim 1 or 2 characterized in that the piezosensitive surface is bonded to the top plate of the force platform 4-Device according to one of claims 1 or 2 characterized in that the piezo-sensitive surface is removable 5-Device according to claim 4 characterized in that the piezosensitive surface is integrated into the cover of the force platform.  6-Device according to claim 5 characterized in that the cells (1) of the piezo-sensitive surface (2) lead to printed tracks (10) arranged on the flexible support, which are folded at right angles in a slot (11 ) and glued to the internal edge of the side of the cover so that, when the cover is placed on said platform (4), the contacts (12) of the printed tracks (10) press on the flexible blades (13) d 'A connector (14) which is fixed on the edge of the top plate (3) of the platform (4)., the connection is thus effected automatically.  7-Device according to any one of the preceding claims, characterized in that it comprises a removable foot support (45) for placing the subject's feet in a standardized position.  8-Device according to any one of the preceding claims, characterized in that it comprises an electronic interface capable of simultaneously recording and managing the information coming from the platform and from a piezosensitive surface. <Desc / Clms Page number 11>

9-Device according to any one of the preceding claims, characterized in that the electronic interface (29) comprises a storage device (37) capable of recording the digital information of the analog-digital converter (36) and of restoring it to Requirement.