FR2792182A1 - PLATFORM FOR MEASURING THE POSITION OF THE CENTER OF GRAVITY IN A STANDING SUBJECT - Google Patents

Abstract

Device for measuring the position and the excursions of the center of gravity of a subject standing on said device. According to the invention, the body of the device is a platform consisting of two plates made of a material of high stiffness, for example of aluminum "AU 4G", sandwiching according to a particular arrangement at least three sensors (11) and incorporating all the electronics. The force sensors used in this invention are miniaturized, of the "button" type. The upper plate (5) rests on the studs of the sensors by means of hard steel cup screws (12). Said screws make it possible to adjust the height of the platform, the attitude of the plate (5) and also make it possible to adjust the distribution of the load between the various sensors. The base of the sensors is either left free on the bottom of the plate (4), or housed on the same bottom thanks to a slight recess, or glued. The two plates are joined while allowing axial deformation of the sensors under the effect of a charge, to express oneself. The connecting device of the plates (4) and (5) allows tightening in abutment or controlled tightening by means of one or more shoulder screws (13).

Description

PLATFORM FOR MEASURING THE POSITION OF THE CENTER OF GRAVITY AT

A STANDING TOPIC

  The present invention relates to a device intended to measure the position and the

  excursions of the center of gravity 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 disorders. The method and device of the present invention can be used in any application, where a determination of the center of mass, force or thrust in a plane is

wanted.

  However, to simplify the presentation of the prior art, the following will be cited

  essentially references to posturology or stabilometry applications.

  In 1985, the French Association of Posturology (A.F.P) founded by the doctor

  P.M. GAGEY publishes standards for the manufacture of a posturology platform.

  For a more in-depth study of these standards, we can usefully refer to the fairly complete publication on the subject, in the eight lessons of posturology published by the A.F.P. The posturology platform described by the A.F.P is the subject of recommendations which essentially relate to the size of the upper plate, the number and arrangement of the sensors, the other choices are left to the discretion of the manufacturers. These choices concern for example: force sensors, platform height, type of connection, toe clips, color, electronics, etc. 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 support on which the subject is

is standing.

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

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 the understanding of this calculation method, an illustration is given on board 1, with weights: Pl = 10kg, P2 = 20kg and P3 = 40 kg respectively attached to the sensors of the vertices A, B, C of the equilateral triangle as well - 2 - defines: the size of the arrows which symbolize the supports are related to the corresponding weights. We show that the coordinates X and Y of the center of the three forces exerted by said weights are given by the relations: X - I (P3 - P2) r Y - = / 3 (2P1 - (P2 + P3))

2PT 6PT

  with 1 = AB = BC = AC = 400mmetPT = Pl + P2 + P3 if we replace in the relationships above, the variables by the numerical values given as an example, we find: X = +57.14 mm and Y = -66 mm The coordinates of this center of forces is related to an orthonormal coordinate system x O y whose origin O is located at the barycenter of the equilateral triangle. But it is easy by simple calculation of transformation of benchmark to relate the center of pressures to a frame of reference attached to the feet of the subject for example on condition however of correctly placing the subject in this new frame of reference. In this calculation method, we have toe clips

  removable on the platform on which the subject is positioned.

  To give a precise idea of the relevance of the invention, it is appropriate to specify

  1 5 the prior art highlighting the technical problems.

  According to a first non-marketed prototype manufacture, the force transducers were compression cells. Each cell (Figure 1) is composed of three transverse beams forming a "(Z": the external faces of the lower beam and the upper beam are the two planes of application of the compressive forces which subject the central beam (2) to a torsional torque. The flexing of said beam, under the effect of said torque, is translated electrically by means of a set of strain gauges.

  resistive (3) bonded to the flexible part of said beam.

  The deformation of such a sensor under the action of compression forces is perpendicular to the compression planes (4) and (5). To be able to determine precisely the X and Y coordinates of the result of the forces exerted on the platform, the bearing surfaces of the three cells rigorously placed at the top of the equilateral triangle must be as punctual as possible. In a first prototype embodiment, the base of the cells was fixed to the bottom of the platform (4) while the upper part was surmounted by a hard steel point (6). The top plate (5) of the platform rested on the tips of the three collectors by means of hard steel plates represented in the drawing in FIG. 1 by a bowl. The top plate is only free in the direction perpendicular to the plane of the plates (one degree of freedom) which corresponds to five connections out of the six to achieve complete immobilization - 3 - of the two plates. The platforms built according to this principle had some drawbacks: - too high a height (greater than 10 cm) of the platform making its use perilous in the event of a fall in unstable subjects - due to its five mechanical connections, the deformations, whether of thermal origin or induced by the load, cannot be expressed, this results in lateral shear stresses at the point-toe plug contacts which distort the measurement - cost price and ratio quality price too high Kode error as defined below: A standing subject is comparable to an inverted pendulum, rotating around the axis of the ankle. There is an error in the measurement of the mechanical moment exerted by the subject standing on the platform. This error according to Kode's calculations (Kode L., 1978) is proportional to the product of the result of the lateral forces and the distance between the horizontal plane containing the axes of the dowels and the horizontal plane defined by the plane of

  range of the sensors. To simplify, we will call this error: "Kode error">.

  This error is zero if the sensors are on the same horizontal plane which contains the axis

ankles.

  In a different embodiment which gave rise to marketing from 1992, by the company Dynatronic, the transducers used (FIG. 2) were with simple bending beam and at constant time, identical to those used for the production of electronic scales. The base of the beams was mechanically fixed to the bottom (4) while the generally mobile ends were fixed to the top plate (5) always at the exercise points. The whole forming the three vertices of the equilateral triangle as shown in Figure 4. Such an embodiment made the two plates integral in the six degrees of freedom possible. Figure 3 shows a side view showing the relative immobilization of the two plates. These simple beam platforms were thinner and less expensive but had the following drawbacks: - less reliable and less precise because of the connections which did not allow to fully realize a deformation in the direction of the force - to the error of measure which increased with the load added the inevitable

  drifts due to the expansion which cannot be expressed.

  -4 - - geometry error by indetermination of the exact point of application of the forces because of the size of the contact surface which cannot be considered as punctual. In a different embodiment (plate 3), which also uses sensors with a simple bending beam, solutions have been provided to release the top plate (5) from

  axial and lateral constraints by the adoption of the Principle of Lord Kelvin "Hole -Trait-

  Plan (TTP) "at the three connections with the top plate (5). In this type of mounting, the movable ends of the sensors are surmounted by a mechanical contact in the shape of a point with rounded tip or by an inlaid steel ball , not shown here, while the internal face of the top plate (5) has three bearing surfaces, made of hard steel, in the same arrangement as the sensors. One of the bearing surfaces is a hole (7), to accommodate a contacts, another bearing is a groove (8), along which the contact can move: Figure 6 shows a section along NN of this groove.The third bearing is a plane (9), on which the contact can move in all directions. The top plate (5) is immobilized in translation and in lateral rotation relative to the

  bottom plate (4). This conception of isostaticism is illustrated by the sections N-

  N (fig. 6) and M-M (fig. 7). These platforms marketed by QFP in Mougins and SATEL in Toulouse are sensitive and precise at low loads but have some drawbacks: thicker platform (more than 8 Cm) due to the addition of spikes and reported surfaces to carry out the "TTP". The risk of falling is greater. - geometrical modification of the place of the supports with the load because of the bending of the beams, which results in an error of geometry - the lateral forces of friction induce hard points which results in thresholds and hysteresis especially for d '' high costs - more complex realization and manufacturing high price and value for money

- significant Kode error.

  In other embodiments, the sensors used are of the piezoelectric type based on quartz or ceramic. These sensors, which can be miniaturized, have the major drawback of electrically drifting and are therefore not suitable for measuring the center of pressure of static or slowly moving parts. We can quote for this

  type of manufacturing, the platform of the Swiss company "Kisler".

  -5 - A solution to the problems posed above is a method of measuring the center of the forces from at least three force transducers of a different type from those mentioned above and from a particular mechanical arrangement in order to simplify manufacturing and overcome parasitic mechanical constraints while giving good precision to the platform. According to the invention, the body of the platform consists of two plates made of a material of high stiffness: aluminum AU4G 10 mm thick for example, sandwiching according to a particular mechanical arrangement the

  sensors and incorporating all the electronics for implementing these so-called sensors.

  The force sensor used in this invention (plate. 4) has the characteristic of being as small as a coin since its diameter d = 16.5 mm, its height h = 5.4 mm. said sensor is also axially deformed and supports

a maximum load of 150 kg.

  Said sensor has in the center of its active face a cylindrical pin (10) of

  length I = 1 mm and diameter t = 1.2 mm, on which the force to be measured must be exerted.

  The sensor excitation and measurement cable has a diameter f = 1.5 mm.

  The base (11) of said sensor can be simply placed or glued to the bottom plate (4) of the fixed frame. We can also practice on the bottom plate (4), a slight recess to accommodate the base of the sensors to facilitate their positioning. The top plate (5) rests on the pins (10) of the sensors by means of screws

  headless "bowl" (12), made of hard steel.

  The depth of the bowl as well as its diameter are adjusted so as to cover

  the head of the sensor nipple without tailgating the base of said nipple.

  The hole in the bowl is slightly flared to allow rapid positioning of the top plate (5) on the sensors. The bottom of the bowl hole is slightly larger than the diameter of the nipple to avoid tightening while retaining a

  precise alignment of the sensor axis with the axis of the "cuvette" screws.

  In addition to their primary function which is to receive the sensor head, these "bowl" screws allow precise adjustment of the height of the top plate (5) relative to the bottom plate (4). A platform height e = 25 mm for example, with plates (4) and (5) 10 mm thick is easy to obtain. The plate of the upper plate (5) can also be easily adjusted to suit by these same screws, likewise

  that the distribution of the weight of said tray on the sensors, if necessary.

  -6 - Thus it is possible by this process to design platforms with more than three sensors as soon as we can very precisely adjust the pressure on all contact points to better distribute the forces and compensate for flatness defects . In the type of embodiment which is the subject of the invention, it should be noted that the base of the sensors (plate 5) is left fairly free on its seat (bottom of the platform 4) while in all cases , the sensor heads remain rigorously attached to their housings, in the "bowl" screws (12). The precision of the place of application of the reaction forces is only determined by the precision of the drilling to accommodate the "bowl" screws. Thus a lateral shift of one plate relative to the other has no effect on the precision of the

measured.

  However, in order to facilitate the transport of the platform, it is preferable to secure the two plates using one or more clamping screws (13). These screws must nevertheless allow a slight axial and transverse play (14) of a plate relative to the other in order to avoid any friction which would hamper the axial deformation induced by the

charge.

  To do this, one or more connecting screws will be available which would sandwich the two plates without tightening them by means of a stop spacer or by the use of shouldered screws (13). In another embodiment, it was possible to reduce this play by the use of spring (15) as illustrated in FIG. 14. Said spring exerts a force which comes to pull the bottom plate (4) towards the top plate ( 5) for

  sandwich the sensors (11) firmly without impeding their deformation.

  In one embodiment, but not the only one, which is in the field of stabilometry, the lower (4) and upper (5) plates will be of sufficient size to include the feet of a subject standing in the rest position. For example, 500mm side plates will be sufficient. In said position, according to the literature, the feet form an angle of approximately 30 degrees with non-contiguous heels spaced 2 centimeters apart. In this standardized position, it may be useful to have removable toe clips (18) but not necessarily, in order to report the measurement of the placements of the center of pressures in a frame of reference linked to the support polygon which is defined by the places of the supports external of the feet, as desired by the recommendations of the French Association of Posturology (AFP-85 Standards). Said toe clip (18) will advantageously be fixed on the upper plate using at least two centering pins which facilitate its installation and its rapid extraction. By way of nonlimiting example, the centering holes may be located on the base of the equilateral triangle materialized by the places of

  contact with the three sensors (11).

  The stability of the platform placed on the ground can be ensured for example using at least three adjustable jacks not shown here and which are screwed onto the lower plate (4). The location of these cylinders should be possible below the sensors to avoid twisting of the lower plate under the effect of a load. Said cylinders will also serve to adjust the horizontality of the platform in order to allow an unbiased measurement of the center of pressures. As such we can advantageously use a spirit level (20), which may for example be embedded in a corner of the upper plate

(5).

  In one of the embodiments according to the invention, the electronics (16) for implementing the sensors is placed between the two plates, close to said sensors so as to ensure a

non-noisy signal flow.

  The information, formatted by the electronic card (16) is transmitted via the connector (17) to a display and processing mode that is not viewed.

  here, which could be, for example, a computer.

  In an embodiment of an electronic interface, FIG. 18 illustrates here a nonlimiting example, consisting of an amplification plate (25) for three sensors with a resistive gauge bridge (30). The gauge bridges (30) are supplied by a supply device (31). The interface (26) is a well-known commercial device called an integrated circuit microcontroller. Said microcontroller integrates inter alia the functions of converting analog signals into digital signals and the functions of interfacing communication of the type for example serial communication well known to electronics engineers. In this embodiment, the circuit (27) and the circuit (28) are galvanic isolation circuits according to this mode of communication and allow the platform to be electrically isolated for safety reasons. The circuit (27) conveys the output information to the processing and display device (29) which may be a computer, while the circuit (28) receives, according to this same mode of communication, the information in origin of the device (29). The energy supply device comes either from the sector through an isolation transformer, in accordance with

  European standards, that is to say an autonomous battery.

  In an embodiment, but not the only one, of the electronic device for implementing and managing the force sensors, the power supply (31) is a possibly rechargeable battery, embedded in the platform. In this embodiment, it is advantageous to preserve the energy, in particular that which is supplied for the gauge bridges of the sensors. According to this mode, the supply of said gauges is continuous or interrupted, so that the energy is only supplied when the measurement signal is collected. According to this mode, the microcontroller (26) also generates the energy pulses in rhythm with the collection of the signal. In an embodiment, but not the only one, of data transmission to the processing and display device, the communication between the device (26) and the device 29 takes place by radio or by optical means. According to these modes, the device (27) is a radio transmitter or an infrared transmitter and the device (28) is a

  radio receiver or infrared receiver.

  For reasons of aesthetics and electrical isolation between the metal plate (5) and the feet of the subject, it is advisable to interpose, between said plate and said feet, a plate or a cover of non-conductive material of electricity or any coating capable of fulfilling this insulation function. An example of embodiment of a cover (21) in plastic material is given in FIG. 17. The top of said cover is in this embodiment, screen printed and drilled (23) to reveal the spirit level embedded on the plate (5) and allow passage (24) for the centering pins (19) of the toe clip (18). The screen printing, in this embodiment, represents a certain positioning of the feet (22), but not only this. Graduations are reported to allow reading

the size of the subject.

Claims (11)

  1 - Device for measuring the position of the center of gravity of a person consisting of a platform on which the person stands, characterized in that it comprises a bottom plate (4) or lower plate resting on the ground on which   at least three piezoelectric force sensors (11) of the button type are arranged.   2 - Device according to claim 1, characterized in that it comprises an upper plate (5) which rests on the pins (10) of the sensors (11) by means of cup screws (12) whose adjustment allows to modify the distance between the upper plate (5) and the lower plate (4), modify the plate of the plate (5), make up for   flatness defects of the platform (5), perfect the electrical zero of the sensors (11) of the platform   empty form. It allows at least one degree of freedom, to allow deformation   without hindrance of the sensors (II) under the effect of a load to be measured.   3 - Device according to any one of the preceding claims,   characterized in that the sensors (1 1) are placed free on the lower plate (4).   4 - Device according to any one of claims 1 or 2, characterized in   that the base of the sensors (11) is housed with recess on the lower plate (4).   - Device according to any one of claims 1 or 2, characterized in   that the base of the sensors (11) is glued to the lower plate (4).   6 - Device according to any one of the preceding claims,   characterized in that it comprises a screw or screws for securing the two plates (4) and (5) provided with a stop (13) to allow a slight axial and lateral play, these screws being by   example of shouldered screws (13) or normal screws with spacers.   7 - Device according to claim 6, characterized in that it comprises a spring (15) interposed between the screw head (13) and its housing (14) in the plate   lower allowing the screw or screws (13) to exert a controlled and limited tightening.   8 - Device according to any one of the preceding claims   characterized in that it comprises a removable device (18) for placing the subject's feet in a standardized position on the platform in order to report the measurements of the   excursions from the pressure center to a reference system linked to said feet.   9 - Device according to claim 8, characterized in that the device (18) comprises at least two centering pins (19), allowing said device to be fixed   on at least two centering holes in the plate (5). / o   - Device according to any one of the preceding claims,   characterized in that the electronic circuit for implementing the sensors is on board   in the platform between the two shelves.   1 l - device according to claim 10, characterized in that it comprises a device (31) consisting of a mains supply isolated by an external transformer 12 - device according to claim 10, characterized in that it comprises a   device (31) consisting of an autonomous battery.   13 - Device according to one of claims 11 or 12, characterized in that it   comprises a circuit (26) interrupting and controlling the device (31)   14 - Device according to any one of claims 11 to 13, characterized   in that it comprises galvanic isolation circuits (27) and (28).   - Device according to any one of claims 1 1 to 13, characterized   in that it comprises a radio transmitter and receiver (27) and (28)   16 - Device according to any one of claims 11 to 13, characterized   1 5 in that it comprises devices (27) and (28) consisting of an infrared transmitter and an infrared receiver.