WO1987001574A1

WO1987001574A1 - Evaluation of the phases of gait

Info

Publication number: WO1987001574A1
Application number: PCT/IT1986/000070
Authority: WO
Inventors: Massimiliano Bugarini; Pietro Chistolini; Luciano Da Cas; Velio Macellari
Original assignee: Istituto Superiore di Sanità
Priority date: 1985-09-19
Filing date: 1986-09-17
Publication date: 1987-03-26
Also published as: IT1182866B; EP0237544A1; IT8548574A0

Abstract

An apparatus for the evaluation of the temporal phases, the geometrical data and the reaction forces of the sole of the feet under dynamical conditions. This apparatus essentially provides a walkway (1) formed of three layers (2-4). The upper layer (2) consists of a flexible printed board having conductive paths (5) parallel to one another and applied to the lower face thereof, the middle layer (3) being formed of a conductive rubber sheet able to vary its resistance in the direction of its thickness when pressure is exerted in the direction perpendicular to the faces of the layer, the lower layer (4) being a printed board having conductive paths (6) parallel to one another and applied to the upper face thereof and extending in a direction perpendicular to the conductive paths of the upper layer (3). The apparatus allows the whole walkway to be scanned in real time during the gait of the subject at a sampling frequency higher or equal to 50 Hz, and the distribution of the support pressures exerted by the sole of the feet to be determined in a full gait cycle (double step).

Description

Evaluation of the phases of gait

DESCRIPTION

The present invention relates to a computer controlled platform responsive to the pressure of the foot sole of a walking subject. The lenght of this platform has to be such as to allow at least a complete gait cycle (double step) to be analyzed. Furthermore, the system has to provide firstly information concerning the geometry of the footprints as a function of the time and the relative location of the footprints of both feet and secondly a point by point information .-.bout the intensity of the pressure exerted by the sole of a feet on the platform.

In the study of the biomechanics of the man's gait an essential moment is that of the measurement of the kinematic and dinamical data. Such data are stored, suitably processed and then used by orthopaedists, physiologists, physiotherapeutists, sports doctors, manufacturers of prothesis a.s.o. as aid in their professional activity. The conventional force plates are by now a standard in the institutes of biomechanics. However, they provide only the resultant force of the reactions, thus losing any information about the distribution of the same throughout the sole of the foot. In 1978 (Hennig and Nicol Registration methods for time-dependent pressure distribution measurements with mats working as capacitors, Biomechanics VI-A.VPP, Baltimore, 361-367) a platform was already set up on the ground of capacitive sensors capable of measuring the pressure distribution under the sole of the foot. Another platform based on capacitive sensors is handled by the German firm NOVEL GmbH. The Israeli firm PALROD produces a platform based on piezooptical sensors, i.e. providing an imagine of the footprint the intensity of which is proportional to the pressure of the foot. Platforms using piezoelectric transducers have been provided by Cavanagh and Hennig (A new device for the measurement of pressure distribution on a rigid surface, ed. Sc. Sport. Exer. 13 (2), 1982) and by D. De Rossi et al. (Biomedical applications of piezoelectric and pyroelectric polymers, Ferroelectrics, Vol. 49 No. 1-4, 1983) .

In all above mentioned cases the platforms have limited size of the order of 30 cm x 30 so as to provide the pressure distribution of one foot at a time and then insufficient for the analysis of the double step.

Furthermore, even if it is desired to analyze the dynamical pressure produced by the single footing during the execution of a single step, the platforms of the prior art would give impaired results. In fact, the walking subject attempting to trample exactly on the centre of the small platform would be disposed to toddle before the latter, thus altering his normal gait with the result that the pressure distribution of the foot in the sole valid footing on the platform would be unnatural. The limited size of the already known platforms is intrinsically due to the tecniques used for manufacturing the sensors. Such tecniques do not allow for different reasons much larger platforms to be manufactured at non-prohibitive price. The man's gait dynamics has for physiological reasons significant spectral components higher than 20 Hz, * whereby it is necessary a sampling frequency not lower than 50 Hz in order to analyze these phenomena. Most of the above mentioned systems operate at a lower sampling frequency. In those systems with capacitive sensors in which an adequate sampling frequency can be reached, this facility is limited to a minimun number of sensors which is not sufficient to analyze the double step. The systems based on piezoelectric sensors besides being much more expensive are not provided with a matrix (lines and columns) but have at least a wire for each sensor, thus providing insurmountable wiring difficulties as the number of sensors increases. The system based on capacitive sensors operate at scanning frequencies higher than 1 MHz and therefore are subject to noise caused by stray capacities between adjacent sensors. Such a circumstance sets bounds to both the sensor reading speed and the density of the sensors itself.

In the system of Hennig and Nicol measurement data are stored on video recorder for reading speed reasons so that data are not allowed to be directly processed by computer.

Also systems based on piezooptical sensors suffer from a similar limit. In fact, in order to process data the imagine thereby generated has to be taken by T.V. means which are interfaced to a computer, thus involving further complications in the data bus and increasing the production cost.

Unlike such systems the platform according to this invention can have a size such as to provide digital pressure information, temporal phases and geometrical data of the whole double step such as the times between two footings, the delays, the distance between footings and the inclination of the feet with respect to the walking direction. The whole system of the present patent application consists of following four main blocks:

1) The sensing mat with the sensor matrix;

2) The electronics scanning the sensing mat and measuring the pressure exerted on it:

3) A storing system where the measured values are stored;

4) A host computer retrieving data from the storing system, processing them and displaying the results.

The sensing mat consists of three rectangular layers laid one upon the other. The lower layer is formed of a rigid or flexible printed board provided 'on the upper surface, with conductive paths equally spaced and parallel to one another and to one side of the sensing mat.. The upper layer is formed of a flexible printed board provided on the lower surface, with conductive paths equally spaced and parallel to one another and orthogonal to the paths of the lower layer. The middle layer is formed of a conductive rubber sheet the electric resistance of which varies point by point only in the direction of the thickness of the layer as a function of the pressure exerted between a point of the upper surface and the corresponding point of the lower surface of the rubber sheet.

A resistive contact matrix is thus provided. The size and the resolving capability of such a matrix depend upon the number and the spacing of the conductive paths of the upper and lower layers. The resistance of each contact point is inversely proportional to the pressure applied by the sole of the foot. The use of resistive contacts and the way to take the information allow the points on which a pressure is applied to be detected without ambiguity and mistakes. It should be noted that problems are avoided that might ensure, for exa ble, by using a decoding similar to that of the computer keyboards (phanton key problem) when more than one key at a time are pressed. Such an aspect will be now better illustrated. The conductive rubber can be schematically represented as a matrix of variable resistances connected among lines and columns of the matrix.

The lines are supplied one after the other with a d.c. voltage Vg and the columns are connected to the input of an operational amplifier acting as current-voltage converter. Due to this configuration each column is connected to the virtual ground of the corresponding operational amplifier. In this way adjacent columns are at the same potential. Such e.n expedient along with the use of resistive contacts would prevent that an undesired current flows in the lines which are not supplied with the voltage Vg_r said current being capable to cause errors of detecting the points on which a pressure has been applied.

The output voltage of each operational amplifier varies as a function of the resistance between the column to which the amplifier is connected and the line selected at that time, i.e. as a function of the pressure applied on that cross point. The use of resistive and non-capacitive contact points avoids a trigger signal at high frequency to be used, thus providing a lower sensitivity to noise caused by stray capacities and a higher measuring rate. As a result, data can be supplied by a great number of sensors at a sampling frequency of at least 50 Hz. The structural simplicity and the low cost allow a sensing mat to be built having a size such as to analyze a double step. This analysis can involve also particular gait conditions such as the running and the walk with a walking-stick or on crutches. The above mentioned measures can be taken without disturbing at all the normal gait and the free carriage of the walking subject across the sensing mat. Once detected, data are transferred to a computer, even of low cost, or to any permanent storing system. A preferred embodiment of the present invention having non-limitative and non-restrictive character will be described herebelow by way of example with reference to the accompanying drawing, in which:

Fig. 1 shows a schematic diagram of the sensing mat with its three layers;

Fig. 2 shows the equivalent electric diagram of the sensing mat with the annexed electronics for detecting signals generated by the application of the pressure on the sensing mat and

Fig. 3 shows a block diagram of the whole apparatus for the evaluation of the support pressures according to the present invention.

Fig. 1 shows the sensing mat according to the present invention which is generally designed by 1. Sensing mat 1 according to a preferred embodiment has size 2.6m x 0.5m and consists of an upper layer 2, a middle layer 3 and a lower layer 4, which are laid one upon the other, and of support means. Upper layer 2 consists of a flexible printed board on the lower surface of which conductive paths 5 spaced apart from (for instance 5 mm)

_» and parallel to one another are applied.

Middle layer 3 consists of a conductive rubber sheet, for example, of the type available under the name VELOSTAT of the firm 3M Italia S.p.A. Lower layer 4 consists of a rigid printed board on the upper surface of which conductive paths 6 spaced apart from (for istance.5 mm) and parallel to one another, are applied, said paths being transversal to the conductive paths 5 of the upper layer 2. In the embodiment of Fig. 1 conductive paths 6 are orthogonal to conductive paths 5. On the whole a crossing path matrix is provided, consisting of 512 lines and 96 columns in case a sensing mat having the above mentioned size is built. If it is used a conductive rubber sheet the surface resistance of which (Fig. 2), measurable between two contacts provided on the same side of the sheet, is low enough, the surface resistance R'_t between two columns of the matrix does not considerably impair the sensing of the pressure signal as the columns are connected to a virtual ground. The surface resistance R"_t between lines can give rise to tolerable errors in many applications, but if it is not negligible to reach the desired precision, it has to be interrupeted by means of a little modification. The conductive rubber sheet is stuck on the lower layer of the sensing mat consisting of conductive paths (the lines of the matrix) by applying the adhesive in the insulating spaces between the- aths. Afterwards, horizontal cuts.are provided (for example, by lancet) along the width of the footboard on the rubber sheet at the median lines of the spacings between adjacent paths. Fig. 2 shows the equivalent circuit diagram of sensing mat 1 consisting of the matrix having lines 6 and columns 5. Schematically shown between lines 6 and columns 5 are the equivalent resistances (R; , T_.2' ^R21' 22._") of the middle conductive rubber sheet 3 at the cross points between lines and columns, said resistances depending upon the pressure applied on those points.

Voltage generator V_s supplies lines 6 through the solid state analogue switch 7 in a cyclic sequence, i.e. one after the other as shown by the arrows in switch 7. The invertent input of each operational amplifiers 8, connected as transimpendance amplifiers in a number of one for each columns, is connected to a column 5 of the matrix footboard 1.

A voltage signal V_TH is applied to the invertent input of the operational amplifiers 8. It determines the threshold level of the operational amplifiers 8. Generator Vg' iⁿ' case no pressure is applied on the sensing mat, is practically a no-load generator as it is connected to the very high resistances of the conductive rubber sheet 3 connecting the cross points of the conductive path matrix (5,6). On the contrary, when a pressure is applied by the foot of the walking subject, the voltage Vg supplied, for example, to line r₂ generates a current only in those columns over which a pressure is applied on the sensing mat, as this pressure causes the variation, i.e. the decreasing, of the resistances between those columns and lines r₂ selected by analogue switch 7. In case of Fig. 2 it is assumed that the line selected during the scanning of all lines is *^~2 and that a pressure is applied on the points associated to resistances R₁₁₍ R₁₂ ^{and R}22" ^{In the} illustrated case the current I flows in line r₂ and only in resistance R₂₂ ^an<^ then to the virtual ground of operational amplifier 8 of column c₂. in fact, current I' which would produce an undesired signal at the output of column ci could be generated only if all columns are not at the same potential.

All points of a line over which no pressure is applied are represented by resistance R shown in phantom, the value of that resistance being practically infinite so that a current cannot be generated, i.e. no or non- valuable pressure is applied on the sensing mat. Each line is selected by a solid state analogue switch 7 that applies the voltage Vg to the lines according to a cyclic sequence. Switch 7 (Fig. 3) is in turn addressed by the output of line counter 9. The same output is fed through port 11 to the output data bus 12 for being stored into RAM 13 connected by parallel interface 14 to computer 15. The whole scanning of the lines is carried out at a^" frequency of 50 Hz. Such a value of frequency corresponds to the maximum sampling frequency at which the pressure applied by the sole of the feet on the whole sensing mat is sensed. Generally each sampling phase is enabled by an outer clock signal S supplied to line counter 9 from computer 15 through interface 14. Such a signal defines the sampling frequency. Line counter 9 is increased by line clock C_rq generated by unit 10. Designated by 19 is a clock generator C_s^ the output signal of which is fed to column counter 20 and controller 21. Column counter addresses each column through analogue multiplexer 16 and is connected to bus 12 through port 15. Operational amplifiers 8 are connected to analogue multiplexer 16, the output of which is connected to analogue-digital converte block 17. The output of converter 17 is connected to store register 22. Digital comparator 18 checks that data contained in register 22 is different from zero. Only if this is true comparator 18 supplies signal E₀ to controller 21. On receiving E_Q controller 21 supplies signals E_χ, E₂, E3 to ports 11, 15, 23, respectively. On receiving said signals E_lf E₂, E₃, said ports enable storing of line and column addresses and the content of register 22 into memory 13 through bus 12. Data are then available through interface 14 to computer 15 processing and displaying them on a video and/or graphic terminal.

During a measurement with a walking subject on sensing mat 1 the scanning of the sensing mat is enabled by a sampling signal S having a frequency of 50 Hz as a maximum. During each sampling the voltage Vg is applied in a cyclic sequence to each line via the solid state analogue switch 7 which in turn is addressed by line counter 9. Line counter 9 after signal S being fed, is increased by clock signal C_RG generated by block 10. During such clock cycle it is established whether the walking subject found a valid footing on at least one of ^"the 96 sensing points of the lines selected at that - 13 - moment. To this end, after line clock C_RG being issued, scanning of the line itself is carried out. Column counter 20 addresses sequentially the outputs of operational amplifiers 8 which are connected to analogue-digital convertor 17 through multiplexer 16. Data at the output of convertor 17 is stored into register 17. Comparator 18 determines if data of register 21 is different from zero. If so, a pressure has been applied on the point addresses by the column and the line selected at that time. In such a case comparator 18 enables controller 21 which in turn enables sequentially gates 11, 15 and 23, thus allowing line address, column address and the value of the pressure to be stored into memory 13. The content of memory 13 is then transferred through interface 14 into computer 15.

If a low number of pressure levels (for example, as coded with one or two bits) are to be detected and a higher sampling frequency is to be reached, the diagram heretofore described can be simplified. Operational amplifiers 8 act as voltage comparator. In this way the output of operational amplifiers 8 is already coded in digital form, the voltage V_TH being constant, so that the multiplexer 16 is of digital type and the analogue- digital converter 17 can be omitted. Therefore the addresses of the points of the sensing mat, on which a pressure greater or equal than that corresponding to the threshold voltage V_TH has been applied, are stored into memory 13. By means of a suitable generator of the threshold signal V_TH which can be succes_sively varied, it is possible to carry out an analogue to digital conversion, which is slow but inexpensive, and then to provide more pressure levels. This solution is particularly advantageous for a two-level conversion (coded with only one bit) and useful for those applications in which it is preferred to know only if on each sensitive point a pressure greater or equal than a reference value is applied.

In order to provide a greater number of pressure levels at a higher sampling frequency, analogue-digital conversion block 18 is formed of more analogue-digital

» converters (for example, 8) operating in parallel.

Scanning of the columns has not to be carried out a column at a time but a group of more columns (for example, 8) are scanned at a time, so that each column is connected to one converter of the block 17.

Claims

1. Apparatus for the evaluation of the temporal phases of the human gait and the instant distribution of the support pressure on the ground, characterized in that it consists of a platform consisting of a mat formed of three layers laid one upon the other and support means of said platform, the upper layer of which being formed of a flexible printed board provided on its lower face with conductive paths spaced apart and parallel to one another, the middle layer being formed of a conductive rubber sheet able to vary its resistance in a direction of its thickness when a pressure is exerted in the direction perpendicular to the faces of the layer, the lower layer being formed of a printed board having on its upper face conductive paths spaced apart and parallel to one another which extend in a direction perpendicular to the conductive path of the upper layer so that the conductive paths of the first and the third layers provide a matrix formed of lines and columns which are connected at the intersection points by the resistive contacts of the middle layer having variable resistance according to the applied pressure, said lines of the matrix being connected to a voltage source by sequence line selection means, and the columns being connected each to a current sensing means able to provide time by time an output signal being responsive to the overcoming of a threshold value by the column currents generated by the pressure exerted on the upper surface of the mat, said output signal containing the information of the pressure intensity, and the whole of said output signals of said current sensing means being fed through analogue-digital converter means to the storing means of a host computer along with the addresses of the corresponding sensitive points before the next line being selected by the sequence line selection means.

2. Apparatus according to claim 1, characterized in that said sequence line selection means consists of a solid state analog switch.

3. Apparatus according to claim 1, characterized in that said lower layer consists of a flexible printed board, so that a flexible sensor matrix is formed.

4. Apparatus according to claim 1 and 2, characterized in that the storing of the pressure distribution on the whole sensitive point matrix of large size such as 96 x 512 points is effected in a time not longer than 20 msec.

5. Apparatus according to claim 1, characterized in that said current sensing means are operational amplifiers acting as current voltage converter, the input of which, that is connected to the respective column of the sensing point matrix of the mat, being at virtual ground.

6. Apparatus according to claim 1 and 5, characterized in that the conductive rubber sheet is provided with cuts oriented in the direction of the lines and interrupting the resistive contat between adjacent lines due to the non-negligible surface conductivity of the conductive rubber sheet.

7. Apparatus according to claim 1 and 5, characterized in that the analogue-digital conversion is carried out connecting the further input of the operational amplifier to a stepwise variable threshold generator.