Abstract:
The invention relates to a transducer arrangement for converting a load variation into one or more electrical output signals. The transducer arrangement comprises at least one transducer element and an evaluation unit operatively connected to the transducer element. The transducer arrangement can be used amongst others, for healthcare applications, sport leisure activities, impact detection for safety applications in the automotive industry as well as for safety surveillance systems in the industry.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to a transducer arrangement for measuring load variations. This invention more specifically relates to a transducer arrangement to be arranged inside a shoe sole or on a shoe inlay for evaluating load variations applied thereon. 
       BACKGROUND ART 
       [0002]    It is known within the art that a load variation applied onto a surface or a body can be measured with the help of a transducer. A transducer converts a variation in one physical quantity, e.g. pressure, quantitatively into a variation in another physical quantity, e.g. voltage. The present invention relates to an improved transducer arrangement with transducers for converting load variations into voltage variations. 
         [0003]    One particular embodiment of such transducers are so-called force-sensing resistors (FSRs). FSRs are well known within the art and can be used in numerous applications. Such force-sensing resistors usually include at least two electrodes with a force-sensitive material arranged there in-between. In case a load is applied onto an FSR the resistance of between the electrodes across the force-sensitive material varies. An electrical circuit coupled to the electrodes monitors the change in resistance. 
         [0004]    According to a preferred application, FSRs can be arranged inside a shoe sole for measuring the force a person applies thereon, during walking, jumping and especially running. US 2010/0063778 reveals such a shoe sensor system with different force-sensing resistors, which are operatively connected to an electrical module. The electrical module is capable of gathering measurements from the different force-sensing resistors and of transmitting the data for further use via a communication port to an external device. 
         [0005]    Yet, force-sensing resistors have a low speed response and can therefore only be used for measuring static loads or quasi-static load variations. They are not sufficiently precise to provide information about the user&#39;s foot anatomy or gait dynamics. This information however can be very useful in athlete monitoring or for healthcare applications, e.g. foot diagnosis and health prophylaxis. 
       BRIEF SUMMARY 
       [0006]    A transducer arrangement is provided that can measure load variations more accurately. 
         [0007]    The invention relates to a transducer arrangement for converting a load variation into one or more electrical output signals. The transducer arrangement comprises at least one transducer element and an evaluation unit operatively connected to the transducer element. The transducer arrangement can be used amongst others for healthcare applications, sport leisure activities, impact detection for safety applications in the automotive industry as well as for safety surveillance systems in the industry. 
         [0008]    The transducer element comprises a combination of a first transducer and a second transducer. The evaluation unit comprises:
       a first evaluation circuit associated to the first transducer for converting static loads or quasi-static load variations, which comprise typical time frames of &gt;0.1 sec, into a first output signal,   a second evaluation circuit associated to the second transducer for converting a highly dynamic load variations, which are typically a factor 10-100 faster than quasi-static signal changes, into a second output signal, and   an output circuit operatively connected to the first evaluation circuit and the second evaluation circuit for outputting the first electrical output signal and/or the second electrical output signal.
 
Thanks to this transducer arrangement, load variations are measured with higher precision.
       
 
         [0012]    This invention more particularly but not exclusively relates to a transducer element, wherein the first transducer or/and the second transducer is foil-based. The first transducer and the second transducer can be embodied as a multilayered arrangement of several foils, e.g. laminated together at least at specific locations thereof. Thanks to the flexibility, lightness and thinness of foils, they can be arranged on or inside a great variety of different materials. 
         [0013]    According to a particular advantageous embodiment of the invention, the first transducer is a foil type pressure sensor. The foil type pressure sensor comprises:
       a first carrier foil,   a second carrier foil that is kept apart from the first carrier foil by one or more spacers arranged between the first carrier foil and the second carrier foil, and   at least two electrodes and a layer of pressure sensitive material arranged in an active area of the first transducer. The pressure sensitive material connects the first electrode and the second electrode.
 
In response to a load variation that is acting onto the active area, the first carrier foil approaches the second carrier foil and an electrical contact is established across the layer of pressure sensitive material between the first electrode and the second electrode so that the resistance measured between the first electrode and the second electrode changes. The spacers can be made of foam or any other material that can be compressed under pressure and that regains its initial size after the compression.
       
 
         [0017]    According to a preferred embodiment of the invention, the first electrode is arranged on the first carrier foil facing the second electrode arranged on the second carrier foil. A layer of pressure sensitive material covers the first and/or the second electrode in the active area of the foil type pressure sensor and connects the first electrode to the second electrode. In response to a pressure acting onto the active area of the foil type pressure sensor the first electrode approaches the second electrode and the resistance between the electrodes across the pressure sensitive material arranged there in-between varies. The variation in resistance is measured by an evaluation circuit operatively connected to the first and second electrodes. 
         [0018]    Alternatively the first electrode and the second electrode are arranged separated one from the other on the first carrier foil, while the layer of pressure sensitive material is arranged on the second carrier foil facing the first carrier foil. The spacer is arranged between the first carrier foil and the second carrier foil, holding the first carrier foil and the second carrier foil apart one from another, when no pressure is applied onto the active area. 
         [0019]    When a force is applied onto the active area, the first and second carrier foils are brought together and the layer of pressure sensitive material is connecting the first and the second electrodes in the active area. In response of the establishment of electrical contact the resistance between the electrodes, connected by the pressure sensitive material varies. The first carrier foil and the second carrier foil can be made out of a very thin electrical insulating material. The first transducer can be embodied as a FSR (force-sensing resistor) or any other transducer capable of measuring low dynamic load variations. 
         [0020]    Each transducer element can be embodied as a separate unit measuring a load variation at a predefined area and transmitting one or more electrical output signals to an output circuit. According to a preferred embodiment of the invention, a plurality of transducer elements can be arranged spatially separated one from another. This is advantageous since the elements can be placed in a specific section, where load measurements are required. In some cases, it is required to measure high-dynamic load variations only in specific sections of the first transducer. Hence, multiple transducer elements can use one common first transducer or/and second transducer. 
         [0021]    The second transducer can advantageously be an electret based pressure sensor or, alternatively, with less dynamic range, a piezoelectric pressure sensor comprising a sensing foil. The sensing foil can have an anode and a cathode with an electret based material or a piezoelectric material arranged in-between the anode and the cathode. Preferably, the anode and cathode are connected to the second evaluation circuit. Such electret-based or, with less sensitivity, piezoelectric sensors are capable of measuring high dynamic load variations. 
         [0022]    According to one embodiment of the invention, the first transducer and the second transducer are arranged one above the other. The first transducer and the second transducer can be very thin, such that a load applied onto one of the transducers is automatically applied onto both transducers. 
         [0023]    Furthermore, the first carrier foil or/and the second carrier foil comprises the sensing foil. The first carrier foil or/and the second carrier foil of the first transducer can be replaced entirely by the sensing foil. 
         [0024]    According to a different embodiment of the invention, the one or more spacers comprise the sensing foil. The sensing foil can replace the one or more spacers partly or entirely. 
         [0025]    Preferably, the first evaluation circuit comprises a R-circuit and the second evaluation circuit comprises a R-C-circuit. The low-dynamic load variation is evaluated by the R-circuit and outputted as a first change in voltage of the first output signal over the time period Δt, and the high-dynamic load variation is evaluated by the R-C-circuit and outputted as a second change in voltage of the second output signal over the time period Δt. Thanks to the separation between the first evaluation circuit and the second evaluation circuit, the first transducer and the second transducer operate in their optimal conditions. The low-dynamic load variation and the high-dynamic load variation can be measured simultaneously or sequentially. 
         [0026]    According to a preferred embodiment of the invention, the evaluation unit comprises a central processing unit. The output circuit is, in operating state, directly or wirelessly coupled to the central processing unit. The output circuit can connect via a wireless communication, e.g. Bluetooth, Wireless Local Area Network or infrared to a smartphone, a personal computer or any other electronic device and transmit the processed first and second electrical output signals. This may be particularly advantageous for athlete monitoring to indicate proper (healthy) foot loading while a person is running, walking or jumping. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
           [0028]      FIG. 1  is a schematic view of a shoe inlay assembly of a sensor arrangement in accordance with a first preferred embodiment of the invention. 
           [0029]      FIG. 2  is a cross-sectional view of a sensor element in accordance with a first preferred embodiment of the invention. 
           [0030]      FIG. 3  is a schematic view of a shoe inlay assembly of a sensor arrangement in accordance with a second preferred embodiment of the invention. 
           [0031]      FIG. 4  is a cross-sectional view of a sensor element in accordance with a second preferred embodiment of the invention. 
           [0032]      FIG. 5  is a schematic view of a shoe inlay assembly of a sensor arrangement in accordance with a third preferred embodiment of the invention. 
           [0033]      FIG. 6  is a cross-sectional view of a third sensor element in accordance with a third preferred embodiment of the invention. 
           [0034]      FIG. 7  is a record of a simultaneously recorded high dynamic load variation and a low dynamic load variation over a time period of 1 second applied onto the sensor arrangement for a person while walking. 
           [0035]      FIG. 8  is a record of a simultaneously recorded high dynamic load variation and a low dynamic load variation over a time period of 1 second applied onto the sensor arrangement for a person while running. 
           [0036]      FIG. 9  is a record of a simultaneously recorded high dynamic load variation and a low dynamic load variation over a time period of 1 second applied onto the sensor arrangement for a person while landing after a jump. 
           [0037]      FIG. 10  is a record of a simultaneously recorded high dynamic load variation and a low dynamic load variation over a time period of 1 second applied onto the sensor arrangement for a person while stamping. 
           [0038]      FIG. 11  is a schematic arrangement of the different components of one sensor arrangement. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    The particularly advantageous but not limiting embodiments of the invention, which will be described with reference to the figures, relate to a transducer arrangement embodied as a shoe inlay to measure a load applied thereon. The shoe inlay can be inserted into a sport shoe for athlete monitoring to measure proper foot load conditions versus athletic performance such as speed, distance or/and acceleration of the person wearing it. Alternatively the transducer arrangement can be arranged directly onto or into the shoe sole. The transducer arrangement converts the load variation applied onto the transducer elements over multiple stages into electrical output signals. For each transducer element the load applied thereon is converted into two electrical output signals. 
         [0040]    In the embodiment of  FIG. 1  multiple transducer elements  4 ,  10 ,  20 ,  30  are arranged on a common carrier foil  2 , such that they cover the foot areas of maximum load variations. The carrier foil  2  has a shape similar to the contact surface of the foot and is made of a film of flexible electrical insulating material. One transducer element  4  is arranged in the toe area of the foot, two transducer elements  10 ,  20  are arranged close to the mid-section of the foot, where the metatarsals connect to the phalanges, and one transducer element  30  is arranged close to the heel area of the foot. In addition to the transducer elements  4 ,  10 ,  20 ,  30  other transducer elements can be used in other areas of the shoe, where an accurate measurement of load variations is required. In order to cope with anthropometric variations in foot anatomy each transducer element  4 ,  10 ,  20 ,  30  is preferably oval-shaped having rounded edges with a radius of 6 mm and each transducer element  4 ,  10 ,  20 ,  30  has a width of 20 mm and a length of 30 mm. The transducer elements  4 ,  10 ,  20 ,  30  are very thin and can thus be arranged inside a shoe without hindering the person wearing it. 
         [0041]    The definitions of bottom and top will be used hereinafter to describe the layer arrangement of the transducer elements shown in  FIG. 1  to  FIG. 6 . The top layer refers to the layer closest to the surface a person&#39;s foot contacts and the bottom layer refers to the layer closest to the shoe sole. The definition of top and bottom is added for intelligibility and cannot be regarded as limiting the scope of the invention. Since the transducer elements measure a load variation, it should be regarded as obvious, that the transducer elements deliver similar results when being turned-over. 
         [0042]    The transducer arrangement as shown in the attached figures is particularly advantageous since it is capable of capturing high dynamic load variations and low dynamic load variations separately. Each of the transducer elements therefore comprises a first transducer for measuring low dynamic load variations and a second transducer for measuring high dynamic load variations. The high dynamic load variations and the low dynamic load variations are evaluated by using two separate evaluation circuits. One evaluation circuit is associated to the first transducer and one evaluation circuit is associated to the second transducer 
         [0043]    The first transducer is combined with a second transducer according to three advantageous embodiments of the transducer arrangement mentioned hereafter. According to a first transducer arrangement in  FIG. 1  and  FIG. 2 , the second transducer is arranged on top of the first transducer. In a second embodiment, illustrated in  FIG. 3  and  FIG. 4  the spacer of the first transducer is partly replaced by a second transducer. In a third embodiment in  FIG. 5  and  FIG. 6  the bottom carrier foil of the first transducer is replaced with the second transducer. 
         [0044]    The first transducer  6 ,  12 ,  22 ,  32  is a foil type pressure sensor having a first carrier foil  42  at the bottom, which corresponds to the first carrier foil  2  in  FIG. 1 . A first electrode  44  is applied onto the first carrier foil  42 . A pressure sensitive material  46 , such as a force-sensitive resistive material, is arranged on the top surface of the first carrier foil  42 . The pressure sensitive material  46  separates the first electrode  44  from the second electrode  48 . The second electrode  48  is applied on the second carrier foil  50  facing the first electrode  44 . Spacers  66 ,  68  are made e.g. of a foam material or a polymer film material, circumferentially arranged around the active area of the first transducer to keep the first carrier foil  42  at a certain distance from the second carrier foil  50 . In response to a low dynamic load variation that is acting onto the active area of the first transducer, the first carrier foil  42  approaches the second carrier foil  50  and the resistance between the first electrode  44  and the second electrode  48  changes. This resistance is preferably measured by a first evaluation circuit operatively connected to the first electrode  44  and the second electrode  48  via the leads  52 ,  54 . 
         [0045]    According to the first transducer arrangement, the second transducers  8 , 14 ,  24 ,  34  are arranged on top of the first transducer  6 ,  12 ,  22 ,  32 . The second transducer in  FIG. 2  comprises an electret material  60 , with quasi-permanent electrostatic dipoles. The electret material  60  is arranged in the active area between the anode  56  and the cathode  58 . The anode  56  and the cathode  58  are operatively connected via the leads  64  and  62  respectively to the second evaluation circuit. If a load is applied onto the transducer element  4  in  FIG. 1 , the capacitance of the electret material and the resistance between the electrodes across the pressure sensitive material changes. The first evaluation circuit measures the change in resistance between the electrodes. The change in capacitance of the electret material is measured by the second evaluation circuit. 
         [0046]    Similar to the first embodiment of the transducer arrangement, the second embodiment of the transducer arrangement as shown in  FIG. 3  comprises four transducer elements  104 ,  110 ,  120 ,  130  arranged on the bottom carrier foil  102  of the first transducers in the areas of maximum load variation. Each transducer element comprises the first transducer of the first embodiment with a second transducer that partly replaces the spacers.  FIG. 4  is a cross-sectional view of one of the transducer elements  104 ,  110 ,  120 ,  130 . The second transducer comprises an electret  160 , arranged in-between an anode  156  and a cathode  158 . The leads  154  and  152  operatively connect the foil electrodes  144 ,  148  to the first evaluation circuit. The spacer of the first transducer is partly replaced by a circumferential electret  160  arranged between the electrode foils  144 ,  148  and the anode  156 , cathode  158 . The second transducer has approximately the same thickness as the spacers  151 ,  153  of the first transducer. The leads  162 ,  164  operatively connect the cathode  158  and the anode  156  to a second evaluation circuit. 
         [0047]    The third embodiment of a transducer arrangement as shown in  FIG. 5  comprises a transducer element having four first transducers  206 ,  212 ,  224 ,  232  and one common second transducer  208 . The second transducer has a shape similar to the contact surface of the foot and carries the four first transducers. The second transducer replaces the first carrier foil of the first transducer. Leads  262 ,  264  connect the anode  256  and the cathode  258  to the second evaluation circuit. Each of the electrodes  244 ,  248  is operatively connected via a lead  252 ,  254  to a second evaluation circuit. The dimensions of the first transducers  206 ,  212 ,  224 ,  232  are equal to the dimensions of the transducer elements  4 ,  10 ,  20 ,  30 ,  104 ,  110 ,  120 ,  130  of the first and the second preferred embodiment of the invention. 
         [0048]    The first and the second evaluation circuits for each of these three embodiments of the invention are necessary to distinguish between the different forms of locomotion of a person on land, e.g. jumping, walking, running and stamping. In a possible embodiment, the first transducer is integrated as a resistor into an R-circuit. The voltage of the R-circuit is measured by the first evaluation circuit and outputted to an output circuit as a first output signal. By applying a load onto the first transducer, the first and second carrier foils are pressed together and the resistance across the force-sensitive resistive material changes. Since foil-based force-sensing resistors are only capable of reliably measuring static loads or quasi-static load variations, a first change in voltage therefore is proportional to the quasi-static load applied onto the active area of the first transducer. 
         [0049]    The second evaluation circuit only detects the high-dynamic load variations by measuring the change in capacitance of the electret material. Thus, the second transducer element is integrated as a capacitor into a RC-circuit. The voltage of the RC-circuit is measured by the second evaluation circuit and outputted to an output circuit as a second output signal. The electret material has a quasi-permanent electric charge or dipole polarization. As the load applied onto the electret changes, the distance between the anode and the cathode changes. This change in distance results in a change in capacitance, which is monitored by measuring the voltage of the second output circuit. 
         [0050]    In  FIGS. 7 to 10 , the output circuit monitors the first output signals  302 ,  402 ,  502 ,  602  and the second output signals  304 ,  404 ,  504 ,  604  simultaneously over a time period Δt of 1 s. Each figure corresponds to a load variation a person applies onto one transducer element of the transducer arrangement while carrying out a specific movement. 
         [0051]    The first output signal  302  and the second output signal  304  in  FIG. 7  correspond to a load variation applied onto a transducer element by a person while walking. 
         [0052]    The first output signal  402  and the second output signal  404  in  FIG. 8  correspond to a load variation applied onto a transducer element by a person while running. 
         [0053]    The first output signal  502  and the second output signal  504  in  FIG. 9  correspond to a load variation applied onto a transducer element by a person while landing after a jump. 
         [0054]    The first output signal  602  and the second output signal  604  in  FIG. 10  correspond to a load variation applied onto a transducer element by a person while stamping. 
         [0055]    For each load variation the voltage of the first output signals  302 ,  402 ,  502 ,  602  differs from the voltage of the second output signals  304 ,  404 ,  504 ,  604 . The second transducer (with the corresponding second output signals  304 ,  404 ,  504 ,  604 ) has a shorter reaction time than the first transducer (with the corresponding first output signals  302 ,  402 ,  502 ,  602 ). The combination of the first output signal and the second output signal enhances the sensing capabilities by providing complementary dynamic and static plantar load information for further analysis. As illustrated in  FIG. 11 , a first output signal  702  and a second output signal  704  can be processed by a central processing unit. In this preferred embodiment the first and the second output signals of the output circuit are transferred via a Bluetooth  706  to a CPU. 
         [0056]    The voltage variation over time of the first output signals  402 ,  502 ,  602  is quite similar for three different load variations. Therefore, it is difficult to assign each of the output signals to one of the load variations without taking the low dynamic load variations into account. Furthermore the second output signals  504 ,  604  mostly comprise peaks of short duration and are thus difficult to associate to one specific load variation. Since a high dynamic load variation is measured in addition to a low dynamic load variation, the invention is particularly advantageous. Each load variation is a combination of the first output signal and a second output signal. By combining the first output signal with the second output signal the load variation can be measured more accurately.