WO1998022836A1 - Device for presence detection - Google Patents

Device for presence detection Download PDF

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Publication number
WO1998022836A1
WO1998022836A1 PCT/SE1997/001821 SE9701821W WO9822836A1 WO 1998022836 A1 WO1998022836 A1 WO 1998022836A1 SE 9701821 W SE9701821 W SE 9701821W WO 9822836 A1 WO9822836 A1 WO 9822836A1
Authority
WO
WIPO (PCT)
Prior art keywords
sensor element
layers
detection
electrically conducting
capacitance
Prior art date
Application number
PCT/SE1997/001821
Other languages
French (fr)
Inventor
Bengt-Allan Bergvall
Original Assignee
Scandmec Ab
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Scandmec Ab filed Critical Scandmec Ab
Publication of WO1998022836A1 publication Critical patent/WO1998022836A1/en

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Classifications

    • BPERFORMING OPERATIONS; TRANSPORTING
    • B60VEHICLES IN GENERAL
    • B60RVEHICLES, VEHICLE FITTINGS, OR VEHICLE PARTS, NOT OTHERWISE PROVIDED FOR
    • B60R21/00Arrangements or fittings on vehicles for protecting or preventing injuries to occupants or pedestrians in case of accidents or other traffic risks
    • B60R21/01Electrical circuits for triggering passive safety arrangements, e.g. airbags, safety belt tighteners, in case of vehicle accidents or impending vehicle accidents
    • B60R21/015Electrical circuits for triggering passive safety arrangements, e.g. airbags, safety belt tighteners, in case of vehicle accidents or impending vehicle accidents including means for detecting the presence or position of passengers, passenger seats or child seats, and the related safety parameters therefor, e.g. speed or timing of airbag inflation in relation to occupant position or seat belt use
    • B60R21/01512Passenger detection systems
    • B60R21/0153Passenger detection systems using field detection presence sensors
    • B60R21/01532Passenger detection systems using field detection presence sensors using electric or capacitive field sensors
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01LMEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00Measuring force or stress, in general
    • G01L1/14Measuring force or stress, in general by measuring variations in capacitance or inductance of electrical elements, e.g. by measuring variations of frequency of electrical oscillators
    • G01L1/142Measuring force or stress, in general by measuring variations in capacitance or inductance of electrical elements, e.g. by measuring variations of frequency of electrical oscillators using capacitors
    • G01L1/144Measuring force or stress, in general by measuring variations in capacitance or inductance of electrical elements, e.g. by measuring variations of frequency of electrical oscillators using capacitors with associated circuitry
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01VGEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V3/00Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
    • G01V3/08Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices
    • G01V3/088Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices operating with electric fields

Definitions

  • the present invention relates to a device for presence detection, according to the preamble of appended claim 1.
  • the invention is particularly intended for use in connection with presence detection of a passenger in a vehicle seat.
  • an airbag for a passenger can cause severe damage if the airbag is activated when, for example, a rear-facing child seat is mounted in front of the airbag or when a passenger who, for example, is not using the seat belt, is leaning forwards at the moment of activation.
  • the known sensor in principle provides an acceptable presence detection
  • the sensor has certain disadvantages. For one thing, there is a risk that the detected capacitance will change depending upon the interfering capacitances which are connected to the upper and the lower layer in the sensor, which, in turn, gives rise to an inferior reliability due to the fact that it will be more difficult to separate a condition regarding "presence” from a condition regarding "non-presence".
  • the capacitance of the sensor constitutes a source of interference which emits electromagnetic radiation which, for example, may disturbe other electronic equipment in the vehicle.
  • a further disadvantage is the risk of possible damp in the compressible layer causing errors in measurement.
  • the object of the present invention is thus to provide an improved capacitive arrangement for presence detection, by means of which the above-mentioned disadvantages can be eliminated and an increased reliability can be provided, particularly in connection with presence detection in vehicle seats. Said object is accomplished by means of an arrangement as initially mentioned, the characterizing features of which will be apparent from appended claim 1.
  • the invention comprises a capacitive sensor element which contains a number of layers having an essentially two- dimensional extension, wherein the capacitance of the sensor element changes during the influence of force.
  • the invention also comprises a detection unit for detection of the difference between the capacitance that, on the one hand, arises when the object is present and, on the other hand, arises when the object is absent.
  • the invention is based on the idea that the sensor element comprises at least two electrically conducting layers which are arranged in such a way that they surround a third electrically conducting layer.
  • intermediate dielectric layers are arranged between the electrically conducting layers.
  • one or both of the dielectric layers can be replaced by a spring arrangement which keeps the electrically conducting layers apart. When pressure is applied, the spring arrangement compresses, which results in that the conducting layers are brought closer to each other.
  • a "screened" arrangement is provided for reliable presence detection which offers low electromagnetic interference.
  • the invention is also used purely as a pressure sensor, which, as a consequence, is essentially insensitive to electrical devices in its vicinity.
  • the sensor element is part of an electric circuit which generates a certain output signal, wherein a parameter of said output signal varies depending upon the capacitance of the sensor element.
  • Said electric circuit may, for example, be constituted by an oscillator, wherein the capacitance of the sensor element in this case is utilized as a frequency determining component in the oscillator circuit.
  • said detection circuit is in this case adapted for detecting the period time of the output signal from the oscillator and for comparing said period time with a certain limit value which indicates the presence of the object in question.
  • Fig. 1 is a cross-sectional view which shows schematically the principle of a capacitive sensor element for presence detection according to the present invention
  • Fig. 2 is a cross-sectional view which shows a capacitive sensor element according to an alternative embodiment
  • Fig. 3 is a cross-sectional view which shows a sensor element according to a further embodiment
  • Fig. 4 is a cross-sectional view which shows a sensor element according to a further embodiment
  • Fig. 5 is a side view of a vehicle seat which shows how the invention may be utilized
  • Fig. 6 shows schematically a measuring unit which is intended to be utilized during the presence detection according to the invention.
  • the invention is primarily, but not exclusively, intended to be used for detection of the presence or the absence of an individual in a predefined area of a vehicle seat.
  • the detection may, for example, be utilized in connection with systems which are intended to deactivate an airbag if a passenger is not present in the detection area.
  • the airbag can be controlled so that it does not deploy unnecessarily or cause damage due to the fact that, for example, a child seat is mounted in the vehicle seat or that the passenger is sitting inappropriately in the vehicle seat.
  • Fig. 1 shows the basic principle of the present invention, which is constituted by a pressure-sensitive capacitive sensor element 1 that is constructed from a number of layers and is intended to be situated in the seat part of a vehicle seat.
  • the device 1 comprises a first layer 2 which is constituted by a metal layer.
  • a second layer 3 is provided, which is constituted by a compressible dielectric, preferably in the form of plastic foam.
  • a third layer 4 is provided, preferably in the form of a metal foil.
  • a fourth layer 5 is provided, which is constituted by an additional layer of compressible dielectric, preferably in the form of foam.
  • a fifth layer 6 of metal foil is arranged at the top of the sensor 1.
  • the metal layers 2, 4, 6 are constructed in such a way that they are slightly flexible.
  • Both of the outer layers 2, 6 are earthed. In this manner, said layers form two capacitances which are connected in parallel through the layers 2 and 4 and the layers 6 and 4, respectively. Also, the intermediate layer 4 constitutes a "hot" connection for this capacitance, i.e. the intermediate layer conducts a certain alternating voltage potential.
  • the dimensions of the sensor element 1 and the layers forming part of said sensor element are chosen on the basis of the application and depending upon how large the predefined detection area in the vehicle seat in question is required to be and how large the pressure stress is required to be in order for a capacitance difference to be detected when the presence of an object has been confirmed.
  • the outer dimensions of the sensor element 1 are in the size of 200 * 80 * 5 mm.
  • the three conducting layers 2, 4, 6 have a thickness which preferably is in the order of 50 ⁇ m, whereas the two compressible layers 3, 5 have a thickness which preferably is in the order of 5 mm.
  • the different layers 2, 3, 4, 5, 6 are joined together in such a way that they form an integrated unit. The joining may for example be carried out by means of a suitable adhesive.
  • the sensor 1 according to the invention is based on the principle that its compressible dielectric layers 3, 5 will be pressed together if they are influenced by a force. This, in turn, results in the distance between the metal layers 2, 4, 6 being reduced, which increases the capacitance that is created by the metal layers 2, 4, 6. This capacitance change may be detected by means of detection equipment, according to that which will be described in detail below.
  • the sensor layers 2, 4, 6 are arranged in such a way that the two conducting layers 2 and 6 are essentially surrounding the capacitor that is formed by the sensor element 1. In this manner, a screened construction is provided which emits a very low radiated electromagnetic radiation which can affect other components in the vicinity of the sensor element 1. Also, a very low sensitivy for any inwardly radiating interference is obtained.
  • the sensor element 6 may be connected to the sensor layer 2 from an alternating voltage point of view, which results in both of the layers being earthed from an alternating voltage point of view and they form a "screening cage" around the layer 4. In this manner, the layer 4 will be insulated from the surrounding environment .
  • Fig. 2 shows a sensor element 1 according to an alternative embodiment, which, apart from the above-mentioned conducting layers 2, 4, 6, also includes two essentially identical spring element 7 and 8, respectively.
  • the first spring element 7 connects the two conducting layers 2 and 4
  • the second spring element 8 connects the two conducting layers 4 and 6.
  • the spring elements 7, 8 are arranged in such a way that they do not cause any shortcircuiting between each conducting layer 2, 4, 6.
  • the capacitance of the entire sensor element is determined as a result of the distance between the layers, which varies depending upon whether the sensor 1 is influenced by force.
  • Fig. 3 shows a sensor element 1 according to a further embodiment, which comprises an intermediate conducting layer 4 and two outer, conducting layers 2' and 6', respectively, which are connected to earth.
  • the outer layers 2' and 6' are designed having a certain degree of elasticity and are arranged in such a way that they can adopt a rest position at a certain distance from the intermediate layer 4 and an additional position where they have been displaced inwardly towards the intermediate layer 4. This inner position, which thus occurs when the sensor element 1 is influenced by a force, is shown with dotted lines in Fig. 3.
  • the sensor element 1 is constructed having an integrated spring action. Due to the fact that the distance between the outer layers 2 ' , 6 ' and the intermediate layer 4 will change during the influence of a force, this will result in change of the total capacitance of the sensor element 1. This capacitance change can be detected by means of suitable detection equipment, which will also be described in detail below.
  • outer layers 2', 6' are attached to the intermediate layer 4 via a number of distance element 9 which are made of electrically insulating materials.
  • neither of the respective sensor elements 1 comprises any foam which may accumulate damp, which, in turn, could affect the measured capacitance and thus result in errors in measurement. Also, a metallic spring element does not become slack in the same way as foam.
  • Fig. 4 shows a sensor element 1 according to a further embodiment, according to which embodiment the upper conducting layer 6 is electrically indeterminate, i.e. said layer is not provided with any additional connection.
  • the layer 2 is earthed from an alternating voltage point of view.
  • a compressible layer 10 is provided between the layers 2 and 6, a compressible layer 10 is provided.
  • an intermediate layer is provided, which, in turn, is divided into two layer parts 4a and 4b, which are connected in such a way that the sum of the alternating voltages on both of the layer parts, i.e. u(4a) + u(4b), is equal to, or close to, zero volts.
  • Fig. 5 shows a vehicle seat 11 having two suitable locations of a sensor 1, more precisely in the seat cushion 12 of the seat 11 or in the back rest 13 of the seat.
  • the seat 11 may include two or more separate sensors 1, wherein different locations in the seat 11 may be utilized.
  • the respective sensor 1 is connected to a measuring unit 14, which is shown only schematically in Fig. 5 but which will be described in detail hereinafter, with reference to Fig. 6.
  • the variable capacitor 15 (which is formed by means of the sensor element 1 in accordance with, for example. Fig. 1), which is formed by means of the sensor 1 , is connected to the measuring unit 14. More precisely, the capacitor 15 is connected as a frequency determining component in an oscillator circuit 16 which is part of the measuring unit 14. It is per se previously known how oscillators can be constructed having capacitors as frequency determining elements, and the oscillator circuit 16 according to the invention is, adapted for generating an output signal in the
  • I form of an alternating voltage U, having a frequency which varies depending upon the capacitance of the capacitor 15, i.e. it varies depending upon the extent of compression of the sensor element 1 (cf., for example. Fig. 1).
  • the output signal U, of the oscillator circuit 16 is connected to a detection unit 17 which is adapted for detecting the period time of said output signal U,, the frequency of which thus depends upon the capacitance of the capacitor 15.
  • the detection unit 17 may be constituted by a non-computerbased circuit solution, but is preferably constituted by a computerbased unit.
  • a detection of the "presence” or the "non-presence" of an object in a vehicle seat is carried out by means of the measured sensor period time t s of the output signal U] being compared to a certain limit value t 0 , which corresponds to an object (i.e. a passenger) being seated in the seat 11 in question (cf. Fig. 5).
  • the limit value t 0 for the period time is chosen so that it is possible to positively declare whether there is presence or not, i.e. the limit value is set so that the value corresponds to, for example, the capacitance change of the sensor 1 which occurs for a load having a body weight of, for example, 30 kg.
  • the invention is particularly intended for use in motor vehicles, more precisely for deactivation of an inflatable airbag in the event of no confirmation of any presence in the respective seat.
  • the detection unit 17 is connected to a trigger mechanism 18 for an inflatable airbag (not shown).
  • the trigger mechanism 18 is adapted to allow activation of said airbag if presence in the seat has been confirmed.
  • the airbag actually to be inflated other conditions regarding, for example, the retardation of the vehicle also must be satisfied in a known manner.
  • the invention preferably comprises a reference oscillator 19, which is connected to a second input on the detection device 17.
  • the reference oscillator 19 is constructed essentially on the same silicon chip as the oscillator circuit 16, which results in both of the oscillators 16, 19 obtaining essentially the same temperature dependence.
  • the reference oscillator 19 generates an output signal U 2 in the form of an alternating voltage having a predetermined period time t R .
  • the detection circuit 17 being adapted for calculating the relative period time, i.e.
  • the detection circuit is preferably adapted for comparing the calculated relative period time t REL with a predetermined limit value t 0REL which indicates if there is any presence in the seat in question and which is utilized to trigger an airbag in accordance with what has been described ab Iove.
  • the capacitive sensor element 1 may be located in different places and in a variety of quantities in the vehicle seat 11 (see Fig. 5). Also, the invention may be utilized in other contexts than in vehicles, where there is a need for detection of the presence or the absence in a predetermined detection area.
  • the above-described capacitor 15 is a component forming part of an electric circuit across which a voltage is applied and where the capacitance of the capacitor affects the value of an output signal from said electric circuit.
  • Said circuit does not necessarily have to be constituted by an oscillator in accordance with what has been described above, but may also be constituted by other types of circuits where a capacitance is included as a component which determines the output signal.
  • the sensor element 1 can be arranged in a watertight casing, which makes the sensor element 1 insensitive to any possible damp in a vehicle seat.

Abstract

The present invention relates to an arrangement for presence detection of an object in a predetermined area (11), said arrangement comprising at least one capacitive sensor element (1) comprising a number of electrically conducting layers (2, 4, 6) which have an essentially two-dimensional extension and which are arranged at a distance from each other, wherein the relative distance between at least two of the conducting layers will reduce during the influence of force, by means of which the capacitance of said sensor element (1) will change, and a detection unit (14) for detection of the difference between the capacitance that, on the one hand, arises when the object is present and, on the other hand, arises when the object is absent. The invention is characterized in that the electrically conducting layers (2, 4, 6) are arranged in such a way that at least two (2, 6) of the layers are essentially surrounding at least one further layer (4). Preferably, the invention comprises at least one resilient device (7, 8), which is constructed in such a way that at least two of the conducting layers (2, 4, 6) will reduce their relative distance when the sensor element (1) is exposed to the influence of force. By means of the invention, an improved arrangement for presence detection is provided, in particular for detection of the presence of an individual in a vehicle seat.

Description

TITLE :
Device for presence detection.
TECHNICAL FIELD: The present invention relates to a device for presence detection, according to the preamble of appended claim 1. The invention is particularly intended for use in connection with presence detection of a passenger in a vehicle seat.
BACKGROUND OF THE INVENTION:
In different areas, there is a need for detection of the presence or the absence of an object. A number of different technical solutions are available, and the choice of the principal solution depends in many cases upon the application in question. Examples of various solutions are photocells, mechanically adjustable switches and capacitive sensor systems. The last-mentioned technique is per se previously known and is well suited for, among other things, presence detection of individuals on a vehicle seat, with the purpose of activating or deactivating various functions, for example airbags for protection of the passengers in the event of a collision. It is a fact that an airbag for a passenger can cause severe damage if the airbag is activated when, for example, a rear-facing child seat is mounted in front of the airbag or when a passenger who, for example, is not using the seat belt, is leaning forwards at the moment of activation.
Previously known capacitive systems have not, however, satisfied the extensive demands for reliability. One difficulty has involved the separation, in a highly secure way, of the capacitance that arises with the present measuring objects from other capacitances in a vehicle. This is due to the fact that the capacitance between the capacitive sensor element and the other objects in a vehicle (with the exception of individuals) can be large and/or vary considerably. For example, the frame work in practically all vehicle seats is metallic and has a varying distance to the capacitive sensor element in the seat. Since the capacitance from the sensor element to the frame work is approximately as large as the capacitance to the individual, this results in a large diffusion as regards the detection distance.
Furthermore, there is a risk that, during travelling, the extremities of a passenger who sits in the back seat will be pressed into the back of the seat from behind or in the bottom of the seat from underneath. These extremities are electrically conducting and provide, in the same manner as has been described above as regards the frame work, a large diffusion as regards the detection distance to the individual in the front.
Thus, there - is a need for arrangements and methods for presence detection, by means of which the variations of the capacitance in a sensor can be detected in a reliable manner. A previously known type of capacitive presence sensor is described in the US Patent 5 074 583. According to a certain embodiement, this sensor is constructed of three layers, where an upper and a lower layer are constituted by conductors which surround an intermediate layer that is constituted by a compressible synthetical material. The sensor is intended to be situated in a vehicle seat. When the sensor is influenced by a force due to a person sitting in the seat in question, the intermediate layer will be compressed, which results in the distance between the upper layer and the lower layer changing. This, in turn, results in the capacitance of the sensor changing, which may be detected by means of associated detection equipment. In this manner, a condition in which a passenger is seated in a seat can be detected.
Although the known sensor in principle provides an acceptable presence detection, the sensor, however, has certain disadvantages. For one thing, there is a risk that the detected capacitance will change depending upon the interfering capacitances which are connected to the upper and the lower layer in the sensor, which, in turn, gives rise to an inferior reliability due to the fact that it will be more difficult to separate a condition regarding "presence" from a condition regarding "non-presence". Also, the capacitance of the sensor constitutes a source of interference which emits electromagnetic radiation which, for example, may disturbe other electronic equipment in the vehicle. A further disadvantage is the risk of possible damp in the compressible layer causing errors in measurement.
SUMMARY OF THE INVENTION:
The object of the present invention is thus to provide an improved capacitive arrangement for presence detection, by means of which the above-mentioned disadvantages can be eliminated and an increased reliability can be provided, particularly in connection with presence detection in vehicle seats. Said object is accomplished by means of an arrangement as initially mentioned, the characterizing features of which will be apparent from appended claim 1.
The invention comprises a capacitive sensor element which contains a number of layers having an essentially two- dimensional extension, wherein the capacitance of the sensor element changes during the influence of force. The invention also comprises a detection unit for detection of the difference between the capacitance that, on the one hand, arises when the object is present and, on the other hand, arises when the object is absent. The invention is based on the idea that the sensor element comprises at least two electrically conducting layers which are arranged in such a way that they surround a third electrically conducting layer.
Preferably, intermediate dielectric layers are arranged between the electrically conducting layers. Alternatively, one or both of the dielectric layers can be replaced by a spring arrangement which keeps the electrically conducting layers apart. When pressure is applied, the spring arrangement compresses, which results in that the conducting layers are brought closer to each other.
By means of the invention, a "screened" arrangement is provided for reliable presence detection which offers low electromagnetic interference. The invention is also used purely as a pressure sensor, which, as a consequence, is essentially insensitive to electrical devices in its vicinity. I
According to a preferred embodiment, the sensor element is part of an electric circuit which generates a certain output signal, wherein a parameter of said output signal varies depending upon the capacitance of the sensor element. Said electric circuit may, for example, be constituted by an oscillator, wherein the capacitance of the sensor element in this case is utilized as a frequency determining component in the oscillator circuit. Also, said detection circuit is in this case adapted for detecting the period time of the output signal from the oscillator and for comparing said period time with a certain limit value which indicates the presence of the object in question.
Other advantageous embodiments of the invention will be apparent from the appended dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS:
The invention will be further described in the following with reference to a preferred embodiment and with reference to the appended drawings, in which
Fig. 1 is a cross-sectional view which shows schematically the principle of a capacitive sensor element for presence detection according to the present invention;
Fig. 2 is a cross-sectional view which shows a capacitive sensor element according to an alternative embodiment;
Fig. 3 is a cross-sectional view which shows a sensor element according to a further embodiment;
Fig. 4 is a cross-sectional view which shows a sensor element according to a further embodiment;
Fig. 5 is a side view of a vehicle seat which shows how the invention may be utilized, and
Fig. 6 shows schematically a measuring unit which is intended to be utilized during the presence detection according to the invention.
PREFERRED EMBODIMENTS:
The invention is primarily, but not exclusively, intended to be used for detection of the presence or the absence of an individual in a predefined area of a vehicle seat. The detection may, for example, be utilized in connection with systems which are intended to deactivate an airbag if a passenger is not present in the detection area. In this manner, the airbag can be controlled so that it does not deploy unnecessarily or cause damage due to the fact that, for example, a child seat is mounted in the vehicle seat or that the passenger is sitting inappropriately in the vehicle seat.
Fig. 1 shows the basic principle of the present invention, which is constituted by a pressure-sensitive capacitive sensor element 1 that is constructed from a number of layers and is intended to be situated in the seat part of a vehicle seat. According to the embodiment, the device 1 comprises a first layer 2 which is constituted by a metal layer. Above said first layer 2, a second layer 3 is provided, which is constituted by a compressible dielectric, preferably in the form of plastic foam. Above this layer, a third layer 4 is provided, preferably in the form of a metal foil. Above said third layer 4, a fourth layer 5 is provided, which is constituted by an additional layer of compressible dielectric, preferably in the form of foam. Finally, a fifth layer 6 of metal foil is arranged at the top of the sensor 1. In order to allow the dielectric layers 3, 5 to be compressed, the metal layers 2, 4, 6 are constructed in such a way that they are slightly flexible.
Both of the outer layers 2, 6 are earthed. In this manner, said layers form two capacitances which are connected in parallel through the layers 2 and 4 and the layers 6 and 4, respectively. Also, the intermediate layer 4 constitutes a "hot" connection for this capacitance, i.e. the intermediate layer conducts a certain alternating voltage potential.
The dimensions of the sensor element 1 and the layers forming part of said sensor element are chosen on the basis of the application and depending upon how large the predefined detection area in the vehicle seat in question is required to be and how large the pressure stress is required to be in order for a capacitance difference to be detected when the presence of an object has been confirmed. According to the embodiment, the outer dimensions of the sensor element 1 are in the size of 200 * 80 * 5 mm. The three conducting layers 2, 4, 6 have a thickness which preferably is in the order of 50 μm, whereas the two compressible layers 3, 5 have a thickness which preferably is in the order of 5 mm. Furthermore, the different layers 2, 3, 4, 5, 6 are joined together in such a way that they form an integrated unit. The joining may for example be carried out by means of a suitable adhesive.
The sensor 1 according to the invention is based on the principle that its compressible dielectric layers 3, 5 will be pressed together if they are influenced by a force. This, in turn, results in the distance between the metal layers 2, 4, 6 being reduced, which increases the capacitance that is created by the metal layers 2, 4, 6. This capacitance change may be detected by means of detection equipment, according to that which will be described in detail below.
The sensor layers 2, 4, 6 are arranged in such a way that the two conducting layers 2 and 6 are essentially surrounding the capacitor that is formed by the sensor element 1. In this manner, a screened construction is provided which emits a very low radiated electromagnetic radiation which can affect other components in the vicinity of the sensor element 1. Also, a very low sensitivy for any inwardly radiating interference is obtained.
According to an alternative embodiment, the sensor element 6 may be connected to the sensor layer 2 from an alternating voltage point of view, which results in both of the layers being earthed from an alternating voltage point of view and they form a "screening cage" around the layer 4. In this manner, the layer 4 will be insulated from the surrounding environment . Fig. 2 shows a sensor element 1 according to an alternative embodiment, which, apart from the above-mentioned conducting layers 2, 4, 6, also includes two essentially identical spring element 7 and 8, respectively. The first spring element 7 connects the two conducting layers 2 and 4, whereas the second spring element 8 connects the two conducting layers 4 and 6. The spring elements 7, 8 are arranged in such a way that they do not cause any shortcircuiting between each conducting layer 2, 4, 6. This is either achieved by means of the spring elements 7, 8 being constructed of an insulating material or by means of the spring elements 7 , 8 being attached to the respective conducting layer 2, 4, 6 via insulated (not shown) spring mountings. Here, the capacitance of the entire sensor element is determined as a result of the distance between the layers, which varies depending upon whether the sensor 1 is influenced by force.
Fig. 3 shows a sensor element 1 according to a further embodiment, which comprises an intermediate conducting layer 4 and two outer, conducting layers 2' and 6', respectively, which are connected to earth. The outer layers 2' and 6' are designed having a certain degree of elasticity and are arranged in such a way that they can adopt a rest position at a certain distance from the intermediate layer 4 and an additional position where they have been displaced inwardly towards the intermediate layer 4. This inner position, which thus occurs when the sensor element 1 is influenced by a force, is shown with dotted lines in Fig. 3.
Thus, the sensor element 1 according to Fig. 3 is constructed having an integrated spring action. Due to the fact that the distance between the outer layers 2 ' , 6 ' and the intermediate layer 4 will change during the influence of a force, this will result in change of the total capacitance of the sensor element 1. This capacitance change can be detected by means of suitable detection equipment, which will also be described in detail below.
Furthermore, the outer layers 2', 6' are attached to the intermediate layer 4 via a number of distance element 9 which are made of electrically insulating materials.
The advantage of the embodiments which are shown in Figs. 2 and 3 is that neither of the respective sensor elements 1 comprises any foam which may accumulate damp, which, in turn, could affect the measured capacitance and thus result in errors in measurement. Also, a metallic spring element does not become slack in the same way as foam.
Fig. 4 shows a sensor element 1 according to a further embodiment, according to which embodiment the upper conducting layer 6 is electrically indeterminate, i.e. said layer is not provided with any additional connection. The layer 2 is earthed from an alternating voltage point of view. Between the layers 2 and 6, a compressible layer 10 is provided. Furthermore, an intermediate layer is provided, which, in turn, is divided into two layer parts 4a and 4b, which are connected in such a way that the sum of the alternating voltages on both of the layer parts, i.e. u(4a) + u(4b), is equal to, or close to, zero volts. In this manner, the sensor layer 6 will obtain an alternating voltage which is close to zero volts, which is due to the fact that the displacement currents from the layer parts 4a and 4b on the whole will cancel each other out. Thereby, in spite of the fact that the sensor layer 6 will not be connected anywhere, said layer 6 will function as a screen and as insulation from the surrounding environment . Fig. 5 shows a vehicle seat 11 having two suitable locations of a sensor 1, more precisely in the seat cushion 12 of the seat 11 or in the back rest 13 of the seat. The seat 11 may include two or more separate sensors 1, wherein different locations in the seat 11 may be utilized.
Furthermore, the respective sensor 1 is connected to a measuring unit 14, which is shown only schematically in Fig. 5 but which will be described in detail hereinafter, with reference to Fig. 6. As is apparent from this drawing, the variable capacitor 15 (which is formed by means of the sensor element 1 in accordance with, for example. Fig. 1), which is formed by means of the sensor 1 , is connected to the measuring unit 14. More precisely, the capacitor 15 is connected as a frequency determining component in an oscillator circuit 16 which is part of the measuring unit 14. It is per se previously known how oscillators can be constructed having capacitors as frequency determining elements, and the oscillator circuit 16 according to the invention is, adapted for generating an output signal in the
I form of an alternating voltage U, having a frequency which varies depending upon the capacitance of the capacitor 15, i.e. it varies depending upon the extent of compression of the sensor element 1 (cf., for example. Fig. 1). This means, in principle, that the period time of the output signal Uj is also proportional to the weight of a passenger who is seated in a seat in which the sensor 1 is arranged.
The output signal U, of the oscillator circuit 16 is connected to a detection unit 17 which is adapted for detecting the period time of said output signal U,, the frequency of which thus depends upon the capacitance of the capacitor 15. The detection unit 17 may be constituted by a non-computerbased circuit solution, but is preferably constituted by a computerbased unit. A detection of the "presence" or the "non-presence" of an object in a vehicle seat is carried out by means of the measured sensor period time tsof the output signal U] being compared to a certain limit value t0, which corresponds to an object (i.e. a passenger) being seated in the seat 11 in question (cf. Fig. 5). If the measured period time ts is below the limit value t0, this corresponds to there being no presence in the seat. Thus, when the period time ts has increased until it has exceeded the limit value t0, "presence" is confirmed. The limit value t0 for the period time is chosen so that it is possible to positively declare whether there is presence or not, i.e. the limit value is set so that the value corresponds to, for example, the capacitance change of the sensor 1 which occurs for a load having a body weight of, for example, 30 kg.
According to the embodiment, the invention is particularly intended for use in motor vehicles, more precisely for deactivation of an inflatable airbag in the event of no confirmation of any presence in the respective seat. To this end, the detection unit 17 is connected to a trigger mechanism 18 for an inflatable airbag (not shown). The trigger mechanism 18 is adapted to allow activation of said airbag if presence in the seat has been confirmed. Of course, in order for the airbag actually to be inflated, other conditions regarding, for example, the retardation of the vehicle also must be satisfied in a known manner.
When the invention is utilized in connection with motor vehicles, large temperature differences occur between, for example, -25° C to +35° C, which may occur concurrently with the changing of the seasons. In order to reduce a possible temperature dependence for the capacitor 15, the invention preferably comprises a reference oscillator 19, which is connected to a second input on the detection device 17. The reference oscillator 19 is constructed essentially on the same silicon chip as the oscillator circuit 16, which results in both of the oscillators 16, 19 obtaining essentially the same temperature dependence.
Furthermore, the reference oscillator 19 generates an output signal U2 in the form of an alternating voltage having a predetermined period time tR. By means of the detection circuit 17 being adapted for calculating the relative period time, i.e.
tREL = ts / tR,
a measure of the capacitance of the capacitor 15 is obtained, by means of which an insignificant temperature dependence is obtained. When the reference oscillator 19 is utilized, the detection circuit is preferably adapted for comparing the calculated relative period time tREL with a predetermined limit value t0REL which indicates if there is any presence in the seat in question and which is utilized to trigger an airbag in accordance with what has been described ab Iove.
The invention is not limited to the embodiments which are described above and shown in the drawings, but may be varied within the scope of the appended claims. For example, the capacitive sensor element 1 may be located in different places and in a variety of quantities in the vehicle seat 11 (see Fig. 5). Also, the invention may be utilized in other contexts than in vehicles, where there is a need for detection of the presence or the absence in a predetermined detection area.
Furthermore, the above-described capacitor 15 is a component forming part of an electric circuit across which a voltage is applied and where the capacitance of the capacitor affects the value of an output signal from said electric circuit. Said circuit does not necessarily have to be constituted by an oscillator in accordance with what has been described above, but may also be constituted by other types of circuits where a capacitance is included as a component which determines the output signal.
According to an alternative embodiment of the invention, the sensor element 1 can be arranged in a watertight casing, which makes the sensor element 1 insensitive to any possible damp in a vehicle seat.

Claims

CLAIMS :
1. Device for presence detection of an object in a predetermined area (11), said device comprising at least one capacitive sensor element (1) comprising a number of electrically conducting layers (2, 4, 6) which have an essentially two-dimensional extension and which are arranged at a distance from each other, wherein the relative distance between at least two of the conducting layers will reduce during the influence of force, by means of which the capacitance of said sensor element (1) will change, and a detection unit (14) for detection of the difference between the capacitance that on the one hand, arises when the object is present and, on the other hand, arises when the object is absent, c h a r a c t e r i z e d i n that the electrically conducting layers (2, 4, 6) are arranged in such a way that at least two (2, 6) of the layers are essentially surrounding at least one further layer (4).
2. Device according to claim 1, c h a r a c t e r i z e d i n that said capacitive sensor element (1) is constructed from, in turn, a first electrically conducting layer (2) which is earthed from an alternating voltage point of view, a first compressible dielectric layer (3), a second electrically conducting layer (4), a second compressible dielectric layer (5) and a third electrically conducting layer (6), wherein said first and third conducting layers (2, 6) constitute outer layers in the sensor element (1).
3. Device according to claim 1, c h a r a c t e r i z e d i n that said capacitive sensor element (1) is constructed from, in turn, a first electrically conducting layer (2) which is earthed from an alternating voltage point of view, a second electrically conducting layer (4) and a third electrically conducting layer ( 6 ) , wherein the layers are kept apart by means of at least one electrically insulating spring element (7, 8) and wherein said first and third conducting layers (2, 6) constitute outer layers in the sensor element ( 1 ) .
4. Device according to claim 1, c h a r a c t e r i z e d i n that said capacitive sensor element (1) comprises a first electrically conducting layer (2') which is earthed from an alternating voltage point of view, a first electrically insulating distance device (9), a second electrically conducting layer (4), a second electrically insulating distance device (9) and a third electrically conducting layer (6'), wherein said first and third conducting layers (2, 6) are made of a resilient material.
5. Device according to any of the preceding claims, c h a r a c t e r i z e d i n that the sensor element ( 1 ) is part of an electric circuit (16) which generates an output signal (U, ), wherein a parameter of said output signal (U.) varies depending upon the capacitance of the sensor element (1) and wherein said detection circuit (17) is adapted for detecting said parameter.
6. Device according to claim 5, c h a r a c t e r i z e d i n that said electric circuit (16) is constituted by an oscillator circuit (16), in which oscillator circuit the sensor element (1) is included as a frequency determining component, and that the detection unit (17) is adapted for determining the period time (ts) of the output signal (U,) and for comparing said period time with a predetermined limit value (t0) which indicates presence.
7. Device according to claim 6, c h a r a c t e r i z e d i n that said device comprises a reference oscillator (19) which is connected to the detection unit (17) and which generates a second output signal (U2) having a predetermined second period time (tR), and that the detection unit (17) is adapted to compensate for influence from the surrounding environment by means of calculations which include both said period times (ts, tR) .
8. Device according to any of the preceding claims, c h a r a c t e r i z e d i n that said detection unit (17) is connected to a trigger mechanism (18) which is intended for an inflatable airbag and which is adapted to allow activation of said airbag if presence of said object has been confirmed.
9. Vehicle seat (11) comprising a device according to any of claims 1-8, wherein said sensor element (1) is adapted for detecting the presence of said object in a predetermined area in the vehicle seat (11).
10. System for presence detection of an object in a predetermined area (11), said system comprising at least one capacitive sensor element ( 1 ) , the capacitance of which will change as a result of the influence of force, and a detection unit (14) for detection of the difference between the capacitance that, on the one hand, arises when the object is present and, on the other hand, arises when the object is absent, c h a r a c t e r i z e d i n that said system comprises an oscillator circuit (16) for generation of an alternating voltage (U,), wherein said sensor element ( 1 ) is arranged as a frequency determining component in said oscillator circuit (16), and that the detection unit (14) is adapted for detecting the period time (ts) of said alternating voltage (U,) and for comparing said period time with a predetermined limit value (t0) which indicates presence.
11. System according to claim 10, c h a r a c t e r i z e d i n that said system comprises a reference oscillator (19) which generates a second output signal (U2) having a predetermined second period time (tR), and that the detection unit (14) is adapted to compensate for influence from the surrounding environment by means of calculations which include both said period times (ts, tR) .
12. System according to claim 10 or 11, c h a r a c t e r i z e d i n that said detection unit (14) is connected to a trigger mechanism (18) which is intended for an inflatable airbag and which is adapted for allowing activation of said airbag if said object is present.
PCT/SE1997/001821 1996-11-01 1997-10-31 Device for presence detection WO1998022836A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
SE9603988A SE518888C2 (en) 1996-11-01 1996-11-01 Presence sensing device and vehicle seat comprising the device
SE9603988-8 1996-11-01

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Cited By (17)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2001014171A1 (en) * 1999-08-24 2001-03-01 Harald Philipp Occupancy sensor
JP2001287619A (en) * 2000-03-03 2001-10-16 Bayerische Motoren Werke Ag Method of detecting capacitive object in vehicle
JP2001296370A (en) * 2000-03-03 2001-10-26 Bayerische Motoren Werke Ag Method for detecting capacitive object of vehicle
WO2003007476A2 (en) * 2001-07-09 2003-01-23 Nartron Corporation Compressible capacitance sensor for determining the presence of an object
EP1447653A1 (en) * 2001-10-22 2004-08-18 Microjenics, Inc. PRESSURE−SENSITIVE SENSOR AND MONITOR USING THE PRESSURE−SENSITIVE SENSOR
WO2007094993A1 (en) * 2006-02-10 2007-08-23 Milliken & Company Flexible capacitive sensor
US7301351B2 (en) 2006-02-10 2007-11-27 Milliken & Company Printed capacitive sensor
US7368921B2 (en) 2006-02-10 2008-05-06 Milliken & Company Printed capacitive sensor
WO2008141205A3 (en) * 2007-05-10 2009-04-16 Tk Holdings Inc Vehicle seat including sensor
EP2127957A1 (en) * 2008-05-29 2009-12-02 Delphi Technologies, Inc. Seatbelt warning system
US7641618B2 (en) * 2004-03-29 2010-01-05 Sanyo Electric Co., Ltd. Capacitance-type pressure sensor and heart beat / respiration measuring device using the same
NL2001822C2 (en) * 2008-07-17 2010-01-19 2M Engineering Ltd Force sensor and corresponding force monitoring mattress.
US7719007B2 (en) 2008-04-30 2010-05-18 Milliken & Company Flexible electroluminescent capacitive sensor
CN102481886A (en) * 2009-12-30 2012-05-30 高田-彼得里公开股份有限公司 Capacitive sensor assembly comprising a heating element
CN102484474A (en) * 2009-12-30 2012-05-30 高田-彼得里公开股份有限公司 Capacitive sensor assembly
CN102607747A (en) * 2012-03-04 2012-07-25 江南大学 Disk target beating sensor
JP2013156662A (en) * 2013-05-10 2013-08-15 Yamaha Corp Pressure sensor and data input apparatus

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2163814A5 (en) * 1971-12-02 1973-07-27 Luton Michel
US3875481A (en) * 1973-10-10 1975-04-01 Uniroyal Inc Capacitive weighing mat
US5074583A (en) * 1988-07-29 1991-12-24 Mazda Motor Corporation Air bag system for automobile

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2163814A5 (en) * 1971-12-02 1973-07-27 Luton Michel
US3875481A (en) * 1973-10-10 1975-04-01 Uniroyal Inc Capacitive weighing mat
US5074583A (en) * 1988-07-29 1991-12-24 Mazda Motor Corporation Air bag system for automobile

Cited By (34)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2001014171A1 (en) * 1999-08-24 2001-03-01 Harald Philipp Occupancy sensor
JP2001287619A (en) * 2000-03-03 2001-10-16 Bayerische Motoren Werke Ag Method of detecting capacitive object in vehicle
JP2001296370A (en) * 2000-03-03 2001-10-26 Bayerische Motoren Werke Ag Method for detecting capacitive object of vehicle
EP1129893A3 (en) * 2000-03-03 2003-01-15 Bayerische Motoren Werke Aktiengesellschaft Capacitive object recognition method in vehicles
EP1129894A3 (en) * 2000-03-03 2003-01-15 Bayerische Motoren Werke Aktiengesellschaft Capacitive object recognition method in vehicles
JP4658352B2 (en) * 2000-03-03 2011-03-23 バイエリッシェ モートーレン ウエルケ アクチエンゲゼルシャフト Method for capacitive object detection in vehicles
US6577142B2 (en) 2000-03-03 2003-06-10 Bayerische Motoren Werke Aktiengesellschaft Process for the capacitive object detection in the case of vehicles
JP4621369B2 (en) * 2000-03-03 2011-01-26 バイエリッシェ モートーレン ウエルケ アクチエンゲゼルシャフト Method for capacitive object detection in vehicles
WO2003007476A3 (en) * 2001-07-09 2005-05-19 Nartron Corp Compressible capacitance sensor for determining the presence of an object
WO2003007476A2 (en) * 2001-07-09 2003-01-23 Nartron Corporation Compressible capacitance sensor for determining the presence of an object
EP1933461A1 (en) * 2001-07-09 2008-06-18 Nartron Corporation Compressible capacitance sensor for determining the presence of an object
EP1447653A1 (en) * 2001-10-22 2004-08-18 Microjenics, Inc. PRESSURE−SENSITIVE SENSOR AND MONITOR USING THE PRESSURE−SENSITIVE SENSOR
EP1447653A4 (en) * 2001-10-22 2006-05-03 Microjenics Inc Pressure-sensitive sensor and monitor using the pressure-sensitive sensor
US7641618B2 (en) * 2004-03-29 2010-01-05 Sanyo Electric Co., Ltd. Capacitance-type pressure sensor and heart beat / respiration measuring device using the same
GB2448453A (en) * 2006-02-10 2008-10-15 Milliken & Co Flexible capacitive sensor
US7395717B2 (en) 2006-02-10 2008-07-08 Milliken & Company Flexible capacitive sensor
US7578195B2 (en) 2006-02-10 2009-08-25 Milliken & Company Capacitive sensor
US7368921B2 (en) 2006-02-10 2008-05-06 Milliken & Company Printed capacitive sensor
US7301351B2 (en) 2006-02-10 2007-11-27 Milliken & Company Printed capacitive sensor
WO2007094993A1 (en) * 2006-02-10 2007-08-23 Milliken & Company Flexible capacitive sensor
WO2008141205A3 (en) * 2007-05-10 2009-04-16 Tk Holdings Inc Vehicle seat including sensor
US7719007B2 (en) 2008-04-30 2010-05-18 Milliken & Company Flexible electroluminescent capacitive sensor
EP2127957A1 (en) * 2008-05-29 2009-12-02 Delphi Technologies, Inc. Seatbelt warning system
US8198992B2 (en) 2008-05-29 2012-06-12 Delphi Technologies, Inc. Seat belt warning system
WO2010007520A1 (en) * 2008-07-17 2010-01-21 2M Engineering Limited Force sensor and corresponding force monitoring mattress
US20110185824A1 (en) * 2008-07-17 2011-08-04 2M Engineering Limited Force sensor and corresponding force monitoring mattress
NL2001822C2 (en) * 2008-07-17 2010-01-19 2M Engineering Ltd Force sensor and corresponding force monitoring mattress.
US8671782B2 (en) 2008-07-17 2014-03-18 2M Engineering LMT. Force sensor and corresponding force monitoring mattress
CN102481886A (en) * 2009-12-30 2012-05-30 高田-彼得里公开股份有限公司 Capacitive sensor assembly comprising a heating element
CN102484474A (en) * 2009-12-30 2012-05-30 高田-彼得里公开股份有限公司 Capacitive sensor assembly
US8970231B2 (en) 2009-12-30 2015-03-03 Takata AG Capacitive sensor assembly
US8970232B2 (en) 2009-12-30 2015-03-03 Takata AG Capacitive sensor assembly
CN102607747A (en) * 2012-03-04 2012-07-25 江南大学 Disk target beating sensor
JP2013156662A (en) * 2013-05-10 2013-08-15 Yamaha Corp Pressure sensor and data input apparatus

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SE518888C2 (en) 2002-12-03
SE9603988D0 (en) 1996-11-01

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