Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B60—VEHICLES IN GENERAL
  • B60R—VEHICLES, VEHICLE FITTINGS, OR VEHICLE PARTS, NOT OTHERWISE PROVIDED FOR
  • B60R21/00—Arrangements or fittings on vehicles for protecting or preventing injuries to occupants or pedestrians in case of accidents or other traffic risks
  • B60R21/01—Electrical circuits for triggering passive safety arrangements, e.g. airbags, safety belt tighteners, in case of vehicle accidents or impending vehicle accidents
  • B60R21/015—Electrical 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
  • B60R21/01512—Passenger detection systems
  • B60R21/0153—Passenger detection systems using field detection presence sensors
  • B60R21/01532—Passenger detection systems using field detection presence sensors using electric or capacitive field sensors
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L1/00—Measuring force or stress, in general
  • G01L1/14—Measuring 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
  • G01L1/142—Measuring 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
  • G01L1/144—Measuring 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
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
  • G01V3/08—Electric 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
  • G01V3/088—Electric 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.

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• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Remote Sensing (AREA)
• General Physics & Mathematics (AREA)
• Life Sciences & Earth Sciences (AREA)
• Power Engineering (AREA)
• Electromagnetism (AREA)
• Mechanical Engineering (AREA)
• Environmental & Geological Engineering (AREA)
• Geology (AREA)
• General Life Sciences & Earth Sciences (AREA)
• Geophysics (AREA)
• Geophysics And Detection Of Objects (AREA)
• Seats For Vehicles (AREA)
• Air Bags (AREA)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

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Cited By (17)

Citations (3)

• 1996
  • 1996-11-01 SE SE9603988A patent/SE518888C2/sv not_active IP Right Cessation
• 1997
  • 1997-10-31 WO PCT/SE1997/001821 patent/WO1998022836A1/en active Application Filing

Patent Citations (3)

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