SE518888C2 - Presence sensing device and vehicle seat comprising the device - Google Patents

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

518 888 metallic and has a varying distance to the capacitive sensor element in the seat. Since the capacitance from the sensor element to the framework is approximately equal, the consequence will be as large as the capacitance of the person spread in the sensing distance.

Furthermore, the extremities of a seated passenger while driving can be pushed into the back of the seat from behind or into the seat from below. These extremities are electrically conductive and, in the same way as described above, give the framework a large spread in the sensing distance to the person in front.

Thus, there is a need for devices and methods for presence detection by means of which capacitance changes in a sensor can be detected in a reliable manner. A prior art type is shown in the document US 5074583. this sensor consists of three layers, where an upper and a lower layer are constituted by electrical conductors which surround an intermediate layer of a compressible synthetic material. The sensor is intended to be placed in a vehicle seat. According to a certain embodiment, when the sensor is affected by a force as a result of a person sitting in the seat in question, the intermediate layer will be compressed, which means that the distance between the upper layer and the lower layer changes. This in turn causes the capacitance of the sensor to change, which can be detected by associated detection equipment. In this way, a condition in which a passenger is sitting: in a seat can be detected.

Although the known sensor in principle provides an acceptable sense of presence, it is, however, encumbered with certain disadvantages. the risk that the detected one changes depending on the interference capacitances that are connected to a state of "presence" from a state of more difficult to of "non-presence".

In addition, the capacitance of the sensor constitutes a source of interference that emits electromagnetic radiation such as may interfere with other electronic equipment in the vehicle. An additional disadvantage of moisture in it is the risk that any compressible layer will cause measurement errors.

DISCLOSURE OF THE INVENTION = The object of the present is to provide a presence detection in which the above-mentioned disadvantages can be increased reliability, in This so that device for invention is improved capacitive is eliminated and which provides a particular in presence detection in vehicle seats. object is achieved by means of a device of the type mentioned in the introduction, the characteristic features of which appear from the following claim 1.

The invention comprises a capacitive sensor element which comprises a plurality of layers with a substantially two-dimensional extent, wherein the capacitance of the sensor element changes also includes a difference in force. The invention detects a unit for detecting capacitance which is present in both objects present and objects which are absent. at least two electrically conductive layers arranged so as to enclose a third electrically conductive layer.

According to the invention, the sensor element comprises a spring device which keeps the electrically conductive layers apart. At applied pressure, the spring device springs so that the conductive layers are brought closer together.

The invention provides a "shielded" device for reliable presence sensing which exhibits low electromagnetic interference. In addition, the invention is used as a pure pressure sensor which is thereby substantially insensitive to electrical equipment in its vicinity.

According to a preferred embodiment, the sensor element is included in an electrical circuit which generates a certain output signal, wherein a parameter of said output signal varies depending on, for example, this capacitance of the sensor element. electrical circuit consists of an oscillator, whereby the capacitance of the sensor element is then used as a frequency-determining component in the oscillator circuit. In addition, said detection circuit is then arranged to detect the period time of the output signal from the oscillator and compare it with a certain limit value which indicates the presence of the object in question.

Further advantageous embodiments of the invention appear from the following dependent claims.

DESCRIPTION OF THE DRAWINGS: The invention will be explained in more detail below in connection with a preferred embodiment with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view schematically showing the principle of a capacitive sensor element for presence detection, which does not form part of the present invention, Figure 2 is a cross-sectional view showing a capacitive sensor element according to the present invention, Figure 3 is a cross-sectional view showing a sensor element according to a further embodiment, Figure 4 is a cross-sectional view as a sensor element not forming part of the invention, figure 5 is a side view of a vehicle seat showing how the invention can be used, and figure 6 schematically shows a measuring unit to be used in presence detection according to the invention.

PREFERRED EMBODIMENTS / IER: The invention is particularly, but not exclusively, intended for sensing the presence or absence of a person in a predefined area of a vehicle seat. The sensing can be used, for example, in systems which have the purpose of deactivating an air cushion if a occupant is not in the sensing area. In this way it can be ensured that the air cushion is not unnecessarily triggered or causes damage due to e.g. a child seat is fitted or the occupant is sitting inappropriately in the vehicle seat.

Figure 1 shows a pressure-sensitive capacitive sensor element 1 which is built up of a plurality of layers and which is intended to be placed in the seat part of a vehicle seat. This sensor element 1 does not form part of the invention but is described for the understanding of the invention. According to the embodiment, the device 1 comprises a first layer 2 which consists of a metal layer. Arranged on top of this first layer 2 is a second layer 3, which is constituted by a compressible dielectric, preferably in the form of foam plastic. On top of this layer layer 4, preferably in the form of a mesh foil. Arranged on top of the third layer 4 is a fourth layer 5, which is constituted by a further layer of compressible dielectric. Finally, a fifth layer is arranged, a third preferably in the form of foam. layer 6 of metal foil arranged at the top of the sensor 1. In order to allow compression of the layers 3, 5 of dielectric, the metal layers 2, 4, 6 are made so that they are somewhat flexible.

The two outer layers 2, 6 are grounded. In this way, the two capacitances connected in parallel through layers 2 and 4 form layers 6 and 4. "hot" capacitance, i.e. the intermediate layer 4 for a certain respective In addition, the intermediate layer 4 forms a connection for this alternating voltage potential.

The dimensions of the sensor element 1 and its constituent layers are selected according to application and depending on how large a predefined sensing area is necessary in the vehicle seat in question and how much pressure effect is required to detect a difference in capacitance in the presence of an object. According to the embodiment, the outer dimensions of the sensor element 1 are of the order of 200 X 80 X 5 mm. The three conductive layers 2, 4, 6 have a thickness which is preferably of the order of 50 μm, while the two compressible layers 3, 5 have a thickness which is preferably of the order of 5 mm. Furthermore, the different layers 2, 3, 4, 5, 6 are joined together so that they form an integrated unit. The joining can be carried out, for example, by means of a suitable adhesive.

The sensor 1 is based on the principle that its compressible layers 3, 5 of dielectric will be compressed if they are actuated by a force. This in turn means that the distance between the metal layers 2, 4, 6 decreases, which means that the capacitance formed by the metal layers 2, 4, 6 increases. This capacitance change can be detected by means of detection equipment, as will be described in more detail below. The sensor layers 2, 4, 6 are arranged so that the two conductive layers 2 and 6 substantially enclose the capacitor formed by the sensor element 1. This provides a shielded construction which gives a very low radiated electromagnetic radiation. which can affect other components in the vicinity of the sensor element 1. In addition, a very low sensitivity is given to radiating disturbances.

According to an alternative embodiment, the sensor layer 6 can be connected to the sensor layer 2 in terms of alternating voltage, so that the two layers form a "Faraday's cage" grounded and in this way the layer 4 is isolated from the surroundings in terms of alternating voltage.

Figure 2 shows a sensor element 1 according to the present invention, which in addition to the above-mentioned conductive layers 2, 4, 6 also comprises two substantially identical spring and 8, respectively. The first connects the two conductive layers 2 and 4, while the second spring element 8 connects the two conductive layers 4 and 6.

The spring element 7, 8 are arranged so that they do not cause a short circuit between the respective conductive layers 2, 4, 6. This element 7 the spring element 7 is achieved either by the spring elements 7, 8 being made of an insulating material or by the spring elements 7, 8 being fastened in the respective conductive layers 2, 4, 6 via insulated (not shown) spring brackets. The capacitance of the complete sensor element is determined here depending on the distance between the layers, affected by some force. which varies depending on whether the sensor 1 Figure 3 shows a sensor element 1 according to a further embodiment of which intermediate conductive layer 4 and two outer, conductive layers The outer invention comprises a 2 'and 6', respectively, which are connected to earth. the layers 2 ', 6' are arranged so that they can assume a rest position on a certain are formed with a certain elasticity and a distance from the intermediate layer 4 and a further position where they have been moved towards the intermediate layer. 4. This internal position, which thus exists when the sensor element 1 is actuated by a force, is shown in broken lines in Figure 3.

The sensor element 1 according to figure 3 is thus designed with an integrated spring action. Since the distance between the outer layers 2 ', 6' and the intermediate layer 4 changes during a force action, then also the total capacitance of the sensor element 1 will also change. This change in capacitance can be detected with appropriate detection equipment, which will also be described in more detail below.

Furthermore, the outer layers 2 ', 6' are attached to the intermediate layer 4 via a plurality of spacer elements 9 of electrically insulating material.

The advantage of the embodiments shown in Figures 2 and 3 is that the respective sensor element 1 does not comprise any foam which can accumulate moisture, which in turn can affect the measured one thus lead to measurement errors.

In addition, a metallic spring element does not settle so the capacitance and foam can do.

Figure 4 shows a sensor element 1 which does not form part of the invention, the upper conductive layer 6 flowing electrically, i.e. it has no additional connection. Layer 2 is grounded in terms of AC voltage.

Between layers 2 and 6 there is a compressible layer 10.

Furthermore, there is an intermediate layer which in turn is divided into two layer parts 4a and 4b, which are connected so that the sum of the alternating voltages on the two layer parts, i.e. u (4a) + u (4b), is equal to or close to zero volts. In this way, the sensor layer 6 will have an alternating voltage which is close to zero volts, since the displacement currents from the layer parts 4a and 4b substantially cancel each other out.

The sensor layer 6 will thus function as a screen and that no insulation is connected to the surroundings, despite somewhere.

Figure 5 shows a vehicle seat 11 with two suitable locations of a sensor 1, namely in the seat cushion 12 of the seat 11 or in its backrest 13. The seat 11 may comprise two or more separate sensors 1, whereby different locations in the seat 11 can be used.

Furthermore, the respective sensor 1 is connected to a measuring unit 14, which is only shown schematically in figure 5 but which will now be described in more detail with reference to figure 6. From this figure it appears that the variable capacitor 15 (formed by the sensor element 1 according to t .ex figure 1) formed by the sensor 1 is connected to the measuring unit 14.

More specifically, the capacitor 15 is connected as a frequency determining component in an oscillator circuit 16 which It is known per se how included in the measuring unit 14. oscillators can be constructed with capacitors as frequency determining elements, and the oscillator circuit 16 according to the invention is arranged to generate an output signal in form of an alternating voltage U1 with a frequency which varies depending on the capacitance of the capacitor 15, i.e. depending on the degree of compression of the sensor element 1 (cf. eg 1). This period time is also in proportion to the weight of a traveling figure means in principle that the output signal U1 in a seat where the sensor 1 is arranged. is connected to an output unit 17 for detecting the output signal U1 of the oscillator circuit 16, which is the period time of said output signal U1, the frequency of which thus depends on the capacitance of the capacitor 15. The detection unit 17 may be a non-computer-based circuit solution but a computer-based unit. One preferably consists of a detection of "presence" or "non-presence" of an object in a vehicle seat by comparing the measured sensor period ts of the output signal U1 with a certain limit value to which corresponds to against the object (ie a passenger) being placed in the current seat ll (cf. figure 5). If the measured period time ts is less than the limit value to, this corresponds to the presence in the seat. Thus, when the period time ts has increased until it has exceeded the limit value to, "presence" is ascertained. The limit value to for the period is chosen so that it can be determined with certainty whether presence is present or not, i.e. it is set to a value which corresponds, for example, to the capacitance change of the sensor 1 which occurs under load with a body weight of e.g. 30 kg.

According to the embodiment, the invention is particularly intended to be used: in motor vehicles, more particularly; for deactivation of an inflatable airbag in the event that presence in the respective seat has not been established. For this purpose, the detection unit 17 is connected to a release mechanism 18 for an inflatable airbag, the mechanism 18 is arranged to allow an activation said (not shown). Ejaculation airbag on \ presence has been detected in the seat. In a known manner, of course, other conditions regarding e.g. the deceleration of the vehicle is also met in order for the airbag to actually inflate.

When using the invention in connection with motor vehicles, there are large temperature variations, e.g. between -250 C to + 35 ° C, which can occur in step with the changing of the seasons. In order to reduce a possible temperature dependence of the capacitor 15, the invention preferably comprises a reference oscillator 19, which is connected to a reference oscillator 19 constructed in silicon wafer as the oscillator circuit 16, which means that second input of the detection device 17. substantially on the same 35 518 888 ll both mainly temperature dependent. oscillators 16, 19 receive in the same Furthermore, the reference oscillator 19 generates an output signal U2 in the form of an alternating voltage with a predetermined period time tg.

In that the detection circuit 17 is arranged to calculate the relative period time, i.e. tREL = ts / tR a measure of the capacitance of the capacitor 15 is obtained so that a slight temperature dependence is obtained. In the event that the reference oscillator 19 is used, it is preferably detected that the period time tREL with a predetermined limit value t0, REL as the circuit compares the calculated relative indicates presence in the current seat and which is used for controlling an airbag as described in OVaII.

The invention is not limited to the embodiments described above and shown in the drawings, but can be varied within the scope of the following claims. For example, the capacitive sensor element 1 can be placed in different places and in different numbers in the vehicle seat 11 (see figure 5). In addition, the invention can be used in other contexts than in vehicles, where there is a need to detect presence or absence with respect to a predetermined sensing area.

Furthermore, the above-described capacitor 15 is included as a component in an electrical circuit which is applied to a voltage and where the capacitance of the capacitor affects the value of an output signal from said electrical circuit. This circuit does not necessarily have to be an oscillator, as described above, but can also consist of other types of circuits where a capacitance is included as an output determining component. According to an alternative embodiment of the invention, the sensor element 1 can be arranged in a watertight housing, which makes it insensitive to any moisture in a vehicle seat.

Claims (6)

10 15 20 25 30 35 518 888 13 PATENT REQUIREMENTS: Device for sensing the presence of an object in a predetermined area (11) and comprising at least a plurality of substantially capacitive sensor elements (1) comprising an electrically conductive layer (2, 4, 6) with two-dimensional extent and arranged at a distance from each other , wherein the mutual distance between at least two of the conductive layers decreases under the influence of force, whereby the capacitance of said sensor element (1) changes, and a detection unit (14) capacitance present at partly present objects and for detecting the difference in partly absent objects, whereby they electrically conductive layers (2, 4, 6) are arranged so that at least two (2, 6) of the layers substantially enclose at least one further layer (4), characterized in that said capacitive sensor element (1) is built up in turn and arrangement of a first electrically conductive layer (2) which is grounded in alternating voltage, an electrically conductive layer (4), and a third electrically conductive layer The conductive layer layers are kept separate by at least one insulating spring element (7, 8) and wherein second (6), wherein electrically said first and third conductive layers (2, 6) constitute outer layers in the sensor element (1). Device according to claim 1, characterized in that the sensor element (1) is included in an electrical circuit (16) (U1), parameters of said output signal (U1) vary depending on which generates an output signal wherein a sensor element (1) capacitance and wherein said detection unit (14) comprises a detection circuit (17) arranged to detect said parameter. 10 15 20 25 30 518 888 14 Device according to claim 2, characterized in that said electrical circuit (16) is constituted by an oscillator circuit (16) for generating an alternating voltage (01): included as in which oscillator circuit (16) the sensor element (1) determining the frequency component , and that the detection circuit (17) is arranged for determining the period time (ts) of the output signal (U1) and comparing it with a predetermined limit value (to) which indicates presence. Device according to claim 3, characterized in that it comprises a reference oscillator (19) which is connected to the detection circuit (17) and which generates a second output signal (U2) with a predetermined second period time (tR), and that the detection circuit (17) is set up to compensate for the impact of the environment through calculations including both mentioned period times (ts, tR). Device according to any one of the preceding claims, k ä n n e t e c k n a d d ä 1? a v, that said detection unit (14) is connected to a release mechanism (18) airbag and arranged to allow, in the presence of said object of said object, activation of according to an inflatable said airbag. Vehicle seat (11) comprising a device according to any one of claims 1-5, wherein said sensor element (1) is arranged to detect the presence of said object in a predetermined area in the vehicle seat (11).