Process for the capacitive object detection in the case of vehicles

[0002] This application claims the priority of German patent document 100 10 531.9, filed Mar. 3, 2000, the disclosure of which is expressly incorporated by reference herein.

[0003] The invention relates to a process for object detection using capacitive elements.

[0004] Capacitive object detection processes are disclosed, for example, in International Patent Document WO 99/59003 A and German Patent Document DE 19612626 A. In principle, two capacitors (usually plate shaped) are arranged adjacent to the object that is to be detected. For detecting whether or not a vehicle seat is occupied, the capacitors are situated in the upholstery, for example in the seating surface and in the backrest. In this case, the detection is used for triggering safety devices, such as air bags or the like, only when the seat is, in fact, occupied.

[0005] When a person is present, the capacitance of the capacitors changes because of a human being's relatively good conducting capacity. To differentiate between a person and a child seat, the capacitance values must be observed and be evaluated by means of an algorithm.

[0006] It is an object of the present invention to provide a process of the initially mentioned type which, with low equipment expenditures, supplies precise information concerning not only the presence of an object but also about which object is involved.

[0007] This and other objects and advantages are achieved by the detection process according to the invention, in which capacitors arranged in the backrest and in the seat surface are influenced by an object. The capacitors are acted upon by alternating voltage, and their capacitance is determined individually.

[0008] In a first process according to the invention, two evaluations are derived by means of a value table and are compared with one another. If the two evaluations correspond with respect to the type of the object, the safety device is controlled as required (that is, triggered; or for example in the case of a rebound child seat or in the case of an unoccupied seat, not triggered).

[0009] In the second process according to the invention, the two evaluations are mathematically related to one another, and based on the value resulting from the mathematical relationship, a decision table is used to determine whether, as required, an occupant protection device assigned to the seat is activated. The mathematical relationship can preferably be an averaging or an adding of the two evaluations. Only an overall evaluation is used here.

[0010] According to the invention, at least one of the capacitances may consist of two partial capacitances which are formed by two capacitors arranged in the respective seat parts. This is a further refinement of the analysis.

[0011] Finally, in addition, the output signal of another seat occupation detection sensor can be taken into account at least when the two evaluations supply contradictory information with respect to the object.

[0012] Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

[0013] FIG. 1 is a schematic diagram of a system for implementing the process according to the invention; and

[0014] FIG. 2 is a diagram for explaining the process according to the invention.

[0015] FIG. 1 is a schematic representation of a switching arrangement according to an embodiment of the invention.

[0016] In a schematically illustrated vehicle seat 1, capacitors 3 and 2, situated in a seat surface 1′ and a backrest 1″, respectively, are each acted upon by alternating voltages V₁and V₂. In practice, an alternating-voltage frequency between 10 and 150 kHz, preferably 50 kHz, was found to be expedient.

[0017] Each capacitor 2 and 3 consists of two partial capacitors A1 and A2 or A3 and A4. In addition, an electrically conductive plate 8 is assigned to shield each partial electrode, in order to achieve a directional effect toward the measured object.

[0018] The capacitance values (C_exti, i=1, 2, 3, 4) of the four capacitors A1, A2, A3 and A4 are determined. From these values, the values C_vacanti(i=1, 2, 3, 4) for the vacant seat are subtracted. (The latter were determined beforehand for the various types of seats.) From four differential values, the resulting C_objiare used and a classification takes place by means of them.

[0019] Every 500 ms, a new C_objiis determined. From these four values, also every 500 ms, a static algorithm is used to determine the actual rating, as to what is situated on the seat.

[0020] Generally, a high capacitance C_objiindicates that a person is situated in the seat; a low capacitance or no capacitance is typical of an unoccupied vehicle seat or of a vehicle seat occupied by an unoccupied child seat. In the case of a person of a normal sitting posture and normal clothing, high capacitances (approximately 40 pF) will occur at A2 and A3 and slightly lower capacitances (approximately 15 p to 25 p) will occur at A1 and A4.

[0021] In order to take into account various clothing variants and sitting postures, all 4 capacitors contribute to the rating, which is determined alternatively by one of two different methods.

[0022] In a first method, in each case, the capacitance values of the seat surface and backrest are added, and a rating is formed from the sum, separately for the seat surface and the backrest. If the two ratings correspond to one another in terms of to the type of the object (that is, for example, in both ratings, the object is found to be a person or an occupied rebound child seat on the front passenger seat) in the event that it is required, the safety device (for example, a frontal air bag) is activated or not activated. If the ratings reach contradictory results, the latter decision will be made as a function of the variation in time of the ratings.

[0023] For example, after the vehicle has been started or after an interruption of the operation during which the pertaining vehicle door was opened, if at first ratings exist which have the same tendency and which, during a defined time period (for example, 2 seconds) are definite and indicate the presence of a person, and if the two ratings subsequently contradict one another (typically, in the case of a person in the seat who then bends forward), the original information (person present) continues to be used even if the contradictory information lasts for a longer period, for example, 1 min.

[0024] In the second method, separate ratings are determined from the individual capacitance values, and an average value is formed from the four rating values. By means of a table, the average value is used to determine whether activation of the safety system is required.

[0025] Each rating takes place on the basis of a table whose values are graphically illustrated in FIG. 2. FIG. 2 shows the basic course of the rating_i on the basis of the capacitance value C_objiof the capacitor i (i=1, 2, 3, 4) or of the capacitance value present as a cumulative value of the capacitance of the capacitor 2 or 3.

[0026] Herein,

[0027] rtp_i: rating_i when definitely a person

[0028] rtk_i: rating_i when definitely a child seat

[0029] rtm_i: rating_i a person absolutely improbable

[0030] Ci_l: threshold vacant seat: if all C_obji<=Ci_l, then vacant seat

[0031] Ci_m: minimum capacitance of person with an unfavorable sitting posture

[0032] Ci_ku: child seat lower bend. Starting here, no longer 100% correlation to a child seat; the absolute value of the rating_i decreases, while its real value increases from a negative value (for example, −1), to zero.

[0033] Ci_ko: child seat upper bend. Starting here, no longer child seat; rating_i=0

[0034] Ci_pu: person lower bend. Starting here, a person;

[0035] rating_i increases

[0036] Ci_po: person upper bend. Starting here, 100% correlation to a person

[0037] Ci_max: maximal capacitance: one or several C_obj_i>Ci_max=>error

[0038] The assignment table is determined by measurement of the capacitance values C_objiusing many different child seats and persons, with different clothing and in different seats.

[0039] The evaluation of the object is determined on the basis of the average value of the 4 individual capacitor ratings rating i. That is, rating=(rating_—1+rating_—2+rating_—3+rating_—4)/4. If:

[0040] rating<=0: Rebound child seat, safety system should possibly not be triggered

[0041] rating>0: Person, safety system should possibly be triggered.

[0042] This results in reliable information concerning the type of object and the safety measure to be taken if required.

[0043] The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.