ACCELEROMETER AND METHOD OF MANUFACTURING SAME

Abstract: An accelerometer comprising a piezo-electric element, a holder and at least one electrode, said piezo-electric element having a contact surface at which the piezo-electric element is in electric contact with the electrode, and the piezo-electric element having a clamping surface adjoining the contact surface, at which clamping surface the holder can press against the piezo-electric element.

Description: Accelerometer and method of manufacturing same. The invention relates to an accelerometer comprising a piezo-electric element, a holder and at least one electrode, the piezo-electric element having a contact surface where the piezo-electric element electrically contacts the electrode. A piezo-electric element may be composed, for example, of two interconnected, elongated plates of a piezo-electric ceramic material which are polarized in the thickness direction. The element is firmly held on one side. An acceleration or deceleration in a direction perpendicular to said plate will cause the plate to bend, leading to a displacement of electric charge in the element. This brings about a voltage difference in the electrodes situated on either side of the element. The voltage difference is a measure of the acceleration taking place. Such an accelerometer can be applied in vehicles, for example, to measure the deceleration of the vehicle in the case of an emergency stop or a collision. If the signal originating from the accelerometer exceeds a certain value, for example, one or more airbags and/or safety belts may be activated. The accelerations taking place in these cases may be a multiple of the gravitational acceleration g. Another, more recently developed application in vehicles is the measurement of the speed of rotation of the wheels of the vehicle. For this purpose, an accelerometer is provided in one or more wheels of the vehicle, which accelerometer measures the difference in accelerating power between the upward and downward movement of the wheel, which, at a constant speed of the vehicle, is caused by gravity. By counting the number of variations in the signal from the accelerometer, the speed of rotation of the wheel can be derived. As the difference in acceleration between the upward movement and the downward movement is only 2 g, the accelerometer used for this application must be very sensitive. A well-known problem in existing accelerometers of this type is that large shocks, caused, for example, by a collision or by the element being dropped, or prolonged variations in load, as occur in a rotating wheel, may cause the electrodes to become detached from the piezo-electric element. Customarily, the electrodes are glued onto the element, and they come loose first at the boundaries of the contact region. It is an object of the invention to alleviate the above-mentioned problem. The invention further aims at providing a reliable accelerometer comprising few components, which accelerometer is very sensitive and can be manufactured at low cost. To achieve this, the piezo-electric element in accordance with the invention is provided with a clamping surface adjoining the contact surface, at which clamping surface the holder can exert pressure on the piezo-electric element. As a result of this rigid clamping along the edges of the contact surface, the holder can take up the greater part of the forces acting there as well as the associated stresses, so that the contact between the electrodes and the element is relieved, particularly at the vulnerable boundaries of the contact surface. Preferably, the clamping surface at least partly, but preferably entirely, surrounds the contact surface so that substantially all boundary regions of the contact surface are protected against becoming detached. Preferably, the electrode is adhered to the contact surface by means of an adhesive, said adhesive preferably being an electroconductive glue. In an embodiment, the adhesive forms an adhesive element between the electrode and the piezo-electric element, said adhesive element clamping the electrode preferably against the holder. In another embodiment, the holder clamps the electrode against the piezo-electric element. Both embodiments contribute to a firm connection between the electrode, the holder and the piezoelectric element. Preferably, the electrode is partly accommodated in the material of the holder, for example by incorporating the electrode in a synthetic resin holder when the latter is formed by injection molding, or by placing the electrode in a recessed portion of the holder. In this manner, it is precluded that the electrode can move with respect to the holder and hence with respect to the piezo-electric element, as a result of which the contact could be interrupted. In a preferred embodiment, the holder forms a housing which surrounds the piezo-electric element, a bending surface of the piezo-electric element adjoining the clamping surface being freely bendable in a space formed by the housing under the influence of an acceleration. Such a housing protects the element against external influences and makes the accelerometer easier to handle. Preferably, the holder comprises two substantially identical, interconnected holder portions which can exert pressure on the clamping surface on at least two sides of the piezo-electric element. This leads to an efficient accelerometer which can be manufactured at low cost. Preferably, the clamping surface comprises at least four sides of the piezoelectric element, as a result of which the element is firmly clamped. Preferably, the rigidity of the holder is such that the pressure it can exert on the clamping surface is in excess of 5 MPa, preferably in excess of 20 MPa, and more preferably in excess of 40 MPa. This is a characteristic tension range which, in practice, occurs in such accelerometers. The holder should be capable of taking up these tensions in order to relieve the connection with the electrodes. The invention also relates to a method of manufacturing an accelerometer, in which a holder is placed against a clamping surface of a piezo-electric element, and an electric contact between the piezo-electric element and an electrode is formed on at least one contact surface adjoining the clamping surface. Preferably, the holder is composed of two substantially identical holder portions, which are interconnected after they have been placed against the clamping surface. Preferably, the two holder portions are provided with synthetic resin connection strips which are interconnected by means of an ultrasonic welding technique. In this manner, the two holder portions can be readily and forcefully pressed against the element and, simultaneously, they can be connected to each other. The invention also relates to methods of measuring the speed of rotation of a wheel and methods of measuring the deceleration of a vehicle, as described hereinabove, in which methods use is made of an accelerometer in accordance with the invention. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 is a perspective view of an open holder of an accelerometer comprising a piezo-electric element; Fig. 2 is a perspective view of an accelerometer with the holder of Fig. 1 in the closed state; Fig. 3 is a longitudinal cross-sectional view of the axis of the adhesive elements of an accelerometer; Fig. 4 is a cross-sectional view of the axis of the adhesive elements of an accelerometer; Fig. 5 diagrammatically shows a preferred modification of the rigid clamping shown in Fig. 4; and Fig. 6 is a diagrammatic, perspective view of a wheel comprising an accelerometer. With reference to Fig. 1 , an accelerometer 1 in accordance with the invention comprises a synthetic resin holder 2 which is composed of two identical holder portions 3. Holder portions 3 have two recessed portions 5, 6 which, when the holder is closed, form two beam-shaped spaces of different sections, which are in communication with each other. Said spaces can accommodate a ceramic piezo-electric element 7 having the shape of an elongated flat beam. In this manner, the holder 2 forms a housing which protects the element 7 and which makes the accelerometer easily handleable. The cross-section of the space formed by the recessed portions 5 corresponds to the cross-section of the piezo-electric element 7, so that the element can be placed in the recessed portions 5 so as to be clamped widthwise. The cross- section of the space formed by the recessed portions 6 is slightly larger. Both holder portions are provided with holes 8 for making a connection between the recessed portion 5 and the outside of the holder. Fig. 2 shows an accelerometer 1 which is composed of the closed holder 2 accommodating the piezo-electric element 7. On the outside of the holder 2, metal electrodes 9 are attached by means of electroconductive adhesive elements 10 which connect the electrodes 9 to both sides of the piezo-electric element 7 via the holes 8. Figs. 3 and 4 are sectional views of the accelerometer 1 shown in Figs. 1 and 2. In the manufacture of the accelerometer 1, a piezo-electric element 7 is positioned in a holder portion 3 of an open holder 2 in such a way that an end portion of the element 7 is placed in the recessed portion 5 (see Fig. 1). Subsequently, the holder 2 is closed by placing both holder portions 3 against each other. The dimensions of the holder portions 3 are such that, in the closed state of the holder portions, there is a narrow interspace between the edges of both portions. Next, both portions 3 are forcefully pressed against each other. As a result of the space between the holder portions 3, this force is completely transferred to the clamping surface 12 of the element 7, so that the clamping surface 12 is firmly clamped by the surface of the recessed portion 5. While exerting said clamping force, the holder portions 3 are bonded together, for example by means of an adhesive provided along the edges of the holder portions 3. The two holder portions can alternatively be bonded together by providing them, along the edges, with synthetic resin adhesive strips of the same material as the holder itself, said synthetic resin adhesive strips facing each other and contacting each other in the closed state of the holder portions. These synthetic resin adhesive strips are bonded together by means of ultrasonic welding. In this manner, the element 7 is very rigidly clamped in the holder 2. Subsequently, the electrodes 9 are positioned. The electrodes 9 are provided with holes corresponding to the holes 8 in the holder portions 3. These holes are filled with an electroconductive adhesive, which is provided in such a quantity that it overflows slightly. In this manner, rivet-shaped adhesive elements 10 are formed which, on the one hand, adhere to the contact surface 1 1 and, on the other hand, hold the electrodes 9 against the holder 2. By virtue of the rigid construction of the holder 2 and the element 7, the holder portions 3 can exert a pressure on the clamping surface 12 which corresponds to the tensions occurring during use of the accelerometer 1, as a result of which the glued joint of the contact surface 11 is relieved to a substantial degree, particularly at the edges. To ensure that the contact between the electrodes 9 and the adhesive elements 10 is not interrupted, and to counteract rotation of the holder 2 with respect to the electrodes 9, it is advantageous to house the electrodes in a groove formed at the side face of the holder portions 3, the width of this groove being equal to the width of an electrode 9. This can alternatively be achieved by partly accommodating the electrodes 9 in the material of the synthetic resin holder portions 3 during the injection molding process. It is alternatively possible to place the electrodes in a groove on the inside of the holder portions 3, so that the holder 2 presses them against the clamping surface 12 of the piezo-electric element 7. Fig. 5 diagramrnatically shows a further embodiment of the way in which the element 7 is clamped by the holder portions 3. The object of clamping the element 7 is to fix it as rigidly as possible in the holder 2, therefore it is advantageous to clamp the element on four sides. For this purpose, the holder portions 3 are each provided with a stop shoulder 13 which extends from the holder portions 3, said stop shoulders pressing against the side faces of the element 7 when pressure is exerted on the holder portions 3 in the direction indicated by the arrows. Said stopping shoulders are formed such that, at each side face of the element 7, they press against the entire width thereof, so that the element 7, which is substantially composed of two parts which are bonded together, is not damaged by shear forces. Fig. 6 diagramrnatically shows a possible application of the accelerometer 1. A wheel 14 of a vehicle is provided with an accelerometer 1 , in such a manner that this accelerometer can measure the acceleration in the tangential direction of the wheel 14. If the wheel rotates at a constant speed, due to gravity, an acceleration of +1 g will be measured if the accelerometer is situated at the front side of the wheel, and an acceleration of-1 g will be measured if the accelerometer is situated at the rear side of the wheel. By counting these signal variations, the speed of revolution of the wheel can be derived. This information can be used, for example, to activate an anti-lock system (ALS) or an anti-skid control (ASC).

Claims: CLAIMS: 1. An accelerometer comprising a piezo-electric element, a holder and at least one electrode, the piezo-electric element having a contact surface where the piezo-electric element electrically contacts the electrode, characterized in that the piezo-electric element has a clamping surface adjoining the contact surface, at which clamping surface the holder can exert pressure on the piezo-electric element. 2. An accelerometer as claimed in claim 1 , characterized in that the clamping surface at least partly surrounds the contact surface. 3. An accelerometer as claimed in claim 1 or 2, characterized in that the electrode is adhered to the contact surface by means of an adhesive. 4. An accelerometer as claimed in claim 1, 2 or 3, characterized in that the adhesive is an electroconductive glue. 5. An accelerometer as claimed in claim 3 or 4, characterized in that the adhesive forms an adhesive element between the electrode and the piezo-electric element. 6. An accelerometer as claimed in claim 5, characterized in that the adhesive element clamps the electrode against the holder. 7. An accelerometer as claimed in any one of the claims 1 - 4, characterized in that the holder clamps the electrode against the piezo-electric element. 8. An accelerometer as claimed in any one of the preceding claims 1 - 7, characterized in that the electrode is partly accommodated in the material of the holder. 9. An accelerometer as claimed in any one of the preceding claims 1 - 8, characterized in that the holder forms a housing which surrounds the piezo-electric element, a bending surface of the piezo-electric element adjoining the clamping surface being freely bendable in a space formed by the housing under the influence of an acceleration. 10. An accelerometer as claimed in any one of the preceding claims 1 - 9, characterized in that the holder comprises two substantially identical, interconnected holder portions which can exert pressure on the clamping surface on at least two sides of the piezoelectric element. 11. An accelerometer as claimed in any one of the preceding claims 1 - 10, characterized in that the clamping surface comprises at least four sides of the piezo-electric element. 12. An accelerometer as claimed in any one of the preceding claims 1 - 11, characterized in that the rigidity of the holder is such that the pressure it can exert on the clamping surface is in excess of 5 MPa, preferably in excess of 20 MPa, more preferably in excess of 40 MPa. 13. A method of manufacturing an accelerometer, in which a holder is placed against a clamping surface of a piezo-electric element, and an electric contact between the piezo-electric element and an electrode is formed on at least one contact surface adjoining the clamping surface. 14. A method as claimed in claim 13, characterized in that the holder is composed of two substantially identical holder portions, which holder portions are connected to one another after they have been placed against the clamping surface. 15. A method as claimed in claim 14, characterized in that the two holder portions are provided with synthetic resin connection strips which are interconnected by means of an ultrasonic welding technique. 16. A method of measuring the speed of rotation of a wheel, in which an accelerometer as claimed in any one of the preceding claims 1 - 12 is used. 7. A method of measuring the deceleration of a vehicle, in which an accelerometer s claimed in any one of the preceding claims 1 - 12 is used.

Inventors: BONES NORBERT, JANSSEN GERARDUS J, MERTENS WILHELMINA M J, STEIJAERT HANS A M, BONES, NORBERT, JANSSEN, GERARDUS, J., MERTENS, WILHELMINA, M., J., STEIJAERT, HANS, A., M.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: G01P1
IPC codes: G01P1, G01P3, G01P15