ACCELERATION SENSOR

Abstract: An acceleration sensor is disclosed which includes a weight (2), piezoelectric element(s) (4), a charged amplifier (6), an insulation plate (9) and a conductive casing (1). The insulation plate (9) fixedly mounts the weight (2) and piezoelectric element(s) (4) and is fixed to the conductive casing (1) which is maintained at the earth potential. Input terminals (13, 14) of the charged amplifier are connected to the output electrodes (3a, 3b) of the element(s) and a grounding circuit of the amplifier is connected to the conductive casing (1) so that the input terminals of the amplifier are floated from the earth potential.

Description: The present invention relates to an acceleration sensor which employs piezoelectric element(s) adapted to detect vibration in an automobile or the like. A piezoelectric type sensor has been used for detecting vibration in an automobile. This piezoelectric sensor is described, for example, in "Force and Acceleration Sensor (1)" p. 81 - 85, by Keras, Series No. 5, Ceramic Sensor, 1973, No. 32 Ele-Cera Publishing Co. An acceleration sensor employing such a piezoelectric sensor as described above, a weight, and a charge amplifier is applied, for example, to an electric control suspension means adapted to control vibration in an automobile in order to improve the driving and riding feel. In the case that the piezoelectric sensor is applied to the electronic control suspension means, since the piezoelectric sensor has a high impedance and is capacitive, coping with external noise is a problem. Accordingly, there have been proposed such measures as to incorporate an impedance conversion circuit, and to ground one of the electrodes of a piezoelectric sensor to a casing, so that the casing provides a shielding effect. However, in this instance, in such an application as an automobile in which a single battery power source is available for activation and the body of the automobile is grounded, if one of the electrodes of a piezoelectric sensor is grounded to the casing, a signal for detecting acceleration is variable mainly relative to the ground level, while an amplifier is caused to amplify the signal on the basis of the DC level obtained by dividing by two the source voltage. Under this condition, it is difficult to separate the variation in the source voltage from the detected acceleration signals, and therefore the signals are disturbed with the effect that source voltage variation as well as external noises. GB-A-1 258 176 shows an electrical circuit for transmitting and amplifying the output signal of a piezoelectric sensor which is bipolar and screened by a housing and from which an insulated and screened bipolar signal lead extends. The circuit comprises an amplifier circuit which is electrically insulated from earth potential and has two input terminals and two output terminals with the signal lead being connected to the input terminals. A differential amplifier has two input terminals connected to the output terminals of the amplifier circuit. Because this electrical circuit is only earthed at one point, no earth currents flow, so that interference on the high-resistance signal lead is suppressed. Further, US-A-4,085,349 discloses an acceleration sensor comprising weight means, piezoelectric means for generating a signal by a pressure lead caused by said weight means depending on the rate of acceleration of a device. A velocity responsive charge is delivered to a charge converter which generates an electrical signal having a voltage which is proportional to the velocity of the device. The piezoelectric means consists of a stack of piezoelectric crystal elements secured together with the weight means by a connecting tension bolt on a circular base made from non-magnetic steel. This base is fixedly mounted on the device and forms part of the casing surrounding the weight means and the piezoelectric means. The acceleration sensor is only intended to detect acceleration in the vertical direction, i.e. in the direction of the tension bolt. It is to be noted, however, that the piezoelectric means will be subjected to a force caused by a rolling movement due to acceleration in a lateral direction, and as a consequence, the piezoelectric means may generate an output as a result of the rolling movement. It is, therefore, an object of the present invention to provide an acceleration sensor which has a stable detection sensitivity in the vertical direction of acceleration, but a reduced detection sensitivity in the lateral direction. This object is achieved by an acceleration sensor comprising an insulation plate for fixedly mounting the weight means and piezoelectric means and being fixed to the conductive casing, and a metallic rigid means for supporting the weight means and piezoelectric means, wherein the metallic rigid means has a U-shaped portion, the insulation plate is inserted in the U-shaped portion, and the weight means, piezoelectric means and metallic rigid means are together fixed at the inserted portion of the insulation plate by a fixing means. Further, when an acceleration meter employing a piezoelectric sensor and a weight is used in an automobile, it is necessary to cancel by any means the pyroelectricity, or the characteristic that the piezoelectric sensor generates a charge depending on the change of temperature, in order to pick up only acceleration signals stably regardless of a change in the temperature of the environment. Therefore, it is also an object of this invention to provide an acceleration sensor which is most properly used in a wide temperature range and under a severe change of temperature. In order to achieve this object, the piezoelectric means comprises two piezoelectric elements electrically connected in parallel and serially located between the weight means and insulation plate through a part of the metallic rigid means and the fixing means comprises a metallic screw and a nut, whereby the two piezoelectric elements are thermally balanced. Embodiments of an acceleration sensor according to the present invention will now be explained by referring to the drawing. Fig. 1 illustrates a block schematic diagramm showing an embodiment of this invention; Fig. 2 is a circuit diagramm of the embodiment shown in Fig. 1; and Fig. 3 is a sectional view of another piezoelectric means suited for use in a wide temperature range. In the drawing, numeral 1 designates a conductive casing, 2 a metallic weight, 4 a piezoelectric element, 5 and 15 a fastening screw and a nut, 6 a charge amplifier, and 9 an insulation plate which is a printed circuit board. The piezoelectric element 4 and the weight 2 are attached to the insulation plate 9 by the fastening screw 5 and nut 15 and the plate is fixed to a specified location (not shown) on the conductive casing 1. For the insulation plate 9, a printed circuit board comprising an epoxy resin or a ceramic board is used. This plate 9 enables the element 4 and weight 2 to be fixedly mounted to the casing in a non-conductive condition. At the front and rear side surfaces of the element 4, there are provided electrodes 3a and 3b. The electrode 3b at the rear side surface of the element 4 is pressed against the upper surface of the plate 9, while the electrode 3a is interposed between the weight 2 and the element 4. These electrodes 3a and 3b are respectively connected to input terminals of the charge amplifier 6 by way of output lines 13 and 14. The charge amplifier 6 is also attached to the plate 9, and serves to amplify a detection signal from the element 4. The amplifier 6 is used for reducing a fluctuation in a source voltage and an influence by noises, because it is floated from the conductive casing 1 the potential of which is ground level. Numeral 7 designates the amplifier attached to the insulating plate 9. This amplifier 7 is used for amplifying the output from the charge amplifier 6 and has an output terminal connected to the input terminal of an electric control unit 11 by way of a three-terminal capacitor 8b and a through type capacitor 10b. Numeral 12 designates a stabilizing power source which is available from the stabilizing power source circuit (not shwon) of the control unit 11 and serves to provide the piezoelectric acceleration sensor unit including the element 4 and amplifiers 6 and 7 with the power source from its positive pole by way of a feeder line 31, a through type capacitor 10a and a three-terminal capacitor 8a. The negative pole of the stabilizing power source 12 is grounded and also connected to the electrically conductive casing 1 by way of a grounding line 32. The conductive casing 1 is connected to an earth portion printed on the insulation plate 9 by way of a grounding line 16. Thus constituted, the electronic circuits on the insulating plate 9, or the charge amplifier 6 and the amplifier 7, have their ground portions connected to the grounding line 32 by way of the conductive casing 1. So, the shielding effect is utilized by the casing, and thereby the affect of external noise can be reduced. Fig. 1 illustrates an improved method of fixing the element 4 and weight 2, in which detection in a vertical direction of acceleration is stable while unnecessary detection sensitivity in a lateral direction is reduced. According to the embodiment shown in Fig. 1, the acceleration sensor is only intended to detect acceleration in a vertical direction. It is to be noted, however, that the piezoelectric element will be subjected to a force caused by a rolling movement due to acceleration in a lateral direction, and as a consequence, the piezoelectric element may generate an output as a result of the rolling movement. More specifically, there is a problem as to the method of mechanically fixing the piezoelectric element 4 in a reliable manner to the casing in such a way that it is electrically insulated, according to the fixing means. In consideration of this problem, as shown in Fig. 1, a plate 30 for reinforcing the fixing portion is used. The plate 30 is made of a rigid material such as metal, and formed into an U shape. The insulation plate 9 is inserted in the U shape plate 30 and the piezoelectric element 4 and weight 2 are placed on and fixed to the plate 30 incorporating the insulation plate 9 by the fastening screw 5 and nut 15, whereby any rolling movement my be reduced. Since this plate 30 is fixed to the conductive casing 1 by way of the insulation plate 9, it can maintain its insulating function with regard to the casing 1 while it is mechanically fixed to the casing in a reliable fashion by means of screws or the like not shown, whereby the acceleration detection capability in a mechanical sense may be realized as in the prior arts. The charge amplifier 6 is so constituted as shown in Fig. 2. In Fig. 2, it is seen that the piezoelectric element 4 is floated from the grounding potential and the potential of one of the electrodes 3b is fixed to the potential divided by resistors R₁ and R₂. In the amplifier 6, (+) input and (-) input terminals of an operational amplifier 20 having an FET input stage are connected to each other via a resistor R₃. Accordingly, even if the level of the power source fluctuates, their potentials of both the (+) and (-) input terminals move in unison and thereby the affect of any fluctuation in the power source on the charge amplifier 6 may be mitigated. A capacitor C₁ is connected in parallel to the resistor R₃ and the electrode 3a of the piezoelectric element 4 is connected to the (+) input terminals of the operational amplifier 20 while the electrode 3b is connected to the (-) input terminal of the amplifier 20 via the resistor R₄. The capacitor C₁ is inserted in order to reduce the minimum detection frequency of the acceleration to be detected. For example, when the minium detection frequency is set to 0.2 Hz, C₁ = 0.047 µF and R₃ = 15 MΩ may be set. By using the parallel circuit of the capacitor C₁ and resistor R₃, unnecessary drifting voltage due to temperature change may be prevented, even if a special low pass filter is not inserted. It is further possible to set the time constant at a low frequency range optionally and most properly by making the value of the resistor R₃ smaller than the input impedance of the operational amplifier 20. The input impedance of the amplifier 20 having the FET input stage is more than 10⁹ Ω and can therefore meet the above requirement. The resistor R₅ is connected between the output terminal of the operational amplifier 20 and the (-) input terminal thereof. The output of the operational amplifier 20 is connected to the control unit 11 by way of the amplifier 7, the three-terminal capacitor 8b, and the through type capacitor 10b. The grounding electrode of the capacitor 10b is connected to the negative pole of the power source 12 of the control unit 11 by way of the grounding line 32. The power source terminal of the amplifier 7 is connected to the positive pole of the power source 12 by way of the three-terminal capacitor 8a, the through type capacitor 10a and a feeder line 31. Operation of the acceleration sensor shown in Figs. 1 and 2 will now be explained. When the pressure load of the weight 2 is imposed on the piezoelectric element 4 due to acceleration, the change of the charge on the element 4 is generated depending on the rate of the acceleration and applied to the charge amplifier 6. The charge generated by the element 4 is converted to a voltage signal by the capacitor C₁ and applied between the (-) and (+) input terminals of the amplifier 20. The input voltage is amplified by (1 + R₅/R₄) times by the operational amplifier 20 and the resistors R₄ and R₅ and further amplified by the amplifier 7, and thereafter input to the control unit 11 via the three-terminal capacitor 8b and the through type capacitor 10b. The charge amplifier 6 is designed so as to convert the change of the charge caused by the piezoelectric element 4 to voltage and precisely amplify the converted voltage by floating the potential at the piezoelectric element 4 despite fluctuation in the source voltage. As a consequence, the charge amplifier 6 is less affected by fluctuation in the source voltage and does not particularly require a stabilizing power source exclusively for the acceleration sensor unit. Therefore, even if the stabilizing power source 12 of the control unit 11 is used for the sensor unit and the power is supplied from the feeder line 31, the amplifier 6 may be operated in a stabilized condition. Fig. 3 shows another embodiment of this invention wherein two piezoelectric elements 4a and 4b electrically connected in parallel to each other instead of the single piezoelectric element 4 shown in Figs. 1 and 2 are used. In this second embodiment, electrodes 3a and 3a are provided on the upper surface of the element 4a and the lower surface of the element 4b, while an electrode 3b is provided between the lower surface of the element 4a and the upper surface of the element 4b. The U-shaped metallic rigid member 30 is fixed to the insulation plate 9 together with the elements 4a and 4b and weight 2 by the metallic fixing screw 5 and nut (not shown in Fig. 3). In this way, since the metallic weight 2 and metallic rigid member 30 are fixed and thermally connected by means of the metallic fixing means, or the fixing screw 5 and nut 15, they are thermally balanced. In the second embodiment, since there is an air layer inside the casing 1 and the elements 4a and 4b are fixed at the insulation plate 9 so as to be thermally insulated from the casing, any influence on the elements 4a and 4b from the change in the temperature outside the casing 1 is mitigated. This feature is the same as that in the first embodiment. Further, in the second embodiment, since the elements 4a and 4b retain heat due to the metallic weight 2, rigid member 30 and fastening screw 5 and nut 15, the temperature of the elements may be balanced and the gradient of the temperature change thereof may be more gentle. Furthermore, the elements 4a and 4b are located in symmetry as well as being electrically connected in parallel, the pyroelectricity characteristics are thus off-set. Accordingly, by the second embodiment the affect of the pyroelectricity caused by the change in temperature may be mitigated and such pyroelectricity may be off-set by the plurality of piezoelectric elements. More specifically, this embodiment is stable against fluctuation in the source voltage, resistant to temperature drifting and will not amplify any unnecessary noise caused by vibration, the acceleration sensor being most appropriate for an automobile. Although the detection of acceleration in the vertical direction has first been described, a similar effect may be expected in the case of the detection of acceleration in the left and right direction as well as in the forward and backward direction, if the direction of fixing the piezoelectric element (2) and weight is changed. The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of this invention as defined by the appended Claims. For example, in the respective embodiments, the capacitor C₁ for converting the change of the charge at the piezoelectric element(s) to the corresponding voltage change may be selected such that it has a positive capacitive temperature coefficient, whereby a temperature characteristic of the detection may be compensated for. Further, the charge amplifier 6 and metallic rigid plate 30 should not be taken as being limited to those described above, and other constitutions may be employed.

Claims: An acceleration sensor comprising: weight means (2); piezoelectric means (4; 4a,4b) for generating a signal by a pressure load caused by said weight means (2) depending on the rate of acceleration; charge amplifier means (6) connected to said piezoelectric means (4; 4a,4b) for amplifying said signal generated by said piezoelectric means (4; 4a, 4b); a conductive casing (1) the potential of which is maintained at the earth level; and a grounding circuit of said charge amplifier means coupled to said conductive casing (1), so that input terminals of said amplifier (6) keep floating from said earth level, characterized by an insulation plate (9) for fixedly mounting said weight means (2) and piezoelectric means (4; 4a, 4b), and being fixed to said conductive casing (1), and a metallic rigid means (30) for supporting said weight means (2) and piezoelectric means (4; 4a, 4b), wherein said metallic rigid means (30) has a U-shaped portion, said insulation plate (9) is inserted in said U-shaped portion and said weight means (2), piezoelectric means (4; 4a,4b) and metallic rigid means (30) are together fixed at the inserted portion of said insulation plate (9) by a fixing means (5, 15). An acceleration sensor according to claim 1, characterized in that said piezoelectric means comprises two piezoelectric elements (4a, 4b) electrically connected in parallel and serially located between said weight means (2) and insulation plate (9) through a part of said metallic rigid means (30), and said fixing means comprises a metallic screw (5) and a nut (15), whereby said two piezoelectric elements (4a, 4b) are thermally balanced. An acceleration sensor according to claim 1 or 2, characterized in that said insulation plate (9) comprises a printed circuit board onto which said charge amplifier means (6) is mounted.

Inventors: WADA, SHUNICHI C/O HIMEJI SEISAKUSHO, YANO, MASAYUKI C/O HIMEJI JIGYOSHO
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: G01P15
IPC codes: G01P1, G01P15