This invention relates to piezoelectric accelerometers, and especially to piezoelectric accelerometers for detecting the vibrations or the degree of riding comfort of automotive vehicles.
Piezoelectric accelerometers are used conventionally for detecting the vibrations or the degree of riding comfort of automotive vehicles. Let us first describe the overall organization of a peizoelectric accelerometer by reference to FIG. 1, which, by the way, shows the fundamental organization of an accelerometer according to this invention.
In FIG. 1, within an electrically conductive casing 1 is accomodated an electrically insulating plate 9, on which a weight 2 and a piezoelectric element 4 are attached by means a fixing screw 5, extending through the weight 2 and the element 4, and a nut 15 engaing with the screw 5. The insulating plate 9 consists of a printed circuit board made of an epoxy resin or a ceramic. The piezoelectric element 4, disposed between its upper and lower electrodes 3a and 3b, is secured to the casing 1 via the insulating plate 9. The lower electrode 3b bears on the plate 9, while the upper electrode 3a is held between the weight 2 and the piezoelectric element 4; the electrodes 3a and 3b are electrically coupled via the respective output leads 13 and 14 of the piezoelectric element to the input terminals of a charge amplifier 6.
The charge amplifier 6, mounted to the electrically insulating plate 9, pre-amplifies the detector output signal of the piezoelectric element 4. The charge amplifier 6 is utilized for the purpose of reducing the adverse effects of the noises and of the variations in the voltage source level. An amplifer 7, also mounted on the insulating plate 9, amplifies the output of the charge amplifier 6. The output terminal of the amplifier 7 is electrically coupled, via a three-terminal capacitor 8b, a through type capacitor 10b, and a lead 33, to the input terminal of a control device 11.
A stabilizing voltage source 12, comprising a stabilizing voltage source circuit (not shown) of the control device 11, supplies a source voltage to the piezoelectric accelerometer via a voltage supply lead 31, a through-type capacitor 10a, and a three-terminal capacitor 8a. The grounded negative terminal of the stabilizing voltage source 12 is coupled to the electrically conductive casing 1 via a grounding lead 32. Further, the casing 1 is coupled via the grounding lead 16 to the insulating plate 9. Thus, the grounding of the electronic circuits on the insulating plate 9 is effected by means of electrical connections to the casing 1 and thence to the grounding lead 32 of the control device 11, the electronic circuit of the accelerometer being completely enclosed within the electrically conductive casing 1; this provides the shielding effect for the electronic circuit of the accelerometer accomodated within the casing and hence is effective in supressing the adverse effects of the external noises thereon.
The conventional piezoelectric accelerometer, however, suffers from the problem of a low assembling efficiency. Namely, the electrical connections between the charge amplifier 6 and the electrodes 3a and 3b of the piezoelectric element 4 are made by means of the leads 13 and 14; this is a difficult and time consuming process. In addition, the steps for providing electrical shielding and waterproofing of the casing 1 add to the time and labor required for completing the assembling process. A further disadvantage of the piezoelectric accelerometer results from the fact that the electrical characteristics of the piezoelectric element 4 and those of the circuit elements of the charge amplifier 6, etc., are temperature dependent and vary with temperature. Namely, changes in the ambient temperature bring about changes in the electrostatic capacity of the piezoelectric element 4 and in the electrical characteristics of the circuit elements of the charge amplifier 6, etc. Hence, the detector output voltage of the acceleromter corresponding to one and the same acceleration varies with the changing ambient temperature; that is, temperature change results in the generation of error in the detector output voltage of the accelerometer.