Piezoelectric Physical Quantity Detector

This invention relates to a detector for detecting a physical quantity such as motion and stress by use of a piezoelectric device and more particularly to a driving system of a piezoelectric detector capable of measuring signals of physical quantities having a low frequency range including D.C.

Conventionally, a piezoelectric device has been used as a detector of acceleration vibration. In Japanese Patent Laid-open No. 62-81572 (1987), for example, detection can be made down to a low frequency by providing for polarization in parallel with a main plane of the piezoelectric. As another prior art example, Japanese Patent Laid-open No. 61-270665 (1986) makes it possible to detect the acceleration down to a low frequency by use of a center fixed type piezoelectric device.

However, the conventional technique stated above has a fatal drawback in that measurement of the frequency component of the acceleration to the D.C. is impossible. Electrical charges generated by stress applied to the piezoelectric device leak because the impedance between the electrodes of the device is finite.

An object of the present invention is to furnish a piezoelectric detector which makes it possible to measure a signal of a physical quantity such as motion and stress in a detection range of low frequency including D.C. by use of a piezoelectric device.

The above object is attained by using a device for exciting in advance a piezoelectric device by a carrier A.C. signal in a physical quantity detector composed of a piezoelectric device, a weight coupled mechanically to the piezoelectric device and means for taking the output signal obtained at the electrodes of the piezoelectric device in association with a physical quantity such as stress or momentum (e.g. acceleration, speed, displacement) applied to the weight. The output signal is subjected to amplitude-modulation, the carrier which is modulated by the physical quantity is then demodulated and the output signal associated with the physical quantity is removed.

Therefore, in the present invention, as the charges generated by the stress applied to the piezoelectric device are changed to positive or negative according to the carrier A.C., the charge does not leak from the electrode of the piezoelectric device and can be amplified and taken out in the frequency range of very low frequency including D.C.

FIG. 1 is a block wiring diagram showing an embodiment of the present invention;

FIG. 2 is a block wiring diagram showing an another embodiment of the present invention;

FIG. 3 is a block wiring diagram showing another embodiment of the present invention using a selfoscillator; and

FIG. 4 is a block wiring diagram showing still another embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows a first embodiment of an acceleration detector. A detection unit 11 is formed by a disc-like piezoelectric device 1, opposed electrodes 12, 12' and 13, and a weight 2 which is mechanically coupled with the device 1 through the electrode 13. A carrier A.C. voltage 5 is applied to the electrode 12 from an excitation power source 3. An acceleration step signal 6 occurring between the opposed electrodes 12 and 13 of the piezoelectric device modulates the carrier A.C. so as to generate a modulated output signal 7 from the electrode 12', and is amplified by an A.C. amplifier 8 and thereafter is detected synchronously by a demodulator 9. The amplified acceleration step signal 10 is then obtained from the demodulator.

In FIG. 1 the electrode 12 is separated from the electrode 12', but if the electrode 12 is connected to the electrode 12' as one electrode, the operation is the same as that of the embodiment in FIG. 1.

FIG. 2 shows another embodiment wherein two systems of acceleration detectors are fitted to a body of a vehicle in order to determine the vector of motion of the vehicle on a real time basis from the acceleration components in the moving direction of the vehicle and the direction thereto. This apparatus can be used, for example, for determining the angle of inclination of a slope and a horizontal component of a car speed when the vehicle travels on the slope at a low speed or for determining the angle of inclination of the car body and the car speed relative to the ground when abrupt acceleration/deceleration is made on a horizontal road. In the drawing, reference numerals 14 and 15 represent the acceleration detection units in the moving direction of the vehicle and in the direction perpendicular thereto, respectively, 16 and 17 are A.C. amplifiers, 18 and 19 are demodulators and 20 is an excitation power source, which is used in common for the two systems of acceleration detectors. Reference numeral 21 represents an operation unit for operating the output signal levels of the two systems, whereby the inclination angle of the slope, the inclination angle of the vehicle and the car speed to the ground are determined. Reference numeral 22 in the drawing represents schematically the state where the driving circuit portions of the two systems of the acceleration detectors and the operational processing units of the output signals are disposed on a common substrate.

FIG. 3 shows another embodiment for converting the change of acceleration to the change of the oscillation frequency by use of the piezoelectric device having electrodes 31, 32 as a capacitance C for determining the oscillation frequency in a self-oscillator such as R-C oscillator. This is based on the phenomenon that the capacitance between the electrodes changes when stress due to the acceleration is applied to the piezoelectric device. In this case, it is preferred to set the oscillation frequency to a broader side than an audible frequency band so as eliminate offensive noise and to near to a resonance point so as to improve detection sensitivity. The change of the oscillation frequency may be supplied as an analog signal of the acceleration by use of an F-V converter. In the drawing, reference numerals 23 represents the self-oscillators and reference numeral 24 represents the detection unit including the piezoelectric device.

In FIG. 4, two piezoelectric devices 25, 26 of the same kind having electrodes 41, 42 and 43 are coupled mechanically to the weight 2 and are connected to the modulator 4 which modulate carrier A.C. voltage from a power source 3. An amplifier 8 amplifies outputs from the modulator 4 in the same way as in FIG. 1 so that their outputs are amplified mutually complimentarily. In the drawing, the outputs of the piezoelectric devices 25, 26 have the polarities as shown in the drawing.

This embodiment can offset temperature dependence of the two piezoelectric devices by connecting them complimentarily.

As described above, the present invention can measure the frequency detection range of physical quantities such as motion and stress down to the D.C. component. Though using a simple driving circuit construction, the present invention can not only detect economically the physical quantities at a plurality of positions but can also provide a detector which is highly resistant to disturbance such as an electro-magnetic interference.