DESCRIPTION JPH01312425

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DESCRIPTION JPH01312425
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
mechanical quantity sensor for detecting a mechanical quantity correlated with strength such as
vibration or impact, that is, an acceleration caused by vibration, impact or the like. The present
invention relates to a sensor for sensitively detecting vibration, impact and the like in an ultra
low frequency band of about 0, 01 Hz to 50 Hz, which is perceived as The dynamic quantity
sensor of the present invention can be used, for example, for detecting an earthquake, for
controlling an attitude during traveling of an automobile, and for confirming a deceleration effect
at the time of braking. [Prior Art] Various methods can be considered for detecting the
mechanical quantity. However, as an inexpensive and highly sensitive sensor, a piezoelectric
sensor is mainly used. The piezoelectric dynamic quantity sensor in practical use includes one
utilizing the longitudinal effect of the piezoelectric elements 1 and 2 as shown in FIG. 10 or the
piezoelectric element 4 on both sides of the metal plate 3 as shown in FIG. .5 and the like, and
there is one which takes out an electric signal by bending deformation of the disc bimorph 6
pasted together. [Problems to be Solved by the Invention] Of the two types of sensors described
above, the latter one has a larger amount of deformation for the same acceleration, and hence a
larger amount of generated charge, in the low frequency band than the former. Although it has
the feature of excellent sensitivity, the following two points remain as problems. The first is that
in the piezoelectric voltage detection circuit shown in FIG. 12, in order to extract the signal from
the piezoelectric element 7 as a voltage, it must be received with high impedance. Usually, it is
IM Ω or more. This requires sufficient shielding against extraneous noise, and also imposes
certain limitations on the routing of signal lines. Furthermore, the capacitance C of the
piezoelectric element 7 and the input impedance R constitute a bypass filter having a cutoff
frequency determined by the frequency of f−1 / (2πCR) [Hz]. Therefore, when it is desired to
detect acceleration in the very low frequency band, R needs to be made larger, and in that
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respect even higher noise resistance is required, but satisfying both of these requirements is It
was difficult practically. The second is that the piezoelectric element is also a pyroelectric
element. たとえば、０． When an acceleration of about 1 Hz is detected, the pyroelectric voltage
due to an external temperature change may not be negligible. That is, since both the piezoelectric
effect and the pyroelectric effect are phenomena derived from crystal expansion and contraction
in the polarization axis direction, there is a problem that they can not be separated as a single
signal.
Therefore, the present invention has been made to solve the above-mentioned problems of the
prior art, is strong against external noise, is not restricted by detection in a low frequency band,
and is not influenced by the pyroelectric effect. It is an object of the present invention to provide
a dynamic quantity sensor. [Means for Solving the Problems] The mechanical quantity sensor
according to the present invention is characterized by having the following configuration in order
to solve the above-mentioned technical problems. First, a fixing member and a displacement
member at least partially displaced with respect to the fixing member by an external force are
provided. The fixing member is provided with a first ultrasonic transducer. On the other hand, a
second ultrasonic transducer is provided at the displacement portion of the displacement
member. The second ultrasound transducer is disposed to face the first ultrasound transducer via
a space. One of the first and second ultrasonic transducers is a transmitting transducer, and the
other is a receiving transducer. An excitation signal for exciting the transmission side transducer
is supplied from the input signal generation means to the transmission side transducer. The
output of the receiving transducer, which receives the output of the transmitting transducer via
space, is processed by the output processing means. The output processing means includes
means for demodulating the output of the reception side transducer which has received the AM
modulation by changing the distance of the space according to the displacement of the
displacement member. [Operation and Effect of the Invention] In the mechanical quantity sensor
of the present invention, at least a part of the displacement member is displaced with a
predetermined acceleration when an external force or some other external force or vibration is
applied. The amount of displacement is substantially proportional to the magnitude of the
acceleration. Depending on such displacement of the displacement member, the distance
between the first ultrasonic transducer and the second ultrasonic transducer, that is, the distance
between the transmitting transducer and the receiving transducer is Change. The receiving
transducer always resonates in response to the sound pressure from the transmitting transducer,
but when such a change in distance occurs, the output voltage waveform of the receiving
transducer undergoes AM modulation. It will be received. Therefore, if the AM-modulated output
waveform is demodulated, the demodulated output becomes a function of the physical quantity
to be detected. In this way, if the output of the receiving transducer is demodulated, it can be
extracted as a signal representing the mechanical quantity to be obtained.
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In addition, the ultrasonic transducer used in the present invention has a small impedance, and in
particular, in the case of the receiving transducer, it can be, for example, not more than several
tens of Ω. Therefore, when such a transducer is incorporated into a circuit, the influence of
extraneous noise can be reduced. Further, in the mechanical quantity sensor according to the
present invention, only the displacement amount of the displacement member is the source of
the signal, and if this displacement is sustained, the output is also sustained and the voltage value
correlated therewith is basically displayed , DC based measurements are possible. Therefore,
there is no restriction that comes from the electrical cutoff frequency, and vibration, impact and
the like in the very low frequency band can be detected without any problem. Furthermore, the
receiving transducer constantly resonates in response to the sound pressure from the
transmitting transducer, and the charge is constantly repeating charging and discharging.
Therefore, the charge due to the pyroelectric effect due to the external temperature change is
immediately discharged, is not affected by the pyroelectric effect, and high sensitivity detection is
possible. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a perspective view
showing a dynamic quantity sensor as one embodiment of the present invention, and FIG. 2 is a
cross-sectional view of the same. Although the sensor is usually housed in a case surrounding the
outside, such a case is not shown in FIGS. 1 and 2. The sensor includes a base 11 as a fixing
member and a thin metal plate 12 as a displacement member. The sheet metal 12 is cantilevered
on the base 11 so that at least a part thereof, in particular its free end, can be displaced relative
to the base 11 by an externally applied force. The thin metal plate constituting the displacement
member may be replaced by, for example, a plate of carbon fiber reinforced plastic or the like.
That is, the desirable properties required of the displacement member are low mechanical Q (i.e.,
low reverberation vibration) and elasticity. The displacement member may be made of any
material as long as such properties are satisfied. The base 11 is provided with a first ultrasonic
transducer 13 while the free end of the sheet metal 12 is provided with a second ultrasonic
transducer 14. The first and second ultrasonic transducers 12 and 13 are disposed to face each
other via a predetermined space 15. The first and second ultrasonic transducers 13 and 14 have
the same structure as each other in this embodiment, and the funnel-shaped resonators 17a and
17b are disposed on the unimorph diaphragms 16a and 16b, respectively. It is of attached
structure.
Then, as these ultrasonic transducers 13.14, one having a resonance frequency of 40 kHz, for
example, is used. Although which of the first and second ultrasonic transducers 13 and 14 is
used as the transmitting side and the receiving side is entirely arbitrary, in this embodiment, the
first ultrasonic transducer 13 is the transmitting side transformer. The second ultrasonic
transducer 14 is used as a receiving-side transducer. Therefore, the lead wires 18a and 18b of
the transmitter transducer 13 are wired to a circuit on the transmitter side described later with
reference to FIG. 3, while the lead wires 19a and 19b of the receiver transducer 14 are received.
Wired to the circuit on the side. Referring to FIG. 3, the transmission side transducer 13 has a
rectangular wave waveform (40 kHz) as shown by A in FIG. 4 from the oscillation circuit 20 as
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input signal generation means included in the transmission side circuit. An excitation signal is
provided. Thereby, the transmitter transducer 13 is excited and the sound pressure generated by
this excitation is received by the receiver transducer 14 via the space 15. At this time, when the
distance of the space 15 is constant, the receiving transducer 14 outputs a signal having a
sinusoidal waveform as shown by B in FIG. The output from the receiving transducer 14 is
processed by circuitry on the receiving side. As this output processing means, a sensitivity
adjustment circuit 21, an AM demodulation circuit 22, and an amplification / low pass filter
circuit 23 are provided. The bending displacement amount X of the metal thin plate 12 shown in
FIGS. 1 and 2 is represented by the following equation. x- (4F) 3 / (Ebt ') where F is force, section
is effective length, t is thickness of the thin metal plate 12, b is width of the thin metal plate 12,
and E is Young's modulus. Further, assuming that the acceleration is a (m / 5 ee 2) and the mass
is m, since the relationship of -ma is established, the above-mentioned amount of bending
displacement x is x-[(4rrl! 3) / (Eb t ') Since it is represented by a, the bending displacement Hkx
is proportional to the acceleration a. Therefore, the distance of the space 15 between the
transmitting transducer 13 and the receiving transducer 14 changes substantially in proportion
to the acceleration a. The distance of the space 15 is selected to be, for example, about 2 mm.
Experimentally, if the distance of the space 15 is increased, the output voltage of the receiving
side transducer 14 is decreased and the shape of the sensor is also increased. It confirmed that
there was almost no distortion.
As described above, when the distance of the space 15 changes, the output signal of the receiving
transducer 14 is subjected to AM modulation. For example, C in FIG. 5 shows an AM-modulated
reception waveform. As described above, the output signal from the receiving-side transducer 14
that has received AM modulation is subjected to impedance conversion or amplification in the
sensitivity adjustment circuit 21 to be converted into a continuous wave of constant amplitude,
and then the AM demodulation circuit 22 , And is output through the amplification / low pass
filter circuit 23 for removing and amplifying the ripple of the carrier wave of 40 kHz. D of FIG. 5
shows a waveform which has been demodulated and amplified through the AM demodulation
circuit 22 and the amplification / low pass filter circuit 23. In practice, vibration of 10 Hz was
applied to the sensor at an acceleration of 0 ° 2 G, 0.4 G, 0.6 G, 0.8 G and 1 ° 0 G by a vibrator.
FIG. 6 is a waveform diagram showing an output voltage when vibrations of accelerations of 0.2
G, 0.6 G and 1.0 G are applied to the sensor. As can be seen from FIG. 6, as the acceleration
increases, the obtained output voltage increases. The relationship between the acceleration and
the output voltage is shown in FIG. For example, at an acceleration of 1.degree. G, as shown in
FIGS. 6 and 7, a peak-to-peak voltage of about 1.5 V is obtained, and as shown in FIG. A linear
relationship is obtained with the voltage. Therefore, according to this embodiment, it is
understood that the sensor can be practically used as a sensor for detecting an acceleration, that
is, a mechanical quantity. The dynamic quantity sensor according to the present invention is not
limited to the structure as shown in FIG. 1 and FIG. For example, as shown in FIG. 8, the thin
metal plate 12a may be peripherally supported relative to the base 11a via the elastic body 24,
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and as shown in FIG. 9, the thin metal plate 12b may be used as the base llb. The peripheral may
be fixed. In FIGS. 8 and 9, parts corresponding to the parts shown in FIG. 2 carry the same
reference numerals, and duplicate explanations are omitted. Further, as the ultrasonic transducer,
in addition to the funnel-shaped resonators 17a and 17b as illustrated, a simple bimorph element
or a unimorph element may be used. In addition to using the thin metal plates 12, 12a and 12b
which can be bent by itself as the displacement member, the displacement member may be
replaced by a member held by another elastic member.
[0002]
Brief description of the drawings
[0003]
FIG. 1 is a perspective view showing a mechanical quantity sensor according to an embodiment
of the present invention.
FIG. 2 is a cross-sectional view of the sensor shown in FIG. FIG. 3 shows an example of a circuit
for driving a sensor and extracting a mechanical quantity detected by the sensor as an analog
output. FIG. 4 shows the waveform (A) of the excitation signal applied to the transmitting side
transducer 13 by the circuit shown in FIG. 3 and the waveform (B) of the output signal output
from the receiving side transducer 14 Show. FIG. 5 shows the waveform (C) of the output signal
of the receiving side transducer 14 subjected to the modulation and the waveform (D) of the
demodulated signal. FIG. 6 shows the waveform of the output voltage when excited by various
accelerations. FIG. 7 is a graph showing the relationship between acceleration and output voltage
achieved by the sensor according to the present invention. 8 and 9 are cross-sectional views
showing a mechanical quantity sensor as another embodiment of the present invention,
respectively. 10 and 11 schematically show the structure of a conventional piezoelectric dynamic
quantity sensor. FIG. 12 shows a piezoelectric voltage detection circuit used, for example, in
connection with the sensor shown in FIG. In the figure, 11. 11 a, 1 1 b are bases (fixed members),
12, 12a, 12b are thin metal plates (displacement members), 13 is a first ultrasonic transducer or
transmitting side transducer, 14 is a second ultrasonic transducer A transducer or a receiving
transducer, 15 is a space, 12 is an oscillation circuit (input signal generating means), and 22 is an
AM demodulation circuit. ｆＯＰｓｅｃ／ｌ！ '+ V main power t 7 i-(roons v 7 a; v) FIG.
Tatsuhiko Tatsumi (Cr)
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