JP2007033196A - Ultrasonic sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To transmit and receive ultrasonic wave with a single piezoelectric vibrator by using a two-terminal type piezoelectric vibrator as it is which is designed to use individual vibrators for transmission and reception, without using a boosting transformer. <P>SOLUTION: The ultrasonic sensor 51 of this invention comprises: a series capacitance (Cs) 6 connected in series with the ultrasonic sensor for elevating the resonance frequency fr of the ultrasonic sensor; and a parallel capacitance (Cp) 11 connected in parallel with the ultrasonic sensor for lowering the antiresonant frequency fa of the ultrasonic sensor. By driving the ultrasonic sensor in the state where only the series capacitance Cs is added, ultrasonic wave is transmitted in the air for a certain time with a transmission timing, and the ultrasonic wave 60 reflected from an object 61 is received the a state where only the parallel capacitance Cp is added with the next reception timing. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an ultrasonic object detection apparatus that detects the presence or absence of a target object and the distance to the target object by measuring the ultrasonic wave reflected from the target object (target) by transmitting an ultrasonic wave in the air. In particular, the present invention relates to a transmission / reception ultrasonic sensor capable of transmitting and receiving ultrasonic waves with one piezoelectric vibrator and transmitting ultrasonic waves with a low driving voltage.

  An ultrasonic sensor used for transmission / reception of ultrasonic waves in the air is generally configured using a piezoelectric vibrator (hereinafter, “ultrasonic sensor” is simply referred to as “piezoelectric vibrator for ultrasonic sensor”). And a case including a “piezoelectric vibrator for an ultrasonic sensor” and a drive circuit thereof).

  FIG. 7 is a perspective view showing the structure of a two-terminal piezoelectric vibrator. In FIG. 7, the two-terminal piezoelectric vibrator 70 includes a piezoelectric plate 72, a terminal 75 connected to the electrode 71 is provided on one surface of the piezoelectric plate 72, and an electrode 73 is provided on the other surface. A terminal 76 connected via a metal plate 74 is provided. The piezoelectric plate 72 is polarized in the thickness direction using these electrodes 71 and 73, and the piezoelectric plate 72 and the metal plate 74 constitute a bimorph or unimorph piezoelectric vibrator. As shown in FIG. 7, a piezoelectric vibrator having two terminals 75 and 76 is called a two-terminal piezoelectric vibrator.

  FIG. 8 is an electrical equivalent circuit of a two-terminal piezoelectric vibrator. In FIG. 8, an equivalent circuit 81 of a two-terminal piezoelectric vibrator is connected between terminals 86 with a series circuit including an inductor L indicated by 82, a capacitance C indicated by 83, and a resistor R indicated by 84, It is represented by an electrical equivalent circuit in which an electrostatic capacity Cd indicated by 85 is connected in parallel as a braking capacity unique to the vibrator.

FIG. 9 is an example of frequency characteristics of input impedance of a two-terminal type piezoelectric vibrator. In FIG. 9, the resonant frequency fr [Hz] indicated by 91 The input impedance becomes the minimum value Zr [Ω], and the input impedance becomes the maximum value Za [Ω] at the antiresonance frequency fa [Hz] indicated by 92.
Here, the resonance frequency fr and the antiresonance frequency fa are given by Equation 1 and Equation 2, respectively, using the equivalent circuit constants L, C, and Cd of FIG.

  In an ultrasonic sensor that detects the presence or absence of an object and the distance to the object by transmitting an ultrasonic wave into the air using such a piezoelectric vibrator or receiving an ultrasonic wave that has propagated through the air, In the case of wave transmission, it is common to drive at a resonance frequency fr that can excite the vibrator with a low drive voltage and to receive at an anti-resonance frequency fa with high impedance. This is because the ultrasonic wave is efficiently transmitted despite the low voltage and the reception sensitivity is increased.

10 and 11 are explanatory diagrams showing the configuration principle of an ultrasonic object detection device which is a conventional ultrasonic sensor.
FIG. 10 shows a case where the ultrasonic sensor 101 on the transmitter side and another ultrasonic sensor 102 on the receiver side are used. A transmission drive circuit 103 is connected to the ultrasonic sensor 101 on the transmitter side, and a reception drive circuit 106 is connected to the ultrasonic sensor 102 on the receiver side. In the transmission drive circuit 103, a transmission drive voltage is supplied from the power supply 104 to the ultrasonic sensor 101 on the transmitter side via the resistor 105, and ultrasonic waves are transmitted from the ultrasonic sensor 101 on the transmitter side to the object 109. Waved.

  In the reception drive circuit 106, the ultrasonic wave reflected from the object 109 is received by the ultrasonic sensor 102 on the receiver side, and the received voltage is output from the output terminal 107 as a voltage drop with respect to the resistor 108. In the configuration shown in FIG. 10, it is necessary to use two ultrasonic sensors in which the resonance frequency fr of the ultrasonic sensor 101 used for transmission and the anti-resonance frequency fa of the ultrasonic sensor 102 used for reception are equal. .

FIG. 11 is a configuration principle diagram when one ultrasonic sensor 111 is used for both the transmission side and the reception side.
In FIG. 11, the primary voltage is supplied to the transformer 115 via the resistor 114 from the power source 113 of the drive circuit 112 during wave transmission. The secondary voltage boosted by the transformer 115 is supplied to the ultrasonic sensor 111 as a transmission drive voltage, and ultrasonic waves are transmitted from the ultrasonic sensor 111 to the object 118. At the time of wave reception, an ultrasonic wave reflected from the object 118 is received by the ultrasonic sensor 111, and the received voltage is output from the output terminal 117.

  In the configuration shown in FIG. 11, an ultrasonic wave is transmitted for a predetermined time at the anti-resonance frequency fa of one ultrasonic sensor 111, and the same ultrasonic sensor 111 is used as a receiver at the next timing. However, at the time of wave transmission, the drive circuit 112 drives the piezoelectric vibrator of the ultrasonic sensor 111 at an anti-resonance frequency with a high input impedance, so a high drive voltage is required, and a step-up transformer 115 is required. .

  The diode circuit D indicated by 116 in FIG. 11 acts as a low impedance due to conduction when a large drive voltage is applied from the drive circuit 112 to the ultrasonic sensor 111 during transmission, and acts on the ultrasonic sensor 101 during reception. When a minute voltage is induced by the reflected ultrasonic wave, the impedance change circuit acts as a high impedance due to non-conduction. The first auto circuit D is a technique often used for detecting a minute voltage using the nonlinearity of a diode.

There is an ultrasonic sensor technique that eliminates the disadvantages of the configurations shown in FIGS. 10 and 11, does not require a step-up transformer, and enables transmission / reception of ultrasonic waves with a single ultrasonic sensor. It is disclosed in Patent Document 1.
FIG. 12 is a perspective view showing the structure of a symmetrical three-terminal piezoelectric vibrator for an ultrasonic sensor disclosed in Patent Document 1, and FIG. 13 is an electrical diagram for explaining the operating principle of the ultrasonic sensor. It is a figure which shows an equivalent circuit.

  In FIG. 12, a symmetrical three-terminal piezoelectric vibrator 119 used for an ultrasonic sensor has electrodes 121, 122, and 124 formed on both surfaces, and a piezoelectric plate 123 polarized in the thickness direction has substantially the same outer dimensions. The metal plate 125 is joined. The electrodes 121 and 122 on the surface are divided into two equal parts by the electrode gap 120 into the electrode 121 and the electrode 122, and the electrode 124 on the bonding surface side of the piezoelectric plate 123 with the metal plate 125 is mechanically and electrically bonded to the metal plate 125. Common terminal 127. An input terminal 126 and an output terminal 122 are provided on the electrodes 121 and 122 whose surfaces are divided.

  The piezoelectric vibrator 119 shown in FIG. 12 is called a symmetric three-terminal piezoelectric vibrator because it has a common terminal 127, an input terminal 126, and an output terminal 128, and its electrical equivalent circuit 131 is shown in FIG. An inductance L indicated by 132 and a capacitance C indicated by 133 and a resistance R indicated by 134 connected in series between both terminal pairs of the input terminal pair 137-138 and the output terminal pair 139-140, and the input terminal pair 137. -138 and the output terminal pair 139-140 are represented by electrostatic capacity Cd indicated by 135 and Cd 'indicated by 136, which are substantially equal braking capacities connected in parallel, respectively.

  In the symmetric three-terminal piezoelectric vibrator 131 represented by the equivalent circuit shown in FIG. 13, the resonance frequency when the output terminal pair 139-140 is opened and the anti-resonance frequency when the output terminal pair 139-140 is short-circuited. Therefore, the ultrasonic sensor is driven for a predetermined time at the resonance frequency fr when the output terminal pair 139-140 is opened to transmit ultrasonic waves into the air, and at the next timing, the output terminal pair 139- 140 is short-circuited to receive the ultrasonic wave reflected from the object.

JP-A-9-166659 (FIGS. 2 and 4)

However, the conventional ultrasonic sensor using the symmetrical three-terminal piezoelectric vibrator 131 shown in FIGS. 12 and 13 can transmit and receive ultrasonic waves without using the step-up transformer 115 as shown in FIG. However, as shown in FIGS. 12 and 13, it is necessary to design the piezoelectric vibrator 119 in a symmetrical three-terminal structure in advance. That is, one of the drive electrodes of the piezoelectric vibrator 119 needs to be divided into two equal parts to have a three-terminal configuration.
Therefore, it is impossible to directly use a two-terminal piezoelectric vibrator designed to use different piezoelectric vibrators for transmission and reception.

  In the present invention, a two-terminal piezoelectric vibrator designed to use different piezoelectric vibrators for transmitting and receiving waves can be used as it is, and a single step-up piezoelectric vibrator can be used. An object of the present invention is to provide an ultrasonic sensor capable of transmitting and receiving ultrasonic waves without using a sensor.

  In order to solve the above problems and achieve the object of the present invention, the ultrasonic sensor of the present invention includes a second capacitance (in a series circuit including an inductance L, a first capacitance (C), and a resistor R). In an ultrasonic sensor using a two-terminal piezoelectric vibrator represented by an electrically equivalent circuit in which Cd) are connected in parallel, in order to increase the resonance frequency fr of the ultrasonic sensor, the ultrasonic sensor is connected in series. A third capacitance (series capacitance Cs) to be connected and a fourth capacitance (parallel capacitance Cp) connected in parallel to the ultrasonic sensor in order to lower the anti-resonance frequency fa of the ultrasonic sensor. ), The ultrasonic sensor is driven with only the series capacitance Cs out of the series capacitance Cs and the parallel capacitance Cp, and the ultrasonic wave is transmitted to the air at a transmission timing for a predetermined time. Reception timing Is to receives ultrasonic waves reflected from the object while adding only parallel capacitance Cp.

The ultrasonic sensor according to the present invention includes a switching unit that simultaneously switches the series connection of the series capacitor Cs or the parallel connection of the parallel capacitor Cp to the ultrasonic sensor, and a control unit that controls switching of the switching unit. It is to be prepared.
In the ultrasonic sensor according to the present invention, the switching timing of the switching unit controlled by the control unit can be arbitrarily adjusted according to the ultrasonic wave transmission or reception state.

  According to the present invention, it is possible to use both of the two-terminal piezoelectric vibrators designed to use different piezoelectric vibrators for transmission and reception, and with one piezoelectric vibrator, An ultrasonic sensor capable of transmitting and receiving ultrasonic waves without using a step-up transformer can be provided.

  Therefore, for an ultrasonic sensor of the same specification, the degree of freedom of whether it is configured as a separate type of transmission / reception type or according to the application, such as performance, dimensions, cost, etc. expands, and as a result, It is possible to increase the production volume of ultrasonic sensors of the same specification, and it is possible to reduce the cost due to the mass production effect.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.
The resonance frequency fr and anti-resonance of the two-terminal piezoelectric vibrator 1 using the two-terminal piezoelectric vibrator electric equivalent circuit 1 similar to the two-terminal piezoelectric vibrator electric equivalent circuit 81 shown in FIG. A method for controlling the frequency fa will be described.

FIG. 1 is an equivalent circuit diagram in the case where a series capacitance Cs is connected in series to a two-terminal piezoelectric vibrator, and FIG. 2 is a parallel capacitance Cp connected in parallel to the two-terminal piezoelectric vibrator. It is an equivalent circuit diagram in the case.
In FIG. 1, an equivalent circuit 1 of a two-terminal piezoelectric vibrator is connected between a terminal 7 and a series circuit including an inductor L indicated by 2, a capacitance C indicated by 3, and a resistor R indicated by 4, This is represented by an electrical equivalent circuit in which an electrostatic capacity Cd indicated by 5 is connected in parallel as a braking capacity unique to the vibrator.
In addition, an electrostatic capacity Cs indicated by 6 is connected in series between one of the terminals 7 and 8 of the equivalent circuit 1 of the two-terminal piezoelectric vibrator, and the other of the terminals 7 and 8 is Commonly connected.

  On the other hand, as shown in FIG. 2, a circuit in which a parallel capacitor Cp indicated by 11 is connected in parallel between terminals 7 of the equivalent circuit 1 of the two-terminal piezoelectric vibrator is also conceivable. Here, the series capacitance Cs shown in FIG. 1 and the parallel capacitance Cp shown in FIG. 2 have a configuration in which the capacitance can be varied.

  In general, the resonance frequency fr is given by a frequency at which the input impedance of the two-terminal impedance element such as the two-terminal piezoelectric vibrator 1 is minimized, and the anti-resonance frequency fa is the frequency of the two-terminal impedance element such as the two-terminal piezoelectric vibrator 1. It is known that the input impedance is given at a frequency that maximizes. Accordingly, by calculating the frequency characteristics of the input impedance using the equivalent circuits of FIGS. 1 and 2, the series capacitance Cs is added to the two-terminal piezoelectric vibrator 1, and the two-terminal piezoelectric is obtained. The resonance frequency fr ′ and the anti-resonance frequency fa ′ when the parallel capacitance Cp is added to the vibrator 1 can be obtained by calculation.

  Table 1 shows the resonance frequency fr, the anti-resonance frequency fa, and the electrical equivalent circuit constants of the ultrasonic sensor using the two-terminal piezoelectric vibrator used for the calculation. As shown in Table 1, the resonance frequency fr is 40.0 “kHz”, the anti-resonance frequency fa is 41.0 “kHz”, the electrical equivalent circuit constant L is 317 [mH], and C is 50 [pF]. ], R is 265 [Ω], and Cd is 1000 [pF].

FIG. 3 is a diagram illustrating the frequency characteristics of the input impedance calculated by changing the series capacitance Cs.
FIG. 3 shows the frequency characteristics of the input impedance calculated by changing the series capacitance Cs shown in 6 using the equivalent circuit constant shown in Table 1 as the electrical equivalent circuit constant of the ultrasonic sensor in FIG. FIG. 6 is a diagram showing how the resonance frequency fr indicated by 32 and the anti-resonance frequency fa indicated by 31 change.

  From FIG. 3, when the value of the series capacitance Cs decreases in the order of 1 [F], 5000 [pF], 3000 [pF], 1000 [pF], 500 [pF], and 200 [pF], the resonance frequency fr becomes smaller. 40.0 "kHz", 40.1 "kHz", 40.25 "kHz", 40.5 "kHz", 40.7 "kHz", 40.8 "kHz" in this order, It can be seen that the anti-resonance frequency fa has hardly changed from 41.0 “kHz”.

  The vertical axis in FIG. 3 indicates the absolute value of the impedance, and the resonance frequency fr is 40.0 “kHz”, 40.1 “kHz”, 40.25 “kHz”, 40.5 “kHz”, 40.7 As the frequency increases in the order of “kHz” and 40.8 “kHz”, the impedance at resonance is 400 [Ω], 500 [Ω], 700 [Ω], 1000 [Ω], 3000 [Ω], and 9000 [Ω]. It becomes larger in order. In addition, it turns out that the impedance of the antiresonance frequency fa has hardly changed from 80000 [Ω].

FIG. 4 is a diagram illustrating the frequency characteristics of the input impedance calculated by changing the parallel capacitance Cp.
FIG. 4 shows the frequency characteristics of the input impedance calculated by changing the parallel capacitance Cp using the equivalent circuit constant shown in Table 1 as the electrical equivalent circuit constant of the ultrasonic sensor in FIG. It is a figure which can know a mode that a frequency and an antiresonance frequency change.

  From FIG. 4, when the value of the parallel capacitance Cp increases in the order of 1000 [pF], 1200 [pF], 1500 [pF], 2000 [pF], 3000 [pF], and 5000 [pF], the anti-resonance frequency fa Decreases in the order of 41.0 "kHz", 40.8 "kHz", 40.7 "kHz", 40.5 "kHz", 40.7 "kHz", 40.25 "kHz". It can be seen that the resonance frequency fr has hardly changed from 40.0 “kHz”.

  Further, as can be seen from FIG. 4, the anti-resonance frequency fa is 41.0 “kHz”, 40.8 “kHz”, 40.7 “kHz”, 40.5 “kHz”, 40.7 “kHz”, 40 The impedance at the time of anti-resonance decreases in the order of 80000 [Ω], 70000 [Ω], 50000 [Ω], 20000 [Ω], 8000 [Ω], and 4000 [Ω] as the frequency decreases in the order of .25 “kHz”. ing. It can be seen that the impedance of the resonance frequency fr has hardly changed from 300 [Ω].

  As described above, as can be seen from FIG. 3, the resonance frequency fr is changed to be higher than the original value by adding the series capacitance Cs to the two-terminal piezoelectric vibrator 1 shown in FIG. Can do. Then, as can be seen from FIG. 4, the antiresonance frequency fa is lowered from the original value by adding the parallel capacitance Cp to the two-terminal piezoelectric vibrator 1 shown in FIG. Can be changed.

  In the two-terminal piezoelectric vibrator 1, when the difference frequency between the resonance frequency fr and the antiresonance frequency fa is Δf, the value of Δf / fr is the magnitude of the electromechanical conversion efficiency of the piezoelectric vibrator. 3, the resonance frequency fr is increased or decreased by the resonance frequency fr increasing and changing means indicated by 32 in FIG. 3 and the anti-resonance frequency fa decreasing and changing means indicated by 41 in FIG. 4. Even in this case, in order to improve the characteristics as a piezoelectric vibrator as much as possible, it is important to keep the value of Δf / fr as large as possible.

  That is, from FIG. 3 and FIG. 4, in order to maximize the value of Δf / fr, the adjusted resonance frequencies fr ′ and fa ′ are opposite to the resonance frequency fr as shown in the following equation (3). It can be seen that the average value of the resonance frequency fa is good.

  Therefore, the procedure for obtaining the optimum values of the series capacitance Cs and the parallel capacitance Cp in order to maximize the value of Δf / fr is shown below. The optimum values of the series capacitance Cs and the parallel capacitance Cp need to be set in advance for the two-terminal piezoelectric vibrator 1 shown in FIG. Since this setting is unique to each two-terminal piezoelectric vibrator 1, it is necessary to perform this setting for each two-terminal piezoelectric vibrator 1.

  The same setting is possible for the two-terminal piezoelectric vibrator 1 having the common resonance frequency fr, anti-resonance frequency fa, and electrical equivalent circuit constant shown in Table 1. This setting can be performed, for example, at the time of designing, manufacturing, and adjustment.

  First, as procedure 1, the electrical equivalent circuit constants L, C, R, and Cd of the two-terminal piezoelectric vibrator 1 are obtained using an impedance analyzer or the like. At this time, the resonance frequency fr and the anti-resonance frequency fa are also obtained at the same time.

  Next, as the procedure 2, the adjusted resonance frequency fr ′ (= the adjusted anti-resonance frequency fa ′) is obtained by the equation (3). Then, as step 3, the constant obtained in step 1 is substituted into the circuit of FIG. 1, the frequency characteristic of the input impedance is calculated by changing the capacitance of the series capacitance Cs, and the frequency at which the input impedance is minimized is adjusted. The value of the series capacitance Cs when it becomes equal to the resonance frequency fr ′ is obtained.

Finally, as step 4, the constant obtained in step 1 is substituted into the circuit of FIG. 2, the frequency characteristics of the input impedance are calculated by changing the capacitance of the parallel capacitance Cp, and the frequency at which the input impedance becomes maximum is adjusted. The value of the parallel capacitance Cp when it becomes equal to the later anti-resonance frequency fa ′ is obtained.
The ultrasonic sensor is configured such that the series capacitance Cs and the parallel capacitance Cp with the capacitance set in this way are connected in series or in parallel to the two-terminal piezoelectric vibrator 1 shown in FIG. .

FIG. 5 is a circuit diagram showing a configuration example of the ultrasonic sensor according to the embodiment of the present invention.
In FIG. 5, it has one ultrasonic sensor 51 that serves both as the transmission side and the reception side, and a drive circuit 52 for the ultrasonic sensor 51 at the time of transmission and reception. The ultrasonic sensor 51 has the same configuration as the two-terminal piezoelectric vibrator 1 shown in FIG.

  The drive circuit 52 includes a power supply 55 that supplies a drive voltage to the ultrasonic sensor 51 via an internal resistance Ro of the power supply indicated by 56, and a series electrostatic capacitance indicated by 6 that is selectively connected in series to the ultrasonic sensor 51. A capacitance Cs, a parallel capacitance Cp indicated by 11 that is selectively connected in parallel to the ultrasonic sensor 51, and a series capacitance Cs for the ultrasonic sensor 51 are switched between a series connection or a short circuit 53. And a switch SW2 indicated by 54 for switching the parallel capacitance Cp to parallel connection or release with respect to the ultrasonic sensor 51.

  In addition, the drive circuit 52 includes a switching control unit 57 that controls switching between the switch SW1 and the switch SW2, a diode circuit D indicated by 58 in which two diodes are connected in opposite directions, and impedance changes according to an applied voltage, And an output terminal 59.

  In FIG. 5, at the time of wave transmission, the drive circuit 52 controls the switching to the open state by turning off both the switches SW1 and SW2 based on the switching control signal from the switching control unit. As a result, the series capacitance Cs is connected in series to the ultrasonic sensor 51 and the parallel connection of the parallel capacitance Cp is opened to the ultrasonic sensor 51.

As a result, the series capacitance Cs is connected in series to the ultrasonic sensor 51 so that the resonance frequency fr shown in FIG. 3 is changed to be higher than the original value. The sound wave 60 can be transmitted.
At this time, a transmission drive voltage is supplied from the power supply 55 to the ultrasonic sensor 51 via the resistor 56, and the ultrasonic wave 60 is transmitted from the ultrasonic sensor 51 to the object 59 at the adjusted resonance frequency fr ′.

  At the time of wave reception, the drive circuit 52 controls both of the switches SW1 and SW2 to be turned on based on a switching control signal from the switching control unit 57 to switch to a short circuit state. As a result, the series connection of the series capacitance Cs is short-circuited to the ultrasonic sensor 51 and the parallel capacitance Cp is connected to the ultrasonic sensor 51 in parallel.

Accordingly, the parallel capacitance Cp is connected in parallel to the ultrasonic sensor 51 to increase the combined capacitance, thereby adjusting the antiresonance frequency fa shown in FIG. 4 to be lower than the original value. The ultrasonic wave 60 can be received at the anti-resonance frequency fa ′.
At this time, the reflected wave of the ultrasonic wave 60 from the object 61 is received at the anti-resonance frequency fa ′ adjusted by the ultrasonic sensor 51, and the received wave voltage is output from the output terminal 59.

  Note that the diode circuit D indicated by 58 is conductive when a large drive voltage for transmitting the ultrasonic wave 60 at the adjusted resonance frequency fr ′ is applied from the drive circuit 52 to the ultrasonic sensor 51 during transmission. When a minute voltage is induced by the ultrasonic wave 60 reflected by the adjusted anti-resonance frequency fa ′ in the ultrasonic sensor 51 at the time of reception, it acts as a high impedance due to non-conduction. Improve voltage detection.

  FIG. 6 is a time chart for explaining the operation of the ultrasonic sensor of the present invention. FIG. 6A shows the switching state of the switch SW1 and the switch SW2, FIG. 6B shows detection signal control during reception, and FIG. 6D is a detection signal 1 without detection signal control, and FIG. 6E is a detection signal 2 with detection signal control. Hereinafter, the operation of the ultrasonic sensor of the present invention will be described with reference to FIG.

First, an operation example at the time of ultrasonic transmission will be described.
At the time of transmission, at time t1, based on a switching control signal from the switching control unit 57 of the drive circuit 52, the switching state of the switches SW1 and SW2 shown in FIG. 6A is opened (OFF). As a result, the series capacitance Cs is connected in series to the ultrasonic sensor 51, and the parallel capacitance Cp is opened.

  In this state, an AC pulse of the transmission drive voltage shown in FIG. 6C having the adjusted resonance frequency fr ′ given by the equation (3) from the power supply 55 of the drive circuit 52 is applied to the ultrasonic sensor 51. The adjusted resonance frequency fr ′ is the resonance frequency of the ultrasonic sensor when the series capacitance Cs is added in series to the ultrasonic sensor 51. Therefore, the resonance frequency fr indicated by 32 shown in FIG. Efficient excitation can be performed with an appropriate transmission drive voltage of the adjusted resonance frequency fr ′ that is changed so as to increase.

Next, an operation example when receiving ultrasonic waves will be described.
At time t2 when the transmission of the ultrasonic wave is completed, the switches SW1 and SW2 shown in FIG. 6A are short-circuited (ON) based on the switching control signal from the switching control unit 54 of the drive circuit 52. As a result, the series capacitance Cs is short-circuited, and the parallel capacitance Cp is added in parallel to the ultrasonic sensor 51.

  In this state, the arrival of the ultrasonic wave 60 reflected on the object 59 is awaited. At this time, the adjusted anti-resonance frequency fa ′ (= fr ′) is the anti-resonance frequency of the ultrasonic sensor when the parallel capacitance Cp is added in parallel to the ultrasonic sensor 51, and therefore 41 shown in FIG. The ultrasonic sensor 51 has the sensitivity of the anti-resonance frequency fa ′ adjusted so that the anti-resonance frequency fa shown in FIG. Can be received well.

  Here, in the detection signal 1 without detection signal control shown in FIG. 6D, noise due to the influence of the transmission drive voltage shown in FIG. 6C and noise due to reverberation vibration are detected after time t2. Next, the main signal by the ultrasonic wave directly reflected on the object is detected and further transmitted, and the signal by the ultrasonic wave reflected by the object behind is detected. Therefore, the detection signal control pulse shown in FIG. 6B rises at time t3 delayed by time Td from time t2 and falls at time t4, thereby detecting only the main signal from the ultrasonic wave reflected from the target object. can do.

  The detection signal control at the time of reception shown in FIG. 6B, the detection signal 1 without detection signal control shown in FIG. 6D, and the detection signal 2 with detection signal control shown in FIG. 6E are provided at the subsequent stage of the output terminal 59 (58). The signal is controlled and detected in a signal control circuit (not shown) provided.

  The time Td from the completion of the transmission at time t2 to the rise of the detection signal control pulse shown in FIG. 6B is included in the detection signal 1 without detection signal control shown in FIG. The output voltage due to the reverberation of the signal is sufficiently attenuated and may be set variably at a time that does not affect the received signal as noise. As a result, as shown in the detection signal 2 with detection signal control shown in FIG. 6E, the necessary received signal can be detected.

  The width Tp of the AC pulse shown in FIG. 6A is determined by how many cycles are included in the pulse width Tp at the frequency of the ultrasonic wave to be transmitted, and the number of cycles is determined by the range of the distance of the object to be detected. It is done. That is, when the pulse width Tp is increased and the number of cycles in the pulse width Tp is increased, the energy of the transmitted ultrasonic waves increases, the received signal voltage also rises, and is almost saturated at a predetermined number of cycles. Increasing the number of cycles increases the AC pulse width Tp, making it difficult to detect short distances. Usually, the cycle number is set from 10 cycles to 20 cycles. However, in the case of detection at a short distance, the attenuation of ultrasonic waves is small, and therefore, several cycles to 5 cycles are suitable.

  Further, the period T for transmitting the ultrasonic waves shown in FIG. 6 can be arbitrarily set according to the distance range to be detected, the moving speed of the object to be detected, and the like. In the above-described embodiment, only the example in which the switch SW1 and the switch SW2 are turned on and off at the same time is shown. However, the switch is not limited to the simultaneous switching, and the transition between the transmission state and the reception state is not necessarily performed. Some marginal period may be included in a range that does not affect the operation.

  Needless to say, the configuration of the present invention is not limited to the above-described embodiment, and the configuration can be changed as appropriate within the scope of the present invention.

It is a figure which shows the equivalent circuit at the time of connecting the serial electrostatic capacitance Cs in series with a 2 terminal type piezoelectric vibrator. It is a figure which shows the equivalent circuit at the time of connecting parallel electrostatic capacitance Cp in parallel with a 2 terminal type piezoelectric vibrator. It is a figure which shows the frequency characteristic of the input impedance with respect to the series electrostatic capacitance Cs. It is a figure which shows the frequency characteristic of the input impedance with respect to the parallel electrostatic capacitance Cp. It is a circuit diagram showing an example of composition of an ultrasonic sensor by an embodiment of the invention. 6A and 6B are time charts for explaining the operation of the ultrasonic sensor of the present invention, in which FIG. 6A shows the switching state of the switches SW1 and SW2, FIG. 6B shows detection signal control during reception, and FIG. 6C shows drive voltage during transmission. 6D shows a detection signal 1 without detection signal control, and FIG. 6E shows a detection signal 2 with detection signal control. It is a perspective view which shows the structure of a two-terminal type piezoelectric vibrator. It is a figure which shows the electrical equivalent circuit of a two-terminal type piezoelectric vibrator. It is a figure which shows the frequency characteristic example of the input impedance of a two-terminal type piezoelectric vibrator. It is a figure which shows the circuit of the conventional ultrasonic sensor. It is a figure which shows the circuit of the other conventional ultrasonic sensor. It is a perspective view which shows the structure of a symmetrical 3 terminal type piezoelectric vibrator. It is a figure which shows the electrical equivalent circuit of a symmetrical 3 terminal type piezoelectric vibrator.

Explanation of symbols

6 ... Series capacitance Cs, 11 ... Parallel capacitance Cp, 51 ... Ultrasonic sensor, 52 ... Drive circuit, 53 ... Switch SW1, 54 ... Switch SW2, 55 ... Power source, 56 ... Internal resistance of power source, 57 ... Switching control unit, 58 ... diode circuit D, 59 ... output terminal, 60 ... ultrasonic wave, 61 ... object

Claims (3)

In an ultrasonic sensor using a two-terminal piezoelectric vibrator represented by an electrical equivalent circuit in which a second capacitance is connected in parallel to a series circuit including an inductance, a first capacitance, and a resistor,
A third capacitance connected in series with the ultrasonic sensor to increase the resonant frequency of the ultrasonic sensor;
A fourth capacitance connected in parallel to the ultrasonic sensor to reduce the anti-resonance frequency of the ultrasonic sensor;
With
Among the third capacitance and the fourth capacitance, the ultrasonic sensor is driven in a state where only the third capacitance is added, and the ultrasonic wave is transmitted to the air for a certain time. The ultrasonic sensor receives the ultrasonic wave reflected from the object in a state where only the fourth capacitance is added at the next reception timing. A switching unit that simultaneously switches the third capacitance in series connection with the ultrasonic sensor or the fourth capacitance in parallel connection;
A control unit for controlling switching of the switching unit;
The ultrasonic sensor according to claim 1, further comprising:   The ultrasonic sensor according to claim 2, wherein the switching timing of the switching unit controlled by the control unit can be arbitrarily adjusted according to an ultrasonic transmission or reception state.

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