JP2013250169A - Ultrasonic sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress reverberation vibration of a vibrator of an ultrasonic sensor in spite of temperature variation of the vibrator and individual differences, and further to lower the manufacturing cost.SOLUTION: An ultrasonic sensor 1 includes a reverberation suppression circuit 4, which includes a transformer 41 boosting and outputting voltage, input to a primary side, from a secondary side to a vibrator 2, and a variable capacitance circuit 42 connected to the primary side of the transformer 41 in parallel. Even if the vibrator 2 varies in natural frequency owing to temperature variation or has variance in natural frequency due to individual differences, the resonance frequency of the reverberation suppression circuit 4 can be adjusted substantially to the natural frequency of the vibrator 2 by varying the electrostatic capacitance of the variable capacitance circuit 42. Reverberation vibration by the vibrator 2 can, therefore, be suppressed by utilizing a resonance phenomenon. Further, the variable capacitance circuit 42 is provided on the primary side of the transformer 41 which is lower in voltage than the secondary side, and a low-breakdown-voltage capacitor is thereby usable for the variable capacitance circuit 42.

Description

  The present invention relates to an ultrasonic sensor that transmits and receives ultrasonic waves.

  Conventionally, an ultrasonic wave is transmitted by driving a vibrator, and an ultrasonic wave reflected by an object (hereinafter referred to as a reflected wave) is received by using the vibrator. 2. Description of the Related Art Ultrasonic sensors that measure time) and measure the distance to an object based on the measurement time are known. In this ultrasonic sensor, when an electrical transmission signal is input to the vibrator, the vibrator vibrates based on the transmission signal, and an ultrasonic wave is oscillated. On the other hand, when receiving the reflected wave, the vibrator vibrates due to the reflected wave, and outputs an electrical reception signal based on the vibration due to the piezoelectric effect.

  In this ultrasonic sensor, transmission of ultrasonic waves and reception of reflected waves are performed using a common vibrator. Therefore, as shown in FIG. 11, during transmission of ultrasonic waves based on the transmission signal S1, a reception signal S2 corresponding to the transmission signal S1 is output from the vibrator. Even after the transmission of the ultrasonic wave, the reception signal S3 is output due to the reverberation vibration of the vibrator. Therefore, when the object is far from the ultrasonic sensor and the reception signal S4 due to the reflected wave is output after the convergence of the reverberation vibration, the reception signal S4 can be detected and the distance to the object is accurately measured. it can. On the other hand, it is assumed that the object is near the ultrasonic sensor and the reflected wave is received during the reverberation vibration convergence time T1 until the reverberation vibration converges. In that case, it is difficult to distinguish whether the reception signal output from the vibrator is the reception signal S4 due to the reflected wave or the reception signal S3 due to the reverberation vibration, It is difficult to accurately measure the distance.

  Therefore, an ultrasonic sensor having a configuration that suppresses reverberation vibration is known in order to accurately measure the distance to the object even when the object is at a short distance. This ultrasonic sensor includes a transformer that boosts the power supply voltage input to the primary side and outputs the boosted voltage from the secondary side to the vibrator. This ultrasonic sensor generally utilizes the fact that the frequency of reverberation vibration is approximately equal to the mechanical natural frequency (self-resonance frequency) of the vibrator, and the electrical resonance frequency of the transformer is the natural frequency of the vibrator. Are set to be the same. Therefore, the energy of the reverberation vibration is absorbed by the transformer using the resonance phenomenon, and the reverberation vibration is suppressed.

  However, in general, the vibrator has temperature characteristics, and as the temperature rises, the capacitance decreases and the natural frequency also changes. For this reason, in the ultrasonic sensor, when the temperature changes, a difference occurs between the natural frequency of the vibrator (= frequency of reverberation vibration) and the resonance frequency of the transformer. Therefore, the energy of the reverberation vibration cannot be sufficiently absorbed by the transformer, and the effect of suppressing the reverberation vibration may be reduced.

  Therefore, in the ultrasonic sensor, even if the natural frequency fluctuates due to a temperature change, a capacitor for causing the electrical resonance frequency to follow the fluctuation and eliminating the difference is connected in parallel to the primary side of the transformer. Those are known (for example, see Patent Document 1). In this ultrasonic sensor, the capacitor has a temperature characteristic, and the capacitance of the capacitor changes so that the resonance frequency follows the natural frequency according to the temperature. According to this configuration, reverberation vibration can be suppressed regardless of the temperature change of the vibrator. In addition, the capacitor for suppressing the reverberation vibration is provided on the primary side having a lower voltage than the secondary side of the transformer, and may be a low withstand voltage capacitor, thereby reducing the manufacturing cost.

JP 2005-83935 A

  By the way, vibrators have individual differences in mass, shape, and piezoelectric elements constituting the vibrator. Due to the individual differences, capacitances vary between vibrators, and the natural frequency of vibrators varies. is there. Therefore, in the ultrasonic sensor as described in Patent Document 1, due to the variation, the natural frequency of the vibrator and the preset resonance frequency of the reverberation suppression circuit including a transformer and a capacitor are obtained. A difference may occur between them, and the suppression effect of reverberation vibration may decrease.

  The present invention has been made to solve the problem, and provides an ultrasonic sensor capable of suppressing reverberation vibration of a vibrator regardless of temperature changes and individual differences of the vibrator and reducing manufacturing costs. The purpose is to do.

  In order to achieve the above object, an ultrasonic sensor of the present invention boosts a voltage input from a power source to a primary side and outputs an ultrasonic voltage from the secondary side to a vibrator. The sensor includes a variable capacitance circuit connected in parallel to the primary side of the transformer.

  By shifting the level of the voltage applied to the variable capacitance circuit and controlling the amplitude of the voltage, the voltage is adjusted to be within a positive range with the peak value of the original voltage as the upper limit. It is desirable to further include a voltage level shift circuit that performs this.

  A resistor for suppressing reverberation vibration by the vibrator connected in parallel to the vibrator is further provided, and the impedance of the voltage level shift circuit is such that its second-order equivalent impedance is larger than the resistance value of the resistor. It is desirable to be set to.

  It is desirable that the impedance of the voltage level shift circuit is also used as a resistor for suppressing reverberation vibration by the vibrator.

  The variable capacitance circuit is preferably composed of a GIC (Generated Immittance Converter) circuit.

  The variable capacitance circuit is preferably composed of a plurality of capacitors connected in parallel to each other and a plurality of switches for turning on and off the plurality of capacitors.

  According to the present invention, reverberation vibration of a vibrator can be suppressed regardless of temperature changes and individual differences of the vibrator, and the manufacturing cost can be reduced.

The circuit block diagram of the ultrasonic sensor which concerns on one Embodiment of this invention. The block diagram of the variable capacitance circuit of the said ultrasonic sensor. The one-side logarithm graph which shows the time change of the peak-to-peak voltage value of the received signal output from a vibrator | oscillator at the time of the ultrasonic transmission in the said ultrasonic sensor. The block diagram of the variable capacitance circuit of the ultrasonic sensor which concerns on the 1st modification of the said embodiment. The circuit block diagram of the ultrasonic sensor which concerns on the 2nd modification of the said embodiment. (A) is a signal waveform diagram at point A in FIG. 5, and (b) is a signal waveform diagram at point B in FIG. 5. The circuit block diagram of the ultrasonic sensor which concerns on the 3rd modification of the said embodiment. The equivalent circuit diagram of the ultrasonic sensor. The circuit block diagram of the ultrasonic sensor which concerns on the 4th modification of the said embodiment. The equivalent circuit diagram of the ultrasonic sensor. The figure which shows the time change of the both-ends voltage of the vibrator | oscillator at the time of the ultrasonic transmission in the conventional ultrasonic sensor.

  An ultrasonic sensor according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows the configuration of the ultrasonic sensor of this embodiment. The ultrasonic sensor 1 includes a vibrator 2, transmits ultrasonic waves to the vibrator 2, and transmits ultrasonic waves (hereinafter referred to as reflected waves) reflected by an object among the transmitted ultrasonic waves. 2 is received. The ultrasonic sensor 1 detects the presence of the object by receiving it, and further measures the time required from transmission to reception of the ultrasonic wave, that is, the time of flight of the ultrasonic wave. Measure the distance to the object. The ultrasonic sensor 1 is mounted on a moving body such as a vehicle, for example, detects the presence of an object such as an obstacle around the moving body, and further measures the distance from the moving body to the object.

  In addition to the vibrator 2, the ultrasonic sensor 1 includes a drive circuit 3 that transmits ultrasonic waves to the vibrator 2, a reverberation suppression circuit 4 that suppresses reverberation vibration after ultrasonic transmission by the vibrator 2, vibration And a receiving circuit 5 for detecting a received signal caused by the reflected wave from the received signals output from the child 2. The ultrasonic sensor 1 includes a control circuit 6 that performs drive control of the vibrator 2 by the drive circuit 3, suppression control of reverberation vibration by the reverberation suppression circuit 4, and distance calculation based on the received signal detected by the reception circuit 5. Prepare.

  The vibrator 2 is composed of a piezoelectric element. When a transmission signal is input from the drive circuit 3, the piezoelectric element vibrates by repeatedly expanding and contracting at the frequency of the input transmission signal, and transmits ultrasonic waves having substantially the same frequency as the transmission signal. Further, when the piezoelectric element is vibrated by the reflected wave, an electric reception signal corresponding to the vibration is output to the receiving circuit 5 due to the piezoelectric effect.

  The drive circuit 3 is configured by a switching element that turns on and off the supply of the power supply voltage Vcc from the DC power supply P <b> 1 to the vibrator 2 based on a drive signal input from the control circuit 6. The drive circuit 3 inputs a pulse-shaped transmission signal having the peak value of the power supply voltage Vcc to the vibrator 2 by turning on and off the supply. The frequency of the transmission signal is substantially equal to the drive signal input from the control circuit 6. The switching element is configured by a transistor, an FET, or the like. The power supply voltage Vcc is set to 10 [V], for example.

  The reverberation suppression circuit 4 includes a capacitor component (not shown) of the vibrator 2 itself, a transformer 41, a variable capacitance circuit 42, and a resistor R1, and is disposed between the vibrator 2 and the drive circuit 3. The transformer 41 boosts the voltage as a transmission signal input to the primary side of the transformer 41 and outputs the boosted voltage from the secondary side of the transformer 41 to the vibrator.

  The transformer 41 is constituted by, for example, a single-winding transformer, and the transformer is a shunt winding 41a common to the primary side and the secondary side, and a series winding 41b used only on the secondary side. The transmission signal is input to the shunt winding 41a from the primary side. When the voltage value of the transmission signal is temporarily approximately 0, that is, when the supply of the power supply voltage Vcc is temporarily turned off, the shunt winding 41a has the power supply voltage Vcc due to electromagnetic induction. A substantially equal induced electromotive force (back electromotive force) is generated. Accordingly, when the transmission signal is input to the shunt winding 41a, the voltage across the shunt winding 41a becomes an AC voltage whose peak value is the power supply voltage Vcc.

  Here, it is assumed that the turn ratio of the series winding 41b to the shunt winding 41a is N-1 (N> 1). In this case, the voltage across the series winding 41b is a value obtained by multiplying the voltage across the shunt winding 41a by (N-1), and is output from the shunt winding 41a and the series winding 41b to the secondary side. The total voltage is a value obtained by multiplying the voltage across the shunt winding 41a by N. Therefore, when the transmission signal is input from the primary side to the shunt winding 41a, an AC transmission signal having a peak value of N × Vcc is output from the secondary side to the vibrator 2. Therefore, when the turn ratio N = 5 and the power supply voltage Vcc = 10 [V], the transmission signal output from the secondary side to the vibrator 2 is an AC transmission signal having a peak value of 50 [V]. become. The variable capacitance circuit 42 is connected in parallel with the shunt winding 41a. The resistor R1 is connected to the vibrator 2 in parallel.

The self-inductance of the series winding 41 b and the capacitance of the variable capacitance circuit 42 are set in advance so that the resonance frequency of the reverberation suppression circuit 4 matches the natural frequency of the vibrator 2. By the way, the resonance frequency can be obtained on the basis of the capacitance of the vibrator 2 itself, the self-inductance, and the secondary equivalent capacitance obtained by converting the capacitance on the secondary side of the transformer 41. it can. Therefore, conversely, by determining the value of the resonance frequency, the self-inductance and the second-order equivalent capacitance can be derived so that the resonance frequency becomes the value. Further, when the capacitance of the variable capacitance circuit 42 is C, its secondary equivalent capacitance is expressed as C / N ² (N: turn ratio). Therefore, the electrostatic capacity C can be calculated by performing a reverse calculation based on the derived second-order equivalent electrostatic capacity. The resistor R1 absorbs the energy of reverberation vibration of the vibrator 2, converts it into thermal energy, and releases it.

  The reception circuit 5 extracts a reception signal having substantially the same frequency as the ultrasonic wave transmitted by the transducer 2 from the reception signal output by the transducer 2 as a reception signal due to the reflected wave by a filter function, To detect.

  The control circuit 6 can be configured by a microprocessor or the like, and inputs a drive signal having a preset frequency to the drive circuit 3 when the transducer 2 transmits ultrasonic waves. Although not shown, the control circuit 6 can be connected to a capacitance adjusting device for performing an operation of adjusting the capacitance of the variable capacitance circuit 42 via a connection terminal. The control circuit 6 sends a capacitance information signal indicating the current capacitance of the variable capacitance circuit 42 and a reception signal detected by the reception circuit 5 to the capacitance adjustment device while being connected to the capacitance adjustment device. . The capacity adjustment device displays the waveform of the received reception signal on the monitor, and allows the user to adjust the capacitance of the variable capacitance circuit 42 while checking the reverberation vibration convergence time by viewing the waveform. It is configured. The control circuit 6 adjusts the capacitance of the variable capacitance circuit 42 according to the operation content when the capacitance is adjusted by the user using the capacitance adjusting device.

  FIG. 2 shows the configuration of the variable capacitance circuit 42. The variable capacitance circuit 42 is configured by, for example, a GIC (Generated Immittance Converter) circuit including five impedance elements Z1 to Z5 and two operational amplifiers OP1 and OP2, but is not limited thereto. Since the GIC circuit has a general-purpose configuration, a detailed description of the configuration is omitted.

This GIC circuit, the impedance of the impedance element Z1~Z5 and _Z 1 to Z ₅ each, their combined impedance Z is given by the following equation (1).

Here, the impedance element Z1, a capacitor of the capacitance _{C 1,} the impedance element Z2~Z4 are each resistance of the resistance value _R 2 to R _4, the impedance element Z5 is a variable resistance the resistance value _{R 5} And In that case, the impedance Z is a value represented by the following equation (2).

上記の式（３）に示されるように、可変抵抗の抵抗値Ｒ_５を変えることにより、静電容量Ｃを変更することが可能である。 Then, by substituting “Z = 1 / (jωC)” (C: capacitance of the variable capacitance circuit 42) into the above equation (2), the capacitance C is obtained by the following equation (3). Can do.

As shown in the above formula (3), by changing the resistance R ₅ of the variable resistor, it is possible to change the capacitance C.

  The variable resistor is constituted by a switching element Tr1 made of a MOSFET, and the on-resistance value of the MOSFET changes by increasing or decreasing the input voltage value to the gate of the MOSFET while the MOSFET is operating in the linear region. It uses characteristics. The input voltage value to the gate is controlled by the control circuit 6 (see FIG. 1). The variable resistor may be configured by resistors R2 and R3 connected in parallel to each other and switches SW1 and SW2 for switching on and off the energization of the resistors R2 and R3. Under the control of the control circuit 6, the variable resistance is controlled by the switches SW <b> 1 and SW <b> 2 independently of the energization of the resistors R <b> 2 and R <b> 3, thereby increasing or decreasing the overall resistance. The number of resistors and switches constituting the variable resistor having such a configuration is not limited to two, but may be plural.

  In the present embodiment, the natural frequency of the vibrator 2 fluctuates due to a temperature change, the natural frequency varies depending on the individual difference of the vibrator 2, or the temperature characteristics of the transformer 41. It is assumed that the resonance frequency of the reverberation suppression circuit 4 has changed. In any of these cases, the resonance frequency of the reverberation suppression circuit 4 can be adjusted to be substantially the same as the natural frequency of the vibrator 2 by changing the capacitance of the variable capacitance circuit 42. it can. Therefore, the reverberation vibration by the vibrator 2 can be suppressed using the resonance phenomenon regardless of the temperature change and individual difference of the vibrator 2. As a result, even when the object is at a short distance from the ultrasonic sensor 1, the reception signal output from the vibrator 2 when receiving the reflected wave from the object can be accurately detected. It is possible to accurately measure the distance from the object to the object. In addition, the variable capacitance circuit 42 for suppressing the reverberation vibration is provided on the primary side having a lower voltage than the secondary side of the transformer 41. Therefore, a low withstand voltage element is used in configuring the variable capacitance circuit 42. Therefore, the manufacturing cost can be reduced.

  Further, when the variable capacitance circuit 42 is configured by the above-described GIC circuit, the configuration can be simplified and has no voltage dependency. Moreover, even if the variable capacitance circuit 42 requires a capacitance on the order of nF, only one capacitor is required for the configuration. Therefore, even if the capacitors are externally attached to the ASIC when the electrical circuit in the ultrasonic sensor 1 is made into an ASIC (Application Specific Integrated Circuit), the number of capacitors can be reduced to one. Therefore, the circuit can be reduced in size.

Further, as shown in FIG. 3, the reverberation vibration convergence time T1 of the present embodiment is shorter than the reverberation vibration convergence time T1 ′ when the variable capacitance circuit is provided on the secondary side of the transformer 41 (shown by a broken line). can do. The principle will be described. The vertical axis in the figure indicates the peak-to-peak voltage value Vpp of the reception signal output from the vibrator 2, and the horizontal axis indicates time. The scale on the vertical axis is a logarithmic scale. The peak-to-peak voltage value Vpp is approximately 100 [V] at the time of ultrasonic transmission, and does not instantaneously become 0 [V] due to reverberation vibration even when transmission of the ultrasonic wave is finished. Therefore, by operating the variable capacitance circuit 42, the reverberation vibration can be suppressed and the peak-to-peak voltage value Vpp can be lowered. Here, the variable capacitance circuit 42 can suppress the reverberation vibration only when the input voltage to the variable capacitance circuit 42 is equal to or lower than the drive voltage of the variable capacitance circuit 42, and in this embodiment is equal to or lower than 10 [V]. To do. In this case, if the variable capacitance circuit is on the secondary side of the transformer 41, the received signal is input to the variable capacitance circuit 42 as it is, and therefore when the peak-to-peak voltage value Vpp becomes 10 [V] ( Until t _s ′), reverberation vibration cannot be suppressed using the variable capacitance circuit. As a result, the reverberation vibration convergence time T1 ′ is extended. The reverberation vibration convergence time is the time required for reverberation vibration to converge after ultrasonic transmission based on a transmission signal.

On the other hand, in the present embodiment, since the variable capacitance circuit 42 is on the primary side of the transformer 41, the received signal is not applied to the variable capacitance circuit 42 as it is, but the voltage is 1 by the transformer 41. / N (for example, 1/5) and then input to the variable capacitance circuit 42. Therefore, for example, even when the peak-to-peak voltage value Vpp is N × 10 [V] (for example, 5 × 10 [V]) (t _s ), the peak of the received signal input to the variable capacitance circuit 42. • The two-peak voltage is 10 [V]. Therefore, the variable capacitance circuit 42 can be functioned, and the reverberation vibration convergence time T1 can be shortened.

Hereinafter, various modifications of the above embodiment will be described with reference to the drawings. In each modified example, the same components as those in the above embodiment are denoted by the same reference numerals, and description of the same configuration as that in the above embodiment is omitted.
(First modification)
FIG. 4 shows the configuration of the variable capacitance circuit 42 of the ultrasonic sensor according to the first modification. The variable capacitance circuit 42 includes capacitors C2 to C4 connected in parallel to each other and switching elements Tr2, Tr3, and Tr4 (switches) for turning on and off the energization of these capacitors C2 to C4. The variable capacitance circuit 42 is controlled by the control circuit 6 (see FIG. 1), and the energization of the capacitors C2 to C4 is independently controlled by the switching elements Tr2 to Tr4. Increased or decreased. The number of capacitors and switching elements is not limited to three, but may be plural.

  The capacitances of the capacitors C2 to C4 may be the same as or different from each other. The switching elements Tr2 to Tr4 are connected in series with the capacitors C2 to C4, respectively. Each of the switching elements Tr2 to Tr4 is configured by, for example, an FET, the drain is connected to each of the capacitors C2 to C4, and the source is grounded. Each of the switching elements Tr2 to Tr4 switches on and off of the capacitors C2 to C4 by switching between an on state and an off state in accordance with an on / off switching control signal input from the control circuit 6 to the gate.

  In this modification, the configuration of the variable capacitance circuit 42 can be made simpler than the above-described embodiment, and can be made independent of voltage.

(Second modification)
FIG. 5 shows a configuration of the ultrasonic sensor 1 according to the second modification. The ultrasonic sensor 1 of the present modification further includes a voltage level shift circuit (hereinafter simply referred to as a shift circuit) 43 that shifts the level of a voltage (hereinafter simply referred to as an applied voltage) applied to the variable capacitance circuit 42. The shift circuit 43 is arranged between the power supply system L1 from the drive circuit 3 to the primary side of the transformer 41 and the variable capacitance circuit 42, and one end of the shift circuit 43 is supplied via the AC coupling capacitor C1. The other end is connected to the variable capacitance circuit 42. The shift circuit 43 shifts the level of the voltage applied to the variable capacitance circuit 42 by the power feeding system L1.

  6A shows the signal waveform of the applied voltage before adjustment by the shift circuit 43, and FIG. 6B shows the signal waveform of the applied voltage after adjustment by the shift circuit 43. FIG. As shown in FIG. 6A, the applied voltage before adjustment by the shift circuit 43 is a voltage based on an AC transmission signal whose crest value is substantially equal to the power supply voltage Vcc, and the polarity of the voltage is the ground potential (zero potential). ) To switch between positive and negative. As shown in FIG. 6B, the shift circuit 43 shifts the level of the applied voltage by offsetting the applied voltage (indicated by the alternate long and short dash line) in the positive direction by approximately half the value of the power supply voltage Vcc. To do. The applied voltage whose level has been shifted becomes a pulsating voltage that repeats an increase and a decrease alternately about a value of about half of the power supply voltage Vcc. The shift circuit 43 controls the amplitude of the applied voltage by cutting a range higher than the power supply voltage Vcc and a negative polarity range (0 or less range) in the applied voltage. In this way, the shift circuit 43 adjusts the applied voltage so as to be within the positive polarity range with the power supply voltage Vcc (original voltage) as the upper limit value, thereby protecting the variable capacitance circuit 42.

  In this modification, the voltage applied to the variable capacitance circuit 42 falls within the positive polarity range. Therefore, when the variable capacitance circuit 42 has the same configuration as that of the above-described embodiment (see FIG. 2), when driving the operational amplifiers OP1 and OP2 to which the applied voltage is input, each operational amplifier OP1 and OP2 has a positive power source. There is no need to prepare both a negative power source and a negative power source. Therefore, a single positive power supply is sufficient.

  By the way, the operational amplifiers OP1 and OP2 can be driven when the input voltage to them becomes equal to or lower than the drive voltage. Therefore, by setting the upper limit of the input voltage to the power supply voltage Vcc, the operational amplifiers OP1 and OP2 can be driven using the DC power supply P1. Therefore, the operational amplifiers OP1 and OP2 and the vibrator 2 can share the DC power supply P1.

(Third Modification)
FIG. 7 shows a circuit configuration of the ultrasonic sensor 1 according to the third modification. In this modification, the shift circuit 43 has bias resistors R4 and R5 (an impedance of the shift circuit 43) for offsetting the voltage applied to the variable capacitance circuit. The shift circuit 43 includes clamping diodes D1 and D2 that cut a range higher than the power supply voltage Vcc and a negative polarity range in the offset voltage.

  One end of the bias resistor R4 is connected to the DC power supply P1, and the other end is connected to the input terminal 42a for the applied voltage in the variable capacitance circuit 42. The bias resistor R5 has one end connected to the input terminal 42a and the other end grounded. The resistance values of the bias resistors R4 and R5 are equal to each other. With such a configuration, the power supply voltage Vcc is divided and a voltage approximately half the power supply voltage Vcc is applied to the bias resistor R5, and the capacitor C1 is charged with the voltage. As a result, the voltage applied to the input terminal 42a from the power feeding system L1 via the capacitor C1 is offset in the positive direction by the charging voltage of the capacitor C1, that is, approximately half the voltage of the power supply voltage Vcc. Hereinafter, this offset voltage is simply referred to as an offset voltage.

  The diode D1 has a cathode connected to the DC power supply P1 and an anode connected to the input terminal 42a. The diode D1 becomes conductive when the value of the offset voltage exceeds the power supply voltage Vcc, and lowers the offset voltage to a value substantially equal to the power supply voltage Vcc, thereby cutting a voltage in a range higher than the power supply voltage Vcc. On the other hand, the diode D2 has a cathode connected to the input terminal 42a and an anode grounded. The diode D2 becomes conductive when the value of the offset voltage becomes less than 0 and becomes negative, and raises the offset voltage to substantially 0, thereby cutting the voltage in the negative polarity range. Here, the forward voltage of each diode when the diodes D1 and D2 are turned on is ignored.

FIG. 8 shows an equivalent circuit of the ultrasonic sensor 1 shown in FIG. In this equivalent circuit, a secondary conversion resistor R45 (secondary conversion equivalent impedance) is provided on the secondary side of the transformer 41 in place of the bias resistors R4 and R5. The secondary conversion resistor R45 has a secondary equivalent resistance value obtained by converting the combined resistance value of the bias resistors R4 and R5 on the secondary side. The secondary equivalent resistance value is represented by (R / 2) × N ² where R is the resistance value of each of the bias resistors R4 and R5. The secondary equivalent resistance value (R / 2) × N ² is, for example, 10 times or more the resistance value of the resistor R1 so as to be sufficiently larger than the resistance value of the resistor R1 for reverberation vibration suppression. The resistance values of the bias resistors R4 and R5 are set.

  In the present modification, the bias resistors R4 and R5 of the shift circuit 43 can prevent the reverberation vibration from being absorbed by the resistor R1, and a reduction in the reverberation vibration suppressing effect can be prevented.

(Fourth modification)
FIG. 9 shows a configuration of an ultrasonic sensor 1 according to a third modification. The ultrasonic sensor 1 of the present modification example omits the resistor R1 for reverberation vibration suppression in the configuration of the third modification example (see FIG. 7), and further replaces the bias resistors R4 and R5 with variable resistors R6 and R7 ( (Impedance of the shift circuit 43). The variable resistors R6 and R7 are configured by, for example, a MOSFET, and the on-resistance value of the MOSFET changes by increasing or decreasing the input voltage value to the gate of the MOSFET while the MOSFET is operating in the linear region. Is used. The input voltage value to the gate is controlled by the control circuit 6 (see FIG. 5).

  FIG. 10 shows an equivalent circuit of the ultrasonic sensor 1 shown in FIG. In this equivalent circuit, a secondary conversion variable resistor R67 is provided on the secondary side of the transformer 41 in place of the variable resistors R6 and R7. The secondary conversion variable resistor R67 has a secondary conversion equivalent resistance value obtained by converting the combined resistance value of the variable resistors R6 and R7 on the secondary side. When the resistance values of the variable resistors R6 and R7 are changed, the secondary equivalent resistance value also increases or decreases, but within the range in which the resistance can be increased or decreased, the resistor R1 and the secondary equivalent resistor R45 in the third modified example (see FIG. 8). The resistance values of the variable resistors R6 and R7 are set so as to include the combined resistance value of In this way, the variable resistors R6 and R7 are also used as resistors for suppressing reverberation vibration by the vibrator 2 instead of the resistor R1.

  In this modification, the variable resistors R6 and R7 on the primary side lower in voltage than the secondary side of the transformer 41 are also used as reverberation suppression resistors, so that the reverberation suppression resistors are substantially reduced. Is provided on the primary side. Therefore, the variable resistors R6 and R7 can be configured with low withstand voltage elements, and therefore can be built in the ASIC, and the resistance value is adjusted to an appropriate value according to the convergence degree of the reverberation vibration by the vibrator 2. be able to.

  In addition, this invention is not limited to the structure of said embodiment and each modification, A various deformation | transformation is possible according to the intended purpose. For example, the transformer 41 may be a push-pull type transformer or a multi-winding type transformer. Further, the peak value of the transmission signal may be ½ of the power supply voltage Vcc. In this case, even if an ultrasonic wave is transmitted based on the transmission signal and reverberation vibration due to the transmission occurs, the input voltage to the variable capacitance circuit 42 immediately after the occurrence is substantially equal to the power supply voltage Vcc. Therefore, if the drive voltage of the variable capacitance circuit 42 is substantially the same as the power supply voltage Vcc, the variable capacitance circuit 42 can be made to function immediately, and the reverberation vibration convergence time can be further shortened.

  In the above embodiment, the resistor R1 may be omitted. Further, each of the switching elements Tr2 to Tr4 illustrated in FIG. 4 may be configured by an npn transistor, and a collector may be connected to each of the capacitors C2 to C4, and an emitter may be grounded. In that case, each of the switching elements Tr2 to Tr4 may be switched between an on state and an off state in accordance with an on / off switching control signal input from the control circuit 6 to the base.

DESCRIPTION OF SYMBOLS 1 Ultrasonic sensor 2 Vibrator 41 Transformer 42 Variable capacity circuit 43 Voltage level shift circuit R1 Resistance R4, R5, R6, R7 Bypass resistance (impedance of voltage level shift circuit)
C2, C3, C4 Capacitors Tr2, Tr3, Tr4 Switching elements (switches)
P1 DC power supply

Claims (6)

In an ultrasonic sensor including a transformer that boosts a voltage input from a power source to a primary side and outputs the boosted voltage from a secondary side to a vibrator.
An ultrasonic sensor comprising a variable capacitance circuit connected in parallel to the primary side of the transformer.   By shifting the level of the voltage applied to the variable capacitance circuit and controlling the amplitude of the voltage, the voltage is adjusted to be within a positive range with the peak value of the original voltage as the upper limit. The ultrasonic sensor according to claim 1, further comprising a voltage level shift circuit that performs the operation. A resistor for suppressing reverberation vibration by the vibrator connected in parallel to the vibrator;
The ultrasonic sensor according to claim 2, wherein the impedance of the voltage level shift circuit is set such that a second-order equivalent impedance is larger than a resistance value of the resistor.   The ultrasonic sensor according to claim 2, wherein the impedance of the voltage level shift circuit is also used as a resistor for suppressing reverberation vibration by the vibrator.   The ultrasonic sensor according to any one of claims 1 to 4, wherein the variable capacitance circuit is configured by a GIC (Generated Immittance Converter) circuit.   5. The variable capacitance circuit includes a plurality of capacitors connected in parallel to each other and a plurality of switches for turning on and off the plurality of capacitors. The ultrasonic sensor according to claim 1.

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