JP2760079B2 - Ultrasonic sensor - Google Patents

Description

The present invention relates to an ultrasonic sensor that transmits and receives frequency-modulated ultrasonic waves to measure a distance to an object, and the like.
In particular, the present invention relates to an ultrasonic sensor having a structure in which a transmitter and a receiver are provided independently.

<Conventional technology> An ultrasonic sensor detects the distance to an object and the presence or absence of an object using ultrasonic waves.
Recently, the FM-CW method has been proposed.

The ultrasonic sensor based on the FM-CW method generates an ultrasonic wave transmission signal whose frequency continuously changes and transmits the ultrasonic wave to the object, and receives the ultrasonic wave reflected by the object and transmits the ultrasonic wave. Beat frequency f _c defined by the frequency difference between the wave signal and the received signal
, And the distance to the object is detected.

The beat frequency f _c is the propagation distance of the ultrasonic wave 2R, the repetition frequency f _m of the frequency modulation, the propagation speed of the ultrasonic v, when the frequency shift width of the frequency modulation and Delta] f, is given by the following expression.

f _c = (4Rf _m Δf) / v f Here, since f _m and Δf, v are known, the propagation distance 2R of the ultrasonic wave can be calculated by measuring the beat frequency f _c ,
Therefore, the distance to the object can be detected.

FIG. 3 shows a specific example of a conventional ultrasonic sensor based on the FM-CW method, and FIGS. 4 and 5 are time charts of the conventional example. In this time chart, the voltage V or the frequency F is plotted on the vertical axis with respect to the time t on the horizontal axis.

The illustrated ultrasonic sensor includes a transmitter 2 for transmitting an ultrasonic wave to an object 1 and a receiver 3 for receiving a reflected wave from the object 1. Receiver 3
Are arranged independently on the holder 4 and inclined inward as shown in FIG. The transmitter 2 and the receiver 3 are ultrasonic transducers having directivity angles α and β, respectively, and a region S indicated by a diamond in the drawing is a detectable region.

Returning to FIG. 3, the signal generator 5 is connected to the transmitter 2 and driven, and the detector 3 is connected to the detector 3 to receive the received signal b.

The signal generation section 5 is a section that generates a continuous wave of frequency-modulated ultrasonic waves, and includes a reference clock generation circuit 7, a conversion circuit 8, a drive circuit 9, and the like.

The reference clock generation circuit 7 includes an oscillation source such as a crystal oscillator, and appropriately divides the oscillation output to generate a clock signal CK.
And a gate signal g described later. The conversion circuit 8 includes a mirror integrated circuit that inputs a clock CK and converts the signal into a triangular wave voltage signal, and a V / F conversion circuit that converts a triangular wave voltage signal into a frequency. A continuously changing ultrasonic wave signal a (FIG. 4 (1)
(Indicated by a solid line). The drive circuit 9 transmits the transmitted signal a
Is used to drive the transmitter 2 at a constant period.
Is reflected by the object 1 and received by the receiver 3.

In this case, the propagation speed of the ultrasonic wave in the air is defined as v, and the distance of the path from the transmitter 2 to the receiver 3 via the object 1 is defined as the distance.
Assuming 2R, the propagation time ΔT is given by the following equation.

ΔT = 2R / v ‥‥ Therefore, for the transmitted signal a, the received signal b (Fig. 4 (1))
(Indicated by a broken line in the middle) is delayed by the propagation time ΔT. 4 (2) is an enlarged view of the transmission signal a, and FIG. 4 (3) is a signal waveform diagram of the reception signal b. The reception signal b has a crosstalk component d described later. Is on.

Next, the detection circuit section 6 is for detecting the distance to the object 1 based on the frequency difference between the transmitted signal a transmitted from the transmitter 2 and the received signal b received by the receiver 3. so,
In addition to the reference clock generation circuit 7, an amplification circuit 10, a frequency difference detection circuit 11, a low-pass filter 12, a counter 13, a D / A
It comprises a converter 14 and an amplifier 15.

For example, a multiplier is used as the frequency difference detection circuit 11, and by inputting and multiplying the transmission signal a and the reception signal b, a signal having a difference between the frequencies f _a and f _b of the two signals a and b is obtained. And the sum signal. The low-pass filter 12 passes only the difference frequency component as distance information to obtain a beat frequency signal c (shown in FIG. 5 (1)). The frequency of the beat frequency signal c (beat frequency f _c ) is given by the following equation.

f _c = | f _a −f _b | は The counter 13 counts the number of repetitions of the beat frequency signal c for a certain period of time T. The counter 13 specifies the counting time from the reference clock generation circuit 7 to the counter 13. A gate signal g (shown in FIG. 5 (2)) is provided. The D / A converter 14 converts the count data by the counter 13 into an analog quantity, and this analog signal is taken out as an output through the amplifier 15.

<Problems to be Solved by the Invention> In the ultrasonic sensor having the above configuration, when the transmitter 2 is driven by a transmission signal, the transmitter 2
Is transmitted to the receiver 3 via the holder 4. The mechanical vibration (referred to as “crosstalk”), which is the resonance between the transmitter 2 and the holder 4, is received as a crosstalk component d as shown in FIGS. 4 (3) and 5 (1). It is mixed with the wave signal b and the beat frequency signal c. Since the crosstalk component d is caused by mechanical vibration, the frequency is not low and does not become a constant frequency standing wave, but is input to the counter 13 through the low-pass filter 12 in a form included in the beat frequency signal c. You.

Due to the crosstalk component d, the beat frequency signal c
Of the counter 13
Occurs below the threshold level TH. For this reason, a counting error by the counter 13 occurs, and accurate distance measurement becomes difficult.

The present invention has been made in view of the above problem, and an object of the present invention is to provide an ultrasonic sensor capable of accurately measuring a distance by removing a signal component caused by the mechanical vibration.

<Means for Solving the Problems> An ultrasonic sensor according to the present invention includes a receiving circuit for detecting a distance to an object based on a frequency difference between a received signal and a transmitted signal. A first filter circuit for removing a signal component caused by mechanical vibration of a transmitter mixed in a signal, and a second filter circuit for passing only a component of a frequency difference between a transmitted signal and a received signal are provided. It was made.

<Operation> Even if the signal component caused by the mechanical vibration of the transmitter is mixed in the received signal, the signal component caused by the mechanical vibration is removed by the first filter in the detection circuit unit. Since only the frequency difference component between the transmitted signal and the received signal is extracted by the second filter circuit, accurate distance measurement is possible.

<Embodiment> Fig. 1 shows an overall configuration of an ultrasonic sensor according to an embodiment of the present invention, in which a transmitter 2, a receiver 3, and a signal generator are provided.
(5) It comprises a detection circuit section 6.

The configurations and arrangements of the transmitter 2 and the receiver 3 are the same as those of the conventional example shown in FIG. 6, and the circuit configuration of the signal generator 5 is also the same as that of the conventional example shown in FIG. Here, the same reference numerals are given to the corresponding components, and the description thereof will be omitted.

The detection circuit section 6 includes a circuit configuration similar to that of the conventional example shown in FIG. 3, and a high-pass filter 16 is provided between the frequency difference detection circuit 11 and the low-pass filter 12.

The high-pass filter 16 is a circuit for attenuating and removing the crosstalk component d as shown in FIG. 4 (3) mixed in the received signal b. As a result, FIG. As shown, a beat frequency signal c from which the crosstalk component d has been removed is obtained.

Now for example, the beat frequency f _c is, 20 kHz at a short distance point shown in FIG. 6, transmit the signal a such that the 50kHz in the far point
Is generated and the ultrasonic wave is transmitted from the transmitter 2, a received signal b obtained by the receiver 3 includes a crosstalk component d due to mechanical vibration between the transmitter 2 and the holder 4. Are mixed, and the signal waveform is as shown in FIG. 4 (3).

When the reception signal b and the transmission signal a are input to the frequency difference detection circuit 11 of the detection circuit section 6, in addition to the signal of the frequency difference between the two signals a and b required as the distance information, the sum of The signal and the crosstalk component d are output from the frequency difference detection circuit 11.

Therefore, in order to pass only signals having a difference of 20 kHz to 50 kHz required as distance information, the detection circuit unit 6 is configured using a low-pass filter 12 having a cut-off frequency of 50 kHz and a high-pass filter 16 having a cut-off frequency of 20 kHz. Then, the beat frequency signal c from which the crosstalk component d has been removed
(Shown in FIG. 2 (1)) can be obtained.

Since the amplitude of the beat frequency signal c is substantially constant, the situation where the peak value falls below the threshold level TH of the counter 13 does not occur. Therefore, when the number of repetitions of the beat frequency signal c is counted for the count time T specified by the gate signal g (shown in FIG. 2 (2)), a counting error unlike the conventional example does not occur, and accurate counting is performed. Data can be obtained.

Also, the above-described low-pass filter 12 and high-pass filter 16
A kind of band-pass filter is constituted by the combination with the above, so that an ultrasonic sensor having excellent electromagnetic noise resistance can be obtained.

<Effect of the Invention> As described above, the present invention provides a detection circuit for detecting a distance to an object or the like based on a frequency difference between a received signal and a transmitted signal. Since a first filter circuit for removing a signal component caused by mechanical vibration and a second filter circuit for passing only a component of a frequency difference between a transmission signal and a reception signal are provided, even if the reception signal Even if the signal components caused by the mechanical vibration of the transmitter are mixed, the signal components are removed in the detection circuit section, so that accurate distance measurement is possible.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the overall configuration of an ultrasonic sensor according to one embodiment of the present invention, FIG. 2 is an explanatory diagram showing the waveforms of a beat frequency signal and a gate signal, and FIG. FIG. 4 is a block diagram showing the overall configuration, and FIG.
FIG. 5 is a time chart of a conventional example, FIG. 5 is an explanatory diagram showing waveforms of a beat frequency signal and a gate signal in a conventional example, and FIG. 6 is a side view showing a configuration of an ultrasonic sensor. 2 ... transmitter, 3 ... receiver 5 ... signal generator, 6 ... detector circuit 16 ... high-pass filter

Claims (1)

(57) [Claims] 1. A signal generator for generating a continuous wave of frequency-modulated ultrasonic waves, a transmitter for continuously transmitting ultrasonic waves to an object, and a reflected wave from the object for the ultrasonic waves. In an ultrasonic sensor including a receiver that continuously receives waves, and a detection circuit unit that detects a distance to an object based on a frequency difference between a received signal and a transmitted signal, the detection circuit unit includes: A first filter circuit for removing a signal component caused by mechanical vibration of a transmitter mixed in a received signal, a second filter circuit for passing only a component of a frequency difference between the transmitted signal and the received signal, An ultrasonic sensor comprising: