JP2007232659A - Ultrasonic flowmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrasonic flowmeter hardly affected by a drift change of a zero-cross voltage and capable of measuring accurately an ultrasonic wave propagation time. <P>SOLUTION: Ultrasonic elements are provided in a flow passage with a fluid flowing therethrough. A prescribed period of driving signal is impressed to the ultrasonic element in a transmission side. The propagation time of an ultrasonic wave in the flow passage is found by detecting a phase difference between a reception signal output from the ultrasonic element in the transmission side and the driving signal. The reception signal is hardly affected by the drift change of the zero-cross voltage, because the reception signal is a waveform having the prescribed period. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an ultrasonic flowmeter.

JP 2004-239868 A

  2. Description of the Related Art Conventionally, an ultrasonic flowmeter that measures flow velocity using ultrasonic waves is known as a flow measurement device that measures the flow rate of a fluid such as city gas or water. In general, a “propagation time difference method” is used as a measurement principle at that time. This is provided with a pair of ultrasonic elements (ultrasonic transmitting / receiving elements) on the upper and lower sides of the fluid flow direction of the flow path, and switching between ultrasonic transmission / reception alternately, is transmitted from the ultrasonic element on the upper side in the flow direction. The time until the ultrasonic wave reaches the ultrasonic element on the lower side in the flow direction (forward propagation time) and the ultrasonic wave transmitted from the ultrasonic element on the lower side in the flow direction enters the ultrasonic element on the upper side in the flow direction. This is a method of measuring the time to reach (reverse propagation time) and obtaining the average flow velocity and flow rate of the fluid flowing through the flow path from the time difference between the two.

  When measuring the flow rate, first, a drive signal is input to one ultrasonic element to transmit an ultrasonic wave. The ultrasonic wave propagates through the flow path and is received by the other ultrasonic element. When receiving an ultrasonic wave, the ultrasonic element outputs an electrical signal (reception signal), and the reception signal is input to the reception circuit. The reception circuit amplifies the reception signal output from the ultrasonic element and inputs the amplified signal to a comparator such as a comparator. Then, the zero cross voltage and the amplified signal are compared and binarized. Using the obtained binarized signal, a zero cross detection signal is output, and a time (ultrasonic propagation time) from when the drive signal is input until the zero cross detection signal is output is measured.

  The comparator compares the zero cross voltage with the ultrasonic reception signal. However, if the zero cross voltage drifts due to noise or the like, there is a problem that the ultrasonic propagation time cannot be measured accurately.

  In addition, the conventional measurement method is easily affected by phase noise of the ultrasonic reception signal and drift due to temperature change, and there is a possibility that accurate measurement cannot be performed.

  The present invention has been made in the background as described above. In particular, it is an object of the present invention to provide an ultrasonic flowmeter which is not easily affected by a drift change of a zero cross voltage and can accurately measure an ultrasonic propagation time. And

Means for Solving the Problems and Effects of the Invention

The invention described in claim 1
A flow path through which fluid flows;
A pair of ultrasonic sensors provided in the flow path for transmitting ultrasonic waves toward the upper side or lower side of the fluid flow direction and then receiving ultrasonic waves coming from the upper side or lower side of the flow direction and outputting a reception signal An ultrasonic element;
A transmission means for sending a drive signal of a fixed period to the ultrasonic element on the ultrasonic transmission side, and transmitting ultrasonic waves to the ultrasonic element;
A phase difference detecting means for detecting a phase difference between the received signal output from the ultrasonic element on the ultrasonic receiving side and the drive signal;
Based on the detected phase difference, time measuring means for measuring the time for the ultrasonic wave to propagate through the flow path,
It is an ultrasonic flowmeter characterized by providing.

  According to the above-described invention, a constant period of ultrasonic waves is transmitted from the ultrasonic element by sending a drive signal of a constant period to the ultrasonic element. When the ultrasonic element on the receiving side receives this ultrasonic wave, it initially vibrates at its own natural frequency, but gradually vibrates at the cycle of the transmitted ultrasonic wave. As a result, a reception signal having the same cycle as the drive signal is output. By measuring the time from when a specific ultrasonic wave is transmitted until it is received and the phase difference between the drive signal and the received signal, the time for the ultrasonic wave to propagate through the flow path can be accurately determined. In addition, this method is not easily affected even if the zero cross voltage changes due to noise or temperature change.

The invention according to claim 2
The phase difference detection means is an ultrasonic flowmeter that obtains the phase difference from an exclusive OR of a phase shift signal obtained by shifting the drive signal by a specific phase and the received signal. According to the present invention, the phase difference can be accurately obtained even if the zero cross voltage is shifted.

The invention described in claim 3
A plurality of phase shift signals having different phases from each other are prepared in advance, and a phase shift signal having the smallest phase shift with the received signal is selected from the plurality of phase shift signals, and the exclusive OR with the received signal is selected. And an ultrasonic flowmeter that measures the ultrasonic propagation time by measuring the width of the obtained signal using a high-resolution time measuring means.

  According to the above invention, since an exclusive OR of the phase shift signal and the received signal is taken and the width of the obtained signal is measured using the high resolution time measuring means, the ultrasonic propagation time can be measured accurately.

The invention described in claim 4 is a specific example of the invention described in claim 3.
An ultrasonic flowmeter using a ring oscillator as the high-resolution time measuring means. When a ring oscillator is used, it becomes a simple digital circuit, so an inexpensive ultrasonic flow meter can be realized.

The invention according to claim 5 is a concrete implementation of the invention according to claim 3,
An ultrasonic flowmeter using a triangular wave circuit as the high-resolution time measuring means. If it does in this way, it will become possible to obtain | require integrated time only by repeating charging / discharging of a triangular wave in multiple times, and then calculating | requiring a triangular wave once, and a simple ultrasonic flowmeter can be implement | achieved.

The invention described in claim 6
It is an ultrasonic flowmeter which corrects the measurement time by the high resolution time measurement means using the cycle of the drive signal.

  The measurement time of the high-resolution time measuring means varies depending on the temperature and the circuit voltage. However, as described above, if the correction is made using the cycle of the drive signal, the time can be stably measured.

The invention described in claim 7
A flow path through which fluid flows;
A pair of ultrasonic sensors provided in the flow path for transmitting ultrasonic waves toward the upper side or lower side of the fluid flow direction and then receiving ultrasonic waves coming from the upper side or lower side of the flow direction and outputting a reception signal An ultrasonic element;
A transmission means for sending a drive signal of a fixed period to the ultrasonic element on the ultrasonic transmission side, and transmitting ultrasonic waves to the ultrasonic element;
The period of the drive signal is φ0, the phase difference between the phase shift signal shifted by a phase difference φa with respect to the drive signal and the reception signal is φ, and the period from the transmission of the drive signal to the reception of the ultrasonic wave is detected. When the number of pulses in the driving cycle is N and the average value obtained by measuring the phase difference φ m times (m is a natural number of 2 or more) is Σφ / m, the time Tud during which the ultrasonic wave propagates through the flow path is
Tud = φ0 · N + φa + Σφ / m
A time measurement means obtained from
It is an ultrasonic flowmeter characterized by providing.

  According to the above-described invention, a constant period of ultrasonic waves is transmitted from the ultrasonic element by sending a drive signal of a constant period to the ultrasonic element. When the ultrasonic element on the receiving side receives this ultrasonic wave, a reception signal having the same period as the drive signal is output. When the ultrasonic propagation time is measured using such a received signal, the influence of drift of the zero cross voltage due to noise or the like is reduced. In addition, since the phase difference φ between the phase shift signal and the received signal is measured a plurality of times (m times) and the average is taken, the measured value is accurate.

Embodiments of the present invention will be described with reference to the drawings.
FIG. 1A is a block diagram of an embodiment of an ultrasonic flowmeter used as a general residential gas meter or the like. A flow measurement gas (fluid) flows in the flow direction shown in the flow path 2 for flow measurement of the ultrasonic flowmeter 1. In the flow path 2, an upper ultrasonic element 3 is provided on the upper side in the flow direction, and a lower ultrasonic element 4 is provided on the lower side in the flow direction. These ultrasonic elements 3 and 4 are composed of piezoelectric elements and the like, and an ultrasonic transmission function for transmitting an ultrasonic wave when a driving voltage is applied, and an ultrasonic reception function for outputting an electric signal (reception signal) when receiving an ultrasonic wave. In combination.

  When the measurement target is gas, the measurement channel 2 is closed, and for example, a circular shape, an elliptical shape, a square shape, a rectangular shape, or the like is adopted as the axial cross-sectional shape. The flow path 2 of this embodiment is formed in a rectangular shape, and the upper-side ultrasonic element 3 and the lower-side ultrasonic element 4 are attached to the upper wall 11a as shown in a sectional view (FIG. 1B). The ultrasonic wave transmitted from the upper ultrasonic element 3 or the lower ultrasonic element 4 is reflected by the bottom wall 11b at an angle θ and reaches the counterpart ultrasonic element. In FIG. 1, a V-shaped structure in which ultrasonic waves are reflected by the bottom wall 11b is used. For example, the upper ultrasonic element 3 is arranged on the upper wall 11a, and the lower ultrasonic element 4 is arranged on the bottom wall 11b. In addition, a structure (Z type) that transmits and receives ultrasonic waves without reflecting them can also be employed.

  As shown in FIG. 1A, the ultrasonic flowmeter 1 is provided with a transmission means 12, a transmission side switch 8, a reception side switch 9, a control means 10, an amplification means 5, a phase difference detection means 6, and a time measurement means 7. It has been. The transmission means 12 is a circuit for inputting a drive signal to the ultrasonic elements 3 and 4. By switching the transmission side switch 8 and the reception side switch 9, the transmission side ultrasonic element and the reception side ultrasonic element are switched. The control means 10 performs overall operation control of the ultrasonic flowmeter 1, and performs control of drive signal transmission timing, flow rate calculation, switching control of the switches 8 and 9, and the like.

  As shown in FIG. 1A, by connecting the transmission side switch 8 to the a side and the reception side switch 9 to the b side, the upper ultrasonic element 3 is set to the transmission side, and the lower ultrasonic element 4 is set to the reception side. To. Then, a driving signal is transmitted from the transmitting means 12 to the upper ultrasonic element 3. Thereby, an ultrasonic wave is transmitted from the upper ultrasonic element 3, propagates through the flow path 2, and is reflected by the bottom wall 11b. When the lower ultrasonic element 4 receives the reflected ultrasonic wave, a reception signal is output, and the reception signal is transmitted to the amplification means 5 through the switch 9. Thereafter, using the received signal, the time during which the ultrasonic wave propagates through the flow path 2 (forward ultrasonic wave propagation time Tud) is measured. Details of the method will be described later.

  Thereafter, the transmission side switch 8 is switched to the b side and the reception side switch 9 is switched to the a side, so that the upper ultrasonic element 3 is set to the reception side and the lower ultrasonic element 4 is set to the transmission side. And a drive signal is transmitted from a transmission means, The reverse direction ultrasonic propagation time Tdu is calculated | required by performing the operation | movement similar to the above-mentioned.

In FIG. 1A, assuming that the average flow velocity of gas is v, the speed of sound propagating in the gas is c, the reflection angle of the ultrasonic wave is θ, and the propagation distance of the ultrasonic wave is L, the forward ultrasonic wave propagation time Tud and the reverse ultrasonic wave Each propagation time Tdu is expressed as follows.
Tud = L / (c + v · cos θ) (1)
Tdu = L / (cv · cos θ) (2)
Taking the reciprocal of equations (1) and (2) and taking the difference, the following equation is obtained.
1 / Tud−1 / Tdu = 2v · cos θ / L (3)
Therefore, from the measurement of the forward ultrasonic propagation time Tud and the reverse ultrasonic propagation time Tdu, the average gas flow velocity v and flow rate Q are obtained by the following equations. However, A is a cross-sectional area of the flow path 1.
v = L / 2 cos θ (1 / Tud−1 / Tdu) (4)
Q = v · A (5)
In this way, by removing the sound velocity c depending on the gas temperature, the contained component, etc. from the equation (4), the measured value (ultrasonic propagation time Tud, Tdu) and a constant value (propagation distance L, reflection angle θ). From these, the flow velocity v and the flow rate Q can be calculated.

  Next, the principle of measuring the ultrasonic propagation time will be described with reference to FIG. First, a plurality of drive signals Va are input from the transmission means 12 to the upper ultrasonic element 3. This drive signal Va has a constant period φ0. As a result, an ultrasonic wave with a period φ0 is transmitted from the upper ultrasonic element 3. This ultrasonic wave propagates through the flow path 2 and reaches the lower ultrasonic element 4. The lower ultrasonic element 4 that has received the ultrasonic wave outputs a reception signal Vb as shown in FIG.

  The reception signal Vb is difficult to detect because the amplitude is small at the beginning of receiving the ultrasonic wave. Therefore, reception of ultrasonic waves is continued until the amplitude becomes sufficiently large. Immediately after receiving the ultrasonic wave, the lower ultrasonic element 4 vibrates at a resonance frequency unique to the element, and therefore vibrates at a period different from the ultrasonic period φ0, and the period is unstable. When waiting until the amplitude becomes sufficiently large, the period of the reception signal Vb is synchronized with the period of the ultrasonic wave φ0 and becomes stable.

  For example, when 50 ultrasonic waves are transmitted from the upper ultrasonic element 3, a sufficiently stable reception signal can be obtained at the 20th. Therefore, if the time from when the upper ultrasonic element 3 transmits the 20th ultrasonic wave to when the lower ultrasonic element 4 outputs the 20th received signal is measured, the ultrasonic wave flows. The time for propagating the road (forward propagation time Tud) is obtained. In FIG. 2, for the sake of simplicity, the reception signal Vb is described as being stable when the fourth ultrasonic wave is received.

  Here, if the ultrasonic wave is not emitted continuously but is transmitted in a single shot, the lower ultrasonic element 4 vibrates at a resonance frequency unique to the element. In this state, since the amplitude is small, it is difficult to detect the reception signal Vb and it is easily affected by noise and the like. For this reason, in the present invention, an ultrasonic wave having a constant period is transmitted and the waiting signal until the lower ultrasonic element 4 is synchronized with the ultrasonic wave, thereby stabilizing the received signal Vb and making it less susceptible to noise and the like.

  The received signal Vb is amplified by the amplifying means 5 (FIG. 1A) to become an amplified signal Vc. This amplified signal Vc is binarized by a comparator described later, and a binarized signal Vd is obtained as shown in FIG. The (1) to (6) waves of the binarized signal Vd correspond to the (1) to (6) waves of the drive signal Va, respectively.

  On the other hand, as shown in FIG. 4, a plurality of phase shift signals Ve having the same period φ0 as the drive signal Va and slightly different phases are prepared. This phase shift signal can be output from the transmission means 12, for example.

  Returning to FIG. A phase shift signal having a minimum phase difference from the binarized signal Vd and not 0 is selected from the plurality of phase shift signals Ve, and an exclusive OR of the phase shift signal Ve and the binarized signal is selected. Take. As a result, the EXOR signal Vf shown in FIG. 2 is obtained.

  Assuming that the period of the reception signal Vb is stable when the lower ultrasonic element 4 receives the fourth ultrasonic wave, as shown in FIG. 2, the fourth driving signal Va is transmitted. From this, the forward ultrasonic propagation time Tud can be measured by measuring the time until the fourth binarized signal Vd is received. That is, after the fourth driving signal Va is transmitted, the phase shift signals are counted, and the number N is obtained (9 in FIG. 2). Then, the phase difference φ between the phase shift signal Ve and the binarized signal Vd is measured using the width of the EXOR signal Vf. This measurement is performed using a ring oscillator or a triangular wave circuit. That is, as shown in FIG. 6, a signal having a short period clk is output using a ring oscillator or the like, and the signal φ is counted to measure the width φ of the EXOR signal Vf. At this time, the measurement is not performed only once, but is measured a plurality of times (m times), and the addition average is taken. Thereby, the width φ of the EXOR signal Vf can be accurately obtained.

  Returning to FIG. As described above, the phase shift signal Ve having the smallest phase difference φ with respect to the binarized signal Vd and not 0 is selected from the prepared plurality of phase shift signals, and exclusive with the binarized signal Vd. The logical sum is taken. The reason why the phase difference φ is minimum is that the number of counts when the time measurement is performed using a ring oscillator or the like is small. If a phase difference φ of 0 is selected, φ cannot be measured. For this reason, the phase shift signal Ve whose phase difference φ is not 0 is selected.

Σφ / m is the addition average obtained by measuring the width φ of the EXOR signal Vf m times, the phase difference between the phase shift signal Ve and the drive signal Va is φa, the period of the phase shift signal Ve is φ0, and the count number of the phase shift signal Ve is N , The forward ultrasonic propagation time Tud is
Tud = φ0 · N + φa + Σφ / m (6)
Can be obtained as The backward ultrasonic propagation time Tdu can be measured by transmitting an ultrasonic wave from the lower ultrasonic element 4 and receiving it by the upper ultrasonic element 3.

  In the above description, the fourth signal is detected from the continuously transmitted binary signal Vd. However, since the waveform of the received signal Vb immediately after reception is unstable, the first signal is It may not be detected reliably. Therefore, there is a possibility that the fifth signal is mistakenly detected as the fourth signal. However, in practice, the measurement of the ultrasonic propagation time is repeated many times, and if it is detected by mistake, it can be corrected because the ultrasonic propagation time suddenly becomes longer. For example, when the period φ0 of the drive signal Va is set to 5 μs, when the wavelength is shifted by one wavelength in a household ultrasonic flowmeter, the change is 5000 L / h in terms of flow rate. It can be determined that such a rapid flow rate change is a false detection. This determination can be made by the control means 10 described above, for example.

  Next, specific configurations of the amplifying unit 5, the phase difference detecting unit 6, and the time measuring unit 7 are shown in FIG. As described above, the amplifying unit 5 includes the amplifier 51 (operational amplifier), and the phase difference detecting unit 6 includes the comparator 61 and the EXOR circuit 62. The time measuring means 7 includes a high resolution time measuring means 7a (a high resolution pulse counter circuit 71 and a high resolution clock pulse generation circuit 72) and a pulse counter circuit 73. The high resolution clock pulse generation circuit 72 is composed of a ring oscillator or a triangular wave circuit.

  The reception signal Vb output from the ultrasonic elements 3 and 4 is input to the amplifier 51, where the voltage is amplified and output as an amplified signal Vc. The amplified signal Vc is input to the comparator 61. The comparator 61 compares the amplified signal Vc with the zero cross voltage, and outputs a signal (binarized signal Vd) obtained by binarizing a portion where the voltage is higher and lower than the zero cross voltage. The binarized signal Vd is input to the EXOR circuit 62. On the other hand, the phase shift signal Ve is input from the transmission means 12 to the EXOR circuit 62, and the EXOR circuit 62 takes the exclusive OR of the binarized signal Vd and the phase shift signal Ve and outputs the EXOR signal Vf.

  The high resolution time measuring means 7a measures the width φ of the EXOR signal Vf. That is, a pulse is output from the high resolution clock pulse generation circuit 72, and the pulse is counted by the high resolution pulse counter circuit 71. The obtained result (that is, the phase difference φ between the binarized signal Vd and the phase shift signal) is transmitted to the control means 10. The phase difference φ is measured a plurality of times as described above, and the control means 10 calculates the addition average Σφ / m.

  On the other hand, the pulse counter circuit 73 counts the number of pulses N of the phase shift signal Ve and transmits the result to the control means 10. The control means 10 calculates the forward ultrasonic propagation time Tud from these results, and obtains the fluid flow rate.

  Next, the advantage of taking the exclusive OR will be described with reference to FIG. As described above, the received signal Vb and the zero cross voltage are compared and the binarized signal Vd is output. When the zero cross voltage is not affected by noise or the like, the binarized signal Vd becomes as shown in (1), and from the exclusive OR of this and the phase shift signal Ve, the EXOR signal Vf becomes as shown in (1). Become. In this case, the signal widths φx and φy are almost equal.

When the zero cross voltage is offset due to noise or the like, the binarized signal Vd becomes as shown in (2) and the EXOR signal Vf becomes as shown in (2). In this case, φx ′ is shorter than φy ′,
φx + φy = φx ′ + φy ′ (7)
The relationship is established. Therefore, if the addition average of φx ′ and φy ′ is taken, the influence of drift of the zero cross voltage due to noise or the like can be avoided.

Next, correction of the high resolution time measuring means 7a will be described with reference to FIG. As described above, the high-resolution time measuring unit 7a uses a ring oscillator, a triangular wave circuit, or the like. These pulse generation circuits tend to change in frequency under the influence of circuit voltage and temperature. Therefore, the period φ0 of the drive signal Va is counted using the oscillation frequency of the ring oscillator. Since the driving signal Va uses a crystal oscillator or the like, the period φ0 is relatively stable. If the oscillation frequency of the ring oscillator is clk, and φ0 is measured using the oscillation frequency, the count number is N0.
φ0 = clk · N0 (8)
It is. from now on,
clk = φ0 / N0 (9)
Is required. On the other hand, as shown in FIG. 6, assuming that the count number when the width φ of the EXOR signal Vf is measured by the pulse of the ring oscillator is N1,
φ = clk · N1 (10)
Substituting equation (9) into equation (10),
φ = φ0 · N1 / N0 (11)
It becomes. Since there is no ring oscillator transmission frequency clk in the equation (11), the width φ of the EXOR signal Vf can be stably measured without being affected by the temperature and the circuit voltage.

  Next, a specific example of the control means 10 is shown in FIG. The control means 10 is a microcomputer, for example, and includes a CPU 14, a ROM 15, a RAM 16, an I / O 17, and a bus line 13 for connecting them. As shown in FIG. 9, the ROM 15 stores various programs such as a propagation time calculation program 15a, a phase shift signal generation / selection program 15b, a Σφ / m calculation program 15c, a flow rate calculation program 15d, and a correction program 15e. The propagation time calculation program 15a is a program for calculating the ultrasonic propagation time using the above-described equation (6). Further, the phase shift signal generation / selection program generates a plurality of phase shift signals Ve as shown in FIG. 4, and selects one having a minimum phase difference from the binarized signal Vd and not 0 among them. It is a program. The Σφ / m calculation program is a program for obtaining the addition average of the width φ of the EXOR signal Vf. The flow rate calculation program 15d is a program for obtaining the flow rate of the fluid using the above-described equations (4) and (5). Further, the correction program 15e is a program for obtaining the width φ of the EXOR signal Vf using the equation (11).

  FIG. 10 shows a conventional example. Conventionally, the drive signal is transmitted to the ultrasonic element in a single shot. For this reason, the ultrasonic waves are transmitted in a single shot, and the ultrasonic element on the receiving side outputs a reception signal having the waveform shown in FIG. Since the transient response at the time of ultrasonic reception is determined by the resonance frequency, anti-resonance frequency, Q value, etc. specific to the element, it is difficult to reflect the characteristics of the individual elements on the flowmeter when mass producing ultrasonic flowmeters. Also, since the beginning of the received signal is more susceptible to noise, for example, a zero cross signal is generated when the third wave is generated, and the time from when the drive signal is transmitted until the zero cross signal is output is used. The ultrasonic propagation time was calculated. However, since the received signal is relatively weak in this method, if the zero-cross voltage drifts due to noise or the like, for example, the zero-cross signal is generated when the fifth wave is generated, and the ultrasonic propagation time cannot be measured accurately. It was. On the other hand, the present invention transmits a drive signal having a fixed period a plurality of times, so that the ultrasonic propagation time can be measured when the amplitude of the received signal becomes large and the period becomes stable. For this reason, even if the zero cross voltage drifts due to noise or the like, it is hardly affected.

The block diagram of the ultrasonic flowmeter which concerns on this invention. The figure which shows the relationship between a drive signal, a received signal, a phase shift signal, and an EXOR signal. A specific example of a receiving circuit. The figure explaining the state in which the several phase shift signal is prepared. The figure explaining the phase difference with a phase shift signal when the zero cross voltage of a received signal fluctuates. Relationship between ring oscillator pulse and EXOR signal The figure which correct | amends the measurement time of a ring oscillator using the period of a drive signal. Configuration example of control means 10 Example of ROM15 Conventional example

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ultrasonic flowmeter 2 Flow path 3 Upper-side ultrasonic element 4 Lower-side ultrasonic element 5 Amplifying means 6 Phase difference detecting means 7 Time measuring means 8 Transmitting side switch 9 Receiving side switch 10 Control means

Claims (7)

A flow path through which fluid flows;
A pair of ultrasonic sensors provided in the flow path for transmitting ultrasonic waves toward the upper side or lower side of the fluid flow direction and then receiving ultrasonic waves coming from the upper side or lower side of the flow direction and outputting a reception signal An ultrasonic element;
A transmission means for sending a drive signal of a fixed period to the ultrasonic element on the ultrasonic transmission side, and transmitting ultrasonic waves to the ultrasonic element;
A phase difference detecting means for detecting a phase difference between the received signal output from the ultrasonic element on the ultrasonic receiving side and the drive signal;
Based on the detected phase difference, time measuring means for measuring the time for the ultrasonic wave to propagate through the flow path,
An ultrasonic flowmeter comprising:   The ultrasonic flowmeter according to claim 1, wherein the phase difference detection unit obtains the phase difference from an exclusive OR of a phase shift signal obtained by shifting the drive signal by a specific phase and the received signal.   A plurality of phase shift signals having different phases from each other are prepared in advance, and a phase shift signal having the smallest phase shift with the received signal is selected from the plurality of phase shift signals, and the exclusive OR with the received signal is selected. The ultrasonic flowmeter according to claim 2, wherein the ultrasonic propagation time is measured by measuring the width of the obtained signal using a high-resolution time measuring means.   The ultrasonic flowmeter according to claim 3, wherein a ring oscillator is used as the high-resolution time measuring means.   4. The ultrasonic flowmeter according to claim 3, wherein a triangular wave circuit is used as the high resolution time measuring means.   The ultrasonic flowmeter according to any one of claims 3 to 5, wherein a measurement time by the high-resolution time measurement unit is corrected using a cycle of the drive signal. A flow path through which fluid flows;
A pair of ultrasonic sensors provided in the flow path for transmitting ultrasonic waves toward the upper side or lower side of the fluid flow direction and then receiving ultrasonic waves coming from the upper side or lower side of the flow direction and outputting a reception signal An ultrasonic element;
A transmission means for sending a drive signal of a fixed period to the ultrasonic element on the ultrasonic transmission side, and transmitting ultrasonic waves to the ultrasonic element;
The period of the drive signal is φ0, the phase difference between the phase shift signal shifted by a phase difference φa with respect to the drive signal and the reception signal is φ, and the period from the transmission of the drive signal to the reception of the ultrasonic wave is detected. When the number of pulses in the driving cycle is N and the average value obtained by measuring the phase difference φ m times (m is a natural number of 2 or more) is Σφ / m, the time Tud during which the ultrasonic wave propagates through the flow path is
Tud = φ0 · N + φa + Σφ / m
A time measurement means obtained from
An ultrasonic flowmeter comprising:

Images