JP2005172518A - Circuit for arrival time detection of ultrasonic waves in ultrasonic sensor, and ultrasonic flowmeter using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a circuit for arrival time detection of an ultrasonic waves in a ultrasonic sensor, capable of always securing stabilized and high detection accuracy, and to provide an ultrasonic flowmeter which uses it. <P>SOLUTION: The circuit for arrival time detection 2 is provided with a charge amplifier 3, which changes and outputs the electric charge generated according to reception of an ultrasonic wave from each ultrasonic sensor B1, B2 composed of piezoelectric elements into a voltage signal, and a zero-cross point detector 5 which detects the zero-cross point of the voltage signal outputted from the charge amplifier. And besides, one pair of ultrasonic sensors B1, B2 are arranged mutually and opposite with a prescribed distance between them in the passage through which a fluid flows. The circuit for arrival time detection 2 is so formed that the ultrasonic flowmeter 1 is constituted. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an ultrasonic arrival time detection circuit in an ultrasonic sensor, and an ultrasonic flowmeter using the same.

  As shown in FIG. 6, the conventional ultrasonic flowmeter includes a pair of ultrasonic sensors B1 and B2 made of piezoelectric vibrators, and one ultrasonic sensor in a flow path through which a fluid such as gas or liquid flows. There is a configuration in which B1 is disposed on the upstream side (left side in the drawing) and the other ultrasonic sensor B2 is disposed to face each other in a state of being separated by a predetermined distance L on the downstream side (right side in the drawing). In this case, each of the ultrasonic sensors B1 and B2 has both functions of transmitting and receiving ultrasonic waves.

  Here, when the fluid is flowing at a constant speed, an ultrasonic pulse is transmitted by applying an excitation pulse to each ultrasonic sensor B1 or B2. At that time, the time required for the ultrasonic wave transmitted from the upstream ultrasonic sensor B1 to be received by the downstream ultrasonic sensor B2 is t1, and the ultrasonic wave transmitted from the downstream ultrasonic sensor B is Assuming that the time required for reception by the ultrasonic sensor A on the upstream side is t2, t1 <t2, and the difference Δt (= t2−t1) between the times t1 and t2 is approximately proportional to the fluid flow velocity v. Yes. Therefore, by detecting the time difference Δt, the flow velocity v of the fluid can be measured, and if the cross-sectional area of the flow path is known, the flow rate can be measured from the flow velocity v.

  By the way, in the ultrasonic flowmeter described above, the ultrasonic wave transmitted by exciting the ultrasonic sensors B1 and B2 generally has a waveform that repeats the amplitude. Therefore, from the ultrasonic sensor B1 or B2 on the side that receives the ultrasonic wave. The output reception signal also has a waveform that repeats the amplitude. For this reason, on the receiving side, it becomes a problem which time point of the received signal is regarded as the time when the ultrasonic wave is received.

  In this case, since it is actually difficult to detect the time immediately after the ultrasonic sensors B1 and B2 receive the ultrasonic waves, the reverse of the reception signals output from the ultrasonic sensors B1 and B2 after a certain peak has passed. A time point at which the signal level becomes zero when going to the peak (so-called zero cross point) is detected and set as a reception time (see, for example, Patent Document 1).

  Further, since the reception signals output from the ultrasonic sensors B1 and B2 are relatively weak, conventionally, as shown in FIG. 7, an operational amplifier is provided before the zero-cross point detection circuit 5 for detecting the zero-cross point of the reception signal. A voltage amplifying circuit 4 composed of a combination of Amp1 and resistors R1 to R4 is provided, and a received signal output from the ultrasonic sensor B1 or B2 is amplified by the voltage amplifying circuit 4 and then given to the zero cross point detecting circuit 5. ing. C1 is a coupling capacitor for connecting the voltage amplification circuit 4 to the ultrasonic sensors B1 and B2.

  Assuming that the flow of fluid is stopped and the flow rate is zero, the ultrasonic wave is transmitted from each ultrasonic sensor, and then the zero cross point of the received signal is detected by the zero cross point detection circuit 5. The time difference Δt required until then should be zero. However, in actuality, after the ultrasonic waves received by the ultrasonic sensors B1 and B2 are converted into electric signals, the time difference (hereinafter, referred to as the time difference between the received signals being input to the zero-cross point detection circuit 5). Δ1, Δ2) (referred to as offset time).

  The following factors can be considered for this. First, since each ultrasonic sensor B1, B2 is configured by forming electrodes on both surfaces of a plate-like piezoelectric body, there is a capacitance between the two electrodes. Further, a resistance value exists between the ultrasonic sensors B1 and B2 and the voltage amplifier circuit 4. Therefore, a delay circuit composed of a capacitance and a resistance is formed between the ultrasonic sensors B1 and B2 and the input terminal of the zero cross point detection circuit 5, and this causes offset times Δ1 and Δ2.

If the offset times Δ1 and Δ2 generated in association with the individual ultrasonic sensors B1 and B2 are different from each other, even if the flow rate is zero, the time difference Δt is not zero and an error remains. That is, since the time required for each of the ultrasonic sensors B1 and B2 to transmit an ultrasonic wave until the zero cross point of the received signal is detected by the zero cross point detection circuit is (t1 + Δ1) and (t2 + Δ2), respectively. The time difference Δt of
Δt = (t2 + Δ2) − (t1 + Δ1)
= (T2-t1)-(Δ2-Δ1) (1)
It becomes. That is, even if the flow rate is zero and t2 = t1, the time error Δe (= Δ2-Δ1) remains.

  As described above, when the time error Δe occurs due to the presence of the offset times Δ1, Δ2 of the ultrasonic sensors B1, B2 constituting the ultrasonic flowmeter, the flow rate of the fluid cannot be accurately measured. Therefore, it is necessary to reduce such time error Δe as much as possible.

  For this reason, in the prior art, a part of the electrodes of the ultrasonic sensors B1 and B2 are trimmed so as to adjust the capacitances C1 and C2 held by the ultrasonic sensors B1 and B2 to substantially coincide with each other. Doing the ring.

Japanese Patent No. 2656396

  If the pairing is performed for each of the ultrasonic sensors B1 and B2 as in the prior art, the capacitances C1 and C2 possessed by both the ultrasonic sensors B1 and B2 can be substantially matched. Δ2 can be brought close to each other, and the time error Δe can be reduced to some extent.

  However, when performing such pairing, it is necessary to trim the electrodes of the ultrasonic sensors B1 and B2 and measure each time whether or not the capacitances after trimming match each other. In addition to the time and effort required for adjustment, this is a factor in increasing the cost of the apparatus.

  Moreover, even if the capacitances of the ultrasonic sensors B1 and B2 can be matched by pairing, stray capacitance exists between the wiring connected to the ultrasonic sensors B1 and B2 and the ground. However, these stray capacitance values may be different. At that time, since the offset times Δ1 and Δ2 generated in association with the individual ultrasonic sensors B1 and B2 are different from each other, the time error Δe remains.

  The conventional circuit using the charge amplifier described in Patent Document 2 is for increasing the amplitude of the output signal from the piezoelectric element to increase the detection sensitivity, and is influenced by the offset times Δ1 and Δ2 described above. Is not intended to be as small as possible. Therefore, this conventional technique is still not effective in that the detection accuracy of the apparatus is increased by reducing the time error Δe as much as possible when detecting the zero-cross point of the received signal of the ultrasonic sensor to determine the time difference Δt. It is enough.

  The present invention has been made in order to solve the above-described problems, and an ultrasonic wave that reduces the time error as much as possible by reducing the influence of the offset time of the ultrasonic sensor and always ensures a stable and high detection accuracy. It is an object to provide a circuit for detecting the arrival time of ultrasonic waves in a sensor, and an ultrasonic flowmeter using the same.

  In order to achieve the above object, an ultrasonic arrival time detection circuit in an ultrasonic sensor according to the first aspect of the invention includes an ultrasonic sensor composed of a piezoelectric element and the ultrasonic sensor in response to reception of the ultrasonic wave. A charge amplifier that converts the electric charge generated from the acoustic wave sensor into a voltage signal and outputs the voltage signal, and a zero-cross point detection circuit that detects a zero-cross point of the voltage signal output from the charge amplifier are provided.

  An ultrasonic flowmeter according to a second aspect of the invention comprises a circuit for detecting the arrival time of ultrasonic waves in the ultrasonic sensor according to the first aspect, and the flow path through which the two ultrasonic sensors flow a fluid. It is characterized in that they are opposed to each other with a predetermined distance therebetween.

  According to the ultrasonic arrival time detection circuit of the ultrasonic sensor according to the first aspect of the invention, the difference in the capacitance of the ultrasonic sensor, the wiring connected to the ultrasonic sensor, and the stray capacitance are different. Even in this case, the charge amplifier outputs a voltage signal from which the influence of such delay factors has been removed, so when detecting the zero-cross point, the influence due to the difference in offset time associated with each ultrasonic sensor is output. Can be effectively removed.

  According to the ultrasonic flowmeter of the invention described in claim 2, since the influence due to the difference in the offset time associated with each ultrasonic sensor is effectively removed when detecting the zero cross point, the ultrasonic wave is forward and backward. Thus, it is possible to reduce as much as possible the time error that occurs when the difference between the times required to transmit and receive in the respective directions is obtained. For this reason, the flow rate of the fluid can be measured with higher accuracy than in the past.

  In addition, since adjustment of pairing such as trimming the electrodes of the ultrasonic sensor is unnecessary, the cost of the apparatus can be reduced.

  FIG. 1 is a block diagram showing the overall configuration of the ultrasonic flowmeter in this embodiment, and FIG. 2 is a circuit diagram showing a specific configuration of an arrival time detection circuit constituting the flowmeter.

  The ultrasonic flowmeter 1 in this embodiment includes a pair of ultrasonic sensors B1 and B2 disposed in a fluid pipe (not shown) through which a fluid such as gas or liquid flows. Both ultrasonic sensors B1 and B2 are composed of piezoelectric vibrators having electrodes formed on both sides of a plate-like piezoelectric body. For example, one (left side in the figure) ultrasonic sensor B1 is an inner wall on the upstream side of the fluid pipe. In addition, the other (right side in the figure) ultrasonic sensors B2 are attached to the inner wall on the downstream side of the fluid pipe so as to face each other in a state of being separated from each other by a predetermined distance.

  A first switching circuit SW1 and a second switching circuit SW2 are connected to both ultrasonic sensors B1, B2. Both switching circuits SW1 and SW2 are complementarily switched by the control of the arithmetic control circuit 7 described later. For example, when the first switching circuit SW1 is connected to one ultrasonic sensor B1, the other ultrasonic sensor B2 is connected. When the second switching circuit SW2 is connected to the other ultrasonic sensor B2 and the first switching circuit SW1 is connected to the other ultrasonic sensor B2, the second switching circuit SW2 is connected to the one ultrasonic sensor SW1. Yes.

  A driving pulse generation circuit 6 that generates a driving pulse for exciting and driving the ultrasonic sensors B1 and B2 is connected to the common contact of the first switching circuit SW1. On the other hand, the arrival time detection circuit 2 and the calculation control circuit 7 are sequentially connected to the common contact of the second switching circuit SW2. The time detection circuit 2 includes a charge amplifier 3, a voltage amplification circuit 4, and a zero cross point detection circuit 5. The voltage amplification circuit 4 is connected to the output side of the charge amplifier 3 via a coupling capacitor C1, Further, a zero cross point detection circuit 5 is connected to the output side of the voltage amplification circuit 4.

  The charge amplifier 3 is generated from the ultrasonic sensors B1 and B2 without being affected by delay factors such as a difference in capacitance between the ultrasonic sensors B1 and B2, wirings connected to the ultrasonic sensors B1 and B2, and stray capacitance. The electric charge is directly converted into a voltage signal and output. The operational amplifier Amp2, the feedback capacitor C2 connected between the inverting input terminal (−) and the output terminal of the operational amplifier Amp2, and the feedback capacitor C2 The feedback resistor R5 is connected in parallel. The non-inverting input terminal (+) of the operational amplifier Amp2 is connected to the ground. In addition, the voltage amplifier circuit 4 is configured by combining an operational amplifier Amp1 and a plurality of resistors R1 to R4 as in the prior art.

  The zero cross point detection circuit 5 detects a zero cross point of the received signal, and this circuit itself is well known. The arithmetic control circuit 7 is constituted by a microcomputer, for example, and outputs a drive pulse generation trigger signal to the drive pulse generation circuit 6 and controls switching of the first and second switch circuits SW1 and SW2. Do. Further, the arithmetic control circuit 7 measures the time required until the zero cross point of each received signal is detected by the zero cross point detecting circuit 5 after outputting the trigger signal to the drive pulse generating circuit 6. The fluid flow rate is calculated based on the measurement result.

  The operation in the case of measuring the flow rate of a fluid such as gas or liquid flowing in the fluid pipe in the ultrasonic flow meter 1 having the above configuration will be described.

  First, in order to transmit an ultrasonic wave from one ultrasonic sensor B1, the first switching circuit SW1 is connected to the ultrasonic sensor B1, and the second switching circuit SW2 is connected to the other ultrasonic sensor B2. .

  In this state, when a trigger signal is given from the arithmetic control circuit 7 to the drive pulse generation circuit 6, a drive pulse is generated from the drive pulse generation circuit 6 in response to this, and one super Since it is added to the sonic sensor B1, an ultrasonic wave is transmitted from the ultrasonic sensor B1 toward the other ultrasonic sensor B2.

  When an ultrasonic wave is received by the other ultrasonic sensor B2, a charge corresponding to the magnitude of the received ultrasonic wave is generated from the ultrasonic sensor B2, and this generated charge is generated by the arrival time detection circuit 2. Join the charge amplifier 3.

  The charge amplifier 3 is generated from the ultrasonic sensors B1 and B2 without being affected by delay factors such as a difference in capacitance between the ultrasonic sensors B1 and B2, wirings connected to the ultrasonic sensors B1 and B2, and stray capacitance. The electric charge is directly converted into a voltage signal and output. That is, the level of the voltage signal output from the charge amplifier 3 varies depending on the difference in capacitance between the ultrasonic sensors B1 and B2, the difference in wiring connected to the ultrasonic sensors, and the difference in stray capacitance. The charge amplifier 3 outputs a voltage signal that is not affected by these delay factors.

  Thus, the voltage signal output from the charge amplifier 3 is amplified by the voltage amplifier circuit 4 and then applied as a reception signal to the zero cross point detection circuit 5. Therefore, the zero cross point detection circuit 5 detects the zero cross point of the reception signal. To do. Therefore, the influence of the delay factor is already removed from the zero cross point of the received signal by the charge amplifier 3. Then, the detection output of the zero cross point detection circuit 5 is given to the arithmetic control circuit 7 at the next stage.

  Similarly, when transmitting an ultrasonic wave from the other ultrasonic sensor B2, the first switching circuit SW1 is connected to the other ultrasonic sensor B2, and the second switching circuit SW2 is connected to the one ultrasonic sensor SW1. Connected. Then, an ultrasonic wave is transmitted from the ultrasonic sensor B2 toward one ultrasonic sensor B1 by the driving pulse from the driving pulse generation circuit 6.

  When an ultrasonic wave is received by one ultrasonic sensor B1, a charge corresponding to the magnitude of the received ultrasonic wave is generated from the ultrasonic sensor B1, and this generated charge is added to the time detection circuit 2. It is done. The subsequent signal processing operation of the arrival time detection circuit 2 is the same as described above.

  The arithmetic control circuit 7 outputs a trigger signal to the drive pulse generation circuit 6 and detects the zero cross point after the ultrasonic wave transmitted from the upstream ultrasonic sensor B1 is received by the downstream ultrasonic sensor B2. The time t1 required until the zero cross point of the received signal is detected by the circuit 5 and the ultrasonic wave transmitted from the downstream ultrasonic sensor B2 by outputting the trigger signal to the drive pulse generating circuit 6 are upstream. The time t2 required until the zero cross point of the received signal is detected by the zero cross point detection circuit 6 after being received by the ultrasonic sensor B1 is measured.

  Subsequently, the arithmetic control circuit 7 obtains a difference Δt (= t2−t1) between the two times t1 and t2 thus measured. In this case, since the influence of the offset times Δ1 and Δ2 caused by delay factors associated with the ultrasonic sensors B1 and B2 is excluded from the measurement times t1 and t2, the time indicated by the above-described equation (1). The error Δe becomes extremely small, and an accurate time difference Δt is calculated. Therefore, based on this time difference Δt, the flow rate is calculated with higher accuracy than before.

  A set of two ultrasonic sensors B1 and B2 arbitrarily extracted (here, 47 sets) are made, and each set of ultrasonic sensors B1 and B2 is used to detect the arrival time of the present invention shown in FIG. An ultrasonic flow meter 1 having a circuit 2 and an ultrasonic flow meter having a conventional arrival time detection circuit (FIG. 7) were respectively configured. In this case, pairing is not performed for each pair of ultrasonic sensors B1 and B2.

  Then, using the ultrasonic flowmeter of the present invention and the conventional ultrasonic flowmeters, the times t1 and t2 required to transmit and receive the ultrasonic waves in the forward and reverse directions are measured under the condition that the fluid flow velocity is zero. The time difference between the two (hereinafter referred to as arrival time difference) Δt was measured as shown in the above equation (1). The result is shown in FIG.

  As can be seen from FIG. 3, the dispersion of the arrival time difference Δt measured by the ultrasonic flowmeter 1 having the arrival time detection circuit 2 of the present invention (FIG. 3A) is the difference of the arrival time difference Δt measured by the conventional apparatus. It is extremely small compared to the variation ((b) in the figure).

  In addition, the relationship between the capacitance difference ΔC of the pair of ultrasonic sensors B1 and B2 and the arrival time difference Δt of the ultrasonic waves was examined for the case of the present invention and the conventional case, respectively. The results are compared and shown in FIG.

  As apparent from FIG. 4, when the arrival time difference Δt is measured by the ultrasonic flowmeter 1 having the arrival time detection circuit 2 of the present invention, the difference in capacitance difference ΔC between the pair of ultrasonic sensors B1 and B2 The arrival time difference Δt shows a substantially constant value without being affected by. On the other hand, when the arrival time difference is measured by the conventional apparatus, it can be seen that the arrival time difference Δt is also greatly influenced by the difference in capacitance between the pair of ultrasonic sensors B1 and B2. In addition, in the case of the conventional apparatus, the variation in the arrival time difference Δt due to the capacitance difference ΔC between the pair of ultrasonic sensors B1, B2 is also large.

  The electrostatic capacitances of the ultrasonic sensors B1 and B2 are temperature-dependent. Therefore, it is conceivable that the offset times Δ1 and Δ2 increase depending on the temperature and affect the arrival time difference Δt. Therefore, the amount of change in the arrival time difference Δt when using the present invention and each of the conventional ultrasonic flowmeters at that time when the temperature is changed in the range of −30 ° C. to 70 ° C. with reference to the arrival time difference at normal temperature. I investigated. The result is shown in FIG.

  As is apparent from FIG. 5, the amount of change in the arrival time difference Δt measured by the ultrasonic flowmeter 1 having the arrival time detection circuit 2 of the present invention (FIG. 5A) is the arrival time difference Δt measured by the conventional apparatus. Is very small compared to the amount of change ((b) in the figure). Specifically, the standard deviation σ is conventionally 1.9, but in the present invention, the standard deviation σ is as small as 0.9. Therefore, the ultrasonic flowmeter according to the present invention can reduce the temperature dependence of the arrival time difference Δt to about 1/3 of the conventional one. As described above, it is understood that the ultrasonic flowmeter having the reception signal detection circuit of the present invention is more advantageous because it is hardly affected by the temperature change.

  In the prior art, when a piezoelectric element is used as various detection sensors, an output signal from the piezoelectric element appears as a change in charge. Therefore, a charge amplifier that amplifies the change in charge and outputs it as a voltage change is provided. There is something.

  However, the charge amplifier in this case is not intended to reduce the influence of the offset time of the ultrasonic sensor and eliminate the occurrence of a time error as in the present invention.

It is a block diagram which shows the whole structure of the ultrasonic flowmeter in Embodiment 1 of this invention. It is a circuit diagram which shows the specific structure of the arrival time detection circuit which comprises the same flowmeter. It is a normal distribution diagram which shows the result of having investigated the dispersion | variation in the arrival time difference of an ultrasonic wave, respectively about the case where the ultrasonic flowmeter provided with the circuit for arrival time detection of this invention is used, and the case where a conventional apparatus is used. The characteristic which shows in comparison the result which investigated the relation between the capacitance difference of a pair of ultrasonic sensors, and the arrival time difference of an ultrasonic wave about the ultrasonic flowmeter provided with the arrival time detection circuit of the present invention, and the conventional device FIG. Normal distribution diagram showing the results of examining the temperature dependence of the variation in the arrival time difference of each ultrasonic wave when using the ultrasonic flowmeter equipped with the arrival time detection circuit of the present invention and when using the conventional apparatus It is. It is principle explanatory drawing in the case of measuring the flow volume of the fluid using an ultrasonic flowmeter. It is a circuit diagram which shows the structure of the circuit for arrival time detection in the conventional ultrasonic sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ultrasonic flowmeter B1, B2 Ultrasonic sensor 2 Arrival time detection circuit 3 Charge amplifier 5 Zero cross point detection circuit

Claims (2)

An ultrasonic sensor composed of a piezoelectric element, a charge amplifier that converts the electric charge generated from the ultrasonic sensor into a voltage signal in response to reception of the ultrasonic wave, and a zero cross of the voltage signal output from the charge amplifier An ultrasonic arrival time detection circuit in an ultrasonic sensor, comprising: a zero-cross point detection circuit for detecting a point. An ultrasonic arrival time detection circuit in the ultrasonic sensor according to claim 1 is provided, and the two ultrasonic sensors are arranged to face each other in a state where they are separated by a predetermined distance in a flow path through which a fluid flows. Ultrasonic flow meter characterized by.

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