JP2008164329A - Ultrasonic flow meter - Google Patents

Abstract

The output of an ultrasonic sensor is weak and has a voltage of 10 ⁻³ V and a current of 20 × 10 ⁻⁶ A. Therefore, when electrical noise (common mode noise) from the outside is superimposed on this output, high-precision measurement cannot be performed.
A flow path for guiding a fluid, ultrasonic sensors 51 and 52 provided in the flow path, and a control unit are provided. The control unit transmits a transmission signal to the ultrasonic sensor. And a reception circuit 56 for detecting a reception wave of the ultrasonic sensor, the reception circuit has a constant voltage source 56, the reception circuit has a low impedance termination 57, and a low impedance The termination 57 is configured such that a part of the output of the ultrasonic sensor is terminated at a low impedance without being shared with the reference potential of the receiving circuit.
[Selection] Figure 1

Description

  The present invention relates to an ultrasonic flowmeter that measures the flow rate of a fluid such as city gas or LP gas, particularly by ultrasonic waves.

  A conventional ultrasonic flowmeter is described in Patent Document 1, for example.

  FIG. 4 is a control block diagram showing a conventional ultrasonic flowmeter described in Patent Document 1. In FIG.

  In FIG. 4, a first vibrator 5 that transmits ultrasonic waves and a second vibrator 6 that receives ultrasonic waves are disposed in the flow direction along the fluid conduit 4. 7 is a transmission circuit to the first vibrator 5, and 8 is an amplification circuit for a signal received by the second vibrator 6. This amplified signal is compared with a reference signal by the comparison circuit 9, and the time from transmission to reception is shown. Is calculated by the time counting means 10 such as a timer counter, the flow rate value is obtained by the flow rate calculation means 11 in consideration of the size of the pipeline and the flow state according to the ultrasonic propagation time, and the value of the flow rate calculation means 11 is obtained. To adjust the timing of signal transmission to the trigger means 13 of the transmission circuit 7.

  Next, the operation will be described. The ultrasonic signal transmitted from the trigger circuit 13 from the transmission circuit 7 and transmitted from the first vibrator 5 is transmitted through the flow and received by the second vibrator 6, and is received by the amplification circuit 8 and the comparison circuit 9. Signal processing is performed, and the time from transmission to reception is measured by the time measuring means 10.

If the sound in the static fluid is c and the flow velocity of the fluid is v, the propagation speed of the ultrasonic wave in the forward direction of the flow is (c + v). When the distance between the transducers 5 and 6 is L, and the angle formed by the ultrasonic transmission axis and the central axis of the pipe is φ, the time T that the ultrasonic wave reaches is:
T = L / (c + vcosφ) (1)
From the equation (1), v = (L / Tc) / cosφ (2)
If L and φ are known, the flow velocity v can be obtained by measuring T. From this flow velocity, if the flow rate Q is S and the correction count is K,
Q = KSv (3)
It becomes.

  FIG. 5 is a control block diagram showing a fourth embodiment of the conventional ultrasonic flowmeter described in the publication, and repeats transmission and reception by the number of times set by the repeat setting means 16 by the repeat means 15; Further, after switching between oscillation and reception by the switching means 17, the same is repeated. That is, an ultrasonic wave is generated from the first vibrator 4 by the oscillation circuit 7, and this ultrasonic wave is received by the second vibrator 5 and reaches the comparison circuit 9 via the amplifier circuit 8. The transmission circuit 7 is triggered. This repetition is performed the number of times set by the repetition setting means 15, and when the set number of times is reached, the time required for the repetition is measured by the time measuring means 10. After that, the transmission and reception of the first vibrator 4 and the second vibrator 5 are reversely connected by the switching means 17, and this time, the ultrasonic waves are sent from the second vibrator toward the first vibrator 5 and the same as described above. The flow time value is calculated by the flow rate calculation means 11.

If the sound in the static fluid is c and the flow velocity of the fluid is v, the propagation speed of ultrasonic waves in the forward direction of the flow is (c + v), and the propagation speed of the reverse direction is (cv). If the distance between the transducers 7 and 8 is L, the angle between the ultrasonic transmission axis and the central axis of the pipe is φ, and the number of repetitions is n, the forward and reverse repetition times T1 and T2 respectively. Is
T1 = n × L / (c + v cos φ) (4)
T2 = n × L / (c−v cos φ) (5)
From the equations (4) and (5), v = n × L / 2 cos φ × (1 / T1-1 / T2) (6)
If L and φ are known, the flow velocity v can be obtained by measuring T1 and T2. However, the difference between T1 and T2 is extremely small when the flow rate is small and the number of repetitions is small, and it is difficult to measure accurately. Therefore, when the number of measurements is set large to reduce the error relatively, and the flow rate increases, T1-T2 Therefore, the measurement becomes easy, and in this case, the number of repeated settings is reduced, the sampling interval is increased, and the error is reduced.

  That is, the number of repetition setting means 15 is changed by the flow rate calculation means 11.

  Patent Document 1 is a method of calculating flow rate by using two transducers to switch between transmission and reception, obtaining a flow velocity from ultrasonic propagation time obtained from each received waveform, and calculating a flow rate.

Further, in Patent Document 2, the vibrator and the amplification circuit (receiver circuit) for the signal received by the vibrator are determining factors for the shape of the received waveform, so that the flow measurement accuracy is greatly influenced by these. There is a problem, and for this reason, it is described that a difference in characteristics of a plurality of vibrators and a difference in reception waveform caused by a difference in circuit impedance are reduced. Specifically, a transmission circuit having a voltage source and a reception circuit having a low impedance termination element constituted by an operational amplifier are used.
JP-A-8-122117 JP 2004-144700 A

  The flow rate is obtained from the flow velocity, and the flow velocity is expressed by the difference between the reciprocal times T1 and T2 in the forward direction and the reverse direction from the equation (6). The reciprocal difference can be approximated as (T1-T2).

Depending on the size of the conduit (also referred to as the flow path) through which the fluid flows, if the size of an ultrasonic flowmeter that measures gas of 2000 liters / hour to 6000 liters / hour, (T1-T2) is If it is 1 × 10 ⁻⁹ seconds, it corresponds to 2 liters / hour, and it is necessary to measure the time with great accuracy when attempting to measure with an accuracy of 1 liter / hour.

Further, the outputs of the first and second vibrators (also referred to as ultrasonic sensors) are weak and have a voltage of 10 ⁻³ V and a current of 20 × 10 ⁻⁶ A. Therefore, if electrical noise from the outside is superimposed on this output, high-precision measurement cannot be performed. For this reason, the conventional ultrasonic flowmeter has taken noise countermeasures such as using a noise filter and a shield wire for the lead wire of the ultrasonic sensor, but it has been found to be weak against common mode noise.

  FIG. 6 is a schematic diagram showing the connection between an ultrasonic sensor in a conventional ultrasonic flowmeter and a receiving circuit having a low impedance termination. The ultrasonic sensor 20 is shown in a cross-sectional structure, and has a structure in which a piezoelectric vibrating body 21 having electrodes on upper and lower surfaces is provided in a metal case 22, and the upper surface of the piezoelectric vibrating body 21 is electrically connected to the metal case 22. It is connected. The electrode on the lower surface is connected to a terminal 23 that is insulated from the metal case 22. A terminal 24 having the same potential as that of the metal case 22 is also provided. The terminals 23 and 24 are connected to the shield wire 25. The shield wire 25 is also shown in a cross-sectional structure.

  The terminal 23 is connected to the core wire of the shield wire 25, and the terminal 24 is connected to a mesh line provided also on the outer periphery of the shield wire 25. The mesh line on the outer periphery of the shield line 25 is connected to a reference potential 27 of a receiving circuit 26 having a low-impedance termination element composed of an operational amplifier. The core wire of the shield wire 25 is input to the operational amplifier of the receiving circuit 26. Of the high-frequency electromagnetic wave noises 28 and 29 superimposed in common mode on the reference potential 27 and the shield line 25 of the reception circuit 26, the high-frequency electromagnetic wave noise 29 does not enter the reception circuit 26, but the high-frequency electromagnetic wave noise 28 does not enter the reception circuit 26. In order to enter and cause measurement errors, there is a problem that the receiving circuit must be made strong against common mode noise.

  Since common mode noise is generally superimposed on two electric wires in the same manner, the common mode noise is eliminated by taking the difference in signal between the two electric wires.

  The receiving circuit that receives the signal of the ultrasonic sensor of the present invention has a low impedance termination, and the low impedance termination shares a part of the two signal lines of the ultrasonic sensor with the reference potential of the receiving circuit. Instead, the two wires are each terminated with a low impedance so that the signal difference between the two wires is taken. Further, the transmission circuit is configured to have a constant voltage source so as to reduce the difference in the received waveform of the two ultrasonic sensors caused by the difference in the characteristics of the two ultrasonic sensors.

(Embodiment 1)
A flow path for guiding a fluid; an ultrasonic sensor provided in the flow path; and a control unit, wherein the control unit includes a transmission circuit that provides a transmission signal to the ultrasonic sensor, and a received wave of the ultrasonic sensor. The transmitter circuit has a constant voltage source, the receiver circuit has a low impedance termination, and the low impedance termination is a signal of the ultrasonic sensor. A part of the line is terminated not with the reference potential of the receiving circuit but with a low impedance.

  FIG. 1 is a circuit diagram showing an example of the ultrasonic flowmeter according to the first embodiment of the present invention. A flow path 50 is a pipe through which a fluid passes, and ultrasonic sensors 51 and 52 are provided on the way. The ultrasonic sensors 51 and 52 are connected to a printed board on which the control unit 53 is provided by lead wires. A transmission circuit 54 for supplying transmission signals to the ultrasonic sensors 51 and 52 to the printed circuit board via lead wires, and a reception circuit for transmitting signals from the ultrasonic sensors 51 and 52 via the lead wires 55, a first-stage amplifier circuit A59 that amplifies the output of the receiving circuit 55, a second-stage amplifier circuit B60, and a comparison circuit 64 that compares the output of the amplifier circuit B60 with the DC potential supplied from the voltage source 62. The control part 53 which consists of is provided. The switch group 65 is provided for switching between transmission and reception of the ultrasonic sensors 51 and 52, and in the figure, the ultrasonic sensor 51 is connected to the transmission circuit 54 and the ultrasonic sensor 52 is connected to the reception circuit 56.

  Two signal lines of the ultrasonic sensor 52 connected to the receiving circuit 56 are connected to a low impedance termination 57 of the receiving circuit 58, respectively. The low-impedance termination 57 is composed of an operational amplifier. The operational amplifier is fed back from the output to the negative terminal with a resistor, and a DC voltage is applied to the positive terminal from a voltage source. For this reason, the output potential of the operational amplifier changes so that the negative terminal is kept at the DC voltage of the voltage source. This is virtually the same as when the voltage source is connected to the negative terminal. Since the impedance of the voltage source is minimal, it is regarded as a low impedance termination.

Thus, since the negative terminal of the operational amplifier which is the low impedance termination 57 is connected to the ultrasonic sensor 52 and the positive terminal is connected to the voltage source, one of the ultrasonic sensors 52 is connected to the reference potential 58 of the receiving circuit. Without being common, both the two signal lines of the ultrasonic sensor are terminated with low impedance. The low impedance termination composed of an operational amplifier outputs a voltage having a magnitude proportional to the current of the ultrasonic sensor 52.

  The transmission circuit 54 includes a transistor group driven by a voltage source 56 and an oscillation circuit 65, and generates a rectangular wave of about 500 kHz that drives the ultrasonic sensor 51. That is, it is the voltage source 56 that is connected to and driven by the ultrasonic sensor 51. With such a configuration, it is possible to reduce the difference in received waveform between the two ultrasonic sensors, which is caused by the characteristic difference between the two ultrasonic sensors.

  FIG. 2 is a schematic diagram showing the connection between the ultrasonic sensor 52 and the receiving circuit 55, and the same reference numerals are used for the parts common to the conventional example of FIG. The ultrasonic sensor 52 and the two-wire shielded wire 67 are connected, and the two wires of the two-wire shielded wire 67 are connected to the receiving circuit 55. The mesh line on the outer periphery of the two-wire shield wire 67 is connected to the reference potential 58 of the receiving circuit. High-frequency common mode noises 68 and 69 superimposed on the two-wire shield line 67 with respect to the reference potential 58 enter the reception circuit 55, but the reception circuit 55 flows between the two lines of the two-wire shield line 67. In order to output a voltage proportional to the current, the common mode noises 68 and 69 are canceled at the output of the receiving circuit 55.

(Embodiment 2)
The output of the receiving circuit having a low impedance termination element can be amplified by a differential amplifier circuit. FIG. 1 is a circuit diagram showing an example of an ultrasonic flowmeter according to Embodiment 2 of the present invention. In the figure, the output of the receiving circuit 55 is connected to a differential amplifier circuit A59, and the output of the amplifier circuit A59 is connected to a differential amplifier circuit B to perform signal amplification. Since both of the amplifier circuit A59 and the amplifier circuit B60 are configured as differential amplifier circuits, they have the ability to remove common mode noise and can obtain performance with high noise resistance.

(Embodiment 3)
Amplification is composed of at least two differential amplifier circuits A and B whose amplification degree can be varied, and the amplification degree of the amplification circuit A is changed from a state where the amplification circuits A and B are set to a total amplification degree n. Also, the amplification degree of the amplification circuits A and B is set so as to include the amplification degree that can be made the same as the original amplification degree n by changing the amplification degree of the amplification circuit B. FIG. 1 is a circuit diagram showing an example of an ultrasonic flowmeter according to Embodiment 3 of the present invention. The differential amplifier circuit A59 and the differential amplifier circuit B60 are configured such that the feedback resistor can be varied so that the amplification degree can be changed.

  The amplification degrees (multiples) of the amplification circuit A59 and amplification circuit B60 are set as shown in the amplification degree table shown in FIG. The amplification circuit A has an amplification factor of 1, 2 times, 3 times, 4 times, 5 times, the amplification circuit B has a time of 1, 2, 3, 4 and 5 times. Become. Amplification circuit A is included in the double column, and the total of 3, 4, 5, and 6 times is also included in the total in the column of amplification circuit A of 1. In addition, among the totals in the column for the amplification circuit A, 4 times, 5 times, 6 times, and 7 times are included in the total in the column for the amplification circuit A 3 times.

The magnitude of the received signal of the ultrasonic sensor changes depending on the magnitude of the flow rate and the temperature. Also, the magnitude of the received signal when the ultrasonic sensor 51 is set to the transmitting side and the ultrasonic sensor 52 is set to the receiving side, and the magnitude of the received signal when the ultrasonic sensor 52 is set to the transmitting side and the ultrasonic sensor 51 is set to the receiving side. May vary depending on the magnitude of the flow rate. Since it has such characteristics, in order to simplify the signal processing of the received signal, the output of the amplifier circuit is kept constant by changing the amplification degree of the amplifier circuit. For this reason, it is necessary to change the amplification degree of the amplifier circuit. Now, when there is a change in the magnitude of the received signal such that the amplification degree of the amplification circuit A is 2 and the amplification degree of the amplification circuit B is in the range of 4 and 5 (a total range of 6 and 7), When it is necessary to increase the amplification degree, the amplification degree of the amplification circuit A shifts to the range of 3 and the amplification degree of the amplification circuit B shifts to the range of 4 and 5 (a total range of 7 and 8). At this time, when a change in the received signal that requires a decrease in the amplification degree occurs, the amplification degree of the circuit A is 3 and the amplification degree of the amplification circuit B is 3 or 4 (total of 6 or 7). Move on. Again, when it is necessary to increase the amplification degree, the amplification degree of the amplification circuit A is 3, and the amplification degree of the amplification circuit B is in the range of 3, 4, and 5 (total of 6, 7, and 8). Can work. In this case, it is possible to cope with the amplification degree of the amplification circuit A being 3 as it is.

  Considering the case where the same thing occurs in the case as shown in FIG. 5B, when trying to obtain a total amplification degree of 6 or 7, the amplification degree of the amplification circuit A is 1 and the amplification circuit B When the amplification degree is 5, and the amplification degree of the amplification circuit A is 6 and the amplification degree of the amplification circuit B is only 1, and the amplification degree needs to be increased, the amplification degree of the amplification circuit A is 6 Thus, the amplification degree of the amplifier circuit B shifts to a range of 1 or 2 (a total range of 7 or 8). At this time, when a change in the received signal that requires a decrease in the amplification degree occurs, the amplification degree of the circuit A is 1, the amplification degree of the amplification circuit B is 5 (total 6), and the amplification degree of the amplification circuit A 6 and the amplification degree of the amplifier circuit B shifts to 1 (7 in total). Again, when it is necessary to increase the amplification degree, the amplification degree of the amplification circuit A is 1, the amplification degree of the amplification circuit B is 5, the amplification degree of the amplification circuit A is 6, and the amplification degree of the amplification circuit B is 1. 2 (total of 6, 7, and 8), and the amplification degree of both the amplifier circuits A and B changes. The amplification circuit may change the distortion of the waveform when the amplification degree changes, and even if it is very small, it may not be negligible for high-precision measurement.

  For this reason, it is desirable that the measurement can be made within the change in the amplification degree of only the amplification circuit B, rather than the amplification degree of both of the amplification circuits A and B being changed. Therefore, as shown in FIG. 5A, the amplification is made up of at least two differential amplification circuits A and B whose amplification degree can be varied, and the amplification circuits A and B are set to a total amplification degree of n. Even if the amplification degree of the amplification circuit A is changed, the amplification degree of the amplification circuits A and B is set so as to include the amplification degree that can be made the same as the original amplification degree n by changing the amplification degree of the amplification circuit B.

(Embodiment 4)
The final configuration of the amplifier circuit is configured to output the amplified ultrasonic sensor signal so as to change with respect to the DC potential. FIG. 1 is a circuit diagram showing an example of an ultrasonic flowmeter according to Embodiment 4 of the present invention. In the figure, the final 61 of the amplifier circuit is composed of an operational amplifier in which the DC potential of the voltage source 62 is applied to the plus terminal. The operational amplifier outputs a difference between signals applied to the plus terminal and the minus terminal, and the signal is a signal on which the DC potential of the voltage source 62 is superimposed. With this configuration, reception detection can be performed by the comparison circuit at the next stage. The amplification degree of this operational amplifier is determined by the resistance in the final 61 of the amplifier circuit.

(Embodiment 5)
The final output of the amplifier circuit is compared with the direct current potential determined by the comparison circuit to generate a signal for determining the propagation time of the ultrasonic wave. The predetermined direct current potential is the direct current voltage used at the end of the amplifier circuit. Use the same thing as the place. As described above, in order to perform high-precision measurement, the potential of the voltage source serving as a reference to be compared in the comparison circuit and the direct current potential of the voltage source superimposed on the signal at the end of the amplification circuit are such as temperature. Due to the influence, if drift (change) individually, an error occurs in the measurement of the propagation time. For this reason, the same potential is used as the reference potential of the voltage source to be compared by the comparison circuit and the direct current potential of the voltage source superimposed on the signal at the end of the amplifier circuit.

FIG. 1 is a circuit diagram showing an example of an ultrasonic flowmeter according to Embodiment 4 of the present invention. In the figure, the final output 63 of the amplifier circuit is input to the minus terminal of the comparison circuit 64, and the voltage source 62 is connected to the plus terminal. This voltage source 62 is the same as the voltage source superimposed on the signal of the ultrasonic sensor in the final 61 of the amplifier circuit. Therefore, even if the DC potential of the voltage source 62 drifts (changes) due to temperature, the DC potential serving as a reference for comparison applied to the plus terminal of the comparison circuit 64 also changes in the same manner, so that an error occurs in the comparison. It becomes difficult to do.

  As described above, the ultrasonic flowmeter according to the present invention can make electrical (electromagnetic wave) noise from outside hardly affect the measurement. It can be used for commercial and household ultrasonic gas flow measurement devices (gas meters) that measure the flow of liquefied petroleum gas.

The circuit block diagram which shows the structure of the ultrasonic flowmeter of this invention The schematic diagram which shows the connection structure of the ultrasonic sensor of this invention, and a receiving circuit The figure explaining the setting of the amplification degree of an amplifier Control block diagram showing the configuration of a conventional ultrasonic flowmeter Control block diagram showing the configuration of a conventional ultrasonic flowmeter Schematic diagram showing the connection structure between a conventional ultrasonic sensor and a receiving circuit

Explanation of symbols

DESCRIPTION OF SYMBOLS 50 Flow path 51,52 Ultrasonic sensor 53 Control part 54 Transmission circuit 55 Reception circuit 56 Constant voltage source 57 Low impedance termination 58 Reference potential of reception circuit 59 Amplifier circuit A
60 Amplifier circuit B
61 Final of amplifier circuit 62 DC voltage 63 Final output of amplifier circuit 64 Comparison circuit

Claims (5)

A flow path for guiding a fluid; an ultrasonic sensor provided in the flow path; and a control unit, wherein the control unit includes a transmission circuit that provides a transmission signal to the ultrasonic sensor, and a received wave of the ultrasonic sensor. The transmitter circuit has a constant voltage source, the receiver circuit has a low impedance termination, and the low impedance termination is the output of the ultrasonic sensor. The ultrasonic flowmeter is configured such that a part thereof is terminated with a low impedance without being shared with the reference potential of the receiving circuit. 2. The ultrasonic flowmeter according to claim 1, wherein an output of a receiving circuit including a low impedance termination element can be amplified by a differential amplifier circuit. Amplification is composed of at least two differential amplifier circuits A and B whose amplification degree can be varied, and the amplification degree of the amplification circuit A is changed from a state where the amplification circuits A and B are set to a total amplification degree n. The ultrasonic flowmeter according to claim 2, wherein the amplification degree of the amplification circuits A and B is set so as to include an amplification degree that can be made the same as the original amplification degree n by changing the amplification degree of the amplification circuit B. 3. The ultrasonic flowmeter according to claim 2, wherein the final output of the amplifier circuit is an output so that the amplified ultrasonic sensor signal changes with reference to a predetermined DC potential. The final output of the amplifier circuit is compared with a predetermined DC potential by a comparison circuit to generate a signal for determining the propagation time of the ultrasonic wave, and the predetermined DC potential is the DC power used at the final stage of the amplifier circuit. The ultrasonic flowmeter according to claim 4, wherein the same one is used as the position.