JP2008164330A - Ultrasound flowmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To ensure highly precise measurement by constituting the terminal board and the noise filter part shielded for inhibiting the electric noise from external superimposing on the output of the ultrasound sensor because of weak output of the ultrasound sensor, the output of which are voltage of 10<SP>-3</SP>V and current of 20×10<SP>-6</SP>A. <P>SOLUTION: The ultrasound flowmeter is constituted of a terminal board 58, and a noise filter part 57, both of which are shielded. Moreover, it is constituted so that the radiated noise hardly superimpose on lead wires 65 of ultrasound sensors 52, 53 and lines on a printed board 55, because of the superimposed noise on the lead wires and the lines affect to the measurement. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an ultrasonic flowmeter that measures the flow rate of a fluid such as city gas and propane gas by ultrasonic waves.

  A conventional ultrasonic flowmeter is described in Patent Document 1, for example. FIG. 5 is a control block diagram showing a conventional ultrasonic flowmeter described in Patent Document 1. In FIG.

  In FIG. 5, a first vibrator 5 that transmits ultrasonic waves and a second vibrator 6 that receives ultrasonic waves are arranged in the flow direction in the middle of 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. 6 is a control block diagram showing a fourth embodiment of the conventional ultrasonic flowmeter described in the above publication, and repeats transmission and reception by the number of times set by the repetition setting means 16 by the repetition 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 based on this difference.

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. Japanese Patent Application Laid-Open No. 10-30947 discloses a measurement method that uses ultrasonic waves to measure ultrasonic propagation time with high accuracy in a short time and with low power consumption.

  As shown in FIG. 7, the transducer 25 that transmits / receives an ultrasonic signal, the comparison unit 29 that compares the AC reception signal of the transducer 25 with a threshold over a plurality of periods, and the transmission unit of the transducer 25 to the comparison unit 29 A time measuring unit 31 for measuring a plurality of propagation times for each detection and a time calculating unit 32 for calculating the propagation time from the average value of the time measured values of the time measuring unit 31 are provided. As a result, a measurement value obtained by comparing with the comparison means many times by one ultrasonic transmission / reception can be obtained. Therefore, by obtaining the average value, a highly accurate propagation time measurement value can be obtained in a short time, and the consumption is low. It is described so that measurement can be performed with electric power.

Patent Document 1 and Patent Document 2 both use two transducers to switch between transmission and reception, calculate the flow rate from the propagation time of the ultrasonic wave determined from each received waveform, and calculate the flow rate It is.
JP-A-8-122117 Japanese Patent Laid-Open No. 10-30947

  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 1 × 10 ⁻⁹ seconds corresponds to about 2 liters / hour, and it is necessary to measure the time with great accuracy in order to obtain a resolution 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 includes a flow path for guiding a fluid, an ultrasonic sensor provided in the flow path, a signal processing unit of the ultrasonic sensor, a control unit for controlling the signal processing unit, The control unit is provided with a terminal block provided for receiving an external signal, and a noise filter unit is provided between the terminal block and the control unit. Furthermore, in order to perform sufficient noise removal, a configuration in which a noise filter unit is also provided in a cord connecting the terminal block and an external device, and there is a problem that the configuration of the noise filter is large.

  We analyzed the mechanism of how noise affects measurement. As a result, the following became clear. A cord for connection with an external device attached to the terminal block serves as an antenna, and a high-frequency electromagnetic wave that becomes noise is superimposed, propagates through the cord, and enters the housing of the ultrasonic flowmeter from the terminal block. Although this noise is attenuated by the noise filter unit, electromagnetic wave is further radiated by the noise filter unit, which is radiated to the control unit and the signal processing unit to have an influence. For this reason, when noise of a certain intensity or more enters the noise filter, the effect is not so much improved even if the attenuation capability of the noise filter is increased. Therefore, it is necessary to reduce radiation from the noise filter unit. Furthermore, when noise of a certain intensity or more enters from the terminal block, the radiation from the noise is radiated to the control unit and the signal processing unit, so the effect of the noise filter is lost.

  For this reason, the ultrasonic flowmeter of this invention is set as the structure which shields a terminal block and a noise filter part. Further, since the noise that is radiated is superimposed on the lead wire of the ultrasonic sensor or the line on the printed circuit board, the measurement is affected. Therefore, the noise is hardly superimposed on the line or the lead wire.

  According to the present invention, it is possible to withstand strong electric field noise of about 30 V / m without adopting a large-scale noise filter configuration. Naturally, the noise filter portion required for the cord connecting the terminal block and the external device can be reduced.

(Embodiment 1)
A flow path for guiding a fluid, an ultrasonic sensor provided in the flow path, a signal processing unit for processing a signal from the ultrasonic sensor, a control unit for controlling the signal processing unit, and an external to the control unit A terminal block provided for receiving a signal from the terminal, and a noise filter provided between the terminal block and the control unit, the terminal block covering a plurality of surfaces with a shield member, and a shield The member is electrically connected to the housing directly or via a capacitor.

  FIG. 1 is a schematic cross-sectional view showing the configuration of the ultrasonic flowmeter according to Embodiment 1 of the present invention. A flow path 51 is a pipe through which a fluid passes, and ultrasonic sensors 52 and 53 are provided on the way. The ultrasonic sensors 52 and 53 are connected to a printed circuit board 55 provided with a signal processing unit 54 with lead wires. The printed circuit board 55 is also provided with a control unit 56, a noise filter unit 57, and a terminal block 58. The terminal block 58 covers a plurality of surfaces with a shield member 59. The shield member 59 is electrically connected to the metal housing 60 directly or via a capacitor. Since the problematic noise is high frequency, if the capacitor has a certain capacity, the impedance of the capacitor at high frequency becomes small, and the effect similar to that of the direct connection can be obtained. A cord 61 is connected to the terminal block 58 to exchange electric signals with an external device.

  The high frequency electromagnetic wave noise from the outside enters the ultrasonic flowmeter using the cord 61 as an antenna. However, since the terminal block 58 is surrounded by the shield member 59, the impedance at high frequency between the housing 61 and the terminal block 58 is small. For this reason, the high frequency electromagnetic wave noise which penetrate | invades from the cord 61 is reduced.

  FIG. 2 is a schematic perspective view, in which FIG. 2A is a terminal block 58, FIG. 2B is a shield member 59, and the terminal block 58 has pins 70 soldered to the printed circuit board 55. FIG. The shield member 59 has a box shape without an upper lid and is made of metal. The terminal block 58 is fitted into the shield member 59 and soldered to the printed board 55.

  3 also shows the terminal block 58 and the shield member 59. FIG. 3A is a side view of the terminal block 58, and FIG. 3B is a top view of the shield member 59. FIG. The shield member 59 is provided with a hole 71 through which the terminal block pin 70 passes. As described above, the terminal block 58 is surrounded by the shield member 59 on the side surface and the bottom surface. The more surfaces surrounding the terminal block 58, the more advantageous is that the impedance at high frequency between the housing 60 and the terminal block 58 becomes smaller. In particular, the shield member 59 portion on the bottom surface of the terminal block 58 is important because the pattern of the printed circuit board 55 is close.

(Embodiment 2)
The noise filter unit covers a plurality of surfaces with a shield member, and the shield member is electrically connected to the housing directly or via a capacitor.
In FIG. 1, the noise filter unit 57 covers a plurality of surfaces with a shield member B62.

  This is to shield the high frequency electromagnetic wave noise radiated from the noise filter unit 57. The shield member B62 and the casing 60 are electrically connected to the casing 60 directly or via a capacitor. The impedance at a high frequency between them is reduced to reduce high frequency electromagnetic noise.

(Embodiment 3)
The noise filter unit, the control unit, and the terminal block are provided on a single printed circuit board, the noise filter unit is shielded by a shield member from above and below the printed circuit board, and the shield member has a reference potential provided on the printed circuit board. Electrical connection to the pattern at multiple points.

  In FIG. 4, the noise filter unit 57, the control unit 56, and the terminal block 58 are provided on one printed circuit board 55. The noise filter unit 57 is covered from above and below the printed circuit board 55 with a metal shield member B 62, and the shield member B 62 is printed. It is electrically coupled to a reference potential pattern provided on the substrate 55 at a plurality of locations. Further, the reference potential pattern and the housing 60 are electrically connected directly or via a capacitor.

  For this reason, since the impedance in the high frequency between the shield member B62 and the housing | casing 60 is reduced, the high frequency electromagnetic wave noise reduces. In this case, the printed board 55 is desirably thin.

(Embodiment 4)
The signal processing unit is configured on a printed circuit board, and the reference potential pattern of the signal processing unit is electrically connected to the housing from a location near the signal processing unit directly or via a capacitor.

  In FIG. 1, the signal processing unit 54 is configured on a printed circuit board 55, and a reference potential pattern 63 of the signal processing unit 54 is electrically connected to a housing 60 from a location 64 close to the signal processing unit 54 directly or via a capacitor. To do. When the signal processing unit 54 is electrically connected to the housing 60 from a location far away from the signal processing unit 54 directly or via a capacitor, the potential of the reference potential pattern at that location and the potential of the reference potential pattern at a location close to the signal processing unit 54 are obtained. However, it becomes different due to the influence of high-frequency noise, leading to deterioration in measurement accuracy.

(Embodiment 5)
The ultrasonic sensor and the signal processing unit provided on the printed circuit board are connected by a shielded lead wire, the shield is connected to a reference potential pattern on the printed circuit board, and the signal is output on the printed circuit board. A reference potential pattern is provided in the vicinity of the line led to the processing unit.

  In FIG. 1, a shielded lead wire 65 is used as a lead wire used for connection between the ultrasonic sensors 52 and 53 and the printed board 55, and a shield (a mesh wire is used) provided on the outer periphery of the lead wire serving as a shield. Is connected to a reference potential pattern 63 on the printed circuit board 55. Since high-frequency electromagnetic noise may be superimposed on the lead wire that guides the signal of the ultrasonic sensor that is a minute signal and may enter the signal processing unit 54, it is necessary to lower the impedance between the lead wire and the housing. is there. For this purpose, the lead wire is shielded. Further, the shield of the shielded lead wire is connected to the reference potential pattern 63 on the printed circuit board 55 so as to have the same potential as the casing.

(Embodiment 6)
It is equipped with element parts such as an ultrasonic sensor, a battery, a shut-off valve that stops fluid from entering and exiting the flow path, and a sensor that detects pressure and temperature. The lead wire is electrically connected to the print board provided with the lead wire, and the lead wire connecting the ultrasonic sensor and the print board connects the shut-off valve, the sensor for detecting pressure and temperature, and the print board. Separate from the lead wire.

  In FIG. 1, the shutoff valve 66 uses a lead wire 67 for electrical connection with a printed circuit board 55 provided with a control unit 56 for controlling the shutoff valve 66. Similarly, the pressure sensor 68 uses a lead wire 69. Further, the battery 40 is connected by a lead wire to supply power to the printed circuit board 55. Since the shutoff valve 66 and the pressure sensor 68 have relatively high impedance, their lead wires 67 and 69 are likely to become antennas, and high-frequency electromagnetic noise is likely to be superimposed. For this reason, the propagation of noise is prevented by arranging the lead wire 65 of the ultrasonic sensors 52 and 53 and the lead wires 67 and 69 separately. Since the battery has a relatively low impedance, its lead wire is unlikely to become an antenna, and therefore it is easy to allow the lead wires 65 of the ultrasonic sensors 52 and 53 to approach this lead wire.

(Embodiment 7)
A discharge gap is provided between the terminal block and the shield member of the terminal block. Although it is desirable that the shield member exists up to the vicinity of the pins of the terminal block, whether or not to discharge when a lightning surge voltage is generated is determined by the distance between the two. By setting the distance to discharge when lightning surge voltage is generated, the lightning surge energy is reduced by the discharge, and the surge absorber, resistor, and coil components that absorb the lightning surge energy configured in the noise filter section Energy capacity can be reduced. That is, it can be reduced in size or reduced.

  In FIG. 3, the voltage to be discharged is determined by the relationship between the diameter of the pin 70 and the diameter 71 of the hole of the shield member 59. This is because the shield member 59 is electrically connected to the housing 60 directly or via a capacitor, and the lightning surge voltage is applied between the housing and the pin 70 of the terminal block 58. If lightning surge voltage occurs in this part, if it is set to discharge, lightning surge energy is consumed by discharge, so surge absorbers and resistors that absorb lightning surge energy configured in the noise filter section, The energy tolerance of the coil components can be reduced.

  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.

Cross-sectional schematic diagram showing the structure of the ultrasonic flowmeter of the present invention (A) Schematic cross-sectional view showing the structure of the terminal block (b) Schematic cross-sectional view showing the structure of the shield member of the present invention (A) Side view of terminal block (b) Top view of shield member of the present invention Cross-sectional schematic diagram showing the configuration of the printed circuit board of the present invention Control block diagram showing the configuration of a conventional ultrasonic flowmeter Control block diagram showing the configuration of a conventional ultrasonic flowmeter Control block diagram showing the configuration of a conventional ultrasonic flowmeter

Explanation of symbols

51 Flow path 52 Ultrasonic sensor A
53 Ultrasonic sensor B
54 Signal Processing Unit 55 Printed Circuit Board 56 Control Unit 57 Noise Filter Unit 58 Terminal Block 59 Shield Member 60 Case 62 Shield Member B
63 Reference potential pattern 65 Shielded lead wire 66 Shut-off valve 67 Shut-off valve lead wire 68 Pressure sensor 69 Pressure sensor lead wire

Claims (7)

A flow path for guiding a fluid, an ultrasonic sensor provided in the flow path, a signal processing unit for processing a signal from the ultrasonic sensor, a control unit for controlling the signal processing unit, and an external to the control unit And a noise filter unit provided between the terminal block and the control unit, wherein the terminal block covers a plurality of surfaces with a shield member. Flowmeter. The ultrasonic flowmeter according to claim 1, wherein the noise filter portion covers a plurality of surfaces with a shield member. The noise filter unit, the control unit, and the terminal block are provided on a single printed circuit board, the noise filter unit is shielded by a shield member from above and below the printed circuit board, and the shield member has a reference potential provided on the printed circuit board. The ultrasonic flowmeter according to claim 2, wherein the ultrasonic flowmeter is electrically connected to the pattern at a plurality of locations. The signal processing unit is configured on a printed circuit board, and a reference potential pattern of the signal processing unit is electrically connected to a housing from a location near the signal processing unit directly or via a capacitor. Ultrasonic flow meter. The ultrasonic sensor and the signal processing unit provided on the printed circuit board are connected by a shielded lead wire, the shield is connected to a reference potential pattern on the printed circuit board, and the signal is output on the printed circuit board. The ultrasonic flowmeter according to claim 1, wherein a reference potential pattern is provided in the vicinity of a line led to the processing unit. It is equipped with element parts such as an ultrasonic sensor, a battery, a shut-off valve that stops fluid from entering and exiting the flow path, and a sensor that detects pressure, temperature, etc., and these element parts are controlled by lead wires and control parts and signal processing parts. The lead wire is electrically connected to the print board provided with the lead wire, and the lead wire connecting the ultrasonic sensor and the print board connects the shut-off valve, the sensor for detecting pressure and temperature, and the print board. The ultrasonic flowmeter according to claim 1, wherein the ultrasonic flowmeter is separated from the lead wire. The ultrasonic flowmeter according to claim 1, wherein a discharge gap is provided between the terminal block and the shield member of the terminal block.