JP2008014840A - Ultrasonic flowmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrasonic flowmeter capable of heightening measurement accuracy of a flow rate by removing a measurement error. <P>SOLUTION: In this ultrasonic flowmeter, a pair of ultrasonic vibrators 2a, 2b are provided at a fixed distance on the upstream side and on the downstream side of a channel, and an ultrasonic wave is transmitted/received by switching the transmission/reception direction between the forward direction and the reverse direction, and a flow rate is determined based on an arrival time of the ultrasonic wave in each direction. The flowmeter includes a transmission circuit 4 for generating a transmission signal to be transmitted to one ultrasonic vibrator, a reception detection circuit 5 for detecting a reception signal from the other ultrasonic vibrator, and a transmission/reception changeover switch 3 for switching mutually one ultrasonic vibrator to/from the other ultrasonic vibrator. The flowmeter has equalized values, namely, a resistance value of a route for the transmission signal when transmitting/receiving the ultrasonic wave in the forward direction, a resistance value of a route for the reception signal when transmitting/receiving the ultrasonic wave in the forward direction, a resistance value of a route for the transmission signal when transmitting/receiving the ultrasonic wave in the reverse direction, and a resistance value of a route for the reception signal when transmitting/receiving the ultrasonic wave in the reverse direction. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an ultrasonic flowmeter that measures a flow rate of gas or the like using ultrasonic waves.

  FIG. 6 is a diagram showing a configuration of a conventional ultrasonic flowmeter. This ultrasonic flow meter includes a flow path 1, a pair of ultrasonic transducers 2 a and 2 b, a transmission / reception changeover switch 3, a transmission circuit 4, a reception detection circuit 5, and a control circuit 6.

  A fluid to be measured flows through the flow path 1. The pair of ultrasonic transducers 2a and 2b are respectively arranged at a certain distance on the upstream side and the downstream side of the flow path 1, and the ultrasonic transducer 2a is directed toward the ultrasonic transducer 2b (hereinafter referred to as “the ultrasonic transducer 2b”). (Referred to as “forward direction”) and ultrasonic waves transmitted from the ultrasonic transducer 2b. Further, the ultrasonic transducer 2b transmits ultrasonic waves toward the ultrasonic transducer 2a (hereinafter referred to as “reverse direction”) and receives ultrasonic waves transmitted from the ultrasonic transducer 2a.

  The transmission / reception selector switch 3 switches whether the ultrasonic waves are transmitted in the forward direction or the reverse direction. Specifically, the transmission / reception selector switch 3 sends the transmission signal sent from the transmission circuit 4 to the ultrasonic transducer 2a and sends the reception signal obtained from the ultrasonic transducer 2b to the reception detection circuit 5, or Then, the transmission signal sent from the transmission circuit 4 is sent to the ultrasonic transducer 2b to switch whether the reception signal obtained from the ultrasonic transducer 2a is sent to the reception detection circuit 5.

  FIG. 7 is a circuit diagram showing a specific configuration of the transmission / reception selector switch 3. The transmission / reception selector switch 3 includes four first switches 31, second switches 32, third switches 33, and fourth switches 34. The first switch 31 is disposed between the transmission circuit 4 and the ultrasonic transducer 2a, and controls transmission of a transmission signal from the transmission circuit 4 to the ultrasonic transducer 2a. The second switch 32 is disposed between the transmission circuit 4 and the ultrasonic transducer 2b, and controls transmission of a transmission signal from the transmission circuit 4 to the ultrasonic transducer 2b. The third switch 33 is disposed between the reception detection circuit 5 and the ultrasonic transducer 2a, and controls transmission of a reception signal from the ultrasonic transducer 2a to the reception detection circuit 5. The fourth switch 34 is disposed between the reception detection circuit 5 and the ultrasonic transducer 2b, and controls transmission of a reception signal from the ultrasonic transducer 2b to the reception detection circuit 5.

  The transmission circuit 4 is driven by sending a transmission signal to the ultrasonic transducer 2a or 2b on the transmission side according to the trigger signal from the control circuit 6. The reception detection circuit 5 detects a reception signal received by the ultrasonic transducer 2 a or 2 b on the reception side and sends it to the control circuit 6. The control circuit 6 measures the propagation time from when the transmission signal is transmitted until the reception signal is received. That is, since the propagation time difference of the ultrasonic wave is generated depending on whether the flow of the fluid in the flow path 1 is the forward direction or the reverse direction, the control circuit 6 measures the difference in the propagation time between the forward direction and the reverse direction. And the flow rate is measured based on the obtained flow rate.

  By the way, due to manufacturing variations of the ultrasonic transducers 2a and 2b, even when the fluid is not flowing, there is a difference in the propagation time of the ultrasonic wave between the case where the ultrasonic wave is transmitted in the forward direction and the case where the ultrasonic wave is transmitted in the reverse direction. Occurs.

  In order to solve such a problem, Patent Document 1 discloses that ultrasonic waves are transmitted by alternately switching the transducers arranged facing the upstream side and the downstream side of the pipeline through which the fluid to be measured flows. In an ultrasonic flowmeter that performs wave and wave reception and measures the flow rate of fluid, a transmission circuit that exhibits an output impedance smaller than the series resonance impedance of a piezoelectric vibrator used in the transducer and energizes one transducer, An ultrasonic flowmeter that has an input impedance substantially equal to the output impedance of the transmission circuit and receives a signal from the other transducer, each transducer operating near a series resonance state; Yes.

  In this ultrasonic flowmeter, the impedances of the transmission circuit and the reception circuit related to the transducer are both smaller than the series resonance impedance of the piezoelectric vibrator, and each transducer operates near the series resonance state. Therefore, even if the characteristics differ depending on the frequency characteristics, temperature characteristics, and the structure of the backing material of the piezoelectric vibrator used in the transducer, the offset amount based on the propagation time difference due to switching of the ultrasonic wave propagation direction can be reduced.

In particular, the offset amount is minimized when impedance matching between the transmission circuit and the reception circuit is achieved. In the series resonance state, both the transmission circuit and the reception circuit have low impedance, so that the influence of interference noise is reduced and the signal-to-noise ratio is improved. Furthermore, since no input resistor is used in the receiving circuit, a signal with a low signal level and no thermal noise is generated, and a signal with an excellent signal-to-noise ratio is added, which can be reproduced for various fluids in small-diameter pipes. The flow rate can be measured with improved performance and stability and reduced measurement errors.
No. 7-10253

  In the conventional ultrasonic flowmeter described above, as shown in Patent Document 1, when the input impedance of the reception detection circuit 5 (reception circuit in Patent Document 1) is low, the internal capacitance of the ultrasonic transducer, the transmission / reception selector switch 3 The signal delay time is determined by the internal resistance and the wiring resistance from the transmission / reception selector switch to the ultrasonic transducer.

  By the way, each of the 1st switch 31-the 4th switch 34 which comprises the transmission / reception change-over switch 3 shown in FIG. 7 has internal resistance. The wiring from the first switch 31 or the third switch 33 to the ultrasonic transducer 2a has a wiring resistance. Similarly, the wiring from the second switch 32 or the fourth switch 34 to the ultrasonic transducer 2b has a wiring resistance.

  Now, R is the sum of the internal resistance value of the first switch 31 and the wiring resistance value from the first switch 31 to the ultrasonic transducer 2a, the internal resistance value of the second switch 32 and the ultrasonic transducer from the second switch 32. The sum of the wiring resistance values up to 2b is R + Ra, the sum of the internal resistance value of the third switch 33 and the wiring resistance value from the third switch 33 to the ultrasonic transducer 2a is R + Rb, and the internal resistance value of the fourth switch 34. And R + Rc is the sum of the wiring resistance value from the fourth switch 34 to the ultrasonic transducer 2b. The internal capacitance of the ultrasonic transducer 2a is C + Ca, and the internal capacitance of the ultrasonic transducer 2b is C.

  When transmitting and receiving ultrasonic waves in the forward direction, the first switch 31 and the fourth switch 34 are turned on. When transmitting and receiving ultrasonic waves in the reverse direction, the second switch 32 and the third switch 33 are turned on. Each of the forward delay time and the reverse delay time can be expressed as follows.

Forward delay time ∝ (C + Ca) R + C (R + Rc)
Reverse direction delay time C (R + Ra) + (C + Ca) (R + Rb)
Therefore, between the forward delay time and the reverse delay time,
Difference in forward / reverse delay time ∝C (Ra + Rb-Rc) + CaRb
Occurs.

  If the difference between the forward and reverse delay times is constant, no measurement error occurs, but the internal capacitances of the ultrasonic transducers 2a and 2b, the internal resistance value of the transmission / reception selector switch 3, and the transmission / reception selector switch 3 and the ultrasonic transducer 2a. Alternatively, the wiring resistance value between 2b and 2b varies depending on the temperature, for example, so that the conventional ultrasonic flowmeter has a problem that changes in internal capacitance, internal resistance value and wiring resistance value due to temperature appear as measurement errors. is there.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an ultrasonic flowmeter that can improve measurement accuracy of flow rate by eliminating measurement error due to temperature change. .

  In order to solve the above-described problems, the present invention is provided with a pair of ultrasonic vibrators at a certain distance on the upstream side and the downstream side of the flow path, and is opposed to the forward direction and the flow along the fluid flow. This is an ultrasonic flowmeter that transmits and receives ultrasonic waves by switching the transmission and reception directions to the opposite direction, and determines the flow rate based on the arrival time of ultrasonic waves in each direction, and generates a transmission signal to be sent to one ultrasonic transducer Circuit, a reception detection circuit for detecting a reception signal from the other ultrasonic transducer, and a transmission / reception selector switch for switching one ultrasonic transducer and the other ultrasonic transducer to each other. The resistance value of the path of the transmission signal when transmitting and receiving the forward direction, the resistance value of the path of the reception signal when transmitting and receiving the ultrasonic wave in the forward direction, and the path value of the transmission signal when transmitting and receiving the ultrasonic wave in the reverse direction Resistance value and when sending and receiving ultrasound in the opposite direction Characterized by being equal to the resistance of the path of the signal signal.

  According to the present invention, the resistance value of the path of the transmission signal when transmitting and receiving the ultrasonic wave in the forward direction, the resistance value of the path of the reception signal when transmitting and receiving the ultrasonic wave in the forward direction, and the ultrasonic wave in the reverse direction Since the resistance value of the path of the transmission signal when transmitting and receiving is equal to the resistance value of the path of the reception signal when transmitting and receiving ultrasonic waves in the opposite direction, the forward and reverse delays are made regardless of the difference in the characteristics of the ultrasonic transducer. The time difference can be made zero, and the measurement error can be reduced. Even if the internal capacitance of the ultrasonic transducer changes due to temperature changes, or the internal resistance of the transmission / reception changeover switch and the wiring resistance between the transmission / reception changeover switch and the ultrasonic transducer change, the forward / reverse delay time Measurement error can be reduced.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description, the same or corresponding components as those of the ultrasonic flowmeter described in the background art column are denoted by the same reference numerals as those used in the background art column.

  The configuration and operation of the ultrasonic flow meter according to the first embodiment of the present invention are the same as the configuration and operation of the conventional ultrasonic flow meter described with reference to FIG. The same. Below, it demonstrates centering on a different part from the conventional ultrasonic flowmeter.

  FIG. 1 is a circuit diagram showing a configuration of a transmission / reception selector switch 3 applied to the ultrasonic flowmeter according to the first embodiment of the present invention. The transmission / reception selector switch 3 includes a first switch 31, a second switch 32, a third switch 33, and a fourth switch 34.

  Each of the first switch 31, the second switch 32, the third switch 33, and the fourth switch 34 constituting the transmission / reception selector switch 3 has an internal resistance. Further, the wiring from the first switch 31 to the ultrasonic transducer 2a, the wiring from the second switch 32 to the ultrasonic transducer 2b, the wiring from the third switch 33 to the ultrasonic transducer 2a, and the wiring from the fourth switch 34 to the ultrasonic transducer 2a. Each wiring to the sonic transducer 2b has a wiring resistance.

  In the ultrasonic flowmeter according to the first embodiment, switches having uniform temperature characteristics of internal resistance are employed as the first switch 31, the second switch 32, the third switch 33, and the fourth switch 34. Further, the wiring from the first switch 31 to the ultrasonic transducer 2a, the wiring from the second switch 32 to the ultrasonic transducer 2b, the wiring from the third switch 33 to the ultrasonic transducer 2a, and the wiring from the fourth switch 34 to the ultrasonic transducer 2a. Each of the wirings up to the sonic transducer 2b has the same wiring length, and each wiring is configured to have substantially the same wiring resistance value.

  FIG. 2 shows that the first switch 31, the second switch 32, the third switch 33, the fourth switch 34, the transmission circuit 4 and the reception detection circuit 5 are integrated into an integrated circuit 51 in order to make the temperature characteristics and wiring lengths of the internal resistance uniform. (Hereinafter, abbreviated as “IC”), an example in which the IC 51 and the ultrasonic transducers 2a and 2b are mounted on a substrate and a part of the ultrasonic flowmeter is configured will be shown. In FIG. 2, 52 indicates an upper layer wiring prohibited area in the IC, 53 indicates wiring on the substrate, 54 indicates upper layer wiring in the IC, and 55 indicates lower layer wiring in the IC.

  The first switch 31, the second switch 32, the third switch 33, and the fourth switch 34 are composed of semiconductor switches, and are formed on the same semiconductor wafer. Thereby, the internal resistance and temperature characteristics of each of the first switch 31, the second switch 32, the third switch 33, and the fourth switch 34 can be made substantially equal.

  Further, in the IC 51, since the upper layer wiring resistance, the lower layer wiring resistance, and the connection resistance between the upper layer and the lower layer are different from each other, the length of the upper layer wiring 54, the length of the lower layer wiring 55, and the wiring width are changed from the first switch 31 to the ultrasonic wave. A path to the connection terminal to the transducer 2a, a path from the second switch 32 to the connection terminal to the ultrasonic transducer 2b, a path from the third switch 33 to the connection terminal to the ultrasonic transducer 2a, and a fourth switch It is configured to be substantially equal in each of the paths from 34 to the connection terminal to the ultrasonic transducer 2b.

  Also, the number of connections between the upper layer and the lower layer is the path from the first switch 31 to the connection terminal to the ultrasonic transducer 2a, the path from the second switch 32 to the connection terminal to the ultrasonic transducer 2b, the third switch The same number is provided for each of the path from 33 to the connection terminal to the ultrasonic transducer 2a and the path from the fourth switch 34 to the connection terminal to the ultrasonic transducer 2b.

  Furthermore, the length and the wiring width of the wiring 53 on the substrate from the connection terminal of the IC 51 to the ultrasonic transducer 2a to the ultrasonic transducer 2a are the same as those of the connection terminal of the IC 51 to the ultrasonic transducer 2b. The length and the wiring width of the wiring 53 on the substrate up to 2b are substantially equal to each other.

  With the above configuration, the wiring resistance value of the transmission signal path from the first switch 31 to the ultrasonic transducer 2a, the wiring resistance value of the transmission signal path from the second switch 32 to the ultrasonic transducer 2b, the third switch Each of the wiring resistance value of the path of the received signal from 33 to the ultrasonic transducer 2a and the wiring resistance value of the path of the received signal from the fourth switch 34 to the ultrasonic transducer 2b have substantially the same resistance value R. Have been adjusted so that.

  Now, the sum of the internal resistance value of the first switch 31 and the wiring resistance value from the first switch 31 to the ultrasonic transducer 2a, the internal resistance value of the second switch 32 and the second switch 32 to the ultrasonic transducer 2b. Of the third switch 33, the sum of the internal resistance value of the third switch 33 and the wiring resistance value from the third switch 33 to the ultrasonic transducer 2a, the internal resistance value of the fourth switch 34 and the fourth switch 34. It is assumed that all of the sum of the wiring resistance value up to the ultrasonic transducer 2b is adjusted to the resistance value R. The internal capacitance of the ultrasonic transducer 2a is C + Ca, and the internal capacitance of the ultrasonic transducer 2b is C.

  When transmitting and receiving ultrasonic waves in the forward direction, the first switch 31 and the fourth switch 34 are turned on. When transmitting and receiving ultrasonic waves in the reverse direction, the second switch 32 and the third switch 33 are turned on. Each of the forward delay time and the reverse delay time can be expressed as follows.

Forward delay time ∝ (C + Ca) R + CR
Reverse delay time ∝CR + (C + Ca) R
Therefore, the difference between the forward delay time and the reverse delay time is
Difference between forward and reverse delay time = 0
It becomes.

  Therefore, the difference in forward and reverse delay times can be made zero regardless of the difference in the characteristics of the ultrasonic transducers 2a and 2b, so that the measurement error can be reduced. Further, even if the internal capacitances of the ultrasonic transducers 2a and 2b change due to temperature changes, the internal resistance of the first switch 31 to the fourth switch 34 and the transmission / reception selector switch 3 and the ultrasonic transducers 2a and 2b Even if the wiring resistance changes between them, there is no difference between the forward and reverse delay times, and the measurement error can be reduced.

  FIG. 3 is a circuit diagram showing a configuration of the transmission / reception selector switch 3 applied to the ultrasonic flowmeter according to the second embodiment of the present invention. The transmission / reception selector switch 3 is configured by adding a first variable resistor R1, a second variable resistor R2, a third variable resistor R3, and a fourth variable resistor R4 to the transmission / reception selector switch of the first embodiment.

  The first variable resistor R1 is disposed between the first switch 31 and the ultrasonic transducer 2a, and is used to adjust the wiring resistance value of the path from the first switch 31 to the ultrasonic transducer 2a. The The second variable resistor R2 is disposed between the second switch 32 and the ultrasonic transducer 2b, and is used to adjust the wiring resistance value of the path from the second switch 32 to the ultrasonic transducer 2b. The The third variable resistor R3 is disposed between the third switch 33 and the ultrasonic transducer 2a, and is used to adjust the wiring resistance value of the path from the third switch 33 to the ultrasonic transducer 2a. The The fourth variable resistor R4 is disposed between the fourth switch 34 and the ultrasonic transducer 2b, and is used to adjust the wiring resistance value of the path from the fourth switch 34 to the ultrasonic transducer 2b. The

  Each of the first switch 31, the second switch 32, the third switch 33, and the fourth switch 34 has an internal resistance. Further, the path from the first switch 31 to the ultrasonic transducer 2a, the path from the second switch 32 to the ultrasonic transducer 2b, the path from the third switch 33 to the ultrasonic transducer 2a, and the superordinate from the fourth switch 34 The path to the acoustic wave vibrator 2b has a wiring resistance. Further, as the first variable resistor R1, the second variable resistor R2, the third variable resistor R3, and the fourth variable resistor R4, those having the same temperature characteristics of the resistors are employed.

  The sum of the internal resistance value of the first switch 31 and the wiring resistance value from the first switch 31 to the ultrasonic transducer 2a is R and the second switch, as in the ultrasonic flow meter described in the background section. The sum of the internal resistance value of 32 and the wiring resistance value from the second switch 32 to the ultrasonic transducer 2b is R + Ra, the internal resistance value of the third switch 33 and the wiring resistance from the third switch 33 to the ultrasonic transducer 2a Assume that the sum of the values is R + Rb, and the sum of the internal resistance value of the fourth switch 34 and the wiring resistance value from the fourth switch 34 to the ultrasonic transducer 2b is R + Rc.

  Further, the first variable resistor R1 is adjusted to the resistance value R, the second variable resistor R2 is adjusted to the resistance value R-Ra, the third variable resistor R3 is adjusted to the resistance value R-Rb, and the fourth variable resistor R4 is adjusted to the resistance value R-Rc. It shall be. Furthermore, the internal capacitance of the ultrasonic transducer 2a is C + Ca, and the internal capacitance of the ultrasonic transducer 2b is C.

  When transmitting and receiving ultrasonic waves in the forward direction, the first switch 31 and the fourth switch 34 are turned on. When transmitting and receiving ultrasonic waves in the reverse direction, the second switch 32 and the third switch 33 are turned on. Each of the forward delay time and the reverse delay time can be expressed as follows.

Forward delay time ∝2 (C + Ca) R + 2CR
Reverse delay time ∝2CR + 2 (C + Ca) R
Therefore, the difference between the forward delay time and the reverse delay time is
Difference between forward and reverse delay time = 0
It becomes.

  Therefore, regardless of the difference in the characteristics of the ultrasonic transducers 2a and 2b, the difference between the forward and reverse delay times can be made zero, and the measurement error can be reduced. Further, even if the internal capacitances of the ultrasonic transducers 2a and 2b change due to temperature changes, the values of the internal resistances of the first switch 31 to the fourth switch 34 and the first variable resistance R1 to the fourth variable resistance R4 are also changed. Even if it changes, there is no difference in forward and reverse delay times, so that measurement errors can be reduced.

  FIG. 4 is a circuit diagram showing a configuration of the transmission / reception selector switch 3 applied to the ultrasonic flowmeter according to the third embodiment of the present invention. In the transmission / reception change-over switch 3, in Example 1, the sum of the internal resistance value of the first switch 31 and the wiring resistance value from the first switch 31 to the ultrasonic transducer 2 a is R, and the internal resistance value of the second switch 32. And the sum of the wiring resistance value from the second switch 32 to the ultrasonic transducer 2b is R + Ra, and the sum of the internal resistance value of the third switch 33 and the wiring resistance value from the third switch 33 to the ultrasonic transducer 2a is R, the sum of the internal resistance value of the fourth switch 34 and the wiring resistance value from the fourth switch 34 to the ultrasonic transducer 2b is R + Ra.

  That is, in the ultrasonic flowmeter according to the third embodiment, the first switch 31 and the third switch 33 employ a switch having a uniform temperature characteristic of the internal resistance, and the second switch 32 and the fourth switch 34 have an internal Switches with uniform temperature characteristics of resistors are used. Also, the wiring lengths of the wiring from the first switch 31 to the ultrasonic transducer 2a and the wiring from the third switch 33 to the ultrasonic transducer 2a are aligned, so that each wiring has substantially the same wiring resistance value. The wiring lengths of the wiring from the second switch 32 to the ultrasonic transducer 2b and the wiring length from the fourth switch 34 to the ultrasonic transducer 2b are aligned, and each wiring has substantially the same wiring resistance. Configured to have a value.

  Now, the sum of the internal resistance value of the first switch 31 and the wiring resistance value from the first switch 31 to the ultrasonic transducer 2 a is R, the internal resistance value of the second switch 32 and the ultrasonic transducer from the second switch 32. The sum of the wiring resistance values up to 2b is R + Ra, the sum of the internal resistance value of the third switch 33 and the wiring resistance value from the third switch 33 to the ultrasonic transducer 2a is R, and the internal resistance value of the fourth switch 34. And the wiring resistance value from the fourth switch 34 to the ultrasonic transducer 2b is R + Ra, the internal capacitance of the ultrasonic transducer 2a is C + Ca, and the internal capacitance of the ultrasonic transducer 2b is C.

  When transmitting and receiving ultrasonic waves in the forward direction, the first switch 31 and the fourth switch 34 are turned on. When transmitting and receiving ultrasonic waves in the reverse direction, the second switch 32 and the third switch 33 are turned on. Each of the forward delay time and the reverse delay time can be expressed as follows.

Forward delay time ∝ (C + Ca) R + C (R + Ra)
Reverse delay time ∝C (R + Ra) + (C + Ca) R
Therefore, the difference between the forward delay time and the reverse delay time is
Difference between forward and reverse delay time = 0
It becomes.

  Therefore, regardless of the difference in the characteristics of the ultrasonic transducers 2a and 2b, the difference between the forward and reverse delay times can be made zero, and the measurement error can be reduced. Further, even if the internal capacitances of the ultrasonic transducers 2a and 2b change due to the temperature change, the internal resistance of the first switch 31 to the fourth switch 34 and the transmission / reception changeover switch 3 and the ultrasonic transducer 2a. And 2b, even if the wiring resistance changes, there is no difference between forward and reverse delay times, so that the measurement error can be reduced.

  FIG. 5 is a circuit diagram showing a partial configuration of the ultrasonic flowmeter according to the fourth embodiment of the present invention. The ultrasonic flowmeter 3 is a conventional ultrasonic flowmeter in which a first capacitor C1 is connected in parallel to the ultrasonic transducer 2a, and a second capacitor C2 is connected in parallel to the ultrasonic transducer 2b. It is configured.

  The first capacitor C1 has the same capacity as the internal capacity of the ultrasonic transducer 2a and has a reverse characteristic of its temperature characteristic. Here, the reverse characteristic is a characteristic in which the capacity decreases as the temperature rises, whereas the capacity increases as the temperature increases. Similarly, the second capacitor C2 has the same capacitance as the internal capacitance of the ultrasonic transducer 2b and has a reverse characteristic of its temperature characteristic.

  Now, R is the sum of the internal resistance value of the first switch 31 and the wiring resistance value from the first switch 31 to the ultrasonic transducer 2a, the internal resistance value of the second switch 32 and the ultrasonic transducer from the second switch 32. The sum of the wiring resistance values up to 2b is R + Ra, the sum of the internal resistance value of the third switch 33 and the wiring resistance value from the third switch 33 to the ultrasonic transducer 2a is R, and the internal resistance value of the fourth switch 34. And the wiring resistance value from the fourth switch 34 to the ultrasonic transducer 2b is R + Ra. The internal capacitance of the ultrasonic transducer 2a is C + Ca, the internal capacitance of the ultrasonic transducer 2b is C, the capacitance of the first capacitor is C + Ca, and the capacitance of the second capacitor is C.

  When transmitting and receiving ultrasonic waves in the forward direction, the first switch 31 and the fourth switch 34 are turned on. When transmitting and receiving ultrasonic waves in the reverse direction, the second switch 32 and the third switch 33 are turned on. Each of the forward delay time and the reverse delay time can be expressed as follows.

Forward delay time ∝ (2C + 2Ca) R + 2C (R + Rc)
Reverse delay time ∝2C (R + Ra) + (2C + 2Ca) (R + Rb)
Therefore, the difference between the forward delay time and the reverse delay time is
Difference in forward / reverse delay time ∝2C (Ra + Rb−Rc) + 2CaRb
≒ 2C (Ra + Rb-Rc) (However, C >> Ca)
It becomes.

  Here, even if the internal capacitances of the ultrasonic transducers 2a and 2b change depending on the temperature or the like, the first capacitor C1 and the first capacitor C1 having the inverse characteristics of the temperature characteristics of the internal capacitance in parallel with the ultrasonic transducers 2a and 2b. By connecting the two capacitors C2, respectively, the measurement error can be reduced since 2C does not change with temperature.

  In the ultrasonic flow meter according to Example 4, the first capacitor C1 is connected in parallel to the ultrasonic transducer 2a of the conventional ultrasonic flow meter, and the second capacitor C2 is connected in parallel to the ultrasonic transducer 2b. The first capacitor C1 is connected in parallel to the ultrasonic transducer 2a of the ultrasonic flowmeter according to the first to third embodiments described above, and the second capacitor is connected in parallel to the ultrasonic transducer 2b. C2 can also be connected.

  The ultrasonic flow meter according to the present invention can be used for a flow meter for measuring a flow rate of a fluid such as a gas meter or a water meter.

It is a circuit diagram which shows the structure of the transmission / reception selector switch applied to the ultrasonic flowmeter which concerns on Example 1 of this invention. In the ultrasonic flowmeter which concerns on Example 1 of this invention, it is a figure which shows the example which comprised the transmission / reception changeover switch, the transmission circuit, and the reception detection circuit with IC. It is a circuit diagram which shows the structure of the transmission / reception selector switch applied to the ultrasonic flowmeter which concerns on Example 2 of this invention. It is a circuit diagram which shows the structure of the transmission / reception selector switch applied to the ultrasonic flowmeter which concerns on Example 3 of this invention. It is a circuit diagram which shows the structure of a part of ultrasonic flowmeter which concerns on Example 4 of this invention. It is a figure which shows the structure of the conventional ultrasonic flowmeter. FIG. 7 is a circuit diagram showing a specific configuration of the transmission / reception selector switch shown in FIG. 6.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Flow path 2a, 2b Ultrasonic vibrator 3 Transmission / reception changeover switch 4 Transmission circuit 5 Reception detection circuit 6 Control circuits 31-34 1st switch-4th switch R1-R4 1st variable resistance-4th variable resistance C1 1st capacitor C2 Second capacitor

Claims (6)

A pair of ultrasonic transducers are provided at a certain distance on the upstream and downstream sides of the flow path, and ultrasonic waves are transmitted and received by switching the transmission / reception direction between the forward direction along the fluid flow and the reverse direction against the flow. And an ultrasonic flowmeter for obtaining a flow rate based on the arrival time of the ultrasonic wave in each direction,
A transmission circuit for generating a transmission signal to be sent to one ultrasonic transducer;
A reception detection circuit for detecting a reception signal from the other ultrasonic transducer;
A transmission / reception changeover switch for mutually switching the one ultrasonic transducer and the other ultrasonic transducer;
The resistance value of the path of the transmission signal when transmitting and receiving ultrasonic waves in the forward direction, the resistance value of the path of the reception signal when transmitting and receiving ultrasonic waves in the forward direction, and the transmission signal when transmitting and receiving ultrasonic waves in the reverse direction The ultrasonic flowmeter is characterized in that the resistance value of the path of and the resistance value of the path of the reception signal when transmitting and receiving the ultrasonic wave in the opposite direction are made equal. The transmission / reception selector switch controls transmission of a transmission signal from the transmission circuit to one ultrasonic transducer and transmission of the transmission signal from the transmission circuit to the other ultrasonic transducer. A second switch that controls, a third switch that controls transmission of a reception signal from the one ultrasonic transducer to the reception detection circuit, and a reception signal of the reception signal from the other ultrasonic transducer. A fourth switch for controlling the transmission of
The sum of the internal resistance value of the first switch and the wiring resistance value from the first switch to the one ultrasonic transducer, the internal resistance value of the second switch, and the other ultrasonic vibration from the second switch A sum of wiring resistance values to the child, an internal resistance value of the third switch, a sum of wiring resistance values from the third switch to the one ultrasonic transducer, an internal resistance value of the fourth switch, and the 2. The ultrasonic flowmeter according to claim 1, wherein a sum of wiring resistance values from the fourth switch to the other ultrasonic transducer is made equal. The transmission / reception selector switch controls transmission of a transmission signal from the transmission circuit to one ultrasonic transducer and transmission of the transmission signal from the transmission circuit to the other ultrasonic transducer. A second switch that controls, a third switch that controls transmission of a reception signal from the one ultrasonic transducer to the reception detection circuit, and a reception signal of the reception signal from the other ultrasonic transducer. A fourth switch for controlling the transmission of
A first variable resistor is provided between the first switch and the one ultrasonic transducer, a second variable resistor is provided between the second switch and the other ultrasonic transducer, and the third switch And a third variable resistor between the first ultrasonic transducer and a fourth variable resistor between the fourth switch and the other ultrasonic transducer,
By varying the first variable resistor, the second variable resistor, the third variable resistor, and the fourth variable resistor, the internal resistance value of the first switch and the resistance from the first switch to the one ultrasonic transducer A sum of values, an internal resistance value of the second switch, a sum of resistance values from the second switch to the other ultrasonic transducer, an internal resistance value of the third switch, and the one from the third switch The sum of resistance values to the ultrasonic transducer, the internal resistance value of the fourth switch, and the sum of resistance values from the fourth switch to the other ultrasonic transducer are made equal. Item 2. The ultrasonic flowmeter according to Item 1. A pair of ultrasonic transducers are provided at a certain distance on the upstream side and downstream side of the flow path, and ultrasonic waves are transmitted and received by switching the transmission / reception direction between the forward direction along the fluid flow and the reverse direction against the flow. And an ultrasonic flowmeter for obtaining a flow rate based on the arrival time of the ultrasonic wave in each direction,
A transmission circuit for generating a transmission signal to be sent to one ultrasonic transducer;
A reception detection circuit for detecting a reception signal from the other ultrasonic transducer;
A transmission / reception changeover switch for mutually switching the one ultrasonic transducer and the other ultrasonic transducer;
The sum of the resistance value of the path of the transmission signal when transmitting and receiving ultrasonic waves in the forward direction and the resistance value of the path of the reception signal when transmitting and receiving ultrasonic waves in the forward direction, and when transmitting and receiving ultrasonic waves in the reverse direction An ultrasonic flowmeter characterized in that a sum of a resistance value of a transmission signal path and a resistance value of a reception signal path when transmitting and receiving ultrasonic waves in the opposite direction is made equal. The transmission / reception selector switch controls transmission of a transmission signal from the transmission circuit to one ultrasonic transducer and transmission of the transmission signal from the transmission circuit to the other ultrasonic transducer. A second switch that controls, a third switch that controls transmission of a reception signal from the one ultrasonic transducer to the reception detection circuit, and a reception signal of the reception signal from the other ultrasonic transducer. A fourth switch for controlling the transmission of
The internal resistance value of the first switch, the wiring resistance value from the first switch to the one ultrasonic transducer, the internal resistance value of the fourth switch, and from the fourth switch to the other ultrasonic transducer The sum of the wiring resistance value is the internal resistance value of the second switch, the wiring resistance value from the second switch to the other ultrasonic transducer, the internal resistance value of the third switch, and the third switch The ultrasonic flowmeter according to claim 4, wherein the ultrasonic flowmeter is equal to a sum of wiring resistance values to one ultrasonic transducer. A pair of ultrasonic transducers are provided at a certain distance on the upstream side and downstream side of the flow path, and ultrasonic waves are transmitted and received by switching the transmission / reception direction between the forward direction along the fluid flow and the reverse direction against the flow. And an ultrasonic flowmeter for obtaining a flow rate based on the arrival time of the ultrasonic wave in each direction,
A transmission circuit for generating a transmission signal to be sent to one ultrasonic transducer;
A reception detection circuit for detecting a reception signal from the other ultrasonic transducer;
A transmission / reception changeover switch for mutually switching the one ultrasonic transducer and the other ultrasonic transducer;
An ultrasonic flowmeter comprising a capacitor connected in parallel to each of the pair of ultrasonic transducers and having a reverse characteristic to the temperature characteristic of the internal capacitance of each ultrasonic transducer.

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