JP2016156664A - Ultrasonic flow meter - Google Patents

Abstract

[PROBLEMS] To realize an ultrasonic flowmeter that can reduce the design man-hour by expanding the selection range of electronic parts without changing the circuit configuration, reducing the number of design steps, and reducing the cost. To do.
An ultrasonic flowmeter configured to measure a flow rate by transmitting and receiving an ultrasonic signal to a fluid to be measured via an ultrasonic transducer,
The ultrasonic transducer is driven by a driving signal of a small signal class.
[Selection] Figure 1

Description

  The present invention relates to an ultrasonic flow meter, and more particularly to an ultrasonic signal transmission circuit.

  FIG. 4 is a circuit diagram showing an example of an ultrasonic signal transmission circuit used in a conventional ultrasonic flowmeter. In FIG. 4, the control unit 1 outputs waveform data to the waveform generation unit 2 and controls the waveform generation in the waveform generation unit 2. The waveform generator 2 generates a pulse waveform for driving an ultrasonic oscillator 13 described later.

  The drive unit 3 drives the MOSFETs 4 and 5 based on the pulse waveform generated and output from the waveform generation unit 2, and the output signal is connected to the gates of the MOSFETs 4 and 5.

  The positive voltage source 6 supplies a transmission waveform voltage + V1, and the anode is connected to the source of the MOSFET 4 and the cathode is connected to a common potential point.

  The negative voltage source 7 supplies the transmission waveform voltage −V2, the cathode is connected to the drain of the MOSFET 5, and the anode is connected to the common potential point.

  The drain of the MOSFET 4 is connected to the anode of the protective diode 9 via the protective resistor 8 when the transmission waveform is generated, and the cathode of the diode 9 is connected to the core wire 10 a of the coaxial cable 10.

  The source of the MOSFET 5 is connected to the cathode of the protective diode 12 via the protective resistor 11 when the transmission waveform is generated, and the anode of the diode 12 is connected to the core wire 10 a of the coaxial cable 10.

  An ultrasonic oscillator 13 (hereinafter also referred to as PTZ) made of, for example, PZT (lead zirconate titanate) is connected to the end portion of the core wire 10a of the coaxial cable 10 and the outer conductor 10b.

  The connection point between the cathode of the diode 9, the anode of the diode 12, and the core wire 10 a of the coaxial cable 10 is connected to a common potential via a series circuit of a protection resistor 14 at the time of damping and a switch 15 for discharging the charge of PZT. Connected to a point.

  In the configuration of FIG. 4, ultrasonic waves are generated from the ultrasonic oscillator 13 to measure the flow rate of the measurement target fluid. The PZT built in the ultrasonic oscillator 13 is driven with a predetermined pulse waveform. As driving conditions, for example, the frequency is set to several MHz which is the resonance frequency of PZT, and the voltage is set to several tens of volts with a ± power supply. It is not necessary to have the same voltage for ±, but generally the same voltage is used for operation. PZT operates almost as a capacitive load.

As an initial state, the switch 15 is turned off (opened).
(1) The control unit 1 sends transmission waveform data to the waveform generation unit 2.
(2) The control unit 1 starts waveform generation from the waveform generation unit 2.
(3) The output waveform of the waveform generator 2 is level-shifted as an appropriate waveform that allows the drive unit 3 to drive the MOSFETs 4 and 5 to drive the MOSFETs 4 and 5.

(4) The output waveforms of the driven MOSFETs 4 and 5 are substantially the same as the transmission waveform of the waveform generator 2, and the PZT in the ultrasonic oscillator 13 is driven via the cable 10 to output ultrasonic waves.
(5) When the driving is completed, the waveform generator 2 turns on the switch 15 (short circuit), and releases the charge of PZT in the ultrasonic oscillator 13.

  In the series of steps (1) to (5), the driving of the ultrasonic oscillator 13 by the transmission waveform for one pulse is completed. However, the actual ultrasonic flowmeter needs these series of steps at appropriate intervals. Repeat as many times as necessary.

  Here, the resistors 8, 11, 14 and the diodes 9, 12 are protective elements, and their influence on the basic operation is small.

  FIG. 5 is a timing chart for explaining the operation of FIG. 4, (a) shows the output waveform of the waveform generator 2, (b) shows the operation of the MOSFET 4, (c) shows the operation of the MOSFET 5, d) shows the current of the MOSFET 4, (e) shows the current of the MOSFET 5, (f) shows the operation of the switch 15, (g) shows the current of the switch 15, and (h) shows the current of the ultrasonic oscillator 13. A voltage waveform is shown.

The operation of each step (1) to (5) will be described in detail.
<(1) Transmission of waveform data>
The control unit 1 outputs the following waveform data to the waveform generation unit 2.
1) Pulse width data corresponding to the ON time of the MOSFET 4 shown in (b) 2) Pulse width data corresponding to the ON time of the MOSFET 5 shown in (c) Note that the ON times of (b) and (c) are not the same. May be.
3) Pulse width data corresponding to the ON time of the switch 15 shown in (f)

The following data may be used supplementarily due to differences in the characteristics of semiconductor elements and loads.
-Standby time until MOSFET 4 is turned off and MOSFET 5 is turned on-Standby time until MOSFET 5 is turned off and switch 15 is turned on In FIG. 5, the standby time is shown as "0".

<(2) Start transmission waveform>
The control unit 1 instructs the waveform generation unit 2 to generate a waveform signal, as shown as “start” in FIG.

<(3) Driving of ultrasonic detector by transmission waveform 1>
The drive unit 3 turns on the MOSFET 4 with the signal shown in FIG. The ultrasonic oscillator 13 is driven by a pulsed drive signal of amplitude voltage + V1 shown in (h) through the path of positive voltage source 6 → MOSFET 4 → resistor 8 → diode 9 → cable 10. (D) is a current waveform flowing into the PZT of the ultrasonic oscillator 13, and the PZT has a peak + Ip1 in this way because of a capacitive load. This peak value Ip1 is determined by the supply voltage and the equivalent resistance value of the path formed by the positive voltage source 6 → the MOSFET 4 → the resistor 8 → the diode 9 → the cable 10.

<(4) Driving of ultrasonic detector by transmission waveform 2>
The MOSFET 4 is turned OFF, and the MOSFET 5 is similarly turned ON by the drive unit 3. In some cases, after the MOSFET 4 is turned off, the MOSFET 5 is turned on after some time has passed. In FIG. 5, the idle time is set to “0”. The ultrasonic oscillator 13 is driven by a pulsed drive signal having an amplitude voltage −V2 shown in (h) through the path of the negative voltage source 7 → MOSFET 5 → resistor 11 → diode 12 → cable 10. This peak value Ip2 is determined by the supply voltage and the equivalent resistance value of the path formed by the negative voltage source 7 → the MOSFET 5 → the resistor 11 → the diode 12 → the cable 10.

  The peak currents + Ip1 and -Ip2 are determined by the equivalent resistance values of the path to the output stage and the ultrasonic oscillator 13, but the ON resistances of the MOSFETs 4 and 5 vary widely, and the peak currents + Ip1 and -Ip2 are limited only by the ON resistances. It is difficult. Therefore, resistors 8 and 11 are connected in series to MOSFET 4 and MOSFET 5.

<(5) Damping by switch 15>
When the MOSFET 5 is turned off, the switch 15 is turned on to connect the core wire 10a of the cable 10 to the common potential point, and the vibration of PZT is damped and stopped. In consideration of the fact that the PZT in the ultrasonic oscillator 13 continues to vibrate to some extent even when the MOSFET 5 is turned off, the vibration is damped to be stationary. In some cases, after the MOSFET 5 is turned off, the switch 15 is turned off after some time. In FIG. 5, the idle time is set to “0”. As a damping function of the PZT, a resistor may be connected in parallel with the PZT in the ultrasonic oscillator 13 instead of the switch 15, and may be stopped by spontaneous discharge.

  Non-Patent Document 1 describes the measurement principle and configuration of an ultrasonic flowmeter of a propagation time difference method based on the transmission method similar to the present invention.

Satoshi Fukuhara, 3 others, “Ultrasonic Flowmeter US350”, Yokogawa Technical Report, Yokogawa Electric Corporation, January 20, 2004, Vol. 48 No. 1 (2004) p. 29-32

  However, according to such a conventional configuration, the withstand voltage of the output stage composed of the MOSFET 4, the resistor 8, the diode 9 and the MOSFET 5, the resistor 11, and the diode 12 needs to be several hundred volts or more, and FIG. The current capacity required for the peak current + Ip1 shown in d) and the peak current -Ip2 shown in FIG.

An example of calculating the pulse width and the peak current based on the conventional configuration will be described.
Assuming that the capacitance C of PZT is 10 nf, the resonance frequency is 1 MHz, and the driving voltage of PZT is 60 V, the conventional driving waveform shown in FIG. 5B and the equivalent resistance R that determines the peak current are calculated.

  The pulse width T of the conventional driving waveform shown in FIG. 5B is ½ of the period from the resonance frequency and becomes 500 nsec. Assume that the time to the peak of the voltage waveform of PZT is, for example, three times the time constant τ (= RC).

In this case, R is
R = T / 3 / C = 17Ω
It becomes.

Further, the peak current Ip1 shown in FIG.
Ip1 = V / R = 3.5A
It becomes.

  Here, the selection of semiconductors for design is a medium and high withstand voltage product (currently these are almost DIP products) that can pass a current of several A, and those that can withstand pulses of several A with a resistance of 17Ω are special products and surface mount products. But it's a big resistance.

  The steady power is relatively small because it is a burst wave (the period of one pulse to be generated is an interval of several tens to several hundred times the pulse width), but due to this peak current, the outer shape of each element is relatively large. .

  As for the damping of the switch 15, since the pulse width can be set relatively long, the peak current Ip3 is relatively small as shown in FIG. 5 (g), and a smaller element than the output stage can be selected this time. It doesn't matter.

  As described above, since the components of the medium and high withstand voltage level must be used as the above-described elements constituting the output stage, the following problems occur.

  Each element in the output stage has a large outer shape and requires a heat dissipation mechanism for the heat generating elements (mainly semiconductor elements), so that the mounting space for these elements becomes large.

  Since the ultrasonic oscillator 13 is multi-channel, it has a configuration in which each element having a large outer shape and requiring a heat dissipation mechanism is used.

  In selecting the P-type MOSFET 4 and the N-type MOSFET 5, the specifications of the PZT used as a load are taken into consideration, and the same performance is required as the P-type MOSFET and the N-type MOSFET. Then, since the electrical characteristics of the N type are generally excellent, it is difficult to select a desired element based on the same shape conditions of the same manufacturer. Unavoidable.

  As described above, the parts become large, and it is difficult to select the parts, and there are considerable restrictions on the optimum design including the electrical characteristics and the mounting space, and the design man-hours increase.

  The present invention solves such problems, and its purpose is to devise the pulse width of the transmission waveform, thereby expanding the selection range of electronic components without changing the circuit configuration and selecting small components. The object is to realize an ultrasonic flowmeter that can reduce the design man-hours and reduce the size, thereby reducing the cost.

In order to achieve such an object, the invention described in claim 1 of the present invention is
In the ultrasonic flowmeter configured to send and receive an ultrasonic signal to the fluid to be measured via an ultrasonic transducer and measure the flow rate,
The ultrasonic transducer is driven by a driving signal of a small signal class.

The invention according to claim 2 is the ultrasonic flowmeter according to claim 1,
The ultrasonic transducer is gradually charged with a plurality of pulses of the driving signal.

The invention according to claim 3 is the ultrasonic flowmeter according to claim 1 or 2,
The polarity of one of the drive signals of the ultrasonic transducer is turned on and then the other is turned on.

The invention according to claim 4 is the ultrasonic flowmeter according to claim 3,
A damping operation is performed after the other polarity is turned on.

  As a result, it is possible to realize an ultrasonic flowmeter that can select a small component as a circuit component, reduce the number of design steps, and reduce the cost by reducing the size.

It is a circuit diagram which shows one Example of this invention. 2 is a timing chart illustrating the operation of FIG. 1. It is a waveform example diagram of a single pulse. It is a circuit diagram which shows an example of the transmission circuit of the ultrasonic signal in the conventional ultrasonic flowmeter. 5 is a timing chart for explaining the operation of FIG. 4.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the present invention. The circuit configuration of FIG. 1 is substantially the same as that of FIG. 4, except that the circuit A surrounded by the broken line in FIG. 1 is small in the timing chart as shown in FIG. That is, it is composed of low-voltage components for signals (100 mA).

  2 is a timing chart for explaining the operation of FIG. 1, (a) shows the output waveform of the waveform generator 2, (b) shows the operation of the MOSFET 4, (c) shows the operation of the MOSFET 5, d) shows the current of the MOSFET 4, (e) shows the current of the MOSFET 5, (f) shows the operation of the switch 15, (g) shows the current of the switch 15, and (h) shows the current of the ultrasonic oscillator 13. A voltage waveform is shown.

  The difference between the timing chart of FIG. 2 and the timing chart of FIG. 5 is the operation waveform of the MOSFET 4 shown in FIG. When attention is paid to the operation waveform of the MOSFET 4, the pulse width of the operation waveform of the MOSFET 4 in FIG. 2B is wider than that of the MOSFET 4 in FIG. 5B.

  Comparing the operation waveforms of the MOSFET 4 and the MOSFET 5, in the timing chart of FIG. 5, both pulse widths are set equal, but in the timing chart of FIG. 2, the pulse width of the MOSFET 4 is several times the pulse width of the MOSFET 4 of FIG. It is set.

  The pulse width is conventionally calculated based on the frequency of the resonance frequency of PZT, but is determined according to the circuit element to be selected in the present invention.

  When the PZT is considered to be “capacitance”, the electrical operation of the PZT has the same behavior as when the capacitor is voltage-pulse driven. That is, it is necessary to charge with a spiked large current to raise the voltage sharply and to discharge with a spiked large current at the fall. When a limit is applied to this spike-like current, the rise time and fall time of the voltage become dull.

  As shown in FIG. 2A, + Ip1, which is a spike-like current peak value at the time of rising, is slowly charged with the ON pulse width of the MOSFET 4 shown in FIG. 2B until PZT becomes the voltage + V1.

  After that, the PZT in the ultrasonic oscillator 13 is pulse-driven by the pulse of the voltage −V2 having a predetermined resonance frequency with the ON width of the MOSFET 5 shown in FIG. 2C, and an ultrasonic signal is oscillated and output.

  With such a circuit configuration, circuit components having a small external shape can be used as the MOSFETs 4 and 5, the resistors 8 and 11, and the diodes 9 and 12 in FIG. 1. The pulse width can be set.

A waveform necessary for driving the PZT will be described.
The PZT itself can be driven by a single pulse as shown in FIG. 3, and the pulse width is ½ of the period of the resonance frequency. However, as an ultrasonic flow meter, PZT does not operate stably with this driving, and practical application is difficult. Therefore, it is normally driven by a ± power supply.

  Further, since the PZT continues to vibrate only with the waveform shown in FIG. 5, it is actually necessary to provide a damping mechanism such as the switch 15 shown in FIG. 1 to stop the PZT after the pulse driving shown in FIG. .

  From these facts, practically, it is driven with a ± voltage waveform as shown in FIGS. 2 and 5, and a damping waveform is added.

The equivalent circuit of FIG. 1 will be described.
If the total resistance in the current path from the positive voltage source 6 to the cable 10 through the MOSFET 4, the resistor 8 and the diode 9 is R, and the capacitance of the ultrasonic oscillator 13 is C, the pulse response of a simple primary RC circuit is obtained. The rise time is
tr = V0 × (1-exp (−t / RC))
Indicated by V0 is the voltage of the positive voltage source 6 and is about ± several tens to ± 100V.

  The pulse width and peak current of the waveform shown in FIG. 2B for driving the PZT according to the present invention will be described. This pulse width is calculated not based on 1 MHz of PZT but based on selection of a small signal class element (for example, 100 mA product).

Here, in consideration of the margin of the element, Ip1 is suppressed to 35 mA, which is 1/100 of the conventional example. When the driving voltage of PZT is 60 V, the peak current Ip1 shown in FIGS. 2 and 5 is 35 mA, so the resistance R is
R = V / Ip1 = 60/35 mA = 1.7 kΩ
It becomes.

Similarly, if the pulse width in this case is considered to be, for example, three times τ,
Pulse width = 1.7K × 10nF × 3 = 51 μsec
It becomes.

  The pulse width set here is several tens to several hundred times that of the conventional example. If this pulse width is long enough to overlap with the next drive pulse, there will be a problem in operation, but the interval is actually wider than the set interval, which is not a problem in practical use.

  The required absolute maximum rating value of the component selected in this way requires a conventional value for the voltage, but the current can be small because the peak can be kept extremely low. As a result, a small small-signal SMD product can be selected as the semiconductor or resistance component.

  Although the improvement of the + side circuit in the output stage has been explained, since PZT is a capacitor with no polarity in terms of circuit, the drive waveform is first turned on the-side, then turned on the + side, and finally the damping operation You may let them. From the above, it is possible to reduce the size of the components, and to reduce the mounting space of the circuit of the output stage.

  Since the ultrasonic flowmeter has only a reduction effect on either the positive side or the negative side in the ± output stage, the cost effect is small. However, an actual ultrasonic flowmeter has many channels (for example, four channels). As a whole, the effect of reducing the size of the component and reducing the mounting space can be obtained, and the size can be reduced.

  In terms of circuit design, it becomes possible to select components mainly based on electronic component characteristics when designing the output stage, and the design becomes very easy.

  Conventionally, the selection of electronic components in the output stage of an ultrasonic flowmeter has been determined mainly based on the characteristics of PZT. Since PZT as an ultrasonic oscillator has different driving frequencies and driving voltages due to differences in piping to be measured, several types of PZTs are required.

  It is necessary to select a part that corresponds to the type of PZT and includes the balance of electrical characteristics of the part (MOSFET) used in the ± output stage. It was a restriction when the external size was large and miniaturized.

  By devising the drive waveform based on the present invention, the design of at least one of the + side and the-side of the output stage can be selected mainly based on the characteristics of the electronic components, not PZT, and is relatively small. Parts can also be included in the selection range.

  Thereby, it is possible to select an electronic component with high availability without depending on PZT, greatly increasing the degree of design freedom, and reducing the number of man-hours, and the ultrasonic flow meter can be operated without any problem.

  In addition, when applied to existing products, it can be implemented without changing the basic hardware configuration simply by changing the drive pulse width, which is digital data, and the flexibility of element selection in circuit design is increased. It is easy to create a combined drive waveform, which can be expected to reduce the size and design man-hours.

  As described above, according to the present invention, by devising the pulse width of the transmission waveform, the selection range of electronic components can be expanded without changing the circuit configuration so that small parts can be selected, thereby reducing the design man-hours. In addition, an ultrasonic flow meter that can be reduced in terms of cost can be realized by downsizing.

DESCRIPTION OF SYMBOLS 1 Control part 2 Waveform generation part 3 Drive part 4, 5 MOSFET
6 Positive voltage source 7 Negative voltage source 8, 11, 14 Resistance 9, 12 Diode 10 Coaxial cable 13 Ultrasonic oscillator 15 Switch

Claims (4)

In the ultrasonic flowmeter configured to send and receive an ultrasonic signal to the fluid to be measured via an ultrasonic transducer and measure the flow rate,
The ultrasonic flowmeter, wherein the ultrasonic transducer is driven by a driving signal of a small signal class.   The ultrasonic flowmeter according to claim 1, wherein the ultrasonic transducer is gradually charged with a plurality of pulses of the drive signal.   3. The ultrasonic flowmeter according to claim 1, wherein the polarity of the drive signal of the ultrasonic transducer is turned on after the polarity of the drive signal is turned on.   4. The ultrasonic flowmeter according to claim 3, wherein a damping operation is performed after the other polarity is turned on.

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