JPH0361129B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ultrasonic flowmeter used to measure the flow rate of large-diameter water pipes and the like.

FIG. 1 is a block diagram showing the configuration of an ultrasonic pulse generation circuit and its peripheral circuits in a conventional ultrasonic flowmeter. In the figure, 1 is a voltage controlled oscillator (hereinafter referred to as VCO). The oscillation frequency f of VCO1 is determined by the control voltage Vc supplied from a time difference voltage conversion circuit (not shown), which is the ultrasonic propagation time T from when the ultrasonic wave is transmitted to when it is received.
It is controlled to be N (constant) times the reciprocal of 1/T. That is, the oscillation frequency f is the propagation time T
is inversely proportional to. Then, the oscillation frequency f of VCO1 when the ultrasonic wave is emitted in the flow direction of the fluid to be measured
1 and when ultrasonic waves are emitted in the opposite direction of the flow.
The flow rate is measured from the difference with the oscillation frequency f2 of the VCO1. The pulse signal S1 (frequency f) output from this VCO1 is transmitted to the synchronous circuit 2.
and counter 3. Synchronous circuit 2
is the start signal STRT applied at regular intervals
It is triggered by the pulse signal S1a supplied from the VCO 1 immediately after the signal becomes "H" level, and outputs the excitation signal EXC in synchronization with the pulse signal S1a. This excitation signal EXC is supplied to the counter 3 and the ultrasonic pulse generation circuit 4. counter 3
When it receives the excitation signal EXC, it immediately starts counting pulse signals S1, and when it counts N pulse signals, it outputs a signal and supplies it to the time difference voltage conversion circuit. This time difference voltage conversion circuit outputs a control voltage Vc based on the difference between the ultrasonic propagation time T and the time N/f that elapses while counting N pulse signals S1, so that T=N/ f=
Control VCO1 so that it becomes N/T. Also,
As shown in FIG. 2, the ultrasonic pulse generation circuit 4 generates an ultrasonic transducer drive pulse (hereinafter referred to as drive pulse) V that is synchronized with the rise of the excitation signal EXC. The waveform of this driving pulse V is a unit impulse waveform as shown in FIG. 2B. This driving pulse drives an ultrasonic transducer (hereinafter referred to as a transducer), not shown, to generate ultrasonic waves.

By the way, the above-mentioned unit impulse drive pulse contains many frequency components other than the resonant frequency fr of the vibrator, resulting in the following inconvenience.

(1) Ultrasonic flowmeters generally use vibrations in the thickness direction of the vibrator (the frequency of this vibration corresponds to the resonant frequency f _r of the vibrator mentioned above) as a sound source, but When the vibrator is driven with a wide drive pulse, radial vibration of the vibrator is also generated, which becomes a noise wave.
It becomes an obstacle to stable measurement.

(2) Exciting a vibrator that has sharp resonance characteristics at a certain frequency with a signal that has a wide frequency band will lead to a decrease in the effectiveness of the excitation.

(3) If the vibrator is driven with a unit pulse-like drive pulse, unnecessary ringing will last for a long time.

And in order to eliminate these inconveniences,
It is known that the resonator can be driven by several (several waves) drive pulses having a frequency equal to (or almost equal to) the resonant frequency f _r of the resonator (Reference: Fukukita et al. "Ultrasonic probe with matching layer" IEICE Technical Report US75-28 (1975-11)).
By the way, in an ultrasonic flowmeter, it is essential that the generation of drive pulses and the start of pulse signal counting of a counter are performed in synchronization with a start signal, due to the need for highly accurate time measurement. However, if a signal from an external oscillator were to be used as an excitation signal or drive pulse, there was a problem in that it was difficult to achieve synchronization. For example, if you start an external oscillator in synchronization with the start signal and try to use that signal, the signal that is actually needed is a few pulses (or waves) immediately after synchronization (that is, immediately after the start signal is generated). Despite this, the initial oscillation (rise of oscillation) of the signal supplied from this external oscillator is not sufficiently stable and cannot be used as an excitation signal.

In view of the above circumstances, the present invention provides an ultrasonic flowmeter that can perform stable measurements using ultrasonic waves with little noise. It is characterized by being driven by a drive pulse. More specifically, the present invention divides the pulse signal output from the VCO to generate an excitation signal (which can be amplified to obtain a drive pulse) having a frequency equal to or almost equal to the resonant frequency of the vibrator. It is characterized by comprising an excitation signal generator that generates an excitation signal, and a wave number limiting circuit that controls the number of pulses (wave number) of the excitation signal by operating or stopping the excitation signal generator in synchronization with a start signal. shall be.

Embodiments of the present invention will be described below based on the drawings.

FIG. 3 is a block diagram showing the configuration of the main parts of the first embodiment of the present invention, and in the figure, VCO1,
The counter 3 and the ultrasonic pulse generation circuit 4 have the same configuration and function as those shown in FIG. Further, 7 is an excitation signal generator, which divides the pulse signal S1, which is the output of the VCO 1, into 1/10 to generate an excitation signal EXC with a duty ratio of 50%, which is sent to the counter 3 and the ultrasonic pulse generation circuit. 4, respectively supplied to the wave number limiting circuit 8. The wave number limiting circuit 8 is cleared by the start signal STRT, and supplies an "L" level enable signal ENB to the excitation signal generator 7 to operate the excitation signal generator 7, while counting the wave number of the excitation signal EXC. When the number of waves reaches a certain number (1.5 waves in this embodiment), the enable signal ENB is set to "H" level to stop the operation of the excitation signal generator 7.

In such a configuration, when the start signal STRT (FIG. 4B) is applied to the wave number limiting circuit 8 at time t ₁ shown in the time chart of FIG. 4, the wave number limiting circuit 8 is cleared and the enable signal that is its output is
ENB (D in the figure) becomes "L" level. As a result, the excitation signal generator 7 is enabled, and the pulse signal S1a shown in FIG.
a is the first output from the VCO 1 after time _t1 ), the output signal of the excitation signal generator 7,
That is, the excitation signal EXC (FIG. 3C) becomes "L" level. From then on, every time five pulse signals S1 are applied, the excitation signal generator 7 changes from "L" to "H" to
Outputs an excitation signal EXC whose level changes to "L" (see Fig. 4, c). The frequency f _e of this excitation signal EXC is made to match the resonant frequency f _r of the vibrator (this will be described later). The excitation signal EXC thus formed is supplied to the counter 3 and the wave number limiting circuit 8. And counter 3
starts from the pulse signal S1a and sequentially counts the pulse signal S1, stops counting when the count value reaches N (constant), and supplies the output signal to a time difference voltage conversion circuit (not shown). In addition, the wave number limiting circuit 8 counts the excitation signal EXC, and the point in time when the excitation signal EXC changes from "L" to "H" to "L" and then changes to "H" level (see FIG. 4). The enable signal ENB is set to "H" level at the time _t2 ) shown in FIG. That is, when the excitation signal generator 7 outputs 1.5 waves of the excitation signal EXC, the wave number limiting circuit 8 sets the enable signal ENB to "H" level to stop the frequency dividing operation of the excitation signal generator 7. The excitation signal EXC thus obtained is amplified by the ultrasonic pulse generation circuit 4, becomes a drive pulse V (FIG. 4(e)), and is supplied to a vibrator (not shown). In this way, the vibrator is driven by a driving pulse V having a frequency equal to its resonant frequency f _r and a wave number of 1.5, so it emits ultrasonic waves with less noise and ringing. The ultrasonic flowmeter according to this embodiment can perform stable flow measurement.

Next, FIG. 5 is a block diagram showing the configuration of the main parts of a second embodiment of the present invention. This second embodiment differs from the first embodiment shown in FIG. 3 in the configuration of a wave number limiting circuit 9. In FIG. )9b and FF9c. Here, the inverter 9a inverts the excitation signal EXC and supplies it to the clock terminal CK of the FF9b.
9b is inverted every time the inverted excitation signal falls, its J and K input terminals are set to the "H" level, and the _Q1 output is supplied to the clock terminal CK of FF 9c. The J and K input terminals of FF9c are set to "H" level and "L" level, respectively, and when the _Q1 output falls, the _Q2 output, that is, the enable signal ENB becomes "H" level, and the excitation signal generator The frequency dividing operation of 7 is stopped. Also, clear terminal of FF9b, 9c
CLR is supplied with a start signal STRT.

In such a configuration, when the start signal STRT goes to "H" level at time _t1 shown in the time chart of FIG. 6, FF9b and 9c are cleared, thereby causing the _Q2 output, that is, the enable signal ENB to It becomes L'' level and enables the excitation signal generator 7. Then, the excitation signal generator 7 divides the pulse signal S1 into 1/10 as described above,
An excitation signal EXC whose level changes every 5 pulses is output (Fig. 6 C), and the excitation signal EXC is output from the third pulse.
As in the case of the first embodiment shown in the figure, the signal is supplied to the counter 3 and the ultrasonic pulse generation circuit 4, and performs the same functions. On the other hand, FFs 9b and 9c are maintained in a reset state while the start signal STRT is at the "H" level. Then, when the start signal STRT becomes "L" level at time _t2 shown in FIG. 6, FF9b is inverted by the fall of the inverted excitation signal that occurs at time _t3 shown in FIG. 6, and the _Q1 output becomes "L" level. H"
level. Then, at time t ₄ , the inverted excitation signal
When EXC falls again, the Q ₁ output is inverted again and goes to "L" level, but this falling of the Q ₁ output causes the Q ₂ output of FF9c, that is, the enable signal.
ENB is set to the "H" level, thereby causing the excitation signal generator 7 to stop the frequency dividing operation. In this way, the excitation signal generator 7 continues frequency division between times t ₁ and _{t 4} and outputs an excitation signal EXC consisting of 3.5 waves.
In this way, the length of the start signal STRT is “H”
The wave number of the excitation signal can be changed by changing the time at the level (time t ₁ to _{t 2} in FIG. 6).

In the two embodiments described above, in order to match the frequency f _e of the excitation signal EXC with the resonant frequency f _r of the vibrator, the frequency f of the pulse signal S1 and the frequency division ratio of the excitation signal generator 7 must be adjusted appropriately. or choose a resonant frequency f _r that matches the achievable frequency f _e
It is sufficient to use a vibrator having

In addition, the pulse signal S1 output from VCO1
Although the frequency f changes depending on the flow velocity, this rate of change is extremely small and can be ignored. For example, if the center value of the frequency f (the value of f when the flow velocity is 0) is 10 MHz, and the span proportionality constant (the value of f when the flow velocity is 1
The change in f value when mm/S changes) is 5Hz/
(1 mm/S), even if the flow velocity changes by ±10 m/S, the value of f will only change by about ±50 KHz (change rate of 0.5%). Therefore, this pulse signal S
The frequency f _e of the excitation signal EXC obtained by dividing 1 into 1/10
is 1MHz±5KHz, and considering the resonance characteristic Q of the driven vibrator, it can be said that this excitation signal EXC has sufficient stability.

Further, the ultrasonic pulse generation circuit 4 includes, for example, a seventh
As shown in the figure, there is a FET 11 driven by an excitation signal EXC, and an LC resonant circuit 12 connected to the output side of the FET 11 (the resonant frequency of this resonant circuit 12 is set to match the resonant frequency f _r of the vibrator). ), it is possible to efficiently transmit energy to the vibrator.

Finally, the excitation signal generator 7 and the wave number limiting circuit 8
It is not necessarily necessary to have the configuration as in the above embodiment; for example, a microprocessor may be used. Also, the frequency division ratio of the excitation signal generator 7 (1/10 in the above embodiment) and the wave number count value of the wave number limiting circuit 8 (1.5 waves in the above first embodiment)
can be set to any value.

As explained above, the present invention obtains several waves of excitation signals having a frequency equal to (or almost equal to) the resonant frequency of the vibrator from the output pulse signal of the VCO, and drives the vibrator based on this excitation signal. , it is possible to efficiently generate ultrasonic waves with little noise in synchronization with the start signal. Moreover, this provides the advantage that stable flow rate measurement is possible.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the configuration of an ultrasonic pulse generation circuit and its peripheral circuits in a conventional ultrasonic flowmeter, and Fig. 2 shows the waveforms of input and output signals of the ultrasonic pulse generation circuit 4 shown in Fig. 1. Figure A shows the waveform of the excitation signal EXC, which is the input signal to the ultrasonic pulse generation circuit 4, and Figure B shows the waveform of the drive pulse V, which is the output signal of the ultrasonic pulse generation circuit 4. FIG. Further, FIG. 3 is a block diagram showing the configuration of the main part of the first embodiment of the present invention, and FIG.
The figure is a time chart for explaining the operation of the second embodiment, FIG. 5 is a block diagram showing the configuration of the main part of the second embodiment of the present invention, and FIG. 6 is for explaining the operation of the second embodiment. FIG. 7 is a circuit diagram showing an example of the configuration of the ultrasonic pulse generation circuit 4. 1... VCO (voltage controlled oscillator), 7... Excitation signal generator, 8... Wave number limiting circuit.

Claims (1)

[Claims] 1. An ultrasonic transducer that is driven by an excitation signal and emits ultrasonic waves, and a voltage-controlled oscillator that outputs a pulse signal with a frequency that is inversely proportional to the ultrasonic propagation time from when the ultrasonic wave is emitted until it is received. In an ultrasonic flowmeter that measures the flow rate based on the frequency of the pulse signal, the pulse signal is frequency-divided to generate the excitation signal that is equal to or approximately equal to the resonance frequency of the ultrasonic vibrator. An ultrasonic flowmeter comprising: an excitation signal generator; and a wave number limiting circuit that stops generation of the excitation signal when the wave number of the excitation signal reaches a certain number.