JPH0421807B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to an ultrasonic flow rate measuring device, and particularly to an ultrasonic flow rate measuring device using the sing-around method.

[Prior art and its problems]

Ultrasonic flow velocity measurement devices utilize the fact that the apparent velocity of ultrasound propagating through a fluid varies depending on the velocity of the fluid. FIG. 1 shows the prior art. In this FIG. 1, reference numeral 1 indicates a tube. It is assumed that a fluid to be measured (for example, water) is flowing inside the pipe 1 at a velocity V in the direction of arrow A along the pipe path.

A pair of ultrasonic transducers 2 and 3 for transmitting and receiving are installed in two sections of the tube 1, one on the upstream side and the other on the downstream side, separated by a predetermined distance. The ultrasonic transducers 2 and 3 are connected to a transmitting/receiving circuit section 5 via a changeover switch 4. Then, an ultrasonic pulse emitted from one ultrasonic transducer is received by the other ultrasonic transducer, and then a second transmission wave is emitted from one ultrasonic transducer.

To explain this in more detail, the transmitting/receiving circuit section 5 includes a transmitting pulse generating circuit 6, a receiving circuit 7, and a synchronizing circuit 8. Then, first, the synchronization circuit 8 outputs a start pulse to the transmission pulse generation circuit 6 by orbit operation. The transmission pulse is output to one of the ultrasonic transducers 2 via the changeover switch 4. Ultrasonic transducer 2
converts electrical signals into acoustic signals and emits ultrasonic pulses that propagate through water.

When this ultrasonic pulse is received by the other ultrasonic transducer 3, it is converted into an electrical signal and sent to the receiving circuit 7. After the received wave is amplified by the receiving circuit 7, it is output to the synchronizing circuit 8. This synchronization circuit 8 has a function of performing self-synchronization at a predetermined trigger level and generating a single pulse, and a synchronized pulse signal is sent to the pulse generation circuit 6. Using this transmission pulse, operations such as emission, underwater propagation, reception, and amplification of the ultrasonic pulse are repeated many times in the same manner as described above.

On the other hand, the pulse signal output from the synchronization circuit 8 is sent to the frequency multiplier circuit 9, and pulse oscillation is performed at a frequency corresponding to the pulse repetition frequency in the transmitter/receiver circuit section 5. Separately, when ultrasonic pulses are repeatedly transmitted and received in the same direction for a predetermined period of time, the changeover switch 4 is switched in response to a command from the transmission/reception switching circuit 10, and this time the ultrasonic transducer 3 is switched to the ultrasonic transducer 2.
Ultrasonic pulses are transmitted and received in the direction of. Thereafter, the switching operation of the transmission/reception direction is similarly performed at regular intervals.

Here, the flow velocity is V [m/s], and the underwater sound velocity is C.
[m/s], the distance that the ultrasonic wave travels between the ultrasonic transducers 2 and 3 is L [m], and the angle between the ultrasonic transmission direction and the water flow direction is θ, then from the ultrasonic transducers 2 to 3 When transmitting and receiving from the ultrasonic transducer 3 to 2, the pulse repetition frequency fd of the transmitting/receiving circuit section 5 is fd=1/Td=(C+V・cosθ)/L; conversely, when transmitting and receiving from the ultrasonic transducer 3 to 2, the pulse repetition frequency fd of the transmitting/receiving circuit section 5 is as follows. 5 pulse repetition frequency fu
is, fu=1/Tu=(CV・cosθ)/L. Letting the multiplication factor of the frequency multiplier circuit 9 be M, and find the difference Δf between the two oscillation frequencies "M・fd" and "M・fu" accompanying the switching of the transmission/reception direction of the frequency multiplier circuit 9, Δf= M·fd−M·fu = {(2V·cosθ)/L}·M, which is a value proportional to the flow velocity V. An up-down counter 11 for calculating the flow velocity is connected to the output side of the frequency multiplier circuit 9. This up-down counter 11 is connected to the transmission/reception switching circuit section 10.
When transmitting and receiving ultrasonic pulses from the ultrasonic transducer 2 to the ultrasonic transducer 3 in response to a switching signal sent from the ultrasonic transducer 2, the mode is set to up mode, and vice versa. In this case, the output pulses of the frequency multiplier circuit 9 are counted while switching to the down mode, the above-mentioned Δf is calculated, and the flow velocity or flow rate data is output.

However, since such conventional technology is configured to immediately transmit an ultrasonic pulse upon receiving it, small-diameter pipes in particular are susceptible to multiple reflections and reverberations such as sound waves propagating through the pipe wall, making measurement difficult. Easy to cause errors. For example, caliber 10 [mm],
Propagation time ΔT obtained when flow velocity is 1 [m/s]
is ΔT=Tu−Td, which is only about Δt=40 [nS]. For this reason, when reverberation as an interference wave is combined with a received wave, there will be almost no measurement error if the phase between the received wave and the reverberation is the same regardless of the direction of transmission and reception, but the phase may differ depending on the direction of transmission and reception. If it is on,
As shown in FIG. 2, the timing at which the received wave reaches the synchronization trigger level of the synchronization circuit varies by Δt depending on the transmission and reception direction. This has resulted in a large error in flow rate measurement, which is disadvantageous in that it cannot be put to practical use.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned drawbacks of the prior art.The present invention provides a delay time between receiving an ultrasonic pulse and transmitting it again, and by multiplying this delay time by jitter, the pulse repetition period can be changed. It is an object of the present invention to provide an ultrasonic flow rate measuring device that can eliminate the influence of the reverberation without significantly slowing down the flow rate, and can accurately measure flow rate or flow rate even in a small diameter pipe.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 3 to 5. FIG. 3 is an overall block diagram showing an ultrasonic flow velocity/measuring device according to an embodiment of the present invention, and FIG. 4 is a timing chart showing its operation.

In FIG. 3, the transmitting/receiving circuit section 5A includes a transmitting pulse generating circuit 6 which is selectively connected to the ultrasonic transducers 2 and 3 attached to the tube 1 via the changeover switch 4.
and a receiving circuit 7, and a delay circuit 20 and a synchronization circuit 21 connected in series to the receiving circuit 7. When receiving the received wave amplified by the receiving circuit 7, the delay circuit 20 functions to provide a certain time width, that is, a delay time τ, and to delay the output of the next signal. The delay time τ of the delay circuit 20 is controlled by an irregular waveform signal output from the jitter circuit 22, so that the value of the set time (delay time width) changes. For this reason, the time (delay time τ) between input of one received wave and output of the next transmitted signal is irregularly different each time a received wave is input.

FIG. 5 shows a specific example of the delay circuit 20 and the jitter circuit 22. Here, jitter will be explained in detail.

Jitter itself originally refers to the amplitude of the signal waveform,
It is defined as an undesirable rapid and intermittent change in period, phase shift, or pulse width (JIS/C1002 electronic measuring instrument terminology). For this reason, jitter itself is known as an undesirable phenomenon, such as distortion of observed waveforms due to jitter in the trigger of an oscilloscope. The jitter circuit 22 in this embodiment is used as a circuit that uniquely and intentionally generates jitter. FIG. 5 shows an example.

In FIG. 5, reference numeral 22A indicates a Zener diode. This Zener diode 22A
is reverse-biased with a weak current of about several [μA] through the resistance element R ₀ , so that a voltage of about 10 [mV] is generated across the Zener diode. This noise signal A changes irregularly with respect to time. This noise signal A is
The signal is AC-coupled to the amplifier _22B via the capacitor C1, amplified to a predetermined voltage, and sent to the delay circuit 20 as the output signal of the jitter circuit 22.

The output of the jitter circuit 22 is actually sent to the delay circuit ₂₀ via a capacitor C2 as shown in FIG.
is input to. This capacitor _C2 functions to remove DC components such as the offset voltage of the noise signal A.

The delay circuit 20, for example, as shown in FIG.
A resistive element R ₁ , which is equipped with a PNP transistor T and a monostable multivibrator 20A, and which applies a predetermined bias to the signal from the jitter circuit 22 described above by dividing the voltage and inputs it as the base voltage of the transistor T; It has R ₂ . Further, this delay circuit 20 includes a transistor T
A resistor element _R3 is provided at the emitter terminal of the monostable multivibrator 20A, and a capacitor _C3 is connected in parallel to the monostable multivibrator 20A. That is,
In this delay circuit 20, the impedance between the emitter and collector of the transistor T changes irregularly over time due to the action of the jitter circuit. Therefore, each time a symbol is given to the trigger terminal of a monostable multivibrator, the unique value that determines the time constant of the monostable multivibrator is
Due to R ₃ , C ₃ and the irregularly changing impedance between the emitter and collector, the mono-multivibrator 20A outputs a pulse train whose pulse width changes irregularly.

The output pulse of the delay circuit 20 is sent to the synchronization circuit 21. This synchronization circuit 21 includes a delay circuit 20
It enters a preset state at the trailing edge of a pulse signal sent from the oscillation circuit 26, and forms a single pulse at the rising timing of an oscillation pulse output from a voltage controlled oscillation circuit 26, which will be described later, and outputs it to the transmission pulse generation circuit 6. This transmission pulse generation circuit 6 outputs a transmission pulse in response to a signal sent from the synchronization circuit 21. Under the operating state of the changeover switch 4 shown in FIG. 3, ultrasonic pulses are output from one ultrasonic transducer 2. After this ultrasonic pulse propagates through water, it is received by the other ultrasonic transducer 3, and a received signal is sent to the receiving circuit 7. Thereafter, similar operations are repeated.

The transmitting/receiving circuit section 5A configured as described above is basically intended to eliminate the influence of reverberation on the received waves by providing a delay time between reception and transmission. By multiplying this, the timing of the output of the transmission pulse is changed irregularly each time (each time), and the effect of the reverberation described above can be effectively avoided with a relatively short delay time.

Specifically, since the delay time set in the delay circuit 20 is irregularly changed by the jitter circuit 22 as described above, the position of the received waveform is irregular before and after a specific reverberation force. Therefore, the influence of reverberation can be effectively avoided. Even if the received signal and reverberation overlap, the reverberation level will either be added or subtracted, so when averaged, the effect of the reverberation will be approximately "±"
0”.

Receiving circuit 7 and synchronous circuit 2 of transmitting/receiving circuit section 5A
A variable frequency oscillator 23 is connected to the output side of the oscillator 1 . The variable frequency oscillator 23 has a function of oscillating at a frequency inversely proportional to the ultrasonic pulse propagation time T (see FIG. 4) between the ultrasonic transducers 2 and 3 attached to the tube 1. Specifically, the variable frequency oscillator 23 includes a counter 27 that is reset by the pulse signal output from the synchronization circuit 21 and generates a frequency-divided pulse when the output pulse of the voltage controlled oscillation circuit 26 is multiplied by _Nt . An error detection circuit 24 inputs the output pulses of the counter 27 and the received pulses of the reception circuit 7, and outputs a pulse signal whose polarity changes depending on the order of pulse generation of these two input signals and whose width changes according to the time difference; Error detection circuit 24
Integrating circuit 2 that generates a control signal that averages the output of
5 and a voltage-controlled oscillation circuit 26 whose output frequency is controlled in the direction that this control signal is always zero, forming a closed loop. It's controlled.

Here, if the count number of the counter 27 is M, it is assumed that this M is set in advance. The counter 27 starts counting by the transmission pulse signal output from the periodic circuit, and when it counts M, outputs a predetermined pulse as described above.

The output of the error detection circuit 24 is sent to an integration circuit 25. The oscillation frequency of this voltage-controlled oscillation circuit 26 can be varied by the output voltage of the integration circuit 25.

Further, if the output of the delay circuit 20 (delayed transmission signal) is used as it is as a transmission signal to the transmission pulse generation circuit 6, the count start signal of the counter 27 and the transmission signal are not necessarily synchronized. Therefore, the synchronization circuit 21 synchronizes the transmission signal with the count start signal of the counter 27 to perform transmission.

On the other hand, the changeover switch 4 is switched between the transmission and reception directions at regular intervals by the transmission/reception switching circuit 10, as in the prior art described above. Then, the up-down counter 11 serving as a flow rate calculation counter adds and subtracts the oscillation pulses of the variable frequency oscillator 23 while switching between up mode and down mode according to the transmission/reception direction. Td is the propagation time when transmitting from the ultrasonic transducer 3 side to the ultrasonic transducer 3 side, and conversely the propagation time when transmitting from the ultrasonic transducer 3 side to the ultrasonic transducer 2 side.
Tu, it is possible to obtain a fractional difference Δf' proportional to the flow velocity or flow rate as follows: Δf'=(N/Td)−(N/Tu)=(2V·cosθ/L)·N.

〔Effect of the invention〕

As described above, according to the present invention, by the action of the delay circuit and the jitter circuit, the influence of reverberation on received waves can be effectively removed without increasing the delay time from reception to transmission too much. Continuous flow rate and flow measurements can be performed with little measurement error. In addition, since the interval between reception and transmission can be increased, automatic gain adjustment can be easily applied to the received waves, and measurement accuracy can be further improved, making it possible to provide an unprecedented and excellent ultrasonic flow velocity measuring device.

[Brief explanation of drawings]

FIG. 1 is an overall block diagram showing a conventional example, FIG. 2 is a line diagram showing received waves, FIG. 3 is an overall block diagram showing an ultrasonic flow rate measuring device according to the present invention, and FIG. Timing chart showing the operation of
This figure is a circuit diagram showing a specific example of the delay circuit and jitter circuit in FIG. 3. 1... Tube, 2, 3... Ultrasonic vibrator, 4... Changeover switch, 5, 5A... Transmission/reception circuit section, 10
. . . Transmission/reception switching circuit, 11 . . . Up-down counter as a counter for calculating flow velocity, 20 .

Claims (1)

[Scope of Claims] 1. Two ultrasonic transducers for transmitting and receiving ultrasonic waves installed at two locations along the central axis of a pipeline through which a fluid to be measured flows, separated by a predetermined distance, and each of these ultrasonic transducers. a transmitting/receiving circuit unit that operates one of the ultrasonic transducers for signal transmission and the other for signal reception; and a transmitting/receiving switching circuit that switches and sets the transmitting function and receiving function of the two ultrasonic transducers at regular intervals. for calculating the flow velocity of the fluid to be measured by calculating the time difference between the transmission and reception timings outputted from the transmission and reception circuit section every switching period by the transmission and reception switching circuit. The transmission/reception circuit section is equipped with a delay circuit that appropriately delays the output timing of the transmission signal, and the delay circuit is equipped with a jitter circuit that irregularly changes the time width of the delay time by the delay circuit. An ultrasonic flow velocity measuring device characterized by being attached.