JPH0260967B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ultrasonic flow measuring device that measures the medium flow rate by measuring the propagation time of ultrasonic waves in a medium to be measured in the forward and reverse directions with respect to the flow. More specifically, the present invention relates to an ultrasonic flow rate measuring device that compensates for measurement errors due to temperature changes in a medium to be measured through which ultrasonic waves are propagated.

As an example of a conventionally known ultrasonic flow measuring device,
A closed loop is configured to control the oscillation frequency f of the voltage controlled oscillator so that the time for counting N pulse signals of frequency f oscillated by the voltage controlled oscillator matches the propagation time of the ultrasonic wave in the fluid. death,
In synchronization with the oscillation of one of the transducers, the output of a counter that counts N pulses of frequency f is input to the time difference detection circuit, and on the other hand, the ultrasonic reception signal is also input to this time difference detection circuit, and the time difference is A closed loop is constructed so that the time difference between the two outputs is zero. An example of such a conventional ultrasonic flow rate measuring device is shown in FIG. In the figure, a voltage controlled oscillator circuit 10 has two voltage controlled oscillators (hereinafter referred to as VCO) 13 and 15 whose oscillation frequency changes depending on the magnitude of a control voltage. Oscillation output signal 21 of VCO 13 (frequency f ₁ )
and the oscillation output signal 23 (frequency f ₂ ) of the VCO 15 are switched by a switch 25, and the resulting output signal 29 of the voltage-controlled oscillation circuit 10 is supplied to the pulse generation circuit 27. The pulse generating circuit 27 generates a pulse signal 31 in synchronization with the signal 29 and also generates a counting start signal 33. Based on this pulse signal 31, the transmitter circuit 35 generates a transducer drive signal 37.

Two transducers (vibrators) 41 and 43 convert an electrical signal into an acoustic signal (ultrasonic waves 45 or 47) in response to transducer drive signals 37 alternately supplied by switching a switch 45. one acts as a transmitter, converting the acoustic signal into an electrical signal, and the other acts as a receiver, converting the acoustic signal into an electrical signal. An electrical signal obtained by receiving this acoustic signal is introduced into a receiving circuit 55 as a received signal 53 via a changeover switch 51. The reception circuit 55 supplies a reception detection signal 57 to the time difference detection circuit 59 in response to the reception signal 53.

Further, the counter 61 activated by the counting start signal 33 counts the output signal 29 of the voltage controlled oscillation circuit 10. When the counting state reaches a preset value N, this counter 61 supplies a pulse output signal 63 to the time difference detection circuit 59, and is then reset. The time difference detection circuit 59 detects the time difference between both signals 57 and 63, and generates a control signal 65 with a voltage corresponding to the time difference. This control signal 65 is switched by a switch 67,
It is introduced into either one of both VCOs 13 and 15 in the voltage controlled oscillation circuit 10 to control its oscillation frequency f ₁ or f ₂ .

FIG. 2 shows the state in which both transducers are attached to the pipe line of the fluid to be measured, and also shows the propagation of ultrasonic waves. In the figure, the ultrasonic waves emitted from one of the upstream transducers 41 disposed opposite to each other are as follows:
A plastic wedge 73 is propagated obliquely into a tube 71 and a fluid to be measured 75 is passed through the tube 71.
and again via the wall of tube 71 and another plastic wedge 77 to the other downstream transducer 43. In this case, the ultrasonic wave 4 is transmitted from the transducer 41 to the transducer 43.
The forward propagation time T ₁ of 5 is given as T ₁ =D/cosθ/C _W +Vsinθ (1). Conversely, the reverse propagation time T ₂ of the ultrasonic wave 47 from the transducer 43 to the transducer 41 is given as T ₂ =D/cosθ/C _W -Vsinθ (2). Here, D is the inner diameter of the tube 71,
C _W is the sound velocity in the fluid 75 when the fluid 75 is stationary, V is the flow velocity of the fluid 75, and θ is the incident angle at which the ultrasonic wave enters the fluid 75.
Note that both wedges 73 and 77 and the pipe 7 are shown here.
The time it takes for the ultrasonic wave to propagate through the pipe thickness section 1 is ignored.

Next, referring to FIGS. 1 and 2, the fluid 7
5. Flow rate measurement will be described. Note that this measurement principle is a well-known one that utilizes a phase lock loop, so it will be briefly explained. First, all the changeover switches 25, 45, 51, and 67 are connected to contact a.
Connect to the side for forward mode. In this case, the propagation time of the _ultrasonic wave 45 is
T ₁ is expressed by the above-mentioned equation (1). Also, the time T until the counting state of the counter 61 reaches the set value N
is N/f ₁ . A phase lock loop is formed to feedback control the oscillation frequency f ₁ of the VCO 13 so that this time T and the previous propagation time T ₁ have a predetermined relationship (in this case, they are equal). Therefore, once this system is stabilized, N/f ₁ =T ₁ holds, so the relationship f ₁ =N(C _W +Vsin θ)/D/cos θ (3) holds true.

Further, all the changeover switches 25, 45, 51 and 67 are respectively switched to the contact b side to set the reverse direction mode. In this case as well, as mentioned above (2)
The phase lock loop including the VCO 15 is set so that the propagation time T ₂ of the ultrasonic wave 47 expressed by the formula and the time T during which the counting state of the counter 61 reaches the set value N have a predetermined relationship (in this case, they are equal). The system becomes stable. Therefore, the oscillation frequency of VCO15
f ₂ is expressed as f ₂ =N(C _W −Vsinθ)/D/cosθ (4).

Taking the difference Δf (=f ₁ − f ₂ ) between these two frequencies, Δf=2Nsinθ/D/cosθ・V=Nsin2θ/D・V(5
) is given as Therefore, if the angle of incidence θ is constant, the frequency difference Δf is given as a function only of the flow velocity V of the fluid 75, so if both signals 21 and 23 are counted and the difference Δf between these two frequencies is found, then from that value, The flow velocity V of the fluid 75 can be calculated. Therefore, the flow rate of the fluid to be measured 75 can be measured.

However, such a flow rate measuring device is effective only within a range where changes in the incident angle θ can be ignored. For example, if the temperature of the fluid 75 changes significantly, this change in the angle of incidence θ cannot be ignored, especially in the tube 75.
The smaller the inner diameter D of 1, the greater this effect becomes, resulting in a drawback that the measurement results include large errors.

An object of the present invention was to solve the above-mentioned drawbacks, and even if the temperature of the medium to be measured changes or the flow path of the medium to be measured has a small diameter,
An object of the present invention is to provide a device that measures the flow rate of a medium to be measured without error.

According to the present invention, this purpose is achieved by a pair of transducers installed upstream and downstream with respect to the flow of the medium to be measured, a first oscillator with a constant oscillation frequency, and a second oscillator with a variable oscillation frequency. , a counter, the time for the ultrasonic wave to propagate in the first direction in the medium to be measured between the pair of transducers, and the time for the counter to count the output signal of the first oscillator up to a set value are predetermined; A first step that variably controls the set value of the counter so that the relationship is
a control means, a time for the ultrasonic wave to propagate in the medium to be measured between the pair of transducers in a second direction opposite to the first direction; and a counter controlled by the first control means. a second control means for variably controlling the oscillation frequency of the second oscillator so that the time during which the counter counts the output signal of the second oscillator up to a set value has a predetermined relationship; and the first and second oscillators. This can be achieved by comprising means for measuring the flow rate of the medium to be measured based on both oscillation frequencies.

Hereinafter, the present invention will be explained in detail based on the drawings.

First, the basic principle of the present invention will be explained using calculation formulas.

From equations (3) and (4), the relationship f ₁ +f ₂ =2C _W /D/cosθ·N (6) holds true.

From equation (6), 1/D/cosθ=f ₁ +f ₂ /2C _W ·N (7) Substitute equation (7) into equation (5).

Δf=2Nsinθ/D/cosθ・V=2Nsinθ/2C _W・N(f
₁ +f ₂ )・V = (f ₁ +f ₂ ) sin θ/C _W・V (8) Here, let C _S be the ultrasonic propagation velocity in both wedges 73, 77 and the thick part of the tube 71, and apply Snell's law. Accordingly, by replacing the incident angle θ in equation (5) above with the angle φ in the wedge, it can be expressed as sinθ/C _W =sinφ/C _S (9).

When this equation (9) is substituted into equation (8), the relationship Δf=(f ₁ +f ₂ )sinφ/ _CS ·V (10) is established.

Now, looking at the above equation (10) derived based on Snell's law, it is expressed as a function of the angle φ in the wedge. sinφ is unique to the transducer and is considered to be almost constant, and the ultrasonic propagation velocity C _S in the thick part of the pipe only shows a change of a few percent with respect to a temperature change of 100°C. during the ceremony
It can be assumed that sinφ/C _C is kept approximately constant.

From these facts, it can be seen that the term f ₁ +f ₂ in equation (9) depends on the temperature of the fluid and is a major factor causing measurement errors.

Therefore, in some way, the sum of frequencies (f ₁ +
If f ₂ ) is kept approximately constant, the temperature dependence of Δf can be made smaller. The present invention has been made from this point of view, and by adopting the following configuration, it is possible to obtain the flow velocity V in a manner such that the sum of frequencies is substantially constant. This is what I did.

That is, the ultrasonic flow measurement device according to the present invention fixes the frequency of the first oscillator to a constant (f ₁ ),
A forward loop that controls N so that the time required to count N times (N/f ₁ ) of the frequency f ₁ matches the forward propagation time T ₁ of the ultrasonic wave in the fluid, and a variable oscillation frequency. The frequency of the second oscillator, f ₂
A loop in the reverse direction that controls f ₂ so that the time required to count N numbers determined by the loop in the aforementioned direction (N/f ₂ ) matches the backward propagation time T ₂ of the ultrasonic wave in the fluid. It consists of and.

Here, by transforming equation (3), it can be expressed as f ₁ =NC _W /D/cos0 (1+V/C _W sinθ)...(11), and this equation (11) and the above equation (6) can be expressed as The following equation (12) can be obtained from

f ₁ = f ₁ + f ₂ /2 (1 + V / C _W sin θ) ... (12) Further transforming equation (12), f ₁ + f ₂ = f ₁ = 2 / (1 + V / C _W sin θ) ... (13) can be expressed.

Looking at equation (13) above, the relationship holds between the actual V and C _W : V≫C _W (V _nax = several m/s, C _W = approximately 1000 m/s), and V/C The term _W sinθ is considered to be extremely small, and in the present invention
Since f ₁ is kept constant, f ₁ + f ₂ is almost constant.

From the above explanation, in the present invention, the frequency sum (f ₁ +f ₂ ) is held substantially constant.

FIG. 3 shows an embodiment of the invention. Here, the difference from FIG. 1 is that the VCO 13 is replaced with an oscillator 131 with a constant frequency (for example, 2 MHz) f ₁₁ , the time difference detection circuit 59 is replaced with a variable time difference detection circuit 591, and a new controller 311, N setting This is because a processor 313 and a central processing unit (hereinafter referred to as CPU) 315 are provided. The same reference numerals indicate circuits having similar functions.

FIG. 4 shows a specific example of the variable time difference detection circuit 591 in FIG. 3. Here, the received wave detection signal 57 and the output signal 63 of the counter 61 are supplied to the exclusive OR gate 401.
The output signal 403 controls the on/off state of the semiconductor switch 405. From one end of this switch 405 to a resistor 421 (resistance value R ₄₂₁ ), a resistor 422
(resistance value R ₄₂₂ ) and resistor 423 (resistance value R ₄₂₃ )
are connected to the contacts p, q, and r of the changeover switch 441, respectively, through the respective contacts p, q, and r. Here, it is assumed that R ₄₂₁ > R ₄₂₂ > R ₄₂₃ . Further, a constant standard voltage _-ES is supplied to the switching terminal of the switch 441. The other end of the semiconductor switch 405 is connected to the inverting input terminal of the operational amplifier 445.
An integrating capacitor 447 and a discharge switch 449 are connected in parallel between the inverting input terminal and the output terminal of the amplifier 445, and the non-inverting input terminal of the amplifier 445 is grounded.

5A to 5J are signal waveform diagrams showing the operation of FIG. 3.

Reference will be made to FIGS. 3 to 5 A to J below. First of all,
A control signal 321 from the controller 311 connects the changeover switch 441 to contact q. It also controls the N setter 313 to provide a corresponding setting signal 3.
23, the set value N of the counter 61 is initialized to _N0 . Next, changeover switches 25, 45, 5
1 and 67 are connected to the contact a side for forward mode. In this state, the same operation as in the forward mode described in connection with FIG. 1 is performed.

That is, at time _t1 , the forward ultrasonic wave 45 is emitted and the counter 61 starts counting the signals 211. Assume that thereafter, at time _t2 , the counting state of the counter 61 reaches the set value _N0 . After that, it is assumed that the ultrasonic wave 45 is received at time _t3 , and both signals 57 and 63 in that case are shown in FIG.
and shown in B. Between times _t2 and _t3 , an integrated output signal 65 having a slope _τ1 determined by E _S (capacitance value of capacitor 447×R ₄₂₂ ) is obtained (see FIG. 5C).
If the integrated voltage E _I1 exceeds the reference value E _r1 at time _t3 , the controller 311 commands the control signal 321 to switch the changeover switch 441 to the contact point P.
If it is turned down, switch the switch 441 to the contact r side.

With the changeover switch 441 connected to the contact p, the ultrasonic wave 45 is emitted in the forward direction again.
The integrated voltage of the integrated output signal 65 in this case
Although the slope τ ₂ of E _I2 becomes smaller, the voltage E _I2 at time t ₃ is still higher than the reference value E _r1 . Therefore, the controller 31
1, the N setter 313 is controlled by the control signal 321, and the set value N of the counter 61 is increased by 1 by the setting signal 323 accordingly. Then, as shown by the output signal 63 of the counter 61 in FIG. 5D, the counting end time _t2 is delayed. On the other hand, since the integral gradient τ ₂ is constant, the voltage value of the integral output voltage E _I at the ultrasound reception time t ₃ becomes low. These operations are repeated until the integrated output voltage E _I at time t ₃ becomes approximately equal to the reference value E _r1 , and the obtained integrated output signal 65 is shown in FIG. 5E. In this case, the set value of the counter 61 is N _C , the integral slope τ _C , and the counting end time t ₂₁ .

Note that even if the changeover switch 441 is switched to the contact r, if the integrated output voltage E _I at the reception time t ₃ is still lower than the reference value E _r1 , the set value N of the counter 61 is decreased by 1. Then, the above-described operation is repeated to obtain the optimal set value N _C.

Next, each of the changeover switches 25, 45, 51, and 67 is switched to the contact b side to set the reverse direction mode. Operation in this reverse mode is similar to that described with respect to FIG.

That is, while emitting ultrasonic waves 47 in the opposite direction, the counting operation of the output signal 23 (frequency f ₂ ) of the VCO 15 is started (time t ₁ ). The variable time difference detection circuit 591 starts the integration operation at the counting end time t ₂₂ as shown by the count output signal 63 in FIG. 5G, and at the reception time t ₃₂ as shown by the reception detection signal 57 in FIG.
The integral operation ends with . The integrated output signal 65 is shown in FIG. 5H. Note that the integral gradient is the same gradient τ _C at the end of the operation in the forward mode. When the integrated output voltage E _I at the end of integration (time t ₃₂ ) exceeds the reference value E _r2 , the signal 65 indicating the voltage E _I outputs VCO1.
Lower the oscillation frequency f ₂ of 5. Then, since the set value N of the counter 61 remains at N _C , when the next backward ultrasonic wave 47 is emitted, the counting end time is
_t22 will be late. Such frequency variable control operation,
This is repeated until the integrated voltage E _I at the ultrasound reception time t ₃₂ becomes approximately equal to the reference value E _r2 . The counting output signal ₆₃ in that case is shown in FIG.
is shown in Figure 5J.

Both reference values E _r1 and E _r2 in the above description are determined based on the standard state of the flow rate measuring device. Therefore, if the above-described procedure is followed, the error due to the change in the incident angle θ is compensated for.

Therefore, the CPU 315 measures both the oscillation frequencies f ₁₁ and f ₂ of the OSC 131 and the VCO 15, and calculates the frequency difference Δf (=f ₁₁ −f ₂ ). After that, based on equation (5), the flow velocity V of the fluid to be measured 75 is determined.
can be calculated to determine the flow rate.

In addition, as shown in FIG. 4, resistors 421 and 42
The integral slope is changed by switching between 2 and 423. This is because the set value N of the counter 61 changes digitally, and the width of the digital change value is interpolated. If both frequencies f ₁₁ and f ₂ of oscillator 131 and VCO 15 are made very high (for example, 1 GHz), there is no need to change the integral slope.

As described in detail above, according to the present invention, it is possible to realize an ultrasonic flow rate measuring device that can perform accurate flow rate measurement by compensating for errors caused by temperature changes in the medium to be measured and other factors.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional ultrasonic flow rate measuring device, Fig. 2 is an explanatory diagram of the state in which a transducer is attached to the pipe line of the fluid to be measured and the propagation path of the ultrasonic waves, and Fig. 3 is an illustration of the ultrasonic wave propagation path of the present invention. FIG. 4 is a block diagram of an ultrasonic flow rate measuring device according to an embodiment, and FIG. 4 is a block diagram showing a specific example of the variable time difference detection circuit of FIG. 3.
FIGS. A to J are signal waveform diagrams of various parts for explaining the operation of FIG. 3. DESCRIPTION OF SYMBOLS 13, 15... Voltage controlled oscillator, 27... Pulse generating circuit, 35... Transmitting circuit, 41, 43... Transducer, 55... Receiving circuit, 59... Time difference detection circuit, 61... Counter, 71... Tube, 75... Fluid ,
131... Oscillator, 311... Controller, 315... Central processing unit, 591... Variable time difference detection circuit.

Claims (1)

[Claims] 1 A pair of transducers installed upstream and downstream with respect to the flow of the medium to be measured, a first oscillator with a constant oscillation frequency, and a second oscillator with a variable oscillation frequency.
An oscillator, a counter, a time for the ultrasonic wave to propagate in the medium to be measured in a first direction between the pair of transducers, and a time for the counter to count the output signal of the first oscillator to a set value are predetermined. a first control means for variably controlling the set value of the counter so that the relationship is such that the ultrasonic wave propagates in the medium to be measured between the pair of transducers in a second direction opposite to the first direction; and the time to
The oscillation frequency of the second oscillator is variably controlled so that the time required for the counter to count the output signal of the second oscillator up to the set value of the counter controlled by the first control means has a predetermined relationship. An ultrasonic flow rate measuring device comprising: second control means; and means for measuring the flow rate of the medium to be measured based on both the oscillation frequencies of the first and second oscillators.