JPS5832333B2 - Ultrasonic detection type Karman vortex flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an ultrasonic detection type Karman vortex flow rate system that detects the generation frequency of a Karman vortex street generated in accordance with the flow velocity of a fluid by the phase modulation received by an ultrasonic wave passing through the vortex street. This is related to the improvement of the meter.

A method for measuring the flow rate of fluid flowing in a conduit by installing a vortex generator in a conduit and detecting the generation frequency of the Karman vortex street generated downstream using ultrasonic waves was proposed in Utility Model Application No. 51-24154. .

When detecting the Karman vortex street using this method, the ultrasonic waves emitted from the ultrasonic transmitter are modulated by the Karman vortex street and received by the receiver.

The phase difference between the transmitting and receiving waves at this time is the first phase difference 1 determined by the distance between the ultrasonic transmitting and receiving elements, the temperature or speed of the fluid.
and a second alternating-current phase difference 2 that advances and lags in phase in accordance with the Karman vortex street, with the first phase difference 1 as the central value.

The first phase difference 1 changes relatively slowly in a direct current manner due to temperature changes in the fluid, etc., and the second phase difference 2 changes depending on the period and strength of the Karman vortex street, and changes extremely. fast.

Therefore, the average value of the entire phase difference is the first phase difference 1.

Now, Figure 1 shows a circuit diagram when an exclusive OR circuit and a low-pass filter are used as a phase comparator.
A phase difference output characteristic diagram is shown in FIG.

In FIG. 1, 11 is an exclusive OR circuit, 12 is a resistor, and 13 is a capacitor.

At the input of the exclusive OR circuit in Figure 1, the phase difference △
The average output voltage when φ is applied by two people is △φ is 00
~3600, it changes as shown in FIG.

When the aforementioned first phase difference 1 is, for example, 90°, the output Vout of the phase detector when △φ1 of phase difference 2 is superimposed is △φ1 as shown in FIG. Compatible△
■1 is obtained.

When the first phase difference 1 is 180°, the output Vout due to △φ2 of the superimposed phase difference 2 is as shown in △■2, △
This results in a demodulated waveform different from φ2.

Such a problem occurs when the phase difference is O', 180',
The first phase difference 1 occurs at the point where the output of the phase detector bends like 360°, etc., but the first phase difference 1 is mainly caused by the distance between the ultrasonic transmitter and receiver, the speed of sound in the fluid, Determined by the ultrasonic transmission frequency.

By the way, when demodulating phase modulation due to Karman vortices using the conventional method, one of the parameters that determines the first phase difference 1, for example, when the sound speed changes due to a change in the temperature of the fluid, demodulation failure occurs. Since the method passes through points of possible phase difference, it has the disadvantage that measurements cannot be made at specific temperatures.

Furthermore, there is the drawback that measurement is impossible even when a certain phase difference is determined by the geometry of the conduit due to factors other than the characteristics of the phase detector, such as standing waves or resonance. there were.

The present invention has been made to eliminate the above-mentioned drawbacks, and when the average phase difference between transmitting and receiving waves, the first phase difference 1, and the second phase difference 2 due to thinness are superimposed, However, by always controlling the average phase difference between transmitting and receiving waves to an appropriate value so that the average phase difference can be demodulated, Karman can be stably maintained without being affected by changes in the temperature of the fluid being measured. The purpose of the present invention is to provide an ultrasonic detection type Karman vortex flowmeter that can detect the frequency at which vortices occur and can operate within a desired frequency range immediately after power is turned on.

An embodiment of this invention will be described below with reference to the drawings.

In Fig. 3, 31 is an ultrasonic receiver, 33 is a preamplifier, 32 is an ultrasonic transmitter, 34 is a driver for 32, 35 is a comparator, 36 is a variable resistor for phase setting, and 37 is a resistor. , 38 is a capacitor, and 39 is a timer IC, such as NE555 from Signetakes.
, 40, 41.

42 is a resistor, 43.47 is a capacitor, 44 is a resistor, and 4
5 is a capacitor, and 46 is a Schmitt circuit for pulse conversion.

When the power is first turned on to the ultrasonic Karman vortex flowmeter that has been stolen in this way, the initial charge of the capacitor 38 is 0, so the initial value, such as 2/3 cc, is set. A control voltage is applied to the oscillation frequency control terminal of the timer IC 39 through the resistor 42 and the noise cutting capacitor 43, and oscillation is started at the initially set frequency.

This output pulse waveform is amplified by the driver 34 and drives the ultrasonic transmitting transducer 32.

A transmission wave generated by the ultrasound transmission transducer 32 is received by the ultrasound receiver 31 and converted into a pulse waveform by the preamplifier 33.

The phase difference between the transmitted and received waves is detected by the above-mentioned exclusive OR circuit 11, and the average phase difference is detected by a low pass filter consisting of a resistor 12 and a capacitor 13.

Furthermore, the phase difference between the transmitted and received waves is detected.

Further, the output of the phase difference between the transmitted and received waves is converted into a pulse output by a low-pass filter and Schmitt circuit 46 consisting of a resistor 44 and a capacitor 45, which converts the modulated output by the second phase difference 2 into a pulse output, and outputs an eddy frequency.

Now, the average phase difference is compared with a predetermined phase difference set by the resistor 36 by the comparator 35, and the resistor 37 of the integral element,
Via the capacitor 38, the oscillation frequency control terminal of the timer IC 39 is controlled so that a predetermined phase difference is achieved.

Note that the time constant of the integral element may be a relatively long time constant that does not respond to the second phase difference 2 but responds only to the first phase difference 1.

By the way, in the feedback method for controlling the oscillation frequency shown in the embodiment shown in Fig. 3, the oscillation frequency corresponding to the desired phase difference to become the stable point of the system is the point at which the phase output is shifted by an integer multiple of 2π. In order to match, any number of points can exist.

However, for example, in ultrasonic transducers using PZT,
Since it operates as a transmitter/receiver only within a narrow band near the resonance point, it is necessary to turn on the power to the circuit and first stabilize the oscillation frequency at which feedback occurs.

Incidentally, the time constant of the integral element of the feedback loop that controls the first phase difference 1 needs to be sufficiently long so as not to affect the output of the 20th phase difference 2.

Therefore, in the case of FIG. 3, the resistor 37 and capacitor 38 have large values and capacitances.

By the way, the oscillation frequency control terminal of the timer IC39 is
When controlling by voltage through the resistor 42, it is necessary to apply an initial voltage to oscillate the center frequency in order to offset a certain center frequency to high or low frequencies in relation to the power supply voltage.

It is necessary to quickly charge the large capacity capacitor 38 with a desired initial voltage (for example, 2/3 Vcc) i immediately after turning on the power, and then perform a feedback operation at a stable frequency near the appropriate center frequency.

For this purpose, the timer IC 39 should be made to oscillate stably at the center frequency at an oscillation frequency near the resonant frequency of the vibrator before the feedback starts and reaches a stable point.
It is necessary to preset the initial charge or potential of the capacitor 38.

This can be done, for example, by biasing the ground terminal of the capacitor 38 to an initial voltage so that the timer IC 39 oscillates at the center frequency when the capacitor 38 has no charge, as shown in FIG.

In the above embodiment, the distance between the transmitting and receiving elements was fixed, and the oscillation frequency was changed to perform feedback control so that the average phase difference between the transmitting and receiving waves became the desired value. , by increasing or decreasing the distance between the transmitter and receiver.
Feedback control may be performed so that the average phase difference between received waves becomes a desired value.

In this case, in order to control the position of the vibrator 32, for example, an electric to position converter such as a motor or a bimetal with a heater is used, and the electric input is controlled according to the average phase difference between transmitting and receiving waves. You can do as you like.

As described above, according to the present invention, feedback control is performed so that the average phase difference between transmitting and receiving waves is within the operable phase difference of the phase detector, regardless of temperature changes in the fluid to be measured. At the same time, it is possible to obtain an ultrasonic detection type Karman vortex flowmeter that can perform stable measurements and operate in the desired frequency range immediately after turning on the power.

[Brief explanation of drawings]

FIG. 1 is a general average phase difference detection circuit, FIG. 2 is an operational characteristic diagram of the circuit shown in FIG. 1, and FIG. 3 is an electric circuit diagram showing an embodiment of the present invention. In the figure, 11 is an exclusive OR circuit, 12 is a resistor,
13 is a capacitor, 31 is an ultrasonic receiver, 32 is an ultrasonic transmitter, 33 is a preamplifier, 34 is a dry nozzle,
35 is a comparator, 36 is a variable resistor, 37 is a resistor, 3
8 is a capacitor, and 39 is a timer IC. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A Karman vortex flow meter that detects the frequency at which Karman vortices occur based on the phase difference between sending and receiving ultrasonic waves passing through a fluid. Means for detecting the average value of the phase difference, a direct current first phase difference that has a relatively long time constant and changes relatively slowly depending on the distance between the ultrasonic transmitter and receiver, and the temperature or speed of the fluid. and an integral element that does not respond to the alternating current second phase difference that changes depending on the period and strength of the Karman vortex street, and the output of this integral element determines the average value of the phase difference between the transmitting and receiving waves. , an ultrasonic detection type Karman vortex flowmeter characterized by controlling the average phase difference between transmitting and receiving waves so that it is within the effective measurement phase difference of a phase detector. 2. Claim 1, characterized in that the oscillation frequency of the ultrasound transmitting transducer is controlled so that the average value of the phase difference between transmitting and receiving waves is within the effective measurement phase difference of the phase detector. Ultrasonic detection type Karman vortex flow meter as described in . 3. A patent characterized in that the distance between the ultrasonic transmitting transducer and the ultrasonic receiver is controlled so that the average value of the phase difference between transmitting and receiving waves is within the effective measurement phase difference of the phase detector. An ultrasonic detection type Karman vortex flowmeter according to claim 1. 4 Set the initial charge of the capacitor in the integral element to the charge during normal operation, or set the bias value of the capacitor so that the charge on the capacitor during normal operation is near zero, and then turn on the power switch. 3. The ultrasonic detection type Karman vortex flowmeter according to claim 2, wherein feedback control is enabled immediately after a predetermined frequency.