RU2389981C1 - Method of compensating for errors in ultrasonic level gauge measurements - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method involves measurement of the time interval between two signals, conversion of the input analogue signal to a digital code with frequency not less than ten times greater than the frequency of the input signal and its storage. Further, at least three minimum points and three maximum points in neighbouring periods of the stored signal are determined. One input signal envelope is plotted using three maximum points and a second input signal envelope is plotted using three minimum points. The time coordinate of the point of intersection of these signal envelopes is determined, which is taken as the onset of the echo pulse and is used to determine the distance to the reflecting surface through multiplication with propagation speed of ultrasound in the controlled medium.

EFFECT: compensation for ultrasonic level gauge measurements caused by uncontrolled time interval between the onset of the echo pulse and operation of the threshold device.

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Description

The invention relates to ultrasonic location level meters of liquid and bulk products in tanks at gas stations and tank farms, as well as in the chemical, oil, food and other sectors of the economy.

A known method of compensating for errors of acoustic location level gauges (RF patent No. 2129703, IPC G01F 23/28, publ. 04/27/1999), including radiation and reception of ultrasonic pulses, the formation of reference and measuring time intervals, their digital conversion, respectively, using synchronous and counting pulses and indication of the distance from the acoustic sensor to the measured level.

The disadvantage of this method is the low accuracy of the measurement, due to the inability to take into account the time interval between the beginning of the reflected ultrasonic pulse and the moment of operation of the threshold device, which can vary in a turbulent dispersing gas or liquid medium, as well as in environments with a varying attenuation coefficient.

The closest set of features to the proposed method is a method of compensating for errors of acoustic location level gauges (RF application No. 2006109659, IPC (2006.01) G01B 17/00, publ. 10.10.2007), including measuring the time of bi-directional passage of an ultrasonic pulse from the emitter to the reflector and vice versa in order to eliminate the dependence of the measurement result on the ambiguity of determining the moment of occurrence of the first echo pulse, measure the time interval between the peak of the maximum amplitude of the first echo pulse and the peak of maximum amplitudes of opposite polarity of the second echo pulse, which is directly proportional to the measured length.

The disadvantage of this method is the low stability of the measurement, due to the possible absence of a second reflected ultrasonic pulse in environments with a high attenuation coefficient.

The invention solves the problem of creating a method that reduces the error and improves the stability of measurements in a turbulent dispersing gas or liquid medium, as well as in environments with a high attenuation coefficient.

The problem is solved due to the fact that in the method of compensating for errors in the measurement of an ultrasonic level gauge, the time interval between two signals is measured, it is converted into a digital code and the distance to the reflecting surface is calculated by multiplying the propagation speed of ultrasound in the controlled medium by the measured time interval.

According to the invention, after measuring the time interval between the emitted and received signals, the input analog signal is converted to a digital code with a frequency of at least ten times the frequency of the input signal, it is stored, at least three minimum points and three maximum points are determined in adjacent periods of the stored signal, build one envelope of the input signal at these maximum points and a second envelope of the input signal at these minimum points, determine the temporal coordinate of the intersection point these envelopes, which is taken as the beginning of the echo pulse and is used in determining the distance to the reflective surface.

Converting the input analog signal to a digital code with a frequency of at least ten times the frequency of the input signal allows you to determine the coordinates of the maximums and minimums in adjacent periods with an error of no more than 20%, since 10 samples will be made in one period of the input signal, and on one period of the input signal, there are two extrema. A decrease in the conversion frequency of the input analog signal will lead to an increase in the error in determining the coordinates of the minimum and maximum points. The construction of one envelope of the input signal at least three maximum points in adjacent periods and a second envelope of the input signal at least three minimum points in adjacent periods and finding the time coordinate of the intersection point of these envelopes, which is taken as the beginning of the echo pulse (Fig. 1 ), allows you to compensate for the measurement error of the ultrasonic level gauge. The exclusion of the second reflected echo pulse, which may not be in a turbulent dispersing gas or liquid medium, as well as in media with a high attenuation coefficient, from the measurement process increases the stability of the measurement.

Figure 1 presents a diagram illustrating the claimed method.

Figure 2 presents a diagram of a device for implementing the proposed method.

Figure 3 presents an example of an oscillogram of an echo pulse and two received envelopes.

A device that implements the proposed method contains a control and indication unit 1 (Fig. 2), the output of which is connected to the generator 2 and the input of the time interval forming unit 3. The generator 2 is connected to the emitter 4. The receiver 5 is connected to the amplifier 6, the output of which is connected to the inputs of the threshold device 7 and the analog-to-digital converter 8. To the other input of the threshold device 7 is connected a reference voltage source 9 (ION). The output of the threshold device 7 is connected to the input of the time interval forming unit 3, the output of which is connected to the inputs of the control and indication unit 1, trigger 10 and the delay unit 11. The output of the delay unit 11 is connected to another input of the trigger 10 and to the control and indication unit 1. Output trigger 10 is connected to the input of the address generation unit 12 and the analog-to-digital Converter 8 (ADC). The output of the analog-to-digital Converter 8 (ADC) is connected to the data input of random access memory 13 (RAM). The second output of the analog-to-digital converter 8 (ADC) is connected to the address generation unit 12, the output of which is connected to the address input of the random access memory 13 (RAM), the output of which is connected to the control and display unit 1, which is connected with the address generation unit 12.

The control and indication unit 1 can be performed on an ATMEGA16 microcontroller and seven-segment indicators of the type DA56-11SRWA; an internal timer-counter is used to calculate the time interval. The unit for the formation of the time interval 3 is made on a standard chip K1554TM2. As a threshold device 7, a comparator K521CA3 was used. Generator 2 can be performed according to the scheme with the discharge of the storage capacitance on KU104G type thyristors. The receiver 5 and the emitter 4 can be made of any piezoceramics, for example TsTS-19. The amplifier 6 can be performed on an operational amplifier, for example K544UD2. The reference voltage source 9 (ION) is selected as standard REF 192 by ANALOG DEVICES in the standard inclusion, the analog-to-digital converter 8 (ADC) is selected as standard from the condition that the conversion time should be at least ten times less than the period of the input analog signal, for example, an input signal with a frequency of 1 MHz, you can use the chip AD9057BRS40, random access memory 13 (RAM) is selected from the requirements of the maximum data recording time, which should be less than the conversion time of analog-to-digital convert To 8 (ADC), and the volume of stored data, which must be greater than 10 * (T _c / t _ADC), for example a frequency of 1 MHz can be applied chip K565RU5, the trigger 10 may be formed on a typical chip, e.g. K1554TM2 unit the delay can be performed on the K1554AG1 chip in a typical inclusion, the delay time must be greater than or equal to five periods of the input signal, the address generating unit 12 can be performed on typical binary reversible counters with a switching delay shorter than the conversion time of the analog-to-digital converter 8 (ADC), e.g. K1554IE7.

The device operates as follows.

The control and indication unit 1 gives permission for autonomous operation of the address generation unit 12 and generates a start pulse for the ultrasonic generator 2, with the same pulse, the time interval 3 formation unit is set to the state of a logical unit. The generator 2 excites the emitter 4. The emitted ultrasonic pulse propagates through the controlled environment and is received by the receiver 5, amplified by the amplifier 6 and fed to the input of the analog-to-digital converter 8 (ADC). At the same time, the signal from the output of the amplifier 6 is fed to the input of the threshold device 7. The voltage from the reference voltage source 9 (ION) U1 is supplied to the second input of the threshold device 7. As soon as the voltage at the output of the amplifier 6 exceeds the voltage U1, the output of the threshold device 7 will switch to the logical 1 state and reset the time interval 3 forming unit to the logical zero state (point t _I , Fig. 1). This signal is fed to the input of trigger 10 and the input of delay unit 11, a logic unit appears at the output of trigger 10, which allows the operation of the address generation unit 12 and analog-to-digital converter 8 (ADC), which converts the analog signal into a digital form with a frequency of ten times the input frequency. This data is fed to the data input of random access memory 13 (RAM) and recorded at the address formed by the address generation unit 12. At the same time, the analog-to-digital converter 8 (ADC) generates a pulse to the address generation unit 12 to generate the next address, and during the analog conversion -digital converter 8 (ADC) at the output of the address generation unit 12, the following address will be generated. After the operation of the delay unit 11, a pulse appears at its output, which enters the control and display unit 1, signaling the end of the data recording process, and resets trigger 10 to the state of logical zero. A zero level at the output of the trigger prohibits the operation of the block forming the address 11 and the analog-to-digital converter 8 (ADC). After that, the control and display unit 1, in accordance with the program, sequentially samples data from random access memory 13 (RAM) to determine three maximum points and three minimum points. At these points, a system of three quadratic equations with three unknowns is formed:

where y ₁ and x ₁ , y ₂ and x ₂ , y ₃ and x ₃ are the coordinates of the three points A, B and C, respectively.

Solves this system of equations and finds the coefficients a ₁ , b ₁ and c ₁ for the equation of the first envelope:

,

,

It similarly finds the coefficients for the equation of the second envelope, but it uses the coordinates of the points D, E and F:

,

,

Then it finds the intersection point of these envelopes by equating the equations and its solution:

,

,

The time coordinate of this point is taken as the beginning of the echo pulse and is used in calculating the distance to the reflecting surface.

As an example, consider the determination of distance by the proposed method. A receiver 5 was installed in water at a distance of 250 cm from the emitter 4. The frequency of the ultrasonic signals was 600 kHz, respectively, the wavelength λ was 2.5 mm. As an analog-to-digital converter 8, an AD9057BRS40 chip with a conversion frequency of 40 MHz was used. The radiation and reception of ultrasonic signals was carried out using a device that implements the proposed method, and for comparison was observed using a GDS820G oscilloscope at the output of amplifier 6 of this device. The obtained maximum points had coordinates A (0.71; 1701), B (0.89; 1702.6), C (1.02; 1704.5). The minimum points had the coordinates D (-0.59; 1700.2), E (-0.79; 1701.9), F (-0.98; 1703.7).

Using the coordinates of these points, the control and display unit 1 determined the coefficients a ₁ , b ₁ s ₁ and a ₂ , b ₂ , s ₂ . The temporal coordinate of the intersection point, determined by these coefficients, is equal to:

x = 1696.64 μs.

For comparison, figure 3 shows the waveform of the received signal and two received envelopes, from which it can be seen that the calculated time coordinate of the point of intersection of the two envelopes does not coincide with the beginning of the signal. The time t _p measured by the GDS 820G oscilloscope was 1697 μs.

The error in measuring the level Δh was:

Ah = C · (1697-1696.64) = (1.5 · 10 ⁶ ) · (0.36 · 10 ^-6 ) = 0.54 mm,

where C is the speed of propagation of ultrasound in water.

Thus, it was experimentally established that the level measurement error does not exceed λ / 2.

Claims (1)

A method of compensating for the measurement error of an ultrasonic level gauge, including measuring the time interval between two signals, converting it into a digital code and calculating the distance to the reflecting surface by multiplying the propagation velocity of ultrasound in a controlled environment by a measured time interval, characterized in that after measuring the time interval between the emitted and the received signals convert the input analog signal into a digital code with a frequency of not less than ten times increasing the frequency of the input signal, remember it, determine at least three minimum points and three maximum points in adjacent periods of the stored signal, build one envelope of the input signal at these maximum points and a second envelope of the input signal at these minimum points, determine the time coordinate of the intersection point of these envelopes , which is taken as the beginning of the echo pulse and is used in determining the distance to the reflective surface.

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