RU2471158C1 - Method of compensating for ultrasonic level gauge measurement errors - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: to compensate for ultrasonic level gauge measurement errors, the time interval between emitted and received signals is measured, after which the initial section of the received signal which is pre-processed by a peak detector is digitised and stored. Further, stored data are used to determine the end of the rising interval of the received signal and coefficients of the expression which describes a second-order envelope curve which passes through two extreme points of the rising interval of the received signal and a point lying in the middle of that interval are found. The time at which the value of the expression which describes the envelope curve is equal to zero is determined. Said time is considered the start of echo pulse and is used when determining distance to the reflecting surface by multiplying the speed of ultrasound in the controlled medium by the measured time interval.

EFFECT: reducing minimum frequency of analogue-to-digital conversions while maintaining high measurement accuracy.

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Description

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

A known method of compensating for the error of acoustic location level gauges (RF patent 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 clock and counting pulses and an 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.

A known method of compensating for the measurement error of an ultrasonic level meter (RF patent No. 2389981, IPC G01F 23/28, published on 05/20/2010), selected as a prototype, including measuring the time interval between emitted and received signals, converting the input analog signal to a digital code with a frequency , at least ten times higher than the frequency of the input signal, remembering it, determining at least three minimum points and three maximum points in adjacent periods of the stored signal, constructing one envelope of the input signal by e their points of maximum and the second input signal envelope to these minimum points, determination of the time coordinate point of intersection of these envelopes, using it as the start of the echo pulse and application in the determination of the distance to the reflective surface by multiplying the propagation velocity of ultrasound in a controlled environment on a measured time interval.

The disadvantage of this method is the need for analog-to-digital conversion with a frequency exceeding the frequency of the input signal by at least ten times.

The objective of the invention is to provide a method of compensating for the error of an ultrasonic level gauge, which reduces the minimum frequency of analog-to-digital conversions while maintaining high measurement accuracy.

The problem is solved due to the fact that in the method of compensation of the measurement error of the ultrasonic level gauge, as in the prototype, the time interval between the emitted and received signals is measured, the input analog signal is converted into a digital code, it is stored and the distance to the reflecting surface is determined by multiplying the speed propagation of ultrasound in a controlled environment over a measured time interval.

According to the invention, after measuring the time interval, the initial portion of the received signal previously processed by the peak detector is digitized and saved, the moment of the end of the rise interval of the received signal is determined from the stored data, the coefficients of the expression are described that describe the second-order envelope passing through the two extreme points of the rise interval of the received signal and the point lying in the middle of this interval, determine the point in time at which the value of the expression describing the envelope nd is zero, taking it as the origin of the echo pulse and is used in determining the distance to the reflecting surface.

Determining the point in time at which the value of the expression describing the envelope of the signal processed by the peak detector is zero and using it as the point in time corresponding to the beginning of the echo pulse allows us to compensate for the measurement error of the ultrasonic level gauge. Signal processing by a peak detector before its digitization allows one to reduce the minimum conversion frequency by an order of magnitude in comparison with the prototype.

Figure 1 presents a diagram of a device that implements the claimed method.

Figure 2 presents a diagram illustrating the claimed method.

A device that implements the claimed method (figure 1) contains a generator 1, the output of which is connected to the emitter 2. The input of the generator 1 is connected to the output of the control and display unit 3 and the input of the counter 4. The output of the receiver 5 is connected to the input of the pre-amplifier 6, the output of which connected to the input of the peak detector 7. Another input of the peak detector 7 is connected to the output of the control and indication unit 3, the input of the counter 4 and the input of the memory access control system 8 (SUD). The output of the peak detector 7 is connected to the input of the analog-to-digital converter 9 (ADC) and the input of the threshold device 10. The other input of the analog-to-digital converter 9 (ADC) is connected to the output of the memory access control system 8 (ADCS), the input of which is connected to the output of the unit control and indication 3. The output of the analog-to-digital converter 9 (ADC) is connected to the control and indication unit 3 and random access memory 11 (RAM), the input of which is connected to the memory access control system 8 (SUD). The output of the memory access control system 8 (AMS) is connected to the control and indication unit 3. The output of the threshold device 10 is connected to the memory access control system 8 (AMC) and the counter 4. The output of the counter 4 is connected to the input of the control and indication unit 3. Output the clock generator 12 is connected to the input of the memory access control system 8 (SUDP), the input of the counter 4 and the input of the analog-to-digital converter 9 (ADC).

The control and indication unit 3 can be performed on a microcontroller, for example ATmega16, and semiconductor seven-segment indicators, for example DA56-11SRWA. Generator 1 can be performed according to the scheme with the discharge of the storage capacitance on KU104G thyristors. The emitter 2 and the receiver 5 can be made of piezoceramics, for example TsTS-19. Preamplifier 6 and peak detector 7 can be implemented on operational amplifiers, for example AD8061AR. The analog-to-digital converter 9 (ADC) is selected so that its sampling frequency is comparable to or higher than the frequency of the emitted ultrasonic signal, for example, for a signal with a frequency of 5 MHz, the ADC1175CIMTC chip can be used. As a counter 4, any standard counter can be used with a speed not lower than that of an analog-to-digital converter 9 (ADC), for example, 74VHC4040 and a logic chip CD74НСТ00. The memory access control system 8 (SUD) can be implemented on the counter 74NS590 and logic circuits CD74HCT4075 and CD74HCT00. As random access memory 11 (RAM), a static memory chip with a maximum recording time not exceeding the sampling period of the analog-to-digital converter 9 (ADC), for example IS61C256AL, can be used. The threshold device 10 can be implemented on a high-speed comparator MAX912. As a clock generator 12, a MAX7375 silicon oscillator can be used.

The device operates as follows.

To bring the circuit to its initial state, the control and indication unit 3 generates a pulse resetting the peak detector 7, counter 4, and memory access control system 8 (SUD). Then, the time interval between the emitted and received signals t _{POR is} measured (Fig. 2), for which the control and indication unit 3 generates a signal that starts generating an ultrasonic frequency pulse by generator 1 and a counter 4 counting pulses of clock generator 12. The emitter 2 converts the generator generated 1 pulse into ultrasonic vibrations and emits them into a controlled environment. The ultrasonic signal reflected from the reflecting surface reaches the receiver 5, is converted by it into electrical oscillations, which are then amplified by the pre-amplifier 6. The signal from the output of the pre-amplifier is processed by the peak detector 7 and fed to the input of the analog-to-digital converter 9 (ADC) and the input of the threshold device 10 When the amplitude of the signal from the output of the peak detector 7 reaches the set value U _POR (Fig. 2), the threshold device 10 generates a signal that stops the counter 4. After measurement I of the time interval, digitize and save the initial portion of the received signal previously processed by the peak detector, for which the signal from the output of the threshold device 10, which stops the counter 4, simultaneously starts the memory access control system 8 (SUD), which controls the process of analog-to-digital signal conversion and recording the results of conversion to random access memory 11 (RAM), while the memory access control system 8 (SUD) generates the address of the current memory cell and all necessity to random access memory 11 (RAM) and an analog-digital converter 9 (ADC) control signals. After the digitization and storage process is completed, the memory access control system 8 (SUD) generates a pulse that is fed to the input of the control and indication unit 3 and initiates data sampling from the random access memory 11 (RAM), while the address of the current cell is formed by the access control system memory 8 (SUDP), incrementing it after each control pulse coming from the control unit and indication 3. According to the data read from the random access memory 11 (RAM), the control and indication unit 3 is determined there is a moment of the end of the slew interval of the received signal based on the condition of decreasing the slew rate:

0.05 · (U ₂ -U ₁ ) ≥ (U _n -U _n-1 ),

where U ₁ is the value of the signal amplitude obtained as a result of the 1st analog-to-digital conversion,

U ₂ - the value of the signal amplitude obtained as a result of the 2nd analog-to-digital conversion,

U _n-1 - the value of the amplitude of the signal obtained as a result of analog-to-digital conversion at the point in time preceding the moment taken as the end of the interval of rise of the signal,

U _n is the value of the signal amplitude obtained as a result of analog-to-digital conversion at a time taken as the end of the signal rise interval.

Then, the control and indication unit 3 finds the coefficients of the expression describing the second-order envelope passing through the two extreme points (A and C) of the rise interval of the received signal, and point B lying in the middle of this interval (Fig. 2), solving the system of equations:

,

,

where t _A , t _B , t _C - time points corresponding to points A, B and C,

U _A , U _B , U _C - signal amplitudes at points A, B and C, respectively,

a, b, c - coefficients of the expression describing the envelope of the second order.

Then, solving the equation

,

,

the control and indication unit 3 finds the time moment t _COMP (Fig. 2), at which the value of the expression describing the envelope is zero (point K), takes it as the beginning of the echo pulse and uses it to determine the distance to the reflecting surface by multiplying the propagation velocity ultrasonic pulse in a controlled environment for a measured time interval.

As an example, consider determining the distance of the claimed method. A receiver 5 was installed in water at a distance of 32 cm from the emitter 2. The frequency of the ultrasonic signals was 3 MHz, respectively, the wavelength λ was 0.5 mm. The radiation and reception of ultrasonic signals was performed using a device for compensating the error of measurement of an ultrasonic level gauge (Fig. 1). For comparison, the signal from the output of the preamplifier 6 was observed using a GDS-820C digital oscilloscope.

Using the claimed error compensation method, the control and indication unit 3 determined the time instant corresponding to the beginning of the received signal equal to 214.66 μs, while the time measured by the oscilloscope was 214.5 μs.

The measurement error was 1500 · (214.66 · 10 ^-6 -214.5 · 10 ^-6 ) = 0.24 mm.

Thus, it was experimentally established that the 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 the emitted and received signals, converting the input analog signal to a digital code, storing it, determining the distance to the reflecting surface by multiplying the speed of propagation of ultrasound in a controlled environment by a measured time interval, characterized in that after measuring the time interval, digitization is carried out and preservation of the pre-processed peak detector is accepted of the signal, according to the stored data, determine the end time of the rise interval of the received signal, find the coefficients of the expression describing the second-order envelope passing through the two extreme points of the rise interval of the received signal and the point lying in the middle of this interval, determine the time at which the value of the expression, describing the envelope is equal to zero, take it as the beginning of the echo pulse and is used to determine the distance to the reflecting surface.

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