RU2351903C1 - Level indicator - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: level indicator contains acoustic line 2 placed in protective housing 1 made from magnetostrictive material in the form of string fixed from both sides for obtaining of full reflexion of impulse. Besides the inferior extremity of acoustic line is spring-actuated using spring 3, constant magnets 4, 5, 6 and 7, one of them 4 (basic) is mounted in the bottom of acoustic line and has the fixed Hop distance to the two-tapped reading coil 8, and other three are located on floats of different buoyancy (floats of level 6, densities 7 and section of mediums 5). Transformation block contains differential amplifier 9, comparator with switched operation threshold 10, the shaper of current impulse 11 and microprocessor processing scheme 12. Thus inputs of the differential amplifier 9 are connected with the two-tapped reading coil 8, the first input of the comparator 10 with switched operation threshold is joined to output of the differential amplifier 9, the second input with the second output of the microprocessor processing scheme 12, the input of the microprocessor processing scheme 12 is joined to output of the comparator with the switched operation threshold 10, and the first output with an input of the current impulse shaper 11 which outputs are joined to the extremities of the acoustic line 1.

EFFECT: increase of measuring accuracy, expansion of device functionality, decrease in its cost price at maintenance of basic technical characteristics.

5 cl, 5 dwg

Description

The invention relates to the field of measuring technology, automation and computer technology and can be used in systems for measuring the level, density and separation level of various liquid media in tanks during their dispensing, reception and storage.

The existing technical means of monitoring and measuring the level of liquids are based on various physical principles and technical solutions. One of the most accurate and promising are meters, the principle of which is based on the magnetostrictive effect. These meters contain a sound duct in the form of a string of magnetostrictive material and a float with a permanent magnet installed with the possibility of movement along the sound duct. In a sound duct, a pulsed magnetic field is created in various ways (longitudinal or transverse, depending on the design of the meter). At the place of installation of the float, the pulsed field and the field of the permanent magnet collide, forming a longitudinal or transverse (torsional) impulse in the sound duct under the action of the magnetostrictive effect. This pulse propagates through the sound duct and, with the help of a receiving device, is fixed and processed by the electronic unit of the meter. The signal in the sound pipe propagates at a constant ultrasonic speed. This allows after processing the signal to accurately determine the distance to the magnet of the float, and therefore the liquid level, which corresponds to the position of the float.

Level gauges can be additionally equipped with sensors - density meters, signaling devices (meters) of the medium separation level. The signal receiving device is a piezoelectric element or an inductive coil. In view of the difference in the designs of the level gauges, their capabilities in terms of the measured level, upper and lower unmeasured levels, and measurement accuracy are different. The operational characteristics of the level gauges and their telemechanical capabilities also noticeably differ. (For example, the level indicator according to the patent of the Russian Federation No. 2298156, IPC G01F 23/28, 2007)

A device for measuring the level of petroleum products is described in RF patent No. 2087874, IPC G01F 23/28, 1996. This device contains a sound duct of magnetostrictive material with a damper at one end, a read coil installed in front of the damper and connected to the amplifier-former, three permanent magnets, one of which is fixed at the end of the sound duct from the side of the read coil, and the other two are located on floats of different buoyancy, two keys connected to different ends of the sound duct and connected to a common the bus, moreover, the sound duct at the point located between the fixed magnet and the read coil is connected to the power source, and through the first to the measuring channel, including a register with parallel input and sequential shift of information, a counter of pulse counters, a counter, a memory register, two pulse shapers startup, address decoder, ready signal shaper, blocking pulse shaper and exchange bus. This device allows the measurement of two levels of liquids with different densities using two floats of different buoyancy. However, this device does not allow density measurement.

The closest in technical essence to the proposed level gauge is a device for measuring the level and density described in RF patent No. 2138028, G01F 23/68, 1998. The device contains a sound duct in the form of a string of magnetostrictive material with a damper at its first end, a read coil, installed under the damper, and the first permanent (reference) magnet located under the read coil, the second float (level float) with a second permanent magnet, mounted under the first permanent magnet with the ability to move along the sound duct, the first float (density float) with a third permanent magnet located between the second float and the second end of the sound duct with the ability to move along the sound duct, the lower part of the first float (density) is made in the form of a cylinder, and the upper one contains the same pitch located on the end of the cylinder along the circumference of the cylinder n rods, where n≥2, the second float is installed with the possibility of free movement between the rods of the first float, the conversion unit includes an amplifier-shaped the one whose inputs are connected to the read coil outputs, a pulse generator, a counter, a memory register, a register with parallel input and sequential shift of information, two start pulse shapers, an address decoder, a ready signal shaper, a lock shaper, two keys, the input of the first of which is connected with the second end of the sound duct, and the input of the second is connected to the first end of the sound duct, and the exchange bus, the output of the amplifier-former is connected to the clock input of the register with a parallel input and serial by shifting information, the output of the first trigger pulse generator is connected to the counter reset input, the counter input of which is connected to the output of the counter pulse generator, the counter information output through the memory register is connected to the exchange bus, which is connected to the input of the address decoder, the first output of which is connected to the read permission input of the memory register, the second and third outputs of the address decoder are connected respectively to the input of the first trigger pulse shaper and the second input of the signal shaper te, the first input of which is connected to the enable input of the memory register and the output of the register with parallel input and sequential shift of information, the fourth output of the address decoder is connected to the write input of the register selection code with parallel input and serial shift of information, parallel inputs for input of which information and output the ready signal shaper is connected to a common bus, the control inputs of the first and second keys are connected respectively to the outputs of the first and second pulse shapers the trigger, and the outputs are with a common bus, the inputs of the second trigger pulse generator and the blocking pulse generator are connected respectively to the fifth and second outputs of the address decoder, and the output of the blocking pulse generator is connected to the register blocking input with parallel input and serial shift of information, the sound pipe at the point, located between the first magnet, fixed at a fixed distance from the read coil, and the read coil, connected to a power source.

Common features of this device with the claimed technical solution are an electronic unit (conversion unit), a sound duct installed in a protective casing in the form of a string of magnetostrictive material, a read coil, a first permanent magnet (reference magnet), and a first float mounted for movement with it located on it magnet (level float), a second float with a second permanent magnet with the ability to move along the sound duct (density float), the lower part of the second float is made in in the form of a cylinder, and the upper one contains n rods located at the end of the cylinder, where n≥2, the first float is installed with the possibility of free movement between the rods of the second float.

The disadvantages of this device include the presence of an additional measurement error associated with a change in the signal level in the sound pipe over time and with a change in ambient temperature, the complexity of the design and the presence of limitations in the operational characteristics imposed by the design.

The problem to which the invention is directed, is to increase the accuracy of measurements, simplify the design, reduce the cost of the device while maintaining the basic technical characteristics.

To solve the problem in a known device that contains a conversion unit, a sound duct in the form of a string of magnetostrictive material installed in a protective casing, a read coil, a support magnet, a level float and a density float with permanent magnets installed with the ability to move along the sound duct, the lower part the density float is made in the form of a cylinder, and the upper one contains n rods located at the end of the cylinder, where n≥2, the level float is installed with the possibility of movement between using the density float, the read coil is made of two identical sections, spaced along the sound duct by a distance of 1/4 to 1/2 of the wavelength or a distance corresponding to 1/2 to 1 of the duration of the pulse excited in the sound duct, the conversion unit contains a differential amplifier, the inputs of which are connected to a two-section reading coil, a comparator with a switchable threshold, one input of which is connected to the output of a differential amplifier, a current pulse shaper, the outputs of which are connected connected to the ends of the sound duct, a microprocessor processing circuit, the input of which is connected to the output of the comparator, one output is connected to the input of the current pulse shaper, and the other to the second input of the comparator. A cover is installed on the rods of the density float, eliminating the roll of the float, and a permanent magnet is located on the cover. A permanent magnet interface section float is introduced, located under the density float and can be moved along the sound duct, instead of the damper, the upper end of the sound duct is rigidly fixed to obtain full momentum reflection, the lower end of the sound duct is spring-loaded and fixed to obtain full momentum reflection, the support magnet is either absent or installed above the lower end of the sound duct.

The essence of the invention is illustrated by drawings. Figure 1 shows a functional diagram of the device, figure 2 - the principle of formation of a bipolar signal, figure 3 - waveforms of a signal device with a single-section and two-section read coil, figure 4 - the design of the device for measuring level and density, in fig. 5 illustrates the operation of the device when measuring density.

The device contains a sound duct 2 installed in the protective casing 1 from magnetostrictive material in the form of a string fixed on both sides to obtain a complete reflection of the impulse, in addition, the lower end of the sound duct is spring-loaded 3, permanent magnets 4, 5, 6 and 7, one of which 4 (reference) is installed at the bottom of the sound duct and has a fixed hop distance to the read coil 8, and the other three are located on floats of different buoyancy (floats of level 6, density 7 and medium section 5). The conversion unit contains a differential amplifier 9, a comparator with a switched threshold 10, a pulse shaper 11 and a microprocessor processing circuit 12. Moreover, the inputs of the differential amplifier 9 are connected to a two-section read coil 8, the first input of the comparator 10 with a switched threshold is connected to the output of the differential amplifier 9 , the second input with the second output of the microprocessor processing circuit 12, the input of the microprocessor processing circuit 12 is connected to the output of the comparator with switchable threshold 10, and the first output with the input of the current pulse shaper 11, the outputs of which are connected to the ends of the sound duct 1. The read coil 8 is made of two identical sections spaced along the sound duct at a distance of 1/2 wavelength or at a distance corresponding to 1 duration of sound duct pulse. The lower part of the density float 7 is made in the form of a cylinder, and the upper one contains the rods located on the end of the cylinder 4, the level float is installed with the ability to move between the density float rods, a cover 13 is installed on the density float rods, which excludes the roll of the float.

The device operates as follows. The operation is initiated by the microprocessor processing circuit 12 by applying a pulse to the current pulse shaper 11. At the same time, the countdown starts in the microprocessor processing circuit 12. The current pulse shaper 11 creates a current pulse of a fixed amplitude and duration in the sound pipe 2. The current pulse in the sound pipe creates a pulsed magnetic field. This field, colliding with the magnetic field of magnets 4, 5, 6, and 7, creates a transverse (torsional) strain impulse at their location, which propagates with ultrasonic speed to the ends of the sound duct. At the ends of the sound duct, pulses are reflected. Thus, the excited pulses circulate in the sound duct until they are completely damped. When the device is operating, information is read out only about direct pulses coming from magnets 4 ... 7 to the coil, as well as about reverse pulses going to the coil from magnets 5 ... 7 after reflection from the lower end of the sound duct 2. Each pulse passing under the read coil 8 , induces an electric signal on it (Villari effect). The signals from the sections of the read coil 8 are amplified by a differential amplifier 9. Since the coil sections are spaced along the sound duct, the pulses of the sections are spaced in time. Processing the pulses of the sections with a differential amplifier, we convert a unipolar signal into a bipolar signal (figure 2). This signal is fed to the comparator 10 with a switchable threshold. When the threshold is exceeded at the rising edge of the signal, the comparator 10 generates a leading edge of the output pulse and sets the threshold for operation equal to zero. When the signal passes through zero at the falling edge, the comparator 10 forms a trailing edge of the output pulse, along which the microprocessor processing circuit 12 counts the propagation time of the pulse in the sound pipe. The microprocessor processing circuit 12 captures the propagation times of the strain pulses from magnets 4 ... 7 to the coil and calculates the corresponding distances, and therefore, the liquid level, density and the level of medium separation.

The use of a two-section coil, a differential amplifier and a comparator with a switchable threshold of operation gives a number of advantages, namely:

- in the circuit with a single-section coil and a fixed threshold Ufor. corresponding to the reference, when the signal level changes due to a change in temperature or absorption of the magnetostrictive wave in the sound pipe, a reading error Δt occurs (Fig. 3). The use of a two-section coil and a differential amplifier converts a unipolar signal into a bipolar signal. In this case, the point at which the signal passes through zero does not depend on the signal level and corresponds to the moment the pulse passes the middle of the two-section coil. This point is taken as a reference point. Due to this, the error associated with a change in the signal level is minimized (figure 3). Therefore, two thresholds are used in comparator 10. One of the thresholds Uпор.l serves to isolate the signal against the background of noise, the second - the threshold Uпор.2, equal to zero, to determine the reference point,

- a two-section coil and a differential processing circuit allow to suppress external (equally induced) interference, which, compared with a single-section coil, increases the signal-to-noise ratio, reduces the requirements for shielding the coil, amplifier and communication wires between them, simplifies their design. There is no need to remove the coil from the digital conversion unit.

The distance between the sections should correspond from 1/4 to 1/2 of the wavelength or from 1/2 to 1 of the duration of the pulse excited in the sound pipe. The pulse duration tand is related to the wavelength λ and the propagation velocity V:

When the distance between the sections is equal to λ / 2, the maximum amplitude of the output pulse is obtained, all other things being equal (coil diameter, coil wire diameter, etc.), which is due to the waveform, as well as the fact that the length of the coil section is maximum, close to optimal (λ / 2) and the coil, ceteris paribus, can have a maximum number of turns. When approaching the distance between the sections, equal to λ / 4, the length of the sections decreases, the signal amplitude decreases, but the steepness of the incident pulse front increases, which is positive, since the threshold of the comparator has a temperature, time drift, and also reduces the influence of interference imposed by to the signal.

The introduction of a current pulse shaper instead of the keys that create a voltage pulse on the sound pipe gives the following advantages. Depending on the length of the sound duct and its temperature, its resistance and its own inductance change.

When a voltage pulse is applied, a change in resistance leads to a change in current and, accordingly, an exciting magnetic field. A change in the inductance leads to a tightening of the edges of the current pulse, and with significant lengths of the sound duct when applying a rectangular voltage pulse, you can get a trapezoidal or triangular current pulse in it, which will also change the magnetic field of the excitation. The use of a current pulse preserves the magnetic field (the condition for signal excitation), making it independent of the length of the sound duct and temperature. Thus, the stability of the signal level and, accordingly, the measurement accuracy are improved.

In addition, there is no need to connect the sound duct with power at a point between the coil and magnets to organize magnetization of the sound duct under the coil. Since a current pulse is supplied, independent of the length of the string and temperature, the sound duct under the coil is equally magnetized each time. The implementation of the above connection is a serious task, because it is necessary to ensure good galvanic contact and not violate the conditions of wave propagation in the sound duct. The presence of this compound in any case will cause signal changes due to reflection of part of the pulse from it, and this change will depend on many factors.

Installing the reference magnet 4 at the bottom of the sound duct provides the following benefits. The device determines the propagation times of pulses from magnets 7, 6, 5 to the coil:

Given the constancy of the wave propagation velocity in the sound duct and knowing the distance to the Hop reference magnet, it is possible to accurately determine the distances to the magnets:

As can be seen from the formulas, a change in the speed of propagation of a wave in a sound pipe when a temperature or other factors change does not affect the measurement results.

Similarly, if you put a reference magnet at the top of the sound duct, above the other magnets at a distance Hop from the coil and use as a reference the propagation time of the pulse from the reference magnet to the lower end of the sound duct and after reflection to the coil, then:

but

where A is the distance from the lower end of the sound duct to the coil.

As can be seen from the formulas, a change in the wave propagation velocity in a sound pipe with a change in temperature or other factors does not affect the measurement results, but with a poor fixation of the lower end of the sound pipe, the reflection point can change over time and temperature (parameter A changes), which leads to an error .

At the same time, with good fixation of the end of the sound duct, when the reflection point does not change, the total propagation time of the forward pulse from the magnet to the coil and the reverse pulse from the magnet to the lower end of the sound pipe and to the coil is:

where t1 ', t2', t3 'is the propagation time of the reverse pulse from magnets 7, 6, 5, respectively, to the lower end of the sound duct and after reflection to the coil.

As can be seen from the formulas, the total propagation time of the forward and reverse pulses are the same. Taking this time for the reference and knowing the distance A, we get:

the measurement result also does not depend on the speed of wave propagation in the sound duct.

Thus, in case of poor fixation of the end of the sound duct, it is advisable to place the support magnet in order to reduce the error at the bottom of the sound duct, and if it is good, it is completely unnecessary.

Consider the remaining innovations in more detail. The damper in the known design is replaced by rigid fixation of the upper end of the sound duct. The implementation of the damper is a difficult technical task, because the damper should gradually change the propagation conditions of the wave, otherwise it will not be absorption, but reflection of the wave. The damper must be made of elastic material, but it must have good contact with the surface of the sound duct and not change its properties over time and temperature. It is simpler to fix the upper end of the string, but the problem arises of eliminating the influence of a pulse reflected from the upper end of the string. This problem is solved using the microprocessor processing circuit 12 by applying a blocking pulse to the comparator 10 during the passage of the pulse reflected from the upper end of the sound duct under the read coil. In addition, the damper must have a sufficient length, several wavelengths, to gradually change the conditions of wave propagation. Thus, replacing the damper with rigid fixation, ceteris paribus, reduces the area in the upper part of the protective cover of the level gauge, where measurements are not possible for structural reasons (upper unmeasured level).

Springing the lower end of the sound pipe with a spring 3 (figure 4) is advisable for the following reasons. As the temperature changes, the geometric dimensions of the string change. If the temperature coefficient of linear expansion of the sound duct does not coincide with the corresponding coefficient of the device in which the sound duct is fixed, then the string tension and, consequently, the level of the pulse excited in the string will change with temperature. This will especially occur with large lengths of the sound duct. The introduction of the spring 3 allows you to keep in a stable state the mechanical (magnetostrictive) properties of the sound duct.

Let us consider in more detail the density float. In the known construction, the lower part of the float is made in the form of a cylinder, and the upper part contains n rods located at the end of the cylinder with the same pitch along the circumference of the cylinder, where n≥2. The principle of operation of the float is similar to the operation of the hydrometer in accordance with the law of Archimedes, i.e. its immersion depth depends on the density of the liquid (figure 5).

The density of the liquid in the device, provided that the cross section of the rods is constant in the float, is calculated by the formula:

where ρ _{1 et} , ρ _{2 et.} - the density of the reference liquids (ρ _{1 et} <ρ _{2 et.} ) used in the calibration;

h _{1st floor} h _2et. - the distances between the magnets of the density float 7 and level 6 float, determined during calibration, in liquids with densities ρ _{1 et.} and ρ _{2 et.} . respectively;

h is the measured distance between the magnets of the density float 6 and the level 5 float. The distance between the magnets is determined by:

h = X2-Xl.

To increase accuracy, reference liquids are selected with densities equal to or close to the edges of the measuring range.

Obviously, for a given range of density measurements and the error in measuring the distance between the magnets, the error in the density measurements will depend on the stroke of the float (length of the rods). The larger the stroke (longer than the rods), the smaller the increment in density corresponds to the increment in the immersion depth of the float and, accordingly, the smaller the error in density measurements.

In the known construction, increasing the length of the rods without increasing the diameter of the float is complicated by the fact that the center of gravity of the float is shifted up and the float begins to roll. The roll of the density float prevents the level float from moving freely between the rods of the density float. The roll of the float can be removed by introducing ballast at the bottom of the density float, but with significant lengths of rods and small diameters of the density float this is not effective.

Introduction to the known design of the cover 13 with guides that exclude the roll of the float, ceteris paribus gives a smaller diameter of the float density, which is essential during operation, and with equal diameters, a smaller error due to the increase in the stroke of the float (length of the rods). The condition that the rods should be located at the end of the cylinder with the same pitch along the circumference of the cylinder becomes insignificant, because it is necessary to exclude the above roll. In addition, the presence of the cover 13 allows you to install a magnet 7 on it. In this case, the magnet is located above the liquid level and does not attract metal particles that may be in the liquid and will impede the movement of the float, as well as change its weight.

Claims (5)

1. A level gauge containing a conversion unit, a sound guide in the form of a string of magnetostrictive material installed in a protective casing, a read coil, a level float and a density float with permanent magnets installed with the ability to move along the sound duct, the lower part of the density float is made in the form of a cylinder, and the upper one contains n rods located at the end of the cylinder, where n≥2, the level float is mounted for movement between the rods of the density float, characterized in that the read coil The unit is made up of two identical sections spaced along the sound pipe by a distance of 1/4 to 1/2 of the wavelength or by a distance corresponding to 1/2 to 1 of the duration of the pulse excited in the sound pipe, the conversion unit contains a differential amplifier, the inputs of which are connected to a two-section a read coil, a comparator with a switchable threshold, one input of which is connected to the output of a differential amplifier, a current pulse shaper, the outputs of which are connected to the ends of the sound duct, microprocessor a processing circuit whose input is connected to the output of the comparator, one output is connected to the input of the current pulse shaper, and the other to the second input of the comparator. 2. The level gauge according to claim 1, characterized in that a support magnet is installed above the lower end of the sound duct. 3. The level gauge according to claim 1, characterized in that a medium section float with a permanent magnet is inserted in it, located under the density float and can be moved along the sound duct. 4. The level gauge according to claim 1, characterized in that a cap is installed on the density float rods, and a permanent magnet is located on the cap. 5. The level gauge according to claim 1, characterized in that the upper end of the sound duct is rigidly fixed, and the lower end is spring-loaded and fixed to obtain complete reflection of the pulse.

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