RU2431808C1 - Fluid level optoelectronic measurement system - Google Patents

Abstract

FIELD: physics. ^ SUBSTANCE: proposed system comprises vessel with fluid, photo receiver, LEDs arranged in symmetry about photo receiver optical axis at angle , collecting lenses aligned with photo receiver and LEDs, and mirror reflector. Note here that mirror reflector is arranged in fluid while angle is defined from new relation: , ^ where frs is focal distance of LED lens, mm; is radiation source aperture angle, degrees. Note also that number of LEDs makes at least four. ^ EFFECT: contactless measurement in inflammable liquids, simplified design and higher reliability. ^ 3 dwg

Description

The invention relates to instrumentation and can be used to measure the liquid level on the products of rocket and space and aviation equipment, as well as in various sectors of the economy.

Known methods for measuring the liquid level using capacitive level gauges, which are various kinds of capacitors, between the gaskets of which the liquid level changes, which leads to a change in the dielectric constant of the medium between the gaskets of the condenser and, accordingly, to a change in capacitance [Design of sensors for measuring mechanical quantities. / Ed. E.P. Osadchy. - M: Engineering, 1979. - 480 p.].

The disadvantage of such measurement methods is in increased danger when measuring the level of spark-explosive liquids.

This disadvantage is eliminated by measuring the liquid level by non-contact methods, which include optical measurement methods. Most often, sensitive optical elements (in the form of various kinds of optical fibers) are used to measure the current liquid level, the principle of which is based on the violation of the conditions of total internal reflection when their surfaces come into contact with the liquid [Murashkina TI Fiber optic liquid level warning device. // Radio engineering. - 1995. - No. 10. - S.34-35]; [RF patent RU 2297602 C1, cl. G01F 23/22, No. 2297602 published. 04/20/07].

The disadvantage of these technical solutions is that they are used for discrete changes in the liquid level, since in them the bulk of the light flux goes into the liquid at the first contact of the sensing element with the liquid.

The closest in design and construction to the proposed optoelectronic system for measuring the liquid level is an optical displacement sensor containing a photodetector in series and four LEDs arranged symmetrically relative to its optical axis, an optical collecting system of five lenses and a reflector, the optical axis of the LEDs being located under the design angle to the optical axis of the photodetector [RF patent 2044274, G01B 11/00].

The disadvantage of this sensor is that its design parameters are calculated from the conditions for measuring the movement of the reflecting surface in a homogeneous medium, and, accordingly, are not designed to measure the liquid level. When measuring the liquid level, this sensor will have a low conversion sensitivity of the optical signal and, consequently, low measurement accuracy in a large measurement range (up to 2000 mm).

Thus, the prototype does not achieve a technical result, expressed in non-contact measurement of the current value of the liquid level in large measuring ranges with high accuracy.

A new optoelectronic system is proposed for implementing the proposed method for measuring the liquid level, devoid of the above disadvantages.

The optoelectronic system for implementing the proposed method comprises a container with a liquid, a photodetector, LEDs located symmetrically relative to the optical axis of the photodetector at an angle α, collecting lenses located coaxially with the photodetector and LEDs, a mirror reflector, characterized in that the mirror reflector is located in the liquid, the angle α is determined from the following relation:

where f _AI - the focal length of the lens LEDs, mm;

γ is the aperture angle of the radiation source, deg,

more than four LEDs.

Thus, the present invention is a technical solution to the problem, which is a new, industrially applicable and inventive step, i.e. the present invention meets the criteria of patentability.

The figure 1 shows a simplified structural diagram of the optoelectronic system, in figures 2, 3 - structural diagrams for determining the geometric parameters of the optoelectronic system.

The optoelectronic system for measuring the liquid level contains a container 1 with a liquid 2, the level of which is measured, a photodetector 3, at an angle α, collecting lenses 5 located coaxially with the photodetector 3, a mirror reflector 7. The reflector 7 is located in the liquid 2, usually at the bottom of the tank 1 (figure 1). LED 4 is in the focus of the collecting lens 5 to form a parallel beam of light 8. The photodetector 3 relative to the collecting lens 6 is shorter than the focal length of the lens 6 so that a bright spot is formed that is comparable in size to the receiving area of the photodetector 3.

The optoelectronic system for measuring the liquid level works as follows.

The beams of parallel rays 8 pass at an angle α through the first medium 9 (for example, air) with a refractive index n ₁ path equal to l ₁ defined by the expression

where h is the distance from the radiating surface of the lens 5 to the interface between two media 10 (for example, "air-water");

then, having refracted at the interface 10, at an angle β through the second medium 2 (for example, water) with a refractive index n ₂ , a path equal to l ₂ passes, defined by the expression

where X is the distance from the interface of two media 10 to the mirror reflective surface 7;

to the mirror 7 and, reflected from it, passes the return path to the photodetector 3. In this case, the optical radiation power is attenuated in accordance with Bouguer’s law:

where P ₀ , P is the power of optical radiation at the beginning and at the end of the path, respectively;

ρ is the reflection coefficient of the mirror surface 7;

τ ₀ , τ ₁ - transparency coefficients of the first medium 9 (for example, air) and the second medium 2 (for example, water), respectively

where χ ₁ , χ ₂ are the extinction coefficients (loss coefficients of optical radiation due to absorption and scattering of light) of the first and second media, respectively;

l ₁ , l ₂ - the path traveled by the light flux in the first and second environments, respectively

[Yakushenkov Yu.G. Theory and calculation of optoelectronic devices: Textbook for universities. - Ed. 2nd rev. and add. - M.: Soviet Radio, 1980, 392 p.]. Taking into account dependencies (6) and (7), expression (5) will be rewritten:

If the optoelectronic system operates in the atmosphere or under vacuum in the infrared region of the spectrum, then χ ₁ ≈0. Then

Or, taking into account expressions (3) and (4)

Thus, the dependence P = f (X) is nonlinear (exponential). In addition, if the losses in the second medium are insignificant (χ ₂ → 0), then the conversion sensitivity will be small.

была скомпенсирована нелинейностью второго сомножителя K(X). Этого можно добиться изменением крутизны зависимости K=f{X) путем выбора на стадии проектирования параметров α и d, где d - расстояние между оптическими осями источников и приемника получения (фиг.2).To linearize the dependence P = f (X) and increase the sensitivity of conversion to expression (10), it is necessary to introduce the coefficient K (X). To linearize the dependence Ф (X), it is necessary that the nonlinearity of the first factor

was compensated by the nonlinearity of the second factor K (X). This can be achieved by changing the slope of the dependence K = f {X) by choosing the parameters α and d at the design stage, where d is the distance between the optical axes of the sources and the receiving receiver (Fig. 2).

The linearization mechanism of the output conversion function and increasing the conversion sensitivity is based on artificially reducing the light flux loss at the beginning of the measurement range (with a minimum liquid level) and increasing them at the end of the measurement range. This can be achieved by changing the illumination of the radiation receiver by shifting the light spot reflected from the mirror relative to the photosensitive surface of the radiation receiver.

To do this, the rays of light 8 are directed to the mirror 7 so that when the liquid level value is X = 0, they converge at a point A lying on the optical axis at a distance L from the upper boundary of the container 1 (see FIGS. 2 and 1). With such a course of rays, a light spot of area S _II in the receiving plane (at the end of the path) equal to the reflected spot with area S _OTP will move relative to the receiving area of the photodetector 3 in the Z direction, and the area of the radiation receiver illuminated by the reflected light flux will change, then is S _OSB = f (X).

Then:

Given expression (8), expression (11) is rewritten as follows:

Or in relative units:

The photosensitive area of the receiving optical system can be rectangular (square) or round.

If the photosensitive area of the receiving optical system is rectangular (square), then

where b, h is the length and width of the rectangle (figure 3).

If the platform is round, then

where R _PI is the radius of the photodetector 3.

In the first case, S _OCB is the area formed by the mutual intersection of a circle of radius r _II and chords AB of length a (Fig. 3), and

.Where

.

Respectively

Then:

- at Z <r _II

- at Z> r _AI

Where

where Z = 0 ... 2r _and .

Given the dependencies (14), (18) and (19), the expression (13) will be rewritten:

- at Z <r _II

- at Z> r _AI

In these expressions, the parameter Z is determined by the design features of a particular optical system.

In the particular case under consideration, this parameter is determined by the distance d between the optical axes of the radiation source and receiver.

From the triangle ΔBSD

The distance that the beam travels in the Z direction in air is

and in liquid:

In accordance with figure 3 and taking into account (24), (25) and (4), you can write:

Where from

By changing the design parameters α, d, H, it is possible to improve such metrological characteristics as the linearity of the conversion function, the conversion sensitivity of the optical system, and change the measuring range of the level of liquid media in accordance with customer requirements.

будет равен 1. Тогда выражения (21) и (22) перепишутся следующим образом:If the measurement range is small (no more than 2000 mm), then in expressions (21) and (22) χ ₁ ≈0, respectively, the factor

will be equal to 1. Then the expressions (21) and (22) will be rewritten as follows:

- at Z <r _II

- at Z> r _AI

where Z is determined by expression (27).

The angle α is selected from the following considerations.

At the beginning of the measurement range, the smallest luminous flux should fall on the radiation receiver, and as large as possible at the end of the range, which compensates for its sharp attenuation in space with the removal of the interface. It follows that the angle between the optical axes of the radiation source and the radiation receiver must be converging at the end of the measurement range.

The final dimensions of the structural components also impose certain requirements on the specified angle.

To find the angle α, we use expression (26). If you do not limit the conversion function to a linear section, then in this expression Z can be taken equal to the diameter d _{L of the} lens 5 (see Fig. 1), i.e.

To reduce the transverse dimensions, it is necessary that the radiation sources and receiver are located as close as possible to each other. The indicated distance depends on the outer diameters of the lenses 5 and 6. Then

At the maximum value of the liquid level X _max equal to the measuring range D, equation (31) will be rewritten:

where d _A ≈2f _AI tg (γ / 2),

where f _AI - the focal length of the lens 5, mm;

γ - aperture angle of LED 4, deg.

Finally we have

Since the measurement range D in the recommended embodiment lies within 300 ... 2000 mm, then finally the angle α is determined by the expression (1).

In accordance with formula (33), it was found by calculation that the larger the measurement range, the smaller the angle α. In the recommended version, α≈0.5 ... 3 ° (ZL107B LEDs were used as radiation sources, and an FD 20-32K photodiode and collecting lenses with a diameter of 14 mm and focus f = 15.7 mm were used as a photodetector, while the overall dimensions of the optoelectronic node amounted to 40 × 30 mm).

The proposed optoelectronic system provides safe non-contact measurement of the current level of spark-explosive liquids in large measuring ranges with high accuracy, simple and technological in manufacturing, does not require complex technological, alignment and measurement operations in the manufacture of the optical part of the converter, has a cheap component base: infrared light and photodiodes.

The use of four or more light-emitting diodes and one photodiode ensures a decrease in the measurement error during fluctuations in the interface between the liquid-gas medium.

Claims (1)

An optoelectronic liquid level measuring system containing a container with a liquid whose level is measured, a photodetector, LEDs located symmetrically relative to the optical axis of the photodetector at an angle α, collecting lenses located coaxially with the photodetector and LEDs, a mirror reflector, characterized in that the mirror reflector is located in the liquid , the angle α is determined from the following relation:

where f _AI - the focal length of the lens LEDs, mm; γ is the aperture angle of the radiation source, deg, and the number of LEDs is at least four.

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