RU2327959C2 - Fiber optic indicator of fluid level - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention concerns measurement technologies and is designed to high-precision definition of boundary between two media with different refraction index. Fiber optic indicator of fluid level includes emitter and receiver of optical radiation placed in the data processing unit; transmitting and receiving optic fibers connected optically to the radiation emitter and receiver respectively and having common end; and retroflector fiber, one end of which is connected optically to the common end of transmitting and receiving optic fibers.

EFFECT: high measurement efficiency and operational reliability along with simple design.

2 cl, 1 dwg

Description

The invention relates to measuring equipment and is intended to determine with a high degree of accuracy the interface between two media with different refractive indices.

Known fiber-optic sensor of liquid media [1], containing two fibers, the ends of which at one end lie in the same plane and are optically connected by a sensitive element made of a transparent material, the surface of which is limited by the apertures of the fibers, made in the form of two intersecting symmetrically located relative to the axis of the sensor of ellipsoids of revolution, the first foci of which coincide with the centers of the ends of the optical fibers, and the second coincide with a point lying on the axis of symmetry of the sensor, located on a certain nnom distance from the end plane of the optical fibers. In this sensor, the radiation of the source is introduced into the transmitting optical fiber and sent to the sensing element. The beam of rays emerging from the end of the transmitting fiber is reflected by the left elliptical surface of the sensing element and forms an image of the end of the transmitting fiber in the focus area. Propagating further, the beam of rays is reflected by the right surface of the sensing element and hits the end of the receiving fiber. The surface shape of the sensor is two intersecting ellipsoids of revolution symmetrically located relative to the axis of the sensor. Therefore, almost all the rays of the output beam of the transmitting fiber fall on the end of the receiving fiber and enter the photodetector.

The level of the received optical signal depends on the reflection conditions on the front surface of the sensor. When a sensitive element is immersed in a liquid, the reflection conditions change, most of the radiation goes into the medium, the level of the optical signal received by the photodetector falls, which indicates a change in the level of the liquid medium relative to the sensor element.

The disadvantages of this signaling device include the complexity of manufacturing the sensitive element and the even greater complexity of the alignment and subsequent fixation of the sensitive element relative to the common plane of the ends of the two transceiving optical fibers.

Closest to the proposed invention is a fiber optic liquid level switch described in [2].

The signaling device comprises a radiation source and receiver, transmitting and receiving optical fibers, a housing in which the common end face of the transmitting and receiving optical fibers and a retroreflective element are rigidly fixed. A distinctive feature of the signaling device is a retroreflective element, which uses an optically transparent rod of circular cross section in contact with the liquid, the end of which is a spherical segment, which is simultaneously a sensitive element of the entire signaling device as a whole. The cylindrical part of the rod is fixed in the housing together with the common end of the transmitting and receiving optical fibers using a connecting component, and the spherical segment extends beyond the housing. The radius of the spherical segment is uniquely associated with the angle of entry of the light flux into the rod and the angles at which it is reflected from the interface between the rod and the external environment. In this signaling device, the light flux from the radiation source through the transmitting fiber at an appropriate aperture angle is introduced into the rod and transmitted along the rod by re-reflection from the boundaries of two media to the spherical surface, and then, reflected from it, returns through the rod and the receiving fiber to the radiation receiver. The luminous flux propagates along the rod in such a way that in the zone of contact with air or adhesive, the condition for total internal reflection is fulfilled, and when it comes into contact with the liquid, this condition is violated. This causes a part of the light flux to exit from the light guide. It has been experimentally shown that the output power of a signal decreases by a factor of 2–3 when a sensitive element is immersed in a liquid.

The disadvantages of this signaling device include the process of manufacturing the sensitive element, which is still rather complicated, related to the need to manufacture a spherical segment with a uniquely defined radius, depending on several parameters.

All these shortcomings of the indicated liquid level signaling devices arise due to the fact that the functioning of their sensitive elements is based on the violation of the principle of total internal reflection.

The manufacture of liquid level signaling devices can be simplified if the basis of their design is a sensitive element operating on a different principle - the principle of measuring the change in luminous flux caused by Fresnel reflection from the interface between two media with different refractive indices.

The objective of the invention is to create a simple to manufacture and reliable (due to the simplicity of the design) in operation fiber-optic liquid level switch, the principle of which is based on measuring the change in luminous flux caused by Fresnel reflection from the interface between two media with different refractive indices.

The technical result is achieved by the fact that in a fiber-optic liquid level switch containing a radiation source and receiver located in an optoelectronic module, transmitting and receiving fibers, optical connectors and a retroreflective element, the latter is made of a fiber whose input end is optically connected to the common end of the receiving and transmitting optical fibers, and the output end in contact with the controlled medium (liquid) is a sensitive element of the signaling device.

Simplicity in manufacturing and reliability in operation are ensured by the simplicity and reliability of the retroreflective element, which is a light guide, the length of which is determined by the particular problem being solved.

Fiber-optic liquid level switch (see drawing) contains a radiation source 1 and 2, located in an optoelectronic module 3, equipped with an optical connector 4, a transmitting optical fiber 5, a receiving optical fiber 6, the ends of which are in the same plane and fixed in the connector 4, retroreflective fiber 7 with an optical connector 8 located on the input end, and the output end 9, which is a sensitive element of the signaling device.

Fiber optic liquid level switch operates as follows. The radiation from the source 1 through the transmitting optical fiber 5 enters the retroreflective optical fiber 7 (optical fibers 5 and 7 are optically connected by elements 8 and 4), propagates through it to the output end and is radiated into the surrounding space. However, four (approximately) percent radiation propagating through the optical fiber 7, according to Fresnel, is reflected from its output end 9, which is the interface between the two media (fiber core material / environment, in this case air), and the same fiber 7 optically coupled by elements 8 and 4 to the light guide 6, and the light guide 6 is directed to the radiation receiver 2. The described process occurs when the output end 9 is in air, i.e. out of a controlled environment. As soon as the output end 9 of the retroreflective fiber 7 enters a controlled medium (for example, a liquid), the Fresnel reflection disappears (to one degree or another) and there is no reflected radiation. Depending on this or that case, the optoelectronic module gives a signal of the presence or absence of liquid.

The described design of the signaling device, when the receiving and transmitting optical fibers have a common end and are optically connected to the retroreflective optical fiber terminated by an optical detachable connector (tip) by means of an optical connector (sleeve or socket), are not the only possible design variant of the signaling device. Another practically implemented is a signaling device, the measuring light guide of which is made on the basis of a Y-shaped coupler.

Here are the quantitative parameters of the first version of the signaling device, made on the basis of polymer-polymer fibers with a diameter of ⌀ 400 μm, from which a 7-fiber converter with external modulation, a polymer-polymer fiber with a diameter of ⌀ 1000 μm and a length of 1.5 m, an AL-emitter 107, photodetector FD-256, detachable optical connectors and FC sockets.

In the absence of contact with liquid (water), i.e. When a retroreflective fiber (polymer-polymer, 1.5 mi long with a diameter of ⌀ 1000 μm) was located in air, radiation with a power of 79-81 nW was received at the photodetector. Upon contact with water, the radiation power was 13-15 nW.

The fiber-optic signaling device has shown high measuring efficiency, reliability in operation with a simple design. His serial production is being prepared.

INFORMATION SOURCES

1. SU No. 1613871 A1 G01F 23/28, 1987

2. T.I. Murashkina Fiber Optic Liquid Level Switch, Radio Engineering, No. 10, 1995

Claims (2)

1. Fiber-optic liquid level switch containing a source and a receiver of optical radiation located in the measuring information processing unit, optically coupled to a radiation source and receiver, respectively, transmitting and receiving optical fibers having a common end face and a retroreflective element, characterized in that the retroreflective element made of a fiber, one of the ends of which is optically connected to the common end of the transmitting and receiving fibers, and the other end to the environment. 2. The fiber optic liquid level switch according to claim 1, characterized in that the transmitting, receiving and retroreflective fibers are made in the form of a Y-shaped coupler.