JPS61237027A - Optical sensor - Google Patents

Abstract

PURPOSE:To reduce the detection errors of an optical sensor, by independently detecting light passing through parts at which a compression stress and a tensile stress are caused when a bent moment is applied to a photoelastic material to determine a measured value. CONSTITUTION:Light outputted from a light emitting element 1 is transmitted to a microlens 3 through an optical fiber 2 and is made incident into a photoelastic material 50 via a polarizer 40. A bent moment works on the photoelastic material 50 through a lower fulcrum 51 and a upper load point 52 corresponding to the weight of a load 53 and a phase difference is caused between two polarization components orthogonal to each other due to the load. Moreover, the phase difference between two polarization components orthogonal to each other with a 1/4 wavelength plate 6 is added thereto by a fixed amount and the resultant phase difference is converted into the intensity of light with a polarizer 70. The light passing through the upper portion of the photoelastic material 50 and the light passing through the lower part thereof are made incident into large-dia. fibers 20 and 21 respectively to be converted in to electrical signals with light receiving elements 12 and 13. Thus, the ratio between the output of the light receiving element 12 and the output of the light receiving element 13 is determined to obtain the measured value of the level of the load.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a so-called optical sensor that detects a physical quantity using an optical method.

More specifically, the present invention relates to an optical sensor using a photoelastic material whose birefringence value changes depending on physical quantities in the outside world.

<Prior art> Optical sensors that utilize birefringence are said to be able to detect with high sensitivity by converting the physical quantity to be detected into a minute difference in the refractive index of light in two orthogonal directions by using polarized light. Has characteristics. Among them, optical sensors that utilize temporary birefringence based on the photoelastic effect are capable of detecting various mechanical quantities, such as pressure, load, acceleration, strain, and sound.

An important point in producing an optical sensor is to configure it so that detection errors do not occur even if the intensity of light reaching a photodetector such as a light receiving element changes due to disturbance. Here, the disturbance may be a fluctuation in the output intensity of the light emitting element, a coupling loss between the light emitting element and an optical fiber, an optical fiber and various optical components, or other factors.

FIG. 4 shows a conventionally used configuration of an optical sensor that suppresses errors against the above-mentioned disturbances. Light C output from light emitting element L, optical fiber 2
, and enters the first polarizing beam splitter 4 through the microlens 3. Here, only light with a single polarization component passes through the photoelastic material 5.4 wavelength plate 6 and reaches the second polarization beam splitter 7. Here, the photoelastic material 5 has the effect of changing the ratio of two orthogonal polarization components according to its internal stress, and the polarization beam splitter 7 separates the two polarization components into separate optical paths. It has an effect. One of the two separated polarized lights passes through the microlens 8 and the optical fiber IO and reaches one of the light receiving elements 12, and the other □ passes through the microlens 9 and the optical fiber 11 and reaches the other light receiving element 13. Since the outputs of the light receiving elements 12 and 13 increase or decrease at the same rate with respect to the disturbance before the polarizing beam splitter 7, it is possible to suppress the detection error due to the disturbance to a low level by taking the ratio of these two outputs. Become.

The disadvantage of this configuration is that it uses a large number of expensive optical components. In particular, the polarizing beam splitter is a very expensive component, but in this configuration, the polarizing beam splitter 7 cannot be directly replaced with an inexpensive polarizing plate or other polarizer.

Furthermore, it is extremely inconvenient to use the fibers facing in three different directions with respect to the photoelastic material 5 as shown in Figure 4, and it is actually necessary to bundle the three fibers into one. However, for this purpose, it is necessary to further add a right-angle prism and other optical components, which poses the problem of further increasing the number of components.

<Object of the Invention> The present invention has been made in view of the above points, and provides an optical sensor using a photoelastic effect that has small detection errors, a small number of optical parts, and is easy to handle. The purpose is to provide.

<Example> The present invention will be described in detail below with reference to FIGS. 1 to 3.

FIG. 1 (4) shows a load sensor according to the first embodiment of the present invention, in which light output from a light emitting element 1 is transmitted to a microlens 3 via an optical fiber 2, and is further transmitted to a polarizer 40. The light enters the photoelastic material 50 through the . The photoelastic material 50 is configured such that a bending moment corresponding to the weight of the load 53 is applied to the lower fulcrum 51 and the upper load point 52. A phase difference occurs between the two orthogonal polarization components due to the load, but a certain amount of phase difference is added between the two orthogonal polarization components to the quarter-wave plate 6, and the polarizer 70 creates a phase difference between the two orthogonal polarization components. to convert the phase difference into light intensity. The light that has passed through the upper part of the photoelastic material 50 and the light that has passed through the lower part of the photoelastic material 50 are incident on the large-diameter fibers 20 and 21, respectively.
．． The light receiving elements 12, 13 connected to the light receiving element 21 convert the light into an electrical signal. By determining the ratio between the output of the light receiving element 12 and the output of the light receiving element 13, a measured value of the load amount can be obtained. In this case, the detection error will be very small.

In this embodiment, optical glass was used as the photoelastic material 60, but other types of glass such as quartz glass,
GaP, LiNbO3, LiTaO3 and other optical crystals or optical ceramics, phenolic resin, celluloid, epoxy resin, diallyl phthalate polymer, styrene-polyester copolymer, methyl methacrylate polymer, polycarbonate, gelatin, polyurethane rubber, epoxy rubber, -silicon resin , polycyclohexyl methacrylate, polyacrylonitrile, polyvinyl chloride, unsaturated polyester photosensitive resin, and other polymer materials can be used.

Here, the stress applied to the photoelastic material 50 will be explained. ) in FIG. 1 is a cross-sectional view in the xy plane direction of the peripheral portion of the photoelastic material 50 in FIG. 1(2). The photoelastic material 5'0 is in a state called 4-point bending or uniform bending with two lower supports 51 and two upper load points 52, and the bending moment between the two upper load points 52 is Uniform.

This bending causes compressive stress in the upper part of the photoelastic material 50,
In the lower part, tensile stress is generated in the direction of the arrow in FIG. 1 (2).

As shown in FIG. 1 [F]), when the x and y directions are determined, the light incident on the photoelastic material 50 has polarized light components in the However, within the photoelastic material 50, a phase difference occurs between the two polarization bases due to the photoelastic effect. Since this phase difference is proportional to stress, a phase difference of opposite sign occurs between the light passing through the upper part of the photoelastic material 50 and the light passing through the lower part.

Hereinafter, for convenience of explanation using mathematical formulas, idealization will be performed as follows. It is assumed that the two light beams incident on the optical fibers 20 and 21 have the same intensity before reaching the polarizing plate 70 and have no spread.

For each of the two light beams, the phase difference between the two orthogonal polarization components caused by passing through the photoelastic material 50 is assumed to be θ and -0.

First, let us consider the case where there is no 74-wave plate 6. The amount of light reaching the light receiving element 12 through the optical fiber 20 is A1, and the amount of light reaching the light receiving element 13 through the optical fiber 21 is B,
Assuming that the optical loss after entering the optical fiber 20 or 21 is the same, it is known that, where I is the proportionality constant, the following equation is obtained. The form of the equation is clearly A=
B, and as it is, there are two light amounts A.

It makes no sense to detect B independently.

A /4 wavelength plate 6 is actually included, but this serves to add a common phase difference of 90° to the two lights. At this time, the output shows separate changes such as θ+90° I A ''I 1-1n2=-(1-)-sin θ)-θ+90' I B = I 5in2-=-(1-sane).These two light amounts A From and B, θ can be determined regardless of the proportionality constant 1.

That is, A-B.

;Shelf θ A+B. Disturbances such as light emission intensity fluctuations and connection loss fluctuations are all fluctuations with a proportionality constant of 1, so by using the light quantities A and B, it is possible to sufficiently suppress the influence of disturbances between the light emitting element l and the polarizer 70. It becomes possible. Furthermore, since the output (A-B)/(A+B) is approximately proportional to θ when θ is a small value, the processing circuit can be greatly simplified.

The actual measurement results are shown in Figure 2. The two curves are outputs A and B, respectively, and both change linearly in a small load region. Here, the light emission intensity of the light emitting element 1 is electrically controlled so that A+B=constant. As a result, the value of AB can be taken as it is as an output, and the calculation of dividing by A + B can be omitted, but this significantly simplifies the electrical processing of the signal.

Next, a pressure sensor according to a second embodiment of the present invention is shown in FIG. 3 (A) CB). The light output from the light emitting element 1 is transmitted through the optical fiber 2. Microlens 3. The light passes through the polarizer 40 and enters the photoelastic material 55 . Since the photoelastic material 55 has a diaphragm-shaped counterbore on the bottom surface, stress due to bending is generated in accordance with the pressure difference between the inside and outside of the diaphragm. Here, the light is totally reflected twice within the photoelastic material 55, so that the optical path is bent by 1800 degrees, but at this time a phase difference occurs between the two orthogonal polarization bases.

In other words, the role of the quarter-wave plate 6 in the first embodiment is fulfilled by these two total reflections. The light enters the microlens 100. This microlens loo has its end face particularly processed as shown in the side view in FIG. It functions to introduce the light that has passed through the lower part into the optical fiber 11.Optical fiber 1
The light introduced into 0.11 is transmitted to each light receiving element 12.1.
8 and by processing its output electrically,
Accurate pressure measurements can be obtained.

Here, the phase difference between two orthogonal polarization bases caused by total reflection within the photoelastic material 55 will be explained. In general total reflection, the phase difference δ that occurs between linearly polarized light having an electric field oscillating within the total reflection plane and linearly polarized light oscillating in a direction orthogonal thereto is expressed by the following equation.

Here, n is the relative refractive index of the medium, and ψ is the angle that the incident light makes with respect to the perpendicular to the reflective surface.゛In this example, n is the refractive index of the photoelastic material 55, and φ = 45°, and the phase difference δ in this case is as follows. In order to have the same function as the 174 wavelength plate 6 in the embodiment, 2δ=9
00, and the refractive index of the photoelastic material 55 that satisfies this requirement may be n=1.554. In reality, the phase difference 2δ does not necessarily have to be 90°. When a phase difference of 2δ is given by total internal reflection, if we perform the same idealization as in the first embodiment for explanation using mathematical formulas, the amount of light A reaching the light receiving element 12 and the amount of light B reaching the light receiving element 18 are respectively as follows. Become. Also, A-B=ISin2δiθ A+B=I(1-(2)2δ(2)θ), and (A
-B)/(A+B) is a signal independent of I, that is. The fact that this signal is stable against disturbances is the same as in the first embodiment. Further, the value of (A-B)/(A+B) in the first embodiment corresponds to the case where 2δ=90° is substituted in the above equation.

Here, the first embodiment is a load sensor, and the second embodiment is a pressure sensor. However, by leaving the respective optical systems as they are, the first embodiment is a pressure sensor, and the second embodiment is a load sensor. It can also be a sensor.

In addition, optical sensors using photoelastic materials can be used not only as load sensors or pressure sensors, but also as acoustic, strain, and displacement sensors.In addition, photoelastic materials can be combined with materials with different coefficients of thermal expansion. It is also possible to make a current sensor 1 magnetic sensor by combining a temperature sensor and an electrostrictive material with a voltage sensor and a magnetostrictive material.

<Effects of the Invention> As described above, the optical sensor of the present invention utilizes the fact that both compressive stress and tensile stress are generated when a bending moment is applied to a photoelastic material. It is characterized by obtaining measured values by independently detecting light, which provides the advantages of a small detection error as a sensor and a small number of component parts.

This advantage is extremely useful for the widespread use of optical sensors.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an optical sensor according to an embodiment of the present invention, FIG. 2 is an explanatory diagram showing the output obtained by the optical sensor of FIG. 1, and FIG. FIG. 1 is a configuration diagram of an optical sensor according to an embodiment. FIG. 4 is a configuration diagram showing a conventional optical sensor.

Claims (1)

[Scope of Claims] 1. Light that passes a polarized component of the light through a portion receiving compressive stress and a portion receiving tensile stress due to the bending moment of a photoelastic material to which a bending moment is applied according to the physical quantity to be measured. An optical application sensor comprising a transmission system and a photodetector that individually detects each of the lights that have passed through the photoelastic material, the output of the photodetector being made to correspond to the physical quantity. . 2. The optical sensor according to claim 1, wherein a phase difference is imparted to orthogonal polarization components of the light passing through the portion receiving the compressive stress and the portion receiving the tensile stress.