SU1649315A1 - Photoelastic measuring transducer - Google Patents

Abstract

The invention relates to a measurement technique and can be used to measure forces, strains, pressures, and accelerations. The aim of the invention is to improve the reliability and simplification of structures. The photoelastic measuring transducer contains a photoelastic sensing element in the form of a prism, the entrance face of which is optically connected to a radiation source, and the output one to a photoelectric receiver, the output of which is electrically connected to an amplifier recording unit, with the input and output faces of the prism of the photoelastic sensing element formed by planes the diagonal vertices of the prism base at the Brewster angle to the base plane, and the optical axes of the radiation introduced into the input g earlier and output from the output face, are normal to the planes of these faces. 6 or ID (L

Description

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The invention relates to a measurement technique and can be used to measure forces, strains, pressures, and accelerations.

The purpose of the invention is to increase reliability and simplify the design.

FIG. 1 shows a photoelastic measuring transducer, general view; in fig. 2 - diagonal section of the sensing element; in fig. 3 - equivalent optical design of the device; in fig. 4 is a diagonal section of a prism with a quarter-wave plate; in fig. 5 - photoelastic measuring transducer with a quarter-wave plate, general view; in fig. 6 is equivalent to optical

quarter-wave plate converter circuit.

The photoelastic sensing transducer (Fig. 1) contains a radiation source 1, a light guide 2, a sensing element 3 with an input 4 and an output 5 edges, a sensing light guide 6, a photoelectric receiver 7, an amplifier-recording unit 8.

FIG. 1 the plane P. is rocked. Cutting the top of the base of the prism at the Brewster angle f5.

The diagonal section (Fig. 2) coincides with the plane of propagation of the luminous flux, and at the points A, B, C, D, E there is repeated reflection of perspiration. by the corner

CO 01

Brewster to the base plane of the prism. The equivalent optical design of the device shown in FIG. 3, is a series connection of identical risk fields with parallel polarizers, the role of a polarizer being played by the area of reflection of the light beam from one of the base faces (in Fig. 2, points A, B, C, D, E), and by a photoelastic element on the equivalent circuit, the sensitive element is cut along the path of light propagation between two reflections from opposite bases: AB, BC, CD, DE.

FIG. Figure 4 shows a diagonal section of the prism of the sensing element, on one of the bases of which a quarter-wave plate 9 is placed. The polarizers in this case are only the reflection areas from the lower edge of the sensing element (points AI, C, EI).

The photoelastic elements in FIG. 6 as in the diagram of FIG. 3, are the segments of the sensing element along the path of light propagation between the two reflections from the opposite faces A | Bi, BiCi, CiDi, D | E |, and the points Bi and DI (Fig. 4) are shown as I / 4 elements.

The device works as follows.

The light at source 1 (Fig. 1) passes through the radiating light guide 2 to the input cut-off plane 4 of the prism 3, passes the optical environment of the prism and reflects from the base of the prism at point A (Fig. 2). Since the light falls on the entrance cut plane normally, and the cut plane itself is the Brewster angle with the base plane, the reflected beam completely polarizes. With each new reflection at points B, C, D, E, the polarization is preserved. Under the action of a load (force F, Fig. 1) applied to the face faces of the prism, the linearly polarized light passing through the optical medium of the prism becomes elliptically polarized as a result of the photoelastic effect. The degree of ellipticity depends on the magnitude of the load. Reflected last time at point E (Fig. 2), the light becomes linearly polarized and through the output cavity 5 of the slice (Fig. P enters the receiving light only ti) through which it is fed to the photoelectric converter ib 7, where it is transformed into an electrical signal proportional to the actual load, and then amplified and processed in the amplifying-recording unit 8 to obtain information in the required form.

The cut-off depth of the top of the prism base is determined by the design conditions for the input and output of radiation. For example, when using light guides with a rope diameter d, the cutting depth along the lateral face h can be taken

h l, 2d sincpg

d:

0

The prism bases should be predominantly square, which is caused by

The following considerations. In a square prism, a diagonal plane is located at an angle of 45 ° to the lateral faces m.

Under the action of a measured force applied at an angle of 45 ° to the plane of incidence of light, the light falls into two orthogonal modes, the polarization direction of which is an angle of 45 ° with the polarization direction specified by polarizers A, B, C, D, E ( Fig. 2). Due to the photoelastic effect, these two modes propagate in

photoelastic medium with different velocities, which leads to the accumulation of the phase difference between qimi. As is known, the intensity of light transmitted through a photoelastic articom, clamped between two parallel oriented polarizers, is determined by the expression

: Kx: iocosJ-Ј-, s

(2)

where Γ is the phase shift accumulated by two light modes in the photoelastic plate. For an optically isotropic medium, the phase shift is given by

(ds-qi2) Ep, 3

Where p o is the index of refraction;

qn nqi2- piezo-optic constants;

0- external mechanical stresses; l is the wavelength of light;

1 is the distance that light travels between two reflections.

From expression (2) it follows that the intensity of the light passing through the system (Fig. 1-3) is determined by the expression

g, g

If the external influence of F is small and it is possible to accept - G | ., the expression (4) takes the form

% (1-C1 ± G + Gz ± G) (5)

For small Γ, the quadratic dependence 3 (Γ) determines a relatively low sensitivity. At the same time, expression (4) has an area close to the linear dependence P (D), in which the sensitivity to small external influences is much higher. Usually, the output to the linear section O (H) is carried out by placing a phase plate K / 4 or

3/4 I

In this situation, such a case is realized in the construction of FIG. 1, if additionally enclosed is an external constant

-, -, ft2 To fl9 / U,.

:) Cty-tloCOS- - COS -COS2 --COS-f (4

load or place the phase plate I, / 4 or 3 / 4X, (Fig. 5). An A. / 4 or 3 / 4A ,, plate made of a birefringent crystal, whose eigenmodes are polarized at an angle of 45 ° to the plane of incidence, is glued to the side face of the sensing element along the line connecting the cut edges. Sveta. For a sensing element with an X / 4 plate at a constant load giving a phase shift of l / 2, the intensity at the exit from face 5 of sensing element 3 is expressed as follows:

hl

g + t

Sound 30С052

G -1-I

) cos2 (r2 + z) x

(

l ± i) cos2 (l ± Z2

Tl

(6)

which is equivalent to

(±) (

I-sirjUix 11-sinF24

y rl-sinlV X (-g- K

For small measurable mechanical effects, expression (7) takes the form

I-P-G2-G3-G4,

J in s: J /

(eight)

In case a constant load causes a 3 / 4X phase shift. or a 3 / 4X phase plate is installed, then, in particular, instead of expression (8) we have

70 (l ± Il ± § ± l5 ± Il). (9)

Cj

In addition, the sensitivity of the proposed converter depends on the direction of application of force in the plane of the base 35 cutting off the diagonal vertices of the osnoni of the sensitive element.

When the force coincides with the line connecting the cut edges, for example 4 and 5 in FIG. 2, the polarization of one of its own light modes coincides with the polarization,

specified by the polarizer, and the action of force does not create a birefringence in the photoelastic medium, since only this one mode propagates in it. Therefore, the intensity of light in the described system does not depend on the force applied by the indicated method. The maximum sensitivity of the element is achieved to direct the application of the force shown in fig. 1. If the direction of application of the force is an angle φ with the optimal direction and lies in the plane of the base, the sensitivity is given by

, reos2 p,

;Yu)

where Copt is the sensitivity to direct the application of the force shown in FIG. one.

In the proposed photoelastic measuring transducer there are no polarization elements in the form of films, prisms, etc.

Claims (1)

Invention Formula 25 A photoelastic measuring transducer containing a photoelastic sensitive element in the form of a prism, the entrance face of which is optically connected to the radiation source and the output face to the photoelectric receiver, the output of which 30 is electrically connected to an amplifier-recording unit, characterized in that, in order to increase reliability and simplify the design, in it the input and output faces of the prism of the photoelastic sensing element are formed by planes,  cutting the diagonal vertices of the prism base at the Brewster angle to the base plane, and the optical axes of the radiation introduced into the input face and output from the output face are normal to the planes of these faces. Fng. I R IW ACU / Fng. 2 V BC With CD D DE FIG. 3 B, R -XVys A XN 1 D, C, Ј Fayr 4 a, ft FIG. 5 4 8, С, С, CD-Vf 1) LE, FIG. b