SU1651106A1 - Device for measuring object vibrations parameters - Google Patents

Abstract

The invention relates to a measuring and control technique and is intended for measuring par-meters of oscillations of diffusely reflecting objects. and. The intention is to increase the information content by determining the relative amplitude of oscillations and increasing the noise immunity by compensating for the displacement of the object as a whole. Through the diffraction grating 2 and the objective 3 from the coherent radiation source 1, two beams parallel to each other are formed, which, successively reflected from the object 9 and the plane-parallel plate 4, receive the photodetector 5, which converts the intensity into an electrical signal processed in block 6. 1 .P. f-ly, i ill.

Description

The invention relates to a measurement technique and is intended to measure the amplitudes and phases of oscillations of diffuse-reflecting objects using photoelectron speckle interferometry in real time.

The aim of the invention is to increase the information content by measuring the relative amplitude of the normal vibrations of two points of the test surface located at a given distance from one another, and to increase the noise immunity of measurements by compensating for the displacements of the object as a whole.

The drawing shows schematically a device for measuring the vibration parameters of a diffuse reflecting object.

The device consists of a source of 1 coherent radiator (laser); a diffraction grating 2 with a diffraction angle a; lens 3 mounted at a distance of f - t / 2 ctg o. from the diffraction grating 2, where f is the focal length of the lens, t is the distance between the studied points of the object;

a plane-parallel plate 4 installed at an angle l / A to the optical axis and a thickness of d rt n2 - 05, where n is the refractive index of the plane-parallel glass plate; Photo-receiver 5 with low inertia and aperture D, which is installed along the optical axis at a distance L d D / A from the focal plane of the lens, where d is the diameter of the illuminating beam in the focal plane of the lens. a recording unit 6, the input of which is electrically connected to the output of the photodetector 5; compensatory plane-parallel glass plates 7 and 8. Position 9 denotes a controlled object.

The device works as follows.

The radiation from source 1 is incident on diffraction grating 2, after which two beams are formed as a result of diffraction. These beams fall on the lens 3 mounted at the focal distance f from the grating 2. The lens 3 converts the beams as follows. that their axes become parallel and located at a distance of t 2f between each other. The beams are focused on points on the surface of the object 9 and, reflected from it, fall onto the plane-parallel plate 4. Due to the orientation of the plate at an angle of 7Г / 4 to the axis of propagation of the beams and the choice of its thickness equal to d stupid - 0.5, reflected from its faces are aligned in the registration plane to obtain wide interference fringes. The intensity distribution in the recording plane is fixed by a photodetector 5 with low inertia set at a distance L d D / A from the focal plane of objective 3, in which the object under study is located, where d is the beam diameter in the focal plane of objective 3, D the diameter of the photoreceiver aperture 5, A - the wavelength of the radiation source 1.

 To register the photoreceiver 5 with a change in the intensity of the interference pattern formed by two speckle fields of waves reflected from two points of the diffusely scattering object, it is necessary that, first, the diameter of the photoreceiver aperture does not exceed the width of one interference band; secondly, the size of the speckle structure must be comparable with the size of the photodetector aperture.

The fulfillment of the first condition is ensured by the fact that the installation of a plane-parallel plate of a given thickness at an angle l: f / 4 to the optical axis allows one to combine the image of the points studied and to obtain interference wide enough bands in the recording space. In addition, in order to obtain an interference speckle pattern with sufficiently wide stripes (the width of the stripes is larger than the photomultiplier aperture diameter), compensating plane-parallel glass plates 7 and 8 are installed in the path of the interfering waves. The second condition is satisfied by the fact that the optical system is diffraction grating - the lens allows significantly (40-50 times) to reduce the diameter of the beam illuminating the surface and, thus, the minimum diameter of speckles e - increases significantly. In addition, the photodetector with aperture D is located at a certain minimum possible distance L from the object along the optical axis of the device. Significantly reducing the photodetector aperture or increasing the distance L from the photodetector to the object does not allow the limited sensitivity of the photodetector and the power of the laser used. Beam focusing

laser radiation on the surface of the object, in addition, allows to reduce the surface area over which the measured amplitude of oscillations is averaged.

The change in the intensity of the interference pattern formed by the waves reflected from two points of the investigated surface, detected by the photo-receiver 5 of small inertia, is converted into an electrical signal. According to the time evolution of the electric signal, the relative amplitude of the harmonic oscillations of the two studied points is determined for the case when the initial phase difference of optical waves is w / 2

and K + (- 1) Karcsin If / i0,

where A is the wavelength of the laser used;

And the value of the signal at the point 2тг ЛУ2; f is the frequency;

p is the phase of relative oscillations; io is the maximum value of the electrical signal;

K 0, 1,2, ... is the number of minima for a half period on the oscillogram.

The relative phase of the oscillations of two points can be determined as follows:

p arccos

a + a§ -1

2ag2

where ai, 32 are the oscillation amplitudes of the two points studied, which are predetermined by any of the known methods.

Claims (2)

Claim 1. Device for measuring object vibration parameters, comprising a coherent radiation source and a lens, an object holder, a photodetector and a recording unit located along the radiation path, characterized in that, in order to increase the information content and noise immunity, it is provided with a diffraction grating installed along radiation between the coherent radiation source and the lens, and the plane-parallel plate d 2ftg - 0.5 thick. where f is the focal length of the lens, a is the angle of the diffraction grating, n is the plate's refractive index, placed between the lens and the object holder and oriented at an angle of / 4 to the direction of radiation, the object holder is installed in the focal plane of the lens, and the photodetector is placed on distance L along the radiation path from the focal plane lens, satisfying the ratio of L d D / A, where d is the diameter of the radiation beam in the focal plane of the objective, D is the diameter of the aperture of the photodetector, A is the wavelength of the coherent radiation. 2. The device according to claim 1, that is, in order to measure the parameters oscillations over the entire surface of the volume, the object holder is mounted for movement in three mutually orthogonal directions, one of which is oriented along the optical axis of the lens.