WO2008033049A1

WO2008033049A1 - Method and device for contactlessly (optically) measuring the speed of an object

Info

Publication number: WO2008033049A1
Application number: PCT/RU2006/000485
Authority: WO
Inventors: Stanislav Fedorovich Rastopov; Evgeny Sergeevich Voevodenko; Michail Sergeevich Voevodenko
Original assignee: Stanislav Fedorovich Rastopov; Evgeny Sergeevich Voevodenko; Michail Sergeevich Voevodenko
Priority date: 2006-09-15
Filing date: 2006-09-15
Publication date: 2008-03-20

Abstract

The invention relates to measuring engineering, in particular to optical velocimeters for measuring, for example, the speed of an automobile, to which said device is fastened, with respect to a road. The inventive method consists in forming an amplitude and/or phase modulation of the object image, in detecting it by means of optical receivers and in determining the speed of the object according to the frequency of an output electric signal. Said invention is characterised in that the frequency does not depend on the distance to the object and makes it possible to perform a high-sensitive measurements.

Description

Method and device for non-contact measurement of object speed

The invention relates to the field of measuring technology, in particular, to optical speed meters, for example, a car on which a meter is mounted, relative to the road, the linear speed of rotating bodies relative to a stationary meter, etc.

Optical speed meters generally include a illuminator of the measurement object, an optical system for forming its image, and photosensitive elements (PSE) for analyzing the speed of image movement. By the speed of the image, knowing the increase in the optical system, judge the speed of the object. For example, speed meters are known in which two PSEs are used to analyze the image, located at some distance from each other along the direction of the object’s movement near the plane of its image. Electrical signals with PSEs are recorded simultaneously and the speed of movement is judged by the delay time of one signal relative to the other, for example, by calculating their mutual correlation function (US patent US2001046042 from 11.29.2001). The disadvantage of this method is the low accuracy of the measurements, since the signals with PSE are not the same. To measure the speed of an object, it is also known to use the so-called spatial filters that extract spatial periodicity from the stochastic structure of the surface of an object. When moving the image of an object along such a structure, an alternating electrical signal is formed, the frequency of which is directly proportional to the speed of the object. Such a filter can be implemented by a periodic array of PSEs located near the image plane (US patent US4921345 dated 01/05/1990, JP patent JP52143081 dated 11/29/1977), the spatial filter consists of a PSE line, the elements of which are combined through one, the signals of which connected to various inputs of a differential amplifier. The signal from the average brightness of the object is subtracted, and the signal proportional to the speed is in antiphase at the inputs of the amplifier and is added to its output.

Closest to the claimed invention is a speed meter, which uses a periodic amplitude or phase grating, the strokes of which are perpendicular to the direction of movement of the image. Such gratings divide the image into two or more, which are focused by an additional optical system into two or more PSEs (US patent US6403950 dated November 6, 2002, prototype). The spatial shift of the image in the prototype is chosen equal to either

SUBSTITUTE SHEET (RULE 26) less, preferably 2 times, the distance between the PSE. Signals with PSE are combined in pairs through one, the signals of pairs of PSE are subtracted to reduce the background component, and the frequency of the useful signal is proportional to the speed of the object.

The disadvantage of the prototype, as for all of the above devices, is the dependence of the frequency (or delay time) of the signal on the distance to the object, since the image scale changes with the distance to the object. To reduce this dependence, telecentric optical systems are used (strong aperture of the input beam in the focal plane of the image). However, this sharply reduces the amplitude of the useful signal, since the aperture of the optical system decreases. Also, direct measurements of the distance to the object by individual rangefinders or by the phase shift of the speed signals are used. To do this, in the prototype, each PSE is divided into 2 elements, the total signals from the left elements are compared with the total signals of the right elements and their phase difference is measured, which determines the distance to the object.

The aim of the invention is to increase the accuracy of measuring the speed of an object by eliminating the frequency dependence of the signal on the distance to the object while maintaining high sensitivity measurements, i.e. without reducing the amplitude of the useful signal.

This goal is achieved by using an optical system (OS) and an amplitude and / or phase spatial structure (PS) that forms an image or a sequence of images of an object spatially modulated in amplitude and / or shifted in the direction of motion of the object. The image (s) are recorded using two or more photodetectors (FP), the electrical signals from neighboring FPs are subtracted, and the speed of the object is judged by the frequency of the resulting electrical signal, with the PS located in the main plane of the OS in the image space, or as close to it as possible, the FP positioned at such a distance C from the main plane of the OS in the image space at which an increase (decrease) in the angular velocity V = C / G with a decrease (increase) in the distance G from the main plane of the OS in the space meta to the object is compensated by a corresponding increase (decrease) in X - the period of the spatial shift of images and / or the period of amplitude modulation of the image in the plane of the FP location. Then the frequency of the signal from the phase transition, proportional to the speed of the object, does not depend on the distance G. Optimal

SUBSTITUTE SHEET (RULE 26) the distance S between the centers of the photodetector areas of the FP in the direction of the measured speed is chosen from the condition

S = x / 2

With this S, the amplitude of the difference signal from the phase-proportional phase transition is maximum, since the signals of neighboring phase-transitions are out of phase and the maximum subtraction of the parasitic common-mode signal from the constant illumination level of the object is ensured.

The invention is illustrated by two figures. Fig.l illustrates the implementation of the method in General, where

1, 2 - elements of the lens lattice, 3, 4 - optical axes of the elements 1 and 2,

5, 6 - the main planes of elements 1 and 2 in the space of objects and images, respectively,

7 - amplitude grating, 8, 9 - photodetectors (FP),

10 - plane of the location of the FP,

11 - focal plane

12 - image plane,

13 - block processing electrical signals (differential amplifier). Solid lines without arrows indicate the course of the rays forming the image.

Figure 2 presents a block diagram of a device - a special case of the method, with the same notation.

The method is implemented as follows. In general, both phase and amplitude modulation of an object image can be used. To obtain phase modulation, for example, a lens array is used, the optical axes of the elements of which are spatially shifted in the direction of the measured velocity. In Fig. 1, for simplicity, only two of its elements 1 and 2 are shown with the corresponding optical axes 3 and 4 with the main planes common to all lenses in the space of objects 5 and images

6. For amplitude modulation of the image, an amplitude grating 7 is used, which is located as close as possible to plane 6 — on the surface of lenses 1 and 2, since it can generally pass inside the lenses. Photodetectors 8 and 9 (for simplicity, only two are shown) are located in some FP 10 plane between the focal plane 11 and the image plane 12, the distance from which to plane 6 REPLACEMENT SHEET (RULE 26) is as follows. When an object moves (indicated by a curved arrow on the left) at a distance G from plane 5, its images move (indicated by curly arrows on the right) and the speed of movement in plane 10 is

V = C / G (1)

The distance X between the images (and / or the amplitude modulation period) in the plane 10 is expressed through G, the shift value L of the axes 3 and 4, the focal length F, the distance B to the plane 12 and the phase (amplitude) grating period D using standard lens formulas and geometric formulas like

X = (L + L * B / G + (B-C) * (D / B-L / G)) (2)

The frequency of the FP signals 8 and 9 is directly proportional to V (1) and inversely proportional to X (2). To ensure the independence of this frequency from G, the condition V / X = cst must be satisfied. Calculating the derivative of this expression with respect to C and equating it to zero, we obtain

C = F * (L / D + L) (3)

To increase the useful signal Uout and reduce the contribution of a constant level of illumination of the object to the resulting signal, the signals from the FP 8 and 9 are subtracted in the usual way, for example, by a differential amplifier 13. Moreover, the optimal distance S between the centers of the photodetector areas of the FP 8 and 9 is S = X / 2 (4) where, in this case, by G we mean the average working distance to the object. With this S, signals with a phase transition of 8 and 9, proportional to the speed, are exactly in antiphase and add up to the maximum when subtracting the stray common-mode signal from a constant level of illumination of the object.

The simplest device in which the inventive method is implemented, for the case L = O, is shown in FIG. 2. It consists of OS 1, for example, a focusing lens, an amplitude grating 7, located in the main plane of the lens 6 and FP 8 and 9, located in the focal plane 11 of OS 1 at a distance S between the centers of the photodetector areas of the FP in the direction of the measured speed and the processing unit

SUBSTITUTE SHEET (RULE 26) electrical signals 13, the inverting and non-inverting inputs of which are connected to neighboring FPs (there may be more than two).

The device operates as follows. When an object moves, its amplitude-modulated image also moves and creates an alternating electric signal at FP 8 and 9. When the distance G to the object changes, the speed of the image changes in accordance with formula (1) and the image modulation period in accordance with formula (2) . When the AF is located in the focal plane, these changes are precisely compensated, in accordance with formula (3), i.e. the frequency of the signals from the AF does not depend on G. To increase this useful signal and reduce the background signal from constant illumination, the signals from the AF should be in antiphase, which is ensured at a distance S = F * D / (2 * G), which follows from the formula ( 4) and standard algebraic transformations. Here, G should be understood as the average expected distance to the object. To obtain the maximum sinusoidal signal, the optimal slit width of the amplitude grating 7 is D / 2, which follows from symmetry considerations. Since the image is amplitude-modulated only across the strokes of the grating 7, the length of the photodetector areas of the FP in the direction along the gratings of the grating can be much larger than their width, which increases the amount of detected light, i.e. increases the amplitude of the useful signal with AF.

SUBSTITUTE SHEET (RULE 26)

Claims

Claim

1. The method of non-contact measurement of the speed of an object, including the formation of an image of a lit object or several images spatially modulated in amplitude or shifted in the direction of movement of the object using an optical system (OS) and a periodic spatial phase and / or amplitude structure (PS), recording the intensity image using two or more photodetectors (FP) and the formation of an alternating electrical signal by subtracting the signals of neighboring FP, the frequency of which is judged with object speed, characterized in that the PS is located in the main plane of the OS in the image space, or as close to it as possible, the FPs are located at a distance C from the main plane of the OS in the image space

C = F * (L / D + L) where F is the focal length of the OS, L is the resulting shift of the optical axes of the OS and the PS (if multiple images are formed), D is the period of the PS, and the optimal distance S between the centers of the photodetector areas of the FP in the direction measured speed is chosen from the condition

S = X / 2 where X = (L + L * B / G + (BC) * (D / BL / G)) is the period of the spatial shift of the images and / or the period of the amplitude modulation of the image in the plane of the FP location, B = F * G / (GF) - distance from the main OS plane in the image space to the image plane, G - distance from the main OS plane in the space of objects to the object.

2. A device for non-contact measurement of the object’s speed, consisting of an object illumination source, a focusing OS, an amplitude grating, two or more phase converters and an electrical signal processing unit, whose inverting and non-inverting inputs are connected to neighboring phase transitions, characterized in that the amplitude grating is located in the main the OS plane in the image space, or as close as possible to it, the FPs are located in the focal plane of the OS at a distance S between the centers of the photodetector areas of the FP in the direction of the measured speed

S = F * D / (2 * G).

3. The device according to claim 2, characterized in that the width of the slots of the amplitude grating is D / 2.

4. The device according to claim 2, characterized in that the length of the photodetector areas of the FP along the direction of the bars of the grating is much larger than their width.

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