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Description  |
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FIELD OF THE INVENTION
Our present invention relates to a method of and to an apparatus for
determining the position of an element with respect to some references,
e.g. a scale or a point and specifically for the determination by the use
of optical means for detecting the position of a surface element which has
at least limited reflectivity capability.
The invention is specifically directed to a method of and an apparatus for
determining the position of an element utilizing a technique whereby a
beam of polychromatic light having a plurality of a light wavelengths is
focused such that each wavelength has a respective focal point and the
wavelength of the light whose focal point corresponds to the location of
the element is detected.
BACKGROUND OF THE INVENTION
Position or displacement sensing devices using optical techniques are known
from German patent document (Patent Application-Auslegeschrift) No.
1,962,515 and from British patent document (Published Application) No.
2,077,421.
The German patent document No. 1,962,515 describes an optical distance
sensor of the contactless type in which a luminous beam is focused to a
plurality of distinct foci. This sensor determines the position of an
object which is located between two such foci. In this system, the
respective intensities of two light wavelengths reflected by the object
and converging at these foci are compared and the position of the object
with respect to the sensor is defined when these wavelengths are of equal
intensity. The sensor is displaced to obtain such equality. The final
position of the sensor fixes the location of the object. The sensor is in
effect a hybrid sensor which comprises a mechanical measuring system for
establishing its location in space and an optical measuring system for
locating the element with respect to the sensor.
The British patent document describes an optical detector and a process for
measuring the displacement of an object in which two monochromatic beams
of different colors but equal intensities are focused along a common axis.
This yields two distinct focal points situated at equal distances from a
place of reference. The relative intensities of the respective beams after
reflection by the object can then be measured. This relative intensity can
be a difference or quotient of the intensities of the light of the two
wavelengths and the change in the value of this relative measurement is a
function of the displacement of the object from which the two
monochromatic rays are reflected.
In both cases, therefore, it is necessary to compare two optical signals
and the precision of the measurement and hence of the comparison is a
function of the intensities of these signal. As a consequence, the
precision and resolving power of the detectors are clearly connected to
the optical properties of the surface of the object whose location or
position is to be measured and, specifically, the ability of the object to
reflect adequate amounts of the different incident light beams. This
system is also sensitive to nonlinear variations in light intensity which
may cause a disproportionate intensity for two wavelengths upon any
variation of source intensity.
OBJECTS OF THE INVENTION
It is the principal object of the present invention to provide an improved
method of and apparatus for determining the position of an object using
the optical principles mentioned previously but without the drawbacks of
earlier systems. A more specific object of this invention is to provide a
method of and an apparatus for detecting the position of an object
utilizing the selective focusing of different wavelengths whereby the
precision and resolving power of the technique is not dependent to a
significant extent upon the surface properties of the object whose
position is to be detected as long as that object can reflect light to
some extent.
SUMMARY OF THE INVENTION
These objects and others which will become apparent hereinafter are
attained, in accordance with the present invention, by a method which
involves producing a polychromatic beam of light, resolving this
polychromatic beam into a plurality of monochromatic rays and focusing
said rays at a succession of foci each located at a distance from a
reference which is characteristic of the wavelength of that ray, measuring
the luminous densities of the wavelength constituting the spectrum and
comparing these densities with the densities of reflected rays from an
object which can be disposed at a location along the line of foci, and
determining the wavelength whose density is maximally reflected, i.e.
whose reflected density is in the highest ratio to the incident density
or, where the densities of incident light are all the same, which has the
highest density, and from this wavelength determining the position of the
surface element with respect to the reference scale.
Preferably, therefore, the luminous light beam includes a multiplicity of
light rays of different wavelengths and substantially equal amplitudes
which derive from a single source which can be disposed in a plane
substantially orthogonal to the axis of the beam while means is provided
transversely of this axis to resolve the beam into the individual rays and
to focus these rays to the respective focal points along the axis. Each
focal distance, i.e. distance from a reference which may be the resolving
element or device is thus characteristic of the wavelength of a respective
ray. An object located at any such focal point will maximally reflect that
wavelength and by detecting the reflected wavelength of greatest
intensity, we can readily calculate from knowledge of this wavelength, the
spacing of the object from the reference. According to a feature of the
invention, a portion of the rays constituting the beam reflected by the
surface element are selected before decomposing the beam into its spectrum
with successive foci.
An apparatus for carrying out the method of the invention can comprise at
least one source of polychromatic light generating a beam including a
multiplicity of light rays of different wavelength but substantially equal
amplitudes, a lens capable of focusing each ray to a distinct focal point
characteristic of the respective wavelength, thereby establishing a line
of foci upon resolution of the beam into its discrete rays, the surface
element whose position is to be measured being disposed along this line.
Means are provided to analyze the spectrum of the beam reflected by the
surface element. This means determining the wavelength of the component of
this spectrum for which the luminous intensity is a maximum, and means for
determining the focal point at which the object is located from the
determination may be the ray whose reflected intensity is at a maximum
According to a feature of the invention, the focusing lens is a holographic
lens with concentric circular lines and this holographic lens can also
have concentric elliptical lines and can be placed in a plane inclined
with respect to a plane normal to the axis of the luminous beam. In
another alternative, the focusing lens for each state of a given
wavelength can be a reflective lens having high chromatic aberration and
in still another alternative at least one reflective lens can be located
between the focusing lens for each luminous ray of given wavelength and
the post at which the ray is focused. The apparatus can also comprise a
disphragm for selecting a portion of the wavelengths constituting the beam
of rays reflected by the surface element.
BRIEF DESCRIPTION OF THE DRAWING
The above and other objects, features and advantages of the present
invention will become more readily apparent from the following
description, reference being made to the accompanying drawing in which:
FIG. 1 is a diagram illustrating one embodiment of an apparatus for
carrying out the method of the invention;
FIG. 2 is an explanatory diagram in the form of a calibration curve; and
FIG. 3 is a diagram of a second embodiment of the apparatus.
SPECIFIC DESCRIPTION
The apparatus shown in FIG. 1 comprises a polychromatic light source 1
which generates a beam 1a of polychromatic light surrounding an axis 2.
This source can be, for example, a tungsten filament lamp, a concentrated
arc lamp or the like.
The axis of the beam is trained upon a surface element 4 whose position is
to be determined and which is capable of reflecting at least in part any
incident light thereon.
A holographic lens with concentric circular lines 3 of known type lies
perpendicular to the axis 2 in the path of the beam 1a to resolve this
beam into individual rays of respective focal lengths .lambda..sub.1,
.lambda..sub.2 . . . .lambda..sub.n having respective focal points
F.sub.1, F.sub.2 . . . F.sub.n along the axis if one considers only first
order diffraction.
The device thus generates a focal line F which intersects the object O at
the surface element 4 whose position is to be determined.
For this type of holographic lens the focal distance is, at a first
approximation, inversely proportional to the wavelength of the light ray
focused thereby.
A beam splitter for the luminous radiation, such as a semitransparent
mirror 5 directs the reflected beam from the surface element 4 and focused
thereon by the lens 3 as shown at 5a onto a concave diffraction grating 6.
This diffraction grating decomposes the reflected spectrum of beam 5a and
causes the individual rays of wavelength .lambda..sub.1, .lambda..sub.2 .
. . .lambda..sub.n, respectively to converge upon a linear network of
photodetectors 7. Each of these photodetectors can be considered to have
point-like dimension, i.e. dimension so small that for even focal point
P.sub.1, P.sub.2 . . . P.sub.n on this detector, a respective output
I.sub.1, I.sub.2 . . . I.sub.n representing the light intensity at that
point can be outputted. A lattice network or like point photodetector can
be a charged-coupled device CCD circuit.
The diffraction grating separates the light rays in accordance with the
relationship:
a.multidot.(sin .alpha.+sin .beta.)=k.lambda.
where
a is the distance separating two lines of the diffraction grating 6,
.alpha. is the angle of incident of the beam 5a,
.beta. is the angle of diffraction of the ray of the particular wavelength,
k is a whole number (for first order diffractions a=1), and
.lambda. is the wavelength of the ray contained in the incident beam and
focused on the respective point.
The rays of the spectrum of the reflected light, diffracted upon the grate
6 thus converge at points upon the point area of photodetectors P.sub.1,
P.sub.2 . . . P.sub.n which thus receive intensities of which the greatest
intensity will be that of the light ray reflected by the object O when the
surface element 4 is located at the focal point (e.g. F.sub.2) of that
ray. In this case, this light ray has a wavelength of .lambda..sub.2.
An analyzer 8, which can be any spectral analyzer, compares the intensities
l.sub.1, l.sub.2 . . . l.sub.n from the photodetectors and delivers a
signal representing the wavelength. Here 2, of maximum intensity is a
calculating circuit 9 which can have a transfer function corresponding to
a calibration curve as shown in FIG. 2 and which produces an output X,
representing the distance between the object O and a reference, here the
location of the lens 3, from the wavelength .lambda..sub.2. The transfer
function has been represented at r(.lambda.). A display may also be
provided as shown at 9a.
In this embodiment only a single light source has been provided although it
should be understood that a plurality of light sources can be used, for
example side-by-side in a plane substantially perpendicular to the axis 2
of the beam 1a. These sources can be aligned in one or more rows and can
each have a respective spectrum of different wavelengths or a single
wavelength. In that case, the intensities of the sources can be measured
so that the analyzer 8 can receive inputs from such measuring devices as
well and determine the relatively most intense reflected ray, i.e. the
reflected wavelength whose ratio at its source wavelength is greatest
among the wavelengths analyzed.
This system, moreover, allows selection of certain wavelengths to make up
the incident beam 1a. Such selectivity has been represented by an arrow 1b
through the light source 1.
This arrangement also allows comparison of the respective densities or
intensities of a plurality of images each corresponding to a group of
zones along one or more rows.
FIG. 2 shows a calibration curve for a holographic lens having 270
concentric circular lines and whose outermost line has a diameter of about
5 mm. The focal distances have been plotted along the abscissa and the
wavelengths of the light focused at these distances has been plotted along
the ordinate. To obtain this curve it is possible to focus 6 monochromatic
light beams of known wavelengths through the same lens to form calibration
points, thereby calibrating the lens. A mirror can be placed at each of
the focal points to determine the distance for each beam and the position
of the mirror is recorded at which the intensity of reflected light is a
maximum for each monochromatic source. The preferred monochromatic sources
are lasers.
The calculator produces a signal X=r(.lambda..sub.2) which is
characteristic of the position of the surface element 4 from a reference
point or on a reference scale. While the preferred reference point is the
lens 3, any other reference point or reference scale can be provided along
the focal line 4 with suitable adjustment of the transfer function
r(.lambda.).
For certain models of holographic lenses with circular lines it is also
possible to measure second order diffractions as well as diffractions of
greater than second order.
A second order diffraction focuses a portion of the rays of wavelength
.lambda..sub.1, .lambda..sub.2 . . . .lambda..sub.n at focal point F.sub.1
', F.sub.2 ' . . . F.sub.n ' along a secondary focal line F' located
between the focal line F and the lens. The intensity of the rays focused
along the line F' is much less than that of the first order focuses along
the line F. It is nevertheless possible to analyze the reflected light
spectrum for an object disposed at one of these secondary focal point.
Consequently, at least two focal ranges can be defined for each
holographic lens and two distinct measuring domains can be established for
the same detector.
A portion of the light beam emitted by the source 1 is reflected by the
holographic lens 3. This parasitic line may be superimposed by the light
reflected from the surface element 4 and thus represents a source of
single noise which may not be negligible. In a second embodiment of the
invention, this problem can be eliminated by replacing the lens 3 having
concentric circular line by a lens having concentric lines which are
somewhat elliptical and which is inclined at least several degrees with
reference to a plane normal to the axis 2 of the beam 1a. This arrangement
direct the parasitic reflections from the lens away from the mirror 5 and
out of the path of the reflected beam from the surface element 4.
Otherwise this device closely resembles the device shown in FIG. 1.
In a third embodiment of the invention a diaphragm 10 having a circular
orifice can be disposed between the sensor 5 and the concave diffraction
grating 6 of the spectral analysis system to select only a limited portion
of the reflected beam containing the wavelength .lambda..sub.2.
This diaphragm eliminates at least some of the rays of wavelengths which
are not focused on or close to the surface element since these rays appear
on the object in the form of disks of a diameter proportional to the
distance between the focal point of that ray and the surface element.
In other words by calculating only the centermost portion of the reflected
beam we need measure only the most intense zone which includes the same
focused on the object and excludes other rays which have focal points more
remote. This simplifies the analysis since the diaphragm intercepts most
of the reflected rays and thereby accentuates the wavelength
.lambda..sub.2 focused on the surface element.
A fourth embodiment of the invention can utilize a movable classical
optical system having at least one reflective lens and disposed between
the holographic lens 3 and the focal range F. This allows a single lens 3
to be used for a variety of different applications because the optical
system shifts the line F relative to the lens 3.
In a fifth embodiment of the invention, a reflective lens of high chromatic
aberration is utilized in place of the holographic lens 3 to focus the
rays of beam 2 on a plurality of focal points F.sub.1, F.sub.2 . . .
F.sub.n. The focal range of this reflective lens is obviously much shorter
than that of a holographic lens. Consequently, the two types of lenses are
complementary.
In a sixth embodiment of the invention, a holographic lens is used with
horizontal lines in the form of a cylindrical holographic lens. This type
of lens is distinguished from the circular line lens by the form of its
focuses, the focuses of the cylindrical holograph lens being in the form
of line segments parallel to the lines of the lens.
This embodiment can be used to measure the distance separating two adjacent
surface elements a and b which are not coplanar in this case. The spectral
analysis of the spectrum of the reflected bean detects the two frequencies
.lambda..sub.a and .lambda..sub.b which are focused respectively on the
surface elements a and the surface element b. The distance which is
determined, therefore, is the difference between the respective distances
for the surface elements and the lengths. These distances are determined
as previously described using a calibration curve for the parallel-line
lens.
This embodiment allows shifting of the position which the focal point is
determined and provides an indication of the lateral offset along the line
of separation of the surface elements a and b by a comparison of the
relative intensities of the wavelengths .lambda..sub.a and .lambda..sub.b
reflected by each of the surface elements.
FIG. 3 shows a seventh embodiment of the invention utilizing a multi-mode
optical fiber 26 whose core diameter is between about 10 and about 100
microns and which connects a measuring head 20 with an opto-electric
system 21.
The system 21 generates a polychromatic beam and analyzes the reflected
line from a surface element 31 disposed along a line F representing a
measurement range for the position of the surface element 31 to be
measured.
The system 21 comprises a polychromatic light source 22 which generates a
beam 22a which traverses a first reflective lens, i.e. a condensing lens,
which forms the parallel light beam 23a. The light beam 23a traverses the
semitransparent motor 32 and is collected by a second reflective lens 24
focusing the beam on the first end 25 of the optical fiber 26 fixed by a
connector 27 to the system 21.
A second connector 29 secures the opposite end 28 of the optical fiber 26
in the housing of the measuring head 20. The end 28 thus functions as a
point-light-source and directs a light beam 28a onto a holographic lens
30. Thus the light beam 28a incident upon the holographic lens 30, which
may be of any of the types previously described, is polychromatic and thus
contains rays of wavelengths .lambda..sub.1 to .lambda..sub.n.
As previously described, the lens 30 focuses each of these rays as a
functions of the respective wave length upon the line of foci F and, in
the case illustrated, focuses the line of wavelength .lambda..sub.1 at the
surface element 31 whose position is to be detected.
Similarly, the light reflected from element 31 is focused by the lens 30 at
the end 28 of the optical fiber 26. In this case, the end 25 of the
optical fiber forms a point source of the reflected rays from the surface
element 31 and this reflected radiation is directed by the collimating
lens 24 upon the semitransparent mirror disposed between the lenses 23 and
24 and directing the reflected beam, as beam 32a through the reflective
convergent lens 33 onto the curved diffraction grating 34 which is
analogous to the grating 6 of FIG. 1.
The wavelengths diffracted by the grating 34 are focused at different
points on the area of photodetector 35 which is analogous to the area of
photodetector 7 of FIG. 1 and can be a CCD.
The electrical signals from the area 35 are processed by the logic circuit
36 which performs the comparison of the spectral analyzer 8 and the
calculator 9 of FIG. 1.
It can be noted that in this embodiment, a diaphragm such as the diaphragm
of FIG. 1 need not be used because the window formed by the end 28 of the
optical fiber 26 has an effect analogous to that of the diaphragm insofar
as elimination of unfocused radiation from the surface element is
concerned.
For example, it is possible to measure the size of a mechanical object
utilizing a device of the invention. In this case, two measuring devices
are provided on opposite sides of the piece to be measured so that the
width of the piece across the surface elements of the two devices is
determined by the relationship:
x=d-X.sub.1 -X.sub.2
where
X is the dimension sought;
d is the distance separating the two measuring heads which may be of the
type shown in FIG. 3;
X.sub.1 is the distance separating the piece from the first measuring head;
and
X.sub.2 is the distance separating the piece from the second measuring
head.
The measuring devices of the invention also permit dynamic measurement
since modifications of the reflected spectrum can be monitored as a
function of time. Analysis of the reflected spectrum at regular intervals
permits measurement of the displacement of a surface element. This
application of the devices of the invention has been found to allow it to
be used to measure not only position but also speed or acceleration with
great precision, e.g. of an arm of a robot or for a tool or tool holder in
a machine tool or other manufacturing machine.
The applications of the devices of the invention which have been mentioned
should not be considered to be a complete list since the devices can be
used wherever position, distance or movement measurements without contact
are desirable.
Furthermore, the spectral analysis system mentioned above is only an
example since the skilled worker in the art can adopt any other
conventional spectral analysis system to a similar end. It is possible,
for example, to use a spectral analysis system comprising a diffraction
grating which is given an oscillatory movement for successively directing
each diffracted ray or wavelength on a respective unique photodetector or,
more commonly, for sequentially directing the diffracted rays upon a
single photodetector. In the latter case a sequencing arrangement is
required to establish which frequency is impinging upon the photodetector
at any time.
When the photodetector thus measured the maximum intensity, the position of
the grating represents the position of the surface element along the range
of foci of the lens and thus the distance separating the surface element
from the measuring head.
In still another spectral analysis system for the reflected light, the
diffraction grating can be fixed and the photodetector movable, the
principle being the same. Means other than a diffraction grating can be
used to decompose the reflected spectrum and by way of example we may
mention a dispersion prison on a then later spectral filter.
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