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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a profile-measuring light probe, and more
particularly, to improvements in a light probe for measuring a profile of
a subject to be measured in non-contact therewith by optical means through
the utilization of a change in reflection factor in the proximity of a
critical angle of a light. The inventive light probe is suitable for use
as a probe in a coordinate measuring machine.
2. Description of the Prior Art
There have been popularized coordinate measuring machines, in each of which
a subject to be measured, which is rested on a mount, and a probe
supported on a main body of machine are relatively moved in a
tridimensional manner by a moving mechanism, whereby a relative movement
value between the probe and the subject to be measured is detected. This
movement value is processed in a predetermined manner by a data processing
unit, so that a height, a profile and the like of the subject can be
measured.
The probes for the coordinate measuring machines, have been mainly
contact-type probes. However, as the subjects to be measured come to cover
parts made of thin plastics, clay models and the like, necessity has been
voiced for the development of non-contact type probe.
As a consequence, there have heretofore been proposed various non-contact
type probes using optical means. However, there has been presented such a
problem that the non-contact type probe is coarse in resolution as
compared with the contact-type probe.
To improve the resolution, there has been proposed a probe using a critical
angle prism utilizing a change in an internal reflection factor in the
proximity of a critical angle when the total reflection occurs in a prism.
For example, in Japanese Utility Model Kokai (Laid-Open) No. 6707/1986,
there is disclosed such a probe that, to improve the resolution, four
internal reflections are performed in an critical angle prism.
However, since a light from the subject to be measured is collimated by an
objective lens and made to fall directly into a prism, a large critical
angle prism with high accuracy is needed, and such a disadvantage is
presented that optical parts are expensive. Furthermore, since the
critical angle is varied in accordance with the refractive index, such a
problem is presented that the refractive index should be accurately
measured during the manufacture of the prism. Further, since a change in
the distribution of quantity of light in an optical system of the probe to
a change in profile of the subject to be measured, i.e. the sensitivity is
not yet satisfactorily large, an electric amplitude should be increased to
make the resolution fine, and such a problem is presented that electrical
noises tend to mix thereinto.
SUMMARY OF THE INVENTION
The present invention has been developed to obviate the above-described
disadvantages of the prior art and has as its first object the provision
of a profile-measuring light probe capable of utilizing the properties of
the internal reflection of the light without using a large critical angle
prism with high accuracy.
The present invention has as its second object the provision of a
profile-measuring light probe, wherein the sensitivity in the optical
system is satisfactorily high and fine resolution is obtainable without
increasing the electric amplitude.
To achieve the aforesaid first object, the present invention contemplates
in a profile-measuring light probe comprising:
irradiating means for irradiating the subject to be measured with a fine
spot of light at an angle of 90.degree. or less;
an objective lens for forming an image of the fine spot in the proximity of
an image plane;
a relay lens having the focus on the image plane, for converting a diffused
light from the image of the fine spot into a generally parallel ray;
a window plate including an entrance prism having an angle .beta., for
taking in a generally parallel ray emitted from the relay lens, a flat
glass supported at an angle of inclination .alpha. and having a refractive
index n, for subjecting the taken-in generally parallel ray to multiple
internal reflections and transmitting the same, and an exit prism for
outputting the generally parallel ray thus transmitted to the outside,
with said .alpha. and .beta. being set to substantially satisfy the
following relationship
.alpha..perspectiveto.90.degree.-arcsin (1/n)
.beta..perspectiveto. arcsin (1/n); and
a position sensing detector for receiving the generally parallel ray from
the window plate, measuring the distribution of quantity of light and
outputting the measured result to the outside.
A specific embodiment of the present invention is of such an arrangement
that, between the objective lens and the image plane, there are provided a
polarizing beamsplitter for reflecting and dividing a light of an
S-polarized component normal to the plane of incidence and mainly
transmitting therethrough a light of a P-polarized component incorporated
in the plane of incidence, and a coarse position sensing detector for
measuring a position of an image formed by the light of the S-polarized
component thus divided and outputting the measured results to the outside,
thereby achieving the second object in addition to the first object.
Another specific embodiment of the present invention is of such an
arrangement that, between the fine spot of light and the image of the fine
spot, there is provided an interference filter to remove the lights of
distrubance, for selecting a light having a specific wavelength within the
wavelength region of the lights used by the irradiating means.
A further specific embodiment of the present invention is of such an
arrangement that a pin-hole for removing the lights of distrubance is
provided on the image plane.
A still further specific embodiment of the present invention is of such an
arrangement that the irradiating means is a combination of a laser diode
with a focusing lens.
A yet further specific embodiment of the present invention is of such an
arrangement that the magnification of the objective lens is about 5 times
or less.
A further specific embodiment of the present invention is of such an
arrangement that the relay lens is formed of a gradient index lens having
a small diameter and a focal length shorter than the focal length of the
objective lens.
A further specific embodiment of the present invention is of such an
arrangement that the position sensing detector is formed of a lateral
effect photodiode.
A further specific embodiment of the present invention is of such an
arrangement that the position sensing detector is formed of a CCD or MOS
type line sensor.
To achieve the aforesaid second object, the present invention contemplates
in a profile-measuring light probe for measuring a profile of the subject
to be measured through utilization of a change in reflection factor in the
proximity of a critical angle of a light, wherein a light of a P-polarized
component incorporated in a plane of incidence is mainly used.
To achieve the aforesaid second object, the present invention contemplates
in a profile-measuring light probe comprising:
irradiating means for irradiating a fine spot of light onto a subject to be
measured;
an objective lens for gathering the lights from the fine spot and
converting the same into a parallel ray;
a light transmitting member for transmitting a light from the objective
lens through the internal reflections;
polarization selecting means for mainly transmitting a light of a
P-polarized component incorporated in a plane of incidence therethrough to
the light transmitting member; and
a photo-electric transducer element for receiving the light from the light
transmitting member and transducing the same into an electric signal.
A further specific embodiment of the present invention is of such an
arrangement that the light transmitting member is formed of a critical
angle prism.
A further specific embodiment of the present invention is of such an
arrangement that the light transmitting member includes a flat glass
supported at a predetermined angle and having a light transmitted
therethrough by the internal reflections.
In the profile-measuring light probe according to the present invention,
differing from the conventional profile-measuring light probe, wherein a
light from a fine spot on the subject to be measured is converted into a
generally parallel ray and directly subjected to the internal reflections,
an image of a fine spot is produced by an objective lens, the light from
the image is converted into a generally parallel ray by a relay lens, and
the ray is subjected to the multiple internal reflections in a flat glass.
As a consequence, it becomes possible to measure through the utilization
of a change in reflection factor of a light inciding in the proximity of a
critical angle without using a large critical angle prism with high
accuracy. Furthermore, since the long flat glass is used, the number of
multiple reflections is increased, differing widely from the conventional
case, whereby, even with a small change in position of the fine spot, the
distribution of quantity of light varies widely on the position sensing
detector, so that the sensitivity in the optical system can be raised,
thus enabling the resolution to be finer than the conventional case.
Further, since it is easy to finely adjust the inclination angle of the
flat glass, the fine adjustment of the inclination angle of the flat glass
can be empirically and desirably performed in accordance with the
refractive index of the flat glass to select the optimal angle
experimentally for example, thus easily achieving the adjustment.
Furthermore, when the P-polarized component is used as the ray made to fall
into the flat glass, a change in reflection factor in the proximity of the
critical angle is abrupt, so that the resolution can be finer. Use of the
P-polarized component as the incident ray makes it possible to improve the
resolution and prove effective even when the conventional
profile-measuring light probe with no relay lens is used.
Further, when a change in position of the image by the S-polarized
component divided by the polarizing beamsplitter is measured by another
position sensing detector, measuring of the coarse resolution can be
performed, so that both measurements of the coarse and fine resolutions
can be conducted.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view showing the inner construction of one embodiment
of the profile-measuring light probe according to the present invention;
FIG. 2 is a chart in explanation of the state of transmission of the ray in
the probe of the above embodiment;
FIG. 3 is a chart enlargedly showing the state of transmission of the ray
in the flat glass as shown in FIG. 2;
FIG. 4 is a drawing of principle in explanation of the reflection and
refraction of the ray;
FIG. 5 is a chart showing changes in reflection factor to the incident
angles; and
FIG. 6 is a front view, inlcuding a partial block diagram, and showing an
example, wherein the profile-measuring light probe according to the
present invention is applied to a coordinate measuring machine.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiment of the present invention will hereunder be
described with reference to FIGS. 1 to 5.
FIG. 1 shows the inner construction of the embodiment of the
profile-measuring light probe according to the present invention.
This embodiment includes irradiating means 16 constituted by a laser diode
12 and a focusing lens 14. A fine spot 18 of focussed light is irradiated
by the irradiating means 16 onto a subject 10 to be measured, whereby an
image 20a of the fine spot 18 is made to focus by an objective lens 34
constituted by lenses 30 and 32. The magnification of this objective lens
34 is 1 to 5 times.
An angle made between an optical axis of the irradiating means 16 and an
optical axis of the objective lens 34, i.e. an angle of irradiation
.delta. is set at 90.degree. or less.
The light of the laser diode 12 is linearly-polarized and an electric
vector is directed in a direction of 12a, and the polarized state is
varied by reflection at the subject 10 to be measured.
An interference filter 36 is provided between the two lenses 30 and 32,
which constitute the objective lens. The interference filter 36 transmits
therethrough only a light of 780 nanometers as being the wavelength of the
laser diode 12, to thereby exclude the lights of disturbance i.e., light
from sources other than the laser diode 12.
A polarizing beamsplitter 38 is provided between the objective lens 34 and
the image 20a of the fine spot. The polarizing beamsplitter 38 divides the
ray into two including the P-polarized component 40 incorporated in the
plane of incidence and the S-polarized component 42 normal to the plane of
incidence. The reason why this polarizing beamsplitter 38 is used will be
described later.
Here, a flat plane normal to the optical axis of the objective lens 34 and
including the image 20a of the fine spot 18 when the fine spot 18 is on
the optical axis of the objective lens 34 refers to an image plane 50a.
This image plane 50a is provided thereon with a pin-hole plate 52 to
exclude the lights of distrubance. When the diameter of the fine spot 18
is 25 micrometer and the magnification of the objective lens 34 is 2
times, the diameter of a pin-hole in the pin-hole plate 52 is set at about
500 micrometers.
A relay lens 54 having its focus at this image plane 50a is provided
upwardly of the image plane 50a. In the embodiment, there is used a
gradient index lens having a diameter as small as 1.5 mm to 2 mm and a
focal length considerably shorter than the focal length of the objective
lens 34, as the relay lens 54.
Subsequently, a window plate 62 constituted by an incident prism 56 having
a small angle .beta., a flat glass 58 and an exit prism 60 are supported
at an inclination angle .alpha.. The prism 56, 60 and the flat glass 58
each has a refractive index of about 1.52 and attached and fixed to one
another by a glue. The angle .beta. is set at about 42.degree. in the
proximity of arcsin (1/1.52) as being the critical angle, and the
inclination angle .alpha. is set at about 48.degree. in the proximity of
(90.degree. the critical angle), whereby the ray falling perpendicularly
into the incident prism 56 is totally reflected by more than 50%.
The light from the image 20a of the fine spot is converted into a generally
parallel ray through the relay lens 54, falls into the incident prism 56,
is transmitted through the flat glass 56, repeating the internal
reflections, exits to the outside from the exit prism 60 to form the
distribution of light quantity 64. This distribution of light quantity is
measured by a position sensing detector 66a, passed through a processing
unit 68a, and outputted to the outside. When a flat glass having a
thickness W of 5 mm and a length L in the direction of height of about 60
mm is used as the flat glass 58, the number of the total reflections is
about 16 times.
As the position sensing detector (PSD) 66a, a lateral effect photodiode
capable of measuring the center of the distribution of light quantity is
used for example. However, a bi-cell photodiode also can be used as the
position sensing detector 66a.
On a side surface of the polarizing beamsplitter 38, there is provided a
position sensing detector (PSD) 66b for measuring the position of the
image 20b of the fine spot 18 by the S-polarized component, and the
measured result is outputted to the outside through a processing unit 68b.
This position sensing detector 66b refers to a course position sensing
detector in a sense that the detector 66b measures the profile of the
subject 18 by a coarse resolution as will be described later. As this
position sensing detector 66b, there is used a lateral effect photodiode,
too.
Description will hereunder be given of what change in output occurs when
the subject 10 to be measured makes a relative movement in the direction
of X-axis, with reference to FIG. 2.
In FIG. 2, a change in profile of the subject 10 to be measured appears as
a change in the direction of Z-axis, and, when the angle of irradiation
.delta. is not zero, the fine spot 18 of light is varied as 01, 02 or 03.
Here, if the image of 01 by the objective lens 34 is P1, then the image of
02 becomes P2 and the image of 03 becomes P3. Furthermore, images P1', P2'
and P3' are made to focus at positions symmetrical with the polarizing
beamsplitter 38.
In this case, the ray of the image P1 is converted into a parallel ray by
the relay lens 54, totally reflected through the flat glass 58, and forms
a distribution of light quantity Q1 on the position sensing detector 66a.
Subsequently, the light of the image P2 is inclined to become a generally
parallel ray, whereby, as the multiple reflections are made, the position
of the generally parallel ray is greatly deviated to thereby form a
distribution of light quantity Q2. As a consequence, when the subject 10
to be measured is displaced by .DELTA.Z in the direction of Z-axis, on the
position sensing detector 66a, the displacement is enlarged to .DELTA.Z1
which is a value proportional to the number of the total reflections in
the direction of Z1 axis. As a consequence, when the displacement value
.DELTA.Z1 of the center of the distribution of light quantity is outputted
from the processing unit 68a, .DELTA.Z can be calculated by an external
processing unit.
At the same time, on the coarse position sensing detectors 66b, the
position of the image is varied by .DELTA.Z2 in the direction of Z2 axis.
Since this .DELTA.Z2 is of the same order as .DELTA.Z and the enlarging
rate is small, it is convenient when .DELTA.Z is measured by the coarse
resolution in a wide measuring range. As a consequence, the coarse
position sensing detector 66b may be omitted.
Description will hereunder be given of the reason why the inclination angle
.alpha. is set in the proximity of (90.degree.--the critical angle), with
reference to FIG. 3. The light from the image P2 is converted into a
generally parallel ray by the relay lens 54. However, the image P2 is
positioned forwardly of the focus by .DELTA.t, whereby the ray becomes a
slightly diffused ray in actuality. In this case, since a ray t1 tends to
be totally reflected, the reflection factor thereof becomes high. Whereas,
a ray t2 has a small angle of incidence, whereby the reflection factor
thereof becomes small, so that the center of the final distribution of
light quantity Q2 moves by .DELTA.1 to the outer side relative to the
distribution of light quantity Q2' when the reflection factors of the rays
t1 and t2 are equal to each other.
On the contrary, since the light from the image P3 is made to be a slightly
focusing ray by the relay lens 54, the center of the final distribution of
light quantity Q3 thereof moves by .DELTA.2 to the outer side relative to
the distribution of light quantity Q3' when the reflection factors of the
rays are equal to each other. As a consequence, a value in the proximity
of (90.degree.--the critical value) is selected as the inclination angle
.alpha., the sensitivity is raised.
This effect becomes particularly significant when the angle of irradiation
.delta. shown in FIG. 2 approaches zero and the fine spots 01-03 approach
the optical axis, whereby the images P1-P3 of the fine spots approach the
optical axis. More specifically, this is because the center of the
distribution of light quantity Q2'; shown in FIG. 3, when the reflection
factor is not changed by the angle of incidence, approaches the center of
the distribution of light quantity Q1, so that a displacement value
.DELTA.Z1 of the center of the distribution of light quantity Q2 is
approximately determined by the value .DELTA.1 due to the effect of the
critical angle.
Description will hereunder be given of the reason why the polarizing
beamsplitter 38 is used in FIG. 1 and the action thereof, with reference
to FIGS. 4 and 5.
FIG. 4 shows the state where a ray i falling into a boundary surface 58a of
a flat glass 58 having a refractive index n at an angle of incidence
.theta. is divided into two including a reflected light r and a refracted
light t. The incident ray i and the reflected ray r are constituted by a
P-polarized component 40 and a S-polarized component 42, respectively.
Here, FIG. 5 shows changes in a reflection factor Rp of the P-polarized
component 40 and in a reflection factor Rs of the S-polarized component 42
when the angle of incidence .theta. is varied. In the drawing, designated
at .theta. is a critical angle and .theta.2 a Brewster angle, which are
expressed by the following equations.
.theta.1=arcsin (1/n) (1)
.theta.2=arctan (1/n) (2)
As apparent from FIG. 5, the P-polarized component is larger in change of
the reflection factor in the proximity of the critical angle. As a
consequence, when only the P-polarized component is selected by the
polarizing beamsplitter, the movement values .DELTA.1 and .DELTA.2 of the
centers of the distributions of light quantity Q2 and Q3 in FIG. 3 become
larger, so that the sensitivity of measuring can be further raised and the
resolution can be finer. Since the range of measuring becomes shorter
substantially in proportion to the resolution, when rough measuring is
made by use of the coarse position sensing detector 66b, a highly
effective result can be achieved.
Further, when the change in reflection factor in the proximity of the
critical angle is utilized, the larger the number of the internal
reflections is, the abrupter the effect is amplified. As a result,
according to the present invention in which the flat glass 58 shown in
FIG. 2 is used, the sensitivity of measuring is highly raised, and
finally, the resolution of the order of 0.1 micrometer has been achieved.
Additionally, in the above embodiment, the irradiating means has been
formed by combining the laser diode 12 with the focusing lens 14, however,
the arrangement of the irradiating means 16 need not necessarily be
limited to this, and an arrangement, wherein a visible range emission
diode of a high output or a He-Ne laser is used, may be adopted.
Furthermore, such an arrangement may be thought of that, in place of the
focusing lens, the objective lens is commonly used. Further, such an
arrangement may be adopted that a plurality of the irradiating means 16
are provided, whereby the irradiating means is switched in accordance with
the inclination of the subject 10 to be measured.
In the above embodiment, as the position sensing detectors 66a and 66b,
lateral effect photodiodes capable of measuring the centers of the
distributions of light quantity have been used. However, the arrangements
of the position sensing detectors need not necessarily be limited to
these, a CCD type and MOS type line sensors capable of measuring the
distributions of light quantity as they are, may be used.
Further, in the above embodiment, the polarizing beamsplitter 38 and the
coarse position sensing detector 66b have been provided, however, when the
resolution in the position sensing detector 66a need not be so fine, the
polarizing beamsplitter 38 and the coarse position sensing detector 66b
may be omitted. In this case, such an arrangement may be thought of that,
in place of the polarizing beamsplitter 38, a mere deflecting plate is
provided, whereby only the P-polarized component is transmitted to the
window plate 62.
Furthermore, it is desirable to set the magnification of the objective lens
34 to about 5 times because the objective lens 34 tends to detect a
speckle pattern when the magnification thereof exceeds 5 times or
thereabout. However, if the above arrangement is combined with a method of
applying electric filtering to an output signal, then the magnification
exceeding 5 times may be adopted.
Description will hereunder be given of an example of a system wherein the
profile measuring light probe according to the present invention is
provided on a coordinate measuring machine as an example of application of
the present invention with reference to FIG. 6.
In this system the subject 10 to be measured is rested on a mount 101 of a
coordinate measuring machine 90, and a light probe 92 according to the
present invention is engaged with the forward end of a spindle 102 of
Z-axis, to thereby measure a profile of the subject 10.
Position measuring transducers, not shown, are assembled into the
respective axes of a coordinate measuring machine 90, relative movement
values between the subject 10 to be measured and the light probe 92 are
counted by counters 110, 111 and 112, and calculated by a measuring unit
113.
Furthermore, driving signals for driving motors, not shown, are outputted
from a control unit 114 through drivers 115, 116 and 117, so that
measuring can be performed fully automatically. Power is supplied from the
control unit 114 to the probe 92 through a probe power source 120.
Outputted from the light probe 92 to a Z1 register 121 is a movement value
.DELTA.Z1 of the center of the distribution of light quantity as an output
from the position sensing detector 66a shown in FIG. 2. Furthermore, as an
output from the coarse position sensing detector 66b, a movement value
.DELTA.Z2 of the image is outputted to a Z2 register 122.
To start the measuring, the spindle 102 of Z-axis is vertically moved, so
that, first, a value of the Z2 register 122 reaches substantially zero.
Subsequently, the spindel 102 is finely adjusted so that a value of the Z1
register can reach a predetermined tolerance value, and data take-in by
the measuring unit 113 is started.
Thereafter, the spindle 102 is vertically moved so that the value of the Z1
register 121 reaches the tolerance value, with a slider 103 being moved in
the direction of X-axis. Similarly, a column 104 is moved in the direction
of Y-axis. Thus, the entire top surface of the subject 10 to be measured
can be scanned and values of Z relative to the respective values of X and
Y can be obtained, so that the profile can be measured. When there is a
stepped portion on the profile and the value of the Z1 register 121
overflows, the spindle 102 of Z-axis is adjusted so that the value of the
Z2 register 122 approaches zero, to make the rough positioning.
Subsequently, the value of the Z1 register 121 is used, so that the
efficient measuring can be achieved.
The light probe according to the present invention has been described from
the point of view of the application thereof to the coordinate measuring
machine. However, through the utilization of such a characteristic feature
of the light probe according to the present invention that
profile-measuring of high resolution can be achieved in non-contact with
the subject to be measured, this light probe can be used as the probe for
other measuring machines. When used as the probe for a roundness measuring
machine for example, this light probe features that no damages are given
to the subject to be measured because measuring is made in non-contact
therewith, and this light probe can be applied to the subejct made of a
soft material quality. Furthermore, there is a possibility of that this
light probe can be used as the probe for machines for measuring a
thickness of a metallic foil, a thickness of a material of coating and the
like.
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