 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to an optical device for use in measuring the
proximity or nearness of a surface. The invention also relates to the
application of such a device to the plotting or surveying of the profile
or contour of a surface.
Such devices are used in numerous fields and in particular for controlling
robots, which automatically perform certain tasks. They make it possible
to obtain instantaneous information on the relative position of a member
relative to a surface and, if appropriate, the profile of the surface. The
movements of the robot, as well as the function fulfilled by the robot,
can consequently be automatically adapted at all times to the working
conditions which can evolve over a period of time.
When they are used for plotting the profile of a surface, such optical
devices are advantageously used in adoptive welding robots, as well as in
computer-assisted welding. Thus, these two procedures make it necessary to
know the spatial position and also certain geometrical parameters of the
welded joint. This knowledge is obtained as a result of proximity
measuring devices and profile plotting devices.
The prior art optical surface proximity devices are of two different types.
A first group of known optical sensors utilizes the so-called triangulation
principle. According to this principle, a light beam is transmitted
obliquely towards the surface to be detected, in order to form a light
spot thereon. The beam reflected and/or diffused by the surface of
focused, in order to form the image of the spot on a position finding
detector. The latter can in particular comprise a camera with a charge
transfer device. Such an optical sensor is marketed by the Swedish company
SELCOM under the trade name OPTOCATOR.
Like all optical devices, said device requires no material contact with the
surface to be detected. Moreover, its operating distance with respect to
the surface (called the flight height) is satisfactory. Finally, this
optical sensor is protected against electrical interference, which is
highly prejudicial in certain applications and particularly in the case of
welding.
However, triangulation sensors also suffer from the disadvantage of using a
complex, costly position finding detector. In addition, they have a
relatively limited passband.
There are also infrared optical sensors, in which the infrared radiation
emitted by a light-emitting diode is reflected by the surface onto a
detector, such as a photodiode or phototransistor. The detector supplies
an analog signal representative of the light flux reflected by the target
and in particular the distance between the sensor and the target. These
infrared sensors are simpler and less expensive than triangulation
sensors, whilst they also have a wider passband.
However, the responses of these sensors vary with the surface reflection
coefficient and with the angle of incidence of the incident beam on said
surface. Thus, they are difficult to use when the surface state of the
part and/or its orientation relative to the sensor vary, which is
generally the case in robotics.
The present invention specifically relates to an optical surface proximity
measuring device similar to the second type of existing optical sensors
described hereinbefore but which, contrary to the latter, makes it
possible to measure the distance separating the sensor from the facing
surface independently of the reflection coefficient of said surface and
the angle of incidence. As in the case of all optoelectronic sensors, the
sensor according to the invention also has the advantage, compared with
optical triangulation sensors, of replacing a complex, costly detector by
a bottom range optical detector, whilst also increasing the passband.
SUMMARY OF THE INVENTION
The present invention therefore specifically relates to an optical surface
proximity measuring device comprising a radiation emitter forming a light
spot on said surface and a receiver which is sensitive to the light flux
reflected by the surface through a focusing means, wherein the receiver
comprises two detectors associated with two focusing means having
different focal lengths, as well as means for determining the value of the
relationship between the light fluxes measured by each of the detectors,
said value being used to calculate the distance separating the device from
the surface.
Thus, according to the invention, a signal is obtained which represents the
distance and which is independent both of the surface reflection
coefficient and of the angle of incidence of the beam emitted by the
emitter.
In the definition of the invention as given hereinbefore and as given in
the remainder of the description, the expression "light flux reflected by
the surface" can either designate the beam reflected by the surface and/or
the beam diffused by the surface.
According to a first aspect of the invention, the incident beam is a
parallel beam, such that the dimensions of the spot are independent of the
distance separating the emitter from the surface. If it is also assumed
that the optical axes of the incident beam and the beam reflected onto the
receiver by the surface coincide, the light flux variations measured by
the detectors are then directly derived from the variations in the
distance separating the surface from the focusing means associated
therewith.
According to a second aspect of the invention, the incident beam is a
convergent beam. The dimensions of the spot on the surface then vary when
the distance separating the emitter from the surface varies. In the
aforementioned cast, i.e., the optical axes of the incident beam and the
beam reflected by the surface coincide, this phenomenon is added to the
first-mentioned phenomenon. The light flux measured by the detectors then
results from the simultaneous variation of the distances between the
surface and the focusing means associated with the emitter, on the one
hand, and between the surface and the detector, on the other hand.
According to a third aspect of the invention, which is combined with one or
the other of the two aspects referred to hereinbefore, the emitter beam is
angularly displaced relative to the beam reflected by the surface. Apart
from the phenomena referred to hereinbefore, an increase in the distance
between the detectors and the surface then has the effect of a lateral
displacement of the beam reflected by the surface with respect to the
detectors, a variable part of said beam being received on said detectors
of finite surface area. This effect is added to the aforementioned
phenomena. The variation in the light flux received by the detectors
resulting from the variation in the distance between the surface and the
detectors can be still faster.
Preferably, the measuring device according to the invention also comprises
means for controlling the intensity of the radiation emitted by the
emitter as a function of the light flux measured by one of the detectors,
so that said flux remains constant. Furthermore, the determination of the
value of the relationship between the light fluxes measured by each of the
detectors is facilitated.
The noise resulting from the ambient radiation can be eliminated by
equipping the device with at least one filter associated with the
detectors and centered on the emission wavelength. Moreover, means can be
provided for modulating the radiation emitted by the emitter to a given
frequency and for demodulating the signal supplied by each of the
detectors at said frequency.
According to a first embodiment of the invention, the device also comprises
a semitransparent plate positioned between said surface and one of the
detectors in order to deflect part of the light flux towards the other
detector.
According to another embodiment of the invention, the two detectors are
arranged on either side of the emitter, the optical axes of the emitter
and the detectors being positioned in the same plane and the normal to the
surface being located in a plane passing through the optical axis of the
emitter and perpendicular to the plane defined by the optical axes of the
emitter and the detectors.
According to a particularly interesting application of the invention, the
proximity measuring device as defined hereinbefore can be associated with
means for scanning the profile of a surface by said device, in order to
realize an optical device for plotting said profile.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described in greater detail hereinafter relative to
non-limitative embodiments and the attached drawings, wherein:
FIG. 1 shows diagrammatically a first embodiment of the distance measuring
device according to the invention.
FIG. 2 shows diagrammatically, in the form of a table, the position of the
beam reflected by the surface relative to each of the detectors of the
device of FIG. 1, upon varying the distance between the device and the
surface.
FIG. 3 shows in a response curve of each of the detectors of the device of
FIG. 1, as a function of the distance d separating the device from the
surface, for different values of the incidence angle .beta. of the
incident beam relative to the surface.
FIG. 4 graphically depicts the variations of the quotient q of the signal
supplied by each of the detectors, as a function of the distance d between
the device and the surface.
FIG. 5 shows diagrammatically another embodiment of the proximity measuring
device according to the invention, in which two detectors are placed on
either side of the emitter.
FIGS. 6a and 6b are, respectively a front view and a side view, showing the
application of such a proximity measuring device to the checking of the
profile of the surfaces of two members to be welded, during the automatic
welding thereof.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As illustrated in FIG. 1, the device according to the invention comprises
an emitter assembly 10 and a receiver assembly 12, all positioned in a
case (not shown).
The emitter assembly 10 comprises a light-emitting diode (LED) or a laser
diode 14 and two focusing lenses 16, 18. These two lenses make it possible
to focus the radiation emitted by the laser diode at a distance D.sub.0
from lens 18. As a non-limitative illustration, it should be noted that
the convergent beam focused by lenses 16, 18 can define a focusing
distance D.sub.0 of approximately 150 mm for a convergence angle of
approximately 10.degree..
The emitter 14 is chosen in accordance with the application in question and
e.g. emits infrared radiation between 830 and 850 nm.
In the embodiment shown in FIG. 1, the receiver assembly 12 comprises a
lens 20, an interference filter 22, a semitransparent plate 24, two lenses
26, 28 and two detectors 30, 32. Detectors 30 and 32 are photovoltaic
detectors supplying a voltage signal proportional to the light flux
impinging thereon.
For reasons which become apparent in the remainder of the description,
lenses 26 and 28, respectively associated with detectors 30 and 32, have
different focal lengths. However, the focal lengths of lenses 18 and 20
can be identical.
The optical axes defined by the emitter assembly 10 and by the receiver
assembly 12 form between them an angle .alpha. and its influence on the
value of the light flux received by each of the detectors will become
apparent hereinafter.
In the improved embodiment shown in FIG. 1, the amplitude of the light beam
supplied by emitter 14 is modulated by a circuit 34 to a given constant
frequency, e.g. 40 kHz. Conversely, the electrical signals supplied by
each of the detectors 30, 32 are demodulated in appropriate circuits,
respectively designated by reference numerals 36, 38. The demodulation
frequency of the signal supplied by the detectors is the same as the
modulation frequency of the incident beam and is supplied by modulator 34.
In the previously calculated example, this frequency is 40 kHz. In
parallel, the interference filter 22 is centered on the wavelength of the
radiation emitter by emitted 14.
These two combined characteristics make it possible to overcome background
noise resulting from the ambient radiation. It should be noted that the
protection of the proximity measuring device according to the invention
against ambient radiation is particularly important in the application of
such a device to plotting the profile of a surface in a welding robot.
According to an essential feature of the invention, signals D.sub.1 and
D.sub.2 supplied by each of the sensors 30 and 32 and demodulated by
circuits 36, 38 are input into a processing circuit 40, which determines
the ratio q between these signals. As will become apparent hereinafter,
through analyzing the operation of the device according to the invention,
ratio q is solely a function of the distance d separating the reference
point of the device, such as lens 18, from the surface S of member p.
In order to facilitate its subsequent exploitation, signal q is linearized
in a conventional linearization circuit 42 supplying an output signal V,
so that V=k.multidot.d, k being a constant. This signal V, which varies
linearly with distance d, can be directly used or can be used for
controlling a feedback according to the envisaged application.
Preferably, and as is also illustrated in FIG. 1, a feedback circuit 44 is
provided between the emitter 14 and one of the detectors. In the
represented embodiment, said detector is detector 32. Circuit 44 makes if
possible to control the amplitude of the control signal of emitter 14 in
such a way that the flux received by detector 32 remains constant, no
matter what the operating conditions. Moreover, the processing circuit 40
can be simplified, because the divisor of the division q=D.sub.1 /D.sub.2
remains constant.
All the electronic circuits 34, 36, 38, 40, 42 and to 44 are conventional,
well-known circuits, so that no detailed description will be given here.
An explanation will now be given of the operation of the surface proximity
measuring device according to the invention, with reference to FIGS. 1 to
4.
As the orientation of member P is of a random nature, the emitter assembly
10 projects a light spot onto the surface S of said member. The incident
beam is reflected from said surface, on which there is generally also a
diffusion of the projected radiation.
Part of the beam reflected by surface S of part P reaches detectors 30, 32
by way of the different optical components of the receiver assembly 12. In
view of the fact that the focal lengths of lenses 26, 28 differ, the light
spots in the planes of the detectors have different diameters, i.e. the
respective detectors receive different light fluxes.
FIG. 2 shows in tabular form the positions of the light spots formed at
each of the detectors 30, 32 on the light-sensitive surface S.sub.d
thereof, for three different positions of surface S with respect to the
device.
More specifically, the constant light-sensitive surface S.sub.d of each of
the detectors 30 and 32 is represented by a small hatched circle. The
surfaces at the detectors on which the light spots impinge are defined by
the large non-hatched circles.
In FIG. 2, the relative positions of these two surfaces are represented for
each of two detectors and for the following three characteristic positions
of the device of FIG. 1:
distance d is equal to a minimum distance D.sub.min corresponding to the
lower limit of the range of the device,
distance d is equal to the mean reference distance D.sub.0 for which the
radiation emitted by emitter 14 is focused on the surface of member P; and
distance d is equal to a maximum distance D.sub.max corresponding to the
maximum range limit of the sensor.
As illustrated in FIG. 1, the measuring range C is defined between values
D.sub.min and D.sub.max of distance d, said values being defined in an
approximate and not strict manner.
Upon studying FIG. 2 in greater detail, it can firstly be seen that for the
reference distance D.sub.0, the surface areas of the light spots level
with detectors 30 and 32 are equal to or exceed the area of the
light-sensitive surfaces of the detectors.
Moreover, it can be seen that on moving away from said distance D.sub.0 in
one the other direction, two phenomena are superimposed. The variation of
d compared with d.sub.0 leads to defocusing and consequently to an
increase in the surface areas of the light spots at the detectors. If this
first phenomenon is considered in isolation, the light flux received by
each detector will consequently decrease when the distance d moves in one
or other direction away from the reference distance D.sub.0. The variation
in one the other direction of distance d leads to a displacement of the
center of the light spots impinging on the detectors with respect to the
light-sensitive surface S.sub.d of each of detectors. This phenomenon is
due to the angular displacement .alpha. between the optical axes of the
incident beam and the beam reflected by the surfaces. If considered in
isolation, this displacement of the light spots also leads to a reduction
in the light flux received by each detector as the variation of d from
reference distance D.sub.0 increases. However, this reduction in the
immediate vicinity of distance D.sub.0 is only sensitive to the condition
that the surfaces of the light spots are substantially identical to those
of the sensitive surfaces of the detectors when distance d is equal to
D.sub.0.
These two phenomena are combined in the embodiment described hereinbefore
with reference to FIG. 1. Therefore, the responses of these two detectors
30, 32 vary as a function of distance d. However, it is readily apparent
that the invention is not limited to the simultaneous exploitation of
these two phenomena in the same device and in fact also relates to
comparable devices involving only one of these phenomena.
Thus, a further embodiment not shown according to the invention can be
realized in such a way that the incident beam and the beam reflected by
the surface have coinciding optical axes. For example, this result can be
obtained with the aid of semireflecting plates by making use of techniques
which are well known to those having ordinary skills in optics. However,
it is pointed out that such a device suffers from the disadvantage, as
compared with that of FIG. 1, that the semireflecting plates decrease the
useful light energy level 1 by a factor of four as compared with the level
of the signal emitted by the source and that parasitic radiation appearing
by reflection and diffusion at the plate reflecting the incident beam
towards the surface can bring about a deterioration in the signal-to-noise
ratio. Only the first aforementioned phenomenon is involved in the device
of the invention.
A more detailed analysis of this first phenomenon shows that the useful
light energy level is decreased by a factor of two. Thus, in the optical
assembly described with reference to FIG. 1, any variation in one the
other direction of distance d compared with distance D.sub.0 leads both to
a defocusing of the spot formed on the surface S by the incident beam and
a defocusing of the image of said spot impinging on detectors 30 and 32.
It is therefore obvious that the defocusing effect with respect to the
detectors persists when the cross sectional area of the spot formed by the
incident beam does not vary with distance d, i.e. when the incident beam
is a parallel beam.
In a further embodiment, (not shown) the device according to the invention
thus comprises an emitter assembly emitting an incident parallel beam.
This result can easily be obtained by a practitioner having ordinary skill
in the art using appropriate optics.
Obviously, this embodiment also has two variants. According to a first
variant, the incident and reflected beams have two coinciding optical
axes. However, according to the second variant, the optical axes of the
two beams are angularly displaced.
FIG. 3 shows in respectively continuous and broken line form, the responses
of detectors 30 and 32 as a function of distance d, when the incidence
angle .beta. of the incident beam with respect to the surface S is zero
and when said angle is equal to 60.degree.. All these curves were plotted
for a member P, whose surface has a reflection coefficient .eta. of 0.7.
Examination of the curves of FIG. 3 reveals that if the responses of the
detectors are indeed a function of distance d, as the analysis of FIG. 2
has shown, they are also dependent on the incidence angle .beta..
Moreover, on plotting the response curves of the detectors for different
values of reflection coefficient .eta., it can be seen that the responses
of the detectors are also a function of this reflection coefficient. More
detailed observation of these curves shows that they are homothetic
compared with the axis of the abscissas, whatever the reflection
coefficient .eta. and the incidence angle .beta..
Therefore, the division of the signals D.sub.1 and D.sub.2 supplied by
detectors 30 and 32 by the electronic processing circuit 40 results in an
output signal q=D.sub.1 /D.sub.2, which is a function of distance d, as
illustrated by FIG. 4. The curve q=f(d) is a non-linear function, so that
signal q is linearized by circuit 42 to supply a linear signal
V=k.multidot.d, as indicated hereinbefore. Thus, the device according to
the invention makes it possible to perform a contact-free measurement of
distance d.
Obviously, although interference filter 22, modulation circuit 34 and
demodulation circuits 36, 38 make it possible to protect the device from
ambient radiation, they are not indispensable to the invention and can be
eliminated in certain special applications in which said radiation is
negligible. Moreover, the feedback circuit 44 can be eliminated in a
simplified device.
FIG. 5 shows another embodiment of the device according to the invention,
which essentially differs from that of FIG. 1 as a result of the
arrangement of the detectors. Thus, in the device shown in FIG. 5, the two
detectors 130 and 132 are arranged symmetrically with respect to the
optical axis of the incident beam emitted by emitter 114. For
simplification purposes, the electronic part of the device, which can be
identical to that of the embodiment of FIG. 1, has not been shown.
In this configuration, the signals supplied by the detectors are comparable
to those supplied by detectors 30 and 32 in the embodiment of FIG. 1,
provided that the normal to the surface S of member P is in a plane
containing the optical axis of emitter 114 and is perpendicular to the
plane defined by the optical axes of the emitter and the detectors.
This embodiment has the advantage of being sensitive to the surface state
of the member at the light spot and is able to supply information on the
roughness or the local reflection coefficient. This information is
obtained by processing the alternating component of the measuring signal,
the direct component remaining a function of the distance.
Obviously, with the exception of semireflecting plate 24, all the optical
and electronic elements described hereinbefore with reference to FIG. 1
also apply to this embodiment.
Thus, FIGS. 6a and 6b diagrammatically show, in a non-limitative manner, a
particularly interesting application of the device according to the
invention to the plotting of the profile of a surface associated with a
welding robot. Reference numeral 350 designates the head of a welding
robot supporting a welding torch 352.
In this application of the invention, head 350 has a spindle 354 positioned
above the surface of members P.sub.1 and P.sub.2 to be welded so as to be
substantially parallel to the junction line L formed between these parts
and having an axis in the vertical plane of the welding torch 352. As
illustrated in FIGS. 6a and 6b, spindle 354 pivotably supports the case
356 of the proximity measuring device according to the invention.
Obviously, this device can be constructed in accordance with any of the
previously described embodiments. FIGS. 6a and 6b show in exemplified
manner the case where the device is that described with reference to FIG.
1. Thus, it is possible to see the emitter assembly 10 and the receiver
assembly 12.
Device 356 is supported by spindle 354 in such a way that the incident beam
strikes members P.sub.1 and P.sub.2 in front of the traveling welding
torch 352, as illustrated in FIG. 6b.
Scanning means 360, such as an electrical, hydraulic, pneumatic or similar
motor brings about an oscillation of device 356 about spindle 354, so that
the incident and reflected beams scan the profile of the welding zone
slightly upstream of the torch. This displacement corresponds to an angle
2.theta., which is approximately centered with respect to the vertical
plane passing through the junction line L. The voltage V supplied by the
proximity measuring device represents the distance d, so that its changes
during the displacement in accordance with angle 2.theta. represents the
profile of the surfaces of members P.sub.1 and P.sub.2 on either side of
line L.
In a variant (not shown) of the device illustrated by FIGS. 6a and 6b, the
scanning of the surface with the aid of the proximity measuring device is
carried out by different means. These means comprise guidance rails
permitting a linear displacement of the case transverse with respect to
the joint and means such as a motor or jack for controlling this
displacement. Thus, an angular scan is replaced by a linear scan.
In another constructional variant (not shown), the mechanical scan obtained
with the aid of the mechanism diagrammatically shown in FIGS. 6a and 6b is
replaced by an optical scan using known light beam deflection means. Such
an optical scan makes it possible to reach higher frequencies than a
mechanical scan.
It is clear that the information supplied by such a device makes it
possible, with the aid of appropriate data processing, to correct the
trajectory of the welding torch during welding, control during the welding
process the parameters such as the material quantity to be supplied during
a pass, plotting the section to be filled, or position the welding passes
with respect to one another during a multipass welding operation.
Obviously, this application is in no way limitative. Thus, it is possible
to envisage the use of the proximity measuring device according to the
invention for dimensionally checking a member during its manufacture. As
soon as the distance passes outside a predetermined tolerance range, this
is detected by the device according to the invention and defective parts
can be automatically rejected.
* * * * *
|
|
 |
|
 |
Description |
|
