Arc sensor provided with a transparent objective window having a contamination preventing construction

An arc sensor (10) to be attached to an arc welding torch for detecting a welding spot is provided with an optical detecting system having an optical position detector (14), an objective lens (18), and a transparent objective window (20) arranged in front of the objective lens, and a plurality of arc light screening plates (22 and 24) made of an opaque material, and having a light transmitting apertures (22a and 24a), the plurality of arc light screening plates (22 and 24) being arranged in front of the transparent objective window (20) and spaced from one another for screening an arc light but permitting the detecting light beam to pass therethrough.

TECHNICAL FIELD 
The present invention relates to an arc sensor for detecting welding 
positions, i.e., welding spots, by a scanning method, and attached to a 
welding torch of an arc-welding machine capable of an automatic welding, 
such as an automatic arc-welding machine or an arc-welding robot. More 
particularly it relates, to an arc sensor provided with a transparent 
objective window having a contamination preventing construction and 
capable of not only improving the measuring sensitivity thereof but also 
facilitating and simplifying a maintenance operation of the arc sensor, 
which comprises, as principal components, an optical welding spot 
detecting unit including therein a light emitting element and a light 
receiving element, an objective lens disposed in front of the optical 
welding spot detecting unit to project the light emitted by the light 
emitting element onto a region of a work to be welded and to focus the 
light when reflected by the region of the work to be welded, and a 
transparent objective window disposed in front of the objective lens. 
BACKGROUND ART 
Automatic arc-welding machines and arc-welding robots are used for 
unmanned, labor-saving automatic welding operations in, for example, an 
automatic welding line in an automobile assembly shop The welding torch of 
such,.automatic arc-welding machines and arc-welding robots is provided 
with an arc sensor for continuously detecting welding spots, to thereby 
achieve an accurate and precision arc-welding. The arc sensor has a light 
emitting element, a light receiving element and an objective lens, which 
are contained in a container disposed behind a transparent objective 
window made of a transparent material, projects a beam of light for the 
detection of welding spots through the transparent objective window onto a 
work to be welded, receives a beam of the light reflected by the work via 
the transparent objective window, protects the welding spot detecting 
system by the transparent objective window from an ingress of external 
foreign substances such as sputtered particles and soot generated during 
arc welding, and prevents adverse effects of the arc light on the 
transmission of the detecting light beam. The conventional arc sensor 
incorporates various devices including a transparent objective window made 
of a transparent resin or a heat-resistant glass, so that the attaching of 
sputtered particles to the surface of the transparent objective window is 
prevented, and employs a laser beam emitted as a detecting light by a 
semiconductor laser beam emitter and having a frequency within a specific 
frequency band, to thereby permit the detecting light to be transmitted 
through the arc light. 
Nevertheless, the conventional arc sensor of the above-mentioned 
construction has a drawback in that the accuracy and sensitivity of the 
detection are lowered due to a loss of the transparency of the transparent 
objective window resulting from not only the scorching of the transparent 
objective window by sputtered particles scattered from the surface of the 
work during arc welding but also a smoking of the window by soot from the 
arc-welding flux, and thus the transparent objective window must be often 
periodically cleaned. 
Further, the periodical cleaning of the transparent objective window 
requires manual labor to implement such a cleaning operation, and 
therefore, reduces the labor-saving effect of the unmanned, labor-saving 
assembly line. 
DISCLOSURE OF THE INVENTION 
Therefore, an object of the present invention is to solve the above 
problems by eliminating the troublesome periodical cleaning, and to 
provide an arc sensor fitted with a transparent objective window having a 
protective construction and capable of improving the welding spot 
detecting function of the arc sensor. 
In accordance with the present invention, there is provided an arc sensor 
for detecting a welding spot of a work to be welded and able to be 
attached to an arc welding torch, and including a detecting unit provided 
with an arc sensor able to be attached to an arc welding torch to detect a 
welding spot of a work to be welded. The detecting unit includes a 
detecting means provided with an optical position detector provided with a 
light emitting element for emitting a detecting light beam toward the work 
to be welded, and a light receiving element for receiving the detecting 
light beam when reflected from the work to be welded; an objective lens 
arranged in front of the optical position detector; and a transparent 
objective window member arranged in front of the objective lens. The arc 
sensor is characterized in that it further comprises at least two arc 
light screening plate means arranged in front of the transparent objective 
window member of the detecting means and made of an opaque material; the 
at least two arc light screening plate means each being provided with a 
light transmitting aperture formed therein for permitting only the 
detecting light beam to pass therethrough, respectively, and arranged to 
be spaced apart from one another. The detecting light beam emitted from 
the light emitting element is projected through the light transmitting 
apertures of the arc light screening plate means toward a predetermined 
region to be detected, and an arc light is prevented from impinging on the 
surface of the transparent objective window member by the two arc light 
screening plate means. Therefore, the welding position or spot detecting 
sensitivity can be maintained at a necessary level for a long time, and 
the maintenance of the arc sensor can be simplified. In addition, since 
contamination of the arc sensor due to penetration of the sputtered 
particles and soot can be reduced, a high detecting performance of the arc 
sensor having a high signal to noise ratio ( S/N ratio ) of the detecting 
light beam is obtained Further, a pressurized fluid chamber, preferably a 
compressed air chamber, is provided between the transparent objective 
window and the arc light screening plate means, and an air under pressure 
introduced from an external compressed air source into the compressed air 
chamber is spouted from the chamber through the light transmitting 
apertures of the arc light screening plate means toward the work to be 
welded. Therefore, the spouted air under pressure promotes a prevention of 
the penetration of sputtered particles and soot into the detecting system, 
and accordingly, the maintenance of the arc sensor can be further 
simplified and the detecting performance of the sensor can be enhanced.

BEST MODE OF CARRYING OUT THE INVENTION 
Referring to FIGS. 1 and 2, an arc sensor 10 according to an embodiment of 
the present invention includes a casing 12 having one open end and one 
rear wall 12a on which a welding spot detector 14 provided with a light 
emitting element and a light receiving element is mounted, and an electric 
cable 16 for transmitting a signal is connected to the welding spot 
detector 14. Within the casing 12 is disposed a lens 18 in front of the 
welding spot detector 14, to project a detecting beam of light such as a 
laser beam emitted by the light emitting element of the welding spot 
detector 14 onto a surface of a work to be welded, and to focus the light 
beam reflected by the work onto the light receiving element A transparent 
objective window 20 is disposed in front of the lens 18, to protect the 
detecting system from an ingress of sputtered particles and soot 
generating during the welding operation of an arc welding torch to which 
the arc sensor is attached. The transparent objective window 20 is 
preferably made of a transparent resin material or a heat-resistant glass. 
The construction and arrangement of the optical detecting system provided 
with the welding spot detector 14, the lens 18, and the transparent 
objective window 20 are conventional. 
In accordance with the present invention, the arc sensor 10 is further 
provided with a pair of inner and outer arc light screening plates 22 and 
24 arranged in the casing 12 in front of the transparent objective window 
20 in such a manner that a space remains between the arc light screening 
plates 22 and 24. The arc light screening plates 22 and 24 are made of an 
opaque, heat-resistant material, such as an aluminum alloy or an iron 
system material including a stainless steel, and screen an intense arc 
light generated at an arc-welding spot and prevent detrimental substances 
including sputtered particles and soot, from penetrating the welding spot 
detecting system. The arc light screening plates 22 and 24 are provided, 
at respective central portion thereof, with small coaxial apertures 22a 
and 24a, to permit the projected detecting light beam and the reflected 
detecting light beam to pass therethrough. Each of the small coaxial 
apertures 22a and 24a is formed as a small pin-hole, but may be formed as 
an elongated narrow slit suitable for a continuous scanning of a plurality 
of welding spots. In the present embodiment, the small apertures 22a and 
24a are formed as elongated narrow slits as illustrated in FIG. 2. A 
pressure chamber 26 is provided between the transparent objective window 
20 and the inner arc light screening plate 22, and a pressurized fluid 
such as air under pressure is supplied by a suitable pressure conduit from 
an appropriate external compressed air source into the pressure chamber 26 
via an inlet 28. The air under pressure is spouted from the pressure 
chamber 26 through the small apertures 22a and 24a, whereby sputtered 
particles and soot are prevented from penetrating the detecting system 
during the arc welding operation. Note, the pressure of the spouted air 
need not be high, i.e., the pressure may be such that the air under 
pressure is able to expel the sputtered particles and soot. 
In FIG. 3, the arc sensor 10 provided with the transparent objective window 
protecting structure is attached to a body of an arc-welding torch 40 of 
an arc-welding machine or an arc-welding robot. Namely, the detecting 
system is shielded from the arc light, sputtered particles and soot by the 
transparent objective window protecting structure. 
The detecting light beam emitted by the welding spot detector 14, and the 
reflected detecting light beam reflected by a work W travel through the 
transparent objective window 20, and the respective small apertures 22a 
and 24a of the arc light screening plates 22 and 24, as indicated by 
dotted lines, to scan and detect a detectable region D. During the 
scanning and detecting operation by the arc sensor 10, a control is 
conducted so that a welding rod 42 extending from an extremity of the 
arc-welding torch 40 is brought to the region D detected by the arc sensor 
10 and applies an arc welding effect to a welding spot of the work W 
within the detected region D. At this time, an intense arc light, 
sputtered particles and soot are dispersed substantially radially from the 
welding spot, and accordingly, the arc light travels toward the outer arc 
light screening plate 24 of the arc sensor 10, but only a small, limited 
amount of the arc light and the least amount of sputtered particles and 
soot enter the casing 12 through the small aperture 24a. Nevertheless, the 
arc light, the sputtered particles and the soot entering the casing 12 are 
subsequently blocked by the inner arc light screening plate 22, and thus 
the contamination of the surface of the transparent objective window 20 by 
a direct adhesion of the sputtered particles and soot to the surface of 
the transparent objective window 20 can be prevented, and a reduction of 
signal to noise ratio (S/N ratio ) of the detecting light beam due to an 
adverse affect by the arc light also can be prevented. Therefore, the spot 
welding detecting system disposed behind the transparent objective window 
20 is protected by the arc light screening plates 22 and 24. 
Moreover, a discharge of the pressurized fluid, i.e., a spouting of the air 
under pressure through the respective small apertures 22a and 24a of the 
arc light screening plates 22 and 24 from the pressure chamber 26, 
increases the protective effect of the arc light screening plates 22 and 
24. 
The foregoing arc sensor according to the embodiment of the present 
invention is provided with a pair of arc light screening plates 22 and 24. 
The arc sensor, however, may be provided with three or four arc light 
screening plates, as required The shape of the small apertures 22a and 24a 
of the arc light screening plates 22 and 24 may be altered as required For 
example, the respective small apertures 22a and 24a of the arc light 
screening plates 22 and 24 may be pin-holes instead of the illustrated 
elongated slits, when the arc-welding operation is to be performed at 
fixed spots on the work W. 
From the foregoing description, it will be understood that, according to 
the present invention, an arc sensor attached to the arc-welding torch of 
an automatic arc-welding machine or an arc-welding robot and detecting a 
welding spot can be protected from contamination by sputtered particles 
and soot to maintain a normal detecting performance of the arc sensor for 
a long time, while simplifying and facilitating the maintenance work needs 
to guaranty the normal detecting performance of the arc sensor. Thus, the 
arc sensor is able to maintain the welding spot detecting sensitivity at a 
high level for a long term use, enables the detecting light beam to be 
satisfactorily maintained at a high level S/N ratio for a long term use, 
and accordingly, the performance of the automatic arc-welding machine or 
the arc-welding robot can be improved.