Apparatus for manipulating laser beams, particularly power laser beams for use by robots

Abstract: The beam (F) is deflected by a first mirror (4) whose position is selectively adjustable (6, 7) to a second mirror (8) which preferably has a parabolic surface (9). From the second mirror (8) the beam is directed to a third, flat mirror (12) whose position is also adjustable (14, 15) so that the beam is focussed on the region (P) to be treated in a position substantially in alignment with the initial direction (X₁) of propagation. Improvements (19) in the system for supplying screening gas to the region (P) under treatment are also described.

Description: The present invention relates to apparatus for manipulating (typically focussing) laser beams and has been developed with particular attention to its use in robot systems for cutting, welding, etc. In this context, both lens systems and mirror systems are known for focussing power laser beams. Lens systems generally have the disadvantage that they are very vulnerable to phenomena such as scratching, etc. which make it necessary for the damaged lenses to be replaced completely. Mirror focussing system, for example with parabolic mirrors, have the advantage in this respect that, if they have superficial scratching, they can be polished which can restore the mirror surfaces to the desired operative conditions. On the other hand, mirror focussing systems, such as disclosed, e.g. in DE-A1-30 36 427, give rise to greater operative difficulties both as regards the focussing of the beam along the axis of the robot hand or wrist (with consequent repercussions on the numerical control of the robot's movements along its various axes) and because of the need to arrange the various mirrors in very strict optical alignment along the radiation path. The object of the present invention is to provide a powered-mirror device for manipulating laser beams which does not give rise to the aforementioned disadvantages. A further object of the present invention is to provide improvements in the distribution of the so-called screening gas in the region subject to the action of the laser beam. According to the invention, this object is achieved by virtue of apparatus having the characteristics quoted in the claims which follow. The invention will now be described, by way of example with reference to the appended drawings, in which: Figure 1 shows schematically the general principle of the operation of focussing apparatus with a parabolic mirror, Figure 2 shows a first embodiment of apparatus according to the invention intended, in particular, for use in an automated welding system, and Figure 3 shows a second embodiment of the apparatus according to the invention which is particularly applicable to an automated cutting system. In Figures 2 and 3, apparatus for manipulating (focussing) power laser beams generally indicated 1 is intended to be mounted, according to widely known criteria, on the movable end (the so-called hand) of a robot, not shown as a whole. In general, this hand has a principal reference axis X₁ which is intended to be kept generally perpendicular to the surface P which is being treated. The axis X₁ also constitutes the principal axis of a power laser radiation beam which enters the device 1 through an inlet aperture 2a in the metal casing 3 which encloses the device 1. The same casing 3 also has an outlet aperture 2b, also substantially aligned with the axis X₁, at its end opposite the aperture 2, that is at the end which is intended to face the surface P being treated. Drive means (not shown) are generally associated with the robot hand and enable the casing 3, and thus the device 1 as a whole, to be rotated about the axis X₁. A first mirror, indicated 4, has a flat reflective surface 5 located in the path of the laser beam which enters the apparatus 1 through the inlet aperture 2a. The mirror 4 is produced according to known criteria which do not need to be described in detail, particularly as regards the provision of cooling means. For the purposes of an understanding of the invention, the mirror may be considered to be produced, in theory, by the cutting of a cylindrical body with a principal axis X₄ along a plane generally inclined to the axis X₄: the inclined plane defines the surface 5 which is made reflective by a polishing treatment of known type. The reflective mirror 4 is mounted in a respective adjustment assembly 6 so as to be rotatable about its principal axis X₄ as a result of an adjustment action which can be carried out manually by means of a knob 7 or, alternatively, automatically by drive means not shown. The adjustment assembly 6 is mounted in the casing 3, for example by form coupling, so as to be generally pivotable about an axis X₆ which intersects the axes X₁ and X₄ at a point situated on the reflective surface 5; the axis X₆ is generally perpendicular to the axes X₁, X₄, that is, perpendicular to the plane of the paper as seen in Figure 2. The function of the mirror 4 is to deflect the laser beam, which is propagated in the direction indicated by the axis X₁, away from its original direction of propagation through an angle α of a magnitude of the order of 120°C. Naturally, the magnitude of the angle of reflection, like that of similar angles defined below, refers to the angle between the incident beam and the reflected beam. As a result of its reflection by the flat surface 5 of the mirror 4, the laser beam is deflected towards a further mirror 8 with a reflective surface 9. The surface 9 extends over an imaginary parabolic surface S (indicated in chain line) whose principal axis X S is parallel to the line joining the central point on the reflective surface 5 (which identifies the intersection of the axes X₁, X₄ and X₆) and the central point on the reflective surface 9. The mirror 8 (which is referred to in the following description simply as the parabolic mirror) is mounted in a fixed position relative to the casing 3 but can be separated from the apparatus 1 by a pull on its base support 10 which is fixed to the casing 3 by means of fixing elements such as screws 11 or the like. The parabolic mirror 8 is mounted on the casing 3 of the apparatus 1 so that its reflective parabolic surface 9 extends generally in a direction which is approximately parallel to the initial direction of propagation of the beam, identified by the axis X₁. The parabolic mirror 9 reflects the laser beam through a further angle β of approximately 60° which returns the beam towards its original direction X₁ of propagation until it falls on a further flat mirror 12 mounted in the casing 3 in an arrangement substantially similar to the arrangement of the flat mirror 4 but in a complementary orientation. More specifically, the mirror 12 can also be seen as being produced by the theoretical cutting of a cylindrical body with a principal axis X₁₂ along the inclined plane which identifies the reflective surface 13 of the mirror. The mirror 12 is mounted on a respective adjustment device 14 so as to be rotatable about the axis X₁₄. The device 14 is in turn movable relative to the casing 3 so as to be able to pivot about an adjustment axis X₁₄ which intersects the axes X₁ and X₁₂ which are both incident at point situated on the reflective surface 13. The axis X₁₄ is perpendicular to the axes X₁ and X₁₂. In this case the position of the mirror 12 can also be adjusted manually by means of a knob 15 or, alternatively, automatically by drive means not shown. The function of the mirror 12 is essentially to reflect the laser beam coming from the parabolic mirror 8 and to deflect it through an angle γ of the order of 120°, returning it into alignment with its original direction X₁ of propagation. In practice, after reflection by the flat mirror 12, the laser beam is again propagated in alignment with the axis X₁ and emerges from the device 1 through the outlet aperture 2b. The presence of the parabolic mirror means that, as it travels along its path within the device 1, the laser beam is also focussed. This effect is shown schematically in Figure 1 where it can be seen that the beam F, which initially is propagated parallel to the axis X S of the parabolic surface 9 of the mirror 8 and strikes the parabolic mirror 8, is focussed on a point W. Obviously, the flat mirrors 4 and 12 do not take part in the focussing mechanism. Their function is instead to bend, so-to-speak, the two branches of the path of propagation of the beam F, upstream and downstream of a parabolic mirror 8 respectively, so that the overall path of propagation is a straight line, aligned with the axis X₁, with a single "chicane" deflection in correspondence with the position of the mirror 8. Any error in the parallelism of the direction of propagation of the beam F and the axis X₁ at the inlet of the device thus leads to an error in the focussing of the beam, as shown schematically in Figure 1, where the path of propagation of a beam which is incident on the parabolic mirror 8 at an angle δ other than zero to the direction given by the axis X S of the parabola is indicated schematically in chain line at F′. In this case the beam reflected from the parabolic mirror 8 is dispersed around the focus W of the parabola instead of being focussed on it. The fact that the mirror 4 can be adjusted (that is, the device 6 on which the mirror is mounted can be pivoted about its axis X₆ and the mirror 4 can also be rotated about its principal axis X₄) enables the necessary parallelism to be achieved between the direction of propagation of the beam which is incident on the parabolic mirror 8 and the axis X S of the parabola which defines the reflective surface 9 of the mirror. More particularly, for the adjustment to be made, the mirror 8 can be removed from the casing 3 of the device and a flat mirror can be arranged in the reference plane 16 indicated by a thick line in Figure 2. The reference plane 10 is generally perpendicular to the direction of propagation of the beam reflected from the first flat mirror 4: it is thus possible to check (for example with a low-power He-Ne test-laser generator) that the laser beam injected into the apparatus 1 through the aperture 2a, after reflection from the surface 5 of the mirror 4 and the reflective surface of the flat test mirror 16 so as to be returned and reflected again by the surface 5 of the mirror 4, emerges correctly from the inlet aperture 2a of the apparatus 1 exactly parallel to its initial direction of propagation. This condition can be found by the selective adjustment of the flat mirror 4 (axis X₄, axis X₆) until the correct position of adjustment has been achieved. A similar adjustment of the mirror 12 (in this case also by rotation of the mirror about its principal axis X₁₂ and of the respective adjustment device 14 about its axis X₁₄) enables the beam emerging from the apparatus through the outlet aperture 2b to be focussed on the surface P under treatment in alignment with the axis X₁. The solution shown in Figure 3, which is intended mainly for application to a laser cutter, shows how two variants can be used in a structure generally identical to that described above with reference to Figure 2. The first variant concerns the possibility of replacing the parabolic mirror 8 by a flat mirror 8′, that is, by a mirror whose reflective surface 9′, whilst retaining an orientation generally identical to that of the reflective surface 9 of the parabolic mirror 8 of Figure 2, is flat. Naturally, in this case, the desired focussing effect is not achieved by reflection and must therefore be achieved by a lens 17 arranged in correspondence with the outlet aperture 2b. The second variant concerns the possibility of mounting the outlet assembly of the apparatus, constituted by the flat mirror 14 and the members associated therewith (including the lens 17, if any), on an assembly 18 which is translatable generally parallel to the beam leaving the mirror 8′. Due to the movement of the flat output mirror 12 the laser beam emerging from the device, instead of propagating along principal axis X₁, propagates (and is focussed) along an axis X₁′ parallel but distinct with respect to the principal axis X₁. This means that, when the device 1 is rotated about the principal axis X₁, the focal point of the laser beam output thereby describes a generally annular path about the axis X₁. This capability, which is used, for example, in laser cutting operations, enables cutting to be carried out along a circular path, not just at a point. Whichever solution is adopted, it can immediately be seen that the apparatus according to the invention enables the position of the focal point of the output laser beam to be adjusted precisely. This solution is found to be particularly advantageous in the reconfiguration of a robot laser, originally designed to operate with a lens focussing system, for use with a mirror focussing system. Generally, the replacement of lenses by mirrors would require a complete redifinition of the programs for controlling the movements of the robot. With the use of a system according to the invention, it suffices for the focussing (particularly of the output mirror 12) to be finely tuned (with the further possibility that the device may be arranged at will either as a laser cutting unit or as a welding unit), the control logic for the movement remaining unchanged. As is known, the interaction of a focussed power laser beam with a metal surface causes a hole (known as a "key-hole") to be formed in the bath of molten metal. Metal vapours currently known as "plumes" are released from the key-hole and must be removed from the working zone to prevent adverse effects. It is known to blow inert gases into the region under treatment in order to remove the "plume". Very often, however, the inert gases entrain a certain amount of air (and hence oxygen), causing oxidation and defects. Figure 2 shows improved apparatus generally indicated 19, which enables inert gas to be blown in to screen the region P on which the laser beam is acting without giving rise to the problems quoted above. The solution shown in Figure 2 has been illustrated, in particular, with a view to its use in a welding system. Essentially, the device 19 is constituted by a tubular nozzle with an inlet duct 20 to which an inert gas is supplied under pressure (for example 2-3 bars). The nozzle in question includes a central duct 21 which communicates with the inlet duct 20 through a cylindrical chamber 22. More precisely, the wall 23 of the chamber 22 is surrounded by an outer cylindrical casing 24 which creates an outer annular chamber 25 into which the inlet duct 20 opens. The outer chamber 25 communicates with the inner chamber 22 through ducts 26 provided in the wall 23. As well as the central duct 21, on its side facing the surface P, the nozzle has an outer opening 27 for ejecting the gas, which is usually closed by a gas-permeable metal gauze which surrounds the central duct 21 at a certain distance from its free end. The nozzle 19 is mounted on the device 1 so that its principal axis X₁₉ is at an angle of the order of 30° to the principal axis X₁. The free end of the central nozzle 21 is intended to face the surface P and to be kept at a short distance from that surface. The end is surrounded by a bowl- or spoon-shaped deflector 28 with an end part which extends approximately parallel to the surface P. The pressurised inert gas supplied through the inlet duct 20 is thus divided into two flows, that is to say: a central flow which passes through the central nozzle 21 into the deflector 28 to be deflected in a direction perpendicular to the direction X₁ of propagation of the beam so as to flow over the surface P parallel to the surface P itself, and an outer flow constituted by the gas which emerges from the nozzle 19 through the ring 27. The first, central inert-gas flow essentially has the function of removing the plume. The second flow, however, has the function of creating a ring of inert gas around the region under treatment. The gas which forms this ring, which will be entrained away from the working region with the removal of the plume by the first flow, acts positively to prevent air and oxygen from penetrating that region. The variant of Figure 3 (which is preferably intended to be used for cutting operations in which it is essential that the screening gas reaches the surface in a direction perpendicular to the surface) on the other hand, provides for the inert-gas supply duct 20 to open into a hollow conical body 29 which surrounds the outlet aperture 2b of the device. The casing 29 is mounted on the movable body 18 so that it remains coaxial with the beam emerging from the device under all conditions. At its lower end, the body 29 has a small aperture 30 (for example of the order of two millimeters) which is intended to surround the focussed beam which strikes the surface P. The casing 29 in practice behaves like a nozzle which projects the inert gas supplied through the opening 20 on to the region under treatment, the direction of blowing being perpendicular to the surface P.

Claims: Apparatus for manipulating laser beams, the apparatus receiving at its inlet aperture (2a) a laser beam (F) which is propagated in a given direction (X₁), the apparatus transmitting the laser beam substantially in alignment with that direction (X₁) at its outlet aperture (2b), the apparatus comprising: first reflective means (4), whose position is adjustable (X₄, X₆), for deflecting the laser beam for a first time through a first given angle (α) away from the given direction (X₁), second reflective means (8; 8′) for receiving the beam deflected by the first reflective means (4) and for deflecting the laser beam for a second time through a second given angle (β) towards the given direction (X₁), the reflective surface (9) of the second reflecting means (8; 8′) extending in a direction approximately parallel to the given direction (X₁), third reflective means (12), whose position is adjustable (X₁₂, X₁₄), for receiving the laser beam deflected by the second reflective means (8; 8′) and for deflecting the laser beam for a third time through a third given angle (γ) to a condition in which it is aligned with the given direction (X₁). Apparatus according to Claim 1, characterised in that the first given angle (α), the second given angle (β) and the third given angle (γ) have magnitudes defined between the incident beam and the reflected beam, of 120°, 60° and 120° respectively. Apparatus according to Claim 1 or Claim 2, characterised in that the first reflective means (4) and the third reflective means (12) have flat reflective surfaces (5, 13). Apparatus according to Claim 3, characterised in that the flat reflective surfaces (5, 13) are defined theoretically by the intersection of a plane with a cylinder of given principal axis (X₄, X₁₂), the plane being inclined to the principal axis (X₄, X₁₂). Apparatus according to Claim 4, characterised in that each of the flat reflective surfaces (5, 13) can rotate about the principal axis of the respective cylinder (X₄, X₁₂). Apparatus according to Claim 4 or Claim 5, characterised in that the first (4) and third (12) reflective means have associated adjustment means (6, 14) which can pivot the respective flat reflective surfaces (5, 13) about axes (X₆, X₁₄) perpendicular to the principal axes of the respective cylinders (X₄, X₁₂). Apparatus according to any one of the preceding Claims 1 to 6, characterised in that the second reflective means (8) have a parabolic reflective surface (9) which can focus the laser beam. Apparatus according to Claim 1 or Claim 7, characterised in that it includes a housing (3) which, in use, is traversed by the laser beam (F) with an inlet aperture (2a) for the laser beam, in that the second reflective means (8; 8′) are selectively removable from the housing (3) leaving accessible a reference plane (16) generally perpendicular to the direction of propagation of the beam deflected once by the first reflective means (4) and in that flat, reflective calibration means (16) are provided for location in the reference plane to enable the adjustment of the first reflective means (4) by observation of the back-reflection of radiation directed into the housing (3) through the inlet aperture (2a). Apparatus according to any one of Claims 1 to 6, characterised in that the second reflective means (8′) have a flat reflective surface (9′) and in that lens means (17) are provided for focussing the laser beam. Apparatus according to Claim 9, characterised in that the lens means (17) are arranged downstream of the third reflective means (12) in the path of propagation of the laser beam (F) through the apparatus (1). Apparatus according to any one of Claims 1 to 10, characterised in that the third reflective means (12) are generally translatable (18) in the general direction of propagation of the laser beam (F) deflected for the second time by the second reflective means (8, 8′), the translation of the third reflective means (12) enabling the path of propagation of the laser beam deflected for the third time by the third reflective means (8, 8′) to be translated selectively (X′₁) from the given direction (X′₁); the arrangement being such that the general rotation of the apparatus (1) around the given direction (X₁) imparts a generally circular path of movement to the laser beam (F) deflected for the third time by the third reflective means (12). Apparatus according to any one of Claims 1 to 10, particularly for applications in which the interaction of the laser beam with the surface (P) being treated gives rise to fumes (a plume) which are removed by the blowing of gas on to the region under treatment, characterised in that it further includes means (19) for distributing the gas, provided with: a first blow duct (21) with associated deflector means (28) for producing a gas flow close to the region (P) under treatment, and parallel to the treated surface (P) itself for removing the fumes, and a second blow duct (27) for creating, around the region being treated, a ring of gas which can be entrained by the gas flow parallel to the treated surface substantially without drawing air towards the treated region. Apparatus according to Claim 12, characterised in that the first blow duct is defined essentially by a tubular duct (21) generally inclined (X₁₉) to the given direction (X₁) and in that the second blow duct is defined essentially by at least one gas-ejection aperture (27) which surrounds the tubular duct (21). Apparatus according to Claim 12 or Claim 13, characterised in that the deflector means (28) are generally bowl- or spoon-shaped. Apparatus according to any one of Claims 1 to 10, characterised in that it includes a housing (3), which, in use, is traversed by the laser beam (F), with an outlet aperture (2b) for the laser beam, the outlet being intended, in use, to face the surface (P) under treatment, and in that it includes a nozzle element (29) which is generally tapered from the outlet aperture (2b) towards the surface (P) under treatment and in that means (20) are also provided for blowing a gas under pressure into the nozzle (29) in order to direct a gas flow oriented substantially in the given direction (X₁) on to the surface under treatment (P). Apparatus according to Claim 15, characterised in that partition means (17) are provided for substantially sealing the nozzle element (29) from the housing (3). Apparatus according to Claim 9 and Claim 15, characterised in that the partition means are constituted by the lens means (17).

Inventors: MARINONI GIORGIO, PAIRETTI BARTOLOMEO, MARINONI, GIORGIO, PAIRETTI, BARTOLOMEO
Applicants: COMAU SPA, COMAU S.P.A.
Application year: 1990
Main classification: B23K26
IPC codes: B23K26