Abstract:
A laser scanner system for removing a coating from a surface. The system includes a laser source for generating a laser beam; a focusing optic operative to receive, focus and re-direct the laser beam; a rotating multi-faceted mirror for receiving and reflecting the focused laser beam, wherein the rotating multi-faceted mirror repeatedly translates the reflected focused laser beam in one direction along an arc path and through a first focal point, and wherein passage of the laser beam through the first focal point results in divergence of the beam; a reimaging mirror for receiving and reflecting the divergent laser beam toward a work surface, wherein the reimaging mirror produces an image of the first focal point and its path; and wherein the rotating multi-faceted mirror and the reimagining mirror cooperate to produce a beam cross-over region having a minimal cross-sectional area relative to other points along the beam path, and wherein the beam cross-over region is located between the reimaging mirror and the work surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/289,826 filed on Dec. 23, 2009 and entitled “Polygonal Laser Scanner for Coating Removal,” the disclosure of which is hereby incorporated by reference herein in its entirety and made part of the present U.S. utility patent application for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to systems and devices utilizing lasers, and more specifically to a system for removing materials such as paint and other coatings from various surfaces, wherein the system includes a laser scanner having multiple cooperative optics having unique characteristics. 
         [0003]    The application of laser technology for the removal of coatings developed several decades ago and small hand-held devices (100-500 W) are now available commercially for small area coating removal tasks. “Coating” typically refers to all types of unwanted surface material found on a substrate, including paint, rust, oil, grease, adhesives, sealant, barnacles, radioactive contamination, chemical agent contamination, and the like. Laser technology offers numerous advantages over conventional methods for removal of coatings (e.g., media blasting, chemical stripping, etc.), including no use of hazardous materials, no required inventory of expendable material, minimal preparation of the object to be processed, greater precision of coating removal, higher coating removal rates, and minimal post-process cleanup. 
         [0004]    Options for large area coating removal with lasers are currently being developed. To meet the desired coating removal rates for large areas (such as large aircraft surfaces, ships, buildings, and bridges), laser power levels in the 5 to 10-kW range are suitable for applications involving paint coatings. The desired removal rates are in the range of about 1 to 3 ft 2 /min. The typical removal rate that can be achieved with currently available lasers is about 2 ft 2 /min (per 1 mil of coating thickness for each 1 kW of laser power delivered to the surface). This rate of removal presumes that the laser beam is delivered to the surface in an appropriate pattern and is scanned across the surface at rates that remove coating without alteration of the substrate when it is exposed. 
         [0005]    For delicate or unstable substrates, such as 0.020-inch thick aluminum, the system for scanning a laser beam over the surface should achieve clean coating removal without causing thermal damage, charring, or otherwise altering the substrate. Some success has been achieved with galvanometer-based oscillatory scanning mirrors; however, these scans are limited in surface scan speed to typically less than 10 m/s and suffer from dead zones at the end of the scan where the mirror decelerates, reverses, and reaccelerates. These dead zones may be eliminated in practice with beam blockers that limit the scan width and lose average power at the work surface. Other limitations of currently available oscillatory scanners are power handling capability (&lt;6 kW) and weight (&gt;60 lbs). Due to the limitations of known systems, there an ongoing need for an advanced laser scanner system that will meet the requirements of certain industrial or governmental applications requiring laser power levels up to 10 kW. 
       SUMMARY OF THE INVENTION 
       [0006]    The following provides a summary of certain exemplary embodiments of the present invention. This summary is not an extensive overview and is not intended to identify key or critical aspects or elements of the present invention or to delineate its scope. 
         [0007]    In accordance with one aspect of the present invention, a laser scanner system for removing a coating from a surface is provided. This system includes a laser source for generating a laser beam; a focusing optic operative to receive, focus and re-direct the laser beam; a rotating multi-faceted mirror for receiving and reflecting the focused laser beam, wherein the rotating multi-faceted mirror repeatedly translates the reflected focused laser beam in one direction along an arc path and through a first focal point, and wherein passage of the laser beam through the first focal point results in divergence of the beam; a reimaging mirror for receiving and reflecting the divergent laser beam toward a work surface, wherein the reimaging mirror produces an image of the first focal point and its path; and wherein the rotating multi-faceted mirror and the reimagining mirror cooperate to produce a beam cross-over region having a minimal cross-sectional area relative to other points along the beam path, and wherein the beam cross-over region is located between the reimaging mirror and the work surface. 
         [0008]    In accordance with another aspect of the present invention, a laser scanner system for removing a coating from a surface is provided. This system includes at least one laser source for generating a laser beam; at least one focusing optic operative to receive, focus and re-direct the laser beam; at least one rotating multi-faceted mirror for receiving and reflecting the focused laser beam, wherein the rotating multi-faceted mirror repeatedly translates the reflected focused laser beam in one direction along an arc path and through a first focal point, and wherein passage of the laser beam through the first focal point results in divergence of the beam; at least one reimaging mirror for receiving and reflecting the divergent laser beam toward a work surface, wherein the reimaging mirror produces an image of the first focal point and its path; and wherein the rotating multi-faceted mirror and the reimagining mirror cooperate to produce a beam cross-over region having a minimal cross-sectional area relative to other points along the beam path, and wherein the beam cross-over region is located between the reimaging mirror and the work surface; and a nozzle assembly through which the reimaged laser beam passes. The nozzle assembly further includes a first nozzle component, the interior surface of which tapers inward; a second nozzle component connected to the first nozzle component, wherein the interior surface of the second nozzle component tapers outward; and an aperture through which the laser beam exits the scanner. The aperture is located between the first and second nozzle components and is positioned in the vicinity of the crossover region. 
         [0009]    In yet another aspect of this invention, a laser scanner system for removing a coating from a surface is provided. This system includes at least one laser source for generating a laser beam; at least one focusing optic operative to receive, focus and re-direct the laser beam; at least one rotating multi-faceted mirror for receiving and reflecting the focused laser beam, wherein the rotating multi-faceted mirror repeatedly translates the reflected focused laser beam in one direction along an arc path and through a first focal point, and wherein passage of the laser beam through the first focal point results in divergence of the beam; at least one reimaging mirror for receiving and reflecting the divergent laser beam toward a work surface, wherein the reimaging mirror produces an image of the first focal point and its path; and wherein the rotating multi-faceted mirror and the reimagining mirror cooperate to produce a beam cross-over region having a minimal cross-sectional area relative to other points along the beam path, and wherein the beam cross-over region is located between the reimaging mirror and the work surface; and a nozzle assembly through which the reimaged laser beam passes. The nozzle assembly further includes a first nozzle component, the interior surface of which tapers inward; a second nozzle component connected to the first nozzle component, wherein the interior surface of the second nozzle component tapers outward; and an aperture through which the laser beam exits the scanner. The aperture is located between the first and second nozzle components and is positioned in the vicinity of the crossover region. Also included is a housing for containing the at least one focusing optic, the at least one rotating multi-faceted mirror, and the at least one reimaging mirror. 
         [0010]    Additional features and aspects of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the exemplary embodiments. As will be appreciated by the skilled artisan, further embodiments of the invention are possible without departing from the scope and spirit of the invention. Accordingly, the drawings and associated descriptions are to be regarded as illustrative and not restrictive in nature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, which are incorporated into and form a part of the specification, schematically illustrate one or more exemplary embodiments of the invention and, together with the general description given above and detailed description given below, serve to explain the principles of the invention, and wherein: 
           [0012]      FIG. 1  is an exploded, perspective view of an exemplary embodiment of a laser scanner in accordance with the present invention; 
           [0013]      FIG. 2  is a perspective view of the laser scanner of  FIG. 1  shown in an assembled state; 
           [0014]      FIG. 3  is a cutaway perspective view of the laser scanner of  FIG. 2  showing the relative positions of the internal components; 
           [0015]      FIG. 4  is a cutaway perspective view of the laser scanner of  FIG. 2  illustrating the changes in direction of the beam path through the device when in use; 
           [0016]      FIG. 5  is a cutaway side view of the laser scanner of  FIG. 2  illustrating the changes in direction of the beam path through the device when in use; 
           [0017]      FIG. 6  is a perspective view of the laser scanner of  FIG. 2  shown attached to a robotic arm and properly oriented relative to a particular workpiece; 
           [0018]      FIG. 7  is a graph showing laser paint removal conditions used in previously conducted research for scanned continuous and pulsed lasers; and 
           [0019]      FIG. 8  is a graph showing peak surface temperature as a function of surface scan speed. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Exemplary embodiments of the present invention are now described with reference to the Figures. Reference numerals are used throughout the detailed description to refer to the various elements and structures. Although the following detailed description contains many specifics for the purposes of illustration, a person of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following embodiments of the invention are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention. The present invention relates to a system for removing materials such as paint and other coatings from various surfaces, wherein the system includes a laser scanner having multiple optics. With reference now to the Figures, one or more specific embodiments of this invention shall be described in greater detail. 
         [0021]    As best shown in  FIGS. 1-6 , an exemplary embodiment of laser scanner  10  includes a metal housing or body  20  that further includes rear wall  22 , first side wall  24  having apertures  26  formed therein, second side wall  28 , front  30 , top portion  32 , angled support  34  having apertures  36  formed therein, conduit  38 , middle portion  40 , left support  42  having apertures  44  formed therein, right support  46  (not shown) having apertures  48  (not shown) formed therein, bottom portion  50 , angled support  52 , and aperture  54 . NPT plugs  56  are typically inserted into apertures  26  and at least one NPT fitting  58  is typically inserted into rear wall  22 . Laser assembly  60  includes fiber mounting bracket  62 , which is attached to rear wall  22  using connectors  64  and pins  66 . Shaft clamp  68  connects fiber adapter  70  to fiber mounting bracket  62  after fiber assembly  72  has been inserted into fiber adapter  70 . 
         [0022]    The first optic included in exemplary laser scanner  10  is a parabolic mirror or an asymmetric asphere lens or mirror  80 , which is disposed within angled support  52  in body  20  and held in place by mirror mount  82 . Asymmetric asphere mirror  80  is secured to mirror mount  82  by pins  84  and connectors  88 , while mirror mount  82  is secured to angled support  52  by pins  86  and connectors  90  (see  FIG. 1 ). The second optic included in exemplary laser scanner system  10  is a torodial reimaging mirror  100 , which is secured to left support  42  by connectors  102  and pin  104  and to right support  46  (not shown) by connectors  106  (not shown) and pin  108  (not shown). The third optic included in exemplary laser scanner system  10  is a multi-faceted rotating polygonal mirror  110 . Polygonal mirror  110  is secured to mirror spindle  112  by connectors  118 , which typically include a nylon lock and washers  120  and  122  (see  FIG. 1 ). Post  114  on mirror spindle  112  is adapted to receive the drive shaft of servo motor  182 , which extends through aperture  54  and shaft clamp  116  secures the drive shaft within post  114 . Motor assembly  180  and servo motor  182 , in particular, is used to rotate polygonal mirror  110  and is connected to rear wall  22  of body  20  by connectors  184 . 
         [0023]    Nozzle assembly  130  includes top portion  132  having an entrance port  134  and inwardly tapered internal passage, divider  136  having an aperture  138  formed therein, and bottom portion  140  having an exit port  142  and an outwardly tapered internal passage. Aperture  138  is essentially an aerodynamic opening that permits flowing gas from the interior of the device to escape, thereby reducing or minimizing the introduction of dirt and other contaminants into the interior of the laser scanner. The fundamental nature of the re-imaged, polygon-scanned, laser beam permits this aerodynamic window to be small, thus improving its resistance to contaminant intrusion. In some embodiments of the present invention, a shutter  139  is included for selectively closing aperture  138 . As best shown in  FIG. 1 , laser protection cover  150  is secured to body  20  by connectors  152  and nozzle jet  154  is secured to body  20  by connector  152 . The internal components of system  10  are protected by polygon cover plate  158  and main cover plate  160 , which are both secured to body  20  by connectors  162 . Cover plates  158  and  160  are typically metal and are shown in the Figures as being transparent only for purposes of illustration. Flow regulator  172 , tube fitting  174 , T-connector  176 , and tube adapter  178  are components of a gas delivery system that may be used to rotate multi-faceted polygonal mirror  110  if motor assembly  180  is absent or not utilized. In some embodiments of the present invention, an exhaust system is included for removing debris generated during the operation of system  10 . As shown in  FIG. 6 , exhaust nozzle  190  is mounted near bottom portion  50  and is connected to one or more ducts and exhaust motors (not shown) for quickly and effectively removing particulate matter from substrate  200 . Exhaust nozzle  190  is typically of light weight metallic construction and may contain internal air delivery features to assist in complete combustion of the evaporants from the work surface whose coating (e.g. paint) is being removed. In the embodiment shown in  FIG. 6 , laser scanner  10  is controlled by a robotic arm  210  and is attached thereto by bracket  212 . In other embodiments, laser scanner  10  is configured as a hand-held device that may be operated manually. 
         [0024]    With regard to the function of the laser scanner of the present invention, an input laser beam obtained from a beam delivery fiber or from a beam delivery tube is focused by aspheric mirror  80 . The focal spot from this focusing optic may be circular, elliptical, or rectangular in shape, as determined by the figure (i.e., geometry) of the optic itself. As shown in  FIGS. 4-5 , the laser beam is produced by laser assembly  60  and is directed along path A. The focused laser beam is then reflected from aspheric mirror  80  along path B wherein it eventually impinges on multi-faceted, polygonal mirror  110 . The facets of polygonal mirror  110  may be either flat or cylindrical in figure. They may also be tilted at multiple angles relative to the polygon&#39;s axis of rotation such that the resultant scanned laser beam path can be controlled in at least two directions. As previously indicated, rotation of polygonal mirror  110  is provided either by a high velocity flow of air or other gas or by electric motor  182 . As polygonal mirror  110  rotates, the reflected, focused laser beam is repeatedly translated in one direction along an arc path toward reimaging mirror  100 . After reflecting off rotating polygonal mirror  110 , the laser beam passes through a first focal point along path C (see  FIGS. 4-5 ) and diverges toward reimaging mirror  100 , which is typically spherical. Reimaging mirror  100  produces a new image (which may be an enlarged image) of the first focal point and its path. This new image is then directed along path D, through nozzle assembly  130  and onto the work surface, i.e., substrate, where it is repeatedly scanned in one direction as the polygonal mirror rotates. In one exemplary embodiment, the beam spot on the work surface is substantially elliptical, with the elongation of the normally circular spot provided by either a toroidal surface on reimaging mirror  100  or a cylindrical surface on first focusing optic  80  or both. 
         [0025]    An important feature of the scanner of the present invention is that the geometric parameters are selected such that the laser beam spot at the work surface follows a substantially flat path to maintain minimum or constant spot dimensions on a flat work surface (referred to as a “flat field condition”). If the surface has significant curvature in the fast scan direction, system parameters can be adjusted in real time to accommodate for the curvature. In particular, a motorized mechanism may used to make small adjustments in the distance between the intermediate focal plane and the polygon axis to create a curved process trajectory. 
         [0026]    For large area paint removal, high average power levels are required to achieve reasonable rates of removal. Since reliable and economical repetitively pulsed lasers are not available at the 5 to 10 kW average power level, continuous lasers are most appropriate for this task. The scanned continuous laser beam effectively provides pulses at the work surface as discussed below.  FIG. 7  provides a graph showing previous laser research conditions for various paint removal efforts. The graph shows the effective pulse width at the surface as a function of the beam irradiance (W/cm 2 ). Notably, if the irradiance is too small (either low power or large spot areas), the paint tends to char and not remove cleanly. There is a wide range of operating irradiance levels where the paint ablates cleanly. At very high irradiance the beam is blocked by air plasma. Most successful paint removal has occurred at a fluence level of about 6 J/cm 2 , whether an effective pulse from a scanned continuous laser or a pulsed laser. An advantage of higher laser power is the possibility of achieving higher surface irradiance levels than currently available. Higher irradiance levels may improve primer removal rates in the case of the fiber laser. 
         [0027]    It is desirable to have an irradiance at the work surface greater that 10 5  W/cm 2  to achieve clean ablation. At the same time, the scan of the pulse across the surface should be fast enough to avoid substrate alteration when the substrate is exposed. This is illustrated by the thermal estimate shown in  FIG. 8  for a CO 2  laser. The graph shows an estimate of the peak front surface temperature reached locally on the surface of a chromate conversion coated (CCC) aluminum surface with a beam of 1-mm width scanned at various surface scan speeds perpendicular to the long dimension of the spot at 153 kW/cm 2 . Scan speeds less than 10 m/s lead to peak front surface temperatures near those that will cause alteration of the substrate. 
         [0028]    Motion of the entire scanner head perpendicular to the fast scan direction (by robotics, for example) provides area coverage and controls the bulk heating of the substrate, Slow sweeps of the scanner head remove more paint per pass, but risk higher bulk substrate heating when the substrate is exposed. High speed sweeps of the scanner head limit bulk substrate heating but may require more passes of the scanner head over the work surface, A variety of strategies may be employed to achieve high removal rates that may include a combination of robot program sweep speed variations and sensors to shut down laser power when substrate is detected. 
         [0029]    In one embodiment, the polygonal scanner of the present invention includes a power handling capability of up to 10 kW; a laser wavelength of 1070 nm (fiber laser, also compatible with CO 2  laser operation); a surface scan speed of up to 50 m/s; beam spot dimensions at the work surface of 1.4 mm by 4 mm elliptical; a surface irradiance of 114 to 228 kW/cm 2  for power levels of 5 to 10 kW; a scan width of 140 mm; a laser interface having a standard QBH connector; and a weight of &lt;40 lb. 
         [0030]    For optimum performance of laser coating removal for certain applications, it may be necessary to sense in real time the progress of coating removal including detection of the revealing of the primer coat and revealing of the bare substrate itself. Laser device technologies to be employed in coating removal can change beam power on time scales of the order of 50 μs, which, for the scanner of the present invention, is of the order of the time required for the beam spot on the surface to move one beam width. Having appropriate sensing technology incorporated into the scan head permits adjustment of the laser power in real time when substrate is exposed. Multiple sensing approaches including on-axis and off-axis views of the surface are compatible with the present invention. Viewing through the effluent and filtering out plume glare appropriately are addressed by the scanner of this invention. 
         [0031]    With regard to effluent removal, effluent is swept out of the beam path to both avoid laser beam attenuation and route the effluent efficiently to a vacuum collection system. In the past, air knives in combination with large vacuum ducts have worked fairly well. Incorporation of appropriate nozzles and ducts are aspects of the system of the present invention. Another important consideration is the conditions necessary to minimize organic material in the efluent that can combust downstream. The flow parameters of this scanner are varied to achieve high effluent capture efficiency and low organics while maintaining efficient coating removal. 
         [0032]    A first important aspect of the present invention is the inclusion of all reflective optics. In prior art systems, high-thermal conductivity metal mirrors are typically used for transporting, shaping, and scanning the laser beam. Transmissive optics are more easily damaged by high laser beam irradiance and are usually made of low thermal conductivity material. Transmissive material has low absorbance, but can still heat, expand, and distort the laser beam because thermal conductivity is relatively low. Metal reflectors are more damage tolerant and less expensive to fabricate. The noble metals (silver and gold) and copper and aluminum are acceptable reflectors, with copper and aluminum being preferable. If reflective coatings are used, they are fabricated to withstand high laser irradiance. The use of metal reflectors permits using the same scanner for lasers having different wavelengths, which is generally not possible with transmissive optics. 
         [0033]    A second important aspect of the present invention is aerodynamic beam exit aperture  138 . The laser beam path layout includes a crossing point or cross-over region within or near aperture  138  (see  FIG. 4 ) where the scanning beam is confined to a small cross-sectional area independent of the polygon rotation angle. The cross-over region is located between reimaging mirror  100  and the substrate work surface  200 . By locating aperture  138  at the cross-over region, an aerodynamic opening may be implemented at that location for excluding dust and debris from the scanner enclosure without a transmissive window. Most known high power laser scanners include large, expensive transmissive windows to protect the optics inside the scanner housing from dust and debris. These windows are often damaged by laser heating of deposited dust or debris. The aerodynamic aperture  138  of this invention is simply an open port with gas (air, nitrogen, or inert gas) flowing out of the housing to deflect dust and particulate debris. A closure or shutter  139  for aperture  138  maybe used to exclude dust and debris from the optics in the scanner housing (i.e., body  20 ) when the laser is not operating. An electrical interlock switch on shutter  139  may be included to prevent the laser from operating when shutter  139  is in the closed position. 
         [0034]    A third important aspect provided by some embodiments of the present invention is the inclusion of a gas driven polygonal mirror  110 . Most prior art scanners employ motors or galvanometers that require electrical power and add mechanical complexity to the system. The polygon scanner of this invention may be configured to include a simple gas jet (air, nitrogen, or inert gas) directed at recess features on polygonal mirror  110  to impart rotational motion of the mirror. A simple proximity sensor (or the like) may be used to sense the rotational speed and for controlling the flow or a pressure valve in the gas train may be used to maintain a substantially constant speed. The gas jet that drives polygonal mirror also serves to cool the polygon and the other optics. 
         [0035]    A fourth important aspect of the present invention are the tilted facets included on polygonal mirror  110 . To minimize the peak temperature of the substrate surface occurring when substrate  200  is exposed to the scanned beam, it is useful for each scan location of the beam on the work surface to be offset from the previous scan of the beam in a direction perpendicular to the scan direction. This is accomplished by tilting each facet by a slight angle lateral to the plane of the wheel. In this manner, deposited heat will have more time to thermally conduct into the substrate prior to another scan exposure on the work surface. Each rotation of the wheel may expose N different strips on the surface, where N is the number of facets on the polygon wheel. The separation of the strips is dependent on the tilt angle difference from facet to facet. 
         [0036]    A fifth important aspect of the present invention is the beam directing and focusing asymmetric asphere mirror  80 . This mirror combines two functions into one optical element. First the mirror collects all of the beam rays coming from the laser and focuses them to a small spot at an intermediate focal plane and second it accomplishes a right angle turn of the laser beam. The latter is important in two respects: (i) for robotic processing of aircraft with low ground clearance, it permits the scanner to fit in small areas; and (ii) for manual processing of surfaces it permits ease of cable management. The mirror requirements are unique and the solution is a special diamond turned surface that has been optimized to meet these requirements while maintaining optical quality of the beam. 
         [0037]    A sixth important aspect of the present invention is reimaging mirror  100 . Reimaging mirror  100  receives the beam reflected from polygonal mirror  110  and reimages the spot formed at the intermediate focal plane to substrate  200 . At the same time, a toroidal correction to the approximately spherical mirror stretches the original circular spot at the intermediate focal plane into an ellipse at the work surface. The elliptical spot at the work surface enables control of the paint removal process while limiting peak surface temperatures on bare substrates when exposed. 
         [0038]    A seventh important aspect of the present invention is the real-time adjustable field curvature feature. This scanner maintains beam focus on the work surface over the length of the scan. Scanning curved surfaces includes a real time adjustment of the distance between the polygon and reimaging mirror to maintain focus on a curved surface. 
         [0039]    An eighth important aspect provided by some embodiments of the present invention is the inclusion of a rotating joint (not shown in the Figures), which turns the beam at a right angle to the beam entrance direction. This is done while maintaining azimuthal symmetry in the laser beam as it approaches polygonal mirror  110 . This allows a rotating joint to be placed in the housing so that the scan direction can be easily changed by rotating the axis of rotating polygonal mirror  110  without disturbing the laser beam input optical train (optical fiber or articulated arm). 
         [0040]    While the present invention has been illustrated by the description of exemplary embodiments thereof, and while the embodiments have been described in certain detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to any of the specific details, representative devices and methods, and/or illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicant&#39;s general inventive concept.