Patent Publication Number: US-2003222143-A1

Title: Precision laser scan head

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The subject matter disclosed generally relates to the field of laser beam scanners.  
       [0003] 2. Background Information  
       [0004] Manufacturing process equipment may contain a laser to perform work on a piece part. For example, laser micro-machining equipment utilize lasers to ablate material from the piece part. Such processes include the step(s) of scanning a laser beam across a piece part. The scanning process is performed by a laser scanner.  
       [0005]FIG. 1 shows a laser scanner  1  of the prior art. The scanner  1  redirects and moves a laser beam  2  along a linear path. The laser beam  1  is generated by a laser  3  and reflected by bending mirrors  4 . The system  1  includes a first fast steering mirror (FSM)  5  that can be tilted to change the direction of the laser beam  1 . The beam  1  is directed through a scanning lens  6  located at the output of the scanner  1 .  
       [0006] The FSM  5  includes a mirror  7  that is tilted by one or more actuators  8 . The actuators  8  are driven by a mirror controller  9 . The controller  9  also receives position feedback information from a sensor (not shown) that measures the angular position of the mirror  7  relative to the fixed support structure. The controller  9  processes both the input commands and the feedback signals to generate output signals that drive the actuators  8 , tilt the mirror  7  and scan the laser beam  2 .  
       [0007] While the feedback signals and controller servo algorithms may insure that the mirror  7  is at the proper tilt angle, the system shown in FIG. 1 does not compensate for positioning errors separate from the tilt angle of the mirror  7 . For example, the output beam angle from the laser  3  may change over time. The shift in the output angle will result in error in the position of the output beam even though the mirror  7  is at the proper orientation.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008] A light beam scanner that includes a beam centering device that positions a light beam onto a beam scanning device.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] FIGS.  1  is a schematic of a beam scanner of the prior art;  
     [0010]FIG. 2 is a schematic of a beam scanner;  
     [0011]FIG. 3 is a schematic of an embodiment of the beam scanner;  
     [0012]FIG. 4 is a schematic of an alternate embodiment of the beam scanner;  
     [0013]FIG. 5 is a schematic of an alternate embodiment of the beam scanner;  
     [0014]FIG. 6 is a schematic of an alternate embodiment of the beam scanner.  
    
    
     DETAILED DESCRIPTION  
     [0015] Disclosed is an optical beam scanner. The scanner includes a beam centering device that directs a light beam onto a beam scanning device. The beam centering device can compensate for positioning errors in the light beam. The scanning and centering devices may each have feedback loops used to control the scanning and positioning of the beam, respectively.  
     [0016] Referring to the drawings more particularly by reference numbers, FIG. 2 shows an optical beam scanner  50 . The scanner  50  can stabilize and maintain a light beam  52  that is emitted from a light source  54 . The beam  52  travels along an optical path. The light source  54  may be a laser that emits a laser beam. The beam  52  can be reflected by bending mirrors  56 . The scanner  50  may be a separate assembly that is attached to the light source  54  and mirrors  56 . For example, the scanner  50  may be attached to a laser machine.  
     [0017] The scanner  50  includes a beam centering device  58  and a beam scanning device  60 . The beam centering device  58  directs the light beam  52  onto a desired location on the beam scanning device  60 . For example, the device  60  may direct the light beam  52  onto the center of the beam scanning device  60 . The beam scanning device  60  can redirect and angularly displace the beam  52  in a scanning manner. The beam  52  may enter the scanner  50  through an input aperture  62 . The beam  52  may exit the scanner  50  through a beamsplitter  64  and a scanning lens  66 .  
     [0018] A portion of the light beam  52  may be directed onto photodetectors  68  and  70  by beamsplitter  64  and an additional beamsplitter  72 . An imaging lens  74  may focus an image of the beam  52  onto photodetector  68 . Photodetector  68  may be a quad cell device that can be used to determined whether the light beam is at the desired location at the beam scanning device  60 . Photodetector  70  may be a lateral effect detector that is used to sense the actual position of the light beam being scanned by device  60 . Sensing the position of the beam provides a more accurate feedback of the beam position downstream of the scanning device  60  than the mechanical feedback position of the scanning mirror found in optical scanners of the prior art (see FIG. 1).  
     [0019] The photodetectors  68  and  70  are connected to a controller  80 . The controller  80  includes amplifiers  82  and  84  that amplify the output signals of the detectors  68  and  70 . The controller  80  also contains error control and driver circuits  86  and  88  that provide output signals to the compensation devices  60  and  58 , respectively. Circuit  86  also receives input angle commands from an external source.  
     [0020] Each circuit  86  and  88  may include hardware and software/firmware that performs known proportional-integral-derivative control processing. Circuit  86  may process a feedback signal from detector  70  with the input angle command to generate an output signal that causes the beam scanning device to change the output angle of the laser beam  52 . Likewise, circuit  88  can process a feedback signal from detector  68  to generate an output signal that actuates the beam centering device  58  to direct the beam onto the center of the beam scanning device  60 .  
     [0021] In operation, the light beam  52  is directed into the scanner  50  from the light source  54 . The beam centering device  58  directs the light beam  52  onto the center of the beam scanning device  60 . The detector and control circuit  88  insure that the beam  52  is maintained on the center of the scanning device  60 . The downstream detection of the light beam position and the upstream correction of the beam compensates for drift and tilt errors in the system.  
     [0022] The control circuit  86  receives an input command to change the output angle of the light beam  52  and processes this command to generate an output signal to the beam scanning device  60 . The beam scanning device  60  then changes the beam angle to create a linear scan by the beam  52 . The detector  70  provides feedback information on the actual position of the beam  52  so that the circuit  86  can compensate for any deviation between the desired commanded position and the actual position.  
     [0023]FIG. 3 shows an embodiment of the scanner  50  wherein the beam centering device  58  and the beam scanning device  60  are each fast steering mirrors (FSMs). Each FSM includes a plurality of actuators  90  that can tilt a reflective mirror  92 . The actuators  90  are driven by circuits  86  and  88 .  
     [0024]FIG. 4 shows an embodiment of the scanner  50  wherein the beam centering device  58  includes a fast steering mirror (FSM)  100  and a fast steering plate  102  (FSP). The FSP includes a transmissive plate  104  that is pivoted by actuators  106  driven by control circuit  86 . The plate  106  uses refraction and varying impingement angles to vary the lateral position of the beam. This approach will minimize the tilt error that may be created by the single FSM for the embodiment shown in FIG. 3. This embodiment is preferable for monochromatic light beams. A light beam with multiple wavelengths may produce chromatic feedback errors.  
     [0025]FIG. 5 shows another embodiment wherein the beam centering device  58  has a pair of reflective mirrors  110  that are each moved by a linear translator  112  (only one mirror and translator is shown). One mirror  110  may move the beam  52  along an x axis, the other mirror may move the beam  52  along an orthogonal y axis. Each mirror  110  may reflect the beam  52  in an orthogonal direction resulting in 90 degree turn from the input beam  52 . The translators  112  may include voice coil motors.  
     [0026]FIG. 6 shows yet another embodiment where a scan lens  66 ′ focuses the light beam to a point on a work piece  114 . Focusing the beam to a point eliminates the need for the beam centering device and accompanying feedback system.  
     [0027] While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.  
     [0028] For example, although the beam centering device  58  and beam scanning device  60  are shown in the same scanner module  50 , it is to be understood that the devices  58  and  60  may be mounted to different mechanical platforms.