WO2010005394A1

WO2010005394A1 - Laser processing system and method

Info

Publication number: WO2010005394A1
Application number: PCT/SG2008/000249
Authority: WO
Inventors: Chee Yuen Cheng; Chi Kin Denis Law; Bo Ma; Parthasarathy Muthuvel; Qiu Wan; Jian Dong Wei
Original assignee: Aem Singapore Pte Ltd
Priority date: 2008-07-11
Filing date: 2008-07-11
Publication date: 2010-01-14

Abstract

A laser processing system and method. The system comprises a laser source; an imaging device for capturing positional information of a workpiece under processing for controlling a scanning mechanism for directing a laser beam from the laser source onto the workpiece; and the scanning mechanism for directing the laser beam from the laser source to the workpiece along a laser path based on the positional information feedbacked from the imaging device; wherein an optical path of light captured by the imaging device comprises the laser path.

Description

LASER PROCESSING SYSTEM AND METHOD

FIELD OF INVENTION

The present invention relates broadly to laser processing systems and methods with a built-in imaging system.

BACKGROUND

In industrial laser processing applications, such as laser engraving, laser marking, laser cutting/machining or laser ablation, a laser beam is directed on to a surface of a workpiece to create a desired pattern thereon. The movement of the beam may be pre-programmed or manually controlled. Further, the movement of the beam may be facilitated by a set of galvomotor-controlled mirrors such that the laser source remains stationary while the rotation of the mirrors, coupled with a focusing lens, allows the laser to move in the X-Y directions in a focused beam.

In some conventional laser marking/engraving systems, the pattern to be created may be programmed into a controller once at the beginning of the marking/engraving process to control the direction of the laser beam. Subsequently, the same fixed pattern is automatically repeated for each workpiece. However, due to mechanical tolerances, some of the workpieces may be misplaced or misaligned before undergoing laser marking/engraving, thus resulting in inaccuracy or even damages to the workpieces. As such, these systems may require wide tolerances.

On the other hand, as the laser process is automated at high speed, it is disruptive and costly to stop the process to correct individual workpieces.

A need therefore exists to provide a system and method that seek to address at least one of the above problems. SUMMARY

In accordance with a first aspect of the present invention, there is provided a laser processing system, the system comprising a laser source; an imaging device for capturing positional information of a workpiece under processing for controlling a scanning mechanism for directing a laser beam from the laser source onto the workpiece; and the scanning mechanism for directing the laser beam from the laser source to the workpiece along a laser path based on the positional information feedbacked from the imaging device; wherein an optical path of light captured by the imaging device comprises the laser path.

The imaging device is preferably a charge-coupled device (CCD) or complimentary metal-oxide semiconductor (CMOS) camera. A lens of the camera may be advantageously coated with a layer of high-reflective material to the laser source wavelength and anti-reflective material to an auxiliary light source wavelength.

The scanning mechanism may comprise a plurality of mirrors. The mirrors may be coupled to corresponding galvomotors. One of the mirrors may function as an aperture of the camera.

The laser processing system may further comprise a focusing mechanism. The focusing mechanism may comprise an f-theta lens. The f-theta lens may be advantageously coated with a layer of anti-reflective material for the wavelengths of both the laser source and the auxiliary light source.

In accordance with an example embodiment of the present invention, in a calibration mode, the system undergoes a first coarse calibration; an origin of the imaging device and an origin of the mirrors of the scanning mechanism are aligned; the system undergoes a second fine calibration; and a relationship between coordinates for the laser source to mark and the coordinates captured by the imaging device is formed. In the laser processing system of the example embodiment, the first coarse calibration may comprise marking a test pattern of approximately the same size as a work area; measuring dimensions of a pattern produced; determining the differences between the dimensions of the test pattern produced and the dimensions of a reference pattern; and repeating the above steps until the differences are within a coarse specification.

In the laser processing system of the example embodiment, the second fine calibration may comprise (a) capturing positional information of a calibration grid; (b) reproducing, by laser marking, the calibration grid on a blank plate based on calibration grid parameters; (c) capturing positional information of the reproduced grid; (d) determining the differences between the positional information of the reproduced grid and the positional information of the calibration grid; and (e) repeating steps (b), (c) and (d) until the differences are within a fine specification.

In accordance with a second aspect of the present invention, there is provided a laser processing method, the method comprising providing a laser source; providing an imaging device for capturing positional information of a workpiece under processing for controlling a scanning mechanism for directing a laser beam from the laser source; and using the scanning mechanism for directing the laser beam from the laser source to the workpiece along a laser path based on the positional information feedbacked from the imaging device; wherein an optical path of light captured by the imaging device comprises the laser path.

The method may further comprise a calibration process, the calibration process comprising a first coarse calibration; an alignment of an origin of the imaging device and an origin of mirrors of the scanning mechanism; a second fine calibration; and a formation of a relationship between coordinates for the laser source to mark and the coordinates captured by the imaging device.

The first coarse calibration may comprise marking a test pattern of approximately the same size as a work area; measuring dimensions of a pattern produced; determining the differences between the dimensions of the test pattern produced and the dimensions of a reference pattern; and repeating the above steps until the differences are within a coarse specification.

The second fine calibration may comprise (a) capturing positional information of a calibration grid; (b) reproducing the calibration grid on a blank plate based on calibration grid parameters; (c) capturing positional information of the reproduced grid; (d) determining the differences between the positional information of the reproduced grid and the positional information of the calibration grid; and (e) repeating steps (b), (c) and (d) until the differences are within a fine specification.

The positional information of said workpiece may comprise one or more of a group consisting of reference features, reference edges and reference points.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

FIG. 1 is a perspective view of a laser processing system with imaging feedback according to an example embodiment;

FIG. 2 is a schematic diagram of the laser processing system of FIG. 1;

FIG. 3A is a schematic diagram of an optical configuration of a conventional camera;

FIG. 3B is a schematic diagram of an optical configuration of the camera of FIG. 2 according to an example embodiment;

FIGS. 4A and 4B are patterns produced of a square in a first calibration stage; FIG. 5A is a plan view of a calibration grid for the laser processing system of FIG. 1 according to an example embodiment;

FIG. 5B is an enlarged view of a portion of a calibration grid of FIG. 5A;

FIG. 6A, 6B and 6C are illustrations of some example applications of the laser processing system of FIG.1 ;

FIG. 7A is a schematic diagram of a work area in a conventional laser processing system during operation;

FIG, 7B is a schematic diagram of a work area in the laser processing system of FIG. 1 according to an example embodiment during operation; and

FIG. 8 shows a flowchart illustrating a laser processing method according to an example embodiment.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of a laser processing system with imaging feedback 100 according to an example embodiment. FIG. 2 is a schematic diagram of the laser processing system 100 of FIG. 1. The laser processing system 100 comprises a laser source 102; a scanning mechanism 110 for directing a laser beam from the laser source 102 to a workpiece 130 along a laser path; and an imaging device 104 for capturing positional information of the workpiece 130 under processing for controlling the scanning mechanism 110 for directing the laser onto the workpiece 130, wherein an optical path of light captured by the imaging device 104 comprises the laser path. In this embodiment, the system 100 further comprises a plurality of fixed mirrors 106 and a focusing mechanism in the form of an f-theta lens 118. The whole system 100 may also be contained in a housing as shown in FIG.1. The system 100 may also be used with an auxiliary light source 120, which provides background lighting for the digital imaging device 104 such that an optical path of light captured by the digital imaging device 104 comprises the laser path. In this embodiment, the auxiliary light source 120 has an empty central portion similar to a ring light, through which the laser beam can pass before hitting the workpiece 130. It should be appreciated that other types or a different number of lights can be used to provide background lighting.

The laser source 102 may be an industrial-grade CO₂ laser, solid-state laser (diode-pumped or lamp-pumped) or fibre laser, etc., capable of working on a variety of workpiece 130 materials, such as metal, polymer, wood, etc. Examples of such a laser source 102 include a diode-pump, 130W, 4mm Rod, 30-bar pump, Flat Rod laser module from Northrop Grumman (Model No.: RD40-IC2-FA1-0011), or a G3.0 Pulsed Fibre Laser from SPI (Model No.: SP-12P-0011).

The digital imaging device 104 comprises a charge-coupled device (CCD) camera or a complimentary metal-oxide-semiconductor (CMOS) camera, for example, a 0.8 megapixel SVS Vistek camera (Model No.: SVS2041 MFCP). The scanning mechanism 110 comprises a plurality of mirrors 114, each of which is separately controlled and driven by a galvomotor 116 such that each mirror 114 is rotatable about an axis, thereby capable of directing a laser beam in a plane perpendicular to that axis. Thus, it would be appreciated by one skilled in the art that a minimum of two mirrors 114 and galvomotors 116 may be used to independently control the laser beam in the X-Y directions. In an example embodiment, the galvomotor 116 has a +/-12.5deg mechanical range and +/-25deg optical range.

Further, in this embodiment, the digital imaging device 104 is positioned relative to the laser source 102 and the plurality of fixed mirrors 106 such that the digital imaging device 104 captures the positional information of the same area of the workpiece 130 being worked on by the laser beam. Hence, the digital imaging device 104 is effectively coaxial with the laser source 102, i.e. an optical path of light captured by the digital imaging device 104 comprises the laser path. In some example embodiments, a lens of the digital imaging device 104 is coated with a layer of high-reflective material to protect the sensitive CCD or CMOS imaging sensor from any laser light that may be reflected off the surface of the workpiece 130, mirrors 114 and semi-transparent mirror 106a back to the digital imaging device 104. For example, the lens of the digital imaging device 104 may be coated with a material for high-reflection (>90%) for wavelengths between 1000 to

1100 nm and a material for anti-reflection (>95%) for wavelengths between 600 to

900 nm, which includes the wavelength range of the auxiliary light source 120 in the example embodiment. The f-theta lens 118 may also be coated with a layer of anti- reflective material for (>99%) of wavelengths between 1000 to 1100 nm and anti- reflection (>95%) for wavelengths between 600 to 900 nm. The coating may be multi-layered with different materials, or a single layer of a single or compound material with the relevant optical properties.

The digital imaging device 104 also has an optical configuration such that one of the mirrors 114 advantageously acts as its aperture (FIG. 3B). In the optical configuration of a conventional camera as shown in FIG. 3A, the aperture 302 is positioned behind the camera lens 304. The size of the aperture 302 may be reduced or increased, thereby affecting the amount of light entering the CCD. If the aperture 302 is too small, some of the light may be blocked and an image 308 of an object 306 may not be clearly defined. If the aperture 302 is too large, excess light causes the image 308 to be too noisy.

In this example embodiment, the X-mirror 114x (i.e. one of the mirrors 114) acts as the aperture, with only light reflected from the X-mirror 114x entering the camera. It will be appreciated that, choosing the size of the mirrors 114 in relation to the size of the work area can advantageously ensure that, for images taken within the work area, the collected light can be optimized.

A calibration of the system 100 is carried out before system operation and may comprise a number of stages. FIG. 4A and 4B are patterns produced of a square in a first calibration stage. In the first stage, a manual (coarse) calibration of the system 100 is carried out. Two jigs (not shown) are used as centre guides for the laser beam. One jig is placed between the semi-transparent mirror 106a and the mirrors 114; the other is placed between the mirrors 114 and the light source 120, with the f-theta lens 118 removed. By tuning the mechanical adjustments of the laser source mount, the laser beam is adjusted to pass through the centre of both jigs. With the jigs removed and the f-theta lens 118 replaced in position, the laser beam will then emerge from the centre of the f-theta lens 118 when the mirrors 114 are at the origin (i.e. (0,0) position).

The laser source 102 then attempts to mark a test pattern, here a square pattern of approximately the same size (e.g. 160 x 160 mm) as the work area on the working plane. As shown in FIG. 4A, a pattern 410 produced by laser marking may be different from a reference square due to optical distortion. The distortion may be corrected by measuring the actual dimensions and keying the values into a laser calibration file (e.g. a look-up table), where the differences between the dimensions of the pattern 410 and the dimensions of the reference square may be determined. The steps may be repeated until the pattern produced is approximately a square 420, as shown in FIG. 4B, where the differences are within a coarse specification.

In the next calibration stage, the origins of the laser source 102 and the digital imaging device 104 is being aligned. With the mirrors 114 set at the origin (0,0), the laser source 102 marks a point, such as a '+' sign on the working plane.

The digital imaging device 104 captures the '+' sign and sets the coordinates of the

'+' sign as its origin.

FIG. 5A is a plan view of a calibration grid 500 for the laser processing system of FIG. 1 according to an example embodiment. FIG. 5B is an enlarged view of a portion of a calibration grid of FIG. 5A. The calibration grid 500 may comprise rows and columns of repetitive patterns equally spaced from one another. In this example embodiment, the patterns consist of dark dots with a known diameter D and a centre-.to-centre separation S on a white or transparent background. The calibration grid 500 may be approximately the same size as the work area of the laser processing system 100.

The next calibration stage comprises an auto-calibration (fine calibration) of the system 100. With the calibration grid 500 on the work area, the digital imaging device 104 searches the dots and captures their coordinates. The dot density may vary depending on the laser processing applications such that the diameter D separation S are smaller for applications with higher precision and larger for applications with lower precision. The calibration grid 500 is then replaced with a blank plate (not shown), and the laser source 102 reproduces the grid by marking the dots on the blank plate according to the known grid parameters (i.e. diameter D and separation S) of the calibration grid 500, after which the digital imaging device 104 captures the positional information of these dots. The differences between the positional information of the reproduced grid and the positional information of the calibration grid 500 are determined and re-input into e.g. a laser calibration file. The marking by the laser source 102, image capturing by the digital imaging device 104 and determining the differences are repeated until the differences are within a required fine specification.

Using the information from the above stages stored in a laser calibration file, the relationship between the number of pixels and the distance on the working plane (in millimetres) for the digital imaging device 104 may be determined. Finally, the relationship between the laser source 102 and the digital imaging device 104 is calibrated. The laser source 102 mark dots on a blank plate at specific positions. The digital imaging device 104 searches and captures the coordinates of these dots, thus forming a relationship between the coordinates for the laser source 102 to mark and the coordinates captured by the digital imaging device 104.

Returning to FIG. 1 , during operation of the laser processing system 100, the digital imaging device 104 first captures the positional information of a plurality of reference points on the workpiece 130. By capturing the plurality of reference points on the workpiece 130, the actual position of the workpiece 130 is determined. Whether the workpiece 130 is offset in positional or rotational manner, or is at perfect position, the laser is directed to fire on the workpiece with reference to these reference points as programmed via the appropriate rotation of the galvomotors 116.

In applications in which a large amount of processing is done on the workpiece 130, the digital imaging device 104 periodically checks the positional information of the reference points for possible misalignment during operation. Even if the workpiece 130 is moved during operation, the updated positional information of the workpiece 130 can still be captured to adjust the galvomotors 116 and re-direct the laser beam in real-time.

FIG. 6A, 6B and 6C are illustrations of some example applications of the laser processing system of FIG.1. In FIG. 6A, a pattern 636 comprising multiple alphanumeric characters or logos is marked/engraved on a workpiece 630, which may be an integrated circuit (IC) or a chip. Edges 632 and 634 of the workpiece 630 are identified by the digital imaging device 104 (FIG. 2) as reference axes, from which the position of the starting point of the pattern 636 may be determined (i.e. at a distance X from edge 632 and a distance Y from edge 634).

In another example application as illustrated by FIG. 6B, edges 642 and 644 of ICs 646 are identified to direct the laser to cut as close to them as possible without damaging the ICs 646.

Similarly, in the example application of FIG. 6C, the positional information of existing reference features 652 & 654, for example, pre-marked texts or logos of a workpiece 650, may be captured by the digital imaging device 104. A pattern 656 may then be added to the workpiece 650 exactly in between the reference features 652 & 654.

In the above example applications, as the positional information comprise reference features, edges or points on the workpiece, even if the workpiece is misplaced or misaligned, the laser processing system is able to identify these features, edges or points such that the pattern is created with corrected offset and rotation. It should also be appreciated that the above example applications are illustrative and that numerous other applications of the laser processing system are possible, using the real-time compensation feature as described in the example embodiment.

The described example embodiment incorporates an imaging device that is effectively coaxial with the laser path. By integration with a scanning mechanism to search for reference features on a workpiece to be identified by the imaging device, as well as by directing the laser beam, to computated locations, the position of the workpiece can be accurately determined and a very high precision laser processing can be achieved compared to conventional laser processing systems.

FIG. 7A is a schematic diagram showing the effect of offset in a conventional laser processing system during operation. FIG. 7B is a schematic diagram showing the effect of offset in the system of FIG. 1 according to an example embodiment during operation. The system according to the example embodiments may be advantageous in laser material processing where precise positioning is required. As shown in FIG. 7B, the precise laser marking position is automatically adjusted with the help of the imaging system such that a pattern 730' is marked with correct offset.

In a conventional system, due to mechanical tolerances, when a workpiece 710 enters the firing chamber, it is not always at the same angle or position. It is inevitable to have a rotational or positional offset or both. Where conventional systems are used, the laser will mark patterns 730 at locations that have been pre-programmed even as the workpiece 710, hence objects 720 thereon, has been misplaced. If precision is required or if the object is very small, this will either damage the object or cause inaccuracy, as shown in FIG. 7A. In the system according to the example embodiments, the misalignments are compensated automatically such that the patterns 730' are still marked at the desired position and orientation on objects 720' (FIG. 7B).

FIG. 8 shows a flowchart illustrating a laser processing method, generally designated as reference numeral 800, according to an example embodiment. At step 802, a laser source is provided. At step 804, an imaging device for capturing positional information of a workpiece under processing is provided for controlling a scanning mechanism for directing a laser beam from the laser source. At step 806, the scanning mechanism is used for directing the laser beam from the laser source to the workpiece along a laser path based on the positional information feedbacked from the imaging device, wherein an optical path of light captured by the imaging device comprises the laser path.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. A laser processing system, the system comprising: a laser source; an imaging device for capturing positional information of a workpiece under processing for controlling a scanning mechanism for directing a laser beam from the laser source onto the workpiece; and the scanning mechanism for directing the laser beam from the laser source to the workpiece along a laser path based on the positional information feedbacked from the imaging device; wherein an optical path of light captured by the imaging device comprises the laser path.

2. The laser processing system of claim 1 , wherein the imaging device is a CCD or CMOS camera.

3. The laser processing system of claims 1 or 2, wherein a lens of the camera is coated with a layer of anti-reflective material for the auxiliary light source wavelength and high-reflective material for the laser source wavelength.

4. The laser processing system of any one of the preceding claims, wherein, the scanning mechanism comprises a plurality of mirrors.

5. The laser processing system of any one of the preceding claims, wherein the mirrors are coupled to corresponding galvomotors.

6. The laser processing system of any one of the preceding claims, wherein one of the mirrors functions as an aperture of the camera.

7. The laser processing system of any one of the preceding claims further comprising a focusing mechanism.

8. The laser processing system of claim 7, wherein the focusing mechanism comprises an f-theta lens.

9. The laser processing system of claim 8, wherein the f-theta lens is coated with a layer of anti-reflective material for the wavelengths of both the laser soruce and the auxiliary light source.

10. The laser processing system of any one of the preceding claims, wherein, in a calibration mode: the system undergoes a first coarse calibration; an origin of the imaging device and an origin of the mirrors of the scanning mechanism are aligned; the system undergoes a second fine calibration; and a relationship between coordinates for the laser source to mark and the coordinates captured by the imaging device is formed.

11. The laser processing system of claim 10, wherein the first coarse calibration comprises: marking a test pattern of approximately the same size as a work area; measuring dimensions of a pattern produced; determining the differences between the dimensions of the test pattern produced and the dimensions of a reference pattern; and repeating the above steps until the differences are within a coarse specification.

12. The laser processing system of claims 10 or 11, wherein the second fine calibration comprises:

(a) capturing positional information of a calibration grid;

(b) reproducing, by laser marking, the calibration grid on a blank plate based on calibration grid parameters;

(c) capturing positional information of the reproduced grid; (d) determining the differences between the positional information of the reproduced grid and the positional information of the calibration grid; and

(e) repeating steps (b), (c) and (d) until the differences are within a fine specification.

13. A laser processing method, the method comprising: providing a laser source; providing an imaging device for capturing positional information of a workpiece under processing for controlling a scanning mechanism for directing a laser beam from the laser source; and using the scanning mechanism for directing the laser beam from the laser source to the workpiece along a laser path based on the positional information feedbacked from the imaging device; wherein an optical path of light captured by the imaging device comprises the laser path.

14. The method of claim 13, further comprising a calibration process, the calibration process comprising: a first coarse calibration; an alignment of an origin of the imaging device and an origin of mirrors of the scanning mechanism; a second fine calibration; and a formation of a relationship between coordinates for the laser source to mark and the coordinates captured by the imaging device.

15. The method of claim 14, wherein the first coarse calibration comprises: marking a test pattern of approximately the same size as a work area; measuring dimensions of a pattern produced; determining the differences between the dimensions of the test pattern produced and the dimensions of a reference pattern; and repeating the above steps until the differences are within a coarse specification.

16. The method of claims 14 or 15, wherein the second fine calibration comprises:

(a) capturing positional information of a calibration grid;

(b) reproducing the calibration grid on a blank plate based on calibration grid parameters; (c) capturing positional information of the reproduced grid;

(d) determining the differences between the positional information of the reproduced grid and the positional information of the calibration grid; and

17. The method of any one of claim 13 to 16, wherein the positional information of said workpiece comprises one or more of a group consisting of reference features, reference edges and reference points.