GB2215859A

GB2215859A - Laser scanning system having two oscillating mirrors

Info

Publication number: GB2215859A
Application number: GB8902299A
Authority: GB
Inventors: A S Bransden
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1988-02-05
Filing date: 1989-02-02
Publication date: 1989-09-27
Anticipated expiration: 2009-02-02
Also published as: GB8902299D0; GB2215859B; GB8802689D0

Abstract

A system for scanning a laser beam 1 over an area of a workpiece 7, includes first and second scanning mirrors 4, 5 arranged to direct a laser beam at an area 6 of a workpiece 7 to be treated, means for oscillating each of the said mirrors about an axis 4<1>, 5<1> so as to cause the laser beam to scan the area 6 of the workpiece 7 with a phase difference of ninety degrees between the oscillations of the two mirrors and means 2 for causing the laser beam to be incident alternately on each mirror only during that part of its motion when its angular velocity is substantially constant. <IMAGE>

Description

LASER SCANNING SYSTEM The present invention relates to systems for scanning a laser beam over a workpiece.

Laser transformation hardening of metal surfaces requires the generation of heating patterns having a uniformity of + 10% from a nominal value and an area of the order of a square centimetre. Usually, these criteria are achieved by rastering a focused laser beam across a surface to be treated with a frequency sufficiently high to ensure that the interaction time of the laser beam is small compared with the interaction time of the complete heating pattern when traversing the surface beneath it.

The surface thus 'sees' only the time averaged heating pattern. The required rastering of the laser beam can be performed by the use of oscillating mirrors. By suitable control of the amplitude of the raster, the heating pattern can be adapted to conform with the geometry of the workpiece being treated.

A problem which arises when large water-cooled mirrors capable of handling 5 kw of power continuously are used is that their inertia is considerable and causes their angular velocity to vary sinusoidally. This effect causes qreater amounts of power to be deposited upon the work piece at the extremities of the scanning motion. The problem can be overcome by masking those regions of the workpiece from the laser beam, but not only is treatment area lost, but so is power and the overall efficiency of a scanning system can be reduced to as low as 30%.

According to the present invention there is provided a system for scanning a laser beam over an area of a workpiece, including first and second scanning mirrors arranged to direct a laser beam at an area of a workpiece to be treated, means for oscillating each of the said mirrors about an axis so as to cause the laser beam to scan the area of the workpiece with a phase difference of ninety degrees between the oscillations of the two mirrors and means for causing the laser beam to be incident alternately on each mirror only during that part of its motion when its angular velocity is substantially constant.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which, Figure 1 shows diagrammatically an embodiment of the invention and Figure 2 illustrates a number of different scanning motions.

Referring to figure 1, a converging laser beam 1 from a laser which is not illustrated, falls upon a rotatinq slotted chopper wheel 2 the upper surface 3 of which is reflecting. First and second mirrors 4 and 5, respectively, are positioned to receive the laser beam 1 when it has been reflected from the chopper wheel 2 or passed through one of the slots of the chopper wheel 2, respectively. The mirrors 4 and 5 are positioned to cause the laser beam 1 to be incident upon a region 6 of a workpiece 7 which is to be treated by the laser beam 1.

The mirrors 4 and 5 are arranged to be oscillated about respective axes 4' and 5', as shown, so as to cause the laser beam 1 to scan over the same portion of the region 6 of the workpiece 7.

The motions of the mirrors 4 and 5 are arranged to be ninety degrees out of phase and also to be synchronised with the motion of the chopper wheel 2 so that the laser beam 1 falls alternately upon the mirrors 4 and 5 only when they are moving through the linear part of their oscillations.

The width of the scanned region 6 of the workpiece 7 can be varied by altering the amplitude of the oscillations of the mirrors 4 and 5. The length of the scanned region 6 of the workpiece 7 in the perpendicular direction is produced by oscillating the laser beam 1 in a direction perpendicular to the axes of motion of the mirrors 4 and 5 before it is incident upon the chopper wheel 2.

the null positions of the mirrors 4 and 5 are adjustable so that the lines traced out by the laser beam 1 when reflected from the mirrors 4 and 5 can be separated if so desired. This may be advantageous when dealing with certain types of symmetrical workpiece. Figure 2 shows, for example, how both sides of piston ring grooves (a), cutting edges (b), and gear teeth (c) can be treated simultaneously.

Claims

1 A system for scanning a laser beam over an area of a workpiece, including first and second scanning mirrors arranged to direct a laser beam at an area of a workpiece to be treated, means for oscillating each of the said inirrors about an axis so as to cause the laser beam to scan the area of the workpiece with a phase difference of ninety degrees between the oscillations of the two mirrors and means for causing the laser beam to be incident alternately on each mirror only during that part of its motion when its angular velocity is substantially constant.

2 A system according to claim 1 wherein the means for causing the laser beams to be incident alternately on each mirror only during that part of its motion when its angular velocity is substantially constant comprises a wheel havinq alternate reflective and transmissive regions around its periphery so disposed that laser radiation reflected from the wheel will be incident upon one of the said mirrors and laser radiation transmitted through the wheel will be incident upon the other of the said mirrors, and means for rotating the wheel at an angular velocity such that the laser beam is caused to be incident on one or other of the said mirrors at the appropriate part of their motions.

3 An apparatus according to claim 1 or claim 2 wherein the null positions of the motions of the said mirrors are such that the mirrors cause the laser beam to scan different areas of the work piece.

4 An apparatus for scanning a laser beam over an area of a workpiece substantially as hereinbefore described with reference to the accompanying drawinqs.