Laser scan speed calibration

1. A method of verifying a scan speed in a laser scanning system of an apparatus for acting on the surface of articles, the method including:

modulating a laser of the scanning system while scanning a surface of an article with a beam emitted by the laser to form at least one feature, having a periodic pattern, on the article; and

determining, based on a characteristic of the at least one feature, whether the actual speed at which the scanning takes place corresponds to a speed to which the laser scanning system is set.

2. A method as claimed in claim 1 wherein the feature is generally linear.

3. A method as claimed in claim 1 wherein the feature includes a series of spaced dashes dimensioned in accordance with the modulation.

4. A method as claimed in claim 1 wherein the feature is raised relative to a surface on which the feature is formed.

5. A method as claimed in claim 3 wherein the dashes are spaced apart by a given pitch, and the determining comprises measuring the pitch (dx) of the dashes, and

computing

S=dx·f

where S is the actual speed and f is the frequency of the modulation of the laser.

6. A method as claimed in claim 3 wherein the determining comprises forming a further feature or features of a known length (x) alongside the feature having the periodic pattern,

quantifying the number (n) of dashes contained within the length (x), and

calculating the actual speed (S) using the formula

S

=

x

·

f

n

where f is the frequency of the modulation of the laser.

7. A method as claimed in claim 3 wherein the laser is frequency modulated by a gated oscillator having two gating conditions in which the laser is modulated for y periods in the first condition and for y+dy periods in the second condition such that the at least one feature formed on the article comprises condition features each having a periodic pattern and formed under the gating conditions, respectively, and

the determining comprises forming at least one feature or features each of a predetermined length intended to lie between the linear effect of y and y+dy, and

comparing the condition features with the at least one further feature.

8. A method as claimed in claim 1 wherein the laser scanning system is part of an additive layer manufacturing apparatus and builds the at least one feature.

9. A method as claimed in claim 1 wherein the laser scanning system prints the at least one feature.

10. A method of verifying scan speed in a laser scanning system, including:

writing at least one feature on an article using a laser of the system;

modulating the laser using a gated oscillator, as the at least one feature is being formed, to impart a periodic variation in the at least one feature; and

determining, based on a characteristic of the at least one feature, whether the actual speed at which the laser scanning system scans articles with its laser corresponds to a speed to which the laser scanning system is set.

11. A method as claimed in claim 10 wherein the frequency of the gated oscillator is traceable to national standards.

12. A method as claimed in claim 10 wherein the at least one feature is produced in a linear direction, and

the determining comprises quantifying a number of variations produced in the at least one feature, over a given length in said direction, as a result of modulating the laser, and

calculating the actual speed based on the number of variations.

13. A method as claimed in claim 10 wherein the variations are spaced apart in said direction by a pitch, and the determining comprises measuring the pitch of the variations, and calculating the actual speed based on the pitch.

14. A method as claimed in claim 10 further comprising preventing partial pulses from being produced in the output of the laser, using a mono-stable multivibrator in conjunction with the gated oscillator.

15. A method as claimed in claim 14 wherein the writing comprises writing a sequence of lines of progressively increasing length on the article, as a test pattern.

16. A method as claimed in claim 15 wherein some of the lines of the test pattern is are orientated in directions different from those of others of the lines of the test pattern.

17. A method as claimed in claim 16 wherein the system is an additive layer manufacturing system.

18. A method as claimed in claim 10 wherein the period of the gated oscillator is of the order of 1 ms and the pitch of the variations in the pattern is of the order of 1 mm.

19. A method as claimed in claim 11 further comprising preventing partial pulses from being produced in the output of the laser, using a mono-stable multivibrator in conjunction with the gated oscillator.