WO2000041836A1

WO2000041836A1 - Structured multi-spot detecting technique for adaptive control of laser beam processing

Info

Publication number: WO2000041836A1
Application number: PCT/CA2000/000025
Authority: WO
Inventors: Hongping Gu
Original assignee: Powerlasers Limited
Priority date: 1999-01-14
Filing date: 2000-01-14
Publication date: 2000-07-20
Also published as: CA2323803A1; AU3027700A

Abstract

A laser processing control apparatus utilizes plurality of optical fibers (36) to monitor different areas (37) of a weld zone. The individual signals are combined to provide a number of possible control modes including focus, gap, seam following and speed control. The signals may be used as inputs to fuzzy logic or other laser control systems.

Description

STRUCTURED MULTI-SPOT DETECTING TECHNIQUE FOR ADAPTIVE CONTROL OF LASER BEAM PROCESSING

FIELD OF THE INVENTION The present invention relates to laser processing.

RELATED ART

It is well known to use a laser beam for processing a material, either for cutting, welding or surface treatment of the material. In order to provide effective processing it is necessary to control the interaction of the laser beam with the material so that optimum processing conditions are provided. Variations in those conditions may lead to unsatisfactory processing causing rejection or failure of the treated component.

Laser processing has utilized light emission from the plume during processing to provide quality control and monitoring. Conventionally, light emission from the plume is imaged onto an optic fiber which is fed to a photodiode or light sensor. The output signal from the photodiode is used directly as a signal used to indicate the welding condition. However it has been found that the signal intensity alone cannot indicate accurately the welding condition since the light intensity from the weld zone is not a linear response to the beam penetration depth. Moreover, the intensity of the light will vary significantly from system to system and even within the same system it may change significantly if the operating conditions of the laser are changed. Even with stable processing, the light emission itself may fluctuate significantly.

As a result it is difficult to interpret the detected results and accordingly the light emission can only be used effectively as a fault detection. In this case a large change in the detected signal is used as an indication that there is something wrong in the welding process and the process either stopped or the component flagged for further inspection. One proposal to improve the control of laser processing is shown in US Patent 5,517,420 to W. Duley et al. In this arrangement, the image of the weld pool created by the laser is obtained on a CCD array. Information is taken from the image, such as the proportion of white to black pixies and the aspect ratio of the image, and used as an input to a fuzzy logic control system. The control system then adjusts the processing parameters such as weld speed in accordance with the parameters derived from the image. As a further enhancement of this technique as disclosed in PCT application PCT/CA98/00895 the fuzzy logic is combined with a neural network to provide enhanced control of the processing.

The above techniques have been particularly beneficial where relatively long processing has been utilized. For example, in seam welding the edges of a pair of sheets, the welding process is relatively long and continuous to permit acquisition and processing of the image. The feedback provided by the image is therefore suitable for controlling the welding process.

It has been found however that in certain processing conditions, such as relatively short welds, a fast acquisition and control is required. As noted above, the signal intensity alone is insufficient to provide control and moreover the variation in signals may be attributable to a number of factors such as the location of the beam relative to a seam, the gap between two components being processed, and the speed of movement of the beam over the components. There is therefore a need for a laser processing control system which may monitor the laser processing and provide suitable control under a variety of conditions.

SUMMARY OF THE INVENTION

In general terms therefore the present invention provides a laser processing control apparatus in which a plurality of locations adjacent the incident laser beam are monitored and control signals derived from those locations. The monitoring is performed by individual fiber optics whose outputs may be combined to provide different control parameters. Provision may be made for seam tracking, gap detection and weld speed control within the same control apparatus.

DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of a laser processing apparatus Figure 2 is a view on the line 2-2 of figure 1.

Figure 3 is a view on the line 3-3 of figure 2. Figure 4 is a view in the direction of arrow a of figure 1. Figure 5 is a schematic representation of the alternative position shown in figure 1.

Figure 6 is a plot of signal intensity versus height for the apparatus of figure 1 operating in a first mode of operation. Figure 7 is a graph of differential signal intensity versus height derived from figure 6.

Figure 8 is a side view of a profile of components used to test the apparatus of figure 1.

Figure 9 is a representation of the results obtained from conventional apparatus used with the profile of figure 8.

Figure 10 is a photographic representation of the results obtained using the apparatus of figure 1 with the profile of figure 8.

Figure 11 is a representation of an alternative profile of component.

Figure 12 is a photographic representation of the results obtained on the profile of figure 11 using the apparatus of figure 1.

Figure 13 is a representation of the apparatus of figure 1 operating in a second mode.

Figure 14 is a graphical representation of signals obtained using the apparatus of figure 1 in the mode of figure 13. Figure 15 is a view similar to figure 13 of the apparatus of figure 1 used in a further mode.

Figure 16 is a view on the line 16-16 of figure 15.

Figure 17 is the graphical representation of the results obtained from the apparatus shown in figure 15. Figure 18 is a graphical representation of the results shown in figure 17 after further processing.

Figure 19 is schematic representation of the apparatus of figure 1 being used in a further mode of operation.

Figure 20 is a plan view of the apparatus shown in figure 19. Figure 21 is a graphical representation of the results obtained using the apparatus of figure 1 in the mode represented in figure 20. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring therefore to figure 1, a laser processing apparatus 10 includes a laser 12 that propagates a beam 14 to impinge upon a work piece 16. The work piece 16 may be one of a variety of forms but in the examples illustrated comprises a pair of sheet metal components 18, 20 (Figure 4) abutting along respective edges 22, 24 to be welded to one another. It will however be appreciated that the work piece 16 may be a single sheet of material to be cut by laser beam 14 or may be a workpiece to be surface treated by the laser beam 14.

An optical monitoring assembly 26 includes an optical head 28 secured to the laser 12 by means of an arm 30. The optical head 28 includes a housing 32 containing optical elements 34 that are relatively adjustable to provide a variable focus for the monitoring assembly 26.

Optical fibers 36 are secured to the housing 32 and extend to respective optical sensors, preferably photodiodes 38. In the embodiment of figure 1, four optical fibers 36 (indicated at 36a, 36b, 36c, 36d respectively) are provided and arranged in quadrature within the housing 32 and transmit signals to a corresponding one of the photodiodes 38a, 38b, 38c, 38d. It will be appreciated however that more or less optic fibers may be utilized depending upon the mode of control to be implemented as will be described in further detail below. Each of the photodiodes 38 provides an output signal 40 connected to a control

42. Control 42 may operate in one or more of a plurality of modes to extract a control signal for laser 12. The control signal 44 may be used to control the movement to the laser 12 relative to the work piece 16 or the operation of the laser 12 as it moves over the work piece. The laser 12 is moveable along mutually perpendicular axes x, y, z relative to the work piece 16 to permit the beam 14 to follow the desired path along the component 16.

As can best be seen from figure 4, the beam 14 impinges on the surface of component 16 and produces a weld pool 50. As the beam 14 moves along in a direction parallel to the edges 22, 24 it progressively melts the edges which then solidify to weld the two components 18, 20 to one another. Control of the beam 14 is provided by the monitoring assembly 26 in conjunction with the control 42.

The optical head 28 focuses the fibers 36 to respective discreet locations (indicated as 37a,37b,37c,37d) about the weld pool 50. As illustrated in figure 4, one of the fibers 36a is focused in advance of the pool 50 at 37a and another, 36b, focused behind the pool 50 at 37b. The two other fibers 36c and 36d respectively are focused on opposite sides of the pool 50 at 37c and 37d respectively. The respective photodiodes 38 will therefore receive information from the plume, the weld pool and surrounding regions and may use that information to provide control signals to the control 42.

Typically the information received will relate to the intensity of the emissions at the pool 50 and this information may be refined by providing appropriate filters to select specific wave lengths of radiation for transmission to the photodiode 38 or by selecting a photodiode with specific response characteristics. Typically the photodiodes 38 will be responsive to either UN, IR or visible light. The processing of the control signals 40 will depend upon the mode of control selected by the control 42 as will be exemplified below.

One mode of operation that may be utilized by the control 42 is a focus control to maintain the beam 14 focused on the workpiece 16 to accommodate any variations in the relative position of the laser 12 and the adjacent surface of workpiece 16. To provide a focus control, the control 42 selects signals from the fibers 36a and 36b, that is the fibers in advance of and trailing the weld pool 50. As can be seen from figure 5, the focal points of the fibers 36a and 36b are adjusted by the optical elements 34 to be equally distributed to either side of the weld pool 50 when the beam 14 is correctly focused. This is shown in figure 5b. If the workpiece 16 moves closer to the laser 12 as shown in figure 5a, the fiber 36a images the center of the pool whereas the fiber 36b further trails the weld pool 50. As a result, the intensity of the signal received at the photodiode 38a associated with the fiber 36a increases and that 38b, associated with the fiber 36b decreases. In this case, each of the diodes 38a, 38b is responsive to one range of frequencies, either UN or visible light. IR is not generally desirable as the signal is not symmetric.

Similarly, as shown in figure 5c, an increase in the spacing between the laser 12 and the workpiece 16 causes the trailing fiber 36b to be focused at the center of the pool 50 and the lead fiber 36a further in advance of the pool 50. The signals received from the respective diodes 38a, 38b is shown in figure 6 from which it will be seen that as the spacing increases, the signal of the lead photodiode decreases and that of the trailing photodiode increases. Whilst the absolute values of the signals may vary as discussed above, the difference between the two signals provides a stable control signal that may be used to monitor the height of the laser head 12 relative to the component 16.

Referring to figure 7, the differential signal between the lead and trailing fibers 36a, 36b provides a zero crossing corresponding to the beam 14 being focussed on the workpiece 16. The value of the differential signal may therefore be utilized as an input to a control system 42 in which the position of the laser 12 can be changed to compensate for the variations in the relative position of the component 16.

In tests performed with a CO laser, satisfactory results were obtained. The profile of a test component 16 is shown in figure 8 from which it will be seen that the component provides a ramp up followed by a ramp down. Signals from the photodiodes 38a, 38b were sampled simultaneously using a WLNλGps board. The data is stored directly in memory and manipulated in direct memory access. Fifty data points were averaged to generate a control signal and this process took less than five msec to effectively provide a real time control. As can be seen from figure 9, without the feedback from the photodiodes 38a, 38b, the weld was initially satisfactory and then diminished as the apex of the profile was approached. Thereafter the weld was established but intermittent weld and variable weld quality can be seen in figure 9.

By contrast, when the feed back control using the signals from the photodiodes 38 was implemented, a uniform weld was obtained as the laser head 12 traversed the workpiece 16. A uniform width and dimension to the weld can be seen from figure 10 indicating a generally satisfactory welding process. In each of the samples mild steel of thickness 1mm was used and the slope of the ramp of the components was 4° to horizontal.

In further testing on similar material, the configuration shown in figure 11 was utilized giving a plurality of apexes and troughs in the path of the laser 12. The results of the welding process may be seen from figure 12 where again a uniform bead is established along the length of the component 12 indicating that the laser head 12 adjusted along the z axis to maintain the beam 14 in focus.

Although the tests samples show a generally planar segment it will be appreciated that this control may be used on three dimensional curved surfaces in which the fast acquisition time and real time control will maintain the beam 14 in focus.

The control apparatus 26 may also be used to monitor the gap between the edges 22, 24 and provide appropriate control of the laser head 12. As shown in figure 13, the lead fiber 36a is focussed in advance of the weld pool at 37a and the fiber 36b focussed at the center of the weld pool 50 at 37b. The fiber 36a is adjusted so that it views the component directly rather than through the plume, as shown in ghost outline in figure 1. In this mode, the photodiode 38a receives or is responsive to visible or IR emissions whereas the photodiode 38b receives or is responsive to visible or UN emissions. If the edges 22, 24 provide a close fit so that there is no significant gap, then very little light will be received at the lead photodiode 38a. When a gap appears, it serves as a light guide and the light from the weld zone will be detected by the fiber 36a and the corresponding signal produced at the diode 38a. In general the signal intensity increases as the gap increases and therefore the detected signal provides an indication of the width of the gap.

Because of variations in the intensity of the plume, the signal from the fiber 36b is used as a reference level so that the variations in the signal received from the fiber 36a may be used to monitor variations in the gap. Variation may be monitored by subtracting the signals and using the difference as an indication of gap size, or by dividing the gap signal by the reference signal and using the ratio of the signals as an indication of gap variation.

The results obtained from the arrangement shown in figure 13 are indicated in figure 14 where the infra-red signal from the weld zone is monitored. The results shown in figure 14 were obtained from a test sample in which the gap between the edges 22, 24 progressively opened from zero to 0.22 to 0.23 millimeters. The darkest signal indicated #1 indicates the signal from the fiber 36b and it will be seen that this progressively decreases as the gap widens. Conversely the signal from the fiber 36a indicated by #2 progressively increases. By combining the two signals at the control 42, a control signal 44 may be provided to the head 12 to adjust the movement of the head in accordance with the sensed gap. Thus the welding speed may be reduced as the gap widens to maintain the weld pool or alternatively the beam defocused to provide a wider weld zone. Alternatively the welding may be interrupted for remedial measures.

The fiber array 26 may also be used for seam tracking between a pair of sheets 22, 24 of different thicknesses as illustrated in figure 15 and 16. The fiber 36a is focussed at 37a in advance of the weld pool 50 and fiber 36b focussed at 37b at the tail of the weld pool 50. The fiber 36c is focussed at 37c on the lateral edge of the weld zone 50. Each of the corresponding photodiodes 38a, 38b, 38c receives or is responsive to the same emission, either visible or UN.

As the beam 14 shifts laterally relative to the seam, the different thicknesses of material causes the longitudinal profile of the weld pool 50 to change. The three signals obtained from the fibers 36a, 36b, and 36c are able to monitor the change in profile and provide a signal proportional to the lateral positioning of the beam relative to the edges 22, 24. The effect of lateral shifting relative to the edges 22, 24 is shown in figure 17 that represents the results of tests performed on two zinc coated steel sheets, one with a thickness of 1.6 millimeters and the other with a thickness of 0.8 millimeters. In the first set of results shown at Figure 17a, the beam was shifted laterally from the thinner sheet to the thicker sheet. The signal indicated #1 is that associated with the lead fiber 36a and it can be seen that it progressively decreases as it moves from the thin to the thick sheet. Similarly the trailing fiber 36b indicated as #2 progressively increases and the lateral sensor 36c also progressively increases. Conversely in moving from thick to thin sheets 18, 20 the leading signal decreased rapidly and the trailing signal increased rapidly.

The output signals from the fibers 36a, 36b, 36c are combined in an adaptive linear combiner, the results of which are shown in figure 18. The adaptive linear combiner provides an output signal that is the linear combination of the input signals, i.e.

X.

1=0

Where S is the combined output signal, x, is an input signal w, is a weighting factor determined experimentally from test samples under controlled conditions n is the number of input signals

The weighting w, factor may be determined by moving the beam transversely across the seam and analyzing the variation in signals between the extremes of movement. It should be noted from figure 18 that the outputs of the linear combiner provides a progressive decrease as the beam moves from the thin to thick sheet and a progressive increase as it moves from the thick to the thin sheet. Accordingly, variations from the zero crossing may be used to control the lateral position of the beam 14 relative to the edges 22, 24 and cause the beam to follow the edges 22, 24.

A further control may be implemented as shown in figures 19 through 21 to provide a speed control for the movement of the laser head 12 along the workpiece 16. In this embodiment, the signal received by the lead fiber 36a and the lateral fibers 36c and 36d is used to monitor the weld zone 50 and control the welding speed. The width of the weld zone 50 is closely related to the weld depth and a wide weld bead indicates that sufficient energy has been deposited on the surface of the component 16 so that full penetration or deeper penetration is made. As the speed increases a smaller diameter of weld zone is detected and at lower speeds a larger diameter is detected. The photodiode 38a receives or is responsive to the UV content of the signal from fiber 36a and the photodiodes 38b, 38c determine IR content.

At relatively low weld speeds, a large weld pool 50 is established so that each of the lateral fibers 36b, 36c will transmit a relatively strong IR signal. The plume, however, as seen by the lead fiber 36a, has a relatively low UV component with a resultant low signal from photodiode 38a. At relatively high speeds, the weld pool 50 is reduced and the lateral fibers 36b,

36c transmit lower IR signals and the lead fiber 36a a stronger UV signal.

The results of tests performed with the arrangement of figures 19 and 20 are shown in figure 21 in which the signal obtained from the lead fiber 36a is identified as #3. It will seen as the rate of movement of the laser head 12 increased as shown in figure 21a, the signal associated with the lead fiber 36a increased indicating that the keyhole at the weld zone was getting shallower. However at lower speeds it will be noted that the signals from the sensors 36d and 36c were stronger indicating that the weld pool is larger. Similar results are obtained in decelerating as shown in figure 21b. In each test the rate of movement varied from 12.7 millimeters per second to 101.6 millimeter per second over the length of the workpiece, i.e. 200 mm.

To monitor the weld speed, the difference in the signals between the lead and lateral fibers 36 may be used as a feed back signal to adjust the welding speed and maintain a stable weld pool size. As the speed increases, the effect on the weld pool is monitored by the difference in the signals, thereby providing the required feedback signal. Although both lateral fibers 36b, 36c are used it will be appreciated that only one lateral fiber is needed to obtain the feedback signals. It will be seen therefore that the provision of the optical head 26 and the multiple signals received from the weld zone enables a variety of controls to be implemented by the control 42. Although these have been described as being implemented individually it will be appreciated that the control 42 may process the information received in each mode in parallel and provide the outputs to a suitable logic circuit to make the appropriate adjustments. In this regard the output from photodiodes 38 may be implemented in a fuzzy logic system as disclosed in the above noted U.S. Patent with an appropriate rule set derived from the observed parameters.

It will also be appreciated that the photodiodes 38 may be duplicated to derive different spectral components from the signals in the fibers 36 so that infra red spectral component may be monitored and the ultra violet component monitored from the same fiber to perform different control modes from the same detected signal.

Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the scope of the invention as outlined in the claims appended hereto.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A laser process control apparatus for controlling a laser beam in relation to a workpiece, said laser beam being produced by a laser for impinging upon a surface of said workpiece for producing a melt zone, the apparatus comprising: a monitoring assembly associated with said laser for monitoring a plurality of locations associated with said work piece, said monitoring assembly including a plurality of optical fibers, each of said optical fibers adapted to monitor a respective one of said locations and transmit optical signals collected therefrom; and a controller to receive at least one of said optical signals from at least one of said optical fibers and monitor a predetermined processing characteristic of said melt zone to provide an output signal.

2. A laser process control system according to claim 1, wherein said monitoring assembly further includes a focus mechanism for focusing said optical fibers at respective ones of said plurality of locations.

3. A laser process control system according to claim 2, wherein said focus mechanism includes an adjustable optical element.

4. A laser process control system according to claim 1, wherein an optical property of said signal is correlated by said controller to particular conditions in said melt zone.

5. A laser process control system according to claim 4 including a filter to determine inherent characteristics of said optical signal.

6. A laser process control system according to claim 4, wherein said optical property is signal intensity.

7. A laser process control system according to claim 6, wherein said monitoring assembly further includes a plurality of photodiodes coupled to respective ones of said plurality of optical fibers to convert said optical signals to respective electrical signals.

8. A laser process control system according to claim 7, wherein each of said photodiodes provides a control signal to said controller.

9. A laser process according to claim 7, wherein a plurality of photodiodes are associated with one of said optical fibers and each of said plurality of photodiodes is responsive to a different optical characteristic of said optical signal.

10 A laser process control system according to claim 1 , wherein a pair of said signals are combined for providing a control signal to said controller.

11. A laser process control system according to any preceding claim, wherein said output signal is transmitted as a feedback signal by said controller to responsively adjust said beam relative to said workpiece.

12. A laser process control system according to any preceding claim, wherein said plurality of locations are disposed about said melt zone.

13. A laser process control system according to claim 12, wherein said laser beam translates relative to said workpiece along a predetermined path and a pair of said locations are disposed in spaced relationship along said path.

14. A laser process control system according to claim 13, wherein said controller compares signals received from said pair of locations and adjusts said beam to maintain said signals in a predetermined ratio and thereby focus said beam on said workpiece.

15. A laser control system according to claim 14, wherein said controller determines a difference between said signals and adjusts said beam to maintain said difference at a minimum.

16. A laser process control system according to claim 13, wherein one of said fibers is focused in advance of said beam, said controller monitoring variations in the signal received by said one fiber to determine variations in a gap between adjacent edges of said workpiece.

17 A laser process control system according to claim 16 wherein another of said fibers is focused at said weld zone and provides a reference signal to said controller to compensate for variations in the signal received by said one fiber.

18. A laser process control system according to claim 17 wherein a pair of characteristics of said optical signal transmitted by said fibers is different.

19. A laser process control system according to claim 17 wherein the difference between said signals is determined by said controller and changes therein are used as an indication of the variation in gap between adjacent edges of said workpiece.

20. A laser process control system according to claim 17 wherein a ratio of said signals is determined by said controller and changes therein are used as an indication of the variation in gap between adjacent edges of said workpiece.

21. A laser process control system according to claim 13 wherein one of said plurality of locations is to one side of said predetermined path, said controller combining signals received from each of said locations to determine the position of said beam relative to said path.

22. A laser process control system according to claim 21 wherein said controller includes an adaptive linear combiner to sum said signals from each of said locations.

23. A laser process control system according to claim 22 wherein said controller applies a weighting factor to each of said signals.

24. A laser process control system according to claim 1 wherein a pair of said locations are disposed to opposite sides of said melt zone to provide an indication of the lateral extent thereof and said controller monitors said lateral extent to responsively control a speed at which said beam translates relative to said workpiece.

25. A laser process control system according to claim 24 wherein a further location is provided in advance of said melt zone, said fiber associated with said further location providing a signal indicative of the depth of penetration of said beam in said workpiece, said controller combining said signals to control relative movement to maintain a stable melt zone.

26. A laser process control system for monitoring a laser processing operation on a work piece comprising: a laser for producing a laser beam to impinge upon a surface of said work piece for producing a melt zone; a monitoring assembly associated with said laser and including a plurality of sensors, each of which is adapted to monitor a respective one of a plurality of locations associated with said melt zone and transmit a signal collected therefrom; and a controller to receive said signals from said sensors and combine said signals to obtain an indication of a predetermined processing characteristic of said melt zone.

27. A laser process control system according to claim 26 wherein said controller combines a pair of said signals by determining the difference thereof.

28. A laser process control system according to claim 26 wherein said controller combines a pair of said signals by determining the ratio thereof.

29 A laser process control system according to claim 26 wherein said controller utilizes one of said signals as a reference signal to monitor changes in another of said signals.

30. A laser process control system according to any one of claims 26 to 29 wherein said controller provides an output signal operable upon said laser to control impingement thereof on said workpiece.

31. A laser process control system according to claim 30 wherein said laser beam translates relative to said workpiece and said output signal is operable to control translation therebetween.

32. A laser process control system according to claim 31 wherein said laser beam moves relative to said workpiece along a predetermined path and a pair of said signals are obtained from spaced locations along said path.

33. A laser process control system according to claim 31 wherein said laser beam moves relative to said workpiece along a predetermined path and a pair of said signals are obtained from spaced locations to opposite sides of said path.

34. A laser process control system according to any one of claims 26 to 33 wherein said signals are optical signals transmitted to said controller by optic fibers.

35. A laser process control according to claim 34 including a filter associated with respective ones of said fibers to constrain the spectral component of said signal.

36. A method of monitoring a laser process performed upon a workpiece comprising the steps of: impinging a laser beam on a workpiece for producing a melt zone, obtaining a plurality of signals from said melt zone, combining said signals by a controller for providing information about said melt zone, and using said information to monitor a predetermined processing characteristic of said melt zone.

37. A method according to claim 36 wherein said signals are obtained from a plurality of locations disposed about said melt zone.

38. A method according to claim 37 further comprising the step of combining a pair of said signals by determining the difference thereof.

39. A method according to claim 37 wherein said controller combines a pair of said signals by determining the ratio thereof.

40. A method according to claim 37 wherein said controller utilizes one of said signals as a reference signal to monitor changes in another of said signals.

41. A method according to any one of claims 37 to 40 including the step of providing an output signal said controller operable upon said laser to control impingement thereof on said workpiece.

42. A method according to claim 40 including the step of translating said beam relative to said workpiece and utilizing said output signal to control translation therebetween.

43. A method according to claim 42 further comprising the step of translating said laser beam relative to said workpiece along a predetermined path and obtaining said signals from a pair of said locations disposed in a spaced apart relationship along said path.

44. A method according to claim 43 further comprising the steps of comparing signals received by said controller from said pair of locations and adjusting said beam to maintain said signals in a predetermined ratio and thereby the focus of said beam on said workpiece.

45. A method according to claim 44 further comprising the step of determining by said controller the difference between said signals and adjusting said beam to maintain said difference at a minimum.

46. A method according to claim 42 further comprising the step of obtaining one of said signals from a position in advance of said beam and said controller monitoring variations in said one signal.

47. A method according to claim 46 including the step of utilizing a signal from another position relative to said beam as a reference to determine variations in said one signal.

48. A method according to claim 42 including the step of obtaining a pair of signals from opposite sides of said predetermined path and combining said signals to provide an output signal indicative of deviation from said predetermined path.

49. A method according to claim 42 including the step of obtaining a plurality of signals from locations about said melt zone and combining said signals to provide an output signal indicative of the extent of said melt zone relative to an optimum extent.

50. A method according to any one of claims 36 to 49 wherein said signals are indicative of an optical characteristic emanating in said melt zone.

51. A method according to claim 50 wherein said optical characteristic is filtered to constrain the optical characteristics of said signal.