Note: Descriptions are shown in the official language in which they were submitted.
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S'TRUCTiJR)JD MULTI-SPOT DI;TI;CT1NG'fI~CIINLQUr, 1!'OR ADAI''fIVE
C()N'I'RUL, UI' LASI?,R 131~AM I'ItOCI~,SSIN(
hII:LU Ur 1'lll, INVGN'1'IUN
'fhe present invention relates to laser processing.
RELAT I:JD AIt'f
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 rt
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
rmage.
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As a further enlranccmcnt of this tecl~nidue as disclosed in fC'r application
PC'T/CA98/00895 the fuzzy logic is combined with a neural network to provide
enhanced control of the processing.
'1'I~c above tcclririclucs have been particularly beneficial wlrerc relatively
Icng
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
arid
processing of the image. 'fhe 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.
DESCRIFT'ION OF'I'IIE DI~1WINGS
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 I .
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Figure 5 is a schematic rcprcsetttalion of tltc alternative position shown in
figure
Figure G is a plot of signal intensity. versus hcigl~t for tUc apparatus of
figure 1
operating in a first triode of operation.
Figure 7 is a graph of differential signal intensity versus height derived
froth
figure G.
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 lU 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 ltl
a
further mode.
Figure 1G is a view on the line 16-1G 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 ftgure 1 in the mode represented in figure 20.
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I)C'1'AII~I~,U Dh;SCRl1''1'IUN Uh I~M13UD1MI~,N'I'S UI' '1'111; 1NVI~,N'1'1()N
Refewing 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. 'fhe work piece 16
may be
one of a variety of forms but in lire 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 worlcpiece 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 arn~ 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
Cbers may be
utilized depending upon the mode of control to be implemented as will be
described in
further detail below.
Fach of the photodiodes 38 provides an output signal 40 corrected 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
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the fibers 3Ga is focused in advance of tltc pool 50 at 37a and another, 3Gb,
focused
behind the pool 50 at 37b. The two other fibers 3Gc and 3Gd respectively are
focused on
opposite sides of the pool 50 at 37c and 37d respectively. The respective
photodiodes 38
will tltercforc receive information from floe plume, tltc weld pool and
surrounding
regions and may use that information to provide control signals to the control
42.
Typically the infortnalion received will relate io 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 pltotodiodes
38 will be
responsive to either UV, IR or visible light. 1'he 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 rnay be utilized by the control 42 is a focus
control to
maintain the beam 14 focused on the workpiece 1 G to accommodate any
variations in the
relative position of the laser 12 and the adjacent surface of workpiece 1G. To
provide a
focus control, the control 42 selects signals from the fibers 3Ga 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 3Ga and 3Gb 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 Sb. If the workpiece 1G moves closer to the laser 12
as shown in
figure Sa, the fiber 3Ga 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 3Ga increases and that 38b, associated with the
fiber 3Gb
decreases. In this case, each of the diodes 38a, 38b is responsive to one
range of
frequencies, either UV or visible light. IR is not generally desirable as the
signal is not
symmetric.
Similarly, as shown in figure Sc, an increase in the spacing between the laser
12
and the workpiece 1G causes the trailing fiber 3Gb to be focused at the center
of the pool
50 and the lead fiber 3Ga further in advance of the pool 50.
The signals received from the respective diodes 38a, 38b is shown in figure G
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
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stable control signal that may be used to monitor the heiglU of the laser head
12 relative
to the component 1 G.
Refen-ing to figure 7, the differential signal between the lead and trailing
fibers
3Ga, 3Gb provides a zero crossing corresponding to the beam 14 being focussed
on the
workpiece 1G. 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 l 2 can be changed
to
compensate for the variations in the relative position of the component 1 G.
In tests performed with a COz laser, satisfactory results were obtained. The
profile of a test component IG 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 WIN~Gps board. 'fhe 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
cosec 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 interniittent 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 1 G. A uniform width and dimension to the weld can be seen from
figure I 0
indicating a generally satisfactory welding process. 1n each of the samples
mild steel of
thickness Intro 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
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lead fiber 36a is focussed in advance of the weld pool at 37a and tltc fiber
3~~h 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 pltttne, as shown in ghost outline
in ligurc 1.
In this triode, the pltolodiode 38a receives or is responsive to visible or
llt emissions
whereas the photodiodc 38b receives or is responsive to visible or UV
emissions. 1f the
edges 22, 24 provide a close Gt 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 tray 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 froth 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 3Gb 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
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50. Each of the corresponding photodiodes 38a, 38b, 38c receives or IS
rCSpOIISIVC to the
same emission, either visible or UV.
As the beam 14 slrifis laterally relative to the seam, the dif(crent
tllickncsscs 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 t0 IT10r1rtol' 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 thiruter sheet to the thicker sheet. The signal indicated #I is that
associated with the
lead fiber 3Ga 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 3Gc 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 3Ga, 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.
n
S = ~ Wixi
i=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.
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It should he noted li~om iigurc 18 that the outputs of the linear combincr
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
li-om the
zero crossing may be used to control the lateral position of tltc bcant 1 ~l
relative to tl~c
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.
'fhe 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 awangement 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.
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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 lie appreciated that only one lateral fiber
is needed to
obtain tire feedback signals.
It will be seen therefore that the provision of the optical head 2G 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 tray 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 tnay 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.
