Electric heating – Metal heating – By arc
Reexamination Certificate
2002-09-23
2003-12-09
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130330
Reexamination Certificate
active
06660965
ABSTRACT:
TECHNICAL FIELD
This invention concerns the assessment of welding. In particular it concerns an apparatus and a process for determining whether a fault has occurred in a welding process, while the process is under way. The invention is applicable to gas-metal arc welding, tungsten-inert gas welding, pulsed welding, resistance welding, submerged arc welding and to other welding and cutting processes where there is an arc plasma.
BACKGROUND ART
The study of welding and cutting arc phenomena involves observation of both voltage and current signals having periods of milliseconds to seconds, or even micro-seconds. One way of monitoring these signals involves the use of high speed photography, and another is the use of oscillograms. The limitations inherent in the observation techniques and the difficulties in analysing the resulting data, make it difficult to provide a weld quality measurement in real time.
SUMMARY OF THE INVENTION
A Single Welding Signature
The invention is an apparatus for on line welding assessment, comprising:
first sampling means to sample the welding voltage or current to provide a sequence of values for a first signal,
second sampling means to sample the welding current or voltage to provide a sequence of a values of a second signal,
a signal generating means to generate one or more sequences of values for one or more artificial third signals from the first signal and second signals where the artificial signals depend upon values of the first and second signals through generalised discrete point convolution operations,
tripling means to identify corresponding values of the first, second and third signals, and
collection means to collect triplets of values which are useful for quality monitoring into groups or regions. The triplets collected could be visualised to be those that would fall within selected regions of a three dimensional scatter plot of the values of the first, second and third signals. The regions could be drawn on to such a visualisation.
The first signal data sequence may be represented as the sequence D
1
,D
2
, . . . , D
&eegr;−1
, D
&eegr;
, and the second signal data sequence represented as the sequence &Ggr;
1
, &Ggr;
2
, . . . , &Ggr;
&eegr;−1
, &Ggr;
&eegr;
. The total number of data points &eegr; must be 2 or higher and a value of 1000 may be used. The artificial sequence numbered s is the sequence A
1,s
,A
2,s
, . . . , A
&eegr;−1,s
, A
&eegr;,s
. The artificial sequence number s varies from 1 to a maximum value &sgr;, &sgr; must be 1 or higher and a value of 5 may be used. The member n of the artificial sequence numbered s, A
n,s
, may be determined from:
A
η
.
5
=
∑
κ
=
1
η
Ψ
(
1
,
κ
,
n
,
s
,
t
)
D
κ
+
Ψ
(
2
,
κ
,
n
,
s
,
t
)
Γ
κ
(
1
)
The coefficients &psgr; may depend on &kgr;, the location of D
&eegr;
in the first signal data sequence and also the location of &Ggr;
&eegr;
in the second signal data sequence: n, the location of A
n,s
in the artificial data sequence numbered s; s, the artificial sequence number; and t, the time at which D
&eegr;
and &Ggr;
&eegr;
were measured with respect to some specified time origin. The artificial signal generating means applies equation (1) repeatedly to calculate all values of A
n,s
for n varying from 1 to &eegr;, and s varying from 1 to &sgr;. A useful choice for &psgr; is:
&PSgr;(1,&kgr;,
n,s,t
)=
e
(k−n)(T
0−ST
1)
. . . (&kgr;−
n
)<0
&PSgr;(1,&kgr;,
n,s,t
)=0 . . . (&kgr;−
n
)≧0
&PSgr;(2,&kgr;,
n,s,t
)=&THgr; . . . &kgr;=
n
&PSgr;(2,&kgr;,
n,s,t
)=0
. . . &kgr;≠n
(2)
In equation (2) there is no explicit dependence on t. With this choice. equation (1) is close to a convolution of the first signal with a damped or decaying exponential added to the second signal multiplied by &THgr;. The effective damping time constant is given by &tgr;
0
+&tgr;
1
s. The constants &tgr;
0
and &tgr;
1
set the range covered by the time constant as s varies from 1 to &sgr;. The constant &THgr; sets the amount of second signal added.
The inclusion of an explicit dependence of &psgr; on time t or sequence number n is useful when the welding system properties are varying during the sampling for a signature. For example, in resistance spot welding, physical conditions vary substantially during one spot weld for which a single signature may be determined.
Grouping means form all possible sets of values of the type {D
n
, s, A
n,s
}, that is, sets consisting of a first signal data point D
n
, an artificial sequence number s, and the corresponding member of the artificial sequence number s. A
n,s
, n varies from 1 to &eegr;, and s varies from 1 to &sgr;. If there is only one artificial sequence, then s is always set to 1 in the sets of values.
Collection means collect sets of values which are useful for weld monitoring into groups or regions. The sets collected could be visualised to be those that would fall within selected volumetric regions of a three dimensional scatter plot with one axis plotting the value of the first signal, a second axis plotting the sequence number of the artificial sequence, and a third axis plotting the value of the corresponding artificial signal. If there is only one artificial sequence, all points will lie in the plane defined by s=1. The boundaries of the regions could be displayed as closed surfaces on such a visualisation.
The regions need not be of equal size, and they may be smaller where population density is greatest and may be exponentially greater in dimension, in both the first and artificial signal directions, as they progress away from the region of greatest population density. Once the regions are chosen, they are fixed during the weld monitoring process. The regions selected need not be contiguous, and regions may overlap.
Each of the collected sample points that fall within a given region are accumulated in the population of that region. The region populations can be represented by a population density function f
r
which is the population of the region numbered r, with r varying from 1 to &rgr;.
If a given point at {D
n
, s, A
n,s
} falls within region r, accumulation means increase the population f
r
by w
r
(D
n
, &Ggr;
n
, A
n,s
, n, s, t), where t is the time at which D
n
and &Ggr;
n
were measured. w
r
(D
n
, &Ggr;
n
, A
n,s
, n, s, t) is the weight the point is given in region r. If w
r
is always one, for example, the populations are a simple count of the number of points in each region.
To produce the final adjusted region populations p
r
, function application means apply a single valued monotonic function F to each of the f
r
values:
p
r
=F
(
f
r
) (3)
for r=1 to &rgr;.
The complete set {p
1
, p
2
. . . p
&rgr;−1
, p
&rgr;
} of the p
r
collected is a single welding signature.
The weight functions w
r
are chosen to produce a welding signature which contains as much information about the properties of the final weld as possible for a given sampling rate and size. This may be done experimentally, by trial and error adjustment or by knowledge of the physical process. Since there is some statistical noise in the sample, it is useful to choose the w
r
to smooth the welding signature: this may be achieved by defining overlapping regions and decreasing w
r
for points closer to the boundary of region r. The function F is chosen to maximise the sensitivity of the welding signature to faults in the final weld.
The inclusion of a dependence of the weights w
r
on the data point number n permits windowing. Weights may be reduced near the start of the data sequence at n=1 and near the end of the sequence at n=&eegr; for example.
Generating a Combined Welding Signature
For a given process, it may be desirable to generate two single welding signatures using both the current and the voltage as the first signal. For processes such as tandem arc welding, two welding voltages and two cur
Bozicevic Field & Francis LLP
LaSalle Carol
Shaw Clifford C.
The University of Sydney
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