Apparatus and method for sensing welding point in automatic welding apparatus

An automatic welding apparatus comprises a welding torch including an electrode and a fixture for fixing a workpiece, and a relative positional relation of the welding torch and the workpiece fixture is controlled such that the electrode of the welding torch is moved to trace the welding line of the workpiece, whereby automatic welding is achieved. The automatic welding apparatus further comprises a switch for selectively connecting the electrode of the welding torch to a welding power supply or a detecting power supply, and a sensor for sensing a current or a voltage between the electrode and the workpiece when the electrode is switched to the detecting power supply, whereby the relative positional relation between the welding torch and the workpiece is detected in response to the output of the sensor, and thus a welding point or a welding line is sensed. In the case where the automatic welding apparatus is a wire extension type, adapted to successively feed the electrode in accordance with the progress of welding, a collet chuck for clamping the electrode to the welding torch is provided for fixing the length of the electrode from the welding torch on the occasion of the sensing mode.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an automatic welding apparatus. More 
specifically, the present invention relates to a novel automatic welding 
apparatus, wherein a welding torch itself is used for sensing a welding 
line of a workpiece in an arc welding process. 
2. Description of the Prior Art 
An automatic welding apparatus of the play back type has been proposed and 
put into practical use, wherein a relative positional relation in the 
space between a welding torch and a workpiece is controlled in accordance 
with positional information and control information stored in a storage or 
a control apparatus, thereby to achieve an automatic welding process in 
accordance with a program. It has also been known to those skilled in the 
art that a sensor can be employed to make a welding torch trace a welding 
line of a workpiece. A conventional sensor for detecting a welding line of 
the workpiece was provided separately from the welding torch, but in the 
vicinity of the welding torch. As a result, the whole geometry of the 
welding torch was large. Therefore, it could happen that the welding 
torch, and thus the sensor do not reach the innermost recess of the 
workpiece, or the sensor does not effectively act on the welding line when 
the geometry of beveling or the angle of beveling is small. In addition, 
such a conventional apparatus is complicated in structure and expensive in 
cost, leaving various problems to be solved. 
SUMMARY OF THE INVENTION 
Briefly described, according to the present invention, a relative 
positional relation of a welding torch and a workpiece is controlled by 
means of a control means comprising a data processing unit. In a sensing 
mode, an electrode of the welding torch is supplied with a detecting high 
voltage. In the sensing mode, a conduction state between the electrode of 
the welding torch and the workpiece is detected and the control apparatus 
receives an output indicating the conduction state, whereupon a mutual 
positional relation is effected. 
In a peferred embodiment, the electrode length from the welding torch is 
kept constant and, to that end, a scheme for maintaining such a state is 
additionally provided. 
According to the present invention, using the electrode of the welding 
torch per se as a sensor makes it possible to dispense with a structural 
member such as a sensor around the torch, and also make it possible to 
detect any position of the welding line where the torch can enter. The 
above described feature makes the inventive apparatus simple in structure 
and further makes it possible to detect the position of the welding line 
of any geometry. 
The present invention can be applied even to a case where a consumptive 
electrode is employed, by providing a shaping or forcing apparatus, such 
that the tip end of the electrode is always brought to a predetermined 
position with respect to the welding torch, whereby any problem in the 
sensing mode is eliminated. In a preferred embodiment, the detecting 
voltage source is of a high voltage. In such case, the discharge spacing 
remains approximately constant irrespective of the conditions of the 
surface of the workpiece or the tip end of the electrode, which makes an 
accurate sensing operation consistently possible. 
In another preferred embodiment of the present invention, even the sensing 
operation of the welding line is adapted to be achieved in accordance with 
a teaching operation. Therefore, even if the workpiece or base metal is 
changed, the initiating point and the terminating point of the welding are 
automatically sensed and taught. Therefore, the conventional manual 
control technique for determining mutual positional relation between the 
workpiece and the welding torch in the play back operation can be 
dispensed with, and this shortens the time required for a teaching 
operation and makes possible an accurate teaching operation. 
Therefore, a principal object of the present invention is to provide a 
novel automatic welding apparatus, wherein a welding torch itself is also 
used as a sensor. 
Another object of the present invention is to provide an automatic welding 
apparatus, wherein a welding torch is used as a sensor and a stabilized 
sensing operation is achieved. 
A further object of the present invention is to provide an automatic 
welding apparatus, which is simple in structure. 
Still a further object of the present invention is to provide an automatic 
welding apparatus, which is capable of detecting a beveling with ease and 
within a short period of time. 
These objects and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a complete perspective view showing an automatic welding 
apparatus which forms the background of the present invention and wherein 
the present invention can be advantageously employed. The automatic 
welding apparatus shown is arranged such that the necessary degrees of 
freedom are divided between a workpiece fixture and a welding torch 
fixture, so that the position of a workpiece or the welding torch can be 
controlled. Therefore, even in case of three-dimensionally welding a 
workpiece of complicated shape, welding can be performed with an optimum 
attitude of the workpiece, which provides convenient welding conditions, 
and the mechanical structure and the control apparatus are also simple. 
Referring to FIG. 1, the automatic welding apparatus 10 shown is arranged 
such that a fixture 15 for fixing a workpiece (not shown) can be moved 
right and left and back and forth and rotated by an amount .theta. of 
rotation around a horizontal axis H (simply referred to as .theta. axis) 
and a fixture 18 for fixing a welding torch T can be vertically moved and 
rotated by an amount .phi. of rotation around a vertical axis L (simply 
referred to as .phi. axis), there being provided a control box 40 for 
automatically controlling the movement and rotative position of the 
workpiece (not shown) and the torch T. 
A base plate 11 which is L-shaped in plan view has a first frame body 12 
fixed to one side thereof. Mounted on top of the frame body 12 is a 
carriage 13 movable right and left or in the direction of the X-axis shown 
in the figure. A prime mover, not shown, for the carriage 13 may be a well 
known motor equipped with a reduction gear mechanism and a brake, while a 
power transmission means, not shown, for the carriage may be a 
ball-and-nut engagement means, which is often called a ball screw. 
Further, a second frame body 14 is mounted on the top of the carriage 13 
so as to be movable back and forth in the direction of the Y-axis shown in 
the figure. A prime mover and power transmission means for the frame body 
14, not shown, may be a similar motor equipped with a reduction gear 
mechanism and a brake and a ball screw. The workpiece fixture 15 is 
provided on the front of the frame body 14 so as to be rotatable in the 
.theta.-axis direction shown in the figure. A prime mover for the 
workpiece fixture 15, not shown, may also be a known motor equipped with a 
reduction gear mechanism and a brake. 
A third frame body 16 is erected on the other side end of the base plate 
11. The third frame body 16 is provided with an arm 17 movable in the 
vertical direction, i.e. the Z-axis direction shown in the figure. A prime 
mover and power transmission means for the arm 17, not shown, may also be 
a similar motor equipped with a reduction gear mechanism and a brake, and 
a ball screw, respectively. The arm 17 is provided at its front end with 
the fixture 18 for fixing the welding torch T. A prime mover for the 
welding torch fixture 18, not shown, may also be a known motor equipped 
with a reduction gear mechanism and a brake. Further, the position where 
the welding torch T is fixed is selected such that the welding point WP on 
an extension of the center line of the welding torch T is aligned with the 
vertical axis L, while the fixing angle thereof is suitably selected 
according to the manner of the welding to be performed, such as butt 
welding or fillet welding, and according to the shape of the workpiece as 
well. Further, the welding torch T is supplied with a current from a 
welding power source 20. 
The forward and backward translation and rotation, the rate of movement of 
the prime movers (such as motors each equipped with a reduction gear 
mechanism and a brake for the individual parts) and the welding current 
are automatically controlled by means of the control box 40 and the 
welding control device 30 in accordance with a program stored in the 
control box 40, and the relative position between the two fixtures 15 and 
18 is controlled such that the welding point WP may trace a welding line 
on a workpiece, not shown, and thus an automatic welding operation can be 
performed in an attitude which provides optimum welding conditions. A 
remote control panel 50 is provided for manual operation and for 
preloading a program for that purpose. 
In addition, in the example shown, since the welding point WP on an 
extension of the center line of a welding torch T is adapted to be aligned 
with the vertical axis L, the welding point remains unchanged irrespective 
of the rotation of the fixture 18 in the .phi.-axis direction, and the 
attitude of the welding torch T with respect to the welding point can be 
optionally changed by the rotation of the fixture 18 in the .phi.-axis 
direction. In other words, the automatic welding apparatus of the example 
shown has five degrees of freedom, i.e. the amounts X, Y and Z of 
displacement along the X, Y and Z axes, respectively, and the amounts 
.phi. .theta. of rotation about the L and H axes, respectively. 
Although not shown, the welding control device 30 is provided with a 
voltage regulator and a current regulator, each comprising a variable 
resistor and the like, as is well known in the art. Further, in order to 
provide for changeover between a welding machine proper mode and a welding 
robot mode, there is provided a robot changeover switch. This switch is 
normally set for the welding robot mode and operatively associated with 
the workpiece. 
Position control of this type of automatic welding apparatus is described 
in more detail in the copending United States patent application, Ser. No. 
815,783, entitled "Method and System of Velocity Control for Automatic 
Welding Apparatus", filed July 14, 1977 by Shigeo Maruyama et al and 
assigned to the same assignee as the present application. Briefly 
described, a relative positional relation of a welding torch and a 
workpiece fixture is controlled based on positional information concerning 
the present positions of the welding torch and the workpiece fixture and 
command positional information concerning a desired positional relation of 
the welding torch and the workpiece fixture as programmed in advance to 
reach the commanded relative positional relation of the welding torch and 
the workpiece, whereupon next command positional information is obtained 
and the next commanded relative positional relation is reached. The above 
referenced patent application issued on Dec. 18, 1979 as U.S. Pat. No. 
4,179,602 and is incorporated herein by reference thereto. 
It is pointed out that the above described type of automatic welding 
apparatus should not be construed by way of limitation but the present 
invention can be employed in any other well known type of automatic 
welding apparatus. 
FIG. 2 shows a schematic view of a consumptive electrode supply. The torch 
T is mounted rotatably about the axis L by means of the torch fixture 18, 
as described previously. The torch T is supplied with a consumptive 
electrode 209 from a consumptive electrode supply means 201. The 
consumptive electrode supply means 201 is provided in the power supply 
apparatus 20 as shown in FIG. 1, and comprises a consumptive electrode 
shaping apparatus 202. The consumptive electrode shaping apparatus 202 
will comprise a loop like arrangement of a flexible tube for allowing 
passage of the consumptive electrode therethrough, such that the 
consumptive electrode 209 as allowed to pass through the flexible tube is 
given a curl. Alternatively, the shaping apparatus 202 may be a 
straightening roller adapted for sandwiching the consumptive electrode so 
as to straighten the same. The consumptive electrode 209 as supplied from 
the consumptive electrode supply means 201 has applied thereto a voltage 
from a well known voltage applying means 203. The voltage applying means 
203 is selectively connected by means of a switch 204 to a welding voltage 
source 205 or to a discharging high voltage source 206 serving as a 
detecting voltage source. The welding voltage source 205 is a well known 
voltage source for supplying a welding voltage and is adapted to provide 
an electric power of a relatively low voltage but of a large current. On 
the other hand, the discharging high voltage source 206 is adapted to 
provide an electric power of a relatively high voltage of about 100 to 
2000 volts and of a small current. The welding voltage supply 205 is 
directly connected to a workpiece W (via line 210) and the discharging 
high voltage source 206 is connected to the workpiece W through an 
electric current sensor 207 and line 210. The electric current sensor 207 
is structured to detect a current flowing between the consumptive 
electrode 209 of the torch T and the workpiece W. A voltage sensor for 
detecting a voltage may be substituted for the current sensor 207. The 
output signal associated with the current value variation from the current 
sensor 207, or associated with the voltage variation of such a voltage 
sensor, is applied to a control means of a general purpose computer, or to 
a microcomputer or a microprocessor, in the control box 40. The above 
described switch 204 is normally turned to the welding voltage source 205 
as shown in FIG. 2 but is turned to the discharging high voltage source 
206 responsive to a switch command from the control box 40. 
FIG. 3 is an enlarged side view of the torch fixture 18. As seen from FIG. 
3, the torch fixture 18 comprises a horizontal directional arm 18c 
detachably provided to the lower portion of a shaft 18a by means of a bolt 
18b, a vertical directional arm 18d fixedly provided to the tip end of the 
arm 18c, a rod 18e pivotally mounted to the lower end of the arm 18d, and 
a torch holding member 18f pivotally and detachably provided to the tip 
end of the rod 18e. Accordingly, the welding point WP of the torch T can 
be fine adjusted by pivoting the rod 18e and the holding member 18f. 
FIGS. 4 and 5 shows perspective views of workpieces W to which the present 
invention can be applied. More specifically, FIG. 4 shows a case of 
fillet, and FIG. 5 shows a case of butt welding with the resultant 
beveling. 
FIG. 6 is a flow diagram showing one embodiment of the present invention in 
a teaching mode. The operation of the embodiment will be described in the 
following. 
At the outset, a pattern of teaching will be described. Generally, 
workpieces being welded can be classified into several types or patterns 
depending on the precision, geometry and the like thereof. Therefore, in 
the teaching mode, the information concerning the pattern of the workpiece 
is manually entered. At the outset, the patterns of the workpieces to be 
entered in the teaching mode will be described. 
Pattern 1: 
This pattern covers workpieces or base metals of such high precision that 
the initiating point and the terminating point of welding are sensed only 
in the teaching mode, while the same positional information can be used 
when such workpieces are replaced. 
Pattern 2: 
This pattern covers workpieces of less precision as compared with the 
pattern 1, wherein, although the sensing operation must be taught, an 
error is caused only in the parallel direction, and therefore only the 
initiating point of welding must be sensed. 
Pattern 3: 
This pattern covers workpieces wherein, in addition to the case of pattern 
2, the welding line deviates in a direction other than the parallel 
direction, which necessitates the sensing operation in the initiating 
point and the terminating point of the welding. 
First, a data processing unit or a control means, not shown, in the control 
box 40 (FIG. 1) is placed in a teaching mode and, through manual operation 
of operating buttons, not shown, of the remote control panel 50, the 
welding torch T is controllably moved through manual operation to the 
vicinity of the position being sensed, in this case, the vicinity of the 
initiating point of welding, in accordance with a well known play back 
system. The data concerning which of the above described patterns 1 to 3 
are to be implemented with respect to the workpiece being welded is 
entered by means of a pattern selection switch, not shown, provided in the 
control box 40. Further, the data concerning which kind of base metal A 
such as in the case of fillet welding as shown in FIG. 4, or B such as a 
case of butt welding as shown in FIG. 5 is to be used is entered by means 
of an input switch, not shown. A sensing command is then given. 
Accordingly, the control means makes a decision as to which pattern 1, 2 or 
3 is selected (FIG. 6--block 600). If the decision is pattern 1, then the 
control means makes decision as to which kind the base metal or workpiece 
belongs to, A or B (block 602). If the pattern is other than the pattern 
1, i.e. the pattern is the pattern 2 or 3, then the data concerning the 
above described vicinity is stored as initial positional information 
(block 604) and the kind of the base metal is then decided (block 602). If 
the kind of the base metal is A, then the subroutine of the sensing A is 
initiated (block 606). If the kind of the base metal is B, then the 
subroutine of the sensing B is initiated (block 608). 
Now referring to FIGS. 2, 4 and 7, a sensing operation in the case where 
the kind of base metal is A will be described. If the subroutine is 
initiated, a command to the switch 204 is outputted by a system program 
previously entered in the data processing unit (block 700), whereby the 
switch 204 is switched. Similarly, a command for lowering the Z axis, i.e. 
a command for lowering the torch T, is outputted by the system program 
(block 702), whereby the torch T is lowered. A high voltage is applied 
between the electrode of the torch T and the workpiece W from the voltage 
source 206. Therefore, when the tip end of the electrode of the torch T is 
brought to the point P2, where the distance between the workpiece and the 
tip end of the electrode is about 2 mm at the largest, a spark occurs 
therebetween. Thus, the electric current sensor 207 detects the current 
caused thereby, and the data Zs out of the positional information 
concerning the position P2 is loaded in response to the detected signal, 
whereupon the difference .DELTA.Z between the above described data Zs and 
the data Z2 out of the command information in this step is evaluated by 
the control apparatus or the data processing unit (blocks 704 and 706). At 
the same time, the torch T is raised by a predetermined amount of say 1 mm 
to 2 mm by the system program, to the point P3 (block 708). 
The control apparatus makes a decision as to the next sensing direction X 
and the sense (the rightward sense in FIG. 4) based on the angle of the 
.phi. axis being .phi.1 and the difference between the values X1 and X2 
(block 710). The control apparatus provides a command for moving the 
workpiece W in the above described sense, whereby the workpiece W is moved 
rightward (block 712). Likewise, the electrode 209 and the workpiece W 
come close to each other, and a spark occurs at the point P4, whereupon 
the difference .DELTA.X between the position Xs of the point P4 in the X 
direction and the command X2 is evaluated (blocks 714 and 716). The 
workpiece W is then returned by a predetermined amount to the point P5 
(block 718). Thus, completion of the sensing operation is decided and the 
command to the above described switch 204 is cleared in response to the 
decision output, whereby the switch 204 is returned to the original 
position and thus to the welding voltage source 205 (block 720). 
Now referring to FIGS. 2, 5 and 8, a sensing operation in the case where 
the kind of base metal is B will be described. Such embodiment is 
different from the embodiment of FIGS. 4 and 7 and can be applied to a 
case where the angle between two bevel surfaces constituting the welding 
line WL, i.e. the beveling angle, is not known in advance. The welding 
line WL in the X axis direction in the embodiment shown constitutes a butt 
welding line having a beveling, wherein downward welding is intended, and 
the torch T assumes a vertical posture. 
In this subroutine, first a command to the switch 204 is obtained (block 
800), whereby the switch 204 is turned to the high voltage source 206 and 
a high voltage is applied between the electrode 209 of the torch T and the 
workpiece W. A command for lowering the Z axis, i.e. a command for 
lowering the torch T is obtained (block 802), whereby the torch T is 
lowered. A spark occurs at the point P2 (X1, Y1, Z2) and a detected signal 
is applied from the electric current sensor 207 to the control means in 
control box 40 of FIG. 2 (see block 804 of FIG. 8, as well). The control 
means is responsive to the detected signal to receive the positional 
information (X1, Y1, Z2) concerning the point P2 and the stores the same 
in a predetermined location (block 806). 
In accordance with the sensor command indicating the sensing direction -X, 
the direction of the sensing is decided (block 808) and a move command for 
rightward moving the workpiece as viewed in FIG. 5 is obtained (block 
810). A spark occurs at the point P3 (X2, Y1, Z2) and a detected signal is 
applied from the electric current sensor 207 to the data processing unit 
or the control means (block 812). The data processing unit is responsive 
to the detected signal to enter therein the positional information 
concerning the point P3 and stores the same in a predetermined location 
(block 814). Then, a command for returning the workpiece in the sense 
opposite to the previous movement of the workpiece by a predetermined 
amount in the X axis direction is obtained (block 816). The above 
described predetermined amount is properly determined by the magnitude of 
the welding line WL. Accordingly, the workpiece W is moved slightly 
leftward in such a situation. 
If and when the point P4 (X3, Y1, Z2) is reached, a command for lowering 
the Z axis is again obtained (block 818), whereby the torch T is lowered 
(block 820). The detected signal obtained at the point P5 (X3, Y1, Z3) 
from the electric current sensor 207 (block 822) is applied to the data 
processing unit and the positional information concerning the point P5 is 
entered therein, whereby the positional information is stored in a 
predetermined location (block 824). A command for moving the workpiece W 
leftward, i.e. in the same sense as that of the previous movement in the X 
direction, is obtained (block 826) and at the point P6 (S4, Y1, Z3) a 
detected signal is obtained from the electric current sensor 207 (block 
828) and is applied to the data processing unit, whereby the positional 
information concerning the point P6 is entered in the data processing unit 
and is similarly stored therein. 
Thus, completion of the sensing operation is decided based on storage of 
the fourth piece of positional information responsive to the fourth input 
of the detected signal from the electric current sensor 207 (block 832), 
and the intersection of the line extending between the points P2 and P6 
and the line running between the points P3 and P5 is evaluated (block 
834), thereby obtaining the positional information concerning the point 
P7, and the operation returns to the routine shown in FIG. 6. 
Thus, the sensing of the initiating point of welding is completed. Then, 
referring again to FIG. 6, a further decision is made for example, as to 
whether or not it is necessary to correct the initiating position in 
consideration of uneveness of the thickness of the workpiece W (block 
610). If it is not necessary to correct the initiating position, the above 
described initiating position as evaluated by the routine of the sensing A 
or the sensing B is stored. If it is necessary to correct the initiating 
position, a correction value is added to the positional information 
concerning the initiating point thus evaluated to correct the same (block 
614), and, in case of the FIG. 5 example, the initiating point being the 
point P7 is corrected to the point P8, and the positional information 
concerning the initiating point, thus corrected, is stored. Thus, the 
teaching operation of the initiating position is completed. 
Thereafter, a sensing operation of the terminating position of welding is 
carried out. In this case as well, an operation button of the remote 
control panel 50 is operated to bring the welding torch to the vicinity of 
the terminating point (block 616). Thereafter, the data processing unit or 
the control means makes a decision as to which pattern the workpiece 
belongs to, the pattern 1, 2 or 3 (block 618). In case of the pattern 1 or 
2, a further decision is made as to which kind of the base metal the 
workpiece belongs to, A or B (block 620). If the decision is the pattern 
3, the position in the vicinity of the above described terminating point 
is stored for one step (block 622) and then the kind of the base metal is 
decided (block 620). If the kind of the base metal is A, then the terminal 
position is sensed in accordance with the above described routine of the 
sensing A (block 624). If the kind of base metal is B, then the 
terminating position is sensed by the above described routine of sensing B 
(block 626). Thereafter, a decision is made as to whether correction is to 
be made (block 628) and, if correction is made (block 630), the positional 
information concerning the corrected position is stored as positional 
information concerning the terminating point (block 632) if correction is 
not made, then the positional information of the position, not corrected, 
is stored as the positional information concerning the terminating point 
(block 632). The initiating point and the terminating point of the welding 
line are thus sensed and taught. 
After the teaching mode is completed as described above, an automatic 
welding mode as shown in the FIG. 9 flow diagram is started. The shift to 
the automatic welding mode is achieved by switching the control in control 
box 40 of FIG. 2 to an automatic mode and depressing a start button, not 
shown. Then, the control means, not shown, included in the control box 40, 
makes a decision as to whether a sensing command is present (block 900 of 
FIG. 9), i.e. whether sensing is required on the occasion of automatic 
welding, based on which pattern, the pattern 1, 2 or 3, the workpiece 
belongs to. If a sensing command is available, the subroutine (block 902, 
also shown in detail in FIG. 7 or 8) is initiated. 
If a sensing command is not available, then a decision is made as to 
whether or not a sensing completion command is available. If the sensing 
completion command is available, the control means release the command to 
the switch 204 (block 904), thereby to connect the voltage applying means 
203 and the welding voltage source 205 shown in FIG. 2. Once the latter is 
done, or if the sensing completion command is not available, i.e. the 
command is released, then the positional information and control 
information programmed in the above described teaching mode are outputted 
for execution of welding (block 908). 
If and when the subroutine shown in FIGS. 7 and 8 is completed and the 
routine shown in FIG. 9 is resumed, the control means release a command to 
the switch 204 (block 910). At the same time, correction is made if 
correction of the sensed position is required because of uneveness of the 
thickness of the workpiece and the like (block 912), whereupon the 
corrected positional information is applied to execute welding (block 
914). 
According to the embodiment shown in FIGS. 5 and 8, even if the welding 
line WL is of a beveling geometry which is not predetermined, the 
intersectional of the side surfaces is evaluated, whereby the geometry of 
the beveling is decided, whereupon a welding target position most suited 
for the best welding result can be determined. Although the embodiment 
shown was described as executing welding along a straight welding line WL, 
alternatively, the welding line WL may be curved, in which case all the 
target positions of the torch T at the initiating point, the respective 
points along the curve, and the terminating point are evaluated in 
advance, whereupon the respective points are traced in sequence by a PTP 
control. 
FIG. 10 is a sectional view of a portion of a preferred embodiment of the 
welding torch T of the wire extension type. Referring to FIGS. 2 and 10, 
the welding torch T shown comprises a collet chuck 109a for allowing 
passage of a consumptive electrode 209 fed through a hollow portion of the 
collet chuck 109a from the electrode supply means 201, a fastening 
single-acting piston 109b for fastening the collet chuck 109a, a cylinder 
109c for receiving the piston 109b, and a pipe 109d for flow of a high 
pressure fluid, such as a high pressure gas in a shield gas bomb, in and 
out of the cylinder 109c. Referring to FIG. 10, the left side is the tip 
end of the torch T and the right side is connected to a flexible pipe, 
i.e. an electrode shaping apparatus 202, shown in FIG. 2. The consumptive 
electrode 209 fed from the consumptive electrode supply means 201 is 
withdrawn through the hollow portion of the collet chuck 109a and from the 
tip end of the welding torch 109. Normally, the piston 109b is urged 
leftward, as viewed in the figure, by means of a spring 109e, whereby the 
collet chuck 109a is released. Accordingly, the consumptive electrode 209 
can be moved freely in the welding torch T. For example, if it is 
necessary to clamp the consumptive electrode 209 in the sensing mode, then 
a high pressure fluid is fed from the above described pipe 109d, whereby 
the piston 109 is pushed rightward, as viewed in the figure. Accordingly, 
the collet chuck 109a is fastened by the tip internal periphery of the 
piston 109b, with the result that the consumptive electrode 209 is 
clamped. 
In the case where the electrode is a consumptive electrode, the length of 
the electrode portion protruding from the welding torch is not necessarily 
constant at the time of completion of the welding operation. In addition, 
in the case where a flexible pipe 202 is coupled between the fixed 
consumptive electrode supply means 201 and the movable torch such that a 
consumptive electrode 209 is allowed to pass therethrough, there is the 
possibility that the above described length of protruding electrode will 
vary. Accordingly, the protruding length is not necessarily constant, even 
at the start of the welding operation. Therefore, several examples of 
operation for eliminating instability on the occasion of sensing, and thus 
welding, by virtue of the above described fluctuation will be described in 
the following. 
According to one example, the following operation is carried out, prior to 
the sensing operation. More specifically, the welding torch T is brought 
to a predetermined position of the welding apparatus 10, shown in FIG. 1, 
or brought into proximity relative to to a predetermined position, 
whereupon the length of the consumptive electrode 209 protruding from the 
welding torch T is detected. According to a given example, the outer 
periphery of the workpiece fixture 15 shown in FIG. 1 is used as a 
reference position. In this case, as far as the other points are 
concerned, any position may be used wherein the Z axis direction is 
maintained at a constant position. For the purpose of the sensing mode, 
the control box 40 turns the switch 204 to the detecting voltage supply 
206, whereby the workpiece fixture 15 is controllably moved, with respect 
to the Y axis direction and the X axis direction, to predetermined 
positions, and the arm 17 is controllably moved in the up/down direction, 
i.e. in the Z axis direction, thereby to bring the outer periphery of the 
workpiece fixture 15 close to the welding torch T. Because of approach of 
the tip end of the consumptive electrode 209 of the welding torch T and 
the workpiece fixture 15, a spark occurs therebetween, whereby a 
conduction state is detected by the electric current sensor 207. The 
output signal from the electric current sensor 207 is applied to the 
control box 40. The control box 40 brings the welding torch T close to the 
outer periphery of the workpiece fixture 15, i.e. the reference position, 
thereby to evaluate the Z axis position Zc where a spark starts to occur. 
Thus, the difference between the said position Zc and the conduction start 
position Za in the case where the consumptive electrode 209 of a 
predetermined standard length is protruded is evaluated, whereupon the 
same is converted into the length of the electrode. Accordingly, through 
this operation, it follows that a predetermined standard length of the 
consumptive electrode 209 protruding from the welding torch T and the 
error of the actual protruding length are evaluated. The consumptive 
electrode supply means 201 is controlled based on the error thus 
evaluated, whereby the length of the consumptive electrode is calibrated 
to the standard length. Alternatively, command information for positional 
control of the welding torch T may be corrected based on the error thus 
evaluated. In addition, the value of the protruding length, rather than 
the above described protruding length error, may be employed as the data 
for further control. In this case, the consumptive electrode 209 is 
clamped by the collet chuck 109a by means of the piston 109b shown in FIG. 
10. According to such an approach, even if the torch T is rotated on the 
occasion of the sensing mode, the length of the electrode 209 protruding 
from the welding torch can always be kept constant. Accordingly, a correct 
sensing operation can be performed on the occasion of the sensing of the 
welding line. 
It should be appreciated from the foregoing description that the positional 
relation of the tip end of the consumptive electrode to the welding torch 
T is always constant because the consumptive electrode is protruded always 
in the same shape as a function of the shaping apparatus 202 and the 
protruding length thereof is always constant on the occasion of the 
sensing operation, so that no problem is caused as to their function as 
the above described sensor. It is pointed out that if a gas, such as 
CO.sub.2, is adapted to flow along the welding torch T, a much more 
stabilized sensing operation can be performed. 
Depending on the kind of workpieces, a single workpiece could comprise a 
plurality of welding lines. In such a case, it might be necessary to 
effect a sensing operation with respect to each of the welding lines. In 
the above described embodiment, the protruding length of the electrode 209 
as protruded from the welding torch T was made constant prior to the 
sensing operation. If, in such a situation, the protruding length is 
adapted to be defined by positioning each of the welding lines in 
proximity to the outer periphery of the workpiece fixture 15 serving as a 
reference position, then, in the case of such a workpiece having a 
plurality of welding lines, a longer period of time is required for the 
sensing operation, and thus for the welding operation. In the following, 
therefore, a preferred embodiment of the present invention will be 
described which eliminates the necessity of the above described tiresome 
processing required in the case of a workpiece having a plurality of 
welding lines. 
FIG. 11 is flow diagram showing another embodiment of an automatic welding 
mode of the present invention. With reference to FIG. 11 and other related 
figures, an operation of the FIG. 11 embodiment will be described. At the 
outset, the electrode protruding from the welding torch T is selected to 
be a prescribed value of length (block 1100) and clamped in accordance 
with the operation described in conjunction with FIG. 10 and other related 
figures. Then, the control means included in the control box 40 provides a 
sensing command (block 1102). Accordingly, the subroutine of the first 
cycle sensing operation is read out and is executed (block 1104). The 
subroutine of the sensing operation in case of the horizontal fillet 
welding was described with reference to FIG. 7 and the subroutine of the 
sensing operation in case of the butt bevel welding was described with 
reference to FIG. 8. As a result of execution of the sensing subroutine, 
the welding line in the workpiece is sensed. Thereafter, the welding 
command is obtained (block 1104) and the welding operation is performed 
along the sensed welding line. After the welding operation is completed 
(block 1106), a position for flee away is commanded for the purpose of 
bringing the welding torch T to a given flee away position (block 1108). 
Thereafter, a decision is made as to whether or not all the welding 
operation with respect to the workpiece is completed (block 1110). If all 
the welding operation has been completed, a further processing is achieved 
following the welding operation and, if the workpiece still has the 
welding line left unwelded, then a decision is made as to whether it is 
necessary to make the length of the electrode the same as that in the case 
of the previous sensing operation, i.e. whether or not the command for 
defining the protruding length of the electrode is obtained. A command for 
defining the protruding length may be programmed in advance in the 
teaching mode or may be programmed to be provided automatically upon 
completion of a series of welding lines (block 1112). If the command is 
obtained, then a decision is made as to which direction in accordance with 
the ordinate axes is for use in adjusting the protruding length, the 
horizontal or vertical direction, i.e. the X axis (or the Y axis) or Z 
axis. At that time, if the axis is the horizontal direction, i.e. the X 
axis (or the Y axis), then the previous welding position, i.e. X+.DELTA.X 
(or Y+.DELTA.Y) is provided as a command position (block 1116). 
Accordingly, the welding torch T is moved to the command position, with 
those other than the commanded axis being maintained in the previous flee 
away position. If the axis being controlled is the vertical direction, 
i.e. the Z direction, then similarly the previous welding position 
(Z+.DELTA.Z) is provided as a command position (block 1118). Accordingly, 
the welding apparatus is position controlled, with the commanded value for 
only the Z axis, with the other axes being maintained in the previous flee 
away positions. 
Thereafter the control apparatus makes a decision as to whether the welding 
torch T has reached the previous command position (block 1120) by a null 
or clock signal. If the command position has been reached, then the 
control apparatus provides a command for feeding the electrode to the 
consumptive electrode supply means 201 shown in FIG. 2 (block 1122 of FIG. 
11). At that time, the consumptive electrode 209 has been connected to the 
detecting high voltage source 206 by means of the voltage applying means 
203. If an output is obtained from the current sensor 207 (block 1124), a 
command is applied to the supply means 201 to stop feeding the electrode 
(block 1126). Thus, in the second and further sensing operations of the 
welding lines with respect to one workpiece, the surface position of the 
workpiece is used as a reference position for the purpose of defining the 
protruding length of the consumptive electrode 209 prior to the sensing 
operations. The surface position of the workpiece has been decided in the 
previous first sensing operation. Thus, it is not necessary to return the 
welding torch T each time to the vicinity of the reference position member 
such as the workpiece fixture 15, for example, with the result that the 
cycle time required can be shortened. In the foregoing, the FIG. 11 
embodiment was described as embodied in case of a horizontal fillet 
welding as shown in FIG. 4. However, the FIG. 11 embodiment can be equally 
practiced in case of a butt welding operation having the beveling as shown 
in FIG. 5. In the latter described case, however, the defining of the 
protruding length cannot be achieved with respect to the horizontal 
direction, i.e. the X axis (or the Y axis). Accordingly, the positional 
information concerning a given point on the surface of the workpiece in 
the vicinity of the beveling, for example, should be obtained in advance 
as a command position. 
FIG. 12 is a perspective view of another preferred embodiment of the 
welding torch T. The FIG. 12 embodiment comprises a reference portion 116 
provided in addition to the welding torch T shown in FIG. 10. More 
specifically, the welding torch T comprises a cylinder or guide sleeve 111 
provided adjacent thereto and in parallel therewith by means of rings 111a 
and 111b. The guide sleeve 111 comprises a single acting piston or rod 113 
inserted therethrough. The outer side surface of the rod 113 is formed of 
a protrusion 114, such that the protrusion 114 fits into a groove formed 
at the tip end of the guide sleeve 111. In addition, as shown in a 
fragmentary manner in FIG. 12, the guide sleeve 111 receives a spring 112 
therein. The spring 112 is provided in the guide sleeve 111 to exert a 
spring force to urge the rod 113 inserted therein toward the base end of 
the welding torch T, i.e. in the rightward direction as viewed in the 
figure. The spring 112 normally remains expanded and is compressed as 
necessary by means of a high pressure fluid flowing into the pipe 115, for 
example. 
As seen in FIG. 12, the protrusion 114 of the rod 113 is curved at the 
angle of 90.degree. at a given position, thereby to form a so-called 
spiral shape. In addition, the tip end of the rod 113 is formed of a base 
portion or a reference portion 116 curved at the right angle. 
According to the embodiment shown, a further operation to be described 
subsequently is performed before the sensing operation. More specifically, 
the welding torch T as shown in FIG. 12 is brought into proximity with a 
predetermined position of the welding apparatus 10 shown in FIG. 1, 
thereby to make constant the length of the consumptive electrode 209 
protruding from the welding torch T. In the embodiment shown, the 
reference portion 116 shown in FIG. 12 is assumed to be a reference 
position. Therefore, the reference portion 116 is first positioned in 
front of the welding torch 109. More specifically, normally the spring 112 
has been expanded, so that the piston or rod 113 is housed in the guide 
sleeve 111 so as to extend throughout the total length thereof. 
Accordingly, the reference portion 116 provided at the tip end of the rod 
113 is maintained in such a manner as to be directed in the illustrated 
direction at the position as shown in a solid line. As necessary, i.e. 
before the sensing operation, the spring 112 is compressed by allowing a 
high pressure fluid to flow into the sleeve 111 through the pipe 115. 
Accordingly, the rod 113 comes to protrude from the guide sleeve 111. At 
that time, the rod 113 is turned by 90.degree. in accordance with the 
protrusion, because the protrusion 114 of the rod 113 has become engaged 
with the groove of the guide sleeve 111 and the protrusion 114 has been 
curved by 90.degree. at a given position. Accordingly, the reference 
portion 116 of the rod 113 is displaced from the position as shown in the 
solid line in FIG. 12 to the position as shown in the two dotted lines in 
FIG. 12, while the direction is turned by 90.degree.. Accordingly, the tip 
end of the consumptive electrode 209 can be in contact with the reference 
portion 116. It has been adapted such that the length of the rod 113, as 
fully pushed out by means of the high pressure fluid, i.e. the position of 
the reference portion 116 in such situation, will always come to a 
predetermined position spaced apart from the tip end of the torch T. 
Accordingly, the reference portion 116 may be employed as a reference 
position. 
Thereafter, referring to FIG. 2, the control means in the control box 40 
acts on the consumptive electrode supply means 201 in the voltage source 
20, thereby to supply the consumptive electrode 209 to the welding torch 
T. At that time, the switch 204 has been turned to the detecting voltage 
source 206 by means of the control means for the purpose of the sensing 
operation. Accordingly, if and when the tip end of the consumptive 
electrode 209 comes close to the above described reference portion 206, a 
spark occurs between the consumptive electrode 209 and the reference 
portion 116. Therefore, a detected output signal is obtained from the 
current sensor 207 and the supply means 201 is disabled by means of the 
control means. Thus, the reference portion 116 is positioned so as to be 
spaced apart a predetermined distance from the tip end of the welding 
torch T, when the consumptive electrode 209 is brought into proximity with 
the reference portion 116, with the result that the protruding length of 
the consumptive electrode 209 is always constant. Thereafter, the collet 
chuck 109a is fastened by means of the piston 109b, whereby the 
consumptive electrode 209 is clamped. 
In the above described embodiment, inward and outward flow of the high 
pressure fluid by means of the pipes 109d and 115 may be manually 
controlled by a valve, not shown, or alternatively such valve may be 
automatically switched in response to a signal obtainable from a program. 
In another embodiment of the present invention as shown in FIGS. 13 and 14, 
a consumptive electrode cutting means 110 is provided on the upper end of 
a frame 14. The consumptive electrode cutting means 110 comprises a 
grinding stone 110a and a motor 110b for rotating the grinding stone 110a. 
The motor 110b is controllably rotated by means of the control means 
included in the control box 40. The grinding stone 110a may be a thin disc 
of such material as, for example, a resin grinding stone. 
According to the embodiment shown, the operation to be described in the 
following is carried out before the sensing operation. More specifically, 
the protruding length of the consumptive electrode 209 protruding from the 
welding torch T is kept constant by means of the consumptive electrode 
cutting means 110 as shown in FIGS. 13 and 14. To that end, the control 
means included in the control box 40 first controls of position of the 
workpiece fixture 15, and thus the consumptive electrode cutting means 
110, to a predetermined position, spaced apart a predetermined distance 
from the third frame 16, in terms of the X axis direction and the Y axis 
direction. Thereafter, the arm 17 is controlled in the Z axis direction. 
Then the supply means 201 is controlled, whereby a given amount of the 
consumptive electrode 209 is protruded from the tip end of the welding 
torch T. As a result, a given position on the side surface of the 
consumptive electrode 209, as protruded from the tip end of the welding 
torch T, comes to be faced to the periphery or blade edge of the rotary 
grinding stone 110a. The consumptive electrode 209 is clamped by means of 
the collet chuck 109a as actuated by the piston 109b. Thereafter, the 
welding torch T is positioned to the blade edge of the rotary grinding 
stone 110a and the motor 110b is enabled, so that the grinding stone 110a 
is rotated. Accordingly, the consumptive electrode 209 as protruded from 
the body of the welding torch T is cut at a predetermined length. Even if 
the welding torch 109 is rotated in such situation, the length of the 
electrode 209 protruded from the welding torch 109 is kept always 
constant. It is pointed out that the cutting means may be scissors or 
cutters, apart from the above described rotary grinding stone. 
Generally, the optimum values of the welding voltage applied between the 
electrode and the workpiece and the welding current flowing therebetween 
as determined experimentally based on the thickness of the workpiece, the 
state of beveling, the thickness of the electrode and the like, have been 
commanded in the teaching mode. However, unless the protruding length of 
the electrode is constant in the sensing mode, the voltage and current in 
the automatic welding mode are not maintained constant or at optimum 
values, because the automatic welding mode is carried out based on the 
information obtained in the sensing mode. Assuming that the sensing 
operation is performed with the protruding length being shorter, for 
example, then, the resistance of the electrode in the welding operation is 
small, and the current becomes larger and the voltage becomes smaller. As 
a result, the voltage and current values become different from the optimum 
voltage and current, and welding of a desired finishing or quality cannot 
be achieved. 
Therefore, according to a further embodiment of the present invention, the 
collet chuck 109a is fastened by means of the piston 109b shown in FIG. 10 
in a given state, for example, whereby the consumptive electrode is 
clamped at a given constant protruding length, whereupon the sensing 
operation is performed. 
Thereafter, the automatic welding mode is effected. In the automatic 
welding mode, as shown in the FIG. 15, flow diagram, control is effected 
such that an arc occurs between the consumptive electrode 209 of the 
welding torch T and the workpiece, not shown. If no current flows between 
the consumptive electrode and the workpiece at that time (block 1500), a 
decision is made to the effect that the welding is abnormal (block 1502), 
whereupon the abnormality is warned by means of a warning apparatus, for 
example. If a welding current flows (block 1500), then the welding voltage 
at that time is sampled (block 1504) and the data thus obtained is loaded 
in the control means included in the control box 40. The control means 
makes a decision as to whether or not the above described sampled welding 
voltage approximates a predetermined value as preset in the teaching mode 
(block 1506). If the welding voltage is within a predetermined value 
range, then the welding current is sampled (block 1508) and the sampled 
welding current is loaded in the control means. The control means makes a 
decision as to whether or not the above described sampled welding current 
is in the predetermined value range as taught (block 1500). If the welding 
current is also within the predetermined value range, then the welding is 
continued (block 1512) until termination of the welding (decision block 
1514), upon which the next step follows. 
If and when the welding voltage is outside the predetermined value range, 
then the control means makes a decision as to whether the same is larger 
or smaller than a predetermined value (block 1516). Similarly, if the 
welding current is outside the predetermined value range, then the control 
means makes a decision as to whether the same is larger or smaller than 
the predetermined value (block 1518). If the welding voltage is smaller 
than a predetermined value and the welding current is larger than a 
predetermined value, then this means that the tip end of the welding torch 
makes a sensing operation in a state too close to the workpiece, the 
position is controlled responsive thereto, and accordingly the control 
means serves to move the welding torch in the minus direction of the torch 
axis (block 1520). More specifically, since the tip end of the torch is 
too close to the workpiece, the torch is controllably moved away from the 
workpiece. 
If and when the welding voltage is higher than a predetermined value and 
the welding current is smaller than a predetermined value, then this 
indicates that the tip end of the torch is too far away from the 
workpiece. Accordingly, the control means serves to control the welding 
torch such that the same is moved in the plus direction of the torch axis 
(block 1522). In other words, the tip end of the welding torch is 
controllably moved in the direction toward the workpiece. 
If the controlling movement of the welding torch in the minus direction or 
the plus direction is completed, the welding voltage and/or the welding 
current are again sampled, and a decision is made as to whether or not the 
sampled value (s) is (are) within the predetermined value range (blocks 
1506 and 1510). Thus, the welding voltage or the welding current in the 
automatic welding mode is controlled to be in the preset value range of 
the welding voltage or the welding current as preset in the teaching mode. 
This means that the automatic welding operation is performed in such a 
state that the protruding length of the consumptive electrode is protruded 
so as to substantially satisfy the welding voltage and the welding current 
as preset in the teaching mode. 
FIG. 16 is a perspective view showing another example of a technique for 
detecting the beveling for use in the present invention. The detecting 
method of this example will be described in more detail with reference to 
the flow diagram of the subroutine shown in FIGS. 17A and 17B. To that 
end, the surface WS of the workpiece W is set in advance at approximately 
a right angle with respect to the control axis Z or the sensor direction. 
The welding torch T is then controlled to be above the point P1 responsive 
to a position command. The point P1 is predetermined to be at a position 
slighly spaced apart in the direction intersecting the welding line WL in 
the vicinity of one end of the welding line WL, and the data 
representative thereof is programmed in advance. 
When this subroutine is initiated, a command to turn the switch 204 is 
first obtained (block 1700), whereby the voltage of the detecting voltage 
source 206 is applied to the consumptive electrode 209. The Z axis 
position Z1 in the control means in the control box 40 is set to 
-.infin.(block 1702). The information representing the fact that the 
inside of the beveling is not being detected is further stored (block 
1704). The welding torch T is then commanded to be lowered in the Z axis 
direction (block 1706). Alternatively, the workpiece W may be raised in 
the Z axis direction. In the following, the description will be made based 
on the presumption that the welding torch T is moved. The tip end of the 
electrode of the welding torch T approaches the workpiece and detection is 
made as to whether or not a detected signal is obtained. If a detected 
signal is obtained (block 1708), the value Z2 of the welding torch T in 
the Z axis direction is loaded. Then the welding torch T is returned in 
the Z axis direction (block 1710). 
Thereafter, a decision is made as to whether or not the internal side of 
the beveling is being detected by the welding torch based on the loaded 
information in the data processing unit (block 1712). If the welding torch 
T is not detecting the inside of the beveling, then a shift is made to the 
next step (block 1714). If the welding torch T is detecting the inside of 
the beveling, then a jump through B to the step "Z1-Z&lt;Zc?" (block 1738) 
is executed. If the welding torch T is not detecting the inside of the 
beveling, then a decision is made as to the step "Z1-Z2&lt;Zc?" (block 1714). 
It is pointed out that Zc is a constant as inputted in advance to the data 
processing unit or the control means. 
If and when Z1-Z2&lt;Zc is true, then Z1 is set equal to Z2 (block 1716) and 
the welding torch T is moved by .DELTA.L in the sensing direction, or in 
the X direction and in the sense of -X in this situation, with the value 
of Z2 as Z1 (block 1718). The above described step "lower the Z axis" 
(block 1706) is reexecuted to evaluate the following point P2. Thus, the 
same operation is repeated for each of the points P2, P3, . . . . 
If and when the sensor, i.e. the welding torch T reaches the point P6-- in 
other words, the welding torch T comes within the beveling-then the 
relation "Z1-Z2&lt;Zc" becomes. false when the control means is loaded with 
the information indicating that the inside of the beveling is being sensed 
(block 1720), and a decision is made as to whether or not the angles 
.alpha. and .beta. of the beveling have been stored in advance in the data 
processing unit (block 1722). If such angles have been stored in the data 
processing unit, then the welding torch T is moved in the designated 
direction (block 1724), in this case in the -X direction. If the sensor 
signal--i.e., the position signal obtainable at the position P7--is 
obtained from the current sensor 207 (block 1726), then the positional 
information Z3 is loaded. Based on the information thus obtained, 
including (X2, Z2), (X3, Z3), .alpha., .beta., the intersection point PW 
(Xf, Zf) of the beveling line is evaluated (block 1728) and is stored in 
the data processing unit. Then the welding torch T is slightly raised 
(block 1730), and a decision is made as to whether or not the information 
concerning two points of the intersection point PW is stored with respect 
to the welding line WL. If so, a command for moving the welding torch T in 
accordance with a linear interpolation between these two points PW is 
obtained. If two points of such point PW are not available, then the 
welding torch T is moved in the direction of the welding line WL (block 
1734), in this case in the Y direction, so as to be positioned at the 
other end of the welding line WL, whereupon the progress of execution is 
returned to the start through E . If in the previous decision step "the 
angle of beveling given ?" (block 1720) the decision is that the angles 
.alpha. and .beta. have not been loaded, then a further decision is made 
as to whether or not a command for evaluating the point PW through 
calculation is included or not (block 1736). If the point PW is to be 
evaluated through calculation, then the predetermined pattern, for example 
the points P6, P8, P12, P13 in FIG. 15 and the characteristic points in 
the beveling are sensed a necessary number of times (block 1746), 
whereupon the positional information Z6, Z8, Z12, Z13 are loaded. The 
point PW is evaluated through calculation based on these pieces of 
positional information (block 1748) and is loaded. Then the progress of 
execution is returned to the step (block 1732 after "raise the Z axis" 
(block 1730). 
It the point PW is not to be evaluated through calculation, then a decision 
is made as to whether or not "Z1-Z2&lt;0" is met (block 1738). 
If not, the progress of execution is returned to the step (block 1718) "set 
Z1=Z2" (block 1716). If so, by presuming that the welding torch T has 
already crossed the welding line W1--for example moving from the point P11 
to the point P14--the welding torch T is returned in the sensing direction 
by the value (.DELTA.L+l), where l=.DELTA.L/3 (block 1746). However, the 
above described returning value should not be construed by way of 
limitation, inasmuch as any appropriate small value close to the point PW 
may be used depending on the angles .alpha. and .beta.. Thereafter, the 
welding torch T is lowered in the Z axis direction (block 1742) and, if 
the sensing signal is inputted at that time (block 1744), the position a 
that time is deemed as PW and is loaded, whereupon the progress of 
execution is returned to the step (block 1732) "raise Z (block 1730) 
axis". According to this method, assuming that the angles .alpha. and 
.beta. are 45.degree., .DELTA.L is 2 mm and l is 1 mm, and the detection 
accuracy is within .+-.1 mm. 
In the above described embodiment, a switching operation between the 
welding voltage source 205 and the detecting high voltage source 206 is 
achieved by means of a mechanical switch 204, but alternatively the 
embodiment shown in FIGS. 18 and 19 may be considered. Referring to FIG. 
18, the welding voltage source 205 is applied to the voltage applying 
means 203, i.e. the consumptive electrode 209, through a unidirectional 
device 205a such as a diode. The detecting high voltage source 206 is 
coupled to the output side of the diode 205. The high voltage source 206 
may comprise an oscillator for generating a high frequency voltage which 
is enabled in the sensing mode. Accordingly, in the FIG. 19 embodiment, 
the voltage from the welding voltage source 205 and the high frequency 
voltage from the detecting high voltage source 206 are superimposed, and 
the resultant composite voltage is applied to the consumptive electrode 
209 in the sensing mode. Meanwhile, the detecting voltage source 206 is 
disabled in the welding mode. 
According to the FIG. 19 embodiment, the welding voltage source 205 is 
connected to the voltage applying means 203 through a current limiting 
resistor 206a. The current limiting resistor 206a is shunted by a switch 
204a. In the welding mode, the switch 204a is closed, so that the current 
limiting resistor 206a is shunted. Accordingly, the consumptive electrode 
209 is directly supplied with the output of the welding voltage source 
205. In the sensing mode, the switch 204a is opened, so that the current 
limiting resistor 206a is rendered effective. Accordingly, in the sensing 
mode, the voltage from the welding voltage source 205 is applied through 
the resistor 206a to the consumptive electrode 209. The current limiting 
resistor 206a functions to limit the current in the sensing mode to an 
extremely small value as compared with that in the welding mode. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.