Spot welding machine for heavy-duty operations on surface contoured workpieces

A spot welding machine comprising a pair of vertically spaced-apart guide rails rigidly supported on a framed structure a distance from the ground, a slide member slidably mounted on the guide rails, and a robot arm structure having a housing pivoted at an upper pivot point of the slide member, and outer and inner shafts axially movable as a single piece along the longitudinal axis of the housing, the inner shaft being rotatable about the longitudinal housing about which the inner shaft is rotatable. An electrode holder is attached to the forward end of the housing for unitary axial movement with the outer and inner shafts and for unitary rotation with the inner shaft to vertically swing an electrode tip attached thereto. A fluid operated telescopic cylinder is pivotally connected between the lower pivot of the slide member and a forward end portion of the housing for pivoting the robot arm structure at a desired angular position. A backing electrode is provided on the ground to mount a workpiece to which the electrode tip is addressed in response to a set of instruction data to make pressure contact therewith.

BACKGROUND OF THE INVENTION 
The present invention relates to spot welding machines, and in particular 
to such a machine capable of heavy-duty spot welding operations on surface 
contoured workpieces in response to a set of instruction data. 
In automobile manufacturing plants, it is customary to produce a single 
model of cars on a single production line since there do not exist 
multi-purpose machine tools which make it possible to produce different 
models on the same production line. It is desirable however to 
mass-produce a variety of models of cars on a single production line in 
terms of efficiency, space savings and equipment cost. 
Attempts have hitherto been made to employ industrial robot systems because 
of their capability to repeatedly position an object according to a 
prescribed pattern under the direction of instruction data including tool 
position data and operating speed. 
Conventional robot systems comprise a main body installed on the ground and 
an arm pivoted at one end to the main body for holding a tool at the other 
end. In some machining operations such as spot welding, a substantial 
amount of force is applied to a workpiece and the resulting reaction force 
must be borne by the entire structure. However, the conventional robot 
system is not capable of bearing such reaction force and deformation tends 
to occur. 
To avoid such deformation conventional automatic spot welding machines are 
provided with upper and lower arms in an X-shaped or C-shaped 
configuration to distribute the reaction force components between the 
upper and lower arms. However, large sized workpieces would require the 
machine to extend its arm a substantial distance from its standing 
position. Furthermore, the conventional machine employs upper and lower 
electrode tips between which the workpiece is pressure-contacted and 
current is passed between the electrode tips. Therefore, when the machine 
arm extends a substantial distance from its standing position, the 
secondary impedance of the power transformer would increase requiring 
additional electrical power and resulting in non-uniform spot welds. 
SUMMARY OF THE INVENTION 
The invention contemplates the use of a pair of vertically spaced parallel 
guide rails mounted on a rigid frame structure. The spot welder comprises 
a robot arm including a housing, an outer and inner shaft axially movable 
as one piece with the inner shaft being rotatable about its axis, an 
electrode holder mounted on a horizontal pivot shaft rotatably connected 
to a forward end of the inner shaft, and fluid operated cylinders for 
actuating the moving parts of the robot arm to permit it to assume a 
desired angular position. An upper end of the robot arm is pivoted to an 
upper end of a slide member which is slidably mounted on the guide rails. 
A fluid operated telescoping cylinder is connected between a lower end 
portion of the robot arm and a lower pivot point of the slide member. 
During spot welding reaction force is transmitted through the longitudinal 
axis of the robot arm. Part of the transmitted force is conveyed through 
the upper pivot point of the slide member to the upper guide rail, while 
the remainder is conveyed through the telescoping cylinder to the lower 
pivot point of the slide member and thence to the lower guide rail, 
whereby the reaction force is transmitted to the rigid frame structure. 
The forward end of the robot arm is provided with a brake or frictional 
means which firmly holds the electrode holder, so that the latter is not 
subject to bending or deformation. 
The invention further contemplates the use of an additional electrode 
holder which may be mounted on the same robot arm structure in parallel 
with the first electrode holder to pass welding current between the 
electrode tips mounted on the holders through the workpiece and a backing 
electrode. The additional electrode holder may be mounted on an additional 
robot arm structure which is mounted opposite to the first robot arm 
structure. 
The backing electrode preferably comprises a plurality of angularly 
spaced-apart metal members each having its edge contoured in conformity 
with the lower surface of the particular workpiece. The metal members are 
mounted on a single horizontal rotary shaft which is driven by a fluid 
operated cylinder to position a particular metal member to an upright 
position to effect spot welding operation on different types of workpiece. 
An object of the invention is therefore to provide a heavy-duty spot 
welding machine which is capable of welding a large-sized surface 
contoured workpiece. 
Another object of the invention is to provide a spot welding machine 
capable of spot welding two locations simultaneously by pressure 
contacting two electrode tips with the workpiece and passing a current 
between the electrode tips through the workpiece and a backing electrode 
on which the workpiece is placed. 
A further object of the invention is to provide a spot welding machine 
which is capable of spot welding workpieces having different surface 
contours on the same production line.

DETAILED DESCRIPTION 
Referring now to FIGS. 1a, 1b and 1c, numerals 1, 1 designate columns or 
guide supports erected on the ground 1' by the side of an automobile 
assembly line L, on which a car floor F is successively placed for welding 
operations. The vertical supports 1, 1 are connected by a horizontal beam 
1" spaced a distance from the ground 1'. A pair of vertically spaced 
parallel, upper and lower guide rails 5a and 5b is secured to a reinforced 
horizontal beam 2 mounted on the vertical supports 1. A slide member 4 is 
slidably mounted on the guide rails 5a, 5b. On an upper pivot point 10 of 
the slide member 4 is pivoted an upper end of a spot welding robot arm 
structure 3, and between a lower pivot 12 of the member 4 and a pivot 14 
of the robot arm 3 is connected a fluid operated telescoping means or 
cylinder 11 for pivoting the robot arm 3 at a desired angular position to 
position a pair of electrode holders, or fluid-operated gun cylinders 50, 
50 on the workpiece F. The robot arm 3 is also driven along the guide 
rails to take optimum work positions. FT is a hump formed on the car floor 
F to provide a downwardly open channel and E is a backing electrode 
mounted on an insulator G placed on a base J. 
FIGS. 2 through 7 illustrate the details of the robot arm structure 3. In 
FIGS. 2 and 3, the slide member 4 is shown as including a pair of legs 4a, 
4b slidably secured to the guide rails 5a and 5b, respectively, and a 
bracket 7 to which is secured the piston rod 6b of a fluid operated 
telescoping cylinder 6 (see FIG. 3), the cylinder housing 6a thereof being 
connected to the beam 2 for purposes of moving the robot arm structure 3 
along the guide rails. A position translating encoder 8 (FIG. 2) is 
secured to the beam 2 for detecting the position of the slide member 4 by 
converting the horizontal displacement of the slide member 4 with respect 
to a reference point into the angular position of a detector unit 8b by 
means of a sprocket 8a and a chain (not shown). The arrangement of the 
sprocket 8a and chain is of the same construction as an arrangement shown 
in FIG. 6 which will be described later. 
The slide member 4 is also provided with a pair of yokes 4c, 4c which 
pivotally secure the shaft 10 on which flanges 9a, 9a of a robot arm 
housing 9 are mounted (FIG. 3). The angular position of the robot arm 3 is 
detected by a position encoder 15 having a detector shaft 15a which is 
coupled by means of a chain belt 17 and a sprocket 16 secured to the shaft 
10. 
The robot arm housing 9 includes, as shown in FIG. 2, a bearing 18 which 
rotatably supports a spline shaft 19, an inner cylindrical shaft 21 and an 
outer cylindrical shaft 23, each being mounted coaxially with the spline 
shaft 19. Fixed to the rearward end of the spline shaft 19 is a bevel gear 
24 in mesh with another bevel gear 26 secured to a rotary shaft 25 
extending at right angles to the spline shaft 19, so that the latter is 
driven by the rotary shaft 25. A fluid operated telescoping cylinder 28 is 
secured to the robot arm housing 9 and enclosed by a cover 27, in phantom 
line, to drive the rotary shaft 25. The cylinder 28 is provided with a 
pair of oppositely reciprocating piston rods 28a, each end of which is 
linked to each end of a chain belt 32 which is looped around a sprocket 29 
fixed to the rotary shaft 25 and around a sprocket 31 rotatably mounted on 
a shaft 30 pivoted to the forward end of the robot arm housing 9. The 
reciprocating motion of the piston rods 28a in opposite directions is 
transmitted to the rotary shaft 25 via the chain and sprocket arrangement. 
Numeral 33 designates a position encoder for detecting the angular 
position of the rotary shaft 25, that is, the angular position of the gun 
cylinders 50, 50 through the rotation of a detector shaft 33a by means of 
a chain belt 35 in mesh with a sprocket 34 on the rotary shaft 25 and 
another sprocket 36 on the detector shaft 33a. 
The upper end portion of the spline shaft 19 is slidably spline-connected 
with a hub 20 which is connected by a key 20a to a sleeve 21a of the inner 
shaft 21. When the spline shaft 19 turns, the inner shaft 21 also turns 
inside a bushing 22 and moves in the axial direction together with the 
outer shaft 23. As illustrated in FIG. 5, the outer shaft 23 is provided 
with oppositely facing sliding members 37 spaced at 90 degrees from each 
other which are slidably in engagement with sliding member 38 secured to 
the inner walls of the robot arm housing 9, so that the outer shaft 23 is 
not allowed to rotate about its longitudinal axis. The forward ends of the 
inner and outer shafts extend beyond the forward end wall of the housing 
9, the end of the inner shaft 21 being secured to a spindle 39 as best 
seen in FIG. 2 and the end of the outer cylinder 23 secured to a 
connecting arm 41 fixed to a piston rod 40a of a fluid operated 
telescoping cylinder 40 which is shown in FIG. 4. The cylinder housing 40b 
of the cylinder 40 is secured to the housing 9 by a bracket 42 so that the 
cylinder 40 is in parallel to the longitudinal axis of the inner and outer 
shafts 21, 23. The cylinder 40 thus causes the connecting arm 41 and the 
inner and outer shafts 21, 23 to move axially for purposes of lengthening 
and shortening the extent of the gun cylinder 50 beyond the forward end of 
the housing 9. The axial position of the gun cylinder 50 is detected by a 
position encoder 43 which, shown in FIGS. 2 and 6, converts the axial 
movement of the outer shaft 23 into a rotation of a detector shaft 43a by 
means of a chain belt 47 looped around a sprocket 45 mounted on a shaft 44 
pivoted to the housing 9 and a sprocket 46 fixed to the detector shaft 
43a. 
The spindle 39, which is attached to the forward end of the inner shaft 21, 
is enclosed by a housing 49 with its bevel gear 55 in mesh with a bevel 
gear 54 mounted on a horizontal pivot shaft 51 rotatably secured to the 
housing 49 to constitute a wrist portion 48 of the robot arm structure as 
best seen from FIG. 7. The gun cylinder 50 is secured to the pivot shaft 
51, so that by rotation of the inner shaft 21 the gun cylinder 50 is 
caused to swing on a vertical plane. An electromagnetic brake 58 is 
mounted in the housing 49 to hold the pivot shaft 51 in position by reason 
of friction between inner discs 56a fixed to the shaft 51 and outer discs 
56b fixed to the housing 49 when an electromagnet 57 is energized. The gun 
cylinders 50, 50 are provided with air supply hoses 59,59 to respond to 
controlled air pressure to extend or retract their piston rods 50a, 50a. 
To the forward end of each piston rod 50a is attached an electrode tip 60. 
A welding cable R1 is connected from a transformer T to one electrode tip 
60, another cable R2 connecting the transformer to another electrode tip 
60. A pair of water supply hoses 62 is connected to the electrode tips 60, 
60 to pass cooling water therethrough. 
The fluid operated cylinders 6, 11, 28 and 40 are each provided with a 
fluid supply conduit for hydraulically control their piston rods. Numeral 
63 in FIG. 4 designates a boot to prevent dust entry. 
The electrode tips 60, 60 can thus be moved to any position by 
hydraulically controlling the telescoping cylinders 6, 11, 28 and 40, 
respectively. More specifically, the cylinder 6 (FIG. 3) extends or 
retracts its piston rod 6b causing the slide member 4 to move along the 
guide rails 5a, 5b, the cylinder 11 (FIG. 2) controlling the angular 
position of the robot arm 3. As shown in FIG. 8, the robot arm 3 takes an 
angular position through an angle .theta. in response to an axial length 
of the piston rod of the cylinder 11. The cylinder 28 (FIG. 2) rotates the 
sprocket 29 and the rotary shaft 25 by the chain belt 32, and this 
rotation is communicated via the bevel gear 26, bevel gear 24, spline 
shaft 19, inner shaft 21, spindle 39, bevel gear 55 and bevel gear 54 to 
the pivot shaft 51, causing the gun cylinders 50, 50 to swing through an 
angle .theta.2. This permits the electrode tips 60a, 60a to take a desired 
position within an area defined by points a, b, c and d. In addition, the 
hydraulic controlled cylinder 40 (FIGS. 3 and 4) causes the connecting arm 
41 and therefore the wrist housing 49 to axially extend or retract with 
respect to the housing 9 to allow the electrode tips 60a, 60a to travel a 
distance l, so that it takes a desired position within an extended area 
defined by points a, e, f, g, h and d. At this time the inner and outer 
shafts 21, 23 will also axially move as a single piece with the slide 
members 37 sliding along the slide plates 38 of the robot housing 9, 
whereby the outer shaft 23 is prevented from rotating about its axis. The 
gun cylinders 50, 50 provide pressure to the electrode tips 60a, 60a 
against the surface of the workpiece F to achieve a pressure contact 
therewith. 
The electrode tips 60a, 60a can thus be positioned repeatedly at specified 
locations of the workpiece automatically with the use of a predetermined 
program stored in a computer memory. 
FIG. 9 shows a block diagram of the control unit of the spot welder robot 
system. The position encoders 8, 15, 33 and 43 deliver position data to 
the associated fluid-operated cylinders through a feedback control loop. 
Numeral 63 is a central processor unit to which control data S is 
supplied. Encoders 8, 15, 33 and 43 feed positional data to associated 
level shifters 66, 57, 68 and 69 respectively and thence to an 
analog-to-digital converter 70. The latter provides digital position 
signals to an input of a digital comparator 65 on the one hand and on the 
other to a core memory 64. The control system is initially instructed to 
record position data by feeding a set of input data into the core memory 
64. Responsive to the input data the cylinders 6, 11, 28 and 40 are 
actuated resulting in the generation of actual positional data from the 
associated encoders. The actual position data is transmitted via 
associated level shifters to an analog-to-digital converter 70 and applied 
to the digital comparator 65 wherein the input signal is compared with the 
data stored in a register file 71 retrieved from the memory 64. The 
comparator 65 feeds a signal representing the deviation of the position of 
the respective cylinder from the desired position and continues to feed 
that signal until the deviation is reduced to zero. When this occurs, 
digital signals from the converter 70 represent the desired positions and 
are applied to the memory 64 to be recorded therein as a set of 
instruction data for a particular working position with which the encoder 
signals are compared in subsequent welding operations. This process is 
repeated for each welding spot to record a plurality of sets of 
instruction data to effect a series of welding operations on the car floor 
F. 
After the electrode tips 60, 60 are automatically set into a desired 
position, working fluid is supplied to the gun cylinders 50, 50 to enable 
them to apply pressure to the electrode tips, 60, 60 against the surface 
of the workpiece F. Electric current is then passed between the electrode 
tips 60, 60 through a backing electrode located below the workpiece to 
effect spot welding. The reaction force exerted on the electrode tips 60, 
60 is communicated, as shown in FIGS. 10a and 10b, to the gun cylinders 
50, 50, then to the pivot shaft 51, the connecting arm 41 and the cylinder 
40, to the robot arm housing 9. The reaction force so communicated to the 
arm 9 is further transmitted to the reinforced beam 2 through the upper 
pivot shaft 10 to the guide rail 5a on the one hand and through the lower 
pivot point 12 to the guide rail 5b on the other hand. Therefore, it is 
totally unnecessary to bear the reaction force by the robot arm housing 9 
alone. This produces the effect of practically increasing the strength of 
the welding machine, making possible spot welding using a robot system 
which has hitherto been impossible. 
If the surface of the workpiece F is not perpendicular to the electrode tip 
60, as illustrated in FIG. 11, as a result of irregular contour of the 
workpiece, the pivot shaft 51 of the wrist portion, being firmly held in 
position by means of the electromagnetic brake 58, will not cause the 
electrode tip 60 to slip off the surface of the workpiece, and in 
addition, the outer shaft 23, being prevented from rotation by means of 
slide members 37, will not cause the electrode tip 60 to rotate about its 
axis. 
The welding spots may differ in location and size between car floors of 
different type. In such situations it is necessary to change the backing 
electrode in accordance with different car floors. FIGS. 12 and 13 show a 
backing electrode unit 98 designed to serve this purpose. Numeral 85 is a 
base and 86, an electrode support which is rotatable about a horizontal 
rotary shaft 87. On the support 86 are mounted angularly spaced apart 
backing electrodes 88 and 89 whose side configurations are shown in FIGS. 
13a and 13b, respectively. The electrode support 86 is insulated from 
electrodes 88 and 89 by insulators 88b and 89b, respectively. These 
backing electrodes 88 and 89 each have particular contours to conform to 
the contour of different car floors, and each electrode is so inclined as 
to abut the lower surface contour of the channel portion of the car floor. 
Numeral 90 is a hydraulic cylinder having a piston rod 90a with its top 
end pivoted to a bracket 91 secured to shaft 87, and a cylinder housing 
90b having its bottom end pivoted to a bracket 93 on a base plate 94. When 
the piston rod 90a extends, as shown in FIG. 12, the support 86 turns 
counterclockwise to bring the electrode 88 to the top position, and when 
the piston rod retracts, the support is turned to a position indicated by 
broken lines, bringing the electrode 89 to the top position. Numerals 95, 
96 and 97 designate spacers to determine the position of the support. 
With the car floor F being pressure-contacted between the paired electrode 
tips 60, 60 and the associated backing electrode, a welding current is 
supplied from a first terminal of the transformer T to one electrode tip 
60, then to the workpiece, and through the backing electrode and the other 
electrode tip 60, and back to the second terminal of the transformer, 
whereby spot welding is effected simultaneously at two locations as 
illustrated in FIG. 1c. This arrangement thus eliminates the need for 
connecting a cable from the transformer to the backing electrode. 
An alternative embodiment of the present invention is illustrated in FIG. 
14 in which a second robot arm structure 3', identical in construction to 
the robot arm structure 3, is mounted on the vertical support 1 opposite 
to the vertical support on which the first robot arm structure 3 is 
mounted. Each robot arm structure is provided with a single electrode tip 
instead of paired electrode tips. Cable 61 connects a first terminal of 
the transformer T to the electrode tip 60 of the robot arm 3 and cable 61' 
connects a second terminal of the transformer T to the electrode tip 60' 
of the robot arm structure 3'. Therefore, welding current is passed 
between the electrode tips 3 and 3' through the workpiece F and the 
backing electrode 88 or 89, so that spot weld is accomplished 
simultaneously in any two locations as illustrated in solid and phantom 
lines in FIG. 14. 
Another pair of spot welding machines 3" and 3"' may also be employed as 
shown in FIG. 15 to achieve simultaneous spot welding operations in which 
welding current is passed between the electrode tips of these welding 
machines in the same manner as described in FIG. 14.