Industrial robot having counterbalanced arms

An industrial robot is disclosed which includes three primary drive units defining three separately controlled axes of movements, an inner arm connected to the output of one of the drive units, and an outer arm pivotally connected to the inner arm. The outer arm is connected to another of the drive units by a transmission which extends through the inner arm so that the outer arm may be selectively rotated, and both the inner arm and the outer arm are counterbalanced by a pair of torsion coil springs which are operatively connected to their respective drive units.

The present invention relates to an industrial robot of the type designed 
as a replacement for human labor in performing repetitive, hazardous, or 
tiring work. 
Industrial robots generally have the capability of moving through up to six 
axes of movement to manipulate objects, parts, or tools through variable 
programmed motions for the performance of a variety of tasks. 
Reprogrammable robots are also available which incorporate a computer and 
microprocessor whereby the robot may be taught to move from point to point 
using a portable teaching box or the like. 
Industrial robots of the type adapted to move through a large number of 
axes commonly include a primary support member which includes at least two 
drive units, an inner arm connected to the output of one of the drive 
units so as to rotate about a horizontal central axis, and an outer arm 
which is pivotally connected to the inner arm for rotation about a second 
horizontal axis disposed parallel to and laterally spaced from the central 
axis. Also, a suitable transmission is provided which extends through the 
inner arm for operatively interconnecting the outer arm with the second 
one of the drive units, so that the second drive unit rotates the outer 
arm about the second axis. Robots of this general type are disclosed for 
example in commonly-owned U.S. Pat. No. 4,552,505 and copending 
application Ser. No. 443,156, now U.S. Pat. No. 4,636,138. 
As will be apparent, rotation of one of the arms from a substantially 
vertical position to a horizontal position results in the weight of the 
arm, or the weight of the workpiece at the end of the arm, imparting a 
significant torque to its mounting structure, which results in a strain 
being placed on the mounting structure and an additional load being 
imparted to the drive motor. Heretofore, counterbalancing weights have 
been mounted on the arms to alleviate this problem, but such weights 
increase the load on the drive motor, they are necessarily massive and 
require a great deal of space, and they are not readily adjustable to 
accommodate different arm or workpiece weights. It has also been proposed 
to utilize a tension spring for counterbalancing the arm of a robot, note 
for example U.S. Pat. No. 4,500,251, but this prior spring 
counterbalancing system is seen to require a great deal of space and it is 
not readily adjustable. 
It is accordingly an object of the present invention to provide an 
industrial robot having provision for effectively counterbalancing its 
movable arms, and which avoids the disadvantages and limitations of the 
prior systems. 
It is a more particular object of the present invention to provide an 
industrial robot having inner and outer pivotally interconnected arms, and 
which includes an effective system for counterbalancing each of the arms 
with a readily adjustable counterbalancing force. 
These and other objects and advantages of the present invention are 
achieved in the embodiment illustrated herein by the provision of an 
industrial robot which comprises a support member, a reversible drive unit 
mounted to the support member and including rotatable output shaft means, 
and an arm connected to the output shaft means of the drive unit so as to 
be rotatable about a horizontal axis at least between a vertically 
disposed position and a horizontally disposed position by operation of the 
drive unit. Torsion spring means is also provided for applying a torque to 
the output shaft means in an amount which is a function of the rotational 
positioning of the arm and such that a minimum torque is applied when the 
arm is vertically disposed and a maximum torque is applied when the arm is 
horizontally disposed and so as to effectively counterbalance the weight 
of the arm. The torsion spring means includes a coil spring having a 
plurality of windings and first and second opposite ends, means 
operatively interconnecting the first end of the coil spring to the 
support member, and means operatively connecting the second end of the 
coil spring to the output shaft means of the drive unit, such that 
rotation of the output shaft means causes the coil spring to rotate so as 
to either tighten or loosen the windings. 
In a preferred embodiment, the industrial robot includes two drive units 
which are mounted to a common support member, and with the drive units 
including coaxially disposed output shaft means disposed along the central 
axis. An inner arm is connected to the output shaft means of a first drive 
unit, and an outer arm is pivotally connected to the inner arm for 
relative rotation about a second axis which is disposed parallel to and 
laterally spaced from the central axis. Torque transmission means is 
provided for connecting the output shaft means of the second drive unit to 
the outer arm, and such that the outer arm may be rotated about the second 
axis by the second drive unit. In addition, first and second torsion 
spring means are provided, with the first torsion spring means being 
operatively connected to the output shaft means of the first drive unit, 
and the second torsion spring means being operatively connected to the 
output shaft means of the second drive unit. Each of the first and second 
torsion spring means comprises a coil spring as described above, and the 
two coil springs are preferably mounted along parallel axes, which are 
parallel to the central axis of the robot.

Referring more particularly to the drawings, an industrial robot embodying 
the features of the present invention is illustrated generally at 10. In 
the illustrated embodiment, the robot 10 is adapted to move through six 
axes of movement, and it comprises a primary support stand 12 which 
defines a generally vertical axis A (FIG. 1). A waist 14 is rotatable with 
respect to the stand 12 and defines a generally horizontal axis B which is 
perpendicular to and intersects the axis A. A first or inner arm 15 is 
rotatable with respect to the waist 14 about the horizontal axis B, and a 
second or outer arm 16 is rotatable with respect to the inner arm about a 
second horizontal axis C, which is parallel to and laterally spaced from 
the axis B. A hand assembly 18 including a gripper 19 is mounted at one 
end of the outer arm 16, and is adapted to move through three additional 
axes of movement, in the manner further described in copending application 
Ser. No. 443,156. In addition, the support stand 12 of the robot mounts a 
control box 20 mounted adjacent the stand 13 for housing the electronic 
controls for the various drive motors of the robot. 
As best seen in FIG. 6, the robot includes a drive unit 21 having a tubular 
base component 21a and a secondary component 21b which is rotatable with 
respect to the base component about the axis A. The base component 21a is 
fixedly connected to the support stand 12 by a releasable coupling band 24 
as further described in copending application Ser. No. 443,156. 
A support member 26 is fixed to the secondary component 21b of the drive 
unit 21, and the support member 26 in turn mounts a pair of drive units 22 
and 23. The drive units 22 and 23 each include a tubular base component 
22a, 23a, which are coaxially disposed about the axis B, and a surrounding 
secondary component 22b, 23b. The secondary components 22b, 23b, are fixed 
to each other by bolts 27, and fixed by bolts to the support member 26, 
and the tubular base components 22a, 23a are rotatably mounted to their 
respective secondary components so as to rotate about the axis B. 
The drive unit 22 includes output shaft means which includes a crank arm 
22c (FIG. 5) fixed to the end of the tubular base component 22a by bolts. 
The crank arm 22c is coaxially mounted about the axis B and it is in turn 
coupled to the casing of the inner arm 15 by a coupling band 24 of the 
type described above. 
The drive unit 23 includes output shaft means which includes a crank arm 
23c (FIG. 4) which is bolted to the outer end of the base component 23a. 
Also an elongate drive shaft 30 is fixed to the crank sleeve 23c and 
extends coaxially through the base component 23a of the drive unit 23, and 
also through the base component 22a and crank arm 22c of the drive unit 
22. A tubular sleeve 32 is coaxially fixed to the end of the crank arm 
23c, and the sleeve closely surrounds and supports a cable guide tube 33. 
The guide tube is fixed to the support member 26 by the brace 34, and such 
that the sleeve 32 is rotatable about the fixed guide tube 33. The guide 
tube is designed to facilitate the passage of electrical or other cables 
through the drive shaft 30 and into the inner arm 15, for driving the 
components of the outer arm 16. 
Each of the three drive units 21, 22, 23 includes a face gear 21d, 22d, 23d 
respectively, which is coaxially mounted to the base component thereof, 
and a reversible drive motor 36 and brake 37 which are mounted to the 
exterior of the associated secondary component for rotatably driving the 
face gear. As best seen in FIG. 10, which is representative of all three 
drive units, the motor 36 and brake 37 each include an output shaft 
mounting a drive gear 36a, 37a, and the two drive gears engage a pinion 
gear 38 which in turn meshes with the face gear 21d to effect rotation of 
the face gear and base component in a selected rotational direction. 
However, in the case of the drive unit 21, it will be understood that it 
is the secondary component 21b which actually rotates about the fixed base 
component 21a. Alternatively, a drive unit may be employed which comprises 
a rotor and stator which are both coaxially disposed about the rotational 
axis of the motor, and as further described in U.S. Pat. No. 4,552,505. 
The inner arm 15 of the robot comprises a casing 40, having one end which 
is releasably connected to the crank arm 22c as described above. Thus the 
inner arm may be selectively rotated in either direction about the axis B 
by the drive unit 22. Also, the shaft 30 from the drive unit 23 extends 
coaxially into the casing 40 and mounts a sprocket 42 at the free end 
thereof. A second shaft 44 is rotatably mounted at the other end of the 
casing 40, and is disposed coaxially about the horizontal axis C, which is 
parallel to and laterally spaced from the horizontal axis B. The shaft 44 
is directly joined to the casing of the outer arm 16 by a coupler 24, and 
the shaft 44 coaxially mounts a sprocket 45 within the casing 40 of the 
inner arm. A flexible endless toothed belt 46 is operatively entrained 
about the sprockets 42, 45. By this arrangement, a transmission is defined 
which includes the drive shaft 30, sprockets 42, 45, and belt 46, which 
interconnects the base component 23a of the drive unit 23 with the outer 
arm 16, so that the outer arm may be selectively rotated in either 
direction about the axis C by the drive unit 23. Also, since the support 
member 26 is fixed to the secondary component 21b of the drive unit 21, it 
will be apparent that the rotation of the secondary component 21b results 
in the support member 26, drive units 22, 23, and inner and outer arms 15, 
16 all rotating about the vertical axis A. 
The industrial robot 10 further includes first torsion spring means 
operatively connected to the output shaft of the drive unit 22, and second 
torsion spring means operatively connected to the output shaft of the 
drive unit 23. More particularly, and as best seen in FIG. 7, the first 
torsion spring means includes a first coil spring 50 which is disposed 
along a horizontal axis which is parallel to the central axis B, and the 
first coil spring 50 includes a plurality of windings 51 and first and 
second ends 52, 53 respectively. The first end 52 is fixedly 
interconnected to the support member by the structure described below, and 
the second end 53 is connected to the crank arm 22c of the drive unit 22 
such that rotation of the output shaft means of the drive unit 22 causes 
the second end 53 of the coil spring 50 to rotate upon its axis, and while 
the first end 52 is held against rotation. Thus the windings 51 of the 
coil spring 50 are either tightened or loosened. More particularly, the 
second end 53 is interconnected to the crank arm 22c by an arrangement 
which includes an end flange 54 which receives and supports the end 53 of 
the coil spring as best seen in FIG. 8, and a pulley 55 which is fixed to 
the end flange and rotatably mounted to the support member by bearings 56. 
Also, the mounting arrangement includes a mounting pin 56 on the crank arm 
22c at a location radially spaced from the central axis B, and a cable or 
chain 58 which has one end wrapped upon the pulley 55 and fixed thereto by 
a pin 59, and an opposite end which is fixed to the mounting pin 56 of the 
crank arm 22c. 
As best seen in FIG. 5, the secondary component 22b of the drive unit 22 
mounts a pair of limit switches 60, 61, and the crank arm 22c includes a 
raised cam surface 62 which is designed to engage respective ones of the 
limit switches. This arrangement prevents the base component 22a and crank 
arm 22c from rotating beyond its maximum designed range, which is 
typically about 350.degree.. A corresponding pair of limit switches 64, 65 
are mounted on the secondary component 23b of the drive unit 23, note FIG. 
4, which are engaged by the raised cam surface 66 on the crank arm 23c. 
The first end 52 of the coil spring 50 is fixed to an end flange 68 (note 
FIGS. 7 and 8), and the end flange 68 is fixedly mounted to a support 
shaft 69, which extends along the axis of the coil spring 50. A gear 70 is 
in turn coaxially mounted to the shaft 69. A worm 71 is rotatably mounted 
to the support member so as to operatively engage the gear 70, and the 
worm 71 includes a head 72 as best seen in FIGS. 3 and 4, which is adapted 
to be engaged by a suitable turning tool so that the associated gear 70, 
and thus the first end 52 of the coil spring 50, may be selectively 
rotated. 
The second torsion spring means, and which is operatively connected to the 
output shaft of the drive unit 23, comprises a second coil spring 74 which 
is disposed along a horizontal axis which is parallel to that of the first 
spring 50, and the second spring comprises a plurality of windings 75 and 
first and second ends 76, 77 respectively. The first end 76 is fixedly 
interconnected to the support member 26 by means of an end flange 78, 
which in turn is fixed to a shaft 80 which extends coaxially through the 
spring 74. The opposite end of the shaft 80 mounts a gear 81, which may be 
selectively rotated by the worm 82 in the manner described above with 
respect to the gear 70 and worm 71. The second end 77 of the spring 74 is 
connected to the crank arm 23c of the drive unit 23, such that rotation of 
the output shaft means of the drive unit 23 causes the second end 77 to 
rotate. This interconnection includes an end flange 83 which receives and 
supports the end 77 of the spring 74, and a pulley 84 which is fixed to 
the end flange 83 and rotatably mounted to the support member by bearings 
85. Also, there is provided a mounting pin 86 on the crank arm 23c at a 
location radially spaced from the central axis B, and a chain 88 which has 
one end wrapped upon the pulley and fixed thereto by a pin 89, and an 
opposite end which is fixed to the mounting pin 86 of the crank arm 23c. 
The above described mounting arrangement of the two coil springs 50, 74 
results in the first end of each coil spring being fixed to the support 
member and held against rotation, but the first end may be selectively 
rotated by rotation of the worm to thereby either wind or unwind the coil 
spring and thereby adjust the torque imparted to the associated drive 
unit. Such adjustment permits the applied torque to be readily and easily 
changed where the weight of the arm, or the weight of the workpiece being 
manipulated, changes. 
As best seen in FIG. 7, the first and second ends of the two coil springs 
are oppositely oriented so that one chain is positioned on each side of 
the robot, but the worms 71, 82 for the adjustment of the two coil springs 
50, 74 are positioned on a common side of the robot, which is opposite the 
inner and outer arms, to facilitate access by a technician. Also, a cover 
plate 90, 91 (FIG. 4) is mounted at the end of each coil spring which 
mounts the gear and worm, and a mounting post 69a, 80a extends coaxially 
from the associated shaft through the cover plate, and the post in turn 
mounts a dial 92 which is positioned on the external side of the cover 
plate. Each cover plate also includes suitable external indicia, by which 
the adjusted torsion of each coil spring is visually indicated. 
It will be noted from FIG. 7 that the coil spring 50, and which is 
associated with the inner arm 15, is somewhat longer and heavier than the 
coil spring 74 which is associated with the outer arm 16. This is 
desirable, since the inner arm must compensate not only for its own 
weight, but also for the weight of the outer arm and the workpiece. 
Viewing FIG. 11, it will be seen that in the solid line position, the 
mounting pin 56 of the crank arm 22c is aligned along a line extending 
between the central axis B and the tangent to the pulley 55 defined by the 
chain 58. Thus the coil spring 50 imparts no torque to the drive unit 22 
in the illustrated solid line position, and this position corresponds to 
the illustrated vertical upward position of the inner arm 15. In the 
dashed line position, the crank arm 22c has rotated 90.degree. clockwise, 
and the inner arm 15 has rotated to a horizontal position, i.e. the 
position at which maximum torque will be applied to the drive unit 22 by 
the weight of the inner arm. At the dashed line position, a line extending 
between the mounting pin 56 and the central axis B is essentially 
perpendicular to the direction in which the force of the coil spring 50 is 
being applied through the chain 58, and by design, the resulting torque 
applied by the coil spring 50 to the crank arm 22c is sufficient to 
substantially offset the opposite torque applied by the weight of the arm. 
As will also be apparent, the torque applied by the coil spring 50 to the 
crank arm 22c will progressively increase during movement from the 
illustrated solid line position to the dashed line position, to 
effectively counterbalance the arm 15 in all intermediate positions. Also, 
the force will progressively decrease as the crank arm continues to rotate 
in the clockwise direction beyond the dashed line position, as the inner 
arm 15 moves toward a vertical downward position. A similar result will be 
achieved upon rotation in the counter clockwise direction from the 
illustrated solid line position. 
Viewing FIG. 12, it will be seen that in the solid line position, the 
mounting pin 86 of the crank arm 23c is aligned along a line extending 
between the central axis B and the tangent to the pulley 84 defined by the 
chain 88. Also, the outer arm 16 is in a vertical orientation, such that 
it imparts no torque to the drive unit 23. When the outer arm 16 is 
rotated to the horizontal position illustrated in dashed lines, where 
maximum torque is applied by the weight of the arm 16, the crank arm 23c 
is rotated 90.degree. as shown in dashed lines, so that the coil spring 74 
imparts maximum counterbalancing torque to the crank arm 23c. Thus both 
the inner and outer arms are effectively counterbalanced throughout the 
full range of movements. 
In the drawings and specification, there has been set forth a preferred 
embodiment of the invention, and although specific terms are employed, 
they are used in a generic and descriptive sense only and not for purposes 
of limitation.