Robot arm having motion-limiting tether

An electrical power interrupting tether system for a multiple-joint, anthropomorphic robot arm that substantially improves robot arm operational safety. The tether system includes a robot arm and auxiliary equipment power-interrupting electrical robot arm member by a set of braided steel wires. A pair of conventional robot arm and auxiliary equipment power-interrupting limit switches actuated by excessive rotational robot arm movement are also provided. Movement of the robot arm beyond a predetermined safety zone defined by each arm-member-tethering wire and the limit switch pair will cause electrical power interruption to both the robot arm and to auxiliary equipment associated therewith to thereby preclude potentially unsafe movement of the robot arm beyond the above-defined safety zone.

BACKGROUND OF THE INVENTION 
The present invention relates to programmed industrial robots, in general, 
and to means for limiting the movement of a computer controlled, 
anthropomorphic robot arm to within a prescribed spacial envelope to 
improve its operational safety, in particular. 
Industrial robots are programmed mechanical devices that are capable of 
automatically performing material handling tasks, for extended periods of 
time, without human intervention. Properly applied industrial robots can 
increase productivity as well as relieve human operators from tasks that 
are excessively burdensome, boringly routine and/or dangerous. An 
anthropomorphic industrial robot arm has an articulated mechanical arm and 
hand with freedom of movement that is roughly equivalent to a human waist, 
shoulder, elbow and wrist. The robot's hand can be automatically 
positioned to any point within reach and is capable of grasping various 
parts or tools with relative ease. One such industrial robot is sold by 
Unimation, Inc. of Danbury, CT under its trademarks UNIMATE and PUMA. 
Industrial robots are typically controlled by computer software or a set of 
instructions stored in the memory of a digital computer. The robot arm 
executes the material or parts positioning instructions as they are 
received from computer memory. These robot arm position-controlling 
instructions received by the robot arm are in the form of fairly precise 
elecrical signals whose characteristics control the type and extent of 
robot arm movement. Any significant variation in one or more of the 
characteristics associated with these robot arm position-controlling 
signals will produce corresponding variations in robot arm movement. 
A digital computer is an electrically operated device that occasionally 
becomes susceptible to spurious or random electrical signals. These 
spurious signals may enter the computer from its power source or may be 
spontaneously generated within one or more of its system components. 
Whatever the cause of these signals, one of their effects is to cause the 
computer to transmit spurious positioning signals to, for example, a robot 
arm under its control. These spurious positioning signals may produce 
sudden, unpredictable robot arm movement that could cause serious injury 
to personnel in the vicinity of the computer-controlled robot arm. 
One way to protect personnel from injury due to spurious-signal-induced 
robot arm movement would be to enclose the robot arm in a fairly rigid 
protective cage. The protective cage would preclude contact between the 
moving robot arm and any personnel in close proximity thereto under such 
circumstances. A disadvantage associated with this arrangement is that a 
protective cage makes it more difficult for personnel to service the robot 
arm during normal robot arm operation. Another way to protect personnel 
from such injury would be to enclose the robot arm in a system of 
protective light beams. If any of the beams were broken by, for example, 
personnel coming too close to the spacial envelope within which the robot 
arm is capable of operating, electrical power to the robot arm would 
immediately be interrupted. Disadvantages associated with this arrangement 
would be equipment cost and the liklihood of excessive robot arm power 
interruptions due to inadvertent cutting of one or more of the protective 
light beams controlling access to the robot arm. 
A primary object of the invention, therefore, is to improve the operational 
safety of a computer-controlled anthropomorphic robot arm. 
Another object of the present invention is to provide a simplified system 
for limiting the movement of a computer-controlled anthropomorphic robot 
arm to within a predetermined spacial envelope. 
A further object of the present invention is to minimize the effects of 
spurious or random electrical signals on a computer that controls the 
positioning of an anthropomorphic robot arm. 
Other objects and advantages of our invention will be made readily apparent 
by referring to the preferred embodiments thereof described in detail 
below. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an electrical power interruption 
tether system for a multiple-joint, anthropomorphic robot arm is provided 
that substantially improves robot arm operational safety. The tether 
system includes robot arm and auxiliary equipment power-interrupting 
apparatus and means coupling said apparatus to an extendible robot arm 
member. Means coupled to said power-interrupting apparatus for sensing 
when said robot arm is rotated to limits defined by a predetermined angle 
are also provided. Movement of said robot arm to the limits of a spacial 
zone defined by the lenth of said coupling means and by said predetermined 
angular limits activates said power interrupting apparatus to its power 
interrupting mode and thereby precludes potentially unsafe extensible 
and/or rotational robot arm movement beyond said predetermined spacial 
zone and unsafe auxiliary equipment operation, if such robot arm movement 
should occur.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 of the drawings, a partial system diagram of object placement 
apparatus 10 incorporating a preferred embodiment of the present 
invention, is depicted. Object placement apparatus 10 includes, in part, 
mechanical vibrator 12 for separating the objects that are to be placed in 
a particular position and orientation, from one another. Apparatus 10 also 
includes x, y, .theta. table 14 for moving the object in any direction 
within a particular plane and/or about an axis at right angles to said 
plane to a particular orientation as determined by conventional object 
position sensing apparatus (not shown) such as that shown in U.S. Pat. No. 
4,575,637 to Sullivan, Jr. The specification in said Sullivan, Jr. patent 
is specifically incorporated herein by reference. 
When energized, mechanical vibrator 12 moves objects 15 from its 
object-containing hopper 16 down chute 18 and onto x, y, .theta. table 14. 
Object placement apparatus 10 also includes industrial robot 20. When 
placed in a particular position and orientation by x, y, .theta. table 14 
and said conventional object position sensing apparatus, the object is 
picked up by industrial robot 20 and subsequently placed on conveyor belt 
22. Industrial robot 20 is preferably a computer-controlled 
anthropomorphic robot arm system such as that noted above manufactured by 
Unimation, Inc. Conveyor belt 22 moves the precisely oriented and 
positioned object to the first of several workstations where it is 
assembled into a final product. The position and orientation of the 
objects on table 14 is the initial or reference position and orientation 
of objects transferred from table 14 to conveyor belt 22 by industrial 
robot arm 20. Changes made in object position and/or orientation by robot 
20 when transferring an object from table 14 to belt 22 are always made 
with respect to its said initial or reference position and orientation. 
Robot arm 20 is a five degree of freedom device, i.e., it has a total of 
five axes about which various robot arm members may be rotated to place an 
object in a particular position and orientation. Each member of the robot 
arm is connected to another member at a joint, much like a human arm and 
torso. Through each such joint passes one or more axes around which the 
members of the arm rotate. The members of the robot arm, as shown in FIG. 
1, are waist member 24, shoulder member 26, upper arm member 28, forearm 
member 30, wrist member 32 and gripper member 34. The robot arm members 
contain the various servomotors and gear trains necessary to produce the 
required degree of robot arm member movement. 
The axes of rotation of said robot arm members are as follows: waist axis 
36, which is perpendicular to horizontally positioned robot arm mounting 
plate 38 and coincident with the centerline of waist member 24; shoulder 
axis 40, which is perpendicular to and intersects waist axis 36, is 
coincident with the centerline of shoulder member 26; elbow axis 42, which 
is parallel to shoulder axis 40; wrist rotational axis 44 which is 
perpendicular to and intersects elbow axis 42, is coincident with the 
centerline of forearm member 30; and gripper rotational axis 46, which is 
perpendicular to and intersects said wrist axis 44. 
Rotational movement of the above-noted robot arm members of robot arm 20 is 
controlled by robot arm control system 48. System software that controls 
such movement of arm 20 is stored in the memory of a computer (not shown) 
located within robot arm control system 48. The operational controls (not 
shown) for robot arm 20 are also located within said control system 48. 
Robot arm 20 executes all of the preprogrammed movement instructions 
transmitted to it from the memory of the computer within robot arm control 
system 48 through path 50. 
AC power applied to object placement apparatus 10 is supplied by AC power 
source 52 that is schematically depicted in drawing FIG. 2. As shown in 
FIGS. 1 and 2, electrical power from AC power source 52 is applied to 
robot arm 20 and associated auxiliary equipment 54 that includes 
mechanical vibrator 12, x, y, .theta. table 14 and conveyor belt 22, 
through path 56 and series connected electrical switches S1, S2 and S3. 
Switch S1 is a single-pole lanyard-type switch, Model No. 802T-CM 
manufactured by the Allen Bradley Co. of Milwaukee, Wis. Switch S1 
includes actuator 58 for actuating said switch S1 from its normally closed 
to its open or power interrupting position. Switch S1 remains in its open 
position until manually reset back to its normal or closed position by an 
equipment operator. Switches S2 and S3 are conventional single-pole 
momentary switches that are normally spring-force loaded to their closed 
or power-transmitting positions. Switches S2 and S3 remain in their open 
or power interrupting positions so long as a force is applied to each of 
their switch contact opening actuators. 
Actuator 58 of lanyard switch S1 is connected to eye bolt 60 mounted 
intermediate the ends of upper robot arm member 28 by flexible steel cable 
62. Actuator 58 is also connected to collar 64 mounted in a fixed position 
at the gripper 34 end of forearm member 30 of said robot arm 20 by 
flexible steel cable 66. Cable 66, connecting actuator 58 to forearm 
member 30, is preferably routed through the opening in eyebolt 60. Cable 
66 is routed through the opening in eyebolt 60 in order to preclude any 
interference between said cable 66 and the objects to be positioned. 
Momentary switches S2 and S3 are mounted in a fixed position and in a 
laterally spaced relation on the upper end of waist member 24. Elongated 
metal bar 68 is mounted in a fixed position on shoulder member 26 for 
rotation therewith. Rotational movement of shoulder member 26, including 
metal bar 68 mounted thereon about waist axis 36, causes said metal bar 68 
to engage each of the actuators on switches S2 and S3 when shoulder member 
26 is rotated in opposite directions about said axis 36 to predetermined 
switch actuating angular positions. These predetermined angular positions 
are established by the lateral spacing between momentary switches S2 and 
S3 and their physical placement on upright waist member 24. 
In normal operation, robot arm control 48, in drawing FIG. 1, transmits 
robot arm positioning commands to robot arm 20 for object positioning 
purposes. Referring additionally to drawing FIG. 3, during such normal 
operation shoulder member 26 can freely rotate about axis 36, upper arm 
member 28 can freely rotate about axis 40 and forearm member 30 can freely 
rotate about axis 42 in response to object positioning commands from robot 
arm control 48 so long as such rotational movement does not cause the 
actuation of electrical switches S1, S2 or S3 to their open or robot arm 
20 and auxiliary equipment 54 power interrupting positions. If elongated 
metal bar 68 mounted on shoulder member 26 is rotated about axis 36 to 
either position 68A or 68B as shown in drawing FIG. 3, said bar 68 will 
respectively actuate switches S2 or S3 to their open or power interrupting 
positions. Similarly, as shown in FIG. 4, if upper arm member 28 rotates 
in direction 70 about axis 40 to the point where steel cable 62 is pulled 
taut and thereafter transmits such motion to switch S1 actuator 58, switch 
S1 will be actuated to its open or power-interrupting position by the 
continued rotation of said member 28. Also, as shown in drawing FIG. 5, if 
forearm member 30 rotates in direction 72 about axis 42 to the point where 
steel cable 66 is pulled taut and thereafter transmits such motion to 
switch S1 actuator 58, switch S1 will be actuated to its open or 
power-interrupting position by the continued rotation of said member 30. 
It should be noted that the degree of rotation of forearm member 30 
required to actuate switch S1 to its open position is dependent upon the 
rotational position of upper member 28 at the time of such switch 
actuation. 
Robot arm 20 normally operates within a spacial zone or volume defined by 
the lateral rotation of shoulder member 26 about waist axis 36 and the 
vertical movement of upper arm member 28 and forearm member 30 about axes 
40 and 42, respectively, to the extent that, or so long as, lanyard switch 
S1 and momentary switches S2 and S3 are not actuated to their open or 
power-interrupting positions by such robot arm member rotation. If 
erroneous instructions were transmitted to robot arm 20 by, for example, 
control 48 (FIG. 1) commanding said robot arm to move out of the 
above-defined spacial zone and possibly injure nearby personnel, switches 
S1, S2 or S3 would be actuated to their open position thereby interrupting 
robot arm power to thereby preclude potentially injurious robot arm 
movement outside of its said operational zone. If robot arm 20 is 
commanded to move outside of its operational zone and power to robot arm 
20 and auxiliary equipment 54 is interrupted by such movement, robot arm 
control must be manually overridden in order to restore the electrical 
power to robot arm 20 and auxiliary equipment 54. 
It will be apparent to those skilled in the art from the foregoing 
description of our invention that various improvements and modifications 
can be made in it without departing from its true scope. The embodiments 
described herein are merely illustrative and should not be viewed as the 
only embodiments that might encompass our invention.