Compact robot arm member relative movement sensor

Apparatus for determining when the hand of a robot arm, resiliently mounted on a robot arm body, strikes or makes contact with an object that impedes or limits its movement, employs an optical fiber in place of conventional light focusing optics to produce a compact and ruggedized robot arm member relative movement sensor. The apparatus includes a light source mounted in a particular location on the robot arm hand and a light sensitive device spaced from the light source and mounted in a particular location on the robot body, a device that is capable of continuously generating a hand-to-robot arm body relative position signal in response to the particular location of collimated light from the robot hand mounted light source that passes through a light collimating optical fiber and strikes the light sensitive surface of the light sensitive device. Relative movement between the robot arm body and the robot hand resulting from contact between a motion-impeding object and the robot hand causes the light sensitive device to generate a signal indicative of such object contact and/or extent of relative hand-to-robot arm movement. The use of an optical fiber in the light path in place of a conventional lens makes it possible to employ this relative movement sensing apparatus in a vibratory environment and also makes it possible to shorten the light path length and thereby reduce the size of said relative motion sensing apparatus.

BACKGROUND OF THE INVENTION 
The present invention relates to positioning apparatus in general, and to 
relatively compact apparatus for controlling movement of a positioning 
device commonly referred to as an industrial robot, in particular. 
Automated product assembly machines, for example, have been employed in 
manufacturing industries for a great number of years. More recently, 
though, technologically more sophisticated machines have been employed for 
such purposes. These more recent machines are commonly referred to as 
industrial robots. Industrial robots are capable of performing various 
mechanical operations with a high degree of speed and accuracy in response 
to a set of programmed instructions. 
Common uses for industrial robots include the movement of a workpiece from 
one position to another and the performance of repetitive operations with 
a high degree of precision. The use of industrial robots in place of human 
personnel has proven very beneficial in that they have resulted in both 
cost reductions and processing accuracy and have relieved many personnel 
from routine and/or potentially hazardous jobs. Industrial robots are also 
employed in numerous other fields of technology to perform a variety of 
different operations. 
A significant problem associated with industrial robots, especially those 
employed for product assembly purposes, is their inability to recognize 
when they collide or make contact with objects located in their path of 
travel when moving to perform programmed tasks. Inasmuch as most 
industrial robots are capable of generating extremely large physical 
forces, such forces can seriously damage or even destroy such contacted 
objects, objects that have heretofore been undetectable when struck or 
contacted by any portion of a moving robot. A typical object damaging 
situation often occurs when an industrial robot is programmed to place a 
series of identical objects or piece-parts into fairly close tolerance 
openings or recesses in, for example, a series of housings, during product 
assembly, over an extended period of time. During this extended period of 
product assembly time, a gradual misalignment will often result between, 
for example, the center of the housing opening and the center of the 
piece-part that is being inserted into the housing opening. Such 
misalignment has heretofore been undetectable by an industrial robot. The 
consequences of the industrial robot being unable to detect such 
misalignment often are damage to the piece-part and/or to the housing, or 
to the robot itself because of the large physical forces that are 
sometimes generated by a robot as it blindly attempts to place a 
piece-part in a misaligned housing opening where it is incapable of 
detecting such misalignment. 
In U.S. patent application Ser. No. 484,228 by M. KHUSRO, filed on the same 
date as the present application, robot arm member relative movement 
sensing apparatus is disclosed that determines when a relatively movable 
driven robot arm member moves out of positional alignment with a driving 
robot arm member. The apparatus includes a light source mounted on one 
member and a light sensitive device mounted on the other member that is 
capable of continuously generating a member-to-member relative position 
signal in response to light from said light source being optically focused 
on said light sensitive device. Relative movement between these two robot 
arm members causes the light sensitive device to generate an electrical 
signal representative of the extent of member-to-member relative movement 
However, by employing a conventional optical lens to focus a portion of 
the light from the light source onto a surface of the light sensitive 
device, the lens element must be spaced a relatively large distance from 
the light sensitive device surface in order to place the light sensitive 
surface at the lens element focal plane for proper light sensitive device 
operation. Providing this required space necessarily increases relative 
movement sensing apparatus size and/or length. In addition, the light 
focusing lens element mounting must be substantial enough to maintain the 
lens light path in a predetermined, relatively fixed orientation during 
robot arm operation including operations in a mechanical shock or highly 
vibratory environment. A lens element of a size employed in the 
above-noted patent application presents difficult mounting problems for a 
lens that must operate in such an environment. 
A primary object of the present invention is to provide compact apparatus 
for detecting relative movement between members that are resiliently 
attached to one another. 
Another object of the present invention is to provide relative movement 
sensing apparatus that can operate in a vibratory environment. 
Still another object of the present invention is to provide compact 
apparatus for determining when a portion of an industrial robot arm comes 
in contact with a robot arm motion impeding object. 
A further object of the present invention is to provide compact apparatus 
that will enable an industrial robot to apply a predetermined physical 
force to a particular object or workpiece, such as when forcing components 
together during product assembly, with a predetermined force. 
Other objects, features and advantages of the present invention will be 
readily apparent from the following detailed description of the preferred 
embodiment thereof taken in conjunction with the accompanying drawings. 
SUMMARY OF THE INVENTION 
In accordance with the teachings of the present invention, compact 
apparatus is provided for determining when a relatively movable driven 
member moves out of positional alignment with respect to a drive member. 
The apparatus includes means for resiliently attaching said members to one 
another such that they are maintained in a predetermined positional 
relationship relative to one another A light source is mounted in a 
particular location on one member and a light sensitive device capable of 
continuously generating a member-to-member relative position signal in 
response to collimated light through an optical fiber from said light 
source striking the light sensitive surface of said light sensitive 
device, is mounted on the other member. Relative movement between said 
resiliently attached members causes said light sensitive device to 
generate an electrical signal representative of the extent of 
member-to-member relative movement. This signal may be employed to, for 
example, terminate drive member movement, cause a particular force to be 
applied to an object by said movable member or, in the case of an 
industrial robot, to facilitate robot programming.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 of the drawings, conventional industrial robot or robot arm 
assembly 10 incorporating a preferred embodiment of the present invention, 
is depicted Robot arm assembly 10 is a PUMA Model 600 that is manufactured 
by Unimation, Inc. of Danbury, Conn. Robot arm assembly 10 operates in 
accordance with an anthropomorphic coordinate system having six degrees of 
freedom with portions of said assembly being capable of limited rotational 
movement about axes 12, 14, 16, 18 and 20. Robot arm assembly 10 includes 
pedestal 22 having pedestal axis 12 about which all of the upper portions 
of assembly 10 can be rotated. In addition, lower arm 24 which is 
rotatably attached to pedestal 22, is rotatable about lower arm axis 14. 
Upper arm 26 which is rotatably attached to lower arm 24, is rotatable 
about upper arm axis 16. Wrist joint 28 is rotatable about two mutually 
orthogonal axes 18 and 20, said joint being tiltable with respect to upper 
arm 26 and pivotable about axis 20 which is the axis about which 
pneumatically actuated work-piece-holding gripper 30 is rotated. Gripper 
30 is resiliently attached to said wrist joint 28 by relative movement 
sensing apparatus 32 of the present invention. Sensing apparatus 32 senses 
relative movement between wrist joint 28 and work-piece holding gripper 30 
and generates an electrical signal representative of such relative 
movement. Robot assembly 10 is programmed by the so-called training 
method, i.e., gripper 30, that is resiliently attached to upper arm 26 
through sensing apparatus 32, is manually moved to the desired gripper 
position(s) whereupon the coordinates of this particular gripper 30 
position are stored in a memory established in robot control assembly 34 
for the control of robot assembly 10. 
As noted above, pneumatically actuated gripper 30 is resiliently attached 
to upper arm 26 through or by means of relative motion sensing apparatus 
32 which incorporates the preferred embodiment of the inventive concept of 
the present invention. Relative motion sensing apparatus 32 will now be 
described in detail An enlarged elevational view of motion sensing 
apparatus 32 is shown in drawing FIG. 2 and an exploded perspective view 
of said apparatus 32 is shown in drawing FIG. 3A. With reference to 
drawing FIGS. 2 and 3A, photodiode housing 36, photodiode 38 and backplate 
40 are attached to flange portion 42 of wrist joint 28 by a pair of 
mounting screws (only screw 44 shown). Photodiode 38 is a conventional 
dual-axis (X and Y) lateral effect photodiode that is available from 
United Detector Technology of Culver City, Calif. Photodiode 38 is nested 
in a recess in one side of housing 36 and the opposite side of housing 36 
engages said flange portion 42 of wrist joint 28. Backplate 40, having a 
pair of mounting screw accepting openings 46a and 46b therein, engages the 
photodiode recess side of housing 36, thereby sandwiching photodiode 38 
between housing 36 and backplate 40 when mounting screws 44, etc. extend 
through said openings 46a, 46b, through corresponding openings 48a and 48b 
in photodiode housing 36 and are fully threaded into cooperating threaded 
openings in wrist joint flange portion 42. 
Front plate 50 is resiliently mounted on backplate 40 by a set of three 
spring-like, laterally resilient, compliant pads (only two, 52a and 52b, 
are shown). These compliant pads are commercially available from Lord 
Kinematics of Erie, Pa. A sectional view of one of said spring-like 
compliant pads is shown in drawing FIG. 3B. Pad 52a in drawing FIG. 3B 
consists of a plurality of flat, uniformly stacked, laminated steel discs 
54, 56, etc., with each disc having a flat surface thereof adjacent 
another of said discs and with all of said discs being encapsulated in an 
elastomeric material Compliant pad 52a also includes a pair of mounting 
caps at the opposite ends thereof having threaded openings therein for 
mounting the compliant pad to backplate 40 and frontplate 50. The ends of 
compliant pad 52a, for example, are inserted into recesses 54a and 54b in 
backplate 40 and in frontplate 50, respectively, and is attached to said 
plates by screws 56a and 56b, respectively. The other two compliant pads 
are mounted to plates 40 and 50 in different plate recesses in the same 
manner. Compliant pads 54, 56, etc. are relatively incompressible or will 
experience only negligible deformation when force is applied normal to the 
surfaces of each stacked disc, but are flexible or are deformable when 
shear forces are applied in directions generally lateral to said 
compressive forces Laminated elastomeric spring-like compliant pads 52a, 
52b, etc. provide multi-directional flexibility, with the chosen or 
tailored spring constants, in the required directions. Normally, spring 
constants are selected based on the forces required or the forces expected 
to be encountered in each situation. These commercially available pads do 
not require lubrication, require no adjustment, have low hysteresis, can 
take a fair amount of abuse, and have long live. 
High intensity infrared light emitting diode 58 and multi-mode optical 
fiber 60 are mounted in a fixed position in support housing 62 such that 
infrared light from light emitting diode 58 is optically directed or 
focused on one end of said fixedly mounted optical fiber 60. Other light 
sources such as lasers, laser diodes, non-infrared light emitting diodes 
or incandescent and fluorescent lamps may also be employed as a light 
source in place of diode 58. Optical fiber 60 is preferably a multimode 
fiber. However, a single mode fiber or an optical fiber bundle may also be 
employed in place of said multimode fiber 60. Support housing 62 is 
positioned in recesses 64 in intermediate support member 66 and said 
intermediate support member 66 is, in turn, positioned in recess 68 in 
frontplate 50. Endplate 70 is placed against the recess side of 
intermediate support plate 66 thereby confining support housing 62 within 
recess 64 of intermediate support member 66 and a set of three screws pass 
through respective openings in laterally extending endplate 70 tabs 73A, 
73B and 73C and into threaded openings in frontplate 70, thereby 
maintaining light emitting diode 58 and optical fiber 60 in a fixed 
position with respect to frontplate 50. In order to adequately concentrate 
the light from light emitting diode 58 into a relatively high intensity 
homogeneous spot on the photosensitive surface of photodiode 38, the end 
of optical fiber 60 should be close enough to said surface for such 
concentration, but far enough away to avoid physical contact between the 
photodetector and the optical fiber. In this, the preferred embodiment, a 
spacing of approximately two (2) millimeters was found to be adequate. 
Pneumatic housing portion 76 of pneumatically actuated gripper 30 is 
attached to endplate 70 by a pair of screws 78A and 78B that pass through 
openings in endplate 70 and into threaded openings in pneumatic housing 
76. Gripper 30 includes a pair of fingers 80a and 80b that move toward 
each other for workpiece gripping purposes when tube 82 is pressurized 
from a pressure source (not shown) by the actuation of a pressure control 
valve (not shown) in response to a signal from the robot arm control 
system 34 (FIG. 1). 
Movement sensing apparatus 32 is, in part, a compliant device that performs 
around a point about which rotation will occur when a moment is applied 
and pure translation occurs when a force is applied to said apparatus 32. 
FIGS. 4A and 4B schematically show how lateral error is accommodated when 
interference is experienced by apparatus 32, and FIGS. 5A and 5B 
schematically show how apparatus 32 mechanically operates when there is a 
moment-causing axial misalignment between mating parts. 
In FIG. 4A, grippers 80a and 80b attached to one end of robot arm 10 (FIG. 
1) mounted relative motion sensing apparatus 32 attempts to place pin 84 
in opening 86 of receiving member 88 as pin 84 is moved in axial direction 
90, ut is unable to initially do so because of the interference between 
pin 84 and champfered surface 92 at the entrance to opening 86 of 
receiving member 88 due to lateral misalignment of pin 84 with respect to 
said opening 86. As pin 84 continues to be moved in axial direction 90, 
such movement and the reaction from champfered surface 92 initiate lateral 
movement of said pin 84, frontplate 50 and support housing 62, on which 
light emitting diode 58 and light collimating optical fiber 60 are 
mounted, in direction 94. In FIG. 4B, pin 84 has made sufficient lateral 
movement to enable said pin 84 to fully enter the main portion of opening 
86 in receiving member 88. As a direct consequence of this pin 84 lateral 
movement, spring-like compliant pads 52a, 52b, etc., laterally distort to 
enable such movement, thereby causing light from diode 58 passing through 
light collimating optical fiber 60 and falling in the shape of a spot on 
photosensitive surface 95 of photodiode 38 to move to another position on 
said photosensitive surface. The effect of such light spot movement on 
photosensitive surface 95 of photodiode 38 will be explained below in 
detail. For the present, however, it is this light spot movement on said 
light or photosensitive surface that causes photodiode 38 to generate an 
electrical relative movement or position signal 
In FIG. 5A, grippers 80a and 80b attached to one end of robot arm 10 (FIG. 
1) mounted relative movement sensing apparatus 32 now attempts to place 
said pin 84 in opening 86 of receiving member 88 as pin 84 is moved in a 
direction that is at an acute angle to the longitudinal axis of opening 
86. While pin 84 is able to partially enter opening 86, said pin 84 is 
initially unable to fully enter opening 86 until the longitudinal axis of 
pin 84 is aligned (parallel and/or coincident) with the longitudinal axis 
of opening 86, because of this initial motion limiting angular 
misalignment. As pin 84 continues to be moved in the same direction, said 
pin 84, frontplate 50 and support housing 62, on which light emitting 
diode 58 and light collimating optical fiber 60 are mounted, experience 
rotational movement. In FIG. 5B, pin 84 has made sufficient rotational 
movement about axis 96 to enable said pin 84 to fully enter opening 86 in 
receiving member 88. As in the previously described example, compliant 
pads 52a, 52b, etc., laterally distort to enable such rotational movement 
again causing the spot of light from light emitting diode 58 falling on 
the photosensitive surface of photodiode 38 to move to another position on 
said photosensitive surface, the effect of such movement, as mentioned 
above, is to be described below in detail. 
The operation of conventional two-terminal single axis lateral effect 
photodiode 97, similar in operation to one of the two axes of dual axis 
lateral effect photodiode 38 in motion sensing apparatus 32 of the present 
invention, will now be described in detail. A schematic diagram of said 
two-terminal photodiode 97 is shown in drawing FIG. 6A. With reference to 
said FIG. 6A, the current I.sub.s at position S, which is the position on 
the photosensitive surface 98 of photodiode 97 where a spot of 
concentrated light from a light source falls on said surface, is given by 
the equation: 
##EQU1## 
where I.sub.o is Photoinduced current 
I line current 
S position S 
L width of detector 
For a dual axis photodiode such as photodiode 38 in the motion sensing 
apparatus of FIG. 2, whose four external contacts are A, B, C, and D, as 
shown in drawing FIG. 6B, the electrical signal representing the X and Y 
position of a light spot such as light spot 100 in said drawing FIG. 6B, 
may be expressed as: 
##EQU2## 
As mentioned above, motion sensing apparatus 32 can be employed in at least 
two different modes of operation. In one mode, changes in the relative 
position between the robot hand and the robot arm body are monitored for 
the purpose of interrupting or terminating robot arm movement due to robot 
arm interference. In another mode of operation, changes in the relative 
position of the robot hand (gripper 30) and the robot arm body are 
monitored for the purpose of sensing inertial forces or for applying a 
force to a body of a predetermined magnitude. FIG. 7 shows an information 
flow block diagram of the motion sensing apparatus of the present 
invention operating in the robot arm motion interrupting mode. With 
reference to FIGS. 4A, 4B and 7, if, for example, front plate 50 of motion 
sensing apparatus 32 should be laterally moved or translated in the X 
and/or Y direction (102) against the tailored or chosen force of compliant 
pads 52a, 52b, etc., the spot of light from high intensity infrared light 
emitting diode 58 through multimode optical fibers 60 falling on 
photosensitive surface 95 of photodiode 38 will move from its initial 
position on said surface 95 to a position linearly related to the relative 
movement between front plate 50 supporting pneumatically actuated gripper 
assembly 30 and wrist joint 28 mounted backplate 40 on which is mounted 
photodiode 38 (106). This relative movement produces an electrical signal 
(108) representative of such X and/or Y movement. This signal is compared 
with predetermined X and Y electrical signal levels (110) and then signal 
presence in excess of a minimum time duration (112) is determined to 
filter out false or unwanted mechanical vibrations. If the X or Y signals 
are less than a predetermined magnitude or are shorter than a 
predetermined time, the motion sensing apparatus will continue to monitor 
relative robot hand (gripper) to robot arm body relative movement (114) 
without interrupting robot arm movement. However, if the X or Y 
translation signals are more than a predetermined magnitude and persist 
for more than a predetermined time, compliant motion sensing apparatus 32 
will cause robot arm motion to be interrupted (116). 
FIG. 8 shows an information flow block diagram of motion sensing apparatus 
32 of the present invention operating in the robot arm force providing 
mode. With reference to FIGS. 4A, 4B and 8, if, for example, pin 84 should 
be brought into contact with a portion of fixedly mounted receiving member 
88 by the lateral movement of robot arm assembly 10 (FIG. 1), pin 84 
together with front plate 50 of motion sensing apparatus 32 will be 
laterally displaced or translated in an X and/or Y direction (118) against 
the tailored or chosen force of compliant pads 52a, 52b, etc. The lateral 
translation of pin 84 together with front plate 50 of motion sensing 
apparatus 32 in an X and/or Y direction cause compliant pads 52a, 52b, 
etc. to laterally deflect a corresponding distance "d" (120). The spring 
constants K (122) of said compliant pads combine as a product function 
(124) as said compliant pads are laterally deflected. As front plate 50 
moves laterally, the spot of light from high intensity infrared light 
emitting diode 58 through multimode optical fibers 60 falling 
photosensitive surface 95 of lateral effect photodiode 38 will move from 
its initial position on said surface 104 to a position linearly related to 
the relative movement between front plate 50 and backplate 40 on which 
said diode 38 is mounted (126). This relative movement produces an 
electrical signal (128) representative of such X and/or Y movement. The 
signal is compared with desired X and/or Y electrical signal levels (130) 
whose magnitude(s) corresponds to a desired predetermined force. Forces 
that are generated by the robot arm result from robot arm deflection of 
compliant pads 52a, 52b, etc, having a known spring constant, a particular 
distance "d". The deflection of these compliant pads continues (132) until 
the desired predetermined force is established. Once the force is 
established, robot arm motion is terminated (134) and the established 
force is maintained. 
In addition to the position determining or relative movement sensing and 
force generating modes described above motion sensing apparatus 32 can 
also be employed to determine the null or neutral position of robot hand 
or gripper 30. By monitoring the current of lateral effect photodiode 38 
it is possible to precisely determine said null position which can greatly 
reduce robot arm programming time. 
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.