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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns micropositioning systems and, more
particularly, a micropositioning system for a robotic arm.
2. Description of Prior Art
Robotic devices are finding an increaisng number of applications in a
variety of industries. In automotive assembly lines, for example, robotic
devices are now used to perform various welding operations. A number of
assembly operations are also now performed by robotic devices, such as the
assembly of printed card type electronic circuits and even wristwatches.
Robotic devices are also employed in the transport of fairly massive
workpieces in foundries and the like, typically feeding and removing
workpieces from various types of metal forging equipment.
Robotic operations of the type described above usually require a high
degree of positional accuracy. In automotive assembly line welding, for
example, robotic welds are typically required to be within ten to fifteen
thousandths of an inch of a desired weld location. Electronic circuit and
wristwatch assembly operations usually require workpiece placement by the
robotic equipment to within one to five thousandths of an inch of a
desired position. Foundry operations generally require robotic accuracy of
approximately fifty thousandths of an inch.
Once common method for achieving positional accuracy with robotic equipment
is by the measurement of relative angles between various portions of the
robotic structure. Robotic equipment employed in the type of work
described above typically have a working implement (e.g. welding tips,
workpiece grasping elements, etc.) pivotally attached to an arm structure
which is in turn pivotaly atached to a base structure. Given the angles
between the arm, base, and working implement, along with the location of
the base structure, the position of the working implement can be fairly
precisely determined. These positional calculations, however, are still
subject to certain inaccuracies. The weight of the workpiece held by the
robotic device or the weight of the working implement itself may, for
example, cause the robotic arm to deflect. These deflections can cause an
offset in the position of the working implement without affecting the
relative angles between the various robotic elements.
One common approach to avoiding deflection inducted positional inaccuracies
is to simply build stronger and more massive robotic arms. This approach,
however, suffers from several disadvantages. Even more massive robotic
arms are still subject to a certain amount of deflection, thus setting an
upper limit on the positional accuracy available through this approach.
Further, the weight of an arm sufficiently massive to achieve a desired
positional accuracy may be prohibitive for certain robotic applications.
It would, for example, be impractical to use this approach in building a
precsion robotic transport device having substantial mobility and the
capability of handling loads weighing several thousand pounds with a
positional accuracy on the order of a few thousandths of an inch. The
weight of a robotic arm sufficiently massive to avoid deflection induced
inaccuracies within the desired tolerances would severaly restrict the
mobility of the resulting robotic device.
Another approach to avoiding deflecting induced positional inaccuracies is
to measure or calculate the spring constant of a robotic arm structure and
program this information into a "lookup table" computer memory. Sensors
are then attached to the robotic arm to measure the strain on the arm or,
alternatively, the weight of the load being lifted by the arm. A
theorectical deflection can then be obtained from the "lookup table"
memory to offset inaccurate positional information derived from the
relative angles between the elements of the robotic structure. This
approach, however, fails to account for variations in the robotic arm
spring constant resulting from metal fatigue, stress hysteresis, and
similar effects. Changes in the robotic structure spring constant will
cause a discrepancy between the calculated deflection and the actual
structural deflection, resulting in positional miscalculation and
inaccuracy.
Thus, there still exists a need for a robotic micropositioning system which
can compensate for deflection induced positional inaccuracies with greater
accuracy then presently available through "lookup table" deflection
calculations without resorting to prohibitively massive robotic
structures.
SUMMARY OF THE INVENTION
It is therefore a goal of the present invention to provide a precise
micropositioning system for a robotic arm. This goal is achieved in the
present invention by providing an apparatus which determines the actual
deflection of the robotic structure. Precise positional information is
then obtained by measuring the relative angles between various portions of
the robotic structure and offsetting the resulting positional information
by the determined deflections of the robotic structure. Thus, it is a
further objective of the present invnetion to provide an apparatus for
determining the deflection of a load carrying structure.
The various goals and objective of the present invention are achieved by
attaching deflection sensing optical systems to various load bearing
elements of the robotic structure to directly measure the deflection of
these load bearing elements. Each optical system includes a target having
a plurality of light sources jto be disposed at one end of a load bearing
robotic element and a light detecting system to be disposed at an opposing
end of the robotic element. The light detecting systems measure change in
the position and orientation of their associated targets. Each light
detecting system includes a set of perpendicularly oriented linear
detector arrays, each array sensing target motion along a single axis
parallel to the longitudinal axis of the array. Cylindrical lenses are
provided for each detector array to retain a portion of a target image on
the detector arrays irrespective of target motion perpendicular to the
longitudinal axes of the arrays.
The novel features which are believed to be characteristic of the present
invention, together with further objectives and advantages thereof, will
be better understood from the following detailed description considered in
connection with the accompanying drawings, wherein like numbers designate
like elements. It should be expressly understood, however, that the
drawings are for purposes of illustration and description only and are not
intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side view of the deflection sensing optical system of the
present inventive micropositioning system attached to an illustrative
robotic structure.
FIG. 2 is a side view of an exemplary mobile robotic device incorporating
the present invention.
FIG. 3 is a diagrammatic illustration of the control apparatus of the
present invention.
FIG. 4 is a perspective side view of the deflection sensing optical system
of the present invention.
FIGS. 5A-E show several illustrative views of target light source images
observed by the light detecting system of the present invention.
DETAILED DESCRIPTION
Referring to the figures, and more particularly FIG. 1, the present
invention micropositioning system is shown incorporated into an
illustrative robotic structure 10. It should be understood, however, that
the present inventive micropositioning system could be employed with
virtually any type of robotic structure. The illustrative robotic
structure 10 is intended for lifting heavy loads weighing in excess of
several thousand pounds while still being light enough to permit
substantial mobility, allowing, for example, mounting onto some sort of
transport vehicle 12 as shown in Fig. 2. By incorporating the present
micropositioning system of the present invention, however, the mobile
robotic structure 10 can achieve a positional accuracy of a few
thousandths of an inch in the placement of workloads weighing in excess of
several thousand pounds.
The robotic structure 10 includes a load bearing upper arm 14 pivotally
attached to a turret 16 which is in turn rotationally coupled to a base
18. A load bearing forearm 20 is pivotally attached to the upper arm 14 to
provide greater flexibility in radial extensions of the robotic structure
with respect to the base 18. In FIG. 1 the robotic arms 14, 20 are shown
deflected in a greatly exaggerated manner. A lift structure 22 for
engaging a workload is pivotally attached to a wrist structure 24 which is
in turn pivotally attached to the forearm 20. The lift 22 and wrist 24 may
include internal rotational couplings allowing both elements to rotate
with respect to one another about axes perpendicular to their common
pivotal connection.
The present micropositioning system includes an upper arm deflection
sensing system, including a target 25 and an optical detecting system 28
attached to opposing ends of the upper arm 14, a forearm deflection
sensing system, including another target 25 and an optical detecting
system 26 attached to opposing ends of the forearm 20, and a plurality of
angular encoders measuring the relative angles between the various load
bearing elements. Thus, a turret rotation encoder 30 is disposed at the
pivotal connection between the turret 16 and base 18, a shoulder pitch
encoder 32 is disposed at the pivotal connection between the upper arm 14
and the turret 16, and an elbow pitch encoder 34 is disposed at the
pivotal connection between the forearm 20 and the upper arm 14. Encoders
36 are also disposed at the pivotal connections between the forearm 20 and
the wrist 24, the lift 22 and wrist 24, and the internal rotational
couplings within the lift 22 and wrist 24. Angular encoders commonly
provide a signal indicative of the rotational orientation between two
rotary inputs of the encoder and are well known in the prior art. To
compensate for deflections of the base 18 and to allow proper orientation
of the lift 22 with respect to a geographically local vertical reference
frame, a gravity based vertical reference frame 35 may also be
incorporated into the turret 16.
A diagrammatic block represenation of the control system for the present
inventive micropositioning system in shown in FIG. 3. As shown, the
position of the wrist 24 is determined by a wrist state processor 40
receiving vertical orientation information from the gravity based vertical
reference frame 35 and positional information from the turret rotation
encoder 30, shoulder pitch encoder 32, and elbow pitch encoder 34. When
the robotic structure 10 carries a very heavy load both the upper arm 14
and the forearm 20 are subject to deflections which displace the wrist 24
and lift 22 from their initial unloaded positions and orientations. These
deflectional displacements, however, are not detected by the various
encoders providing positional information to the wrist state processor 40.
A deflection compensation system 42 is therefore provided to indicate to
the wrist state processor 40 the extent of deflection in both the upper
arm 14, and forearm 20. The deflection compensation system 42 includes the
upper arm deflection sensing system 44 (including target 25 and the
optical detecting system 28), the forearm deflection sensing system 46
(including another target 25 and the optical detecting system 26), and a
deflection processor 48. Both of the deflection sensing systems 44, 46
provide signals to the deflection processor 48 indicating the relative
positions and orientations of the targets 25. The deflection processor 48
in turn converts changes in the signals from the deflection sensing
systems 44, 46 into separate deflection data for the upper arm 14 and
forearm 20. The wrist state processor 40 then determines the actual
position of the wrist 24 based on the positional data from the various
angular encoders along with the deflection data from the deflection
processor 48. This actual position data is forwarded by the wrist state
processor 40 to several servo processors 50 which compare the actual wrist
position data with positional commands from other control systems (not
shown) to appropriately instruct mechanisms 52, 54 and 56 respectively
controlling rotation of the turret 16, pitch of the upper arm 14, and
radial extension of the forearm 20.
The novel deflection sensing systems 44, 46 of the present inventive
micropositioning system independently measure actual deflections of both
the upper arm 14 and forearm 20 with respect to the five degrees of
freedom shown in FIG. 1. When loaded, both a far end 14a of upper arm 14,
connected to the forearm 20, and a far end 20a of forearm 20, connected to
the wrist 24, are subect to separate translational deflections physically
displacing the respective arm ends 14a, 20a from initial unloaded
positions as shown in an exaggerated manner in FIG. 1. The upper arm
deflection sensing system 44 resolves translational deflections of the
upper arm 14 with respect of two mutually perpendicular axes (respectively
called the upper arm x and y translational axes) which are both generally
perpendicular to the longitudinal axis of the upper arm 14. The forearm
deflection sensing system 46 similarly resolves translational deflections
of the forearm 20 with respect to two mutually perpendicular axes
(respectively called the forearm x and y translational axes) both
generally perpendicular to the longitudinal axis of the forearm 20.
The upper arm ends 14a and 20a are also both subject to separate rotational
deflections in a load carrying state, changing the orientation of the
respective arm ends 14a, 20a from initial unloaded orientations by
phsically rotating the arm ends 14a, 20a. Rotational deflections of the
upper arm 14 result in a positional displacement of both the wrist 24 and
lift 22 while rotational deflections of the forearm 20 result in a
positional displacement of the lift 22. Deflections of the upper arm 14
are resolved by the upper arm deflection sensing system 44 with respect to
three separate rotational axes: roll rotations about the longitudinal axis
of the upper arm 14, pitch rotations about the upper arm x-translational
axis (that is, generally in the direction of the upper arm y-translational
axis), and yaw rotations about the upper arm y-translational axis (that
is, generally in the direction of the upper arm x-translational axis).
Similarly, the forearm deflection sensing system 46 resolves rotational
deflections of the forearm 20 with respect to roll rotations about the
longitudinal axis of the forearm 20, pitch rotations about the forearm
x-translational axis, and yaw rotations about the forearm y-translational
axis.
The elements of the present invention deflection sensing systems 44 and 46
are shown in FIG. 4, including the target 25, having a plurality of light
sources 52A-E, and either of the optical detecting systems 26 or 28. The
optical detecting system 26 shown in FIG. 4 includes a pair of
perpendicularly oriented linear detector arrays 56a,b and a pair of
cylindrical lenses 58 disposed within an opaque structure 60. Light input
from the target 25 is received through focusing lenses 62 provided for
each of the detector arrays 56a,b. The optical detecting system 28 (not
shown) differs from the optical system 26 by including an additional
linear detector array, as discussed more fully below.
The detector arrays 56a,b of the optical detecting system 26 each contain a
plurality of individual light detecting elements or pixels closely
arranged in side by side relationship along a longitudinal axis of the
array. The detector arrays 56a, b individually resolve motion of the
target light sources 52A-E along a single axis parallel to their
respective longitudinal axis. Linear detector arrays are well known in the
prior art and are available from a number of sources. For example, E G &
G, Inc. manufactures a detector array suitable for the present use, having
one thousand twenty four pixels each approximately one thousandth of an
inch apart.
By perpendicularly mounting the detector arrays 56a,b with respect to one
another, the optical detecting system 26 can resolve any two-dimensional
motion of the target 25 generally in a plane formed by the longitudinal
axes of the detector arrays 56a,b. Thus, longitudinal axes of the arrays
56a,b can be used to define the x and y translational axes with respect to
which translational and rotational motions of the robobic arms 14, 20 are
measured.
Use of two perpendicularly oriented linear detector arrays to resolve
two-dimensional motion of the target 25 provides a substantial savings
over conventional two-dimensional planar detector arrays such as those
commonly used in TV cameras. To obtain a resolution equal to two linear
detector arrays of one thousand pixels each, a conventional
two-dimensional planar array would require approximately one million pixel
elements. In addition to the added cost of the detector array itself,
substantially more complex electronics would be required to process the
additional pixel signals. Substantial costs would also be incurred in
attempting to resolve the center of a target light source image disposed
over several pixel elements in a planar array.
The focusing lenses 62 are provided to focus images from the target light
sources 52A-E onto the individual detector arrays 56a,b. The focal lengths
of the lenses 62 are selected so that the field of view of each detector
array 56a,b extends over the maximum range of thearetical deflections to
be experienced by the associated robotic arms 14, 20. Thus, the focal
lengths of the lenses 62 to be employed on a particular robotic arm 14 or
20 can be determined by calculating the sum of the maximum translational
and rotational deflections of the associated arm, dividing that sum into
the width of the respective detector array 56a, or 56b, and multiplying
the result by the separation distance between the target 25 and the
focusing lenses 62. The minimum translational displacement of the targets
25 which can be resolved by the respective detector arrays 56a,b can then
be determined by dividing the focal length of the lenses 62 into the
spacing of the individual pixel elements of the respective arrays 56a,b
and multiplying the result by the distance between the lenses 62 and the
target 25.
Since the images from the target light sources 52A-E generally extend over
more than a single pixel element when focused onto either of the detector
arrays 56a,b, the minimum translational deflection which can be resolved
by the optical detecting system 26 can be improved by applying
conventional algorithms to resolve the centroid of the light source image.
Translational deflections can then be resolved to within a fraction of a
detector array pixel spacing. These algorithsm are well known to those
skilled in the art and need not be discussed here at length.
The cylindrical lenses 58 are included in the optical system 54 to spread
the images of the target light sources 52A-E into lines which are focused
onto the detector arrays 56a,b with an orientation perpendicular to the
longitudinal axes of the respective arrays 56a,b. By spreading the
typically point images into linear images, a portion of the various light
source images will remain focused on the individual detector arrays 56a,b
despite relative motion of the light sources 52A-E perpendicular to the
longitudinal axes of the respective arrays 56a,b. For example, portions of
the images from target light sources 52A-E will therefore remain focused
on the detector array 56a (defining the x-translational axis in FIG. 4)
despite motion of the target 25 along the y-translational axis. To retain
target light source images on the detector arrays 56a,b during maximum
translational motion, the focal lengths of the cylindrical lenses 58
should be chosen so that the resulting linear images (focused on the
detector arrays 56a,b by the lenses 58) are approximately equal in length
to the lengths of the respective detector arrays 56a,b.
Due to the linear refractive character of cylindrical lenses, the target
images will not be displaced by the cylindrical lenses 58 along the
longitudinal axes of the detector arrays 56a,b if the polar axes of the
respective cylindrical lenses 58 are oriented parallel to the longitudinal
axis of their associated detector array 56a or 56b. The polar axes of the
cylindrical lenses 58 should be parallel to the longitudinal axis of their
associated detector array 56a or 56b to within approximately one pixel
spacing over the entire length of the array.
As shown in FIG. 4, the presently preferred embodiment of the target 25
includes target light sources 52A-E with the light sources 52B-E generally
arranged in a common plane and the light source 52A extending beyond the
plane. The target light sources 52B-E are arranged in pairs 52B,C and
52D,E oriented generally parallel to the respective x and y translational
axes. The extended light source 52A is generally centrally disposed among
the light sources 52B-E. It should be noted, however, that this
arrangement of the light sources 52B-E and the exact location of light
source 52A outside the common plane is not essential.
The target light sources 52A-E can be any convenient type of conventional
light source such as, for example, light emitting diodes. In operation,
the light sources 52A-E are individually activated in a repetitive
sequence to provide illumination of the detector arrays 56a,b in time
multiplexed fashion. The particular source of an image detected by the
arrays 56a,b can then be determined by ascertaining in which portion of
the repetitive cycle the image was sensed. The direction of deflection can
then be obtained by determining the directon of movement of an identified
light source.
FIGS. 5A-E illustrate the manner in which translational and rotational
deflections are viewed by the detector arrays 56a,b. Each of these figures
shows the images from the particular light sources 52A-E which are used to
identify the various deflections. During operation, however, only one
light source image would be visible at any given time and all of the light
sources would normally be imaged on both detectors over the periodic
activation cycle of the light sources.
As shown in FIG. 5A, roll rotation deflections of the target 25 and thus
rotation of either of the robotic arms 14 or 20 about their longitudinal
axes are determined by measuring changes in the spacing between images
from light sources 52B and 52C are viewed by the x-translational detector
array 56a. Use of the x-translational detector array 56a to view sources
52B,C is preferable over use of the y-translational detector array 56b
since deflections of the target 25 along the y-translational axis may
remove one of the light sources 52B or 52C from the field of view of the
y-translational detector array 56b. Similar deflections along the
x-translational axis, however, are less likely to remove the images of
light sources 52B,C from the field of view from the x-translational
detector array 56a. It should also be noted that roll rotation deflections
could also be determined by measuring changes in the separation between
the images of light sources 52D,E with the y-translational detector array
56b.
The resolution of roll rotation deflections of robotic arms 14, 20 is, in
part, dependent upon the spacing between the selected light source pair 52
B,C or 52 D,E. The minimum angular resolution of roll rotation deflection
is equal to the inverse sine (arcsine) of the minimum x or y translational
deflection resolution of the appropriate x or y translational detector
array 56a or 56b divided by the spacing between the selected pair of light
sources 52B,C or 52D,E. Thus a maximum spacing between the selected light
source pair 52B,C or 52D,E is desirable. The light source images, however,
need not be viewed by the same detector array. Consequently, the light
sources 52B,C attached to the upper arm 14 of the robotic structure shown
in FIG. 1 are spaced further apart than the field of view of the
x-translational detector array of the upper arm optical detecting system
28. An additional x-translational detector array (not shown) and focusing
lens 62b are therefore provided opposite the y-translational detector
array focusing lens 62c to separately view the upper arm target light
source 52C. The upper arm optical detecting system 28 does not otherwise
differ from the forearm optical detector system 26.
As shown in FIG. 5B, x-translational deflections are determined by
measuring the displacement of images from the light source pair 52B,C
along the x-translational detector array 56a. The displacement of an
average midpoint position (a point midway between the images formed by
light sources 52B,C) is measured instead of the displacement of the images
from either light sources alone to avoid inaccuracies resulting from
displacement of the light sources 52B,C due to roll rotation deflections
of the robotic arms 14,20. As shown in FIG. 5D, y-translational
deflections are similarly determined by measuring the displacement of an
average midpoint position between the images formed by light sources 52D,E
along the y-translational detector array 56b.
Yaw and ptich rotation deflections are both determined by measuring a
change in the distance between an image formed by light source 52A and the
average midpoint positions of the images from the respective light source
pairs 52B,C and 52D,E. As shown in FIG. 5C, yaw rotation deflections are
determined by measuring the distance between the image from light source
52A and the average midpoint position between the images of light source
pair 52B,C as viewed by the x-translational detector array 56a. As shown
in FIG. 5E, pitch rotation deflections are similarly determined by
measuring the distance between the image from light source 52A and the
average midpoint position between the images from the light source pair
52D,E as viewed by the y-translational detector array 56b. Average
midpoint positions between the images from light source pairs 52B,C and
52E,E are again employed to avoid inaccuracies from relative displacement
of any of the light sources 52B-E due to roll rotation deflections of the
target 25.
The resolution of pitch and yaw rotation deflections is partially dependent
upon the distance from the light source 52A and the plane of the light
sources 52B-E, the minimum angular resolution of either pitch or yaw
rotation deflection being equal to the respective inverse sines of the
minimum translational displacement resolution of the y and x translation
detector arrays 56b and 56a divided by the distance between the light
source 52A and the plane of the light sources 52B-E.
The upper arm 14 and forearm 20 of the robotic structure shown in FIG. 1
are hollow, and the deflection sensing system for each arm is disposed
within the respective arms to avoid both debris and light contamination.
As discussed above, however, the present inventive micropositioning system
could be used in conjunction with virtually any robotic structure. It
will, of course, be understood that modifications of the present invention
will be apparent to others skilled in the art. For example, where
detection of rotational deflections if not a concern, the target 25 could
employ only a single light source. It should also be noted that the
present inventive deflection sensing system, comprising the targets 25 and
either of the optical detecting systems 26 or 28, could be employed in the
measurement of deflections by structures other than those associated with
robotic devices. In addition the optical detecting systems 26, 28
employing perpendicularly oriented linear detector arrays 56a,b could be
used in a diverse number of applications in which two-dimensional planar
detector arrays are presently employed. Consequently the scope of the
present invention should not be limited by the particular embodiments
discussed above, but should be defined only by the claims set forth below
and equivalents thereof.
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