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Description  |
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BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates to robotic manipulators, and more particularly, to
such manipulators having drive actuators attached to a base and providing
movement of an end effector with three degrees of freedom (DOF).
Typical industrial robots having several degrees of freedom have actuators
distributed at the joints. This eliminates much of the mechanical
transmission resulting in simpler, more accurate design. Electric motors,
the most commonly used actuators, are by their nature very heavy, and thus
comprise a sizable percentage of the overall structural weight of the
robot arm.
Distributing the motors at the joints places them in high inertial
positions, and their masses become inertial loads on some of the other
motors. The majority of mechanical work done by some robots is
accelerating and decelerating the motors on the manipulator arms.
Modern teleoperator designs centralize the actuators in a low inertial
position. This reduces the inertia and coupling forces on the joints,
resulting in a quicker manipulator with improved control performance. The
torque is transmitted via cables, metal tapes, back drivable gear trains
and/or torque tubes. These transmission methods are inaccurate due to low
mechanical stiffness and backlash, but have relatively little friction.
Accuracy, while crucial in autonomous robots, is relatively unimportant in
teleoperators due to the existence of a person in the control loop.
The recent emergence of direct-drive technology in robot design underscores
ever increasing accuracy requirements of robots. Unfortunately direct
drive servomotors with high output torques are heavier still than an
equivalent gear motor. As a result, a serial link direct-drive
manipulator, such as the one disclosed in Asada, et al, "Control of a
Direct-Drive Arm" ASME J. Dyn. Sys., Meas. and Control, Vol. 105,
September 1983, has a very massive structure.
Work has been done recently on in-parallel actuated kinematic structures,
including arms with up to three to six DOF. These structures have the
potential of greatly improving the mechanical performance of modern
robots. Before this can be accomplished, a more detailed kinematic
analysis of specific linkages must be carried out, with the goal of
developing a practical engineering design. It is unfortunate that linkages
which have such potential advantages also have extremely complex
geometries for which few kinematic algorithms have yet been proposed. The
geometric relationships between joint space and cartesian end effector
space, the Jacobian relationships and the location and nature of
singularities must be studied before a new design can realize its
potential advantages.
By transmitting torque via a linkage which can be very stiff, the accuracy
of direct drive is preserved. Additionally, links can be selected so as to
provide a "gear reduction", that is, reduce speeds and amplify torques. A
planar five-link linkage having simple kinematics has been analyzed
thoroughly. The principle disadvantage of the design is that it only
provides 2 DOF. This linkage is described by Asada, et al., in "Analysis
and Design of a Direct-Drive Arm with a Five-Bar-Link Parallel Drive
Mechanism", ASME J. Dyn. Sys., Meas. & Control, Vol. 106, No. 3,
(September 1984) and in "A Linkage Designed for Direct-Drive Robot Arms,"
ASME J. Mech. Trans., Vol. 107, December 1985. The addition of a third DOF
is achieved by placing the entire mechanism on a rotating base, which is
itself directly driven. With this design, the dynamics of the five-bar
linkage can be completely decoupled resulting in improved control
performance. However, the base rotation will always be dynamically
coupled, and the advantages of fixed motors are reduced, as the 2 DOF
mechanism becomes an inertial load on the base actuator. Another approach
is to have the five-bar linkage move in a horizontal plane and place a
small motor at the manipulator tip for vertical motion. This has the
advantage of simplicity, and is considerably more compact than the large
rotating base, but again reduces the advantage of fixed motors.
The present invention overcomes many of the described disadvantages of the
known devices and designs. In particular, the present invention provides a
3 DOF closed-chain kinematic structure particularly well suited to robot
manipulators. This structure has geometric and Jacobian relationships
which are much more simple than other parallel actuated spatial structures
with three or more DOF. Further, the present invention provides such a
device having a large work space volume with singularities which can be
restricted to the boundary of the work space.
The present invention generally provides the use of a spatial extension of
the planar five-bar linkage by applying a differential-type input to one
of the input links of the structure. This differential-type input uses two
actuators to actively control a 2 DOF input link. A second single-DOF
actuator is connected to another input link, with each of the input links
being respectively connected to a separate output link. The output links
are then connected together, with an end effector or gripper being
associated with one of the output links. In the preferred embodiment, the
three actuators are fixedly attached to a base and the input link
connected to the 2 DOF actuator is connected to the corresponding output
link by the equivalent of a ball joint, the other end of which output link
is connected to the other output link by a universal joint. Finally, the
remaining joint between an input and an output link is a single DOF pin
joint. In alternative embodiments, these joints and actuators may be
varied in configuration.
It will be seen that such a manipulator made according to the present
invention provides the features and advantages described. Further, the
present invention provides a structure which can achieve speeds and
accuracies unattainable with similar serial link designs. Other features
and advantages of the present invention will be realized from a
consideration of the drawings and the following detailed description of
the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are perspectives of a robotic manipulator in different
operational positions and made according to the present invention;
FIG. 3 is a partial fragmentary, side elevation of the manipulator of FIG.
1;
FIG. 4 is a plan view of the manipulator of FIG. 3;
FIG. 5 is a fragmentary elevation of a 2 DOF actuator assembly included in
the manipulator of FIG. 1;
FIG. 6 is a partial fragmentary perspective of one of the motors of FIG. 3
shown with an alternative mounting;
FIG. 7 is a view similar to FIG. 6 showing yet another mounting; and
FIG. 8 is a plot representative of the volumetric coverage of two
embodiments made according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring initially to FIGS. 1-5, a robotic manipulator, shown generally at
10, made according to the present invention is illustrated. Manipulator 10
includes a frame or base 12 on which first and second actuator assemblies
14, 16 are mounted. Assembly 14 uses a pitch-yaw differential gear. It
includes first and second motors 18, 20 fixedly mounted on base 12 with
respective drive shafts 22, 24 which are coaxial about an axis of rotation
26. Shafts 22, 24 are connected to like bevel gears 28, 30, respectively.
These gears mesh with a third bevel gear 32 which rotates about another
axis of rotation 34 when gears 28, 30 rotate at the same speed in opposite
directions about axis 26. Gear 32 is attached to an idler gear 37 which
engages the other side of gears 28, 30, as seen particularly in FIGS. 3
and 4. However, as the gears 28, 30 rotate at at the same speed in the
same direction, then gear 32 is caused to also revolve around axis 26 on
gears 28, 30. Gear 32 is can thus be caused to rotate with 2 degrees of
freedom. It is fixedly attached at a first joint 39 to a first input
manipulator arm link 38 the distal end of which is formed to extend along
an axis 40 perpendicular to axis 34 and intersecting axis 26. Axes 26 and
34 are also referred to herein as first and second axes, respectively.
Actuator assembly 16 comprises a motor 42, which in this preferred
embodiment is fixedly mounted to base 12. Motor 42 has a drive shaft 44
which rotates about a vertical axis 46, also referred to herein as a third
axis. Shaft 44 is rigidly connected at a second joint 41 to an end of a
second input horizontal link 48. Connected to the other end of link 48 is
a second output link 50, which is coupled to link 48 by a pin joint 52
which provides relative rotation between the links about a vertical axis.
Link 50 is connected at its other end to an end of a first output link 54
via a universal, 2 DOF joint 56. Joint 56 preferably provides for rotation
of link 54 relative to link 50 about a vertical axis as well as a
horizontal axis orthogonal to the length of link 54.
Link 38 is also connected to link 54, but at a point intermediate its ends
at a joint 58 which in this preferred embodiment is a 3 DOF joint, such as
provided by a ball joint, or as shown, by a universal joint with a pin
joint having a third orthogonal axis of rotation. At the distal end of
link 54 is an end effector or gripper 60 used to provide the desired
gripping of a workpiece. As can be seen by FIGS. 1 and 2, control of
motors 18, 20 provide for movement of gripper 60 vertically as well as
horizontally. Joints 58, 52, and 56 are also referred to herein as third,
fourth and fifth joints, respectively.
Referring specifically to FIG. 5, the structural detail of actuator
assembly 14 is shown. This embodiment provides for fixedly mounting motors
18, 20 to base 12. However, the same two 2 DOF can be provided by mounting
motor 20 onto the drive shaft of motor 18 so that rotation of shaft 22
rotates motor 20. Drive shaft 24 is then positioned to rotate orthogonally
about an axis perpendicular to the axis of rotation of motor 18, with link
38 being attached to shaft 24. However, this results in the inertia of
motor 20 being controlled by motor 18, thereby reducing its accuracy and
effectiveness.
Alternative arrangements for joints 52, 56, 58 are possible while
maintaining actuator assemblies 14, 16 fixed to base 12. In the embodiment
shown, these joints must provide a total of 12 constraints of degrees of
freedom to result in a net of 3 DOF for manipulator 10. This is determined
from the Grubler relationship which expresses the number of DOF of a
mechanism as a function of the number of links and the number of
constraints in each joint:
No. DOF=6 (No. links-1) minus the sum of the constraints at each joint.
The joints defined by the motors on base 12 represent the constraints on
the actively controlled input links. Since in general, a link has 6 DOF in
space (three translation, three rotation), the 2 DOF and 1 DOF inputs, as
defined by actuator assemblies 14, 16 and the ground link between them,
constrain 4 DOF and 5 DOF, respectively.
It will be seen that the possible combinations for joints 52, 56, 58 may
take on various arrangements of joint types and still provide a resultant
three degress of freedom of movement of gripper 60. That is to say, joint
52 could be a 3 DOF joint, joint 56 could be a 1 DOF joint, and joint 58
could be a 2 DOF joint. As a comparison to relate to the embodiment shown
in FIGS. 1-4, a study was made using a manipulator like manipulator 10
except with the form of joints in joints 56 and 58 exchanged. Joint 56 was
a 3 DOF joint and joint 58 was a 2 DOF joint.
A portion of the results of this study are shown in FIG. 8 which
illustrates the reach of the two embodiments of the manipulators as a
function of reach, R and vertical distance, Z. The reach of manipulator
10, with the joints as shown in these figures, is illustrated by curve 62.
The other case which was studied in which the universal and ball joints
were exchanged, is shown by curve 64. (The link lengths in nominal units
for curves 62, 64 are, respectively--link 38: 0.798, 0.902; link 48:
1.041, 0.917; link 50: 0.728, 0.838; link 54 (between joints): 1.231,
1.013; link 54 (between joint 58 and gripper 60): 1.009, 0.943; offset of
gripper from link 54: -0.600, -0.589. Axes 26 and 46 are assumed to be
coincident.) The ideal volume covered is the torus obtained by rotating
the cross-sectional plot of FIG. 8 about the z axis. The volume is
symmetric with respect to the plane of z=0 as identified by the reach
axis, R. This, of course, is an ideal situation which ignores physical
barriers such as the base and assumes the vertical axes of the two
actuator assemblies are coincident. However, it will be appreciated that a
substantial volume of space may be reached with the specific manipulators
described.
Additional mechanisms of the class illustrated above can be made by
mounting motor 42 with a single-DOF mounting, such as the pin mounting 66,
shown in FIG. 6 which provides for pivoting of motor 42 about axis 68.
Alternatively, motor 42 may be mounted on a double pin or universal
mounting 70 for rotation about a first horizontal axis 72 and an
orthogonal horizontal axis 74, as shown. It will be seen that these
alternative embodiments add 1 and 2 DOF, respectively to the joint
represented by actuator assembly 16. This means that for each case, the
number of constraints provided by joints 52, 56, 58 can be 1 and 2
constraints more, respectively, than described previously. Thus, when
mounting 66 is used, joints 52, 56, 58 need to provide 13 constraints in
order to result in 3 DOF manipulation of gripper 60. This simplifies the
link joints since two of them can be pin joints, and one a ball or 3 DOF
joint, or two of them universal joints, and one a pin joint. Similarly,
with universal mounting 70, these three joints must provide 14
constraints. Thus, in this latter case, two of the joints can be a simple
pin joint, and one of them a universal joint.
It will be appreciated that there is large variety of configurations in
which a manipulator made according to the present invention may be
provided.
Such manipulators all have three actuators mounted to the base and power is
transmitted via five links, including the base link. The link connections
consist of a differential with two drive motors, a single-drive motor, a
universal joint, a ball joint and a pin joint. Other equivalent forms of
joints can be provided as has been described. Since the actuators are
fixed to the support structure their weight is not supported by the arm.
Thus, very large heavy actuators capable of outputting both high speeds
and high torques can be used while maintaining a relatively low inertia
structure. This gives the arm high speed, acceleration and lift
capabilities. The kinematic configuration, which avoids any cables, metal
tapes, belts or bands, allows the design to utilize components with very
high mechanical stiffness. With the mass of the motors removed, additional
mass can be added to the structure to increase the stiffness while still
maintaining a lower inertia than conventional designs. The differential
gear is located in a particularly low inertia position so that its mass
adds little to the structural inertia. Additionally, the closed chain
kinematic structure with two separate ties to ground is inherently more
accurate than an open chain structure with only one tie to ground.
It will therefore be understood by those skilled in the art that various
changes in form and detail may be made therein without departing from the
spirit and scope of the invention as defined in the claims.
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