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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to multiple-degree-of-freedom fine motion devices
and more particularly relates to a magnetically-levitated fine motion
device having programmable compliance as well as programmable motion.
2. Description of the Prior Art
It has long been recognized that robot control simply by tracking position
goals has many limitations when dealing with real-world environments.
Compliance is required; that is, there is a need for ability to yield
elastically when a force is applied. There has been a great deal of work
in the past aimed at giving robot manipulators some form of compliant
behavior, and/or control by tracking force goals, etc. Much of this effort
has failed to provide satisfactory performance, and applications to the
manufacturing domain have been few, if any. Much has to do with the
mechanical nature of the manipulator itself. When compliance or force
control of a standard industrial robot is attempted, the results are
usually dominated by high masses and inertias, as well as friction
effects. These effects are difficult to overcome by the generally weak and
poorly performing actuators. Additional problems lie with the effective
computational bandwidth of the control system.
An approach to this problem is to divide the robot manipulation task into
coarse and fine domains. That is, the manipulator itself has redundant
coarse and fine degrees of freedom. Here, some form of endpoint sensing is
used to measure the directly relevant task parameters and to guide the
manipulator system to achieve the desired goal. This paradigm is described
in R. L. Hollis, and M. A. Lavin, "Precise Manipulation with Endpoint
Sensing," International Symposium on Robotics Research, Kyoto, Japan Aug.
20-23, 1984, and IBM J. Res. Develop. 29, pp. 363-376, July,1985.
For an extremely wide range of robotic assembly tasks especially in the
electronics industry, it is only necessary to provide fine compliant
motion over limited distances. e.g. fine compliant motion over distances
of the order of the features on the parts to be manipulated. It is
explicity not required to have compliant motion capabilities over the
entire range of motion of the manipulator. Thus, in such a coarse-fine
system, the coarse manipulator (CM) can be operated in a strict
position-controlled mode, while the fine manipulator (FM) attached to it
can be operated in compliance mode or force-controlled mode. The mass and
moments of inertia of the FM can be several orders of magnitude smaller
than those of the CM, and the motion of the FM can be made frictionless.
Accordingly, the desired robot behavior is at least theoretically
achievable, assuming a near-ideal FM. The ideal FM should include:
.cndot. 6 degrees of freedom (DOF) redundant with those of the CM;
.cndot. minimal mass to avoid adversely loading the CM;
.cndot. very high acceleration to make it possible to respond to
vibrational disturbances in the environment and maximize job throughput;
.cndot. minimal static friction, since the presence of static friction
causes loss of accuracy and difficulties with control;
.cndot. FM positional resolution much smaller than the CM for high
precision; .cndot.
FM motion range as large as possible, to avoid extra motion of the CM;
.cndot. adequate damping.
PATENT PRIOR ART
U.S. Pat. No. U.S. Pat. No. 3,260,475, Ormsby et al, "Space Vehicle
Directing Apparatus," July 12, 1966, shows the use of a levitated
stainless steel ball (weightless in space) electrostatically suspended (or
suspended magnetically or pneumatically) and corrected for centering, and
used as a base for rotating the entire spacecraft about the sphere by
inducing electric currents in the rotor ball and relying on the reaction
torque to move the spacecraft.
U.S. Pat. No. 3,732,445, Laing, "Rotating Pole Rings Supported in
Contactless Bearings," May 8, 1973, shows a hydrodynamic bearing for a
spherical rotor.
U.S. Pat. No. 4,445,273, Van Brussel et al, "Displacement Control Device,"
May 1, 1984, shows a position-orientation-compliance device with separate
motors synthesizing linear and torsion springs.
U.S. Pat. No. 4,509,002, Hollis, "Electromagnetic Fine Positioner," Apr. 2,
1985, teaches a two-axis fine positioning device based on electromagnetic
principles.
U.S. Pat. No. 4,514,674, Hollis and Musits, "Electromagnetic X-Y-Theta
Precision Positioner," Apr. 30, 1985, teaches a related three-axis
positioner.
U.S. Pat. No. 4,661,737, M. Barri, "Electrical Machines With Multiple Axes
of Rotation," Apr. 28, 1987, shows a motor with a constrained spindle in a
ball rotor which is movable in a range within a socket stator which is
movable within a base member.
U.S. Pat. No. 4,155,169, S. H. Drake, "Compliant Assembly System Device,"
May 22, 1979, teaches a passive compliance remote center robot end
effector for insertion of pegs into holes.
PUBLICATION PRIOR ART
H. Van Brussel and J. Simons, "The Adaptable Compliance Concept and its use
for Automatic Assembly by Active Force Feedback Accommodations," 9th
International Symposium on Industrial Robots, Washington, D.C., 1979, pp.
167-181.
M. R. Cutkosky and P. K. Wright, "Position Sensing Wrists for Industrial
Manipulators," 12th International Symposium of Industrial Robots, 1982,
pp, 427-438.
Andre Sharon and David Hart, "Enhancement of Robot Accuracy Using Endpoint
Feedback and a Macro-Micro Manipulator System" American Control Conference
proceedings, San Diego, California, June 6-8, 1984, pp. 1836-1842.
Kazuo Asakawa, Fumiaki Akiya, and Fumio Tabata, "A Variable Compliance
Device and its Application for Automatic Assembly," Autofact 5 conference
proceedings, Detroit, Michigan, Nov. 14-17, 1983, pp. 10-1 to 10-17. S.
C. Jacobsen, J. E. Wood, D. F. Knutti, and K. B. Briggers, "The
Utah/M.I.T. Dextrous Hand: Work in Progress," Int. J. of Robotics
Research, 3[4], 1984, pp. 21-50.
There have been a number of studies and experimental fine motion devices
with some measure of compliance control, using various actuation
mechanisms. These include a five-axis DC-motor-driven adaptive compliance
system (Von Brussel and Simons); a five-axis hydraulic fine motion robot
wrist based on expandable elastomeric balls (Cutkosky and Wright); a
five-axis hydraulic fine motion robot wrist based on hydraulic cylinders
(Sharon and Hardt); an electrodynamic variable compliance device for
automatic assembly (Asakawa et al); a four-finger robot hand with
compliance adjustable by changing air pressure in pneumatic cylinders
(Jacobsen et al).
The human hand and wrist of course is a masterpiece of multiple-DOF
positioning with selective compliance. The human hand can deliver a sheet
of paper, an egg or a bowling ball, with brain-program-control of skeletal
motion with muscular and opposed-muscular motion and compliance. The arm
is a CM device; the hand is an FM device with selective compliance in many
degrees of freedom.
Magnetically levitated bearings (usually spindle bearings) are known. These
devices commonly are electromagnetic rather than electrodynamic as in the
present invention, and use very narrow gaps to achieve the highest
possible fields. There is no attempt at compliance, since the desire
usually is to spin at high speed without wobble or friction. The control
system is designed as a regular (to maintain position and orientation) and
the devices are not capable of general position and orientation tasks.
It is also known to provide passive damping, by contrary fields set up by
eddy currents in conductive plates, in a magnetic actuator such as the
"homopolar generator."
This complex body of prior art does not, however, teach nor suggest the
invention, which is a robot wrist with programmable multiple degrees of
freedom and with programmable variable compliance in at least one degree
of freedom.
SUMMARY OF THE INVENTION
The object of the invention is to provide a fine motion robot wrist which
can accomplish moves over limited distances and limited angles in each of
six degrees of freedom, such as X, Y, Z, ROLL, PITCH, YAW, in any
combination of these, with programmable compliance in up to six degrees of
freedom.
A feature of the invention is magnetic levitation of the wrist and its end
effector with programmable electrodynamic positioning and with
simultaneous programmable compliance introduced by the control system.
An advantage of the invention is that it accomplishes very high speed
positioning with great accuracy, and with the following characteristics:
.cndot. Full 6-DOF compliant fine motion;
.cndot. Very high performance for light payloads;
.cndot. Extreme simplicity, with only one moving part;
.cndot. Novel combination of actuation, support, sensing, and control
means;
.cndot. Docking mechanism to allow coils to cool during coarse motion;
.cndot.
Noncontact position and orientation sensing with approximately 1 .mu.m
resolution;
.cndot. Multiple possible control modes, including active compliance
control to mimic the behavior of mechanisms.
The foregoing and other objects, features and advantages of the invention
will be apparent from the more particular description of the preferred
embodiments of the invention, as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a six-DOF hexagonal fine motion wrist, having six
forcer units with associated coordinate systems.
FIG. 2 is a diagram of one forcer unit, with flat coil, four permanent
magnets and two return plates.
FIG. 3 is a semidiagrammatic section view through the fine motion wrist.
FIG. 4 is a translated and rotated hexagonal model fine motion wrist
identical to that of FIG. 1, with motion diagram.
FIG. 5 is a diagram of a spherical fine motion wrist.
FIG. 6 is a graphical presentation of the translation and rotation
constraints of both hexagonal and spherical fine motion wrist designs.
FIG. 7 is a simplified plan view showing docking mechanism incorporated in
the wrist of FIG. 3.
FIG. 8 is a diagram of the position and orientation sensing scheme for the
fine motion wrist.
FIG. 9 is a schematic diagram of the control for the fine motion wrist.
FIG. 10 is a composite schematic diagram of representative mechanisms which
may be emulated, under program control, by the fine motion wrist.
DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
FIG. 1 illustrates a hexagonal six-DOF fine motion robot wrist appropriate
for programmable variable compliance in a preferred embodiment of the
invention. The wrist can be mounted on the end of a standard
position-controlled robot to give the coarse motion robot the ability to
execute fine, compliant motion. The robot (CM) provides coarse motion; the
wrist (FM) provides fine motion and selective compliance under program
control. The magnetically levitated FM wrist of the preferred embodiment
features a single moving part, a dynamically levitated movable "flotor"
element 1. A hollow rigid shell-like moving flotor shell 2 contains planar
or quasi-planar curved magnetic flotor coils 3. The flotor unit 1 is the
levitated structure of the wrist; it bears the same relationship to the
fixed structure (stator) as does the more commonly known rotor is a
magnetic bearing, hence "flotor." Note that the relative position of
flotor and stator as moving and fixed elements, respectively, may be
exchanged, but for clarity the coil-bearing element will be designated the
flotor in this text. The flotor 1 structure carries the tool chuck or
gripper (not shown). The tool chuck or gripper, or equivalent, whether
with or without a tool, may be called the "end effector," or simply the
"hand."
HEXAGONAL FLOTOR
FIG. 1 shows a flotor unit 1 which is in the conformation of a prism of
hexagonal cross-section. The flotor coils 3 are integral to "forcer"
elements, each flotor coil 3 interacting with its respective magnet
assembly 4 within the related forcer element 5 to produce motion of the
flotor unit 1. In the preferred embodiment, a flexible ribbon cable
provides electrical connections to the coils 3 without restricting motion
of the flotor 1. There are six forcer elements in FIG. 1, shown in two
interspersed orthogonally situated triads. That is, adjacent forcer
elements are oriented at right angles to each other around the hexagonal
flotor unit 1. The flotor coils 3 operate within large magnetic gaps in a
fixed stator structure containing permanent magnets, along with relative
position sensing devices related to flotor and stator. Suitable control
means for flotor coil 3 currents is provided to produce a fine motion
device capable of moving with high translational and rotational
accelerations over distances and angles limited by the magnetic gaps. The
forcer elements 5 are arranged in such a manner as to provide three
orthogonal translational degrees of freedom (X,Y,Z) and three orthogonal
rotational degrees of freedom (ROLL, PITCH, YAW) developed by coil
currents specified by a control unit not shown in FIG. 1. As shown in FIG.
1, the six forcer elements are not arranged identically, but rather are
rotated 90.degree. from their adjacent forcers. In the preferred
embodiment, they are alternately horizontal and vertical. These may be
parallel to flotor unit 1 top surface as shown in FIG. 1, or may be at
+45.degree., -45.degree., +45.degree., or otherwise to accomplish the same
purpose. The hollow moving shell flotor unit 1 is suspended by actively
controlled magnetic levitation in such a manner that the compliance
(stiffness) can be varied over a wide range of magnitudes and directions
under program control.
PERIPLANAR COIL
The flotor unit 1 has a preiplanar coil (planar to match the rectangular
face of flotor unit 1 with hexagonal preiphery, or curved to match a
different flotor unit 1 configuration with curved periphery.) For six
degrees of freedom, a number (at least six) of flat-wound periplanar (flat
or curve) flotor coils 3, operating in magnetic fields produced by
permanent magnet assemblies are required to produce actuation forces and
torques in three dimensions. The periplanar coils 3 are rigidly
incorporated in the lightweight hollow shell flotor unit 1 which comprises
the moving part of the wrist. Alternatively, for some applications, the
magnets 4 and associated structures can be made to move, with the flotor
unit 1 coil structure fixed, and arrangement which has some advantages for
cooling. The position and orientation of the moving wrist is measured by
sensors 6. Light emitting diodes 7 arrayed about flotor unit 1, are sensed
by lateral effect cells 8 on the stator support unit 9, which is affixed
to the CM device and may be considered as the fixed base as shown
schematically by an earth mark. Power for the light emitting diodes 7 is
provided by the same flexible cable used for the coils 3. The lateral
effect cells 8 provide flotor unit 1 position data feedback for control of
the fine motion wrist, controlled by an analog or digital controller (not
shown in FIG. 1) in real time in such a manner to achieve a task level
purpose such as compliant parts mating in a robotic assembly operation.
The basic electromechanical unit which provides a source of force or (in
pairs) torque to the wrist is a periplanar (flat and curved) coil
electrodynamic drive unit, or forcer element.
CONFIGURATION OF FORCER ELEMENTS
There are many forcer arrangements which could be used to achieve 6-DOF
motion and which would more or less satisfy the above description of the
invention. For example, one could have forcer elements arranged on the six
faces of a cube, or on six mutually orthogonal paddle-shaped wings, or
have eight forcers in an octagonal ring, or have pairs of xy forcers
separated by a rod-like element extending in z, etc. for translation and
rotation in three-dimensional space. The only strict requirement is that
the forcer elements be arranged in such manner that in combination they
exert three linearly independent translational force components and three
linearly independent torque components on the moving element. If the
desired translational force and torque are expressed as a six-element
vector F=[F.sub.x, F.sub.y, F.sub.z, .tau..sub.x, .tau..sub.y,
.tau..sub.z, ]and the magnitudes of the forcer forces are expressed as the
six-element vector .function.=[.function..sub.1, . . . ,.function..sub.6
], they will be related by the 6.times.6 matrix T:
F=T.function..
A necessary and sufficient condition for the wrist to operate in six
degrees of freedom is that T be nonsingular; that is, it must be possible
to calculate T.sup.-1. Further, the "condition number" of the matrix T is
a measure of the design quality. The mathematics of square matrices,
computation of eigenvalues of matrices, and computation of inverse
matrices are known. It will be apparent to those skilled in the art that
numerous arrangements of forcer elements can be configured subject to this
constraint.
The preferred embodiment provides six forcer elements 5 and a ring-like
shell flotor unit 1. This closed configuration makes it convenient for
mounting the wrist on a robot arm and, in turn, for mounting tooling or
other end effectors to the wrist.
FIG. 1 shows six forcer elements 5, alternately arranged vertically and
horizontally about a ring with a hexagonal cross-section. The inner ring
of magnets and return plates are rigidly connected with a ring-shaped
mechanical support (not shown) and similarly for the outer ring of magnets
and return plates. These inner and outer rings form the fixed stator
structure having a closed dual periphery, which attaches to the CM (robot)
arm. The middle ring, or flotor, containing the coils, is free to move
with six degrees of freedom, within the mechanical limits of the flotor
unit 1 periphery nested within the dual periphery of the stator support
unit 9. An hexagonal top plate (not shown) serves as an end effector
mounting platform.
In FIG. 1, the wrist is shown at its zero position, floating in the
magnetic gaps. In this configuration the flotor XYZ and stator X'Y'Z'
frames are coincident. For a wrist approximately 200 mm in diameter,
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