Note: Descriptions are shown in the official language in which they were submitted.
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Title: MECHANI8N~ FO~ ORIFNTING AND PLACIN~ ARTICLB8
Field of the Invention
This invention relates to the field of
mechanisms and particularly to mechanisms suited for use
05 in the field of robotics. The described mechanisms are
not, however, limited to that field but are suited to
applications wherever an article is to be oriented or
displaced within a given workspace.
Backaround to the Invention
Mechanisms are mechanical structures synthesized
with assemblages of joints and links designed to provide
them with predictable structural, kinematic and dynamic
properties. They are the basis for vast numbers of -~-
applications including cars, aircraft, optical
instruments, manipulation systems, etc. and as such are ~ -
particularly important elements of most technological r;
systems. Parallel mechanisms, a vast sub-class of all
mechanisms, offer an opportunity for improved structural
properties with rigidity, light weight and improved
dynamic properties. Parallel mechanisms allow actuators
to be placed at locations where they contribute the
least to an increase o~ inertia. Further, improved
accuracy can be achieved by eliminating the accumulation
of errors.
Unfortunately, most known parallel mechanisms
with more than two or three degrees of freedom suffer
from a reduced usable workspace. The invention reported
herein achieves a significant improvement in this area.
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Mechanisms are synthesized by constraining the
surfaces of pairs of surfaces to fixed relationships by
means of links. A kinematic analysis assumes links to
be ideally rigid. Mechanisms can be described by
OS selecting one output link and one ground link and
defining the elements there-between.
If a chain of links and joints forms loops, then
the mechanism is termed parallel. If a mechanism ~ `~
requires exact geometrical properties to possess ; ~ ,
mobility (degrees of freedom), it is termed over-
constrained. If a mechanism has no mobility, it is ~
called a 'structure'. (The term 'structure' may apply ~ ;
to other notions but should be clear by context). If ~ ~
there are no loops the mechanism is called serial. ~ ;
As joints play a central role in mechanisms and
are needed to describe the invention, they are defined
herein. I~o surfaces of revolution form the revolute
joint which has one angular degree of freedom. With an
additional translational degree of freedom, the joint
becomes cylindrical. Two surfaces shaped as prisms formthe prismatic joint which has one freedom of
translational motion.
A "universal" joint is composed of two
orthogonal revolute joints with a centre of rotation at
the intersection of their axes.
The spherical joint has three degrees of angular
freedom of motion. A spherical joint may be composed of
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a spherical surface in contact, vis, a ball-in-socket; or
can be created by three orthogonal revolute joints with a
centre of rotation at the intersection of their axes.
A "gimbal" joint has three revolute joints ;
05 positioned to rotate about a common centre of rotation.
In a gimbal joint the supported mass may be at the
centre of rotation.
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Actuated joints are equipped to provide -
mechanical power derived from an external source.
Passive joints are left free to move by virtue of the
forces present in the links. Joints may be actuated to ~;
provide rotational or translational motion. ~ ;
Any joint can be instrumented with sensors to
measure position, velocity or acceleration of the
relative motion of links.
Four bar mechanisms having four links and four
joints are used in a bewildering number of applications.
Many function~ can be accomplished by changing the four
kinematic design parameters (link lengths). If the axes
of ~oints are not exactly parallel, the "mechanism"
becomes a structure.
A five-bar mechanism has five links and five
joints. If one link is grounded as a "base" link, then
the joint opposite the grounded link - the "driven
joint" - can be displaced in space through the
manipulation of the links proximate to the base link -
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the "proximal" links. The remaining two links next to
the driven joint may be classified as "distal links".
Prior Art
Two background papers of particular interest to
05 the present invention are: ~ -
- Pierrot F., Dombre, E. 1991. "Parallel Structures
for Robot Wrists". In Advances in Robot
Kinematics. Stifter, S., Lenarcic (eds.).
Sprinter-Verlag. pp. 476-484; and ~ ~
- Inoue, H., Tsukasa, Y., Fukuizumi, T. 1986. ~ ;
"Parallel Manipulator." In The Third International
Symposium on Robotics Research. Faugeras, 0.
Giralt, G. (eds). MIT Press. pp. 321-327.
The Pierrot/Dombre paper describes a series of
15parallel structures commencing with the basic Stewart
platform. One structure, "P4" ~n this paper, introduces
a constraint for the upper platform which includes a
universal and prismatic joint. In this P4 structure,
three symmetrically placed, two-linX supports extend
20between the lower and upper platforms. The joints for
these supports are variously spherical and universal and
actuation is effected through rotary actuators. In the
Inoue/Fukuizumi paper an upper platform is supported by
three symmetrically placed 5-bar linkages called
25"pantograph" links in the paper. The upper platform is
otherwise unconstrained and actuation is effected
through rotary actuators.
,
While both provide interesting designs, these
papers do not suggest the configuration proposed herein
to provide a "wrist" type orienting mechanism of the -~ -
type hereinafter described. ~ -
05 That invention in its general form will now ;
first be described, and then its implementation in terms
of specific embodiments will be detailed with reference
to the drawings following hereafter. These embodiments
are intended to demonstrate the principle of the
invention, and the manner of its implementation. The
invention in its broadest and more specific forms will
then be further described, and defined, in each of the
individual claims which conclude this Specification.
Summary of the Invention ~ ~ `
This invention relates to an orienting assembly
for effecting the rotational displacement about a fixed
point of an end member supported by a rotational joint.
Additionally, tran~lational displacements of the end
member from that fixed point may also be ef~ected.
In order to control the orientation of the
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manipulated end member with either two or three degrees - -
of rotational freedom, two, paired "5-bar" linkage
assemblies are employed, each equipped with actuators.
The 5-bar linkages are each supported by a revolute
joint connected between the base link of the 5-bar
- ~ . .
assembly and a base which serves as a "ground".
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The opposed, "driven" joint coupled by universal or ~
spherical joints to the end member which is to be ~ -
oriented. ~he actuators employed may either be
translational or rotary.
05 In one variant of the invention employing rotary
actuation this mechanism relies upon two rotational actuators
positioned at two of the joints of each 5-bar linkage. ~ -
Preferably the rotary actuators are positioned at the ~-~
two joints located at the ends of the base link.
In another variant of the invention, linear
actuators are placed between the ends of the base link
and the driven joint. In both cases, actuation means
are used to control the position of the driven joint
with respect to the baæe link, and thereby the
orientation of the end member.
In the case of the use of linear actuators these ;
elements may only be positioned between the ends of the
base link and the driven joint.
The driven joint of the 5-bar linkage, opposite
the base link, may be connected to the end member
through either a universal or spherical joint. If this
joint is universal, then the end member may be manipulated
by the actuator to effect pitch and yaw motions. If the
.
connecting joints between the driven joint and end member
are spherical, then roll motions may also be achieved.
All these rotations occur within a reasonable workspace
due to limited interference from the 5-bar linkages.
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In the case where couplings between the driven
joints of each 5-bar assembly and the end member are
universal, providing only two rotational degrees of ;~
freedom, then the mechanism in this degenerate form will
05 still function usefully as a "pointing" mechanism.
Applications for such a device include supporting
microwave antennae, telescopes and directional laser
mounts.
An advantage of this configuration is that the
full weight of the pointed element is carried by the
pivotal mount at the center of rotation. Thus the
actuators associated with the 5-bar linkages never need
to carry any of the load of the apparatus which is being
oriented.
In the rotary variant of the invevtion where
three degree~ Or rotational freedom are desired, four
rotational actuators are preferably provided. These are
preferably mounted, in pairs, at the ends of the base
link of the respective S-bar linkages, just above the
grounded revolute joint to apply a positioning force
between its respective proximal linkage and the base link.
This may be effected by employing a common shaft for the
revolute joints which these proximal links then share, - ;
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The mechanism is inherently light and has low inertia.
Rotational positioning may be provided by tendons. The
mechanism may be utilized in either input or output mode
and may be inverted. In telerobotic applications it
should provide a high band-width level of sensory feed-
OS back to an operator.
Nore generally, the invention may be describedas an actuable mechanism for orienting an end
member with respect to a base, the end member being ;
constrained by a support joint having two or three
rotational degrees of freedom and a centre of rotation
for at least two of said degrees of freedom, such
mechanism comprising two 5-bar linkages each defining a
closed loop and having~
(a) a base link with two ends, such base link being
connected to the base through a revolute ~oint;
~b) first and second proximal link6 connected
respectively to the ends of the base link;
(c) a driven joint positioned opposite to said base
link and joined thereto by first and second distal ~-
links which are respectively coupled through said
proximal links to said base link; and ~-
(d) first and second actuated joints positioned within
said closed loop between said base link and said
driven joint for displacing the driven joint with
respect to the base link,
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g
wherein the driven joints of each of said 5-bar
linkages are connected to the end member at connection
points through twinned joints which are either twinned
spherical joints or twinned universal joints, being
05 twinned with respect to the two distal links, said
connection points being non-coincident with the centre
of rotation for the support joint, thereby to provide
mobility to the end member with respect to said base in
response to said actuators.
The mechanism of the invention may have first ~ ~;
and second actuated joints which are rotational joints,
positioned respectively between the ends of the base
link and the driven joint. More preferably, such first
and second actuated joints are positioned respectively
at the ends of the base link, between such ends and the -~
respective proximal links.
Alternately, a mechanism of the invention may
have first and second actuated ~oints which are sliding
joints, positioned respectively between the proximal and -~
di~tal links.
If the rotational freedom of the end member is
to be limited to that suited for a pointing mechanism,
the said twinned universal joints may each comprise `~
three revolute joints, the first and second of which ;
joints having coinciding axes and being connected to the
distal links; the third of such joints being positioned
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between the first and second joints and the end
member,all of said joints having a common centre of
rotation. In the fully, spherical embodiment, the
twinned spherical joints may each comprise four revolute
05 joints, the first and second of which are respectively
connected to distal links and have coinciding axes, all
of the axes of said joints having a common centre of
rotation.
As an additional option, the mechanism of the
invention may further incorporate a cylindrical joint -
positioned between the end member and the base to permit
the end member to be translationally displaced with
respect to the centre of rotation for the end member.
Some of the features of the embodiments of the
invention may be summarized as follows~
(1) The mechanism of the present invention may have
four d2grees of freedom: three in angular motions and
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one in translational motion. If the translational
motion is not used it reduces to a spherical mechanism.
If one selected rotat`ional degree of freedom is
suppressed, it becomes a pointing mechanism.
(2) If one of the four freedoms is restricted to a
small range, or totally suppressed, the remaining
workspace---the range of motion free of interferences
and inside which the mechanical advantage of the ~ ~
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actuators is kept approximately constant---becomes
large, a property which is extremely unusual in parallel ;
mechanisms. This is particularly remarkable if
additional motion is suppressed. A constructed
05 mechanical model has shown that a workspace in excess of
180 degrees of roll motion combined with 90 degrees of
pitch and yaw motion can be achieved. ~-~
(3) In the case where the needed ranqe of motion is ;~
small, the mechanical advantage of the actuators with
respect to the output link can be selected over a wide
range of values for each principal direction of motion. -
(4) Again in the case of small motions, control is
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facilitated by the fact that each direction of motion
corresponds exactly to the sums and differences of
actuator motions, taken two by two.
(5) In cases where a large range of motion in the
angular workspace is needed, the mechanical advantage of
the actuators taken as a group can be made approximately
constant and equal for each direction of motion,
possibly approaching a condition known as isotropy.
(6) The special organization of the arrangement
makes it particularly easy to achieve high structural
stiffness and accuracy in many designs as the majority ;~
of structural members are in a position to undergo well
defined stresses: either compression-tension, or
bending in a single plane. ;
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(7) Parallel mechanisms with more than one or two
degrees of freedom are often plagued by the
necessity of numerous passive joints. This -
increases the cost of fabrication and if exacting
05 specifications are not respected, this can defeat ~
the claimed advantages of parallel mechanisms. The ~-
present invention achieves a significant reduction ;
of the number of passive joints while retaining the ~ ~
required mobility. ~-
(8) In the case where the mechanism is restricted ~`
to three degrees of freedom, to become spherical for
example, it can be made to create forces within its
structure to eliminate backlash in the joints even in - ` -
the presence of wear or fabrication imprecision.
9) Certain versions of the mechanism can be made to
exhibit advantageous dynamic properties, minimizing the
reaction forces at the ground link in conditions of high
accelerations.
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(10) The mechanism can be easily instrumented and the
sensors can be placed to provide accurate measurementsof the position of the output link. In fact if more
sensors than strictly needed are used, the redundant ~ -
information can be used to increase accuracy.
(11) The mechanism is power efficient as compared to
many conventional mechanisms.
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(12) The mechanism is advantageous from a fabrication
viewpoint as it introduces simplifications which make it ~ -
simpler to achieve mobility and rigidity with a reduced
number of parts, some of them being replicated four
05 times. -~ -~
(13) The mechanism is particularly suited to tendon
control arrangements as wire management connections need
not be made to run throughout the entire structure.
(14) Fabrication is made easy because of the simple type
of stress supported by each joint. In addition, -~
contrary to serial mechanisms, all joints are -~ -
involved in any stress thus sharing the load.
The foregoing summarizes the principal features
of the invention and some of its optional aspects. The
invention may be further understood by the description
of the preferred embodiments, in conjunction with the ; ~ ;
drawings, which now follow.
Summary of the Figures
Figure 1 is a schematic of the links of the
basic mechanism, labelled as to important dimensions.
Figure 2 is a symbolic depiction of the rotary
form of mechanism, identifying the links, joints and -~
actuators (in the form of motors).
Figure 3 is a schematic of a five-bar linkage. -
Figure 4 is a schematic of a constrained six-bar
linkage that functions as a five-bar linkage.
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Figure 5 is a schematic of the links of the
mechanism when linear actuators are employed.
Figure 6 is a symbolic depiction of the linear-
actuated form of the mechanism, identifying the links,
05 joints and actuators (in the form of prismatic
cylinders).
Figure 7 is a schematic of an "inverted" version
of the mechanism of Figure 5 with the linear actuators
grounded.
Figure 8 is a schematic of an inverted version
of the mechanisms of Figure 2 with the ro~ary actuators
grounded.
Figure 9 is a depiction of a shoulder mechanism
equipped with linear actuators.
Figure 10 is a depiction of a rotary cutting
head mounted on a structure according to the invention and
suited for boring operations.
Figure 11 i8 a depiction of a support suited for
orienting optical gratings and the like.
Figure 12 is a depiction of a mechanism for
supporting a joy stick.
Figure 13 is a depiction of an inverted
configuration of the mechanism to provide a leveling
platform for a turret.
Figure 14 is a detail of a joint showing a
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linkage that is of a reinforced form.
Description of the Preferred Embodiment
Operation with Rotary Actuators~
The main links which play a structural role are
05 labelled in Figure 1 as follows: Lo (output link), Lg
(ground link), Lf (fork link), Lm (actuator link), Lc
(connecting link). Five link lengths describe the basic -~
geometry, they are --- kinematic design parameters. In
Figure 2 the principal directions of motion are labelled
R (roll), P (pitch), Y (yaw) and S (slide). Joints labelled
Ml, M2, M3, M4 are rotary joints actuated by motors. By
convention a positive direction of motion is indicated.
Joints labelled Jl, J2, J3, form a passive spherical
joint (or gimbal). Jl, is a cylindrical joint allowing
the output link to slide in and out.
The axes of joints M1, M2, (resp. M3, M4) and J10,11
(resp. J12,J13) do not need to be coincidental. They
are represented or constructed this way for simplicity.
Joints labelled J6, J7, J8, Jg are represented as
cylindrical. In actual practice they may be revolutes. .
There are several ways to implement the four-joint
substructures J10, Jll, J14, J16 and J12, J13, J15, J17
in a manner which is similar to ordinary universal
joints. In the above Figures, several symmetries have
been introduced to simplify analysis and fabrication.
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Referring to Figures 1 and 2, the principles ;
of operation of this rotary-activated embodiment may be
summarized as follows:
- Let M1, M2, M3, M4, rotate in the positive
05 direction: the output link undergoes a pitch motion.
- Let Ml, M2, rotate in the positive direction and ~u
M3, M4, in the negative one: the output link undergoes
a roll motion.
- Let N, M4 rotate in the positive direction and M2, ;
M3 in the negative one: the output link undergoes a yaw
motion.
- Let Ml, M3 rotate in the positive direction and
M2, M4 in the negative one: the output link undergoes a
sliding motion.
Summary of Features - Rotary Actuators:
1) Feature 1: It must be noticed that the two
actuated 5-bar structures appear as six joint/six link
assemblies. The six joints and six links arise from the
fact that the basic S-bar linkage - Figure 3 - includes
a driven joint Jd which is twinned (being either
spherical or universal) and is connected to the end
member through an additional link, La. This sixth link
La is, however, outside the 5-bar loop (Figure 4) and ~ ;
the two revolute joints J10, Jll (resp. J12, J13) share
a common axle connected to the additional link La. As a
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- six joint/six link assembly, this structure appears to
be an over-constrained chain, thereby potentially losing
mobility. However, this type of structure is commonly -~
made to function properly by keeping all axes parallel,
05 which is a simple machining operation. In cases of
exacting specifications, the problem can be dealt with
by introducing suitable elasticity in the links. --~
2) Feature 2: Singularities occur either:
.: ~
(1) wher, the output can undergo finite displacements
while one of the actuator's velocity vanishes:
or,
(2) when the converse condition occurs. -~
Condition (1) occurs for example when the
mechanism is in a position such that points, C,Bl, Al,
align. By design such conditions can be avoided for
large excursion~. In addition, even in such positions
where actuators Ml and M2 lose their influence on the
yaw motion of the output link, N3 and M4 would be
capable of controlling this motion.
Condition (1) also occurs when point A undergoes
a motion in a direction exactly orthogonal to the
principal direction of a connecting link. Proper
functioning has, however, been verified by constructing -
mechanical models and it was impossible to find such ;~
conditions within any workspace free of interferences.
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Condition (2) occurs when one 5-bar linkage
stretches completely. This may put a definite bound on -
the workspace as a degree of freedom is lost.
3) Feature 3: A wide range of mechanical
05 amplification gains or attenuations is achievable by
selecting L5 and the parameters L2, L3, L4, chosen to
vary the angle of incidence of each connecting link in
order to create various lever-arm actions around the
pitch and the yaw directions.
104) Feature 4: The mechanism has the ability to
operate with each effected motion based on the sum and
differences of actuator motion for wide ranges of
designs and in the neighborhood of any operating point.
This property can be exploited by making use of analog ~ -~
electronics to control the device, despite its complex
kinematic st N cture, and thereby achieving very high
control bandwidth. This is because no multiplications
are needed other than by constant
quantities, due to the four way differential nature of
the actuators.
5) Feature 5: If we replace cylindrical joint
Jl, by a revolute joint, eliminating the sliding motion,
the angular workspace can then be made to reach its
maximum. In this case, we are in the presence of a
redundantly actuated mechanism. For a given output
torque, an infinite set of actuator torques can be
cho~en by control. This effect can be applied to
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contribution to fulfill a number of functions. For
example, the set of torques can be selected to create
minimum stress in the structure. Another example is to
select those torques required to minimize the maximum
05 torque in the actuators for a given output, thereby
maximizing efficiency. Yet another example is to create
given bias forces in the joints, thereby cancelling
backlash if any.
This particular effect can be appreciated by -
inspection of Figure 2. If a positive torque i6
created in actuators Ml, N3 while a negative one is
created in M2, M4 (corresponding to the eliminated
sliding motion), the resulting forces cancel out and all
the passive joints are bias-loaded in one well defined
direction. Thus, accuracy can be upheld even in the ~ ~
presence of wear. ~ ;
6) Feature 6: Consider a fixed inertial load
acting vertically on the output link. If its center of
mass lies on the axis of Jl, sliding motions will not
create reaction forces and torques other than exactly in
the direction of motion. If the combined contribution
of the load and links to the inertial tensor of the total
mechanism causes the axes of the corresponding ellipsoid
of inertia to coincide with the principal directions of
motion, and this ellipsoid is centered at the center of -~
rotation, then angular accelerations will create zero
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reaction forces at the ground link, and only reaction
torques. This is even more desirable if all the axes of
this ellipsoid are egual, in which case this effect is
obtained for any direction of rotational acceleration.
05 This feature is particularly useful for high
acceleration, high bandwidth applications.
7) Feature 7: The most obvious place for
sensors to be located is on the same shaft as the
actuators. However, J7, J8, J9, J10 are also excellent
candidates for instrumentation, as well as Jl, J2, J3.
Redundant sensing offers a range of possibilities
including augmentation of accuracy and usage of self-
calibration techniques. The spherical case with co-
located actuators and sensors is sensor-redundant too.
This invention does not suffer from accumulation
of errors as a serial mechanism does. In fact exactly
the opposite occurs, error reduction is obtained as all
sensors are made to measure any motion or position. In
in an analogous way all actuators are made to cause any
motion. In the serial case, each sensor and actuator is
dedicated to each principal direction of motion, and
therefore errors accumulate.
8) Feature 8: Consider a sliding motion for
example. In the serial case only joint J1 contributes
power to this motion. The design of the invention will `~
require the contribution of all four actuators to cause
the same motion. The same argument can be repeated for
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all four principal direction of motion, it thus follows ~
that this design can achieve a factor four in power ~-
efficiency improvement. ~-~
-operation with Linear Actuators
05 Joints labeled Cl, C2, C3, C4 in Figure 6 arè
actuated prismatic joints. By convention the positive
direct~on is taken in the sense of actuator shortening. ~
As in the prior case joints labeled Jl, J2, J3 form a .
passive gimbal, with Jl being a cylindrical joint,
allowing the output link Lo to slide in and out.
The axes of joints J10, Jll, (resp. J12,J13) do
not need to be coincidental. They are represented or
constructed this way for simplicity.
Again, in Figure 6 as in Figure 4, symmetries
have been introduced to simplify analysis and -~
fabrication.
Referring to the Figures 5 and 6, the
principles of operation of this linear-activated
embodiment are as follows~
- Let Cl, C2, C3, C4 translate in the positive
direction: the output link undergoes a sliding motion.
- Let ~1, C2 translate in the positive direction and C3,
C4 in the negative one. The output link undergoes a yaw ~-
motion.
- Let Cl, C4 translate in the positive direction and C2, ~ t~
C3 in the negative one. The output link undergoes a - ~
pitch motion. .
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- Let Cl, C3 translate in the positive direction and C2,
C4 in the negative one. The output link undergoes a
roll motion.
Summary of Features - Linear Actuators:
05 1) Feature 1 is analogous to the previous case.
2) Feature 2 is also analogous except that the
algebraic determination of locus of singularities has
been performed in the spherical case and for L4=0. It
was found that loss of control condition occurs only in
10 the case where points Al, A2 falls in the plane Bl, B2,
B3, B4 which corresponds to a 90 degrees pitch motion.
Condition (2) of previous case never occurs.
3) Feature 3 is exactly analogous to previous case.
4) Feature 4 is exactly analogous to previous
15 case. In fact, the determination of the various
mechanical gains is simpler.
5) Feature 5 is exactly analogous to previous case,
with L4=0. To date a good design has been found for the
following parameters Ll=8, L2=8, L3=12. One
20 disadvantage of the linear actuator design iB the
reguirement to provide room for the actuators to move
free of interference--as their length is by necessity
larger than twice their stroke on the extended
position while their retracted length must be larger
25 than their stroke. However a practical design with
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piston actuators has been physically realizedr
6) Feature 6: Analogously to the rotary case,
as the four actuators undergo axial stress only which
makes it particularly suited for piston actuators.
05 7) Even if the actuators are piston type thus
typically being cylindrical pairs, they are constrained
to undergo strictly translational motions with no twist.
Therefore, position sensors can safely be strapped on
their sides without need for torsion decoupling joints,
simplifying design and construction. In fact the linear
actuator design is even more advantageous for achieving
high rigidity.
8) Feature 8: The mechanical advantage varies
most significantly for roll motions, as both regional
actuator structures extend simultaneously, thus losing
their advantage together. It must be remembered that
the effective range of motion àround this direction is
in excess of 180 degrees irrespective of other motions.
The mechanical advantage changes mildly for yaw motions
as one structure extends while the other contracts. It
changes moderately for sliding motions. ~ -
It remains almost constant for pitch motions.
The worse case occurs for retractions combined with a
roll. Depending on the intended application, many
designs are possible. For a general purpose devicP, one
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should seek angular isotrophy. For example, it is easy
to see that if L5=0, point C falls on the line B1,~2,
the mechanical gain in pitch motions is exactly 1 for
each actuator. The other design parameters can be
05 searched for similar conditions for the other motions.
To date, a good general design has been found for the
following parameters: Ll=4, L2z3,L3=4,L5=O,L4=4
(spherical case).
9) Feature 9: In the case where each nember
under static load conditions ground link Lg undergoes -
composite stress, but since it is stationary, rigidity
can be obtained without penalty on the dynamic response.
Output link Lo also undergoes composite stress, torsion,
bending, compression and tension. But since it is the
last link before the load and because of its simple
shape, it can be designed to sustain the load reaction
forces only. Thus specifications are easy to obtain.
The actuator links Lm undergo bending stress almost
exclusively, and some compression or tension. Thus a
rigid and lightweight link can be optimally designed
using a "wishbone" structure as illustrated in Figure 6.
10) Feature 10: Finally connecting links Lc
undergo pure axial stress, compression or tension. Thus
they can be made ideally light. This is particularly
fortunate since they are the links which reach the -
~ 25 2~ 0~27~ ~ :
highest velocities for any motion thus storing the
largest amount of kinetic energy per unit of mass. In
addition, a slight torsional elasticity can be
introduced in this link to deal with fabrication
05 inaccuracies as the regional structure is over-
constrained, without compromising axial rigidity.
-:
Other Possibilities~
As with any mechanism, it is always possible to
exchange the ground link with the output link. In the
case of this invention, the mechanism can be "inverted"
leading to a situation in which all the actuators are ;~-
completely stationary, at the cost of additional joints
in the links as Figures 7 and 8 show. Now the output
link is the platform that links joints J5, J6, J7, J8.
This configuration suffers from a number of
deficiencies, which will now be listed:
(1) Actuator joints Cl, C2, C3, C4 in Figure 7 are
pri~matic and undergo an axial load combined
with bending as the links arising from them act
as cantilevers.
(2) As the platform tilts, actuator Cl will have to
extend as C2 retracts. If a swiveling motion is
required, interference will occur between the
spherical joints and the connecting reducing the ~
workspace. ~ ;
~.~
~ 21~27i~
- 26 -
(3) The kinematic advantage of the actuator rapidly
diminishes when the platform tilts around the J13
axes, whereas it would remain approximately
constant in the previous designs.
05 (4) The need for four load-bearing spherical joints
is a major drawback as they are typically more
costly than revolute joints, introduce backlash,
wear easily, cause friction, occupy space and
are difficult to protect from environmental
conditions.
(5) The same variation is suggested for rotary-type
actuators as in Figure 8. This version has the
drawback of achieving sufficient workspace only
when the lever-arms stemming from the actuated
joints Ml, M2, M3, M4, are made long enough.
This defeats compactness and structural
integrity.
APPLICaTIONS - HIGH PERFORMANCE SHOULDER MECHANISM ;
The robot manipulator ~oint of Figure 9 is
designed to support large loads (up to 150 Nm around any
axis at 350 N/cm2 pressure supply and with 22.2mm bore
219~27~ ~
- 27 -
diameter cylinders), while featuring a large workspace
(90 degrees, 90 degrees, 180 degrees) and low weight. ~ -~
Because of the various properties claimed earlier, a
very simple fabrication process achieves superior ~;
05 performance and the reduction of parts count. In our
laboratory version, each actuator has been instrumented
with position and force transducers.
APPLICATIONS - MINING & CIVIL ENGINEERING APPLIC~T~ONS
In mining applications, machines with high ~ -
structural stiffness and high strength are required.
For example, in a boring machine a simplified version of ;
the invention as shown in Figure 10 can be applied to
produce three degrees of freedom, all controllable with
high power since all four hydraulic actuators (H) can be
applied to contribute to the forward thrust, as
well as generating lateral orientations.
The general roll motion of the cutter head
support is suppressed by means of a prismatic joint
replacing the original cylindrical joint. The
continuous rotary motion required by the cutting head
can be produced by a dedicated independent motor (not
shown) that can be placed behind the structure. A
simplified kinematic structure results from the
reduction of controlled degrees of freedom but the -~
overall principle remains identical.
Because of the differential nature of the
principle of operation, suitable control can be achieved
with simplicity through the use of four-way hydraulic
~ " ~ '
~ 21~27~
. ., . --.
- 28 -
valves and hydraulic circuitry.
APPLICATIONS - OPTICAL ~ND MICRO PRECISION APPLICATIONS
In Figure 11, micro-motion actuators of the
piezo-electric type marked P are used to displace an
05 output platform marked X with micro precision in all
four degrees of freedom. In an optical instrument for
example, a grating can be micro-rotated around the three
principal directions of motion and translated, all in
one single mechanism.
10All joints may be realized by means of thin
sections and thin rods for elimination of backlash and
high rigidity. In this example, the entire body of the
mechanism can be machined out of one single block of ;~ ;~
material, forming the thin sections first, then the
structure, then the four legs with the actuators bonded
in place last. Using various geometries wide ranges of
mechanical gain can be selected for each direction of
motion.
In a micro-surgery application, the output link
may be extended in one or the other directions by a
cantilevered arm. If the lever arm is long with respect
to the other dimensions, the tip will approximate
closely a manipulator with three degrees of freedom of
translation and one of rotation around its principal axis.
APPLICATIONS - JOYSTICK WITH MOTORIZATION
In aircraft control, robotics, forestry,
. . ...
~. 2 ~ ~; 2 7
- 29 -
excavation, and more generally in the operator/computer-
assisted control of machines, joysticks with multi-
degree of freedom are needed. In advanced applications
joysticks are designed to impart forces in the
05 operator's hand.
The embodiment of the invention of Figure 12
offers an opportunity to design such joysticks with a ;~
high degree of simplicity. In these applications
electric actuators are often a prerequisite. Here four
rotary actuators M are employed mounted two-by-two on ^--
coaxial shafts. Note that other opportunities exist to
place actuators N in more favorable positions and
introduce numerous improvements to this basic desiqn.
APPLICATIONS - EXCAVATION, FORESTRY APPLICATIONS.
Conventional excavators and forestry machines
typically use a turret swivelling around a vertical axis
fixed with respect to the chassis of the vehicle, the
seat of the operator swivelling with the rest of the
machine to provide for visual control. When such
machines are used on uneven terrain, the swivelling axis ;~
will not be vertical. It follows that the operator has
to compensate with bending of this back which results in
injuries. The machine itself must be designed to ~-~
accommodate similar stresses.
In the Figure 13 application we employ the
spherical version of the invention to ~eep the turret
horizontal during swivelling, regardless of
21 0927b
- 30 -
the position of the chassis.
This configuration is "inverted" in that the
output link is the top platform.
Because of the large stroke required from the
05 pistons, the geometry has to be made with a long vertical
dimension. In fact in this case, the roll motion is
maximized, possibly in excess of 200 degrees with a
corresponding decrease in the mechanical advantage
around the roll motion, but it is precisely the
direction which requires the least (or less) torque for
this application.
A version of this mechanism may also be used to
form the shoulder of a high performance robotic arm
which displays isotopy. Then the actuators have to be
placed in a fashion which maximizes the usable stroke
and the implementation is slightly more complicated.
In Figure 15 a rotary-attuated version of the
mechani~m suited to pointing an antenna is depicted.
The antenna 50 is carried by end member 51 that is
supported by the universal (or spherical) joint 52. The
5-bar linkages are connected through the universal (or
spherical)joints 53. Spherical joints would be suitable
if the antenna 50 were to exhibit roll motion, as to
allow for the receipt or transmission of polarized radio
signal~.
~: ~ ,','' .
-` ''
21~27~
- 31
Conclusion
The foregoing has constituted a description of
specific embodiments showing how the invention may be
applied and put into use. These embodiments are only
05 exemplary. The invention in its broadest, and more
specific aspects, is further described and defined in ~ ?
the claims which now follow. ;
These claims, and the language used therein, are
to be understood in terms of the variants of the
invention which have been described. They are not to be
restricted to such variants, but are to be read as
covering the full scope of the invention as is implicit
within the invention and the disclosure that has been
provided herein.
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