CA2109276A1

CA2109276A1 - Mechanisms for orienting and placing articles

Info

Publication number: CA2109276A1
Application number: CA 2109276
Authority: CA
Inventors: Vincent Hayward; Fabienne Reynier
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-10-26
Filing date: 1993-10-26
Publication date: 1995-04-27

Abstract

ABSTRACT
A mechanism for orienting an end member utilizes paired five-bar linkages wherein four joints are actuated. Preferably the actuators are mounted at the base-link of the five bar linkages, such base-links being colinearly aligned. The opposed corner links of each five-bar linkage are coupled through joints to the member to be moved.

Description

210927~ ~
...... , .-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 2l0~27e 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.
.. . . ~
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 -~'"~,'',~''' ~' ~ " '" ' '" '' '' ~ ' ,-' "' ` ,,,", ,,,,> i ~" ,~ "";,,~ ,, ", ~ , " ~ "

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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 .. .- . ~
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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~ 21~- ~37 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 . . ~
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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~ 210~27~ .-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 .. . ~, .- .
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.
, ., ;~ . -

(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 ~ 210~27~ ~

- 15 -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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- 16 - -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 i '. ~ "r~ "~ "", ,;; j ~j , ~ ~

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- 17 -- 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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- 18 -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 ~ 21 0~27~

- 19 - ' --~
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 210~27~

- 20 -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 ''' ~' ~;~

~ 21~27~

- 21 -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 ~`" 210~27~

- 23 -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 ~! 2 1 0 ~ 2 7 ~

- 24 -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.
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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. ~ ;

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(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~ ~

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~
. ., . --.

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 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 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~

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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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY IS CLAIMED AS FOLLOWS:

1. 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 joint;
(b) first and second proximal links 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, wherein the actuated 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 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.

2. A mechanism as in claim 1 wherein said first and second actuated joints are rotational joints positioned respectively between the ends of the base link and the driven joint.

3. A mechanism as in claim 1 wherein said first and second actuated joints are rotational joints positioned respectively at the ends of the base between such ends and the respective proximal links.

4. A mechanism as in claim 1 wherein said first and second actuated joints are sliding joints positioned respectively between the proximal and distal links.

5. A mechanism as in claim 1 wherein the said twinned universal joints each comprise three revolute joints, two of which joints have coinciding axes and are connected to the distal links, all of said joints having a common centre of rotation.

6. A mechanism as in claim 1 wherein the said twinned spherical joints each comprise four revolute joints, the first and second of which are respectively connected to distal links having coinciding axes, all of the axes of said joints having a common centre of rotation.

7. A mechanism as in claims 1, 2, 3, 4 ,5 or 6 further comprising a cylindrical joint positioned between the end member and the base.