FIELD OF THE INVENTION The present invention relates to spherical joint mechanisms which can be used e.g. in relation to assistive exoskeletons. In particular, it relates to such joint mechanisms comprising a double parallelogram mechanism.

BACKGROUND OF THE INVENTION

An exoskeleton is a robotic device that is capable of producing supplementary muscular function of weakened body limbs. This enables the user to lift a greater load or compensate for a lack of strength. In exoskeleton designs, the mechanical structure of the exoskeleton has to duplicate the movement of the human skeleton joint to which it is connected remotely from the human body. Especially, complex joints of the human with more than one degree of freedom, such as the glenohumeral joint at the shoulder joint or the hip joint, can be described as a ball and socket joint. Previous attempts to duplicate a shoulder joint in an exoskeleton design have been inadequate for various reasons. In some cases, like the 3-revolute (3R) serial linkage type mechanisms, the parts of the mechanisms are positioned far from the human body to avoid collision with the human and still maintain the large range of motion required by the joint. Such examples have been presented in "D. Naidu, R. Stopforth, G. Bright, S. Davrajh, A 7 DOF exoskeleton arm :

Shoulder, elbow, wrist and hand mechanism for assistance to upper limb disabled individuals, IEEE AFRICON Conference, Sep.2011, pp. 13-15" and in "J. C. Perry, J. Rosen, S. Burns, Upper-limb powered exoskeleton design, IEEE/ASME Trans. Mechatronics, vol. 12, no. 4, Aug. 2007, pp. 408-417". Alternative approaches, like the mechanisms using a circular guide, can make the design more compact. However, these designs often have a heavy and complicated construction. Such an example was presented in "Y. Jung, J. Bae, Kinematic analysis of a 5 DOF upper-limb exoskeleton with a tilted and vertically translating shoulder joint, 2013 IEEE/ASME International Conference on Advanced Intelligent Mechatronics

(AIM),July 2013, pp. 1643-1648". Conventional designs of spherical shoulder joints, such as for use in an

exoskeleton, use a serial linkage system with 3-revolute joints in which the axes intersect remotely at a point, the centre of rotation. A problem with these 3R mechanisms is their singular configurations that occur when the three axes become coplanar, which results in the loss of a degree-of-freedom. Additionally, this kind of mechanism suffers from a low stiffness, which can lead to the centre of rotation shift, either causing an unexpected motion, or making the user feel uncomfortable.

A further problem with the 3-revolute joint design is its workspace limit. The user of the exoskeleton can only raise the upper arm a small angle in the frontal plane before the shoulder mechanism collides with his/hers shoulder, neck or head. To avoid this problem, some alternative designs of the exoskeletons have been proposed in which they were designed so that so that the singular configurations and collision problem of the 3R mechanism occur at postures that are less likely for the user to reach.

OBJECT OF THE INVENTION

It is an object of at least some embodiments of the present invention to provide a spherical joint mechanism that allows a compact and lightweight design which makes it easier to wear, when the mechanism is incorporated in an exoskeleton. It is an object of at least some embodiments of the present invention to provide a spherical joint mechanism with a relative large range of motion free of

singularities and a high overall stiffness as compared to prior art, thus a more compact and lightweight design can be achieved. It is another object of at least some embodiments of the present invention to provide a spherical joint mechanism which, when incorporated in an exoskeleton, makes it possible to replicate the three rotations in the shoulder joint without the exoskeleton colliding with the person wearing the exoskeleton. It is a further object of the present invention to provide an alternative to the prior art.

In particular, it may be seen as an object of the present invention to provide a spherical joint mechanism that solves the above mentioned problems of the prior art with

SUMMARY OF THE INVENTION Thus, the above-described object and several other objects are intended to be obtained in a first aspect of the invention by providing a spherical joint

mechanism comprising two revolute joints joined by a double parallelogram linkage. By "spherical" is preferably meant that there exists a centre of rotation for the mechanism to produce spherical motions.

In preferred embodiments of the invention, the double parallelogram linkage may comprise a first linkage part hingedly connected to a first revolute joint at a distal end of the first linkage part and a second linkage part hingedly connected to a second revolute joint at a distal end of the second linkage part, the first linkage part may comprise a first link arm and a second link arm, which first and second link arms are arranged to move parallel to each other, the second linkage part may comprise a third link arm and a fourth link arm, which third and fourth link arms are arranged to move parallel to each other, and a proximate end of the first linkage part and a proximate end of the second linkage part may be mutually hingedly connected.

The proximate end of the first linkage part and the proximate end of the second linkage part may be mutually connected via three joints having axes of rotation perpendicular to longitudinal extensions of the first and second linkage parts.

A spherical joint mechanism according to the invention may comprise a third revolute joint connected to the connection between the proximate end of the first linkage part and the proximate end of the second linkage part. In some embodiments of the invention, axes of rotation of the revolute joints may coincide at one remote centre of rotation. A spherical joint mechanism according to the invention may further comprise at least one motor, which at least one motor is arranged to actuate at least one of the revolute joints.

In embodiments comprising at least one motor, the at least one motor may be mounted in line with a base line of the revolute joint actuated by the motor.

In other embodiments comprising at least one motor, the at least one motor may be mounted offset from the base line of the revolute joint actuated by the motor. In a second aspect, the present invention relates to an exoskeleton with a shoulder joint comprising a spherical joint mechanism according to the first aspect of the invention.

In a third aspect, the present invention relates to an exoskeleton with a hip joint comprising a spherical joint mechanism according to the first aspect of the invention.

Thus, the core of the present invention is to design and construct a novel spherical joint mechanism comprising a double parallelogram linkage which connects two revolute joints for three degrees of freedom rotations. The joint mechanism can e.g. be used in a shoulder joint of an exoskeleton as will be explained in further details below and in relation to the figures. In general, it can be used in any similar mechanisms wherein a remote centre of rotation is needed, for example, the remote centre of rotation to guide the instruments in computer assisted surgeries.

The first, second and third second aspects of the present invention may each be combined with any other aspects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE FIGURES

The joint mechanism according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 schematically shows a three-dimensional view of a conceptual design of a spherical joint mechanism with a double parallelogram mechanism. Figure l.a shows the mechanism assembled, and figure l.b shows the mechanism in an exploded view.

Figure 2 schematically shows the kinematic structure for the mechanism in figure 1.

Figure 3 shows a curve of the manipulability index of the spherical mechanism in figure 1. Figure 4 schematically shows an embodiment of the present invention

implemented as a shoulder joint for an exoskeleton.

Figure 5 schematically shows an alternative embodiment of the present invention. DETAILED DESCRIPTION OF AN EMBODIMENT

Figure 1 schematically shows a three-dimensional view of a conceptual design of a spherical mechanism with double parallelogram mechanism according to an embodiment of the invention. Figure l.a shows the mechanism assembled, and figure l.b shows the mechanism in an exploded view. Not all the references are on both figures; this is to obtain more clear figures. In the following, reference will be made to a shoulder mechanism, since that is the application that is illustrated in figure 4. However, as mentioned above, other uses are also covered by the scope of the present invention. The spherical shoulder mechanism in figure 1 comprises two revolute joints, referred to as actuators in the figure, connected by a double parallelogram mechanism (DPM) with a remote centre of rotation. The DPM consists of six links (labelled Link 1, 2, 3, 4, 5 and 6 in figure l. b) that are connected to each other via seven joints (labelled A, B, C, D, E, F and G in figure l.a). As the name implies, the mechanism consists of two parallelograms. The first parallelogram is made up by joints A, B, C and D, while the second

parallelogram is made by joints D, E, F and G. In the embodiment shown in figure 1, Link 1 is fixedly connected to Actuator 1, and Link 6 is fixedly connected to Actuator 3. The mechanism in figure 1 comprises a third driven revolute joint, labelled Actuator 2.

As seen in the figure, the double parallelogram linkage DMP is constituted by a first linkage part 10 hingedly connected to a first revolute joint, driven by Actuator 1, at a distal end and a second linkage part 20 hingedly connected to a second revolute joint, driven by Actuator 3, at a distal end . The first linkage part 10 comprises a first link arm Link 2 and a second link arm Link 3, which first and second link arms are arranged to move parallel to each other. The second linkage part 20 comprises a third link arm Link 4 and a fourth link arm Link 5, which third and fourth link arms are arranged to move parallel to each other. A proximate end of the first linkage part 10 and a proximate end of the second linkage part 20 are mutually hingedly connected via Joints C, D and F. In this embedment, Actuators 1, 2 and 3 each consists of an electric motor and a HarmonicDrive gear. As best seen in figure l. b, Links 2 and 3 are each constituted by two components moving in parallel to ensure a stable design. The kinematic structure of the DPM, as illustrated in figure 2, is made up of six parameters; four link lengths and two offset angles. L and L ₂ are the lengths of the first parallelogram and L ₃ and L ₄ are the lengths of the second parallelogram . The two offset angles φ and φ ₂ offsets the remote centre of rotation (RC) from the links 1 and 6 to optimize the effective range of motion when adding two adjoining revolute joints. The lines L and L ₂ mark the offset in figure 2. The minimum angle, i.e. θ ₂ , of the DPM is the sum of the two offset, since a smaller angle of θ ₂ causes either Joint B to cross Link 5 or Joint E to cross Link 2. The maximum angle of the DPM is constrained by the human body. Increasing θ ₂ from its minimum angle moves Joint D closer to RC and thus the human body. When a mechanism as described above is used in upper limb exoskeletons such as shown in figure 4, the combination of three rotations about the three axes can produce the spherical motions needed, depending on the installation of the embodiment. The actuator 1 will produce the abduction/adduction for the arm of the person wearing the exoskeleton, the joint between the linkage parts follows the arm internal/external rotations, while actuator 3 produces the

extension/flexion.

The manipulability index of the Spherical Mechanism is expressed solely by the angle of the DPM and expressed as:

μ = I sin θ ₂ \

The manipulability index is a measure of the kinematic performance of a robotic mechanism. It is also known as Yoshikawa's manipulability index [T. Yoshikawa, "Manipulability of robotic mechanisms," The International Journal of Robotics Research, vol. 4, no. 2, pp. 3-9, 1985.]. It is a quality measure, which is able to describe the best postures for the mechanism and its singular configurations.

Figure 3 shows a curve of the manipulability index as a function of θ ₂ . From the expression above and the curve in figure 3, it is see that the mechanism in the range of 0° to 180° only has a singular configuration at the extremities, i.e. 0° and 180°. Hence, the mechanism offers a considerable large range of motion free of singularities. Figure 4 schematically shows a spherical joint mechanism according to the invention incorporated in an exoskeleton. The exoskeleton shown in the figure is intended as a part of a portable exoskeleton for elderly people to assist them in their daily activities. For the proposed exoskeleton design, the two revolute joints can be actuated by a Flat DC motor (e.g. EC60 from Maxon motors) and a

Harmonic Drive Gear (CSD-25-2A from Harmonic Drive) each, while the DPM is left passive. As seen from the figure, the novel shoulder mechanism has a compact design without compromising the range of motion. Due to the parallel structure, the design is more compact compared to other known solutions. This allows a closer fit to the human which is needed for portable exoskeletons. Figure 5 schematically shows an alternative embodiment of the present invention. The proposed design consists of two revolute joints that are connected together via four links in the same manner as shown in Fig. 1. The four links form a double parallelogram mechanism DPM, which under the given configuration forms a remote centre of motion mechanism. The proposed design is constructed as a hybrid mechanism. The benefit of the proposed design is that the risk of collision with the user is minimized compared to the classical 3R mechanism having two links in series. Moreover, the structure is more compact, lighter and less complicated compared to the 3R mechanism using a circular guide. In the embodiment in figure 5, Actuator 2 is omitted and has left this connection as a passive one. However, if needed Actuator 2 can have the same configuration as Actuators 1 and 3, i.e. an electric motor and a gear as explained above in relation to figure 1. The kinematics of the proposed mechanism is formulated based on Denavit-

Hartenbergs convention. Cartesian coordinate frames are attached to each link of a manipulator, as shown in Figure 5. The corresponding DH parameters can be obtained as listed in Table I. Using these parameters the rotation matrix is obtained as:

R = where e^ ^and θ ₃ are the joint angles. In addition, c and s stands for cosine and sine functions, respectively.

TABLE I

DENAVIT-HARTENBERG PARAMETERS OF THE PROPOSED MECHANISM

Link, / ^' H

1 0 0 0

2 £ ₃ sin0 ₂ -90° 0 90° + φ ₁ - θ ₂

3 L ₂ 0 0 180° - φ ₁ - φ ₂ + θ ₂

4 L ₃ 0 0 90° + φ ₂ - θ ₂

5 L^in^ 90° 0

The inverse kinematics problem is solved for the three joint angles: θ ₂ = arccos(r ₃₃ )

θ _ί = arctan(-r ₂₃ /s0 ₂ , -r ₁₃ /s0 ₂ )

θ ₃ = arctan(-r ₂₃ /s0 ₂ , r ₃₁ /s0 ₂ ) where r _j stands for the (i,j)th element of the matrix R. It should be noted that there are two possible solutions for the second equation, but given the allowable range of motion only the solution between 0 and 180° is used.

The velocity and singularity analysis of the DPM can be performed by deriving the Jacobian for the angular velocities: where θ = [θ ₁ θ ₂ θ ₃ ] ^τ is a vector with the joint angular velocities, ] _ω is the Jacobian and ω ₆ = [ω _χ ω _γ a½] ^r is the end-effector angular velocities. The Jacobian is found as:

A common measure of the evaluating the performance of a mechanism is the manipulability index μ, which is defined as:

From the last shown equation, it is clear that the kinematic performance of the DPM only depends on the angle of the double parallelogram. The manipulability index over the range of motion of the parallelogram is shown in figure 3, where it is seen that the DPM is at a singular configuration in the case that the joint axes constitute a common plane (θ ₂ = Ο,π). Due to the two offset angles, the proposed design has a minimum angle of 0 ₂ ,mm = Φι + Φι · Thus, the singular configuration of θ ₂ = 0 is not obtainable. A range of motion for shoulder internal/external rotation of 135° is sufficient for most of our activities of daily living. Hence, the mechanism is free of singularities and covers the required range of motion if the maximum angle satisfies the following condition θ _2ιΊηαχ = 0 ₂ ,mm + 135° < 180°. Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. In addition, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.