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Title:
A RECONFIGURABLE COMPLIANT REVOLUTE JOINT MECHANISM WITH NONLINEAR STIFFNESS
Document Type and Number:
WIPO Patent Application WO/2020/052724
Kind Code:
A1
Abstract:
The invention is a new design of a compliant revolute joint mechanism, which has multiple configurations exhibiting variable stiffness performance. The joint mechanism provides compliance between its input and output shafts. Variable stiffness performance, such as soft spring, hard spring, in addition to linear spring, can be achieved by changing the mechanism configuration which is based on the wrapping pattern of elastic material, replacing the elastic material of different stiffness, or tuning their pretention force. The invention is designed with a compact structure. The new joint can be integrated into electric motors to build compliant actuators in assistive exoskeletons, rehabilitation robots and other robots where a safe human-robot interaction is concerned. The innovation can also be applied as a new coupling device, which is commonly used in motion transmissions.

Inventors:
BAI SHAOPING (DK)
LI ZHONGYI (DK)
Application Number:
PCT/DK2019/050268
Publication Date:
March 19, 2020
Filing Date:
September 11, 2019
Export Citation:
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Assignee:
UNIV AALBORG (DK)
International Classes:
B25J9/00; B25J13/08; B25J17/02; B25J19/02; B25J19/06; F16D3/62
Foreign References:
SU1622673A11991-01-23
CN101398036A2009-04-01
US4377386A1983-03-22
US4182139A1980-01-08
CN105643659A2016-06-08
US20180092792A12018-04-05
Attorney, Agent or Firm:
PLOUGMANN VINGTOFT A/S (DK)
Download PDF:
Claims:
CLAIMS

1. A compliant joint mechanism (1), comprising

an input shaft (2) directly or indirectly connected to a primary input shaft element (3) in a fixed relation, wherein the primary input shaft element (3) comprises or is coupled to a plurality of input protruding elements (4) arranged in a first predetermined pattern on/in the primary input shaft element (3),

an output shaft (5) directly or indirectly connected to a primary output shaft element (6) in a fixed relation, wherein the primary output shaft element (6) comprises or is coupled to a plurality of output protruding elements (7) arranged in a second predetermined pattern on/in the primary output shaft element (6),

an elastic element (8) wrapped partly around at least one of the input protruding elements (4) and at least two of the output protruding elements (7) in a compliance pattern indirectly coupling the input shaft (2) to the output shaft (5), such that rotation of the input shaft (2) causes rotation of the output shaft (5) and wherein the input shaft (2) can move relative to the output shaft (5) due to the elastic properties of the elastic element (8).

2. A compliant joint mechanism (1) according to claim 1, wherein the joint mechanism (1) further comprise one or more bearings (9) and/or one or more connecting elements (10) and/or one or more housing elements (11), and/or one or more ring elemets (12).

3. A compliant joint mechanism (1) according to claim 1 or 2, wherein the input shaft (2), the output shaft (4), the one or more input shaft elements (9), the one or more output shaft elements (10), the input protruding elements (4) and the output protruding elements (7) are arranged in a way such that the input shaft (2) and output shaft (5) are configured to rotate around a same axis (13) or parallel axes (13a, 13b).

4. A compliant joint mechanism (1) according to any of the preceding claims, wherein the elastic element (8) is a closed elastic band or string with a pre- determined length and stiffness such as a rubber spring, elastic nylon cable, or thin metal strips.

5. A compliant joint mechanism (1) according to any of the preceding claims, wherein the stiffness of the compliant joint mechanism (1) can be increased by increasing the number of input protruding elements (4) and/or output protruding elements (7) around which the elastic element is wrapped and/or by exchanging the elastic element (8) with another elastic element (8) having a higher stiffness and wherein the stiffness of the compliant joint mechanism (1) can be decreased by decreasing the number of input protruding elements (4) and/or output protruding elements (7) around which the elastic element is wrapped and/or by exchanging the elastic element (8) with another elastic element (8) having a lower stiffness.

6. A compliant joint mechanism (1) according to any of the preceding claims, wherein for each input protruding element (4) the elastic element (8) is wrapped around, the elastic element (8) is wrapped around at least two output protruding elements (7) in a way such that the elastic element (8) extend from a first output protruding element (7a), wrap partly around an input protruding element (4), and extend to a second output protruding element (7b), wherein the elastic element (8) extend parallely between the input protruding element (4) and the first and second output protruding elements (7a, 7b).

7. A compliant joint mechanism (1) according to any of the preceding claims, wherein the input protruding elements (4) are arranged such that they are evenly spaced and have the same distance to a center point of the primary input shaft element (3) and the output protruding elements (7) are arranged such that they have the same distance to a center point of the primary output shaft element (6), wherein the distance from the output protruding elements (7) to the center point of the primary output shaft element (6) is larger than the distance from the input protruding elements (4) to the center point on the primary input shaft element (3).

8. A compliant joint mechanism (1) according to any of the preceding claims, wherein the input protruding elements (4) and/or the output protruding elements (7) are arranged in a circle on the primary input shaft element (3) and the primary output shaft element (5), respectively, in a way such that the center point is located on the axis (11) or axes around which the input shaft (2) and/or output shaft are configured to rotate.

9. A compliant joint mechanism (1) according to any of the preceding claims, wherein the input protruding elements (4) and/or the output protruding elements (7) have a circular cross-section, allowing the elastic element (8) to move and stretch around the protruding elements.

10. A compliant joint mechanism (1) according to any of the preceding claims, wherein the primary input shaft element (3) and/or the primary output shaft element (6) are plates, preferably circular plates

11. A compliant joint mechanism (1) according to any of the preceding claims, wherein the primary input shaft element (3) have a smaller surface area and/or diameter than the primary output shaft element (6).

12. A compliant joint mechanism (1) according to any of the preceding claims, wherein the compliant joint mechanism (1) is configured such that the center of the primary input shaft element (3) and the center of the primary output shaft element (6) are arranged on the same axis (13), which axis (13) is parallel to and/or the same as the axis (13) around which the input shaft (2) and the output shaft (5) can rotate.

13. Exoskeleton comprising one or more compliant joint mechanisms (1) according to any of the preceding claims.

14. Transmission device with coupling between input and output comprising a compliant joint mechanism (1) according to any of preceding claims 1-12.

15. Sensing device, such as a force sensing device, comprising a compliant joint mechanism (1) according to any of preceding claims 1-12.

Description:
A RECONFIGURABLE COMPLIANT REVOLUTE JOINT MECHANISM WITH

NONLINEAR STIFFNESS

FIELD OF THE INVENTION

The present invention relates to compliant joint mechanisms.

BACKGROUND OF THE INVENTION

In rehabilitation robots, wearable exoskeletons, collaborative robots, hopping robots and walking machines, it is essential to ensure a safe human-robot interaction or dynamical adaptability. Traditional stiff robotic joints cannot meet these critical requirements due to the mechanical properties of rigid and fast impacts and high contact forces in interaction with a hard object. An effective solution to the problem is the introduction of compliance in the systems. They can be achieved conventionally by active impedance control or introducing inherent compliance into mechanism of joints.

In compliant joints using active impedance control, the compliance relies on an elaborate controller and proper sensor technologies. The limitation of this type of devices is that the controller may fail and introduce instability to the system. Moreover, the control system with high performance is essential for fast response of controller.

In the joints with inherent compliance, physical flexible element is deliberately included to the interposition between the actuator and its output end. An intrinsic safe interaction between robotic system and environment can be obtained because of the passive compliance ability of flexible element to absorb the shock and impact. Compliance joints can be classified into two groups, the fixed compliance joints and the adjustable compliance joints. In the fixed compliance group, compliant property of joint together with compact structure is achieved easily. However, fixed stiffness in the system restricts its usage for general purpose. For the adjustable compliance group, adjustable stiffness mechanism is used to replace conventional flexible elements with constant stiffness, exhibiting varying stiffness. However, a secondary motor is needed to tune the adjustable stiffness mechanism to achieve stiffness variation. As a consequence, most of adjustable compliance joints are heavy and complicated. Moreover, most joint designs are mainly concerned with adjustment of variable stiffness, while very few works concerned with the regulation of elastic behaviour which can improve the performance of system in some specific applications, such as an application where high stiffness is exhibited when subjected to a low external torque, and maintained low stiffness otherwise.

Hence, an improved compliance joint would be advantageous, and in particular a more efficient and/or reliable joint would be advantageous.

OBJECT OF THE INVENTION

It is an 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 joint that solves the above mentioned problems of the prior art.

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 compliant joint

mechanism, comprising

an input shaft directly or indirectly connected to a primary input shaft element in a fixed relation, wherein the primary input shaft element comprises or is coupled to a plurality of input protruding elements arranged in a first predetermined pattern on/in the primary input shaft element, an output shaft directly or indirectly connected to a primary output shaft element in a fixed relation, wherein the primary output shaft element comprises or is coupled to a plurality of output protruding elements arranged in a second predetermined pattern on/in the primary output shaft element,

an elastic element wrapped partly around at least one of the input protruding elements and at least two of the output protruding elements in a compliance pattern indirectly coupling the input shaft to the output shaft, such that rotation of the input shaft causes rotation of the output shaft and such that the input shaft can move relative to the output shaft due to the elastic properties of the elastic element.

The core of the present invention is the design and construct of a novel compliant revolute joint mechanism with reconfigurability. The invention proposes a compliant revolute joint mechanism with reconfigurability which allows for adjustable stiffness and variable stiffness performance based on different configurations. The design is compact and lightweight, and can be easily integrated into a system where compliance is needed.

The invention has many potential applications. For example, a direction

application of the compliant joint mechanism is to design new joints of robots which have close physical interaction with human (e.g. motion assistance exoskeletons, collaborative robots and rehabilitation robots), where variable compliance can be obtained for safety and comfort. Another application is a new coupling device, which is a very common device in motion transmissions. The invention can also be integrated into electric motors, to build compliant motors. For example the invention could be used in a plurality of servo motors and drive, such as in a sewing machine. The invention could also be developed as a force sensing device.

In a first configuration, rotation of the input shaft causes a substantially identical rotation of the output shaft. In a second configuration, the primary input shaft element moves relative to the primary output shaft element and the elastic element is stretched and/or relaxed. This is how compliance is brought into the compliant joint mechanism of the present invention. The length of the elastic element may be determined such that when no external force is applied to either of the shaft elements, the stretching of the elastic element is in a predetermined equilibrium, but when the primary input shaft element is rotated relative to the primary output shaft element, the elastic element is stretched (or relaxed).

The input shaft is configured to be connected to a first external element, such as a motor, and the output shaft element is configured to be connected to a second external element.

In some embodiments, the joint mechanism further comprise one or more bearings and/or one or more connecting elements and/or one or more housing elements, and/or one or more ring elemets. Such elements may be present to stabilize, optimize or protect the compliant joint mechanism. In some embodiments, the input shaft, the output shaft, the one or more input shaft elements, the one or more output shaft elements, the input protruding elements and the output protruding elements are arranged in a way such that the input shaft and output shaft are configured to rotate around a same axis or parallel axes.

In some embodiments, the input shaft and the output shaft have a circular cross- section.

In some embodiments, the elastic element is a closed elastic band or string with a pre-determined length and stiffness such as a rubber spring, elastic nylon cable, or thin metal strips. In some embodiments, the stiffness of the compliant joint mechanism can be increased by increasing the number of input protruding elements and/or output protruding elements around which the elastic element is wrapped and/or by exchanging the elastic element with another elastic element having a higher stiffness. In the same way, the stiffness of the compliant joint mechanism can be decreased by decreasing the number of input protruding elements and/or output protruding elements around which the elastic element is wrapped and/or by exchanging the elastic element with another elastic element having a lower stiffness. In some embodiments, for each input protruding element the elastic element is wrapped around, the elastic element is wrapped around at least two output protruding elements in a way such that the elastic element extend from a first output protruding element, wrap partly around an input protruding element, and extend to a second output protruding element, wherein the elastic element extend parallely between the input protruding element and the first and second output protruding elements.

In some embodiments, the input protruding elements are arranged such that they are evenly spaced and have the same distance to a center point of the primary input shaft element and the output protruding elements are arranged such that they have the same distance to a center point of the primary output shaft element, wherein the distance from the output protruding elements to the center point of the primary output shaft element is larger than the distance from the input protruding elements to the center point on the primary input shaft element.

In some embodiment, the number of output protruding elements is twice that of input protruding elements. However, further protruding elements may be present. Thus, even though an optimal number of input protruding elements relative to output protruding elements may be 1:2, more input protruding element or output protruding element may be present on/in the primary input shaft element or primary output shaft element.

In some embodiments, the input protruding elements and/or the output protruding elements are arranged in a circle on the primary input shaft element and the primary output shaft element, respectively, in a way such that the center point is located on the axis or axes around which the input shaft and/or output shaft are configured to rotate.

In some embodiments, the protruding elements have a circular cross-section, allowing the elastic element to move and stretch one of the protruding elements.

In some embodiments, the protruding elements are pins.

In some embodiments, the primary input shaft element and/or the primary output shaft element are plates, preferably circular plates. However, the primary input shaft element and/or the primary output shaft elements may also have other forms, such as square triangular, hexagonal or the like. Furthermore, they may be plates with a hollow center.

In some embodiments, the primary input shaft element has a smaller surface area and/or diameter than the primary output shaft element.

In some embodiments, the compliant joint mechanism is configured such that the center of the primary input shaft element and the center of the primary output shaft element are arranged on the same axis, which axis is parallel to and/or the same as the axis around which the input shaft and the output shaft can rotate.

In a second aspect, the present invention relates to an exoskeleton comprising one or more compliant joint mechanisms according to the present invention.

In a third aspect, the present invention relates to a transmission device with coupling between input and output comprising a compliant joint mechanism according to the present invention.

In a fourth aspect, the present invention relates to a sensing device, such as a force sensing device, comprising a compliant joint mechanism according to the present invention. Such a device is very sensitive at small torque (or force), thus optimize its performance of sensitivity over its range of measurement.

Terms used in the application are used in a manner being ordinary to a skilled person. Some terms are, however, elaborated below:

"Input" and "output" are. used to indicate e.g. input and output in the sense that input and output resembles where e.g. a rotational force is applied as input and rotational force is picked-up as output. As will be apparent from the description herein, input and output may be reversed as per desire.

"Elastic properties" refers to the fact that the elastic element can streatch and relax from a predetermined equilibrium. The elastic element is in the

predetermined equilibrium when it is wrapped tightly around the input and output protruding elements to couple the input shaft and the output shaft. The elastic element is not completely relaxed in this equilibrium but will be streatched to a predetermined extend. By increasing the number of protruding elements which the elastic element is wrapped around, streaching of the elastic element is increased to a new predetermined equilibrium, thereby increasing the stiffness of the coupling between the input shaft and the output shaft. However, because the elastic element can streatch, the input shaft is able to move relative to the output shaft under some circumstances, making the joint mechanism compliant. The different embodiments of the present invention may each be combined with any of the other embodiments. 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 present invention and preferred embodiments according to the invention will now be described in more detail with regard to the accompanying figures. The figures show ways of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 is a schematic illustration of a simple embodiment of the present invention,

Figure 2 is a schematic illustration of another simple embodiment of the present invention,

Figure 3 - Concept description of reconfigurable compliant joint mechanism : (a) three-dimensional model, (b) exploded view of the three-dimensional model and (c) detailed view of the compliant mechanism,

Figure 4 - Working principle of reconfiguration of the compliant mechanism (a) A design with n= 6. (b) Another design with n= 3,

Figure 5 - Working principle of the realization of nonlinear stiffness in one branch of the compliant mechanism; (a) basic schematic and (b) the design model in the invention,

Figure 6 -Stiffness performance of (a) K eq l , and (b) K eq 2 ,

Figure 7 - Specific nonlinear stiffness profiles of the reconfigurable compliant joint mechanism where n= 6, and

Figure 8 - An implementation case of the invention which is integrated into an elbow exoskeleton. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is made to Fig. 1, illustrating a three-dimensional view of a compliant joint mechanism 1 according to a simple embodiment of the present invention.

Fig. 1(a) shows the mechanism unassembled, whereas Fig. 1(b) shows the mechanism assembled.

The compliant joint mechanism 1 shown in fig. 1 comprise an input shaft 2 and an output shaft 5. The input shaft 2 is configured to be connected to a first external element, such as a motor, and the output shaft 5 is configured to be connected to a second external element.

The input shaft 2 is in the embodiment shown in Fig. 1 directly connected to a primary input shaft element 3 in a fixed relation. In the same way, the output shaft 5 is directly connected to a primary output shaft element 6 in a fixed relation. In Fig. 1, the primary input shaft element 3 and the primary output shaft element 6 are circular plates. However, the invention would also work with other shapes.

The primary input shaft element 3 illustrated in Fig. 1, comprise six input protruding elements 4 arranged in a first predetermined pattern, in this case in a circular pattern, on the primary input shaft element 3. The input protruding elements 4 are shown with dotted lines, as they are arranged on the surface facing away from the input shaft 2. The primary output shaft element 6 comprise twelve output protruding elements 7 arranged in a second predetermined pattern, in this case also a circular pattern on the primary output shaft element 6. The protruding elements 4, 7 are arranged such that the center point is located on the axis 13 around which the input shaft 2 and/or output shaft 5 are configured to rotate. Furthermore, the input and output protruding elements 4, 7 are arranged such that they are evenly spaced and have the same distance to a center point of the primary input shaft element 3 and the output shaft element 6, respectively. The distance from the output protruding elements 7 to the center point of the primary output shaft element 6 is larger than the distance from the input protruding elements 4 to the center point on the primary input shaft element 3. In the embodiment illustrated in Fig. 1, the input protruding elements and the output protruding elements are pins extending from the primary input shaft element subatantially parallel to the input shaft and output shaft. The pins have a circular cross-section and a flat end section. However, in other embodiments, the pins may have other shapes. In other embodiments, the input protruding elements 4 and output protruding elements 7 may not protrude from the primary input shaft element 3 and primary output shaft element 6, respectively, as in Fig. 1, but may be attached on/in it or coupled to them in a plurality of other ways.

The compliant joint mechanism 1 of the present invention further comprise an elastic element 8. The elastic element 8 should wrap partly around at least one of the input protruding elements 4 and at least two of the output protruding elements 7 in a compliance pattern indirectly coupling the input shaft 2 to the output shaft 5. As a result of the elastic coupling, the rotation of the input shaft will cause the output shaft to rotate too. The compliance of the compliant joint mechanism is a result of the elastic properties of the elastic element 8, coupling the rotation of the input shaft 2 and output shaft 5. Depending on the force applied to the compliant joint mechanism 1 and the stiffness of the elastic element 8, the elastic element 8 may streatch when the primary input shaft element 3 begin to rotate. Thus, the primary input shaft elemenet 3 may rotate a short distance relative to the primary output shaft element 6 before the primary output shaft element 6 rotates too and causes the output shaft 5 to rotate with a delay compared to the input shaft 2.

In Fig. 1, the input shaft 2 and output shaft 5 are configured to rotate around the same axis 13. However, in other embodiments, they may rotate around different but parallel axes 13a, 13b.

The elastic element 8 is not very visible in Fig. 1. Reference is made to Fig. 2, 3 and 4 for an illustration of different compliance patterns. Further compliance patterns not illustrated herein would also work.

Reference is made to Fig. 2 showing another simple embodiment of the present invention. The embodiment is similar to that described in relation to Fig. 1, except that in the embodiment illustrated in Fig. 2, the primary input shaft element 3 have a smaller area and/or diameter than the primary output shaft element 6 and the input protruding elements 4 are arranged on the opposite surface of the primary input shaft element 3. Hence, the compliance pattern of the elastic element 8 is visible in Fig. 2b.

The elastic element 8 is preferably a closed elastic band or string with a pre- determined length and stiffness such as a rubber spring, elastic nylon cable, or thin metal strips. In fig. 2b, the elastic element is shown as a string. The stiffness of the compliant joint mechanism can be increased by increasing the number of protruding elements around which the elastic element is wrapped and/or by exchanging the elastic element with another elastic element having a higher stiffness. Thus, the compliant joint mechanism is adjustable and can be used for many different applications.

In the embodiment illustrated in Fig. 2b, for each input protruding element 4 the elastic element 8 is wrapped around, the elastic element is wrapped around two output protruding elements 7 in a way such that the elastic element 8 extend from a first output protruding element 7a, wrap partly around an input protruding element 4, and extend to a second output protruding element 7b. Thus, in the compliance pattern illustrated in Fig. 2b, the number of output protruding elements are twice that of input protruding elements. Importantly, this pattern introduces compliance into the system. However, other patterns of protruding elements and wrapping of the elastic element can also provide compliance into the mechanism.

Reference is made to Fig. 3c and 4 illustating different compliance patterns with different numbers of input protruding elements 4 and output protruding elements.

In Fig. 3c, a primary input shaft element comprising six input protruding elements arranged in a circular pattern on the primary input protruding element.

Furthermore, twelve output protruding elements are shown arranged in a circular pattern, but without being coupled to the primary output shaft element, for simplicity. The distance from each output protruding element to the center point of the primary input shaft element is greater than that of the input protruding elements. Finally, the elastic element 8 couple the input protruding elements 4 and the output protruding elements 7 in a compliance pattern, where for each input protruding element 4 the elastic element 8 is wrapped around, the elastic element is wrapped around two output protruding elements 7 in a way such that the elastic element 8 extend from a first output protruding element 7a, wrap partly around an input protruding element 4, and extend to a second output protruding element 7b. Preferably, the elastic element 8 extend parallely between the input protruding element 4 and the first and second output protruding elements 7a, 7b as illustrated in Fig. 3c. Due to the elastic coupling of the input protruding elements 4 and the output protruding elements 7, the input protruding elements 4, the primary input shaft element 3 and the input shaft 2 can move/rotate relative to the output protruding elements 7, the primary output shaft element 6 and the output shaft 5, by stretching of the elastic element 8 (or relaxation). An example of movement/rotation of the input protruding elements 4 relative to the output protruding elements 7 can further be viewed in Fig. 5, showing two different configurations. In a first configuration, the elastic element is relaxed (or as relaxed as it can be in the elastic pattern). In a second

configuration, the primary input shaft element moves relative to the primary output shaft element and the elastic element is stretched. This is how compliance is brought into the compliant joint mechanism of the present invention.

In Fig. 4, different configurations are shown and the elastic element is wrapped around a different number N of input protruding elements 4. Thus, even though an optimal number of input protruding elements relative to output protruding elements may be 1:2, more input protruding element 4 or output protruding element 7 may be present on/in the primary input shaft element 3 or primary output shaft element 7, such as illustrated in Fig. 4b, where six input protruding elements 4 and six output protruding elements 7 are present. Fig. 3 shows a preferred embodiment of the present invention. The compliant joint mechanism 1 illustrated in Fig. 3b comprise an input shaft directly connected, in one end, to a primary input shaft element 3 in a fixed relation. The primary input shaft element 3 comprise input protruding elements 4 arranged in a first predetermined pattern on the primary input shaft element 3. The output shaft 5 is indirectly connected to a primary output shaft element 6 in a fixed relation and is configured to be coupled to output protruding elements 7 by reciveing them in holes arranged around its circumference. The primary output shaft element 6 is circular but have a hollow center area and comprise holes around its circumference configured for receiving output protruding elements 7. The primary output shaft element 6 is further configured to be coupled to a connecting element 10a in the form of a housing with a hole in its end wall (not visible), which is again configured to be connected to a connecting element 10b in the form of a plate, directly coupled to the output shaft. The connecting element 10b is configured to be inserted into the hole of connecting element 10a and fastened to it. In that way, the primary output element is indirectly connected to the output shaft 5, through connecting elements 10a, 10b, in a fixed relation. Thus, if the primary output shaft element is rotated, the output shaft rotate too. The output protruding elements 7 are not attached to the primary output shaft element in fig. 3b, but are configured to be fit into the holes around the circumference of the primary output shaft element. In that way the input shaft and the output shaft is coupled through the elastic element 8 wrapped around the input and output protruding elements 4,7.

In other words: he new compliant revolute joint mechanism is shown in Figure 3, wherein Figure 3(a) shows the mechanism assembled. Figure 3(b) presents an exploded view of the mechanism, and Figure 3(c) shows detailed view of compliant mechanism. The compliant revolute joint mechanism in Figure 3 consists of two coaxial shafts (shaft A and shaft B denoting the input and the output of mechanism) which rotate around axis A and axis B, respectively. The two shafts are coupled through elastic element (e.g. rubber spring, elastic nylon cable, or thin metal strips ), which introduces compliance between the input and the output of the joint. In Figure 3(b), the housing parts are designed to support the compliant mechanism and transmission components. As shown in Figure 3(c), the shafts A and B, and pins A and B together with the elastic element form a reconfigurable compliant mechanism. Two ending plates are designed on the shaft A and shaft B separately. A number of pins are evenly mounted on the two end-plates. The elastic element wraps around the pins to couple the rotation from the shaft A to the shaft B. The stiffness of coupling is determined by the elastic element, the number of pins and also the pattern of wrapping, as demonstrated in Figure 4. In other words, the mechanism can be reconfigured to change its stiffness performance. The joint mechanisms is featured with nonlinear stiffness. This is achieved by utilizing a four-bar linkage of zero base link, as illustrated in Figure 5. Refer to Figure 5b, the length of the elastic element in the configuration is:

l = [(2 R 1 + b) a + (2R 2 + b) b + (2R 3 + b) g + 2 l BD + 2l GE ]/2. (1) In Figure 5, and T are the tension force along bar-2 and the external equilibrium torque applied on the joint, respectively. From the static equilibrium analysis, we have

T = J - F, (2) where, J = 3Q is the Jacobian.

Based on Eqn. (2), the equivalent rotation stiffness of joint is given as

The tension force F is associated to the stiffness of elastic element k and the pretension of elastic element F 0 , which is rewritten as

F = k - (l— l\e=o + F Ol e=0 - l r ) = k - (l - I| e=0 ) + F 0 . (4) where 1\ q=0 represents the length of bar-2 when Q = 0, l r is the free length of elastic element, F 0 represents the pretension of elastic element when Q = 0.

In the design, each pair of pins in the input-output shafts stand for a four-bar linkage. Assume that N four-bar linkages are used in the mechanism, the total stiffness of joint becomes

where two terms K eq l and K eq 2 have different influence on the overall stiffness. They behave respectively as a hard spring with zero-stiffness at singularity (Q = 0) and a soft spring within the working range as demonstrated in Figure. 6, where e\ Keq 2=0 stands for the value of Q which satisfies K eq 2 = 0.

In Eqn. (5), F 0 is the pretension force of the elastic element. It is associated to the free length of the elastic element l r , elastic element stiffness k and the mechanism configuration, which is given by

F 0 = N - k - Al. (6) where Al is the elongation of the single spring (elastic element). From Eqns. (5) and (6), it is seen that stiffness performance is influenced by both the mechanism configuration and the elastic element property. Through the changing of mechanism configuration, variable stiffness can be achieved, as shown in Figure 7.

An example is included to illustrate the variable stiffness performance of the invention shown in Fig. 3. A rubber spring (elastic element) with k = 0.07 N/mm and l r = 120mm is used in the invention. The elastic element is selected such that its free length l r is smaller than 1\ q=0 for any given configuration N. Thus, the elastic element is always stretched, and the tension force F increases with the increasing of Q. The stiffness performance of the invention with respect to configuration N and shaft rotation Q is shown in Fig. 7. Stiffness varying in two modes, namely hardening and softening modes can be observed. In addition to changing configuration, the invention can also change its design with different elastic element of different free length and stiffness to achieve varying stiffness performance.

With the invention, reconfiguration is feasible not only by changing the number of pins, but also by pattern of wrapping. Herein, we use the number of pins on the shaft A, n, to describe a reconfigured design. Fig. 4 illustrates two designs of the mechanism where n= 6 and n= 3. In Fig. 4(a), a design with 12 pins on shaft B and 6 pins on shaft A is demonstrated. In this design, six configurations can be implemented by different wrapping. Fig. 4(b) shows another design with 3 pins on shaft A, and three configuration can be implemented.

The invention has many potential applications:

A direction application of the compliant joint mechanism is to design new joints of robots which have close physical interaction with human (e.g. motion assistance exoskeletons and rehabilitation robots), where variable compliance can be obtained for safety and comfort.

Another application is a new coupling device, which is a very common device in motion transmissions.

The invention can also be integrated into electric motors to build compliant motors.

The invention can be developed as a force sensing device.

Robot industry, such as in 4-axis SCARA robots, and 6-axis

general purpose robots Compliant sewing machine servo motor and drives

An implementation case of the invention which is integrated into an elbow exoskeleton is illustrated in Fig. 8. In this case, the elbow joint is driven by a DC motor with worm gear. The invention is mounted on the interposition between the worm gear and the fore-arm link of elbow exoskeleton. Consequently, the invention allows an improvement of the physical human-robot interaction of the elbow exoskeleton due to the compliant and back-drivable property of the new mechanism.

Note that for fig. 6 and 7 0/rad should be understood as 0[rad] and KeQ / l/N^mm/rad should be understood as K eq,l [N*mm/rad].

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. Also, 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.

Itemized list:

1. Reconfigurable compliant revolute joint mechanism comprising input shaft parts and output shaft parts coupled through elastic element (e.g. rubber spring, elastic nylon cable, or thin metal strips).

2. Reconfigurable compliant revolute joint mechanism according to item l, wherein - the input shaft parts comprise an input shaft which transfers the outside torque or rotation to the mechanism and an end-plate designed on the input shaft and a plurality of pins circumferentially mounted on said end- plate,

- the output shaft parts comprise an output shaft which transfers the torque or rotation inside the mechanism to the external and an end-plate designed on the output shaft and two times number of pins of input shaft parts circumferentially mounted on said end-plate,

- the elastic element wraps around the pins of the input shaft parts and the output shaft parts to eventually couple the rotation or torque from the input shaft to the output shaft.

- the input shaft and the output shaft are coaxial.

3. Reconfigurable compliant revolute joint mechanism according to item 1 and 2, wherein reconfigurability is achieved through adjusting the number of pins, such as selecting 6 pins on the input shaft and 12 pins on the output shaft, such as 3 pins on the input shaft and 6 pins on the input shaft.

4. Reconfigurable compliant revolute joint mechanism according to item 1 and 2, wherein reconfigurability is also achieved through pattern of wrapping of elastic element.

5. Reconfigurable compliant revolute joint mechanism according to item 1, 2 and 4, wherein said pattern of wrapping of elastic element is determined by the number of coupled pins between input shaft parts and output shaft parts through the elastic element.

6. Exoskeleton built with compliant joint(s) comprising a reconfigurable compliant revolute joint mechanism according to any of the preceding claims.

7. Transmission devices with coupling between input and output comprising a compliant joint mechanism according to any of itemsl to 5.

8. Compliant joint mechanism according to any of itemsl to 5, is used as new force sensing devices, which is very sensitive at small torque (or force), thus optimize its performance of sensitivity over its range of measurement. LIST OF REFERENCE SYMBOLS USED

1 Compliant joint mechanism

2 Input shaft

3 Primary input shaft element

4 Input protruding elements

5 Output shaft

6 Primary output shaft element

7 Output protruding elements 8 Elastic element

9 Bearings

10 Connecting elements

11 Housing elements

12 Ring elements

11 Axis