Double cardan joint with rolling contact movement through flexible cross elements

Abstract

Joint mechanism (in particular for realizing an external kinematics corresponding to a rolling contact movement) with two rotational degrees of freedom, comprising: a. a basic element (1); b. an end element (2) movable relative to the base element (1); c. a central double universal joint (3) which connects the base element (1) to the end element (2) in an articulated manner and defines two axes of rotation which are essentially orthogonal to one another; characterized by d. at least three flexible cross-clamping elements (4) which are clamped crosswise between the base element (1) and the end element (2) in such a way that they effect an inverse kinematic coupling of the rotational movements of the individual cardan joints formed by the double cardan joint (3); and e. a design of the deflection points of the flexible cross clamping elements (4) and their offset (v) such that the effective length of the clamping elements (4) remains substantially constant during a joint movement.

Description

[0001] The invention relates to a joint mechanism with the external kinematics of a rolling contact joint, also known as a quaternion joint, with two rotational degrees of freedom, preferably for use in robot arms, lightweight robots, handling devices or other kinematic chains.

[0002] Conventional 2-DOF robot joints are often based on serial arrangements of two single rotary joints. This design has disadvantages such as heavy actuators located in the arm, increased inertia, susceptibility to singularities (see gimbal lock), and limited range of motion. Remote drives using belts, cables, or gears can optimize the actuator position, but often result in complex systems and do not solve the singularity problem.

[0003] Alternative approaches utilize the principle of rolling contact. For example, US Pat. No. 3,932,045 A describes one-dimensional rolling contact joints with flexible bands, for example, for prostheses, but these lack the second degree of freedom. A 2-DOF rolling contact mechanism is shown in WO 2016 / 123139 A2 for surgical applications, but it is not designed for the requirements of robotic joints.

[0004] For robotics applications, so-called "omni-wrist" mechanisms (see, for example, EP 1 023 653 B1, EP 0 393 024 B1, EP 0 295 244 B1) are known. These enable 2D rolling contact motion and a wide range of motion of approximately ±90 degrees without actual physical rolling. Like similar solutions, also known as quaternion joints, rolling contact joints, or specific names (High Angle Active Link, i-Wrist, LIMS2-AMBIDEX), they often have a complex mechanical structure with many individual joints and additional elements such as gears. This complicates manufacturing and leads to mechanical play.

[0005] Other concepts, as disclosed in KR 2022 0083815 A, which combine three degrees of freedom in a fixed center of rotation, often offer less space for movement and do not offer the kinematics of a moving center of rotation, which is advantageous for cable feedthroughs, as is the case with rolling contact movements.

[0006] Due to the limitations of existing solutions, there is still a need for a robot joint that combines the advantages of rolling contact motion (large, singularity-free range of motion, good cable routing) with a mechanically simple, cost-effective, and low-backlash design. Especially for lightweight and humanoid robots, optimized solutions that equally combine high performance, low weight, and compactness are lacking.

[0007] The present invention is therefore based on the object of creating a joint mechanism with two rotational degrees of freedom and rolling contact motion that overcomes the disadvantages of the prior art. In particular, the aim is to create a joint mechanism that is characterized by a mechanically simple and cost-effective design, has low or eliminable play, offers good feasibility for a variety of ropes or cables, is highly robust, and enables a large, singularity-free range of motion.

[0008] The above-mentioned task is solved by a joint mechanism characterized by the features described below. The joint mechanism essentially comprises: a. A base element (1) which is provided for the fixed connection of the joint mechanism to a base, for example the floor, or to a preceding link of a kinematic chain or a robot arm. b. An end element (2) that is movable relative to the base element (1) and forms the distal end of the joint. This end element (2) executes the movement corresponding to a rolling motion relative to the base element (1). Subsequent elements of a robot arm, further links of a kinematic chain, or an end effector can be attached to the end element (2). c. A double universal joint (3), which is arranged as the central element forming the main structure between the base element (1) and the end element (2). This double universal joint (3) realizes the rotational degrees of freedom of the joint mechanism. The structurally defined distance between the two articulation points of the double universal joint (3) largely determines the virtual diameter of the balls, whose rolling movement mimics the external kinematics of the joint. The double universal joint (3) ensures a constant distance between the base element (1) and the end element (2) and prevents unwanted relative rotation of these two elements about the longitudinal axis of the joint. It also serves to transmit torque about this central axis. d. Flexible cross-tensioning elements (4), for example designed as flexible bands, cables, or tensile plastic elements. Preferably, two pairs of these tensioning elements are provided, but at least three individual elements, each of which is tensioned crosswise between the base element (1) and the end element (2) in such a way that they effect an inverse kinematic coupling of the rotational movements of the two individual cardan joints formed by the double cardan joint (3). The shape and offset (v) of the connection points or deflection points (bending surfaces) of these cross-tensioning elements (4) on the base element (1) and the end element (2) are designed such that the effective length of the tensioned elements changes only negligibly during movement of the joint across its working range. This helps to maintain an essentially constant preload in the cross-tensioning elements (4). e. Drive cables (5) that actuate the joint mechanism and precisely adjust its position in both rotational degrees of freedom. At least three drive cables (5) are required to achieve a defined spatial orientation of the end element (2). If four drive cables (5) are used, they can advantageously be arranged as two opposing cable pairs. With such a paired arrangement, the total cable length of each pair can be kept constant during a movement of the joint, which enables drive via, for example, two motors, each with a simple cable pulley. f. A central guide (6) located inside the joint mechanism. This central guide (6) serves to keep the flexible cross clamping elements (4) at a defined radial distance from the central longitudinal axis of the double cardan joint (3). This supports the generation of a defined preload of the cross clamping elements (4) around this central axis. Furthermore, the specific shape of the central guide (6) can help to further minimize the change in length of the cross clamping elements (4) across different joint positions.

[0009] Actuation of the drive cables (5) causes the end element (2) to move relative to the base element (1), whereby the joint executes the external kinematics of a 2-DOF rolling contact or quaternion joint. The central double universal joint (3) provides the two orthogonal main axes of rotation, ensures a constant distance between the base element (1) and the end element (2), and prevents relative rotation of the two elements around the longitudinal axis of the joint.

[0010] The inverse coupling of the two individual cardan joints of the double cardan joint (3), required for the characteristic rolling contact movement, is realized by the flexible cross clamping elements (4). These are clamped crosswise between the base and end elements, thus forming a coupling structure—for example, functional antiparallelograms when the clamping elements are arranged in pairs. A movement in one of the cardan joints leads to a defined countermovement in the other cardan joint via this coupling through the cross clamping elements (4).

[0011] If such a coupling structure formed by the clamping elements (e.g., an antiparallelogram structure) moves around one of its effective short sides (corresponding to a rotation in one of the cardan joints), the virtual intersection point of the clamping elements or an equivalent kinematic point rotates on an elliptical path. By specifically designing the bending or rolling points of the clamping elements (4) - which, depending on the considered axis of rotation of the joint, either lie in the plane of one of the cardan joints of the double cardan joint (3) or have a defined geometric offset (v) from this plane - the shape and position of this elliptical path can be optimized so that it coincides very well with an ideal circular path over the relevant range of motion of the joint. This allows calculated changes in the length of the clamping elements (4), for example in a simplified geometric model, to be reduced to a few thousandths of their total length.Any remaining minimal length differences can be further compensated or compensated for by additional details in the rolling shape of the clamping elements themselves, their inherent bending properties, and / or the specific shape of the central guide (6). This precise length compensation is crucial to enable play-free operation of the joint with appropriate preload of the clamping elements (4).

[0012] The structure of the joint allows for the passage of numerous additional cables or wires for downstream actuators or sensors. This facilitates the serial arrangement of multiple joints. While changes in the length of the wires must be taken into account when moving upstream joints, this also creates opportunities for synergistic effects, such as coordinated movements of multiple joints to increase force.

[0013] The joint mechanism according to the invention is characterized by the following advantageous technical effects: • Simplified and cost-effective manufacturing is made possible by the use of standard components and easily manufactured parts, including flexible cross-tensioning elements (4) made from readily available materials such as cable ties. • The kinematic coupling using flexible cross clamping elements (4) reduces the number of components and the number of rigid joints, resulting in reduced mechanical complexity and low susceptibility to backlash. The preload capability and optimized length consistency of the clamping elements (4) also enable essentially backlash-free motion transmission. • The joint's structure ensures excellent access for numerous supply lines and drive cables. This facilitates the construction of modular kinematic chains of serially connected joints and opens up potential for synergistic effects such as coordinated movements or increased force. • The elasticity of the joint can be specifically adjusted by the design of the flexible cross-tensioning elements (4), for example for shock absorption or to adjust the mechanical impedance. • The essentially constant total length of paired drive cables (5) during the joint movement leads to a considerably simplified drive mechanism and control.

[0014] Fig. 1a-d show an embodiment of the joint mechanism according to the invention in different views and positions; Fig. 1a shows a side view of the joint in neutral position; Fig. Figure 1b shows a perspective view of the joint in neutral position; Fig. 1c shows a joint position deflected by 70◦ in side view; Fig. Figure 1d shows a perspective view of a joint position deflected by 70◦.

[0015] The invention is described below with reference to a Fig. 1a to 1d are explained in more detail.

[0016] The illustrated joint mechanism comprises a base element (1) and a relatively movable end element (2). These are connected by a central double universal joint (3), which defines the two orthogonal main axes of rotation, maintains a constant distance between (1) and (2), and prevents rotation about the longitudinal axis.

[0017] In the illustrated embodiment, the inverse kinematic coupling of the individual cardan joint movements is achieved by two pairs of flexible cross-tensioning elements (4). These are offset by 90° to each other and pre-tensioned crosswise between the base element (1) and the end element (2). Their deflection points are designed in such a way that changes in length during the movement (visible in the deflected position in Fig. 1c and Fig. 1d) can be minimized, which supports a backlash-free function.

[0018] The joint is actuated by four drive cables (5), which engage the end element (2) as two opposing pairs, also offset by 90°. This arrangement ensures a substantially constant total cable length per pair and allows for simple drive by two actuators. A central guide (6) determines the radial spacing of the tensioning elements (4). The design allows for the passage of up to 32 additional cables or ropes.

[0019] In a particularly cost-effective miniature variant, the flexible cross-clamping elements (4) can also be realized by means of tension-resistant, commercially available cable ties, which effect the inverse coupling and underline the simple structure of the joint. List of designations 1 basic element 2 end element 3 double cardan joint 4 Flexible cross clamping elements 5 drive cables 6 Central Management QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 3 932 045 A

[0003] WO 2016 / 123139 A2

[0003] EP 1 023 653 B1

[0004] EP 0 393 024 B1

[0004] EP 0 295 244 B1

[0004] KR 2022 0083815 A

[0005]

Claims

[1] Joint mechanism (in particular for realising an external kinematics corresponding to a rolling contact movement) with two rotational degrees of freedom, comprising: a. a basic element (1); b. an end element (2) movable relative to the base element (1); c. a central double universal joint (3) which connects the base element (1) to the end element (2) in an articulated manner and defines two axes of rotation which are essentially orthogonal to one another; characterized by d. at least three flexible cross-clamping elements (4) which are clamped crosswise between the base element (1) and the end element (2) in such a way that they effect an inverse kinematic coupling of the rotational movements of the individual cardan joints formed by the double cardan joint (3); and e. a design of the deflection points of the flexible cross clamping elements (4) and their offset (v) such that the effective length of the clamping elements (4) remains substantially constant during a joint movement. [2] Joint mechanism according to claim 1, characterized by Drive means (5), preferably in the form of at least three drive cables, for moving the end element (2) relative to the base element (1). [3] Joint mechanism according to claim 2, characterized by that four drive cables (5) are provided as two opposite pairs, the total cable length of each pair being kept substantially constant during joint movement, so that each cable pair can be driven by a single actuator. [4] Joint mechanism according to one of the preceding claims, characterized bya central guide (6) which serves to space the flexible cross clamping elements (4) from the central longitudinal axis of the double cardan joint (3), to deflect them in a defined manner and / or to support a pre-tension of the cross clamping elements (4). [5] Joint mechanism according to one of the preceding claims, characterized by that preferably two pairs of flexible cross-clamping elements (4) are provided, which are arranged offset from one another by substantially 90°. [6] Joint mechanism according to one of the preceding claims, characterized by that the flexible cross-tensioning elements (4) are designed as flexible bands, ropes or tensile plastic elements, in particular as cable ties. [7] Joint mechanism according to one of the preceding claims, characterized bythat the elasticity of the joint mechanism can be specifically adjusted by selecting the material, the dimensioning and / or the pre-tensioning of the flexible cross-tensioning elements (4). [8] Joint mechanism according to one of the preceding claims, characterized by that the base element (1) and the end element (2) are predominantly made of parts that can be manufactured using 2D manufacturing processes and / or the double cardan joint (3) consists of standard components, thereby enabling a cost-effective construction. [9] Joint mechanism according to one of the preceding claims, characterized by that the central structure of the joint, in particular the double cardan joint (3), allows a passage for a large number of additional ropes or cables for downstream components, such as additional joints or end effectors, thereby promoting serial arrangements and the realization of synergy effects.

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