Ball bearing for mobile robot with single spherical wheel

Abstract

The present invention relates to a sphere having a spherical outer surface comprising N parts (P1, P2, P3, P4), with N = 2K, K being an integer greater than or equal to 2, the parts all having identical overall dimensions, each part comprising a cap-shaped body having an outer face coinciding with the outer sphere, each part comprising three connecting edges (B1, B2, B3), each edge being delimited by an arc of a circle, a first connecting edge being called equatorial (B1), and second and third connecting edges (B2, B3), the angular range of the arc of the equatorial edge being equal to β, with β = 360° / K, each part comprising reinforcing elements, each edge comprising connecting elements. The present invention also relates to a method of assembling the sphere and a mobile robot of the single-wheel spherical type, using such an assembled multi-part sphere. Abstract figure: Figure 3

Description

Title of the invention: Ball bearing for a mobile robot with a single spherical wheel technical field

[0001] The present invention relates to the field of robots, in particular mobile robots, for example, but not exclusively, humanoid robots. More specifically, the invention relates to a mobile robot with a single spherical wheel, said spherical wheel forming the only contact with the ground when the robot moves autonomously or stands motionless in balance.

[0002] The spherical wheel is also referred to here as a 'rolling ball' or simply a 'ball'. The present invention also relates to the structure of the ball in question and its manufacturing process. STATE OF THE ART

[0003] Robots are known that consist of a single spherical wheel (ball) on which the entire robot rests. These types of robots are commonly called "Ballbots." These robots are mobile by means of the single spherical wheel, in all directions, with stability ensured by controlled actuation means that act on the ball to rotate it. Three roller wheels interacting with the spherical surface of the ball can be used as actuation means.

[0004] In particular, the applicant company presented such robots in humanoid form, called Miroki™ and Miroka™. Document FR3142113 presents certain aspects of these robots.

[0005] The ball bearing for these robots must have sufficient rigidity to allow the servo control loop using the three roller wheels to operate effectively. The ball bearing for these robots is made of metal, for example aluminum or steel, using a construction method where two hemispheres are joined together. The weight and moment of inertia of this metal ball bearing are quite substantial and may be considered excessive for certain dynamic situations. Particular attention is also paid to the cost of the ball bearing.

[0006] The ball must resist mechanical wear caused by rough contact surfaces. A high coefficient of friction on its surface and low inertia are advantageous.

[0007] The inventors sought to reduce the weight and moment of inertia of the ball while maintaining good rigidity and a high-quality, non-slip surface finish to ensure proper interfacing with the drive rollers. The inventors also sought to reduce the cost of producing the ball. PRESENTATION OF THE INVENTION

[0008] In this context, the present invention relates, according to a first aspect, to a ball, intended to form a spherical wheel for the movement of a mobile robot, having a spherical outer surface, of radius R and centered on a center of ball, comprising N pieces, with N = 2 x K, K being an integer greater than or equal to 2, the pieces all having identical general dimensions, each piece comprising a cap-shaped body having an outer face coinciding with the outer sphere, each piece comprising three junction edges, each edge being delimited by an arc of a circle having a center of curvature coinciding with the center of ball, a first junction edge being called equatorial, the second and third junction edges joining at a vertex point, the angular range of the arc of the equatorial edge being equal to p, with p = 360° / K, each piece comprising reinforcing elements, each edge comprising connecting elements.

[0009] It must be understood that the parts in question here are formed as separate parts, which, after assembly, together form the rolling ball.

[0010] Thanks to these arrangements, a multi-part ball is proposed, consisting of at least four parts, each smaller than a hemisphere. As will be seen later, the parts can be assembled together by simple clipping (more generally, by the aforementioned fastening means). The resulting solution reduces the cost of the bearing ball. As will be seen later, the parts in question can be made of plastic.

[0011] It is noted that the meeting of the equatorial edges of K pieces forms a diametrical circle, which allows the final assembly of the ball to be carried out from two hemispheres prepared in advance, as will be seen in detail later.

[0012] It should be noted that the proposed solution also makes it possible to reduce the weight and moment of inertia of the ball, which increases the agility of the robot and makes the stability control servo loop more robust.

[0013] For the 'ballbot' type application, the spherical surface of the ball should preferably be continuously spherical, without hollows or bumps, without protruding elements or holes, without facets.

[0014] Two adjacent edges meet at a corner of the piece. The piece thus has three edges and three corners. The arc of the circle delimiting the second edge has a measure of 90° and the arc of the circle delimiting the third edge also has a measure of 90°.

[0015] The reinforcing elements provide good rigidity and resistance to stress of the assembled ball; they can take several forms which will be discussed later.

[0016] According to one embodiment, each edge comprises a docking plane, the docking plane of the second edge being perpendicular to the docking plane of the first edge, and the docking plane of the third edge being perpendicular to the docking plane of the first edge. At the vertex opposite the first edge, the docking plane of the second edge is angularly separated from the docking plane of the third edge by an angle [3], with [3] = 360° / K. Thus, for K=3, [3] = 120°, which is an obtuse angle. For K=5, [3] = 72°. For K=6, [3] = 60°. For K=2, [3] = 180°. The advantageous case K=4 is discussed later.

[0017] Plane-on-plane abutment at the edges joining two adjacent parts makes it possible to efficiently absorb compressive forces, which may exist in particular when a force is exerted on the ball.

[0018] The two right angles are part of a simple geometric definition. Such a shape is easy to demold. If K=4, the vertex angle between the berthing plane of the 2nd edge and the berthing plane of the 3rd edge is also perpendicular, and consequently there are 3 right-angled corners.

[0019] In one embodiment, N = 8 is chosen, i.e., K = 4, and therefore [3 = 90°. This results in a distinctive shape for each piece, with a right angle at each corner, and an overall equilateral triangular shape. The three edges are geometrically identical, and the piece can be mounted in any orientation. Each edge can be described as 'equatorial' because, in fact, each edge shares a great diametral circle with three other edges in the same plane, which can be called the 'equator'.

[0020] According to one embodiment, the connecting elements comprise a male and a female component. The male and female forms cooperate to lock together during assembly. These forms are easy to recognize and intuitive to assemble.

[0021] According to one embodiment, the connecting elements can take the form of hooks. Any clipping system with irreversible or quasi-irreversible locking can be suitable for forming the means of connecting the edges to each other.

[0022] According to one embodiment, the N parts are strictly identical. Consequently, all the parts can be produced from a single mold. The tooling required is thus reduced, both in number and size. Indeed, the size of the part to be formed is on the order of magnitude of the radius R, and not the diameter D=2R as would be the case if a hemisphere were to be formed.

[0023] This solution of strictly identical parts can be applied for all values ​​of K from 2 onwards, it is not only valid for K=4.

[0024] According to another aspect of the invention, the N parts are made of plastic. The use of a plastic molding process makes it possible to obtain complex shapes in the parts that constitute the ball. The use of plastic also makes it possible to obtain a lighter assembled ball with a lower moment of inertia than the metallic solution. The use of plastic to obtain a plurality of moderately sized parts also results in a lower production cost.

[0025] In one particular embodiment, a coating may be provided, obtained for example by an overmolding or dipping operation, which then forms the outer layer of the ball. In another particular embodiment, the parts are made of polymer, for example, fiber-reinforced polyamide.

[0026] It is noted that the ball does not need to be perfectly balanced given the low rotational speeds involved; there is no imbalance effect.

[0027] According to one embodiment, the reinforcing elements are formed as a network of ribs projecting inwards towards the sphere. Some ribs are parallel to an edge of the part. The rib network can form a matrix of intersecting ribs. These ribs significantly limit the deformation of the part, and particularly its bending.

[0028] According to one embodiment, the ball may further comprise a core, each of the N pieces being radially supported on the core directly or indirectly by means of one or more supports. The core is placed at the center of the ball.

[0029] This contributes to the overall rigidity of the assembled ball. The ball's bending under crushing stress is greatly reduced, due to the diametral transfer of forces.

[0030] A deflection criterion of less than 3 mm, or even less than 2 mm, can be aimed for under a diametral crushing force of 100 Newton for a ball of 40 cm in diameter, without these values ​​being limiting.

[0031] According to one embodiment, advantageously, the ball has a radius R of at least 15 cm and a weight of at most 1.5 kg. This provides a lightweight and rigid ball of sufficient size, well suited to the function of a ground interface for a mobile bale-type robot.

[0032] According to another embodiment, the sphere has a radius R of at least 12 cm and a weight of at most 1 kg. According to yet another embodiment, the sphere has a radius R of at least 18 cm and a weight of at most 2 kg.

[0033] According to one characteristic, the sphere is hollow; it is not solid. A solid sphere would have a higher weight. It is noted that the sphere is not pressurized and contains no active organs.

[0034] The present invention relates, according to a second aspect, to a method for assembling a sphere as defined above, in which K=N / 2 pieces are assembled together to form a first hemisphere, then the remaining N / 2 pieces are assembled together to form a second hemisphere, and then the first and second hemispheres together, the ball thus assembled presenting a spherical outer surface.

[0035] The assembly described above is preferably non-dismantable. It would be necessary to break the surface to access the connecting means in order to unlock them.

[0036] According to one feature, the assembly requires no welding or screwing.

[0037] The present invention relates, according to a third aspect, to a mobile robot of the type with a single spherical wheel, said spherical wheel being formed by a ball conforming to the characteristics described above. PRESENTATION OF THE FIGURES

[0038] The invention will be better understood upon reading the following description, given solely by way of example, and referring to the accompanying drawings given by way of non-limiting examples, in which identical references are given to similar objects and on which:

[0039] The [Fig. 1] is a schematic representation of the lower part of an example of a mobile robot with a single spherical wheel, in which the present invention can be implemented;

[0040] Fig. 2 shows an example of a multi-piece ball, according to a first embodiment, with 8 identical pieces, in exploded view, with one piece in transparency;

[0041] Fig. 3 schematically shows the reference frame of the parts in the sphere, with two opposite parts visible in the upper hemisphere;

[0042] Fig. 4 illustrates a top view of an example of balls with 8 identical pieces;

[0043] Fig. 5 is analogous to Fig. 4 and shows a top view of an example of a ball with 8 identical pieces, with a 45° offset from one hemisphere to the other;

[0044] Fig. 6 shows, according to the first embodiment, one of the parts in isolation, seen from the center of curvature;

[0045] Fig. 7 schematically shows the assembly of the parts in an 8-piece / quarter ball;

[0046] Fig. 8 illustrates a variant of the position of the male and female organs arranged in alternating material;

[0047] Fig. 9 illustrates the angles at the corners of the room;

[0048] Fig. 10 illustrates examples of means of joining edges of parts;

[0049] Figure 11 illustrates an embodiment with a core and support elements radials on the nucleus;

[0050] Fig. 12 illustrates in top view an embodiment with six pieces, i.e. K=3 and [3 = 120°.

[0051] Fig. 13 illustrates in perspective view the embodiment of Fig. 12 with six parts.

[0052] Fig. 14 illustrates in cross-section the embodiment of Fig. 12 with six parts.

[0053] Fig. 15 illustrates in top view an embodiment with 10 pieces, i.e. K=5 and [3 = 72°.

[0054] Fig. 16 illustrates in top view an embodiment with 12 pieces, i.e. K=8 and [3 = 50°.

[0055] Fig. 17 illustrates in top view an embodiment with 16 pieces, i.e. K=8 and [3 = 45°.

[0056] It should be noted that the figures set out the invention in detail to enable implementation of the invention; although not limiting, said figures serve in particular to better define the invention where appropriate. DETAILED DESCRIPTION OF THE INVENTION

[0057] The invention relates to a mobile robot with a single spherical wheel, said spherical wheel forming the only contact with the ground when the robot moves autonomously or stands upright in stationary equilibrium. The lower part of the robot is shown in [Fig. 1]. The robot's leg 10 rests on the ball bearing 1, which rests on the ground. A closed-loop control system with known stability per se allows the robot to remain upright continuously without any other point of support on the ground. The robot's leg 10 bears on the ball bearing 1 via three control roller wheels, designated 11, 12, and 13, respectively. Each roller wheel comprises rollers 16 arranged on the circumference of the wheel with an axis perpendicular to the general axis of the wheel A. The wheel 11 is driven by a motor 15 and exerts a tangential force.The coordinated control of the 3 wheel motors allows the ball to rotate in any direction according to the position correction necessary for stability and maintaining an upright posture.

[0058] The ball 1 must resist mechanical wear caused by the rough contact surfaces of the drive wheel rollers. A high coefficient of friction on its surface and low inertia are advantageous. The ball 1 must exhibit good rigidity and high elastic stiffness. The first mode of deformation corresponds to crushing under the vertical weight of the robot; the deformation must be as small as possible. Stability control is all the more robust if the ball remains perfectly spherical while rotating. Its apparent elastic modulus must be uniform in all radial directions.

[0059] For example, the diameter D of the ball can be between 20 cm and 50 cm, without these values ​​being limiting.

[0060] According to the first embodiment illustrated in [Fig. 2], the ball is composed of 8 identical parts. The parts in question are respectively identified as P1, P2, P3, P4, P5, P6, P7, P8 and generically by reference numeral 2.

[0061] Each part 2 comprises a cap-shaped body having an outer face coinciding with the outer sphere of the ball to be formed. As shown in Figures 6 and 9, each part comprises three junction edges labeled Bl, B2, and B3, respectively. Here, each edge forms the interface with only one adjacent part. It will be seen later that, according to certain variations, an edge can form an interface with two adjacent parts. As shown in [Fig. 6], the first edge Bl and the third edge B3 join at a corner E1, the first edge Bl and the second edge B2 join at a corner E2, and the second edge B2 and the third edge B3 join at a corner E3.

[0062] As can be seen in [Fig. 3], each edge is delimited by a circular arc C having a center of curvature coinciding with the center of ball CB, or in other words a radius of curvature corresponding to the radius R of the ball to be formed.

[0063] Figure 3 also illustrates a spherical coordinate system that can be used to geometrically describe the multi-piece ball. The origin of the coordinate system is the center of the ball CB, and an azimuth angle 0 is defined with respect to an arbitrary reference direction 0=0. An elevation angle is defined with respect to an arbitrary reference direction μ=0. The sphere is defined as all points M with coordinates 0, 0 that are located at a given distance R from the center of the sphere CB. By convention for this description, the equator is denoted as the circle of points with coordinates μ=0. Each of the poles O=+90° and -90° is designated as a 'vertex'.

[0064] A first junction edge Bl is called 'equatorial' purely by convention. The second junction edge B2 and the third junction edge B3 join at a point called here vertex S, which is located opposite the first junction edge.

[0065] The first junction edge Bl has a berthing plane Al. The second junction edge B2 has a berthing plane A2 and the third junction edge B3 has a berthing plane A3. After assembly, the berthing planes of two adjacent edges form a plane-on-plane interface which allows compressive forces to be absorbed without substantial deformation.

[0066] The berthing platforms are also equipped with connecting means to link the opposite sides.

[0067] The angular range of the arc of the equatorial edge Bl is here 90°. The arc of the circle delimiting the second edge B2 and the third edge B3 also has an amplitude of 90°.

[0068] According to a general definition of the present invention, the angular range of the arc of the equatorial edge is an angle called p, with p = 360° / K, with K=4 for the first embodiment.

[0069] According to various embodiments, each part includes reinforcement elements 6 which will be seen in detail later.

[0070] Furthermore, each edge includes elements for connecting to other neighboring edges. The connecting means include positioning elements and retention or locking elements.

[0071] According to the example illustrated in figures 2,3,6,7,10, the retention elements 3 comprise a male organ 5 and a female organ 4. The male and female forms (respectively identified as M and F) cooperate together to lock into each other during assembly.

[0072] The female form can include an orifice with a non-return shoulder 42; the male form 5 can include a hook 52 which comes to lodge against the non-return shoulder of the female form.

[0073] The retention elements 3, once locked, prevent the adjacent edges from separating. The circles delimiting the edges thus remain against each other without any gap or gap.

[0074] Furthermore, positioning elements consisting of complementary shapes in the berthing planes may be provided. For example, as illustrated in [Fig.3], the berthing plane A2 includes a projecting shape 53 (generally a projection or convex shape), and the opposite berthing plane A3 includes a recessed shape 43 (generally a depression or concave shape complementary to the shape of the projection opposite).

[0075] These positioning elements make it possible to avoid a radial offset of one edge relative to the other under a radial force supported by a single edge.

[0076] Figure 10 illustrates variants concerning the retention elements. In the detail on the left, in area 10A, the hook 54 is arranged on the radially inner side of the edge and protrudes tangentially from the cap. The hook is anchored to a notch 44 in the opposite edge.

[0077] On the detail on the right, in area 10B, the retention elements are formed as male hooks 55 which hook onto each other.

[0078] According to an advantageous embodiment of the invention, all the parts can be strictly identical. This is the case in the illustrated example of the first embodiment. This can also apply for K other than 4, as will be seen later.

[0079] With reference to the terrestrial analogy presented in [Fig.3], the piece which is in the southern hemisphere corresponds to the opposite piece in the northern hemisphere after pivoting by 180°, the docking plane passing through the equatorial circle EQ.

[0080] According to various embodiments, the parts are made of plastic material.

[0081] Advantageously, the parts are, for example, manufactured from a single mold if they are all identical. Of course, depending on the different solutions for the connecting elements, there could be two different molds.

[0082] The molding injection points are advantageously located in an inner radial area of ​​the cap, so that the outer spherical surface of the cap is free of sprue and need for rework.

[0083] Preferably, the ball is formed simply by assembling the molded parts. However, it is possible to provide a coating to cover the entire spherical surface of the plastic parts once assembled. This coating can be obtained by overmolding or by dipping. It can serve several purposes, particularly if the ball is used in a context other than a mobile robot with a single spherical wheel.

[0084] The reinforcing elements 6 are formed as a network of ribs projecting inwards towards the sphere. Some ribs are parallel to an edge of the part. The rib network can form a matrix of intersecting ribs.

[0085] The arrangement of the ribs in the radially inner part of the cap complements the berthing planes located at the edges of the part.

[0086] As can be seen in [Fig.7], the cap then has a radial thickness denoted DR which can typically be between 0.1 x R and 0.2 x R.

[0087] According to another configuration illustrated in particular in [Fig. 11], a core 7 can be provided in a central position within the sphere. Each of the parts can include arms 81, 82 directed radially inwards which contact the core 7 as shown on the left half of [Fig. 11]. In the case of an integral molding, there could be only one arm.

[0088] In the right part of [Fig.1 1], another solution has been shown where the radial support is formed by a strut 8 formed as a separate part.

[0089] Since each of the parts is radially supported on the core, it is easy to understand that the first mode of elastic deformation of the ball, namely crushing, is strongly limited by the presence of the diametrical transfer of the forces which pass through the core.

[0090] It is noted that the corner chamfers 25 ensure that the contact of the circular arcs of the opposite edges, and that the contact of the corners at the vertices are not hindered by a contact located further inside the sphere.

[0091] Figures 12 to 14 illustrate a second embodiment with a multi-piece ball of 6 pieces, i.e. K=3. It is noted that the upper hemisphere is offset by 60° with respect to the lower hemisphere, i.e. that the edges between the pieces of the upper hemisphere are not aligned with the edges between the pieces of the lower hemisphere.

[0092] Consequently, in this configuration there are no vertex points with 4 branches / segments, there are only vertex points with 3 segments (for example, intersection 9 in [Fig. 13]). It is noted that a particular edge interfaces two other edges in this configuration.

[0093] Conversely, in the configuration of the first embodiment with K=4, all the vertices are 4-segment vertices (cf [Fig.2] and 4).

[0094] A particular arrangement of the retention means in the form of male and female parts is illustrated in [Fig. 8]. An alternation of male and female parts is observed. The parts can be fitted together when one part is rotated 180° with respect to the median axis of edge PMB. Furthermore, it is observed that if an angular offset of [3 / 2] is applied, the complementary parts end up facing each other and fitting is possible. This allows for a general offset of one hemisphere relative to the other, of an angle of [3 / 2], as illustrated in Figures 5 and 13.

[0095] Thus, for the embodiment shown in figures 12 to 14, all the parts are identical to each other.

[0096] The assembly process includes preparing one hemisphere as illustrated in [Fig.7], then preparing another hemisphere, and then assembling the 2 hemispheres.

[0097] Figure 15 illustrates an embodiment with 10 pieces, i.e. K=5 and [3 = 72°]. Figure 16 illustrates an embodiment with 12 pieces, i.e. K=8 and [3 = 50°]. Figure 17 illustrates an embodiment with 16 pieces, i.e. K=8 and [3 = 45°.

[0098] It is preferable that the ball has a continuously spherical surface, however it is not excluded to provide a very small through hole (diameter of the order of 0.5 mm) to allow a suspension wire to pass through in order to suspend the parts and / or the ball in particular during manufacturing operations.

[0099] It should be noted that the ball proposed here can be used in a context other than that of a spherical wheel for moving a mobile robot.

[0100] It should also be noted that the invention is not limited to the embodiments described above. It will indeed become apparent to a person skilled in the art that various modifications can be made to the embodiments described above, in light of the instruction that has just been disclosed to them.

[0101] In the detailed presentation of the invention given above, the terms used shall not be interpreted as limiting the invention to the embodiment set forth in this description, but shall be interpreted to include all equivalents which can be foreseen by a person skilled in the art by applying their general knowledge to the implementation of the teaching which has just been disclosed to them.

Claims

Demands

1. A sphere (1), intended to form a spherical wheel for the movement of a mobile robot, having a spherical outer surface of radius R centered on a sphere center (CB), comprising N parts (P1, P2, P3, P4), with N = 2 x K, K being an integer greater than or equal to 2, the parts all having identical overall dimensions, each part comprising a cap-shaped body having an outer face coinciding with the outer sphere, each part comprising three joining edges (B1, B2, B3), each edge being delimited by an arc of a circle having a center of curvature coinciding with the sphere center (CB), a first joining edge (B1) being called equatorial, the second and third joining edges (B2, B3) joining at a vertex point (S), the angular range of the arc of the equatorial edge being equal to θ3, with θ3 = 3607K, each part comprising reinforcing elements (6), each edge comprising elements of connection (3).

2. Ball (1) according to claim 1, wherein each edge comprises a berthing plane, the berthing plane (A2) of the second edge being perpendicular to the berthing plane (Al) of the first edge, the berthing plane (A3) of the third edge being perpendicular to the berthing plane of the first edge (Al).

3. Ball according to any one of claims 1 to 2, wherein N = 8, i.e. K=4 and therefore [3 = 90°.

4. Ball according to any one of claims 1 to 3, wherein the connecting elements comprise a male organ (5) and a female organ (4).

5. Ball according to any one of claims 1 to 4, wherein the N parts are strictly identical.

6. Ball according to any one of claims 1 to 5, wherein the N parts are made of plastic material.

7. Ball according to any one of claims 1 to 6, wherein the reinforcing elements (6) are formed as a network of ribs projecting into the interior of the sphere.

8. A ball according to any one of claims 1 to 7, further comprising a core (7), each of the N parts being radially supported on the core directly or indirectly through one or more props (8).

9. Ball according to any one of claims 1 to 8, having a radius R of at least 15 cm and a weight of at most 1.5 kg.

10. A method of assembling a ball according to any one of claims 1 to 9, wherein K=N / 2 pieces are assembled together to form a first hemisphere, then the remaining N / 2 pieces are assembled together to form a second hemisphere, then the first and second hemispheres are assembled together, the ball thus assembled having a spherical outer surface.

11. Mobile robot of the type with a single spherical wheel, said spherical wheel being formed by a ball according to any one of claims 1 to 10.