Prosthetic wrist unit

Abstract

A prosthetic wrist unit includes a drive motor, to be interposed between the distal end of an arm prosthesis and a prosthetic terminal device for positioning the terminal device at desired orientations. A reduction mechanism includes first and second cycloidal reduction stages, connected to each other, the first stage having a central wheel connected to the motor shaft, one or more satellite wheels rotatably placed on a planet carrier, and a fixed outer ring gear, the satellite wheels being in peripheral contact with the central wheel and the outer ring gear, the planet carrier being connected to the second stage. The central wheel, the satellite wheels and the outer ring gear have smooth mutually engaging surfaces, causing a rolling of the satellite wheels on the outer ring gear by friction forces generated by radial interference between the central wheel, the satellite wheels and the outer ring gear.

Claims

1. A prosthetic wrist unit comprising: a drive motor, adapted to be kinematically interposed between a distal end of an arm prosthesis and a prosthetic terminal device, so as to position the prosthetic terminal device at desired orientations with respect to the arm prosthesis; and a reduction mechanism comprising a first epicycloidal reduction stage and a second cycloidal reduction stage, connected to each other, wherein the first epicycloidal reduction stage comprises a central wheel connected to a shaft of the drive motor, one or more satellite wheels rotatably placed around their own axis on a planet carrier, and a fixed outer ring gear, the one or more satellite wheels being in peripheral contact with the central wheel and with the fixed outer ring gear, the planet carrier being connected to the second cycloidal reduction stage, and wherein the central wheel, the one or more satellite wheels, and the outer ring gear have smooth mutually engaging surfaces, in such a way that a rolling of the satellite wheels on the outer ring gear is caused by friction forces generated by radial interference between the central wheel, the one or more satellite wheels, and the outer ring gear.

2. The prosthetic wrist unit according to claim 1, wherein the first epicycloidal reduction stage is placed proximally and is connected to the shaft, and the second cycloidal reduction stage is placed distally and is connected to a terminal device fastening element.

3. The prosthetic wrist unit according to claim 1, wherein the planet carrier is provided with one or more pins for engaging the one or more satellite wheels, the one or more satellite wheels being provided with one or more seats for housing the one or more pins, the one or more housing seats being larger than the one or more engagement pins.

4. The prosthetic wrist unit according to claim 2, wherein the second cycloidal reduction stage comprises an input shaft connected to the first epicycloidal reduction stage and provided with one or more eccentric portions with respect to an axis of the input shaft, a fixed outer cam defining an inner lobing and centered around the axis of the input shaft, and one or more cycloidal elements, the one or more cycloidal elements being engaged on one or more eccentric portions of the input shaft so as to be rotatable by eccentric orbital motion with respect to the axis of the input shaft, and being provided with a satellite outer lobing meshing peripherally on said inner lobing of the fixed outer cam, and having a plurality of holes in which a corresponding plurality of protrusions of an output rotating element engages, the output rotating element being connected to the terminal device fastening element.

5. The prosthetic wrist unit according to claim 4, wherein the second cycloidal reduction stage comprises at least two cycloidal elements, which are offset from each other with respect to the axis of the input shaft.

6. The prosthetic wrist unit according to claim 4, wherein the fixed outer cam is formed by a plurality of dowels parallel to each other arranged to form a crown around the axis of the input shaft and angularly equally spaced apart so as to form cam seats for engaging with the outer lobing of the cycloidal elements.

7. The prosthetic wrist unit according to claim 4, wherein the output rotating element comprises two separate parts, a first part of which is placed proximally with respect to the cycloidal elements, and a second part of which is placed distally with respect to the cycloidal elements, the first part and the second part being connected to each other by said protrusions.

8. The prosthetic wrist unit according to claim 4, wherein the planet carrier of the first epicycloidal reduction stage is integrally engaged with the input shaft of the second cycloidal reduction stage by shape coupling.

9. The prosthetic wrist unit according to claim 1, wherein the second cycloidal reduction stage has a recess in a central area of a proximal end surface, the recess in an assembled condition housing at least part of the first epicycloidal reduction stage.

Description

[0055] These and other features and advantages of the present invention will become clearer from the following description of some non-limiting exemplary embodiments illustrated in the attached drawings in which:

[0056] FIG. 1 shows a longitudinal sectional view;

[0057] FIG. 2 shows a cross-sectional view of the first epicycloidal stage;

[0058] FIG. 3 shows a cross-sectional view of the second cycloidal stage;

[0059] FIGS. 4 and 5 show two different axonometric views of the assembled prosthetic unit.

[0060] In the sectional view of FIG. 1, all the components of the prosthetic wrist unit object of the present invention are visible.

[0061] The prosthetic unit is active and therefore comprises a drive motor 3. The motor 3 can be of any currently known type, preferably it is an electric motor, such as a commercial brushless motor. The motor 3 is provided with an output motor shaft 30, which transmits the motion to the downstream kinematic chains. The prosthetic unit is suitable for being kinematically interposed between the distal end of an arm prosthesis and a prosthetic terminal device, such as a hand prosthesis, for positioning the terminal device at desired orientations with respect to the arm prosthesis. The motor 3 is positioned at the proximal end 6 of the prosthetic unit.

[0062] At the distal end 7 of the prosthetic unit, i.e., the end for the connection with the prosthetic terminal device, a terminal device fastening element 4 is positioned.

[0063] The terminal device fastening element 4 is rotatably actuated by the motor 3 by means of a reduction mechanism, housed within an outer frame 5 made up of a cylindrical casing.

[0064] The reduction mechanism comprises a first epicycloidal reduction stage 1 and a second cycloidal reduction stage 2, connected to each other. The first stage 1 is placed proximally and is connected to the motor shaft 30, and is thus suitable for low driving torques and high speeds; the second stage 2 is placed distally and is connected to the terminal device fastening element 4 and is suitable for being used for high driving torques and low speeds.

[0065] The first stage 1, shown in FIG. 2, comprises a central wheel 10 connected to the motor shaft 30, in particular keyed or forced onto the motor shaft 30. In the preferred exemplary embodiment shown in the figure, the central wheel 10 is made up of a bush with a rated outer diameter of 2.95 mm keyed onto the motor shaft 30 and, therefore, has a smooth coating surface.

[0066] The first epicycloidal stage 1 further comprises three satellite wheels 11 rotatably placed around its own axis on a planet carrier 12, angularly equally spaced apart from each other by 120°. The satellite wheels 11 are placed in peripheral contact with the central wheel 10 and have smooth engaging surfaces. The planet carrier 12 is provided with engagement pins 122 of the satellite wheels 11 suitable for engaging in corresponding housing seats 121 provided in the satellite wheels 11. The housing seats 121 have a diameter greater than the engagement pins 122 so as to create a dimensional gap to allow the satellite wheels 11 to adapt automatically to the real size and tolerances of the central wheel 10 and of the fixed ring gear 13. The satellite wheels 11 are preferably made up of ball bearings. In this case, the housing seats 122 are made up of the central holes of the ball bearings. In the preferred exemplary embodiment shown in the figure, the satellite wheels 11 have a rated outer diameter of 9 mm and rotate about their axis positioned at a diameter of 12 mm on the planet carrier 12.

[0067] The first stage 1 also comprises a fixed outer ring gear 13. The first stage 1 is shaped in such a way that the satellite wheels 11 are in peripheral contact simultaneously with the central wheel 10 and the outer ring gear 13. In the preferred example shown in the figures, the outer ring gear 13 has a rated inner diameter of 20.8 mm. The outer ring gear 13, too, has the engaging surface, i.e., the surface facing inwards, being smooth. The first stage 1 is therefore a roller-based epicycloidal reduction stage, in which the central wheel 10, the satellite wheels 11 and the outer ring gear 13 have smooth mutually engaging surfaces, so that the rolling is caused by the friction forces generated by radial interference between the components.

[0068] The entire first stage 1 is implemented through an appropriate design effort on compacting.

[0069] The planet carrier 12 represents the output of the first stage 1, being connected to the second stage 2. Therefore, in the first stage 1, the satellite wheels 11 rotate on the fixed outer ring gear 13 under the action of the central wheel 10 and transmit the motion to the planet carrier 12 and, accordingly, to the second stage 2. In the preferred example shown in the figures, the planet carrier 12 consists of two distinct parts joined together by means of the engagement pins 122. However, alternatively, the planet carrier 12 may be provided in a single piece and with engagement pins 122 fixed in a cantilevered manner.

[0070] In the preferred exemplary embodiment shown in the figures, the first stage 1 is an epicycloidal mechanism with a reduction ratio of about 8:1.

[0071] The second cycloidal stage 2, shown in FIG. 3, comprises an input shaft 20 connected to the first stage 1. The input shaft 20 is integrally engaged with the planet carrier 12 of the first stage 1 by shape coupling. The input shaft 20 is provided with two eccentric portions with respect to its own axis, a first eccentric portion 201 of which being placed proximally and a second eccentric portion 202 being placed distally and offset by 180°. The two eccentric portions 201 and 202 are placed adjacent to each other for the sake of compactness, but may also be provided spaced apart.

[0072] The second stage 2 further comprises a fixed outer cam 23 defining an inner lobing and centred around the axis of the input shaft 20.

[0073] The second stage 2 further comprises two cycloidal elements 21 and 22 in the form of disks. A first cycloidal element 21 is engaged on the first eccentric portion 201 of the input shaft 20 by means of a ball bearing 211, and a second cycloidal element 22 is engaged on the second eccentric portion 202 of the input shaft 20 by means of a ball bearing 221, and they are therefore both rotatable by eccentric orbital motion with respect to the axis of the input shaft 20. The cycloidal elements 21 and 22 are each provided with a satellite outer lobing meshing peripherally with the inner lobing of the outer cam 23.

[0074] The cycloidal elements 21 and 22 are offset from each other by 180° with respect to the axis of the input shaft 20, the two eccentric portions 201 and 202 on which they are fixed being offset from each other by a corresponding angle.

[0075] The outer cam 23 is formed by a plurality of dowels 230, e.g., of a commercial type, parallel to each other, arranged to form a crown around the axis of the input shaft 20 and angularly equally spaced apart from each other. The dowels 230 are held in place by a supporting fixed outer ring gear 232, integral with the outer frame 5. Compartments or recesses forming cam seats 231 for engaging with the outer lobing of the cycloidal elements 21 and 22 are formed between consecutive dowels 230. The supporting fixed outer ring gear 232 is divided into two parts and is assembled during assembly. Thirty-four dowels 230 are mounted on the supporting fixed outer ring gear 232 and, accordingly, thirty-four cam seats 231 are created between the dowels 230.

[0076] The cycloidal elements 21 and 22, in the preferred exemplary embodiment shown in the figure, have a pitch diameter of 24.75 mm and have 33 lobes and 33 compartments, i.e., one less than those of the fixed outer cam 23, to achieve the desired reduction.

[0077] The two cycloidal elements 21 and 22, therefore, keeping the contact on the dowels 230 fixed in the supporting outer ring gear 232, reduce the motion as they have one less lobe than the cam seats 231 existing between the dowels 230 of the supporting outer ring gear 232; after a complete revolution, each cycloidal element will have been left behind with respect to the supporting fixed outer ring gear 232 of the arc of circle equal to the missing cam seat 231, and the speed at which the two cycloidal elements 21 and 22 lose ground with respect to the outer frame 5 is output by the output rotating element 24 to reduce the incoming motion impressed by the motor 3.

[0078] The two cycloidal elements 21 and 22 have a plurality of holes 210 and 220 placed in a mating position in an assembled condition. A corresponding plurality of protrusions 242 of an output rotating element 24 engages in the holes 210 and 220, being such as to constantly achieve contact during the envelope motion of the cycloidal elements 21 and 22 during their eccentric rotation. The output rotating element 24 is connected to the terminal device fastening element 4. The output rotating element 24 is rotatably engaged with the outer frame 5 by means of two ball bearings 50 and with the input shaft 20 by means of two ball bearings 26.

[0079] The output rotating element 24 comprises two separate parts, a first part 240 of which is placed proximally with respect to the cycloidal elements 21 and 22, and a second part 241 is placed distally with respect to the cycloidal elements 20 and 21. The protrusions 242 consist of six pins 243 which connect the first part 240 and the second part 241 to each other, each pin 243 being covered by a bush 244 with reduced rolling resistance friction. Such bushes 244 have a diameter of 4.5 mm, whereas the holes 210 and 220 in the cycloidal elements 21 and 22 have a diameter of 5.04 mm, thereby considering the 0.26 mm eccentricity.

[0080] In the preferred exemplary embodiment shown in the figure, the second stage 2 is a cycloidal mechanism with a reduction ratio of 33:1.

[0081] Therefore, in the preferred exemplary embodiment shown in the figure, the reduction mechanism of the prosthetic unit has a total reduction ratio of about 264:1.

[0082] The second stage 2 has a recess 25 in the central area of the proximal end surface, in particular obtained by suitably shaping the first part 240 of the output rotating element 24. Such recess 25 in the assembled condition of the first stage 1 and of the second stage 2 houses part of the first stage 1, in particular part of the planet carrier 12.

[0083] FIGS. 4 and 5 show two different axonometric views of the assembled prosthetic unit, wherein the cylindrical frame 5 surrounding the first stage 1 and the second stage 2 is visible.

[0084] The terminal device fastening element 4 has an end nut 40 provided with engagement teeth suitable for connection with the terminal device. Advantageously, the terminal device fastening element 4 is fixed to the second part 241 of the output rotating element 24 by means of through screws which engage with corresponding threaded seats provided in the pins 243, as shown in FIG. 1. The terminal device fastening element 4 also has slots 41 for the passage of control and supply electrical wires.

[0085] The motor 3 is enclosed by a cover 31, which can be fixed to the frame 5 by means of screws. The cover 31 is provided with a plurality of electrical terminals 8 for connecting the motor 3 to a supply unit and/or a control unit, not shown in the figures.

[0086] The reduction mechanism may comprise further reduction stages and may be varied to adapt the reduction required for other application cases.

[0087] Weights may be optimized by using the so-called technopolymer materials.