MECHANICAL JOINT WITH SELECTABLE TRANSMISSION MODE

DESCRIPTION Field of the invention

The present invention relates to a mechanical joint wherein different transmission modes can be selected.

In particular, the invention relates to a mechanical joint which can be fixed to robotic systems, orthosis, prosthesis and exoskeletons .

Description of the prior art

As well known, recently it has become indispensable to make joints capable to provide a certain mechanical yielding in applications where the robotic systems have human interaction .

In fact, with respect to the stiff joints, the joints having mechanical yielding offer the following advantages:

- reducing the severity of the stresses on the transmission, since part of the kinetic energy of the impact turns into potential energy accumulated by the joint (for example, as elastic potential energy) ;

- increasing the safety of the interaction man- machine reducing the damages caused by the collision of the mechanism with human operators; - reducing the need for very high precision in defining paths followed by the robotic arms.

The easiest method to provide a yielding joint is to interpose a passive elastic element in series between motor and load (SEA, Series Elastic Actuator) . Such system is easy to produce but it has a limited versatility, since the amplitude of the supportable load regimes is constrained by the stiffness of the elastic element located in series with the motor.

In order to overcome this limitation, the so-called VSA

{Variable Stiffness Actuator) have been introduced, which have an elastic element of variable stiffness interposed between the motor and the load. The stiffness is made variable by a secondary actuating mechanism that change the configuration of the elastic element changing for example the preload or the shape (and then its stiffness) . These systems, properly controlled, are today the more efficient and are used in the field of the humanoid robotics, of the rehabilitation and also in industry. The continuous control of the stiffness is however rather complex in these systems, which still have limitations regarding weight and size.

Furthermore, in such fields it is essential to simulate the effective yielding of the biological joint, returning in visual and touch an impression more natural both to the user and to the external observer, improving the cosmetics: this element is crucial since it affects the statistics of acceptance of the prosthetic limb by the amputee.

In JP2013047563 is described a friction for switching between two transmission modes by means of a cam mechanism easy to carry out. In particular, by centrifugal effect, the mechanism allows to switch between a constrained position, in which a worm screw is connected to a rotating shaft, and a released position, in which the two elements do not rotate together .

However, notwithstanding the cam mechanism allows to switch continuously and quickly between a mode and the other, the device above described does not allow selecting more than two mode and even less than to select one mode that transmits the movement in a yieldable way. Summary of the invention

It is therefore a feature of the present invention to provide a mechanical joint that allows to vary the transmission modes adapting to loading systems even very different from each other.

It is also a feature of the present invention to provide such a mechanical joint that allows to faithfully simulate the actual yieldability of the biological joint.

It is still a feature of the present invention to provide such a mechanical joint that has size and weights reduced with respect to the prior art. It is a further feature of the present invention to provide such a mechanical joint that is easy to carry out and control .

These and other objects are accomplished by a rotational joint arranged to operate selectively at least two modes of transmission of a rotational movement between an input, arranged to carry out a rotation having angular acceleration ω about a longitudinal axis y , and an outlet, arranged to carry out a rotation having angular acceleration ϋ about the same longitudinal axis y , said rotational joint comprising :

- a driving plate integrally connected to the inlet and arranged to carry out a rotation having angular acceleration ω about longitudinal axis y , said driving plate comprising a plurality of radial housings;

- a transmission ring connected to the outlet and co-axial to the driving plate, said transmission ring arranged to carry out a rotation having angular acceleration φ about the longitudinal axis y ;

- a plurality of connection elements subjected to the resilient force of a plurality of springs, said resilient force arranged to push the connection elements towards a plurality of slots located between the driving plate and the transmission ring, said connection elements being configured, when they are located in the slots, to come into simultaneous contact with the driving plate and the transmission ring, generating by friction a constraint of fixed joint between the driving plate and the transmission ring, said constraint adapted to generate the condition ω = φ;

- a plurality of selectors arranged to slide radially in the radial housings between a free position, where the selectors are adapted to counteract the resilient force and to remove the connection elements by the slots, and a constrained position, where the connection elements are located in the slots;

- a cam equipped with a plurality of elongated holes ;

said selectors comprising a plurality of maneuvering portions constrained to slide in the plurality of elongated holes,

said cam being arranged to carry out a rotation, relatively to the driving plate, about the longitudinal axis y , said rotation generating a sliding of the maneuvering portions in the elongated holes causing the movement of the selectors between the free position, where the acceleration φ is independent from the acceleration ω, and the constrained position, wherein ω = φ,

whose main feature is that the outlet is connected alternatively, to:

- a robotic limb;

- an exoskeleton device;

- an orthosis;

- a prosthesis;

- a combination of the above.

The rotational joint according to the present invention allows therefore to switch between two or more transmission modes by simply rotating the cam. Such transition needs therefore of very low power with respect to the prior art, since it is enough to exceed the resilient force of the springs that push the selectors.

In particular:

- If the outlet is connected to a robotic limb, the joint connects two arms of the robot, and the joint can be active and/or passive;

- If the outlet is connected to an exoskeleton device, such as an arm exoskeleton, the joint is located between the arm structure and the forearm structure, and it is an active joint;

- If the outlet is connected to an orthosis, such as a tutor for the elbow or ankle, the joint is located between the arm structure and the forearm structure, and it is a passive joint;

- If the outlet is connected to a prosthesis, such as an external prosthesis of the elbow, the joint replaces the anatomical joint and connects the prosthetic arm to the socket or to the plant integrated to the bones, and the joint can be active and/or passive.

Advantageously, the transmission ring is resiliently connected to the outlet by an elastic constraint arranged to provide the condition ΰ = k(p , in such a way that:

- when the selectors are in the free position, the acceleration ϋ is independent from the acceleration ω and the outlet can rotate freely with respect to the input, obtaining the "free" transmission mode;

- when the selectors are in the constrained position, there is ϋ = kq> = ko and the outlet rotates in a way according to the inlet but not integrally to it, obtaining the "elastic" transmission mode.

This way, then the joint allows, with very low power, to switch between a "free" transmission mode and an "elastic" transmission mode.

Alternatively, the transmission ring is integrally connected to the outlet obtaining the condition ϋ = φ, in such a way that: - when the selectors are in the free position, the acceleration ΰ is independent from the acceleration ω and the outlet can rotate freely with respect to the input, obtaining the "free" transmission mode;

- when the selectors are in the constrained position, there is ϋ = φ = ω and the outlet rotates integrally to the input, obtaining the "rigid" transmission mode.

This way, then the joint allows, with very low power, to switch between a "free" transmission mode and a "rigid" transmission mode.

In particular, the driving plate comprises a plurality of additional radial housings and the rotational joint comprises furthermore:

- an additional transmission ring integrally connected to the outlet (190) and co-axial to the driving plate, said additional transmission ring arranged to carry out a rotation having angular acceleration φ' = ΰ about the longitudinal axis y ;

- a plurality of additional connection elements subjected to the resilient force of a plurality of additional springs, said resilient force arranged to push the additional connection elements towards a plurality of slots located between the driving plate and the additional transmission ring, said additional connection elements being configured, when they are located in the slots, to come into simultaneous contact with the driving plate and the additional transmission ring, generating by friction a constraint of fixed joint between the driving plate and the additional transmission ring, said constraint adapted to generate the condition ω = φ' ;

- a plurality of additional selectors arranged to radially slide in the additional radial housings between a free position, where the additional connection elements are located in the slots, and a constrained position, where the additional selectors are adapted to counteract the resilient force and to remove the additional connection elements by the slots;

said cam being equipped with a plurality of additional elongated holes and said additional selectors comprising a plurality of additional maneuvering portions constrained to slide in the plurality of additional elongated holes,

said cam being arranged to carry out a rotation about the longitudinal axis y, said rotation generating a sliding of the additional maneuvering portions in the additional elongated holes causing the movement of the additional selectors between the free position, where the acceleration φ' is independent from the acceleration ω, and the constrained position, wherein ω = φ' ,

in such a way that :

- when both the selectors that the additional selectors are in the free position, the acceleration ΰ is independent from the acceleration ω and the outlet can rotate freely with respect to the input, obtaining the "free" transmission mode;

- when the selectors are in the free position and the additional selectors are in the constrained position, there is ϋ = φ' = ω and the outlet rotates integrally to the input, obtaining the "rigid" transmission mode;

- when the selectors are in the constrained position and the additional selectors are in the free position, there is ϋ = kq> = ko and the outlet rotates in a way according to the inlet but not integrally to it, obtaining the "elastic" transmission mode.

This way, then the joint allows, with very low power, to switch between a "free" transmission mode, an "elastic" transmission mode and a "rigid" transmission mode. It is therefore clear that the present invention provides a rotational joint that can change its own stiffness, switching between different transmission modes ("free", "elastic" and "rigid") needing an operating power much less than the prior art devices {Variable Stiffness Actuator) . While the present joint, in fact, needs simply a power sufficient to exceed the resilient force of the springs 141 and 142, in prior art this power must be able to exceed the preloading of the main spring, i.e. that one that confers yieldability to the joint itself. Such spring has a stiffness obviously much higher and therefore the power that must exceed its preloading must be very high.

Furthermore, for the above exposed reasons, the power necessary for changing the stiffness of a VSA of the prior art is dependent from the relative position between inlet and outlet, since the stiffness of the spring changes during the rotation. The mechanical joint provided by the present invention, instead, needs an operating power constant and independent from the relative position between inlet and outlet or by the load applied to the outlet, leading to relevant advantages, especially in case of automation of the rotation of the cam.

Advantageously, the elastic constraint comprises:

- at least one pin connected to the transmission ring and off-center with respect to the longitudinal axis y , said or each pin having a rotation axis x and comprising a fixed portion which is integral to the transmission ring and a movable portion which is adapted to rotate about the rotation axis x with respect to the portion fixed;

- a flat spring co-axial with respect to the transmission ring;

said flat spring comprising:

- a stiff portion which is integrally connected to the outlet and arranged to carry out a rotation having angular acceleration ϋ about longitudinal axis y ;

- at least one elastic branch comprising:

- an root end integrally connected to the stiff portion (175) ;

- an elastic end that can be resiliently deformed;

- an inner wall on which the movable portion is adapted to slide due to the acceleration φ ;

said flat spring being configured in such a way that, when the acceleration φ causes the sliding of the movable portion on the inner wall, the elastic end is deformed and the root end transmits to the stiff portion, and then to the outlet, an acceleration ΰ = k<p .

Such exemplary embodiment of the elastic constraint is extremely advantageous since the flat spring used occupies a very reduced volume and you have the possibility, simply varying the geometric shape, to modify the elastic constant which gives yielding to the joint.

Another advantage resides in the 2D geometry of the flat spring that allows it can be realized with cheap fabrication techniques (e.g. Laser cutting).

In particular, the cam comprises at least one area of selection comprising an elongated hole and an additional elongated hole, each area of selection comprising:

- a first angular sector associated with the "free" transmission mode, in said first angular sector being placed a first portion of elongated hole arranged to cause the movement of a selector in the free position and a first portion of additional elongated hole arranged to cause the movement of an additional selector in the free position;

- a second angular sector associated with the "rigid" transmission mode, in said second angular sector being placed a second portion of elongated hole arranged to cause the movement of the selector in the free position and a second portion of additional elongated hole arranged to cause the movement of the additional selector in the constrained position;

- a third angular sector associated with the "elastic" transmission mode, in said third angular sector arranged a third portion of elongated hole arranged to cause the movement of the selector in the constrained position and a third portion of additional elongated hole arranged to cause the movement of the additional selector in the constrained position.

This way, the cam allows to displace in a synchronized way the selectors and the additional selectors, in order to associate a transmission mode to each sector.

Advantageously, the inlet comprises an actuator arranged to transmit the acceleration ω to the driving plate.

In particular, the actuator is adapted to transmit the acceleration ω to the driving plate by a transmission system comprising two pulleys and a belt.

Advantageously, they are also provided:

- an auxiliary actuator arranged to put in rotation the cam for displacing the selectors and the additional selectors;

- a control unit arranged to operate the auxiliary actuator .

Advantageously, at least one torque sensor is also provided configured to measure the torque acting on the outlet and to send a torque signal to the control unit, in such a way that the control unit can operate the rotation of the cam. In this exemplary embodiment it is possible to program the control unit in such a way that it controls the switch to a different transmission mode as a function of the loads acting on the joint. For example, it is possible to program the control unit in such a way that, in the event of a collision, it controls the switch to the "elastic" mode or "free" mode, reducing the force of impact and the damages to the machine or to external users.

Advantageously, at least one proximity sensor is also provided configured to measure the presence of an obstacle closed to the rotational joint and to send a signal of proximity to the control unit, in such a way that the control unit can operate the rotation of the cam.

In this exemplary embodiment it is possible to program the control unit in such a way that it controls the switch to a different transmission mode as a function of the proximity of an object or an external user. For example, it is possible to program the control unit in such a way that, in case of presence of an operator, it controls the precautionary switch to the "elastic" mode or "free" mode, reducing the force of a possible impact.

Brief description of the drawings

Further characteristic and/or advantages of the present invention are more bright with the following description of an exemplary embodiment thereof, exemplifying but not limitative, with reference to the attached drawings in which:

- Fig. 1 shows an exploded perspective of a first exemplary embodiment of the rotational joint, according to the present invention;

- Fig. 2 shows an exploded perspective of a second exemplary embodiment of the rotational joint, according to the present invention;

- Fig. 3 shows an exploded perspective of a third exemplary embodiment of the rotational joint, according to the present invention, provided by the union of the first two exemplary embodiments;

- Fig. 4 shows in a top plan view some components of the first exemplary embodiment;

- Fig. 4A shows a detail of Fig. 4;

- Fig. 5 shows in a top plan view some components of the second exemplary embodiment;

- Fig. 5A shows a detail of Fig. 5;

- Fig. 6A shows an exploded perspective of the pin present in the elastic constraint;

- Fig. 6B shows in detail one of the selectors present in the first exemplary embodiment;

- Fig. 6C shows in detail one of the additional selectors present in the second exemplary embodiment ;

- Fig. 7 shows in a top plan view the cam of the third exemplary embodiment, equipped with elongated holes and additional elongated holes;

- Fig. 8 shows in a top plan view the flat spring present in the elastic constraint;

- Fig. 8A shows in a top plan view the flat spring of Fig. 8, during the deformation;

- Fig. 9 shows a possible exemplary embodiment of the rotational joint where the inlet comprises a actuator . Description of a preferred exemplary embodiment

In Fig. 1 a first exemplary embodiment is shown of the rotational joint 100, according to the present invention, arranged to operate two modes of transmission of the movement between an input, arranged to carry out a rotation having angular acceleration ω about a longitudinal axis y, and an outlet 190, arranged to carry out a rotation having angular acceleration ΰ about the same axis. In particular, in this exemplary embodiment, the joint can select the "free" transmission mode and the "elastic" transmission mode.

With reference even at Figs. 4, 4A and 6B, the rotational joint 100 comprises a driving plate 110, integrally connected to the inlet and comprising a plurality of radial housings 111, and a transmission ring 121 connected to the outlet 190 and co-axial to the driving plate 110. In particular, the transmission ring 121 is adapted to carry out a rotation having angular acceleration φ about longitudinal axis y . The transmission ring 121 is also constrained in an axial direction by the bearing 180.

A plurality of connection elements 131 is then provided subjected to the resilient force of a plurality of springs 141, in such a way that the connection elements 131 are pushed towards a plurality of slots Al located between the driving plate 110 and the transmission ring 121. In particular, the connection elements 131 are configured, when they are located in the slots Al, to come into simultaneous contact with the driving plate 110 and the transmission ring 121, generating by friction a constraint of fixed joint between them, so that occurs the condition ω = φ .

A plurality of selectors 151 is also provided arranged to slide radially in the radial housings 111 between a free position, where the selectors 151 are adapted to counteract the resilient force and to remove the connection elements 131 by the slots Al, and a constrained position, where the connection elements 131 are located in the slots Al and said constraint of fixed joint is obtained.

With reference even at Fig. 7, the rotational joint 100 comprises then a cam 160 equipped with a plurality of elongated holes 161 in which the maneuvering portions 151' of the selectors 151 are constrained to slide when the cam 160 carries out a rotation, with respect to the driving plate 110, about longitudinal axis y . In particular, when the maneuvering portions 151' slide in the elongated holes 161, the selectors 151 move between the free position, where the acceleration φ is independent from the acceleration ω, and the constrained position, wherein ω = φ .

In this exemplary embodiment, furthermore, the transmission ring 121 is resiliently connected to the outlet 190 by an elastic constraint that implements the condition ϋ = kcp .

This way, when the selectors 151 are in the free position, the acceleration ΰ is independent from the acceleration ω and the outlet 190 can rotate freely with respect to the input, obtaining the "free" transmission mode. When, instead, the selectors 151 are in the constrained position, there is ϋ = k<p = ko and the outlet 190 rotates in a way according to the inlet but not integrally to it, obtaining the "elastic" transmission mode. Such second transmission mode allows the joint 100 to work as elastic joint, owing to the connection by means of an element elastic.

The rotational joint 100 shown in Fig. 1 allows therefore to pass between a "free" transmission mode or neutral and an "elastic" transmission mode simply rotating the cam 160. For this reason it is possible to adapt the rotational joint 100 to different loads, needing very low power, since it is enough that they exceed the resilient force of the springs 141.

In this exemplary embodiment, with reference even at Figs. 6A, 8 and 8A, the elastic constraint comprises a flat spring 170 co-axial with respect to transmission ring 121 and two pins 125 connected to transmission ring 121 and off- center with respect to the longitudinal axis y. Each pin 125 has a rotation axis x and comprises a fixed portion 125a integral to the transmission ring 121 and a movable portion 125b arranged to rotate about its rotation axis x with respect to the fixed portion 125a.

In particular, the flat spring 170 comprises a stiff portion 175 integrally connected to the outlet 190 and arranged to carry out a rotation having angular acceleration ϋ about longitudinal axis y. The flat spring 170 also comprises 4 resilient branches 176, each of which comprises a root end 176a, integrally connected to said stiff portion 175, an elastic end 176b, which can be resiliently deformed, and an inner wall 176c, on which the movable portion 125b is adapted to slide due to the acceleration cp.

This way, when the acceleration φ causes the sliding of the movable portion 125b on the inner wall 176c, the elastic end 176b is deformed and the root end 176a transmits to the stiff portion 175, and then at the outlet 190, an acceleration ΰ = k<p . Such exemplary embodiment of the elastic constraint is extremely advantageous since the flat spring 170 used occupies a very reduced volume and you have the possibility, simply changing the shape geometric, to change the elastic constant which gives yielding to the joint.

Another advantage resides in the 2D geometry of the flat spring that allows it can be realized with cheap fabrication techniques (e.g. Laser cutting) .

In Fig. 2 a second exemplary embodiment of the rotational joint 100 is shown, in which it is possible to switch between a "free" transmission mode, as in an exemplary embodiment previous, and a "rigid" transmission mode, where the inlet is integral to the outlet.

With reference even at Figs. 5, 5A and 6C, the rotational joint 100 comprises a driving plate 110, integrally connected to the inlet and comprising a plurality of radial housings 112, and a transmission ring 122 connected to the outlet 190 and co-axial to the driving plate 110. In particular, the transmission ring 122 is adapted to carry out a rotation having angular acceleration φ' about longitudinal axis y.

A plurality of connection elements 132 is then provided subjected to the resilient force of a plurality of springs 142, in such a way that the connection elements 132 are pushed towards a plurality of slots A2 located between the driving plate 110 and the transmission ring 122. In particular, the connection elements 132 are configured, when they are located in the slots A2, to come into simultaneous contact with the driving plate 110 and the transmission ring 122, generating by friction a constraint of fixed joint between them, so that occurs the condition ω = φ' .

A plurality of selectors 152 is also provided arranged to slide radially in the radial housings 112 between a free position, where the selectors 152 are adapted to counteract the resilient force and to remove the connection elements 132 by the slots A2, and a constrained position, where the connection elements 132 are located in the slots A2 and said constraint of fixed joint is obtained.

The rotational joint 100 comprises then a cam 160 equipped with a plurality of elongated holes 162 in which the maneuvering portions 152' of the selectors 152 are constrained to slide when the cam 160 rotates about longitudinal axis y. In particular, when the maneuvering portions 152' slide in the elongated holes 162, the selectors 152 move between the free position, where the acceleration φ' is independent from the acceleration ω, and the constrained position, wherein ω = φ' .

In particular, in this exemplary embodiment the transmission ring 122 is integrally connected to the outlet 190 obtaining the condition ΰ = φ' . This way, when the selectors 152 are in the free position, the acceleration ΰ is independent from the acceleration ω and the outlet 190 can rotate freely with respect to said inlet. When, instead, the selectors 152 are in the constrained position, there is ϋ = φ' = ω and the outlet 190 rotates integrally to the input, implementing indeed the "rigid" transmission mode.

In Fig. 3 a third exemplary embodiment is shown of the rotational joint 100, wherein the first two exemplary embodiments of the rotational joint 100 are substantially combined.

With reference even at Fig. 7, such exemplary embodiments are combinable since the cam 160 comprises a plurality of areas of selection R, for example 6, each of which comprises an elongated hole 161 and an additional elongated hole 162. In particular, in each area of selection R 3 angular fields SI, S2, S3 are provided.

The first angular sector SI is associated with the "free" transmission mode. When the cam 160 rotates in such a way that the maneuvering portions 151' and 152' slide in this first field SI, a first portion of elongated hole 161a causes the movement of a selector 151 in the free position and a first portion of additional elongated hole 162a causes the movement of an additional selector 152 which are also in the free position. This way, the acceleration ΰ is independent from the acceleration ω and the outlet 190 can rotate freely with respect to the input.

The second angular sector S2 is associated with the "rigid" transmission mode. When the cam 160 rotates in such a way that the maneuvering portions 151' and 152' slide in this second field S2, a second portion of elongated hole 161b causes the movement of a selector 151 in the free position and a second portion of additional elongated hole 162b causes the movement of an additional selector 152 in the constrained position. This way, there is ΰ = φ' = ω and the outlet 190 rotates integrally to the input.

The third angular sector S3 is associated with the "elastic" transmission mode. When the cam 160 rotates in such a way that the maneuvering portions 151' and 152' slide in this third field S3, a third portion of elongated hole 161c causes the movement of a selector 151 in the constrained position and a third portion of additional elongated hole 162c causes the movement of an additional selector 152 in the free position. This way, there is ΰ = k<p = ko) and the outlet 190 rotates in a way according to the inlet but not integrally to it .

In particular, in the latter "elastic" mode the elastic constraint can be implemented by the flat spring 170, like what said for an exemplary embodiment of Fig. 1. This third exemplary embodiment, shown in Fig. 3, allows therefore to switch between three transmission mode: "free", "stiff" and "elastic", by an easy rotation of the cam 160.

It is therefore clear as the present invention provides a rotational joint that can change its own stiffness, switching between different transmission modes, needing a power of operation much lower than the prior art devices {Variable Stiffness Actuator) . While the present joint, in fact, needs simply a power sufficient to exceed the resilient force of the springs 141 and 142, in prior art this power must exceed the preloading of the main spring, i.e. the spring which gives yielding to the joint itself. Such spring has a stiffness obviously much higher and therefore the power needed to overcome its preloading has to be very high.

Furthermore, for reasons above exposed, the power necessary for changing the stiffness of a VSA of the prior art is dependent to the relative position between inlet and outlet, since the stiffness of the spring changes during the rotation. The mechanical joint provided by the present invention, instead, needs a power of operation constant and independent from the relative position between inlet and outlet or by the load applied to the outlet, causing relevant advantages, especially in case of automation of the rotation of the cam. With reference to Fig. 9, in a possible exemplary embodiment, the inlet comprises an actuator 200 arranged to transmit the acceleration ω to the driving plate 110 by a transmission system comprising two pulleys 310,330 and a belt 320.

The foregoing description some exemplary specific embodiments will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt in various applications the specific exemplary embodiments without further research and without parting from the invention, and, accordingly, it is meant that such adaptations and modifications will have to be considered as equivalent to the specific embodiments. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention, it is to be understood that the phraseology or terminology that is employed herein is for the purpose of description and not of limitation.