The present invention relates generally to electromechanical digits and particularly, but not exclusively, to bionic fingers with a linear actuator assembly, as well as to prosthetic hands including the electromechanical or bionic digits.
Prosthetic hands having one or more moveable bionic digits are well known. For example, WO2015138968 discloses a bionic digit comprising a knuckle, a proximal element, a distal element, a force actuator and a rod. The force generator includes an electrical motor that turns a screw. A threaded nut that is coupled to the screw can be forced to move forward or backward along the axis of the screw as the screw is driven to rotate by the motor. These parts are connected to each other by four pivotal connectors. A first pivotal connector connects the proximal element to the knuckle and a second pivotal connector connects a proximal end of the rod to the knuckle, the first and second connectors being spaced apart. The third pivotal connector connects the threaded nut to both the distal and the proximal elements and to the distal end of the rod. The fourth pivotal connector connects the distal element to the proximal element, for allowing the distal element to pivot relative to the proximal element. The third and fourth connectors being spaced apart. As the threaded nut is driven along the screw axis, it acts at the third connector to force the distal element to pivot relative to the proximal element at the fourth connector. The rod ensures that the threaded nut remains at a predetermined distance from the second connector, causing the proximal element to rotate relative to the knuckle as the threaded nut is driven to move along the screw axis.
For prosthetic hands that are provided with electric motors that drive movement of the bionic digits, i.e. extension and retraction of those digits, it is desirable to stop the supply of electrical power to the electric motors when the digits are to be retained in position for an extended period of time, in order to save electrical energy (typically battery power). For example, the electrical supply to the motors will be turned off when the digits on the prosthetic hand are in a retracted position and grasping the handle of a bag. It may be necessary to retain the hand in a grasped position for a long period of time if the bag is being carried over a substantial distance and whilst the bag is being carried its handles will exert a force on the digits. A component of that force will try to move the digits from their retracted position towards their extended position and, if the force is able to move the digits in that direction then the prosthetic hand will open, the digits grasp of the handles will fail and the bag will drop to the floor. Opening of the digits can occur if the force applied to them is sufficient to drive the linear actuator in the opening direction. This is known as back-driving. Therefore, the digits need to be self-locking.
If the linear actuator is a screw drive with a threaded drive nut that is driven in a linear direction by the rotation of a leadscrew, then if back-driving is to occur the force applied to the digit, must be sufficient to overcome the frictional resistance that exists between the drive nut and the leadscrew. In a leadscrew drive, the amount of frictional resistance is determined by the coefficients of friction of the materials from which the drive nut and leadscrew are made and by the geometry of the drive nut and the leadscrew, for example the pitch of the screw thread. There are two means to prevent back-driving from occurring, use of a brake or use of a leadscrew drive with a high frictional resistance. However, neither of these means are desirable. Use of a brake will add complexity to the digit and thus cost and weight. Use of a high frictional resistance leadscrew drive will reduce the efficiency of operation of the linear actuator because when the leadscrew drive is being driven by the motor in either direction, to extend or retract the digit, the frictional force needs to be overcome before the drive nut will move and thus before the digit can be articulated. Consequently, it has been identified that there is a need for an actuator arrangement that can resist back-driving without comprising the operation of the actuator when it is being driven, for example by an electric motor.
Accordingly the present invention provides a bionic digit comprising an intermediate portion, a tip portion and a hinge connecting the tip portion to the intermediate portion, and further comprising a linear actuator assembly located within the intermediate portion that is connected to the intermediate portion and to the tip portion and that is provided with a force generator, to which is connected a rotary drive shaft, and a ball screw connected to the rotary drive shaft for rotation therewith, wherein the ball screw has a helical drive ball raceway extending around its external surface, along at least part of its length, a plurality of drive balls, each drive ball located within the helical drive ball raceway and within a drive ball aperture of a ball retention element that is located around the ball screw and that is moveable relative to the ball screw, wherein each drive ball is also located within an annular groove of a drive collar that is positioned around the ball retention element, the drive collar being rotatable relative to the ball retention element and the ball screw around the longitudinal axis L-L of the ball screw, wherein the drive collar has multiple annular grooves that are parallel to each other and perpendicular to the longitudinal axis of the ball screw, wherein the drive collar is provided with a first engagement element and the tip portion is provided with a second engagement element and the first and second engagement portions are engaged with each other.
Preferably, the ball screw is provided with a proximal end stop and a distal end stop and wherein the ball retention element is provided with a proximal end stop abutment and a distal end stop abutment.
Preferably, the ball screw has a circular cross-sectional profile, the ball retention element is a cylindrical tube with an internal diameter that is larger than the external diameter of the ball screw and the drive collar has a circular cross-section bore with an internal diameter that is larger than the external diameter of the ball retention element.
Preferably, the helical drive ball raceway has a hemi-spherical cross-sectional profile, wherein when a drive ball is placed within the drive ball raceway there is a clearance between at least some portion of the drive ball and the drive ball raceway. Alternatively, the drive ball raceway can have a V shaped profile.
Preferably, the multiple annular grooves of the drive collar have a V shaped cross-sectional profile and wherein the angle of the V is such that when a drive ball is placed in an annular groove at least a portion of the drive ball extends past the open end of the annular groove.
Preferably, the bionic digit further comprises a base portion attached to the proximal end of the intermediate portion by a base hinge.
Preferably, the pitch of the ball screw is between 0.25 mm and 4 mm.
Preferably, the force generator of the linear actuator assembly is an electric motor and gearbox with a rotor that rotates around an axis coaxial with the longitudinal axis L-L, wherein the drive shaft is attached to the rotor and rotates with the rotor and extends outwardly from the electric motor along longitudinal axis L-L, the driveshaft is located within a recess in the ball screw so that the ball screw can move axially relative to the driveshaft along axis L-L but cannot rotate relative to the driveshaft, the ball screw has a circular cross-section, the retention element is a straight-sided cylindrical tube with a plurality of circular drive ball apertures that pass through the wall of the tube and that are equally angularly spaced apart around the circumference of the tube, wherein the drive collar has a circular internal bore provided with a plurality of annular grooves that are located perpendicularly to longitudinal axis L-L, wherein the first engagement element of the drive collar is a peg that extends perpendicularly from the external surface of the drive collar and perpendicularly to a central plane CP through which longitudinal axis L-L passes, the first engagement element being located within the second engagement element which takes the form of a slot in the proximal end of the tip portion.
According to another aspect of the present invention there is provided a prosthetic hand comprising a plurality of bionic digits according to any one of the preceding claims wherein, the prosthetic hand is provided with a palm, the bionic digits are each provided with a base portion and the base portion of each bionic digit is attached to the palm.
The present invention will be described below, with reference to the accompanying figures:
An electromechanical digit, specifically a bionic finger 1 according to an embodiment of the present invention, is shown in
The ball cage 55 is provided with a proximal end stop abutment 67 at its proximal end and a distal end stop abutment 69 at its distal end. The proximal end stop abutment 67 extends outwardly from the ball cage 55 in a proximal direction that is parallel to axis L-L and by a distance that is the same as the diameter of a proximal end stop 71. The distal end stop abutment 69 extends outwardly from the ball cage 55 in a distal direction that is parallel to axis L-L and by a distance that is the same as the diameter of a distal end stop 73. The face of the proximal and distance end stop abutments 67, 69 are each perpendicular to the axis L-L and are opposed to each other circumferentially and diametrically, for example as shown in
The drive collar 53 has two articulation pins 79 located on opposite external sides of the drive collar 53 that extend perpendicularly to axis L-L and that are diametrically opposed to each other. The articulation pins 79 fit within travel restraint slots 81 provided in the intermediate portion 15, for example as shown in
The articulation pins 79 also locate within articulation slots 83 of the tip portion 23 of the bionic finger 1, as shown in
The dimensions of the ball screw 51, the drive collar 53, the ball cage 55, the travel restraint slots 81 and the articulation slots 83 are selected to provide the desired extent of articulation of the tip portion 23 relative to the intermediate portion 15, for example from the fully extended, or fully open, position illustrated in
In use, the tip portion 23 of the bionic finger 1 is articulated relative to the intermediate portion 15 by providing an electric current to the electric motor sub-assembly 47 and driving the electric motor (not shown) in either a clockwise or an anti-clockwise direction (when viewed from its proximal end, i.e. looking from the electric motor sub-assembly 47 towards the drive collar 53). Driving the electric motor in a clockwise direction causes the bionic finger 1 to move towards its fully retracted position and driving the electric motor in an anti-clockwise direction causes the bionic finger 1 to move towards its fully extended position.
Rotation of the electric motor causes the driveshaft 49 to rotate in the same rotational direction and thus causes the ball screw 51 to also rotate in the same direction, because the ball screw 51 is keyed to the driveshaft 49. The ball screw 51 is engaged with the drive collar 53 by means of the drive balls 57 that are contained within the ball cage 55. The helical arrangement of the ball raceway 65 of the ball screw 51 carries the drive balls 57 in an axial direction as the ball screw 51 rotates. Each drive ball 57 is simultaneously engaged with a V-profile groove 77 in the drive collar 53 and with the ball raceway 65 of the ball screw 51. The drive collar 53 cannot rotate because the articulation pins 79 are located within the travel restraint slots 81 and therefore the drive collar 53 is moved axially along axis L-L.
The degree of travel of the drive collar 53 is determined by the distance between the proximal end stop 71 and distal end stop 73 that are provided on the ball screw 51, as will be explained in further detail below. Therefore, rotation of the ball screw 51 causes the drive collar 53 to move along axis L-L. If the ball screw 51 rotates anti-clockwise then the drive collar 53 moves away from the motor sub-assembly 47, if the ball screw 51 rotates clockwise then the drive collar 53 moves towards the motor sub-assembly 47 because the drive collar 53 is located between the proximal and distal end stops 71, 73.
The ball cage 55 moves axially with the drive collar 53, because it is constrained to do so as a result of the drive balls 57 being located in the parallel V-profile grooves 77 in the drive collar 53. The ball cage 55 rotates relative to the ball screw 51 and relative to the drive collar 53.
The operation disclosed above with reference to
Number | Date | Country | Kind |
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2105448.1 | Apr 2021 | GB | national |
Filing Document | Filing Date | Country | Kind |
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PCT/IB2022/053429 | 4/12/2022 | WO |
Number | Date | Country | |
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63173530 | Apr 2021 | US |