A mechanical wrist joint

Abstract

A mechanical wrist lock mechanism 300 comprises: a support member 40 and a lock assembly 30. The support member is pivotally coupled at a hinge axis 50 to a mounting member 11 and comprises an interna

Description

TECHNICAL FIELD The present disclosure relates to a mechanical wrist of a prosthetic hand. In particular, but not exclusively, it relates to a mechanical wrist to allow a prosthetic hand to flex and extend with respect to a prosthetic arm, thus, allowing the prosthetic hand to rotate about a hinge axis with respect to the prosthetic arm. Also, in particular, but not exclusively, it relates to a mechanical wrist allowing the prosthetic hand to be locked relative to the prosthetic arm. Aspects of the invention relate to a flex mechanism of a mechanical wrist and a lock mechanism of a mechanical wrist. BACKGROUND The ability for a prosthetic hand to passively flex and extend is desirable to allow a user to orient the hand in a desired position and manipulate objects or the environment more effectively, minimizing the user’s need to rely on shoulder movements to correctly orient the prosthetic hand. Some prosthetic hands have wrist assemblies that provide for the user to be able to lock the prosthetic hand in a particular flexed or extended position for continual use of the prosthetic hand in said position. However, with conventional locking mechanisms of wrist assemblies it may be difficult to activate and disactivate the locking mechanism, in particular for bilateral users. In conventional locking mechanisms, when the prosthetic hand is in a neutral position, the force of the associated spring assembly is greatly reduced, thereby reducing the ability of the user to accurately position the prosthetic hand when it is close to the neutral position. It is an aim of the present invention to address at least some of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION According to an aspect of the invention there is provided a lock mechanism of a mechanical wrist for supporting a prosthetic hand as recited in claim 1. An advantage of this invention is that the user is able to lock the prosthetic hand into a predetermined position by means of a lock assembly. This invention may also separate the lock mechanism and a flex mechanism, therefore allowing for a strong lock mechanism whilst enabling the wrist to be pivoted to be in an anatomically correct position. The lock button may be configured to move along the hinge axis to move the latch body between the locked position and the unlocked position. The first path may comprise a stable unlock position, a stable lock position, an intermediary unstable lock overtravel position between the unlock position and the lock position and an intermediary unstable unlock overtravel position between the lock position and the unlock position. The intermediary unstable lock overtravel position is engageable when the first pin moves from the unlock position to the lock position. The intermediary unstable unlock overtravel position is engageable when the first pin moves from the lock position to the unlock position. The second pin may interact with the second path to ensures that the fist pin and second pin can only travel in a fixed direction of travel along the respective first path and second path. Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates the positions of a hand in a neutral position, with a wrist extension and with a wrist flexion; Figure 2 illustrates a wrist assembly for a prosthetic hand including a wrist, a lock assembly, a spring assembly, and a hand chassis, according to an embodiment of the invention; Figure 3a illustrates a cross sectional view of the spring assembly of Figure 2, according to an embodiment of the invention; Figure 3b illustrates an exploded view of the spring assembly of Figure 2 and Figure 3a, according to an embodiment of the invention; Figure 4 illustrates an exploded view of the lock assembly in the context of the wrist assembly of Figure 2, according to an embodiment of the invention; Figure 5a illustrates a cross section view of the lock assembly from Figure 2 with a latch body in an unlocked position, according to an embodiment of the invention; Figure 5b illustrates a cross section view of the lock assembly from Figure 2 with the latch body in an overtravel position, according to an embodiment of the invention; Figure 5c illustrates a cross section view of the lock assembly from Figure 2 with the latch body in a locked position, according to an embodiment of the invention; Figure 6a illustrates the latch body and latch spring of Figure 4 with a detailed illustration of a first path within the latch body which is followed by a first pin of a latch spring, according to an embodiment of the invention; Figure 6b illustrates the latch body and latch spring of Figure 4 with a detailed illustration of a second path within the latch body which is followed by a second pin of the latch spring, according to an embodiment of the invention; Figure 7 illustrates both first and second paths on the latch body of Figure 4 with the position of the respective first and second pins within the first and second paths, detailing a lock path and an unlock path of the latch spring as well as the unlock and lock overtravel positions, according to an embodiment of the invention; Figures 8a to 8I illustrate the travel of the first pin of the latch spring, shown as a solid black circle, along the first path bounded by a black outline; and the travel of the second pin of the latch spring, shown as a dotted circle, along the second path illustrated as a filled area, with both the first pin and the second pin of the latch spring being mechanically biased towards each other by the latch spring, according to an embodiment of the invention; Figure 9a illustrates the first path on the latch body, whereby the travel of the section of the latch spring is determined by steps in the first path, according to an embodiment of the invention; Figure 9b illustrates a second path with identical outline to the first path in Figure 9a, but having so steps, according to an embodiment of the invention; and Figures 10a to 10h illustrate the travel of the first pin of the latch spring, shown as a solid black circle, along the first path illustrated in Figure 9a, bounded by a black outline, , according to an embodiment of the invention. In the drawings, like parts are denoted by like reference numerals. DETAILED DESCRIPTION Examples of the present disclosure relate to a flex mechanism of a mechanical wrist for supporting a prosthetic hand and a lock mechanism of a mechanical wrist for supporting a prosthetic hand. In particular, examples of the present disclosure relate to a flex mechanism of a mechanical wrist for supporting a prosthetic hand which allows a prosthetic hand to flex and extend with respect to a prosthetic arm connected to the mechanical wrist. Examples of the present disclosure also relate to a lock mechanism of a mechanical wrist for supporting a prosthetic hand allowing a prosthetic hand to be locked relative to a prosthetic arm connected to the mechanical wrist. Non-limiting examples will now be described with reference to the accompanying drawings. The figures illustrate a mechanical wrist 100 comprising a flex mechanism 200 and a lock mechanism 300. It will be understood that in some embodiments, the mechanical wrist 100 may comprise only the flex mechanism 200 or only the lock mechanism 300, whereas other embodiments may comprise both the flex mechanism 200 and the lock mechanism 300. Figure 1 illustrates three distinct positions that a human wrist may take. Similarly these are the positions that the mechanical wrist 100 may be positioned in. In a central position, the wrist is considered to be in a neutral position 73, and may be considered to lie on a neutral axis. When extended, the back of the hand is moved back towards the arm to provide a wrist extension 71. In such an extended position the hand lies outside of the neutral axis. When flexed, the palm of the hand is moved back towards the arm to provide a wrist flexion 72. In such a flexed position the hand lies outside of the neutral axis. Figure 2 illustrates a mechanical wrist 100, which is configured to retain a prosthetic hand and be connected to a prosthetic arm. The mechanical wrist 100 comprises a flex mechanism 200. As more clearly shown in the exploded view of Figure 4, the flex mechanism 200 comprises a support member 40 pivotally coupled at an axis 50, which may be called the hinge axis 50, to a mounting member 11. This enables the mounting member 11 to rotate about the hinge axis 50 with respect to the support member 40. The support member 40 may be mechanically linked to a prosthetic arm. The support member 40 may be positionally fixed relative to a prosthetic arm. The mounting member 11 may be mechanically linked to a prosthetic hand or a hand chassis 81 for a prosthetic hand. The mounting member 11 may be positionally fixed relative to a prosthetic hand or a hand chassis 81 for a prosthetic hand. As shown in the embodiment of Figure 4, a wrist bearing 31, located between the mounting member 11 and the support member 40, may enable rotation of the mounting member 11 about the hinge axis 50. Thus, wrist extension 71 and wrist flexion 72 can be provided by the combination of the support member 40 and mounting member 11 connected via the wrist bearing 31. The support member 40 may also be coupled to the mounting member 11 via one or more hinges that enable the mounting member 11 to rotate around the one or more hinge axes with respect to the support member 40. For example, the support member 40 may be coupled to the mounting member 11 via two hinges that enable the mounting member 11 to rotate around two hinge axes with respect to the support member 40. Alternatively, the support member 40 may be coupled to the mounting member 11 via a ball and socket joint. This enables the mounting member 11 to rotate in three rotational axes with respect the support member 40. The mechanical wrist 100 comprises a spring assembly 20, as illustrated in the cross-sectional view of Figure 3a and the exploded view of figure 3b. The spring assembly 20 comprises two opposing compression springs, a wrist extension spring 25 and a wrist flexion spring 27. The spring assembly 20 is configured in such a way it that it is mounted between two ball joints, an upper ball joint 21, closest to the wrist extension spring 25, and a lower ball joint 28, closest to the wrist flexion spring 27. The wrist extension spring 25 is configured to apply a mechanical bias resisting clockwise rotation of the mounting member 11 about the hinge axis 50. The wrist flexion spring 27 is configured to apply a mechanical bias resisting anti-clockwise, or counter-clockwise, rotation of the mounting member 11 about the hinge axis 50. The wrist extension spring 25 is configured to apply mechanical bias resisting wrist extension wherein the wrist extension is the clockwise rotation of the mounting member 11 relative to the support member 40 about hinge axis 50. The wrist flexion spring 27 is configured to apply a mechanical bias resisting wrist flexion wherein the wrist flexion is the anti-clockwise, or counter-clockwise, rotation of the mounting member 11 relative to the support member 40 about the hinge axis 50. Once the prosthetic hand is moved from the neutral position 73 to a flexed position 72, only a first one of the compression springs is active, namely the wrist flexion spring 27. When the prosthetic hand is in the extended position 71, only a second one of the compression springs is active, which is an opposing compression spring to the wrist flexion spring 27, namely the wrist extension spring 25. In the neutral position 73 the two compression springs 25, 27 are both active and equally compressed. Both compression springs 25, 27 act on a spring platform 26. The spring platform 26 is located between the wrist extension spring 25 and the wrist flexion spring 27. The spring platform 26 is operably connected or coupled to a spring slider 22. The spring platform 26 may be mechanically linked via a connector, such as a pin 29, which may be in the form of a platform pin 29, to the spring slider 22. The upper ball joint 21 is operably connected or coupled to the spring slider 22. The upper ball joint 21 may be mechanically mated to the spring slider 22. The lower ball joint 28, is operably connected or coupled to a spring body 24. The lower ball joint 28 may be mechanically mated to the spring body 24. Both compression springs 25, 27 act on the spring body 24. The spring slider 22 is configured to move within the spring body 24 such that the upper ball joint 21, which is integral with or operably connected or coupled to the spring slider 22, can be moved relative to the lower ball joint 28, which is integral with or operably connected or coupled to the spring body 24, along an axis from the upper ball joint 21 to the lower ball joint 28, through the centres of the spring slider 22 and spring body 24. In this way, the length of the spring assembly 20 can be adjusted to enable wrist extension 71 and wrist flexion 72. To facilitate relative movement between the spring slider 22 and the spring body 24, the spring body 24 may comprise two longitudinal slots, which are on opposite sides of the spring body 24, for receiving the platform pin 29, such that the spring platform 26 may freely slide up and down the spring body 24 to effect compression of the wrist extension spring 25 and the wrist flexion spring 27 dependent on movement of the mechanical wrist 100. The upper ball joint 21 is linked via a ball and socket joint to a prosthetic hand or a hand chassis 81 for a prosthetic hand, the prosthetic hand or the hand chassis 81 comprising a socket for receiving the upper ball joint 21. The lower ball joint 28 is linked via a ball and socket joint to the wrist or a wrist plate 84, the wrist or wrist plate 84 comprising a socket for receiving the lower ball joint 28. The mechanical wrist 100 may be connected to a prosthetic arm. When in the neutral position 73, the forces applied by the wrist extension spring 25 and the wrist flexion spring 27 are low. Therefore, the mechanical wrist 100 can easily move and may seem unstable to a user. Therefore, in some embodiments the spring assembly 20 comprises a resilient member 23, which may be in the form of a canted coil ring spring 23 that engages with a slot or groove in the spring slider 22. The slot or groove in the spring slider 22 may be in an outer portion of the spring slider. In the neutral position 73 the canted coil ring spring 23 bears on the spring body 24 and engages with a circumferential slot or groove in the spring body 24. The circumferential slot or groove in the spring body 24 is positioned such that when the canted coil ring spring 23 is engaged with both the slot or groove in the spring slider 22 and the slot or groove in the spring body 24 the mechanical wrist 100 is in the neutral position 73, corresponding to the position of the neutral axis. The canted coil spring 23 provides a latched position for the mechanical wrist 100 in the neutral position 73 which indexes an associated prosthetic hand in the neutral position 73. This ensures that the mechanical wrist 100 is stable when in the neutral position 73 and the force applied, or exerted, by the wrist extension spring 25 is low, and the force applied, or exerted, by the wrist flexion spring 27 is low. The canted coil ring spring 23 is configured to apply a mechanical bias resisting wrist flexion 72 and wrist extension 71 wherein the mechanical wrist 100 is in the neutral position 73. That is, the canted coil ring spring 23 is configured to apply a mechanical bias resisting clockwise rotation of the mounting member 11 about the hinge axis 50 and anti-clockwise, or counter-clockwise, rotation of the mounting member 11 about the hinge axis 50 when the mechanical wrist 100 is in a neutral position 73. Alternatively, the resilient member 23 may be an O-ring, in place of the canted coil ring spring 23, to perform the same function. In some embodiments, the spring slider 22 may have a plurality of grooves to index various hand positions from fully extended to fully flexed. That is, the plurality of groves allow for the indexing of various extensions of the spring assembly 20 to allow certain prosthetic hand positions to maintain a level of resilience to movement. As the spring assembly 20 is mounted on ball joints 21, 28 it offers a great deal of flexibility where it can be mounted in the mechanical wrist 100. When the upper ball joint 21 of the spring assembly 20 is mounted such that it is configured to reside on the palmar part or side of a prosthetic hand, when a prosthetic hand is operably connected or coupled to the mechanical wrist 100, extension of the spring assembly 20 equates to wrist extension 71 and compression of the spring assembly 20 equates to wrist flexion 72. The upper ball joint 21 of the spring assembly 20 could alternatively be mounted such that it is configured to reside on the dorsal part or side of a prosthetic hand, when a prosthetic hand is operably connected or coupled to the mechanical wrist 100, meaning the extension of the spring assembly 20 equates to wrist flexion 72 and the compression of the spring assembly 20 equates to wrist extension 71. In one embodiment of the invention the upper ball joint 21 of the spring assembly 20 is mounted on the palmer side of a prosthetic hand. In another embodiment of the invention the upper ball joint 21 of the spring assembly 20 is mounted on the dorsal side of a prosthetic hand. In some embodiments, the mechanical wrist 100, for supporting a prosthetic hand, comprises a lock mechanism 300, as illustrated in Figure 2, and Figure 4 to Figure 10. The lock mechanism 300 selectively locks the mounting member 11 relative to the support member 40, thereby locking a prosthetic hand, when connected to the mechanical wrist 100, relative to the mechanical wrist 100 and may be locked relative to a prosthetic arm that may be connected to the mechanical wrist 100, that is, the prosthetic hand is prevented from moving around, or relative to, the hinge axis 50. In other words, when selectively locked, movement of the prosthetic hand is selectively constrained to the support member 40, and therefore the mechanical wrist 100. As illustrated in Figure 4, the lock mechanism 300 comprises a support member 40 pivotally coupled at a hinge axis 50 to a mounting member 11, the support member 40 having an internal spline 41, which forms a part of the lock mechanism 300. The internal spline 41, is a circular or annular arrangement having teeth which provide apertures therebetween for receiving corresponding teeth of an external spline 42 arrangement, which also forms part of the lock mechanism 300, as will be described further below. It may be considered that the teeth of the internal spline 41 may be arranged to engage with the teeth of the external spline 42, or engage with the apertures between the teeth of the external spline 42, such that the internal spline 41 is meshed with the external spline 42. The lock mechanism 300 also comprises a lock assembly 30. The lock assembly 30 comprises a lock spring 34, a latch body 35, a wrist axle 38, a latch spring 39, and a lock button 36. The lock spring 34 mechanically biases the latch body 35 towards an unlocked position. The latch body 35 is illustrated in more detail in Figure 6a and Figure 6b, where Figure 6a illustrates a first side of the latch body 35 and Figure 6b illustrates a second side, opposite the first side, of the latch body 35. On each side of the latch body 35 is a grooved arrangement, or a groove, defining a path. In the embodiment shown in Figure 6a and Figure 6b, the first side of the latch body 35 comprises a first path 56 and the second side of the latch body 35 comprises a second path 57. The latch spring 39 may be a leaf spring, and may be in the form of a u-shaped, a v-shaped, a w-shaped spring, or another similar shape of spring. The latch spring 39 may have a flat form, that is, extend only on one plane. The latch spring 39 is retained in position using a wrist axle 38, which prevents the latch spring 39 from moving along the hinge axis 50. The wrist axle 38 may also provide a surface against which the lock spring 34 engages to bias the latch body 35 towards the unlocked position. The wrist axle 38 is held in position using a fastener, such as an axle retention bolt 32 through the mounting member 11. The latch spring 39 comprises terminal ends that define bosses, sections, or pins 58, 59, each of which are configured to fit and / or engage with a respective grooved arrangement on respective sides of the latch body 35. The pins 58, 59 are configured to traverse or run within the respective grooved arrangements, or grooves, during operation of the lock mechanism 300. In particular, in Figure 6a the latch spring 39 has one end terminating in a pin 58 that engages with the first path 56 in a first side of the latch body 35, and in Figure 6b the latch spring 39 has a second end terminating in a pin 59 that engages with the second path 57 in the second side of the latch body 35. In the embodiments illustrated in Figure 6a to Figure 8I, the first path 56 has a groove that outlines an approximate heart shape. This first path 56 has at least two stable positions for the latch spring pin 58, a lock position 52 and an unlock position 62. The lock position 52 of the latch spring pin 58 relates to a locked position, or condition, for the mechanical wrist 100. The unlock position 62 of the latch spring pin 58 relates to an unlocked position, or condition, for the mechanical wrist 100. The first path 56 in this embodiment additionally has two overtravel positions, as more clearly shown in Figure 7, a lock overtravel position 54 and an unlock overtravel position 51. The lock overtravel position 54 enables the pin 58 to enter the lock position 52. The unlock overtravel position 51 enables the pin 58 to enter the unlock position 62. The second path 57 contains the movement of the second pin 59 of the latch spring 39. The second path 57 is configured so that the pins 58, 59 in the respective paths 56, 57 can only travel in one direction. In the embodiments shown in Figure 6a to Figure 8I, the direction of travel of the first pin 58 in the first path 56 is anti-clockwise, or counter-clockwise, along the first path 56. The two pins 58, 59 may be opposing, in such a way that when a lateral external force acts on the pins 58, 59, the pins 58, 59 are urged towards each other on a lateral plane, that is a plane parallel to the hinge axis 50. A lateral external force is a force that acts perpendicular to that of the u-shape, v-shape, or w-shape of the latch spring 39. The position of the latch body 35 is partially determined by the position of the first pin 58 along the first path 56 and the position of the second pin 59 along the second path 57, wherein the first pin 58 and the second pin 59 are urged towards each other by the latch spring 39. The lock button 36 is configured to move or actuate the latch body 35 towards a locked position. The lock button 36 comprises, or is mechanically linked to, an external spline 42 which is configured to engage with the internal spline 41 of the support member 40. The external spline 42 is engaged with the internal spline 41 when the latch body 35 is in a locked position and the external spline 42 is disengaged from the internal spline 41 when the latch body 35 is in the unlocked position. The lock button 36 is a sliding button which is mounted on the hinge axis 50 of the mechanical wrist 100 and allowed to move there along, that is, to slide in and out towards the support member 40. The lock button may be retained by a retainer plate 37 and affixed to the latch body 35 via a fixing, such as a bolt 33. The lock button 36, and in particular, the external spline 42 of the lock button 36 has a radially toothed form. For example, the external spline 42 may have thirty six teeth. In the locked position, the teeth of the external spline 42 engage with the internal spline 41 of the support member 40. Where the external spline 42 has thirty six teeth, the internal spline 41 will have a corresponding set of apertures, in particular thirty six apertures configured to engage, or receive, the thirty six teeth of the external spline 42 of the lock button 36. This allows increments for the lock positions. In this embodiment the increments will be ten degree increments. Of course, it will be understood that other numbers of teeth may be provided on the internal spline 41 and external spline 42 to provide different increments for the lock positions. When the teeth of the external spline 42 are mechanically engaged with the apertures of the internal spline 41, the lock button 36 is rotationally locked to the support member 40 relative to the hinge axis 50. Thus, the support member 40 is rotationally locked to the prosthetic hand or a hand chassis 81 for the prosthetic hand. Thus, the prosthetic hand may be rotationally locked with respect to the mechanical wrist 100. In this way, a prosthetic hand may be constrained about the hinge axis 50 to a prosthetic arm. The lock button 36 is biased, via the lock spring 34, towards an unlocked position. The force of the lock spring 34 acts in a direction parallel to the hinge axis 50. Therefore, the external spline 42 is biased, via the lock spring 34, away from the internal spline 41 into an unlocked position. The mechanical wrist 100 is in a locked position when the external spline 42 is engaged with the internal spline 41. The mechanical wrist 100 is in an unlocked position when the external spline 42 is not engaged with the internal spline 41. The position of the lock button 36, and thus the external spline 42, is controlled by a two-position lock mechanism 300. The two-position lock mechanism 300 enables a user to alternate the position of the external spline 42 between a locked position and an unlocked position with a momentary push, as will now be described further in relation to Figure 5a, Figure 5b and Figure 5c. As shown in Figure 5a, in the unlocked condition 63, the lock button 36 is fully withdrawn, with the external spline 42 in free space allowing the mechanical wrist 100 to rotate freely relative to a prosthetic hand, the wrist axle 38, the latch body 35 and the lock button 36. The lock button 36 is biased into this position due to the lock spring 34 bearing between the wrist axle 38 and the lock button 36. The radial teeth of the external spline 42 engage with the corresponding teeth or apertures of the internal spline 41 of the support member 40. At this stage the user has to flex the prosthetic hand to the desired position and then push the lock button 36. As the lock button 36 is pushed by the user, the latch body 35 slides relative to the latch spring 39. Therefore, the pins 58, 59 move in their along their respective paths 56, 57. The first pin 58 of the latch spring 39 moves along a lock path 55, as shown in Figure 7, in the first path 56 from the unlock position 62, when in the unlocked condition 63 to the lock overtravel position 54 when in the lock overtravel condition 62, as shown in Figure 5b, biased by the second pin interfacing with the second path 57. Once the lock button 36 is released by the user, the second pin 59, interfacing with the second path 57 on the latch body 35, stops the pins 58, 59 of the latch spring 39 from travelling in a reverse direction along the lock path 55. The pins 58, 59, therefore, are biased by the lock spring 34 towards the lock position 52 for the locked condition 61, as shown in Figure 5c, leaving the external spline 42 engaged with the internal spline 41, and therefore locking the rotation of the hand chassis 81 for a prosthetic hand, or the prosthetic hand, about the hinge axis 50. With the pins 58, 59 in the lock position 52, if the lock button 36 is pressed again by the user, the pins 58, 59 will travel along respective unlock paths 53 of the first path 56 and the second path 57, biased by the first pin 58 along the first path 56. The pins 58, 59 of the latch spring 39 travel from the lock position 52 to an unlock overtravel position 51. Once the lock button 36 is released by the user, the pins of the latch spring 39 travel along the unlock path 53 to a stable unlock position 62. The lock path 55 is a path that the first pin 58 takes from the unlock position 62 to the lock position 52, via the unstable lock overtravel position 54. The unlock path 53 is a path that the first pin 58 takes from the lock position 52 to the unlock position 62, via the unstable unlock overtravel position 51. The trajectory of the latch spring 39 is partially determined by a first pin 58 of the latch spring 39 urging against a first path 56 of the latch body 35. The trajectory of the pins 58, 59 of the latch spring 39 is partially determined by a second pin 59 of the latch spring 39 urging against the second path 57 of the latch body 35. The trajectory of the latch spring 39 is partially determined by a continuous force applied to the latch body 35 via a lock spring 34. If the pins 58, 59 of the latch spring 39 are not on the same central axis, that is, if they deviate in a plane parallel to the hinge axis 50, they are biased towards each other via a tortional force provided by the latch spring 39. Figures 8a to 8I illustrate, in steps, the travel of the first pin 58 of the latch spring 39, shown as a solid black circle, along the first path 56 bounded by a black outline, and the travel of the second pin 59 of the latch spring 39, shown as a dotted circle, along the second path 57 illustrated as a filled or shaded area, with both the first pin 58 and the second pin 59 of the latch spring 39 being mechanically biased towards each other by the latch spring 39. In Figure 8a, the first pin 58 and the second pin 59 are in the stable unlock position 62. A user may apply a force to the lock button 36 to move the latch body 35 towards the latch spring 39 such that the first pin 58 and the second pin 59 move along their respective paths 56, 57, with the first pin 58 following the lock path 55 as shown in Figure 7. At Figure 8b it is observed that the second pin 59, deviates from the central axis, thereby applying a force to the first pin 58 via the latch spring 39 such that the first pin 58 follows the lock path 55, which in this embodiment is an anti-clockwise, or counter-clockwise, path. At Figure 8c the first pin 58 and the second pin 59 continue to travel along their respective paths until, at Figure 8d, the first pin 58 reaches the start of a channel which provides lock overtravel. At Figure 8e, the first pin 58 is fully engaged in the channel and is therefore located in the lock overtravel position 54, when the lock button 36 is fully depressed. Releasing the lock button 36, the latch body 35 moves towards the latch spring 39 such that the first pin 58 and the second pin 59 move along their respective paths 56, 57, as shown in Figure 8f, to the stable lock position 52 shown in Figure 8g. At this stage the external spline 42 of the lock button 36 is engaged with the internal spline 41 of the support member 40, therefore locking the rotation of the hand chassis 81 for a prosthetic hand, or the prosthetic hand, about the hinge axis 50. Pressing the lock button 36 again, urges the first pin 58 and second pin 59 to follow their respective paths 56, 57, with the first pin 58 following the unlock path 53 as shown in Figure 7. Figure 8h shows the movement of the first pin 58 and second pin 59 towards an unlock overtravel position 51, where the first pin 58 and second pin 59 may be located, as shown in Figure 8i, when the lock button 36 is fully depressed. Releasing the lock button 36 again, the latch body 35 moves towards the latch spring 39 such that the first pin 58 and the second pin 59 move along their respective paths 56, 57, as shown in Figure 8j and Figure 8k, to the stable unlock position 62 shown in Figure 8I. At this stage the external spline 42 of the lock button 36 is disengaged from the internal spline 41 of the support member 40, therefore unlocking the rotation of the hand chassis 81 for a prosthetic hand, or the prosthetic hand, about the hinge axis 50. Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For example, it is envisioned that a similar effect to the arrangement described in relation to Figures 8a to 8I, could be achieved by introducing steps into the first path 56 in such a way that once the pin is over the step it cannot return along the same path. In the embodiment of Figure 9a, that would be back along the path in a clockwise direction. In this embodiment the second path 57, shown in Figure 9b, can have a substantially similar profile to the first path 56 but without any steps. In this embodiment the first pin 58 and second pin 59 are urged towards each other in a vertical direction, that is, with a force applied onto face of the path. Figures 10a to 10h illustrate the movement of the first pin 58 along the first path 56 with the arrangement as noted above with regards to Figures 9a and 9b. In Figure 10a the first pin 58 is in the stable unlock position 62. A user may apply a force to the lock button 36 to move the latch body 35 towards the latch spring 39 such that the first pin 58 and the second pin 59 move along their respective paths 56, 57, with the first pin 58 following the lock path 55. At Figure 10b, the first pin 58 is fully engaged in the channel which provides lock overtravel and is therefore located in the lock overtravel position 54, when the lock button 36 is fully depressed. It is noted that the second pin 59 will also be fully engaged in a corresponding channel in the second path 57 at this point. Releasing the lock button 36, the latch body 35 moves towards the latch spring 39 such that the first pin 58 and the second pin 59 move along their respective paths 56, 57, as shown in Figure 10c, to the stable lock position 52 shown in Figure 10d. At this stage the external spline 42 of the lock button 36 is engaged with the internal spline 41 of the support member 40, therefore locking the rotation of the hand chassis 81 for a prosthetic hand, or the prosthetic hand, about the hinge axis 50. Pressing the lock button 36 again, urges the first pin 58 and second pin 59 to follow their respective paths 56, 57, with the first pin 58 following the unlock path 53. Figure 10e shows the first pin 58 and the second pin 59 located at an unlock overtravel position 51, when the lock button 36 is fully depressed. Releasing the lock button 36 again, the latch body 35 moves towards the latch spring 39 such that the first pin 58 and the second pin 59 move along their respective paths 56, 57, as shown in Figure 10f and Figure 10g, to the stable unlock position 62 shown in Figure 10h. At this stage the external spline 42 of the lock button 36 is disengaged from the internal spline 41 of the support member 40, therefore unlocking the rotation of the hand chassis 81 for a prosthetic hand, or the prosthetic hand, about the hinge axis 50. Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. Whilst endeavoring in the foregoing specification to draw attention to those features 5 of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not particular emphasis has been placed thereon. 13 02 25

Claims

1. A lock mechanism of a mechanical wrist for supporting a prosthetic hand, 5 comprising:a support member pivotally coupled at a hinge axis to a mounting member, the support member comprising an internal spline; anda lock assembly, wherein the lock assembly comprises:a latch body having a first groove defining a first path on a first side of10 the latch body, and having a second groove defining a second path on asecond side of the latch body;a lock spring mechanically biasing the latch body towards an unlocked position;a latch spring having a first pin and a second pin which are urged 15 towards each other, wherein the position of the latch body is partiallydetermined by the position of the first pin along the first path and the position of the second pin along the second path; anda lock button for actuating the latch body towards a locked position, the lock button comprising an external spline configured to engage with the 20 internal spline of the support member, wherein the external spline is engagedwith the internal spline when the latch body is in a locked position and the external spline is disengaged from the internal spline when the latch body is in the unlocked position.25 2. A lock mechanism of a mechanical wrist for supporting a prosthetic handaccording to claim 1 wherein the lock button is configured to move along the hinge axis to move the latch body between the locked position and the unlocked position.

3. A lock mechanism of a mechanical wrist for supporting a prosthetic hand 30 according to claim 1 or claim 2, wherein the first path comprises a stable unlock position , a stable lock position, an intermediary unstable lock overtravel position between the unlock position and the lock position and an intermediary unstable unlock overtravel position between the lock position and the unlock position, wherein the intermediary unstable lock overtravel position is engageable when the first pin 35 moves from the unlock position to the lock position, and the intermediary unstableunlock overtravel position is engageable when the first pin moves from the lock position to the unlock position.

4. A lock mechanism of a mechanical wrist for supporting a prosthetic hand 5 according to claim 3, where the second pin interacts with the second path to ensure that the first pin and second pin can only travel in a fixed direction of travel along the respective first path and second path.13 02 25

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