The present invention relates to a knee joint used for a prosthesis.
Background technology
[0002]
　Generally, the prosthesis is composed of a socket fixed to the cut surface of the foot, a knee joint connected to the lower end of the socket, and a foot portion connected to the lower end of the knee joint to be a ground contact portion. Like the knee joint, the knee joint can be extended and flexed within a predetermined angle range.
[0003]
　There are three types of knee joint drive methods: passive type, electronically controlled type, and active type. In the passive type, the wearer moves the artificial leg, and the knee joint is passively flexed / extended by using the damper and spring force of the hydraulic or pneumatic cylinder in response to the movement of the artificial leg. In the electronically controlled type, the movement resistance in flexion and extension of the knee joint can be adjusted electronically to improve the operation of the knee joint. An example of an electronically controlled knee joint is shown in Patent Document 1 below. Further, in the active type, the bending angle of the knee joint is actively controlled by using a motor to support the movement of the knee joint in movements such as climbing stairs and standing up.
[0004]
　By the way, the conventional active knee joint has a problem that not only is it expensive due to its complicated structure, but also it is easy for the wearer to get tired because it is heavy. In particular, conventional active knee joints require a large capacity battery and tend to be large and heavy because the motor mounted on the knee joint must always be operated and is not energy efficient.
[0005]
　On the other hand, the following Non-Patent Document 1 discloses an active knee joint in which a linear motion by a series elastic actuator (so-called "Series Elastic Actuator") is converted into a rotary motion by a pulley to rotate the knee joint. .. In this technique, by using the spring of the series elastic actuator, the walking energy is utilized to obtain high energy efficiency as compared with the energy efficiency of the conventional active knee joint. However, in this technology, in order to convert the linear motion of the series elastic actuator into the rotational motion of the knee, the belt fixed to the linear elastic element becomes a mechanism to rotate the knee via two pulleys. There is. Belts and pulleys should be placed on the sides of the knee joint to prevent interference between the linearly moving elastic element and the knee. Then, in order to maintain the balance, it becomes necessary to use two belts in one knee joint. Therefore, this technique has a problem that the mechanism becomes very complicated and the number of parts is large. If you try to increase the movable angle (movable range) of the knee joint, the belt and pulley will become large, making it difficult to use in terms of size and weight. Further, since the belt for rotating the pulley has a problem in terms of durability, the cost for maintaining or replacing the belt tends to be high.
[0006]
　Further, Patent Document 2 below describes a configuration in which a knee member is rotatably attached to the upper end of the lower limb member and a foot member is attached to the lower end of the lower limb member. A protrusion is integrally formed on the side of the knee member, and a linear actuator is attached between the protrusion and the lower part of the lower limb member. In this technology, the driving force of this linear actuator can assist the rotational movement of the knee member. However, in this technique, since the linear actuator is directly connected to the knee member without a speed reducer, there is a problem that a high load is applied to the linear actuator when trying to obtain a high drive torque.
Prior art literature
Patent documents
[0007]
Patent Document 1: Japanese Patent Application Laid-Open No. 2004-167106
Patent Document 2: International Publication No. 2004/017872
Non-patent literature
[0008]
Non-Patent Document 1: Elliott J. Rouse, Luke M.Mooney and Hugh M. Herr, "Clutchable series-elastic actuator: Implications for prosthetic knee design" October 9, 2014, doi: 10.1177 / 0278364914545673 The International Journal of Robotics Research November 2014 vol. 33 no. 13 1611-1625
Outline of the invention
Problems to be solved by the invention
[0009]
　The present invention has been made based on the above-mentioned situation. A main object of the present invention is to provide a knee joint that is energy efficient, compact and lightweight, and has a wide range of motion.
Means to solve problems
[0010]
　The means for solving the above-mentioned problems can be described as the following items.
[0011]
　(Item 1) A
　drive unit, a series elastic mechanism, and a crank mechanism are provided.
　The series elastic mechanism includes a driven member, an elastic member, and a linear motion member, and the
　drive unit is the drive unit. The structure is such that the driven member is moved, the
　elastic member is arranged between the driven member and the linear motion member, and the
　linear motion member is the driven member via the elastic member. The crank mechanism is configured to elastically move in at least one direction according to the movement, and the
　crank mechanism is configured to convert the linear motion of the linear motion member into a rotary motion.
[0012]
　(Item 2) Further, the item 　1
　is provided with an upper connection portion for connecting the socket and the knee joint, and the
　crank mechanism is configured to rotate the upper connection portion in the forward and reverse directions.
Knee joint.
[0013]
　(Item 3) 　The knee joint according to item 1 or 2,
　further comprising a frame,
　wherein the linear motion member is movable in at least one direction with respect to the frame .
[0014]
　(Item 4) 　The knee joint according to item 1 or 2 ,
　further
　comprising a frame, wherein the rotation shaft of the crank mechanism is supported by the frame .
[0015]
　(Item 5)
　The drive unit includes a motor, a speed change mechanism, and a ball screw, and the
　motor is configured to rotate the ball screw in the forward and reverse directions via the speed change mechanism. 　The knee joint according to any one of items 1 to 4 ,
　wherein the driven member is configured to move linearly in response to the rotation of the ball screw .
[0016]
　(Item 6)
　The linear motion member includes a first contact portion and a second contact portion arranged so as to face each other with the driven member interposed therebetween, and the
　elastic member includes a first spring and a second spring. A spring is provided,
　the first spring is arranged between the first contact portion and the driven member, and
　the second spring is provided with the second contact portion and the driven member.
　The knee joint according to any one of items 1 to 5 arranged between them.
[0017]
　(Item 7)
　An artificial leg provided with the knee joint according to any one of items 1 to 6.
The invention's effect
[0018]
　According to the present invention, it is possible to provide a knee joint that is energy efficient, compact and lightweight, and has a wide range of motion. Further, according to the present invention, it becomes possible to provide a knee joint which is an active type but relatively inexpensive.
A brief description of the drawing
[0019]
FIG. 1 is a perspective view of a knee joint (flexion angle = 0 °) according to an embodiment of the present invention with the cover removed.
FIG. 2 is a front view of FIG.
3 is a left side view of FIG. 2. FIG.
4 is a plan view of FIG. 2. FIG.
5 is a cross-sectional view taken along the line AA of FIG.
6 is a perspective view of the knee joint of FIG. 1 with a cover attached.
FIG. 7 is a front view of FIG.
8 is a left side view of FIG. 7. FIG.
9 is a plan view of FIG. 7. FIG.
10 is a cross-sectional view taken along the line AA of FIG.
11 is a perspective view of the knee joint of FIG. 1 at a bending angle of 60 °.
12 is a front view of FIG. 11. FIG.
13 is a left side view of FIG. 12. FIG.
14 is a plan view of FIG. 12. FIG.
15 is a cross-sectional view taken along the line AA of FIG.
16 is a perspective view of the knee joint of FIG. 1 at a bending angle of 120 °.
FIG. 17 is a front view of FIG.
FIG. 18 is a left side view of FIG.
19 is a plan view of FIG. 17. FIG.
20 is a cross-sectional view taken along the line AA of FIG.
FIG. 21 is a schematic explanatory view showing an example in which an artificial leg is configured by using the knee joint of FIG. 1.
22 is an explanatory diagram for explaining the operation of the artificial leg of FIG. 21. FIG.
23 is an explanatory diagram for explaining the operation of the crank mechanism in the knee joint of FIG. 1. FIG.
24 is a schematic explanatory view of the crank mechanism of FIG. 23. FIG.
25 is a graph showing an example of the characteristics of the crank mechanism of FIG. 24, in which the horizontal axis is the knee angle (degrees) and the vertical axis is the reduction ratio.
FIG. 26 is a graph showing changes in knee angle during walking, with the horizontal axis representing time (arbitrary unit) and the vertical axis representing knee angle (degrees).
FIG. 27 is an explanatory diagram showing an example in which the offset amount of the series elastic mechanism with respect to the rotating shaft in the crank mechanism is changed.
28 is a graph showing a characteristic example of the crank mechanism of FIG. 27 superimposed on the characteristic example of FIG. 25, in which the horizontal axis is the knee angle (degrees) and the vertical axis is the reduction ratio.
Embodiment for carrying out the invention
[0020]
　Hereinafter, the knee joint according to the embodiment of the present invention will be described with reference to the attached drawings (FIGS. 1 to 10). Of these figures, FIGS. 1 to 5 show a state in which the cover 51 (described later) of the frame 5 is excluded, and FIGS. 6 to 10 show a state in which the cover 51 is attached.
[0021]
　As shown in FIG. 21 (described later), the knee joint 100 of the present embodiment can be combined with the socket 200 and the foot portion 300 to form an artificial leg. Hereinafter, the configuration of the knee joint 100 of the present embodiment will be described.
[0022]
　(Structure of the
　present embodiment) The knee joint 100 of the present embodiment includes a drive unit 1, a series elastic mechanism 2, and a crank mechanism 3. Further, the knee joint 100 includes an upper connecting portion 4, a frame 5, and a lower connecting portion 6.
[0023]
　(Drive unit)
　The drive unit 1 includes a motor 11, a speed change mechanism 12, and a ball screw 13 (see FIG. 5). The motor 11 is configured to rotate the ball screw 13 in the forward and reverse directions via the speed change mechanism 12. The drive unit 1 of the present embodiment includes a battery (not shown), and can drive the motor 11 by supplying power from the battery. However, it is also possible to drive the motor 11 by external power supply (for example, commercial power supply). Further, the drive unit 1 includes a sensor (not shown) that detects the rotation angle of the crank mechanism 3 and the load on the motor 11, and can control the torque and rotation speed of the motor 11 according to the output of the sensor. It has become like. The motor 11, the speed change mechanism 12, and the ball screw 13 of the present embodiment are supported by the frame 5 via an appropriate mounting member or bearing.
[0024]
　(Series Elastic Mechanism)
　The series elastic mechanism 2 includes a driven member 21, an elastic member 22, and a linear motion member 23. Further, the series elastic mechanism 2 of the present embodiment includes a guide shaft 24 for guiding the driven member 21 and the first and second contact portions 231 and 232 of the linear moving member 23.
[0025]
　The driven member 21 is configured to be moved along the guide shaft 24 (in the vertical direction in FIG. 1) by the driving force of the driving unit 1. More specifically, the driven member 21 of the series elastic mechanism 2 in the present embodiment is configured to reciprocate in a linear direction according to the rotation of the ball screw 13 of the driving unit 1.
[0026]
　The elastic member 22 is arranged between the driven member 21 and the linearly moving member 23. More specifically, the elastic member 22 of the present embodiment includes two first springs 221 and two second springs 222 (see FIGS. 1 and 3). The first spring 221 is arranged between the first contact portion 231 (described later) of the linear motion member 23 and the driven member 21, but is not fixed to these members. The second spring 222 is arranged between the second contact portion 232 (described later) of the linear motion member 23 and the driven member 21, but is not fixed to these members.
[0027]
　The linear motion member 23 is configured to elastically move in at least one direction according to the movement of the driven member 21 via the elastic member 22. More specifically, as described above, in the linear motion member 23 of the present embodiment, the first contact portion 231 and the second contact portion 232 and the linear motion rod are arranged so as to face each other with the driven member 21 interposed therebetween. It is equipped with 233. Further, the first contact portion 231 and the second contact portion 232 are connected by a support column 234 (see FIG. 3). The strut 234 penetrates the driven member 21 and allows relative movement between the two. Further, the support column 234 is arranged so as to pass through the insides of the first spring 221 and the second spring 222 of the elastic member 22, respectively. In this example, the lower end of the linear motion rod 233 and the upper end of the support column 234 are connected to each other, and these are integral parts.
[0028]
　The number of guide shafts 24 is two in the present embodiment, and each of the guide shafts 24 is arranged so as to connect the upper base 52 and the lower base 53 (described later) of the frame 5. (See FIG. 2). The two guide shafts 24 are not fixed to the driven member 21, the first contact portion 231 and the second contact portion 233, whereby the driven member 21, the first contact portion 231 and the second contact portion 231 are not fixed. The contact portion 233 can move along the extension direction of the guide shaft 24 (that is, in the vertical direction in FIG. 2).
[0029]
　In the present embodiment, the linear motion rod 233 of the linear motion member 23 penetrates the upper base 52 of the frame 5 and is fixed to the upper surface of the first contact portion 231 (see FIGS. 2 and 3). According to the movement of the first contact portion 231 and the second contact portion 232, the guide shaft 24 reciprocates along the extension direction (that is, in the vertical direction in FIG. 1).
[0030]
　(Crank mechanism)
　The crank mechanism 3 is configured to convert the linear motion of the linear motion member 23 into a rotary motion. The crank mechanism 3 of the present embodiment includes a connecting rod 31, an arm member 32, and a rotating shaft 33.
[0031]
　One end of the connecting rod 31 is pin-coupled to the upper end of the linear motion rod 233 of the linear motion member 23 so as to be mutually rotatable.
[0032]
　The arm member 32 is pin-coupled to the other end of the connecting rod 31 so as to be rotatable with each other. Further, the arm member 32 can swing around the rotation shaft 33. An upper connecting portion 4 is attached to the upper portion of the arm member 32.
[0033]
　In the present embodiment, the rotating shaft 33 is attached to the cover 51 (described later) of the frame 5, and the relative position between the rotating shaft 33 and the frame 5 is fixed.
[0034]
　(Upper connection portion)
　The upper connection portion 4 is for connecting the socket 200 (see FIG. 21 described later) and the knee joint 100. The upper connection portion 4 realizes the extension / flexion operation of the artificial leg by rotating in the forward and reverse directions by the crank mechanism 3. The upper connection portion 4 is also called a pyramid connector, and can be connected to the socket 200 by using an existing method.
[0035]
　(Frame)
　The frame 5 of the present embodiment includes a cover 51 (see FIGS. 6 to 10), an upper base 52, and a lower base 53. In the present embodiment, the linear motion member 23 is movable in at least one direction (specifically, in the vertical direction in FIG. 1) with respect to the upper and lower bases 52 and 53 of the frame 5. Further, in the present embodiment, as described above, the rotary shaft 33 of the crank mechanism 3 is supported by the cover 51 of the frame 5 in a rotatable state. Further, the upper base 52 and the lower base 53 are fixed to the cover 51, respectively, so as not to move relative to each other.
[0036]
　(Lower connection portion)
　The lower connection portion 6 is for connecting the knee joint 100 and the foot portion 300 (see FIG. 21 described later). The lower connecting portion 6 is fixed to the lower base 53 of the frame 5. The lower connection portion 6 is also called a pyramid connector, and can be connected to the foot portion 300 by using an existing method.
[0037]
　(Operation of the present embodiment)
　Next, the operation of the knee joint 100 of the present embodiment will be described with reference to FIGS. 11 to 22.
[0038]
　(Angle adjustment operation of the knee joint ... 0 ° to 60 °)
　In the description of the present embodiment, the extended state shown in FIG. 1 is defined as the angle of the knee joint being 0 °. The operation of bending the knee joint angle to 60 ° from this state will be described below.
[0039]
　First, the motor 11 of the drive unit 1 is rotated. Then, the ball screw 13 rotates via the speed change mechanism 12, and the driven member 21 of the series elastic mechanism 2 moves in one direction (downward in FIG. 1 in this example).
[0040]
　Then, the driven member 21 applies a compressive force to the second spring 222 of the elastic member 22 and moves the linearly moving member 23 in one direction (downward in FIG. 1 in this example) via the spring thereof. When the linear motion member 23 descends due to the spring force, the connecting rod 31 of the crank mechanism 3 descends, and as a result, the arm member 32 rotates about the rotation shaft 33 (see FIGS. 11 to 15). As a result, the upper connecting portion 4 can be rotated by a desired angle. The knee angle in human walking repeats from 0 ° (extended state) to about 60 ° (flexed state). Therefore, after the knee flexion of 60 °, the motor 11 is rotated in the reverse direction and a compressive force is applied to the first spring 221 to return the motor 11 to the extended state of 0 °.
[0041]
　(Knee joint angle adjustment operation ... 60 ° to 120 °)
　The operation when the knee angle is between 60 ° and 120 ° is the operation when the user sits, sits upright, or the knee touches the floor. Is. When sitting, the motor does not operate in particular, but when standing up from the sitting state, assistance can be achieved by the operation of the motor.
[0042]
　16 to 20 show an example in which the knee joint is bent to 60 ° or more. By further rotating the arm member 32 of the crank mechanism 3 in the same manner as described above, the knee joint can be bent up to about 120 ° (ideally up to about 140 °).
[0043]
　By rotating the motor 11 of the drive unit 1 in the reverse direction, the bending angle of the knee joint can be returned to the initial state (angle = 0 °).
[0044]
　In the present embodiment, the bending angle of the knee joint 100 can be dynamically changed by appropriately controlling the torque, the rotation speed, or the rotation angle of the motor 11. The behavior of the prosthesis user (walking and alternating stair climbing) by actively controlling the rotation angle of the knee joint by the driving force of the drive unit 1 when climbing the stairs or standing up from the chair, for example, when the prosthesis is used. And stand-up movement) can be supported.
[0045]
　In addition, the technique described in Non-Patent Document 1 described above has the following problems.
-The mechanism that converts the linear motion of the elastic mechanism into the rotational motion of the knee becomes very complicated, and the number of parts is large. Therefore, it becomes heavy;
・ When trying to widen the movable angle of the knee joint, the pulley becomes large;
・ It is necessary to place the pulley on the side of the artificial leg to prevent interference between the component (elastic element) and the knee. .. Then, in order to maintain balance, it becomes necessary to use two pulley mechanisms (one pulley mechanism consists of two pulleys, one connecting cable and related parts) in one prosthesis
; Has a problem in terms of durability and tends to have a high maintenance cost.
[0046]
　On the other hand, according to the knee joint of the present embodiment described above,
・ Since the crank mechanism is used, it is possible to suppress the increase in size of the entire knee joint even when the movable angle of the knee joint is widened;
・ Pulley mechanism Instead of, a single compact and lightweight crank mechanism can be used to provide a compact and lightweight prosthesis;
• Crank mechanisms are generally more durable than pulleys, thus keeping maintenance costs low. You can take
advantage of being able to.
[0047]
　(Attachment state of knee joint)
　Next, an example in which the knee joint of the present embodiment is attached to the user will be described with reference to FIG. 21. In this example, the lower end of the socket 200 is connected to the upper connection portion 4 of the knee joint 100, and the foot portion 300 is connected to the lower connection portion 6 of the knee joint 100. In the illustrated example, the artificial leg is composed of the knee joint 100, the socket 200, and the foot portion 300. An attachment (not shown) similar to the conventional one can be used for the connection between the upper connection portion 4 and the socket 200 and the connection between the lower connection portion 6 and the foot portion 300.
[0048]
　(Walking Movement Using Prosthesis)
　Next, a walking movement using the prosthesis of the present embodiment will be described with reference to FIG. 22. In this figure, a reference numeral L is attached to the artificial leg.
[0049]
　(FIGS. 22A to 22B)
　When the foot portion of the artificial leg lands on the floor surface, in the knee joint of the present embodiment, the socket 200 is pushed downward to the linear motion member 23 via the crank mechanism 3. Pressure is applied, which causes the first spring 221 of the elastic member 22 to be elastically deformed and its energy is stored. However, since the linear motion member 23 is attached to the ball screw 13 via the driven member 21, the resistance to the movement of the driven member 21 by operating the motor in the direction opposite to the direction in which the ball screw tries to rotate. A force can be created and the energy in the elastic member 22 can be retained.
[0050]
　Here, in the present embodiment, by appropriately setting the spring force and the initial position of the elastic member 22, the bending angle (passive) of the knee joint at the time of contact of the foot (FIGS. 22A to 22B). The bending angle due to deformation) can be set to about 20 °. In this way, there is an advantage that the knee flexion angle of about 20 ° for shock absorption when the heel touches the ground, which is the original of human beings, can be realized, and the user's feeling of use can be improved.
[0051]
　Further, in the present embodiment, the repulsive force from the floor surface applied to the foot portion is transmitted to the user via the elastic member 22, so that the impact at the time of installation can be alleviated and the user can be used. It is possible to reduce the progress of fatigue.
[0052]
　(FIGS. 22 (b) to (d))
　During the subsequent walking motion, the energy stored in the elastic member 22 is released. As a result, the linear motion member 23 can be moved to extend the knee joint.
[0053]
　(FIGS. 22 (d) to (e))
　Then, in the present embodiment, the motor 11 is operated to bend the knee joint (see, for example, FIG. 12). As a result, energy can be stored in the elastic member 22.
[0054]
　(FIGS. 22 (e) to 22 (f))
　When walking further progresses, in the present embodiment, the motor 11 is driven in the opposite direction to extend the knee joint. Here, in the present embodiment, the energy stored in the elastic member 22 assists the extension operation of the knee joint, so that the power required for the motor 11 can be reduced. Therefore, in the present embodiment, it can be expected that the battery will be miniaturized and the battery will last for a long time.
[0055]
　Further, when the frictional resistance between the ball screw 13 and the linear motion member 23 is set low, there is an advantage that the electric power can be regenerated by the motor 11 by using the elastic force of the elastic member 22 described above.
[0056]
　Next, the operation of the crank mechanism 3 of the present embodiment will be described in detail with reference to FIGS. 23 to 28.
[0057]
　First, for the purpose of explaining the operation, the structure of the knee joint of the above-described embodiment is schematically shown in FIG. 23. In this figure, the motor 11, the speed change mechanism 12, and the elastic member 22 are shown as one actuator. The mechanism of FIG. 23 is more schematically shown in FIG. 24.
[0058]
　The reduction ratio in the knee joint using the crank mechanism 3 is expressed by the following equation.
[0059]

[0060]
　Here,
N m : the number of teeth of the pulley on the motor 11 side in the speed change mechanism 12;
N b : the number of teeth of the pulley on the ball screw 13 side in the speed change mechanism 12;
L b : the lead of the ball screw 13;
R: the arm member 32 Turning radius;
K: Deceleration coefficient by the crank mechanism
.
[0061]
　Here, since each variable other than K can be regarded as a constant in the description here, detailed description thereof will be omitted. The deceleration variable K of the crank mechanism can be expressed as follows.
[0062]

[0063]
　Here,
α: the angle of the arm member 32 with respect to the vertical direction (vertical direction in FIG. 24);
β: the angle of the connecting rod 31 with respect to the vertical direction (vertical direction in FIG. 24)
.
[0064]
　Due to the influence of the deceleration variable K, the reduction ratio in the crank mechanism is as shown in FIG. In this characteristic, the reduction ratio changes with the change of the knee angle, and the reduction ratio becomes maximum in the vicinity of the knee angle α = 80 °. That is, in the crank mechanism of the present embodiment, the knee joint can be driven at different reduction ratios as the knee angle changes.
[0065]
　FIG. 26 shows an example of changes in the knee angle with time of human walking. As shown in this figure, the knee angle changes between about 0 ° and 80 ° during walking. Further, for example, when going up and down stairs or standing up from a chair, the knee angle may change drastically from a knee angle of about 80 ° to a knee angle of about 0 °. When such a large angle change from a deep knee angle is required, a large torque is required to rotate the knee joint. In this embodiment, a high reduction ratio can be obtained at a deep knee angle of about 80 °. Then, there is an advantage that a large torque can be applied to the knee joint without imposing a large load on the motor 11.
[0066]
　Further, in the case of a hurry leg, the knee joint is required to have a high rotation speed at a shallow knee angle, but does not require a high torque. The crank mechanism of the present embodiment has an advantage that the rotation speed of the knee joint can be easily increased because the reduction ratio is low in the case of a shallow knee angle (for example, 0 ° to 20 °).
[0067]
　If a pulley mechanism (see Non-Patent Document 1 described above) is used instead of the crank mechanism, the reduction coefficient K does not exist in the pulley mechanism, so that the reduction ratio in the pulley mechanism is constant regardless of the knee angle. Become. Therefore, when a large torque is required, a large load may be generated on the motor. Further, the same problem occurs even when the speed change mechanism is not used (see Patent Document 2 described above). On the other hand, the knee joint of the present embodiment has an advantage that both high torque and high-speed rotation can be achieved by using the crank mechanism.
[0068]
　FIG. 27 shows an example in which the offset amount between the rotation center of the crank mechanism 3 and the series elastic mechanism 2 is changed. The characteristics of the reduction ratio after changing the offset amount are shown by the solid line in FIG. 28. The alternate long and short dash line in FIG. 28 is a characteristic in the example of FIG. 25. As can be seen from this figure, the relationship between the knee angle α and the reduction ratio can be adjusted by changing the offset amount. Therefore, according to the present embodiment, there is an advantage that the maximum torque can be obtained at a required knee angle by adjusting the offset amount.
[0069]
　The content of the present invention is not limited to the above embodiment. The present invention may make various changes to a specific configuration within the scope of the claims.
Description of the sign
[0070]
　1 Drive unit
　11 Motor
　12 Speed ​​change mechanism
　13 Ball screw
　2 Series elastic mechanism
　21 Driven member
　22 Elastic member
　221 First spring
　222 Second spring
　23 Linear member
　231 First contact part
　232 Second contact part
　233 Linear rod
　24 Guide shaft
　3 Crank mechanism
　31 Connecting rod
　32 Arm member
　33 Rotating shaft
　4 Upper connection part
　5 Frame
　51 Cover
　52 Upper base
　53 Lower base
　6 Lower connection part
　100 Knee joint
　200 Socket
　300 Foot
　L Prosthesis

WE CLAIM:
1. A crank mechanism for a knee joint for changing the angle ofthekneeto enable it to convert linear motion to rotational motion, said knee joint consisting of a motor, a linear motion member that reciprocates in a linear direction in accordance with rotation of a ball screw that is rotated by the motor and said crank mechanism, wherein said crank mechanism comprises a connecting rod that is rotatably connected to the linear motion member, an ami member that is attached to the connected rod so as to be capable of rotating, and that swings in accordance with movement of the connecting rod, and a rotation shaft constituting a center of rotation when this arm member swings, and the crank mechanism converts linear motion of the linear motion member to rotational motion; and a reduction ratio of the cranlc mechanism becomes a maximum value when the knee angle is about 80° to 90°, and is a lower reduction ratio than the maximum value when the knee angle is about 0° to 20°, and is set so that reduction ratio becomes lower as knee angle becomes smaller.
2. The crank mechanism as claimed in claim 1, wherein a point of connection between the lineal' motion member and the connecting rod is arranged offset by a specified offset amount from a rotational center of the rotation shaft, and
the specified offset amount is determined so that the reduction ratio becomes a maximum value when the knee angle is about 80° to 90°.
3. The crank mechanism as claimed in claim 1 or claim 2, the knee joint further comprising an upper connection section for connecting a socket and the knee joint, wherein the crank mechanism is connected to the upper connection section, and is configured to cause rotational movement of the upper connection section in forward and backward directions.
4. The crank mechanism as claimed in any one of claims 1 to 3, wherein the linear motion member of the knee joint includes a linear motion rod that extends in a movement

direction of the linear motion member, and the connectiifg" rod is rotatahly comiected to an end part of the linear motion rod. "
5. The crank mechanism, as claimed in any of claims 1 to 4, the knee joint further comprising a frame, wherein a rotational shaft of the crank mechanism is supported by the frame.
6. The crank mechanism as claimed in of any one of claims 1 to 5 whenever incorporated into the knee joint of a prosthetic leg.