CN115229768B - Passive knee joint assistance exoskeleton - Google Patents

Abstract

The present invention discloses a passive knee joint assisting exoskeleton, comprising a shank rod, an inner plate, an outer plate, a Bowden cable, a roller, a tension spring, an upper connecting rod, a lower connecting rod, a ratchet-pawl mechanism and a positive locking mechanism; the shank rod is located between the inner plate and the outer plate, and the shank rod is connected to the outer plate and the inner plate through an upper connecting rod and a lower connecting rod respectively, and the rotation of the shank rod drives the upper connecting rod and the lower connecting rod to rotate, and the shank rod can only rotate along a specific trajectory due to the restriction of the upper limit grooves on the inner plate and the outer plate, so that the instantaneous rotation center trajectory of the exoskeleton knee joint is consistent with the instantaneous rotation center trajectory of the human body's knee joint; in the late swing phase of the gait cycle, when the human body's heel touches the ground, under the action of the positive locking mechanism, the ratchet and pawl of the ratchet-pawl mechanism are meshed, and the upper end of the tension spring is fixed, so that the exoskeleton generates a nonlinear torque to assist; after the sole of the human body completely touches the ground, the ratchet and pawl of the ratchet-pawl mechanism are disengaged, and the upper end of the tension spring is released to prevent the exoskeleton from affecting the swing of the human leg.

Description

A passive knee joint assist exoskeleton

Technical Field

The present invention belongs to the technical field of exoskeletons, and in particular relates to a passive knee joint power-assisted exoskeleton.

Background Art

As a type of exoskeleton, the passive power-assisted exoskeleton mainly achieves the power-assisted effect by collecting and returning the energy dissipated during human movement and improving the energy utilization rate of humans and machines. In the existing lower limb exoskeletons, it is generally believed that the human knee joint is mainly supported by locking the bone structure during walking, so there is no need to assist the knee joint. In the actual walking process of the human body, when the human foot touches the ground, the impact force of the ground will cause the knee joint to bend. In this process, the knee joint will generate torque to resist the impact force generated by the ground when the foot touches the ground and restore the knee joint to straighten. Therefore, it is necessary to assist the knee joint when resisting the impact force of the ground and when recovering to straighten.

According to relevant medical research, the trajectory formed by the instantaneous rotation center of the knee joint during walking is "J" shaped (see Figure 1), and the torque generated by the knee joint will also change at different times. In order to assist the knee joint, it is necessary to avoid the shear force perpendicular to the femur and tibia and the friction along the femur and tibia caused by the inconsistent human-machine movement to hinder human movement. At the same time, it is also necessary to provide a nonlinear torque to assist the knee joint more efficiently.

Existing knee exoskeletons usually use a four-bar mechanism or a cross four-bar mechanism to achieve variable rotation center movement, but these structures have poor flexibility, resulting in poor fitting effects. In addition, existing exoskeletons often apply torque by locking the mechanical structure or superimposing two springs when assisting. The applied torque does not conform to the nonlinear change of human knee joint torque, which not only affects the assist effect of the exoskeleton after wearing it, but also causes poor comfort after wearing the exoskeleton.

In summary, the present invention proposes a passive knee joint assisting exoskeleton, which can provide a nonlinear torque to assist the human knee joint while achieving consistency in human-machine motion.

Summary of the invention

In view of the deficiencies in the prior art, the technical problem that the present invention intends to solve is to provide a passive knee joint assisted exoskeleton.

The technical solution adopted by the present invention to solve the technical problem is as follows:

A passive knee joint power-assisted exoskeleton comprises a calf rod, an inner plate, an outer plate, a Bowden cable, a roller, a tension spring, an upper connecting rod, a lower connecting rod, a first slider, a second slider, a third slider, a fourth slider, a ratchet pawl mechanism and a positive locking mechanism;

The upper part of the calf rod is provided with an L-shaped slide groove, and the middle part is provided with a first shaft hole and a second shaft hole; the upper part of the inner plate is provided with a roller slideway, and the lower part is provided with a first inner plate limit groove and a second inner plate limit groove; the upper part of the outer plate is provided with a first outer plate limit groove, and the lower part is provided with a second outer plate limit groove;

The rear sides and bottoms of the inner and outer plates are connected together through an upper support rod and a lower support rod respectively, and the calf rod is located between the inner and outer plates; the upper and lower parts of the upper connecting rod are rotatably installed with a slider No. 1 and a slider No. 2 through a No. 1 connecting shaft respectively, the slider No. 1 cooperates with the No. 1 limiting groove of the outer plate, the slider No. 2 cooperates with the No. 2 limiting groove of the outer plate, and the position of the upper connecting rod close to the No. 2 slider is rotatably connected to the No. 1 shaft hole of the calf rod through a No. 4 connecting shaft; the upper and lower parts of the lower connecting rod are respectively connected through a No. 2 connecting shaft The connecting shaft is rotatably installed with a slider No. 3 and a slider No. 4, the slider No. 3 cooperates with the limiting groove No. 2 of the inner plate, the slider No. 4 cooperates with the limiting groove No. 1 of the inner plate, and the middle part of the lower connecting rod is rotatably connected with the No. 2 shaft hole of the calf rod through the No. 3 connecting shaft; the human body lifts the leg to drive the calf rod to rotate, and drives the upper connecting rod and the lower connecting rod to rotate, so that the four sliders slide in their respective limiting grooves to limit the rotation of the calf rod, so that the instantaneous rotation center trajectory of the exoskeleton knee joint is consistent with the instantaneous rotation center trajectory of the human body knee joint;

One end of the Bowden cable is connected to the upper end of the tension spring, and the other end is connected to the ratchet of the ratchet and pawl mechanism. The lower end of the tension spring is connected to the roller. One end of the roller cooperates with the roller slideway on the inner plate, and the other end cooperates with the L-shaped slideway on the calf rod. The ratchet and pawl mechanism and the positive locking mechanism are installed on the side of the inner plate adjacent to the calf rod. In the late swing phase of the gait cycle, when the heel of the human body touches the ground, under the action of the positive locking mechanism, the ratchet and pawl of the ratchet and pawl mechanism are engaged to fix the upper end of the tension spring, so that the exoskeleton generates nonlinear torque for assistance. After the sole of the human foot completely touches the ground, under the action of the positive locking mechanism, the ratchet and pawl of the ratchet and pawl mechanism are disengaged to release the fixation of the upper end of the tension spring, so as to prevent the exoskeleton from affecting the swing of the human leg.

Furthermore, the center line of the L-shaped chute satisfies the following formula:

y＝83.78e ^-0.3269x +4.619e ^0.197x ,Δx＝13

In the formula, Δx represents the change of the independent variable x.

Furthermore, the ratchet and pawl mechanism includes a ratchet pressure plate, a ratchet, a torsion spring, a pawl pressure plate and a pawl; the ratchet and the pawl are respectively installed on the inner plate through the ratchet shaft and the pawl shaft, and torsion springs are respectively provided in the ratchet and the pawl. Under the action of external force, the ratchet and the pawl rotate around their respective axes, driving the torsion spring to rotate. When the external force is unloaded, the ratchet and the pawl are reset under the action of the torsion spring; the ratchet pressure plate is installed on the ratchet shaft to axially limit the ratchet; the pawl pressure plate is installed on the pawl shaft to axially limit the pawl.

Furthermore, the positive locking mechanism includes a sleeve, a push rod, a guide rail, a compression spring and a push rod; the sleeve is installed on the inner plate, the lower part of the push rod, the upper part of the guide rail and the push rod are all located in the sleeve, the push rod and the push rod are both matched with the guide rail, and the compression spring is sleeved on the push rod; a round table is provided at the front end of the middle part of the calf rod, and the round table can contact the push rod during the rotation of the calf rod, so that the push rod pushes the push rod to slide upward, so that the push rod pushes the pawl to rotate, and then the pawl engages with the ratchet; when the push rod pushes the push rod to slide upward again, the ratchet and the pawl disengage.

Furthermore, the exoskeleton also includes an end cover of the outer plate limit groove No. 1, an end cover of the outer plate limit groove No. 2, an end cover of the inner plate limit groove No. 1, an end cover of the inner plate limit groove No. 2 and a roller slide end cover. The corresponding slider is limited by each limit groove end cover, and the roller is limited by the roller slide end cover.

Compared with the prior art, the present invention has the following beneficial effects:

1. The calf rod is connected to the inner plate and the outer plate through two connecting rods and four sliders. The two limit grooves of the inner plate and the two limit grooves of the outer plate are used to limit the movement trajectory of the calf rod, so that the instantaneous rotation center trajectory of the exoskeleton knee joint better fits the instantaneous rotation center trajectory of the human knee joint, improves the consistency of human-machine movement, reduces the obstruction to human movement caused by the inconsistency of human-machine movement, and increases the comfort of wearing the exoskeleton.

2. While the calf rod is moving, the roller slides downward along the L-shaped groove and the roller slideway under the action of the calf rod, so that the tension spring is stretched. The shape of the L-shaped groove limits the nonlinear relationship between the downward movement of the roller (which is also the stretching amount of the tension spring) and the rotation angle of the calf rod, thereby making the torque generated by the exoskeleton and the rotation angle of the calf rod have a nonlinear relationship, thereby causing the exoskeleton to generate nonlinear torque, which helps the knee joint to resist the impact force of the ground and to recover the straightening of the knee joint.

3. The ratchet and pawl mechanism and the positive locking mechanism constitute a variable stiffness mechanism, which keeps the stiffness of the human-machine joint consistent, makes the exoskeleton's assistance to the human body smoother, and improves the user's comfort. In the swing phase of the gait cycle, the ratchet and pawl are not engaged, and the upper end of the tension spring is not fixed. The exoskeleton does not generate torque and does not affect the swing of the legs. At the end of the swing phase of the gait cycle, the ratchet and pawl are engaged, the upper end of the tension spring is fixed, and the tension spring is stretched, causing the exoskeleton to generate nonlinear torque to resist the impact force generated by the ground when the human foot lands, and to assist the knee joint.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a trajectory diagram of the instantaneous rotation center of the human knee joint;

Fig. 2 is a schematic diagram of the overall structure;

FIG3 is a schematic diagram of the structure after removing the outer plate;

FIG4 is a schematic structural diagram of a calf rod;

FIG5 is a schematic diagram of the structure of the inner side plate;

FIG6 is a schematic diagram of the structure of the outer plate;

FIG7 is an exploded view of the upper connecting rod and the first and second sliders;

FIG8 is an exploded view of the lower connecting rod and the third and fourth sliders;

FIG9 is an assembly diagram of the upper connecting rod and the outer side plate;

FIG10 is an exploded view of the ratchet and pawl mechanism;

FIG11 is an exploded view of the positive locking mechanism;

FIG12 is a curve diagram comparing the human knee joint torque and the torque generated by the exoskeleton of the present invention;

In the figure: 1, calf rod; 2, inner plate; 3, outer plate; 4, Bowden wire; 5, roller; 6, tension spring; 7, ratchet and pawl mechanism; 8, positive locking mechanism; 9, upper support rod; 10, lower support rod; 11, upper connecting rod; 12, lower connecting rod; 13, connecting shaft No. 1; 14, connecting shaft No. 2; 15, slider No. 1; 16, slider No. 2; 17, slider No. 3; 18, slider No. 4; 19, connecting shaft No. 3; 20, guide wheel; 21, outer plate No. 1 limit slot end cover; 22, outer plate No. 2 limit slot end cover; 23, inner plate No. 1 limit slot end cover; 24, inner plate No. 2 limit slot end cover; 25, roller slideway end cover; 26, connecting shaft No. 4;

101, L-shaped slide; 102, shaft hole No. 1; 103, shaft hole No. 2; 104, round table; 201, roller slide; 202, inner plate limit slot No. 1; 203, inner plate limit slot No. 2; 301, outer plate limit slot No. 1; 302, outer plate limit slot No. 2; 701, ratchet pressure plate; 702, ratchet; 703, torsion spring; 704, pawl pressure plate; 705, pawl; 801, sleeve; 802, push rod; 803, guide rail; 804, compression spring; 805, push rod.

DETAILED DESCRIPTION

The technical solution of the present invention is described in detail below in conjunction with specific implementation methods and drawings, which is not intended to limit the protection scope of the present application.

The present invention is a passive knee joint power-assisted exoskeleton (exoskeleton for short, see FIGS. 2 to 12 ), comprising a calf rod 1, an inner plate 2, an outer plate 3, a Bowden cable 4, a roller 5, a tension spring 6, an upper connecting rod 11, a lower connecting rod 12, a first slider 15, a second slider 16, a third slider 17, a fourth slider 18, a ratchet pawl mechanism 7 and a positive locking mechanism 8;

The upper part of the calf rod 1 is provided with an L-shaped slide groove 101, the middle part is provided with a No. 1 shaft hole 102 and a No. 2 shaft hole 103, and the front end of the middle part is provided with a round table 104; the upper part of the inner plate 2 is provided with a vertical roller slideway 201, and the lower part is provided with an inner plate No. 1 limiting groove 202 and an inner plate No. 2 limiting groove 203 that are at a certain angle to the horizontal direction; the upper part of the outer plate 3 is provided with an outer plate No. 1 limiting groove 301, and the lower part is provided with an outer plate No. 2 limiting groove 302, and the outer plate No. 1 limiting groove 301 and the outer plate No. 2 limiting groove 302 are both at a certain angle to the vertical direction;

The rear sides and bottoms of the inner plate 2 and the outer plate 3 are connected together by the upper support rod 9 and the lower support rod 10 respectively, and the calf rod 1 is located between the inner plate 2 and the outer plate 3; the upper and lower parts of the upper connecting rod 11 are rotatably installed with the first slider 15 and the second slider 16 through the first connecting shaft 13 respectively, the first slider 15 cooperates with the first limiting groove 301 of the outer plate, and the second slider 16 cooperates with the second limiting groove 302 of the outer plate. The position of the lower connecting rod 16 is rotatably connected to the No. 1 shaft hole 102 of the calf rod 1 through the No. 4 connecting shaft 26; the upper and lower parts of the lower connecting rod 12 are rotatably installed with the No. 3 slider 17 and the No. 4 slider 18 through the No. 2 connecting shaft 14, respectively, the No. 3 slider 17 cooperates with the No. 2 limiting groove 203 of the inner plate, and the No. 4 slider 18 cooperates with the No. 1 limiting groove 202 of the inner plate, and the middle part of the lower connecting rod 12 is rotatably connected to the No. 2 shaft hole 103 of the calf rod 1 through the No. 3 connecting shaft 19;

A guide wheel 20 is provided at the upper part of the inner plate 2, and the Bowden cable 4 passes around the guide wheel 20. One end of the Bowden cable 4 is connected to the upper end of the tension spring 6, and the other end is connected to the ratchet 702 of the ratchet pawl mechanism 7. The lower end of the tension spring 6 is connected to the roller 5. One end of the roller 5 cooperates with the roller slideway 201 on the inner plate 2, and the other end cooperates with the L-shaped slideway 101 on the calf rod 1; the ratchet pawl mechanism 7 and the positive locking mechanism 8 are installed on the side of the inner plate 2 adjacent to the calf rod 1, and the positive locking mechanism 8 serves as a trigger mechanism for the ratchet 702 of the ratchet pawl mechanism 7 to engage with the pawl 705. When the push rod 802 of the positive locking mechanism 8 slides upward, the ratchet 702 engages with the pawl 705. When the push rod 802 slides upward again, the ratchet 702 and the pawl 705 are disengaged.

When entering the swing phase of the gait cycle, the human body lifts the leg to drive the calf rod 1 to rotate, so that the roller 5 slides downward in the L-shaped slide 101 and the roller slide 201, and at the same time drives the upper connecting rod 11 and the lower connecting rod 12 to rotate, so that the No. 1 slider 15 slides in the No. 1 limiting groove 301 of the outer plate, the No. 2 slider 16 slides in the No. 2 limiting groove 302 of the outer plate, the No. 3 slider 17 slides in the No. 2 limiting groove 203 of the inner plate, and the No. 4 slider 18 slides in the No. 1 limiting groove 202 of the inner plate; restricted by the four limiting grooves, the calf rod 1 can only rotate along a specific trajectory, so that the instantaneous rotation center trajectory of the exoskeleton knee joint is consistent with the instantaneous rotation center trajectory of the human knee joint, and the human knee The trajectory of the instantaneous rotation center of the joint is shown in Figure 1; in the late swing phase of the gait cycle, the human calf gradually straightens, driving the calf rod 1 to rotate in the opposite direction. When the human heel touches the ground, the round table 4 on the calf rod 1 pushes the push rod 802 of the positive locking mechanism 8 to slide upward. Under the action of the positive locking mechanism 8, the ratchet 702 of the ratchet and pawl mechanism 7 engages with the pawl 705 to fix the upper end of the tension spring 6; after the human foot touches the ground, under the action of the ground impact force, the human calf bends again, causing the calf rod 1 to rotate in the opposite direction again (in the same direction as when the human leg is raised), and the tension spring 6 is stretched to generate elastic force, so that the exoskeleton generates torque to resist the ground impact force, thereby achieving a power-assisting effect.

The ratchet and pawl mechanism 7 includes a ratchet pressure plate 701, a ratchet 702, a torsion spring 703, a pawl pressure plate 704 and a pawl 705; the ratchet 702 and the pawl 705 are respectively mounted on the inner plate 2 via a ratchet shaft and a pawl shaft, and torsion springs 703 are respectively provided in the ratchet 702 and the pawl 705. Under the action of an external force, the ratchet 702 and the pawl 705 rotate around their respective axes, driving the torsion spring 703 to rotate. When the external force is unloaded, the ratchet 702 and the pawl 705 are reset under the action of the torsion spring 703; the ratchet pressure plate 701 is mounted on the ratchet shaft to axially limit the ratchet 702; the pawl pressure plate 704 is mounted on the pawl shaft to axially limit the pawl 705.

The positive locking mechanism 8 includes a sleeve 801, a push rod 802, a guide rail 803, a compression spring 804 and a push rod 805; the sleeve 801 is installed on the inner plate 2, the lower part of the push rod 805, the guide rail 803 and the upper part of the push rod 802 are all located in the sleeve 801, the push rod 802 and the push rod 805 are both matched with the guide rail 803, and the compression spring 804 is sleeved on the push rod 802; the push rod 802 can push the push rod 805 to slide upward, so that the push rod 805 contacts one end of the pawl 705 and the pawl 705 rotates around the pawl shaft, thereby causing the pawl 705 to engage with the ratchet 702; the principle of the positive locking mechanism 8 is the same as that of the push-type ballpoint pen.

The exoskeleton also includes an outer plate limit groove end cover 21, an outer plate limit groove end cover 22, an inner plate limit groove end cover 23, an inner plate limit groove end cover 24 and a roller slideway end cover 25; the outer plate limit groove end cover 21 is installed on the outer plate 3, the No. 1 connecting shaft 13 passes through the outer plate limit groove end cover 21 and is connected to the upper part of the upper connecting rod 11, the end face of the outer plate limit groove end cover 21 is in contact with the end face of the No. 1 slider 15, the No. 1 slider 15 is limited, and it is ensured that the No. 1 slider 15 can only slide in the No. 1 limit groove 301 of the outer plate; the installation method of the remaining limit groove end covers and the roller slideway end covers is similar.

The center line of the L-shaped chute 101 satisfies the following formula:

y＝83.78e ^-0.3269x +4.619e ^0.197x ,Δx＝13

In the formula, Δx represents the change in x.

The working principle and workflow of the present invention are:

The lower part of the calf rod 1 of the exoskeleton is bound to the human calf through a strap, and the inner plate 2 is bound to the human thigh through a strap; the end of the double support phase of the gait cycle is taken as the initial position, at which time the calf rod 1 remains vertical, the ratchet 702 and the pawl 705 are not engaged, the upper end of the tension spring 6 is not fixed, and the roller 5 is located at the upper end of the L-shaped slide 101 of the calf rod 1 and the roller slide 201;

Entering the swing phase of the gait cycle, the human body lifts the leg, and the calf bends to drive the calf rod 1 to rotate clockwise, driving the roller 5 to slide downward in the L-shaped slide groove 101 and the roller slide groove 201, and the ratchet 702 is rotated through the Bowden wire 4, and the torsion spring 703 in the ratchet 702 is twisted; while the calf rod 1 rotates clockwise, it drives the upper connecting rod 11 and the lower connecting rod 12 to move, so that the first slider 15 slides in the first limiting groove 301 of the outer plate, the second slider 16 slides in the second limiting groove 302 of the outer plate, the third slider 17 slides in the second limiting groove 203 of the inner plate, and the fourth slider 18 slides in the first limiting groove 202 of the inner plate. The rotation of the calf rod 1 is limited by the four limiting grooves, so that the instantaneous rotation center trajectory of the exoskeleton knee joint is consistent with the instantaneous rotation center trajectory of the human body's knee joint;

In the late swing phase, the human calf gradually straightens, the calf rod 1 rotates counterclockwise, the roller 5 slides upward in the L-shaped slide groove 101 and the roller slide groove 201, the ratchet 702 rotates in the opposite direction, and the torsion spring 703 in the ratchet 702 resets, so that the roller 5 resets; the calf rod 1 continues to rotate counterclockwise, and in the late swing phase of the gait cycle, when the human heel touches the ground, the round table 104 on the calf rod 1 contacts the lower end of the push rod 802 and pushes the push rod 802 to slide along the guide rail 803, and the push rod 802 pushes the push rod 805 to slide upward, so that the upper part of the push rod 805 pushes one end of the ratchet 705 and drives the ratchet 705 to rotate, so that the other end of the ratchet 705 meshes with the ratchet 702, and at the same time, under the action of the compression spring 804, the push rod 805 is locked with the guide rail 803, which is equivalent to fixing the upper end of the tension spring 6;

During the period from when the heel of the human body touches the ground to when the sole of the foot completely touches the ground, the human calf bends again under the action of the ground impact force, causing the calf rod 1 to rotate clockwise, and the round table 104 on the calf rod 1 is out of contact with the push rod 802. At the same time, under the action of the calf rod 1, the roller 5 slides downward again along the L-shaped slide groove 101 on the calf rod 1 and the roller slideway 201 on the inner plate 2, and the tension spring 6 is stretched to generate elastic force, and the exoskeleton generates torque to resist the ground impact force, thereby achieving a power-assisting effect;

When the sole of the person's foot is completely on the ground, it is no longer affected by the impact force of the ground, and the human calf is straightened again. The calf rod 1 rotates counterclockwise again, and the tension spring 6 contracts to assist the calf rod 1 to rotate, thereby helping the human calf to straighten again; when the human calf is completely straightened, the round table 104 on the calf rod 1 contacts the lower end of the push rod 802 again and pushes the push rod 802 to slide along the guide rail 803. Under the action of the compression spring 804, the push rod 805 is disengaged from the guide rail 803, and the push rod 805 slides downward, and the push rod 805 is disengaged from the pawl 705. The pawl 705 and the ratchet 702 are reset respectively under the action of the torsion spring 703. At this time, the upper end of the tension spring 6 is released and waits to enter the next cycle.

In the above description, clockwise and counterclockwise are the rotation directions of the calf rod when viewed from the right side of the human body.

FIG12 is a curve diagram comparing the human knee joint torque and the torque generated by the exoskeleton of the present invention. It can be seen from the figure that the torque generated by the exoskeleton has a high degree of fit with the human knee joint torque, which not only provides assistance but also improves the wearing comfort.

Any matters not described in the present invention are applicable to the prior art.

Claims (5)

1. A passive knee joint assist exoskeleton, characterized in that the exoskeleton comprises a calf rod, an inner plate, an outer plate, a Bowden cable, a roller, a tension spring, an upper connecting rod, a lower connecting rod, a first slider, a second slider, a third slider, a fourth slider, a ratchet pawl mechanism and a positive locking mechanism; The upper part of the calf rod is provided with an L-shaped slide groove, and the middle part is provided with a first shaft hole and a second shaft hole; the upper part of the inner plate is provided with a roller slideway, and the lower part is provided with a first inner plate limit groove and a second inner plate limit groove; the upper part of the outer plate is provided with a first outer plate limit groove, and the lower part is provided with a second outer plate limit groove; The rear sides and bottoms of the inner and outer plates are connected together through an upper support rod and a lower support rod respectively, and the calf rod is located between the inner and outer plates; the upper and lower parts of the upper connecting rod are rotatably installed with a slider No. 1 and a slider No. 2 through a No. 1 connecting shaft respectively, the slider No. 1 cooperates with the No. 1 limiting groove of the outer plate, the slider No. 2 cooperates with the No. 2 limiting groove of the outer plate, and the position of the upper connecting rod close to the No. 2 slider is rotatably connected to the No. 1 shaft hole of the calf rod through a No. 4 connecting shaft; the upper and lower parts of the lower connecting rod are respectively connected through a No. 2 connecting shaft The connecting shaft is rotatably installed with a slider No. 3 and a slider No. 4, the slider No. 3 cooperates with the limiting groove No. 2 of the inner plate, the slider No. 4 cooperates with the limiting groove No. 1 of the inner plate, and the middle part of the lower connecting rod is rotatably connected with the No. 2 shaft hole of the calf rod through the No. 3 connecting shaft; the human body lifts the leg to drive the calf rod to rotate, and drives the upper connecting rod and the lower connecting rod to rotate, so that the four sliders slide in their respective limiting grooves to limit the rotation of the calf rod, so that the instantaneous rotation center trajectory of the exoskeleton knee joint is consistent with the instantaneous rotation center trajectory of the human body knee joint; One end of the Bowden cable is connected to the upper end of the tension spring, and the other end is connected to the ratchet of the ratchet and pawl mechanism. The lower end of the tension spring is connected to the roller. One end of the roller cooperates with the roller slideway on the inner plate, and the other end cooperates with the L-shaped slideway on the calf rod. The ratchet and pawl mechanism and the positive locking mechanism are installed on the side of the inner plate adjacent to the calf rod. In the late swing phase of the gait cycle, when the heel of the human body touches the ground, under the action of the positive locking mechanism, the ratchet and pawl of the ratchet and pawl mechanism are engaged to fix the upper end of the tension spring, so that the exoskeleton generates nonlinear torque for assistance. After the sole of the human foot completely touches the ground, under the action of the positive locking mechanism, the ratchet and pawl of the ratchet and pawl mechanism are disengaged to release the fixation of the upper end of the tension spring, so as to prevent the exoskeleton from affecting the swing of the human leg. 2. The passive knee joint assist exoskeleton according to claim 1, characterized in that the center line of the L-shaped slide groove satisfies the following formula: y＝83.78e ^-0.3269x +4.619e ^0.197x ,Δx＝13 In the formula, Δx represents the change of the independent variable x. 3. The passive knee joint assisted exoskeleton according to claim 1 is characterized in that the ratchet and pawl mechanism includes a ratchet pressure plate, a ratchet, a torsion spring, a pawl pressure plate and a pawl; the ratchet and the pawl are respectively installed on the inner plate through the ratchet shaft and the pawl shaft, and torsion springs are respectively provided in the ratchet and the pawl. Under the action of external force, the ratchet and the pawl rotate around their respective axes, driving the torsion spring to rotate. When the external force is unloaded, the ratchet and the pawl are reset under the action of the torsion spring; the ratchet pressure plate is installed on the ratchet shaft to axially limit the ratchet; the pawl pressure plate is installed on the pawl shaft to axially limit the pawl. 4. The passive knee joint assisted exoskeleton according to claim 3 is characterized in that the positive locking mechanism includes a sleeve, a push rod, a guide rail, a compression spring and a push rod; the sleeve is installed on the inner plate, the lower part of the push rod, the upper part of the guide rail and the push rod are all located in the sleeve, the push rod and the push rod are both matched with the guide rail, and the compression spring is sleeved on the push rod; a round table is provided at the front end of the middle part of the calf rod, and the round table can contact the push rod during the rotation of the calf rod, so that the push rod pushes the push rod to slide upward, so that the push rod pushes the pawl to rotate, and then the pawl engages with the ratchet; when the push rod pushes the push rod to slide upward again, the ratchet and the pawl disengage. 5. The passive knee joint assisted exoskeleton according to any one of claims 1 to 4 is characterized in that the exoskeleton also includes an outer plate limit groove end cover No. 1, an outer plate limit groove end cover No. 2, an inner plate limit groove end cover No. 1, an inner plate limit groove end cover No. 2 and a roller slide end cover, and the corresponding slider is limited by each limit groove end cover, and the roller is limited by the roller slide end cover.