Nazarbayev University Repository

This work was done in the direction of improving the knee joint mechanism to achieve the best possible alignment between the exoskeleton movement and the human knee joint. The overall assembly of the knee joint mechanism is designed to be compatible with the exoskeleton developed at Nazarbayev University by the research team of Professor Prashant Kumar Jamwal (Figure B-1). Because the development of the models described in will significantly increase the complexity of the knee joint mechanism.

It is also clearly seen that most of these proposed exoskeleton models do not take into account the complex kinematics of the knee joint. Such a mismatch of the rotational mechanism in the knee joint of a robotic exoskeleton creates obvious discomfort for the user. In addition, placement of the knee joint actuator has a high risk of overlapping with the motor platform of the hip joint.

A bio-inspired knee joint (Figure 3-3) designed to reduce the misalignment provides the most accurate results to date [14]. These types of joints are modeled taking into account the femur bone curvature at the knee joint. The second model will use ball bearings at the lower part of the knee joint which will roll.

In both designs, the upper and lower part of the knee joint will be connected via springs and metal rods.

Knee joint concept kinematics

Arcs with a uniform radius make it possible to create a more accurate and compatible curvature profile for a gear concept. To place the axis of a motor on the initial center of rotation of a knee joint, the sketch was scaled twice. This magnification is sufficient to position the AK80-64 BLDC motor (Figure B-2) within the curvature contour and maintain the kinematics of the knee joint motion.

Therefore, it can be assumed that the concept uses two driven joints, the revolute and prismatic. Due to the non-circular curvature profile of the upper part, there will be another. This angle is dependent on 𝜃1 by the factor of the gear ratio which can be calculated by equation 4.1.

Where 𝑁1 = 11and 𝑁2 = 14 is the number of gear teeth on corresponding parts of the knee joint. Taking into account the limits of rotation of a knee joint, there will be certain restrictions on the angles of rotation. Additionally, there is one more non-constant parameter in this setup, 𝐷 which is the distance between the centers of rotation.

This parameter depends on the rotation angle of the rotor 𝜃1, and can be calculated using equation 4.4. Where distances between centers of rotation along the axis 𝑋 (𝐷𝑥) and the axis 𝑌 (𝐷𝑦) are also dependent on the value of 𝜃1.

Knee joint design with gear teeth

Where the distances between the centers of rotation along the axis 𝑋 (𝐷𝑥) and the axis 𝑌 (𝐷𝑦) also depend on the value of 𝜃1. and material consumption rates, it was found that triangular infill patterns can provide maximum strength with the least amount of material used for printing. a) Standard spur gear template marked in brown. A standardized spur gear tooth profile was used to design the top and bottom of this concept (Figure 4-4). This standard model is based on the spur gear design template available in the Power Transmission Tool included in the SolidWorks add-in library.

The modulus coefficient of 3.5 with 36 teeth was sufficient to withstand the forces that will act on each individual tooth during movement. Also, the 20 degree pressure angle and 56.75 mm face width significantly reduce the amount of stress per unit area. To design a prototype that will be compatible with the existing exoskeleton, some plastic models were reused from the actual model of the exoskeleton assembly for gait rehabilitation.

To keep the upper and lower parts connected, stiff springs were placed at the centers of rotation on both sides, and to keep the parts aligned in the sagittal plane, steel rods were rigidly mounted to both parts. After each plastic part was printed and the steel bars were laser cut, the prototype was assembled using mostly M6 bolts, nuts, and associated washers (Figure 4). Later, when the real prototype was assembled, it was found that manually turning the rotor shaft of the AK80-64 engine required significant torque.

As validation of the knee joint kinematics does not require the use of a real engine in practice, it was decided to model a plastic alternative that would have all the threaded holes and axis of rotation corresponding to the AK80-64 engine.

Knee joint design with ball bearings

Regarding the design of a lower part, it was necessary to model a new plastic part with 4 slots for ball bearings (Figure 4-6). It was decided to use four ball bearings as rollers, as this will stabilize the lower part during the movement and significantly reduce the load per unit area at the points of contact between the upper and lower parts.

Results

Design validation in CAD software

The markers on a thigh were named T1 and T2, while on a lower leg they were named S1 and S2, referring to the shaft. Since the thigh is fixed and the only moving part is the lower leg, it is suggested to compare data from lower leg markers and use thigh markers as a reference. Spacing between markers on a human leg was 250 mm, while on the mounting models it was 230 mm, therefore creating a visible offset between marker trajectories.

Practical design validation

The distance between the markers on the human leg was 250 mm, and 230 mm on the assembly models, thus creating a visible offset between the marker trajectories. a) Gear concept (b) Ball bearing concept. To evaluate the developed prototypes, it would be sufficient to measure its kinematics in practice with a motion capture system. First, the human leg flexion and extension motion data were recorded by the IMU sensors (Figure B-9).

Next, IMU sensors were attached to the knee joint prototypes to record the same motion (Figure B-10). Figure 5-4 shows that the recorded movement trajectory of the flexion-extension knee of the human leg has a certain degree of fluctuation due to the involuntary movement of the hip joint. When comparing the conceptual prototypes with each other, it is clear that the trajectories do not match (Figure 5-4(c).

Discussion

Conclusion

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