Material Strength Analysis for Designing a Hip Exoskeleton

Posted: August 25th, 2021

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Material Strength Analysis for Designing a Hip Exoskeleton

Children with neuromuscular disorders usually experience numerous limitations when it comes to walking. In recent times, it is emerging that walkers, crutches and other assistive home use devises do not provide the affected children with optimal and effective walking dosages. Also, physiotherapy accords the affected children with significantly less walking practice when compared to their typically developing peers. An increasing body of research in recent years shows that hip exoskeletons are more useful for improving the walking outcomes for children with neuromuscular defects (Wansoo et al. 23). Adults suffering from similar conditions can also benefit a lot from these exoskeletons. Developing an effective hip exoskeleton very much depends on the mechanical aspects of the materials used. Therefore, in this report, I endeavor to conduct a thorough strength analysis of the materials from which an effective hip exoskeleton can be designed. During operation, ahip exoskeleton is required to withstand varying forces. Hence, there is a need for using materials with a higher tensile strength to avoid incidences of failure and breaking. Some of the analyses that will be conducted include young modulus, elasticity, tensile strength, and stress, and yield. Accompanying equations will be included, and corresponding calculations computed.

Design of the Hip Exoskeleton

The purpose of the hip exoskeleton that I am looking to design in this project is to provide sufficient walking assistance to children with walking disorders. Some of these disorders could emanate from conditions such as cerebral palsy and stroke. To effectively perform walking tests, I aim at ensuring the structure of the hip exoskeleton is designed to be kinematically similar to the anatomy of the human hip and thigh. Also, social – exoskeleton interaction mechanics will be analyzed and optimized to enable natural walking.

            Material Selection. The materials I chose were mainly Aluminum 8020 T slotted extrusions. Upon subjecting them to tensile strength, they exhibited a yield strength of 275 MPa. For 500 million reversed cycles, its fatigue limit under cyclic load was 97MPa. Being that aluminum has a lower weight compared to other metals, aluminum alloys are easy to machine in computer numerical control, thus, allowing for faster prototyping of the hip exoskeleton. I chose this specific alloy due to its relatively high yield strength and low cost. In mechanical design spheres, the higher the yield strength of a material, the smaller the final frame can be. This material selection took into consideration the price and accessibility of the aluminum that can produce a practical and functional prototype of the exoskeleton later on in the project. To realize a functional prototype, each component would need to be machined by the use of Computer Numerical Control (CNC). Even though CNC costs a significant amount of financial resources, the material I chose for this project has a low cost, and thus, can easily and rapidly be machined when compared to other metals.

            In addition to the Aluminum 8020 T slotted extrusions for making the exoskeleton body, I selected the following materials. Upon conducting material strength analysis on each one of them in the lab, their mechanical properties are as computed alongside them.

            Plastic & Metal. These will be used to enable actuation of thigh movements on the hip exoskeleton. To realize this, they will be connected to small motors on the right and left sides of the exoskeleton.

Young’s Modulus, E = Tensile stress/ Tensile Strain

                                    = 2000 MPa/ 10

                                    = 200 MPa

Carbon Fiber. Will support the motors on the exoskeleton’s thigh frame. Upon my analysis, the following results highlighted carbon fiber’s mechanical properties.

Tensile strength = 4400 MPa

Density, 1740kg/m³

Elongation at breaking point = 1.7 percent

Young’s Modulus = 4400/1.7 = 235GPa

1050-1095 Spring Steel. To be used in adjusting the thigh brace height. From my lab analysis and computations, this material possesses the following mechanical properties.

Tensile strength = 685 MPa

Yield Strength = 525 MPa

Elongation at breaking point = 10 percent

Young’s Modulus = 685/10 %

 = 200GPa

Poisson’s ratio = 0.3

Nylon. Will be used in pairs in each thigh brace to fit the users’ size. To effectively serve this purpose, the nylon I intent to use in this project is extremely elastic and with significant amounts of stiffnes to prevent it from breaking.

            Human Body Segments

Several works of research have established the existence of various human body segment parameters (BSP). One of the earliest studies in this field can be traced back to 1862 by E. Harless, who did the first-ever dissection of a child’s cadaver. He developed the traditional proportional methods for computing human body segment parameters for 2D analysis (Jawad et al. 12). These parameters include segment mass, segment center of gravity, and segment length. Segment lengths, for example, are usually expressed as a percentage of the body height. For the accurate design of a useful hip exoskeleton, I made a commitment to understanding children’s lower body segment lengths, usually expressed as a fraction of body height (H).

Another valuable BSP in the designing of the hip exoskeleton is the center of gravity. Studies have shown that the center of gravity and center of mass for human beings are usually located at the same place. Primarily, the Center of gravity is the point at which the motionless body of a child would stay balanced. The center of Gravity can be quantified by assuming all other body segments are rigid and ignoring their shapes and structures. The table below highlights the key lower body segment distance proportions of the Center of gravity concerning segment endpoints. These values are essential in the design of the hip exoskeleton for children.

            Assumptions

            The summation of the operator’s and hip exoskeleton’s weight will be limited to 150 Kg. That is, the operator could be assumed to weigh 75kg and likewise for the hip exoskeleton. The sample child’s height will be limited to 1.85 meters. Also, the specimen’s mass distribution, hip, and upper mass aggregate weight will be 47.4 Kg. By use of the Dempster’s Proportional technique, the sample child’s lower body segment properties will be as shown in the table below.

Hip Exoskeleton Design

From the analysis I conducted, the Aluminum alloy extrusions that I used produced the results elaborated below. Collectively, these are the mechanical properties upon which the successful design of the exoskeleton that I am proposing will depend.

Tensile Yield Strength: 276Mpa

Young’s Modulus (E): Tensile strength/ Elongation at breaking point

= 276/22.5

= 68.9GPa

Elongation at breaking point: 22.5 percent

Poisson’s Ratio: 0.33

Specific Heat Capacity: 895 J/(kg K)

Ultimate Tensile Strength: 310MPa

Machinability: 50 percent

Sheer strength: 207MPa

To accurately design the hip exoskeleton, it is essentialfirst to figure out the relative locations of each joint and also to understand the reaction loads. Basing on the above-discussed results of mechanical strength analysis of the Aluminum 8020 T slotted extrusions were preferred in the final design and with the help of a built-in human model in software NX 11, it will be much more comfortable coming up with a prototype for the hip exoskeleton. Basing on the mechanical strength properties of Aluminum 8020 T slotted extrusions ascertained from my analysis, the hip exoskeleton’s frame will be low in volume and weight. Also, it will have a provision for avoiding direct contact with areas close to the children’s bones in addition to them having a cushioning effect on the users’ muscles to prevent injuries. To achieve this, I will insert polyester material with a high concentration of cotton wool at the hip joint areas of the exoskeleton.

Further, the Aluminum 8020 T slotted extrusions will allow the exoskeleton to be able to support itself to prevent encumbering its users. Moreover, the lightness of aluminum used will enable the user to enjoy all degrees of rotation freedom a significant extent, especially around the hip, knee and ankle areas. The full realization of this aspect gives the user the allowance of performing more natural walking exercises so that he or she can seamlessly traverse various types of terrains. At the design stage, I intend to use the modular approach with several components made from the Aluminum 8020 T slotted extrusions. This approach will allow the hip exoskeleton to be adaptable to different body types. After the mechanical strength analysis of the Aluminum 8020, T slotted extrusions, and I will take the hip exoskeleton’s automatic through a cyclical development process to reach an optimal that pretty much combines all the above-discussed material strength analyses into my design. The cyclical development process I intend to use is as shown below. 

Figure 4: Cyclical Development Process for the Hip Exoskeleton

The 3D model of the hip exoskeleton I am expecting to come up with is as shown below

Conclusion

Failing to properly plan increases the chances of any engineering project to fail. Thus, before designing the final hip exoskeleton to help children with walking disorders, a great deal of planning must go into ensuring that proper materials are selected for the project. For this particular project, I decided Aluminum 8020 T slotted extrusions. Basing on the results of these materials’ mechanical strength analysis discussed above, it is apparent that the exoskeleton to be designed from them will able to support itself to prevent encumbering its users. Moreover, the lightness of aluminum used will allow the user to enjoy all degrees of rotation freedom a significant extent, especially around the hip, knee and ankle areas. That is a perfect solution to the problem for which the exoskeleton will be designed, and thus, the materials are an excellent choice for this project.

Works Cited

Kim, Wansoo, et al. “Mechanical design of the Hanyang exoskeleton assistive robot (HEXAR).” 2014 14th international conference on control, automation, and systems (ICCAS 2014). IEEE, 2014.

Masood, Jawad, et al. “Mechanical design and analysis of lightweight hip joint Parallel Elastic Actuator for the industrial exoskeleton.” 2016 6th IEEE International Conference on Biomedical Robotics and Biomechatronics (BioRob). IEEE, 2016.