Design of Energy Efficient Prosthetic Foot Using Topology Optimization
Mrunmayi Mali1, Sarangi Barodkar1, Prachi Inamdar1, Bharanidharan.R2
1Vellore Institute of Technology, Vellore, India
Email: [email protected]
Compared with the conventional flat foot, the flexible foot is advantageous in implementing human?like walking and much reduces energy consumption. The goal of this work is to design highly versatile, fully integrated lower-extremity powered prostheses that can replicate the biological behaviour of the intact human leg. The design will be modified such that it will use least amount of energy possible for walking. From an anatomical and kinesiological point of view, design of a flexible foot with toes and heels considering frontal toe position and ankle joint position will give desired comfort and mobility. Thus the main focus of the work is to use more advanced tools such as topology optimization, compliant mechanism to design a prosthetic foot that will be beneficial for the amputees. The changes will result into several advantages compared to conventional foot in: walking step, walking speed, range of joint angle, change in angular velocity and joint energy-output.
Keywords—prosthetic foot, amputee gait, compliant mechanism, topology optimization, energy efficient
1. INTRODUCTION –
Prosthetics are intended to restore the normal functions of the missing body part. A trans-tibial prosthesis is an artificial limb that replaces a leg missing below the knee. A trans-tibial amputee is usually able to regain normal movement. The energy storage and return capabilities of prostheses are crucial. The negative effects of high energy consumption results in low activity level. Thus there is an opportunity for a design of energy efficient foot. Poor compliant adaptive ability is the main problem with the prosthetic devices. The design challenge with compliant mechanisms is essentially the optimal distribution of materials in a worked domain. To design compliant mechanism, a topology optimization method can be used.
1. Heel Strike – The heel strike phase starts the moment when the heel first touches the ground and lasts until the whole foot is on the ground
2. Early flatfoot- The beginning of the “early flatfoot ” stage is defined as the moment that the whole foot is on the ground. The end of the “early flatfoot” stage occurs when the heel lifts off the ground
3. Late flatfoot- Once the Body’s centre of gravity has passed in front of the neutral position, a person is said to be in the late flatfoot stage.
4. Heel rise- As the name suggests, the heel rise phase begins when the heel begins to leave the ground. Toe off The toe off stage of gait begins as the toes leave the ground.
Topology optimization (TO) is a mathematical method that optimizes material layout within a given design space, for a given set of loads, boundary conditions and constraints with the goal of maximizing the performance of the system. TO is different from shape optimization and sizing optimization in the sense that the design can attain any shape within the design space, instead of dealing with predefined configurations.
The conventional TO formulation uses a finite element method FEM to evaluate the design performance. The design is optimized using either gradient-based mathematical programming techniques such as the optimality criteria algorithm and the method of moving asymptotes or non gradient based algorithms such as genetic algorithms.
Topology Optimization has a wide range of applications in aerospace, mechanical, bio-chemical and civil engineering. Currently, engineers mostly use TO at the concept level of a design process. Due to the free forms that naturally occur, the result is often difficult to manufacture. For that reason the result emerging from TO is often fine-tuned for manufacturability. Adding constraints to the formulation in order to increase the manufacturability is an active field of research. In some cases results from TO can be directly manufactured using additive manufacturing; TO is thus a key part of design for additive manufacturing.
• Problem Description –
Unilateral trans-tibial amputee gait consumes up to 60% more energy than able-bodied gait. For higher level amputees, research suggests that energy efficiency drops by well over 80%. The negative effects of high energy consumption are compounded by reductions in walking speed of typically 40% for trans-tibial amputees with associated low activity levels. This has a tremendous impact on what amputees can achieve and the consequences for their quality of life. Thus the main objective is to design energy efficient prosthetic leg by using topology optimization.
Design Objectives –
1. To optimize the weight of the prosthetic foot using topology optimization.
2. To design an energy efficient foot in order to help the amputees.
3. To use compliant mechanism in existing prosthetic design to improve the performance.
2. METHODOLOGY –
3. MODELLING –
The solid model of foot was prepared in SolidWorks. The average foot dimensions were taken based on statistical study of body measurements of the soldiers.
4. Material selection –
The material selection for prosthetic devices is important because the material directly affects the comfort of the socket and the level of mobility for the patient. The strength and weight of the material contribute to the comfort of the patient while walking. The material associated with the prosthetic limb has a direct effect on the cost of the limb and choosing a material with a lower cost can make the prosthetic much more affordable. Many different types of materials are used for prosthetics, ranging from advanced carbon fiber materials to more simple co-polymers which are easily manipulated and require less technology to mold. Therefore, the material selection is one of the most important aspects to a prosthetic design. We selected the material as polyurethane foam made up of (polyol + isocynate) and Polyurethane elastomer 2. Physical (Density), mechanical (Elastic Modulus, Shear Modulus, Poisson Ratio, Tensile Strength and Yield Strength) and thermal properties (Thermal Expansion and Thermal Conductivity) 2 values were taken for both the foot and according to that the material was finalized. Properties were given as an input to software as material property before analysis.
The finite element analysis was performed on modelled foot. When various loads were applied to the model in different gait cycle conditions 7, maximum ; minimum stress values were obtained which are shown through graphical representation. Further, the design was topologically optimized in Ansys Workbench 18.
Over the last decade, there has been an adequate increment of computer applications in the field of rehabilitation and prosthetic designing. FEA is also one amongst these revolutionary ways to analyze an existing or future model for prediction of its breakage point, durability, stress bearing capacity and displacement. We performed FEA analysis of foot to test the stress bearing capacity. After analyzing stress and displacement, foot was topologically optimized. We used the compliant mechanism to make the design energy efficient. The polyurethane was selected as a material as its foot displacement threshold is higher due to very high resilience of polyurethane foam and due to that displacement appears to be lower.
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