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Three different types of models were created for loaded gait modelsimulation
- A model without actuator
- A model with path actuators supporting plantar flexion
- A model with path actuators supporting hip extension
The path actuator supporting plantar flexion is attached to heel and tibia, and the pathactuator supporting hip extension is attached to backpack and femur. Loaded mass was added to torso for simplicity.
Unloaded gait model
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Same procedure has been done to unloaded walking data and models. All of these models represent unloaded walking model by empty types of models were created for unloaded gait simulation. The main difference between loaded gait model and unloaded gait model is the mass of torso, and transparancy of backpack.
Optimization
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methodology
The idea to optimize the control input force for the actuator is to take advantage of CMC tool. The main reason we use CMC in OpenSim is to find a most suitable excitations for muscles to create body movement while minimizing activation. To see how it works, refer to
In this project, I make use of the optimization procedure in CMC tool. CMC procedure. CMC different use of the objective function in CMC in order to find optimal input force.
- CMC procedure contains static optimization process, and it tries to minimize the cost function J which can be represented as
- When
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- we add active actuators on OpenSim Model, the activation term in cost function becomes
- Where X_muscle is muscle control and X_actuator is actuator control. X_actuator is part of activation state, and it is also adjusted after the optimization process.
- Now, if we diminish the influnece of X_actuator on J, and run CMC, the optimizer tries to find X_actuator in order to minimize muscle activation.
- We know that minimizing muscle activation correponds to minimizing metabolic cost, so we can we can say that the actuator input force resulted from CMC after diminishing the influence of X_actuator is the optimal actuator input for most efficient metabolic reduction.
Assign large value of maximum force to each actuator to reduce the size of xactuator, so that the influence of actuator to J is diminished.
Result & Discussion
Metabolic cost change
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Loaded walking
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Analysis of optimal input force for ankle actuator
This result shows how muscle force changes when a model has ankle actuators. The graphs show plantar flexor muscle forces, and first row is muscle forces of a baseline model, and the second row is the muscle forces of a model with ankle actuator.
The red line is The muscle forces of gastrocnemius, and it barely change when ankle actuators are added. However, other muscle forces, which are from uniarticular muscles, are significantly decreased. Therefore, we can say that ankle actuator assists uniarticular muscles during loaded walking
If we draw the sum of baseline uniarticular forces and active actuator input force together, we can see that the active actuator force follows baseline uniarticular muscle forces. The redline here is sum of baseline uniarticular forces and blue line is active actuator input force. This force signal is clear and easy to implement real world. However, the maximum actuation force is about 2500 N, which is too high, so we need to deal with it if we want to use this profile.
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