P13002: Ankle-Foot Orthotic Un-Tethered, Mechanical

Systems Design

Table of Contents


Listed below are the developed benchmark matrices for the actuation and microcontroller elements of the device.

Actuation Benchmark Matrix

Micro Controller Benchmark

Concept Development

Our team has developed two general concepts in order to assist the user in dorsi-flexion of the foot.

The first general concept involves the users foot in place before the liftoff stage of gait and releasing the foot after the swing phase of gait. As the user steps forward with his/her normal foot, the affected foot is naturally dorsi-flexed. Prior to lifting that foot, a mechanical device can be used to constrain the person's foot from rotation while off the ground. Using the terrain sensing system, the constraint on the user's foot can be removed at the appropriate time, allowing gravity to plantar-flex the foot.

This can be accomplished using some sort of ratchet device that allows only dorsiflexion. Once the ratchet is reversed or released, plantar flexion may occur. A rotational or linear ratchet can be used.

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Another method to constrain the rotation of the foot would be using a braking mechanism. A hydraulic friction brake can be used on a rod connected to the heal of the AFO. When the brake is applied, rotation of the ankle will be prevented.

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The pitfall with this concept is that the locking of the foot angle will force unnatural gait upon the person using the system and will prevent push-off of the toes.

The second general concept involves using a commercial AFO with built in springs (posterior leaf spring) for dorsi-flexion and adding a method of applying torque in order to plantar-flex as needed. The most promising method of applying a torque to rotate the foot will be to use a linear actuator placed behind the heal and calf of the AFO, mimicking the motion and position of the Achilles tendon. Possible methods of toque application include use of a servomotor, solenoid actuator, power screw, and muscle wire.

A third general concept involves using a hard stop to control how far the foot will be able to move.

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The following table shows the pros and cons of using each method.

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The following concept selection matrix was used to rank the 3 potential concepts with respect to customer needs. According to this matrix, the "Mechanical Locking" mechanism is the best concept.

Concept Selection Matrix

The team brainstormed various methods of "Locking" the foot in place when on/off the ground. Ideas considered include:
Hydraulic System
Piston/Cylinder Arrangement (with air or a fluid)
Linear Ratchet Mechanism

The following matrix was used to rank and select our final concept.

Locking Mechanism Selection Matrix

Functional Decomposition

The functional decomposition displays the requirements of the system and the elements by which the requirements can be assessed. The main function of the device will be to mitigate the effects of foot drop through the active ankle foot orthotic.

Functional Decomposition

Systems Architecture

The image below displays the established system architecture for the device. The main elements of the system include the Physical AFO/Framework, Power Supply, Terrain Sensing System, Controlling Device, and Actuation Device.

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Risk Assessment

The risk assessment developed up to the System Design Review is posted below. For the most recent version, refer to the risk assessment link under the Project Review tab.

Risk Assessment

Systems Design Review

The presentation used for the System Design Review, which took place on October 5th 2012 (week 5 of fall quarter) can be viewed below. The system design review covered the developed project foundation, which included constraints, concepts, selections, risk assessment, and schedule planning. Also included were notes taken throughout the presentation, which highlighted the questions and feedback mentioned.


Design Review Feedback Notes

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