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Design of a Passive Dual Joint Stance Assistance Knee Exoskeleton


Lower Extremity Exoskeleton technology has been geared towards benefiting medical and augmentation fields. Functional modularity in exoskeleton design allows practitioners to prescribe exoskeletons which can be geared towards the user's needs and abilities. While most modular knee exoskeleton technology either live in the realm of medicine or augmentation, a stance assistive knee exoskeleton can benefit both able bodied individuals as well as individuals with decreased quadriceps function or knee weakness.

Fully passive systems are lower cost but have limited functionality. Powered systems have diverse functionality but are expensive and large. Microcontroller controlled resistive knees are more functionally diverse than fully passive system, but only provide system resistance to motion and can be expensive.

Passive microcontroller controlled systems can lead to a low cost and functionally versatile system. By embedding some of the required functionality into the mechanical hardware of the system, the burden on the microcontrollers and need for sensors is reduced. To determine the required functionality the characteristic behavior of the knee during level walking and stair descent were studied. Characteristics for a stance assistive knee joint exoskeleton are derived using biomechanics data.

These characteristics are

1) The joint provides resistance to flexion during stance

2) The joint does not impede extension during stance

3) The joint can enter free mode under load at any knee angle

4) The joint has very low impedance / is free while extending and flexing during swing

A dual jointed architecture with a clutch connected across one joint and a torque generator connected across the other is developed to mechanically achieve requirements 1 and 4. By requiring the clutch or torque generator to provide resistance in one direction but no impedance in the other, characteristic 2 is achieved. By controlling the locking and unlocking of the clutch, characteristic 3 is achieved. The torque generator and clutch requirements can be fulfilled by several passive mechanical components.

Several passive mechanical components were analyzed. It was determined that using a wrap spring clutch as the clutch and using a gas spring as the torque generator provides the required characteristics. This combination requires only a small actuator to lock and unlock the clutch. This combination also allows the restoration of energy during extension after it is stored during the resistance phases. This hardware combination results in a stance assistance knee exoskeleton which is relatively small, light and can further help reduce the metabolic cost associated with wearing an exoskeleton. A simplified controller was implemented to gauge system performance.

Additional applications for the hardware architecture are proposed. These applications include use of the knee exoskeleton with a modular hip. The hardware is not limited to use as a knee exoskeleton. It can be used to reduce the erector spinae muscles forces in the back, if it is installed at the user's hip. Experiments are proposed to evaluate the effect of the device on the biological joint torque and user's metabolic cost.

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