Design of Wearable Robots for the Hip and Knee for Gait Assistance with Selective Mechanical Transparency

Abstract

In recent years, research on wearable assistive devices has shown increasing promise for enhancing human mobility. However, several barriers remain to widespread adoption of these devices for practical real-world use. The work presented in this thesis focuses on one of these barriers: transparency, or how well a device can avoid impeding the wearer’s natural motion when not actively applying assistance. This thesis contributes to the field of wearable robotics by introducing various new mechanisms and methods for interfacing to the body, and applying them to the design of assistive devices for the hip and the knee that can be highly transparent as needed.

First, I present a rotary joint mechanism for rigid exoskeletons which enables them to be completely backdrivable as needed in situations where zero-torque control would be challenging, e.g. systems with high transmission ratios or transmissions with significant friction. The mechanism accomplishes this without requiring any additional active clutching or actuation components. A Bowden cable-driven exoskeleton for assisting hip flexion and extension was built using this mechanism.

The remainder of the work expands the design space of the already highly-transparent soft exosuit approach so it can be applied to a wider range of joints in more flexible ways. First is a lightweight, hinge-free wearable robot for assisting knee extension, which takes a primarily exosuit-based approach but with modifications to address the unique challenges of interfacing with the knee joint, including integration of rigid components to extend the moment arm, coupling of the knee assistance with hip abduction assistance to limit suit drift, and implementation of a hyperextension protection module in the controller. Testing with healthy subjects validated that the device could be completely transparent with no significant impact on user kinematics, but when active could reduce the biological knee power by applying power-based assistance profiles during sloped walking. In addition, preliminary testing with chronic post-stroke individuals shows initial promise for the potential of the knee device to support the paretic leg during stance.

The final component of this thesis revisits transparent bidirectional actuation but with soft exosuits, addressing how current exosuits require twice the number of motors to apply the same assistance as an equivalent rigid exoskeleton. I designed a compact and lightweight bidirectional exosuit actuation unit that uses a single motor to control two ropes in a conjoined manner, as well as the rope transmission routing and bracing methods necessary to interface it to the body. The actuator was integrated into an exosuit for combined hip flexion and hip extension assistance and demonstrated to be on par with previous “unidirectional” exosuits in terms of interface stiffness and efficiency while approximately maintaining their transparent nature.

Overall, the designs explored in this thesis advance the area of lightweight and selectively transparent wearable robots, and highlight the opportunities and challenges involved in making such devices be minimally restrictive to the wearer.