Browsing by Author "Wu, Molei"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Design and control of powered lower limb prostheses
    (University of Alabama Libraries, 2016) Wu, Molei; Shen, Xiangrong; University of Alabama Tuscaloosa
    In the development of powered lower-limb prostheses, providing sufficient power and torque to support amputees’ locomotion is a major challenge, considering prostheses’ weight and size limits. Furthermore, regulating the power delivery during locomotion is equally important that gives amputees safe and natural movements. This dissertation aims to address these challenges by investigating new approaches in the actuation and control of powered lower-limb prostheses, with the overarching objective to obtain compact, powerful lower-limb prostheses that interact with amputees and the environment in a coordinated manner. The initial efforts were focused on the design and control of transfemoral (TF, also known as above-knee) prostheses powered by pneumatic muscles, an extraordinary actuator with superb power-to-weight ratio. The first prototype incorporates powered knee and ankle joints in a volumetric profile similar to that of human leg. The unique feature is a single-acting-spring-return mechanism, in which a single pneumatic muscle drives the motion in the torque-demanding direction, while a set of mechanical springs drives the motion in the opposite direction. A finite-state impedance controller has been developed for this prosthesis, which was demonstrated to provide a natural gait. Based on previous success, a novel type of pneumatic muscle, namely double-acting sleeve muscle (DASM), was examined to replace traditional pneumatic muscle. Incorporating a second chamber, the DASM is able to provide additional extensional force without using return springs. Therefore, the prosthesis can be significantly simplified into a more compact and lightweight device. Compared with pneumatic muscles, traditional cylinder-type actuators are more technologically mature. Therefore, the subsequent efforts were to develop a pneumatic cylinder-actuated TF prosthesis, which has great potential for real-world applications. All peripheral components were integrated, including a carbon fiber air tank as the energy source, and the prosthesis’ capability of independent, untethered operation was demonstrated in human walking test. In addition to the improvement of prosthetic design, control methods were also investigated. The results include an integrated walking – stair climbing controller and a sit-to-stand controller. Both were developed based on biomechanical analysis of the knee dynamics in human locomotion. In the walking – stair climbing control system, an improved finite state impedance controller was constructed, which incorporates a unique time function to enable gradual energy injection during weight acceptance phase. An intuitive thigh position-based switching condition was introduced to merge the walking and stair climbing controllers into one system. In the sit-to-stand controller, a similar controller was established, which eliminates the need for a state machine and significantly simplifies the controller tuning and implementation. The human testing was conducted with results demonstrating the effectiveness of both control systems.
  • Thumbnail Image
    Item
    Obtaining Natural Sit-to-Stand Motion with a Biomimetic Controller for Powered Knee Prostheses
    (Hindawi, 2017) Wu, Molei; Haque, Md Rejwanul; Shen, Xiangrong; University of Alabama Tuscaloosa
    Standing up from a seated position is a common activity in people's daily life. However, for transfemoral (i.e., above-knee) amputees fitted with traditional passive prostheses, the sit-to-stand (STS) transition is highly challenging, due to the inability of the prosthetic joints in generating torque and power output. In this paper, the authors present a new STS control approach for powered lower limb prostheses, which is able to regulate the power delivery of the prosthetic knee joint to obtain natural STS motion similar to that displayed by healthy subjects. Mimicking the dynamic behavior of the knee in the STS, a unified control structure provides the desired control actions by combining an impedance function with a time-based ramp-up function. The former provides the gradual energy release behavior desired in the rising phase, while the latter provides the gradual energy injection behavior desired in the loading phase. This simple and intuitive control structure automates the transition between the two phases, eliminating the need for explicit phase transition and facilitating the implementation in powered prostheses. Human testing results demonstrated that this new control approach is able to generate a natural standing-up motion, which is well coordinated with the user's healthy-side motion in the STS process.
  • Thumbnail Image
    Item
    Pneumatic Variable Series Elastic Actuator
    (ASME, 2016) Zheng, Hao; Wu, Molei; Shen, Xiangrong; University of Alabama Tuscaloosa
    Inspired by human motor control theory, stiffness control is highly effective in manipulation and human-interactive tasks. The implementation of stiffness control in robotic systems, however, has largely been limited to closed-loop control, and suffers from multiple issues such as limited frequency range, potential instability, and lack of contribution to energy efficiency. Variable-stiffness actuator represents a better solution, but the current designs are complex, heavy, and bulky. The approach in this paper seeks to address these issues by using pneumatic actuator as a variable series elastic actuator (VSEA), leveraging the compressibility of the working fluid. In this work, a pneumatic actuator is modeled as an elastic element with controllable stiffness and equilibrium point, both of which are functions of air masses in the two chambers. As such, for the implementation of stiffness control in a robotic system, the desired stiffness/equilibrium point can be converted to the desired chamber air masses, and a predictive pressure control approach is developed to control the timing of valve switching to obtain the desired air mass while minimizing control action. Experimental results showed that the new approach in this paper requires less expensive hardware (on-off valve instead of proportional valve), causes less control action in implementation, and provides good control performance by leveraging the inherent dynamics of the actuator.