For the controller of wearable lower-limb assistive devices, quantitative understanding of human locomotion serves as the basis for human motion intent recognition and joint-level motion control. Traditionally, the required gait data are obtained in gait research laboratories, utilizing marker-based optical motion capture systems. Despite the high accuracy of measurement, marker-based systems are largely limited to laboratory environments, making it nearly impossible to collect the desired gait data in real-world daily-living scenarios. To address this problem, the authors propose a novel exoskeleton-based gait data collection system, which provides the capability of conducting independent measurement of lower limb movement without the need for stationary instrumentation. The basis of the system is a lightweight exoskeleton with articulated knee and ankle joints. To minimize the interference to a wearer’s natural lower-limb movement, a unique two-degrees-of-freedom joint design is incorporated, integrating a primary degree of freedom for joint motion measurement with a passive degree of freedom to allow natural joint movement and improve the comfort of use. In addition to the joint-embedded goniometers, the exoskeleton also features multiple positions for the mounting of inertia measurement units (IMUs) as well as foot-plate-embedded force sensing resistors to measure the foot plantar pressure. All sensor signals are routed to a microcontroller for data logging and storage. To validate the exoskeleton-provided joint angle measurement, a comparison study on three healthy participants was conducted, which involves locomotion experiments in various modes, including overground walking, treadmill walking, and sit-to-stand and stand-to-sit transitions. Joint angle trajectories measured with an eight-camera motion capture system served as the benchmark for comparison. Experimental results indicate that the exoskeleton-measured joint angle trajectories closely match those obtained through the optical motion capture system in all modes of locomotion (correlation coefficients of 0.97 and 0.96 for knee and ankle measurements, respectively), clearly demonstrating the accuracy and reliability of the proposed gait measurement system.
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.
BackgroundStanding from a seated position is a common, yet dynamically challenging task. Due to the vertical ascent of the body center of gravity, sit-to-stand (STS) transition requires high torque output from the knee. As a result, STS transition poses a major barrier to the mobility of individuals with lower-limb issues, including the transfemoral (TF, also known as above-knee) amputees. A study showed that unilateral TF amputees suffer from high asymmetry in ground reaction forces (53~69%) and knee moments (110~124%), while the asymmetry for healthy controls is less than 7% [1]. Note that, although a powered TF prosthesis (Power Knee TM ) was used in this study, it generated resistance in the STS and thus produced similar results as the passive devices. The inability of existing prostheses in generating knee torque and regulating the torque delivery in the STS seriously affects the mobility of TF amputees in their daily life.Motivated by this issue, researchers have developed numerous powered TF prostheses (e.g., Vanderbilt powered TF prostheses [2]). These devices are able to generate torque and power for challenging tasks such as STS transition. Making full use of such capability, however, requires an effective controller. Currently, walking control for powered prostheses has been well established, but STS control is much less investigated. Varol et al. developed a multi-mode TF prosthesis controller, in which STS is treated as a transitional motion between sitting and standing states [2]. However, no details were provided on the rationale of the STS controller structure or the determination of the control parameters.In this paper, a new prosthesis control approach is presented, which regulates the power and torque delivery in the STS process. Inspired by the biomechanical behavior of the knee in the STS motion, the new controller provides two desired functions (gradual loading of the knee at the beginning, and automatic adjustment of the knee torque according to motion progress) with a single equation. Combined with a simple yet reliable triggering condition, the proposed control approach is able to provide natural STS motion for the powered knee prosthesis users. MethodsTo provide a natural control experience for amputee users, an STS controller should replicate the biomechanical behavior of the knee in this process. However, exactly replicating the motion of the biological knee is unfeasible, since such position control approach precludes the interaction between the prosthesis and its user and results in a poor control experience. Biomechanical data, on the other hand, provide insight to the dynamic behavior of the knee and thus can be used as an inspiration for the controller.Unlike cyclical motion modes such as walking, STS is a typical transitional motion with clearly defined start (seated position) and end (standing position). The typical joint position and torque trajectories are shown in Fig. 1 (data from [3]). The entire process can be divided into two phases, separated by the instant of Seat-Off...
Mobility impairment is becoming a challenging issue around the world with a rapid increase on aging population. Existing tools of walking assistance for mobility-impaired people include passive canes or wheeled rollators which increase energy consumption on the users and disturb the users' walking rhythm, and powered wheeled chairs which could preclude the muscle activities and accelerate the degeneration of the lower limbs. The research in this paper aiming at helping mobility-impaired people proposes a novel robotic platform with quadrupedal locomotion. With motorized actuation, the quadruped robotic platform could accompany the user at the center and provide protection and possible walking assistance if needed. As the robotic platform is equipped with a leg locomotion, it can enlarge the user's activity environments, such as both indoor flat floor and outdoor uneven terrain. It can even assist the user to involve in some mobility challenging activities, such as climbing stairs. In this paper, we illustrate the mechanical design of the robotic platform. A continuous gait planning is proposed to create a smooth locomotion for the robot. To quantify the performance, a system-level walking experimentation was conducted, and the results showed that quadruped robotic platform can maintain a statically stability which demonstrate the feasibility and capability of the robotic application for walking assistance.
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