A Review of Lower Limb Exoskeletons

In general, exoskeletons are defined as wearable robotic mechanisms for providing mobility. In the last six decades, many research work have been achieved to enhance the performance of exoskeletons thus developing them to nearly commercialized products. In this paper, a review is made for the lower limb exoskeleton concerning history, classification, selection and development, also a discussion for the most important aspects of comparison between different designs is presented. Further, some concluding remarks are withdrawn which could be useful for future work. Keywords: Exoskeletons, Lower extremity exoskeleton, Wearable robot

Design, implementation and control of an overground gait and balance trainer with an active pelvis-hip exoskeleton

Human locomotion is crucial for performing activities of daily living and any disability in gait causes a significant decrease in the quality of life. Gait rehabilitation therapy is imperative to improve adverse effects caused by such disabilities. Gait therapies are known to be more effective when they are intense, repetitive, and allow for active involvement of patients. Robotic devices excel in performing repetitive gait rehabilitation therapies as they can eliminate the physical burden of the therapist, enable safe and versatile training with increased intensity, while allowing quantitative measurements of patient progress. Gait therapies need to be applied to specific joints of patients such that the joints work in a coordinated and repetitious sequence to generate a natural gait pattern. Six determinants of gait pattern have been identified that lead to efficient locomotion and any irregularities in these determinants result in pathological gaits. Three of these six basic gait determinants include movements of the pelvic joint; therefore, an effective gait rehabilitation robot is expected to be capable of controlling the movements of the human pelvis. We present the design, implementation, control, and experimental verification of AssistOn-Gait, a robot-assisted trainer, for restoration and improvement of gait and balance of patients with disabilities affecting their lower extremities. In addition to overground gait and balance training, AssistOn-Gait can deliver pelvis-hip exercises aimed to correct compensatory movements arising from abnormal gait patterns, extending the type of therapies that can be administered using lower extremity exoskeletons. AssistOn-Gait features a modular design, consisting of an impedance controlled, self-aligning pelvis-hip exoskeleton, supported by a motion controlled holonomic mobile platform and a series-elastic body weight support system. The pelvis-hip exoskeleton possesses 7 active degrees of freedom to independently control the rotation of the each hip in the sagittal plane along with the pelvic rotation, the pelvic tilt, lateral pelvic displacement, and the pelvic displacements in the sagittal plane. The series elastic body weight support system can provide dynamic unloading to support a percentage of a patient's weight, while also compensating for the inertial forces caused by the vertical movements of the body. The holonomic mobile base can track the movements of patients on flat surfaces, allowing patients to walk naturally, start/stop motion, vary their speed, sidestep to maintain balance, and turn to change their walking direction. Each of these modules can be used independently or in combination with each other, to provide different configurations for overground and treadmill based training with and without dynamic body weight support. The pelvis-hip exoskeleton of AssistOn-Gait is constructed using two passively backdrivable planar parallel mechanisms connected to the patient with a custom harness, to enable both passive movements and independent active impedance control of the pelvis-hip complex. Furthermore, the exoskeleton is self-aligning; it can automatically adjust the center of rotation of its joint axes, enabling an ideal match between patient's hip rotation axes and the device axes in the sagittal plane. This feature not only guarantees ergonomy and comfort throughout the therapy, but also extends the usable range of motion for the hip joint. Moreover, this feature significantly shortens the setup time required to attach the patient to the exoskeleton. The exoskeleton can also be used to implement virtual constraints to ensure coordination and synchronization between various degrees of freedom of the pelvis-hip complex and to assist patients as-needed for natural gait cycles. The overall kinematics of AssistOn-Gait is redundant, as the exoskeleton module spans all the degrees of freedom covered by the mobile platform. Furthermore, the device features dual layer actuation, since the exoskeleton module is designed for force control with good transparency, while the mobile base is designed for motion control to carry the weight of the patient and the exoskeleton. The kinematically redundant dual layer actuation enables the mobile base of the system to be controlled using workspace centering control strategy without the need for any additional sensors, since the patient movements are readily measured by the exoskeleton module. The workspace centering controller ensures that the workspace limits of the exoskeleton module are not reached, decoupling the dynamics of the mobile base from the dynamics of the exoskeleton. Consequently, AssistOn-Gait possesses virtually unlimited workspace, while featuring the same output impedance and force rendering performance as its exoskeleton module. The mobile platform can also be used to generate virtual fixtures to guide patient movements. The ergonomy and useability of AssistOn-Gait have been tested with several human subject experiments. The experimental results verify that AssistOn-Gait can achieve the desired level of ergonomy and passive backdrivability, as the gait patterns with the device in zero impedance mode are shown not to significantly deviate from the natural gait of the subjects. Furthermore, virtual constraints and force-feedback assistance provided by AssistOn-Gait have been shown to be adequate to ensure repeatability of desired corrective gait patterns

DEVELOPMENT OF A ROBOTIC EXOSKELETON SYSTEM FOR GAIT REHABILITATION

Ph.DDOCTOR OF PHILOSOPH

Advanced technology for gait rehabilitation: An overview

Most gait training systems are designed for acute and subacute neurological inpatients. Many systems are used for relearning gait movements (nonfunctional training) or gait cycle training (functional gait training). Each system presents its own advantages and disadvantages in terms of functional outcomes. However, training gait cycle movements is not sufficient for the rehabilitation of ambulation. There is a need for new solutions to overcome the limitations of existing systems in order to ensure individually tailored training conditions for each of the potential users, no matter the complexity of his or her condition. There is also a need for a new, integrative approach in gait rehabilitation, one that encompasses and addresses all aspects of physical as well as psychological aspects of ambulation in real-life multitasking situations. In this respect, a multidisciplinary multinational team performed an overview of the current technology for gait rehabilitation and reviewed the principles of ambulation training

System Identification of Bipedal Locomotion in Robots and Humans

The ability to perform a healthy walking gait can be altered in numerous cases due to gait disorder related pathologies. The latter could lead to partial or complete mobility loss, which affects the patients’ quality of life. Wearable exoskeletons and active prosthetics have been considered as a key component to remedy this mobility loss. The control of such devices knows numerous challenges that are yet to be addressed. As opposed to fixed trajectories control, real-time adaptive reference generation control is likely to provide the wearer with more intent control over the powered device. We propose a novel gait pattern generator for the control of such devices, taking advantage of the inter-joint coordination in the human gait. Our proposed method puts the user in the control loop as it maps the motion of healthy limbs to that of the affected one. To design such control strategy, it is critical to understand the dynamics behind bipedal walking. We begin by studying the simple compass gait walker. We examine the well-known Virtual Constraints method of controlling bipedal robots in the image of the compass gait. In addition, we provide both the mechanical and control design of an affordable research platform for bipedal dynamic walking. We then extend the concept of virtual constraints to human locomotion, where we investigate the accuracy of predicting lower limb joints angular position and velocity from the motion of the other limbs. Data from nine healthy subjects performing specific locomotion tasks were collected and are made available online. A successful prediction of the hip, knee, and ankle joints was achieved in different scenarios. It was also found that the motion of the cane alone has sufficient information to help predict good trajectories for the lower limb in stairs ascent. Better estimates were obtained using additional information from arm joints. We also explored the prediction of knee and ankle trajectories from the motion of the hip joints

A performance evaluation of overground gait training with a mobile body weight support system using wearable sensors

Overground gait training under body weight support (BWS) for patients who suffer from neurological injuries has been proven practical in recovering from walking ability. Conventionally, skilled therapists or additional robots are required to assist the patient’s body weight and pelvis movement, making the rehabilitation process physically and economically burdensome. We investigate if a BWS walker using only two actuators can support the user’s body weight and simultaneously protect/assist the transverse pelvis rotation, improving natural gait with minimal motion compensation. In this paper, a BWS strategy called transverse pelvis rotation support (TPRS) is proposed to enable the BWS system to generate cable tension in the forward direction, as a purpose to support transverse pelvis rotation in addition to our previously proposed static or variable BWS. Wearable sensory devices, including instrumented shoes and harness, were developed to provide real-time ground reaction force and pelvis rotation signals simultaneously. Ten non-disabled participants were unloaded with 0% ~ 15% BWS under four different controls. Vertical ground reaction force, transverse pelvis kinematics, and user experience were compared using proposed controls. One-Way repeated measures ANOVA analysis assessed if control strategies generally affect the performance. All proposed controls enable the walker to support part of the user’s body weight. SBWS-TPRS and VBWS-TPRS control enable users to achieve a significantly improved pelvic motion and prolonged single support phase than pure static BWS or variable BWS, although users perceive a higher workload under them. The proposed BWS controls show the potential to become a complementary method in gait rehabilitation

Robotic Functional Gait Rehabilitation with Tethered Pelvic Assist Device

The primary goal of human locomotion is to stably translate the center of mass (CoM) over the ground with minimum expenditure of energy. Pelvic movement is crucial for walking because the human CoM is located close to the pelvic center. Because of this anatomical feature, pelvic motion directly contributes to the metabolic expenditure, as well as in the balance to keep the center of mass between the legs. An abnormal pelvic motion during the gait not only causes overexertion, but also adversely affects the motion of the trunk and lower limbs. In order to study different interventions, recently a cable-actuated robotic system called Tethered Pelvic Assist Device (TPAD) was developed at ROAR laboratory at Columbia University. The cable-actuated system has a distinct advantage of applying three dimensional forces on the pelvis at discrete points in the gait cycle in contrast to rigid exoskeletons that restrict natural pelvic motion and add extra inertia from the rigid linkages. However, in order to effectively use TPAD for rehabilitation purposes, we still need to have a better understanding of how human gait is affected by different forces applied by TPAD on the pelvis. In the present dissertation, three different control methodologies for TPAD are discussed by performing human experiments with healthy subjects and patients with gait deficits. Moreover, the corresponding changes in the biomechanics during TPAD training are studied to understand how TPAD mechanistically influences the quality of the human gait. In Chapter 2, an ‘assist-as-needed’ controller is implemented to guide and correct the pelvic motion in three dimensions. Here, TPAD applies the correction force based on the deviation of the current position of the pelvic center from a pre-defined target trajectory. This force acts on the pelvic center to guide it towards the target trajectory. A subject in the device experiences a force field, where the magnitude becomes larger when the subject deviates further away from the target trajectory. This control strategy is tested by performing the experiments on healthy subjects with different target pelvic trajectories. Chapter 3 describes a robotic resistive training study using a continuous force on the pelvis to strengthen the weak limbs so that subjects can improve their walking. This study is designed to improve the abnormal gait of children with Cerebral Palsy (CP) who have a crouch gait. Crouch gait is caused by a combination of weak extensor muscles that do not produce adequate muscle forces to keep the posture upright, coupled with contraction of muscles that limit the joint range of motion. Among the extensor muscles, the soleus muscle acts as the major weight-bearing muscle to prevent the knees from collapsing forward during the middle of the stance phase when the foot is on the ground. Electromyography, kinematics, and clinical measurements of the patients with crouch gait show significant improvements in the gait quality after the resistive TPAD training performed over five weeks. Both Chapters 2 & 3 present interventions that are bilaterally applied on both legs. Chapter 4 introduces a training strategy that can be used for patients who have impairments in only one leg which results in manifests as asymmetric weight-bearing while walking. This training method is designed to improve the asymmetric weight bearing of the hemiparetic patients who overly rely on the stronger leg. The feasibility of this training method is tested by experiments with healthy subjects, where the controller creates an asymmetric force field to bring asymmetry in weight bearing during walking. In summary, the present dissertation is devoted to developing new training methods that utilize TPAD for rehabilitation purposes and characterize the responses of different force interventions by investigating the resulting biomechanics. We believe that these methodologies with TPAD can be used to improve abnormal gait patterns that are often observed in cerebral palsy or stroke patients

Feasibility of novel gait training with robotic assistance : dynamic entrainment to mechanical perturbation to the ankle

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 149-156).Rehabilitation of human motor function is an issue of the utmost significance, and the demand for the effective rehabilitation service is even growing with the graying of the population. Robotic technology has provided promising ways to assist recovery of the motor function of upper extremities. In contrast, current robotic therapy for lower extremities has shown inferior efficacy. In this thesis, the source of the limited efficacy of current robotic walking therapy is addressed. Essential mechanical components for robustly stable walking are identified as energy dissipation and proper compensation. Based on these essential components, design criteria of effective robotic walking therapy are suggested as foot-ground interaction and ankle actuation. A novel strategy of robot aided walking therapy reflecting the design criteria is proposed; dynamically entraining human gait with periodic ankle torque from a robot. Experiments with normal subjects and neurologically impaired subjects support the feasibility of the proposed rehabilitation strategy. The gait period of subjects entrain to the periodic mechanical perturbation with a measurable basin of entrainment, and the entrainment always accompanies phase-locking so that the mechanical perturbation assists propulsion. These observations are affected neither by auditory feedback nor by a distractor task for normal subjects, and consistently observed in impaired subjects. A highly simplified one degree of freedom walking model without supra-spinal control or an intrinsic self-sustaining neural oscillator (a rhythmic pattern generator) encapsulated the essence of these observations. This suggests that several prominent limit-cycle features of human walking may stem from peripheral mechanics mediated by simple afferent feedback without significant involvement of supra-spinal control or central pattern generator. The competence of the highly simplified model supports that the proposed entrainment therapy may be effective for a wide range of neurological impairments.by Jooeun Ahn.Ph.D

Gait Dynamic Stability Analysis with Wearable Assistive Robots

abstract: Lower-limb wearable assistive robots could alter the users gait kinematics by inputting external power, which can be interpreted as mechanical perturbation to subject normal gait. The change in kinematics may affect the dynamic stability. This work attempts to understand the effects of different physical assistance from these robots on the gait dynamic stability. A knee exoskeleton and ankle assistive device (Robotic Shoe) are developed and used to provide walking assistance. The knee exoskeleton provides personalized knee joint assistive torque during the stance phase. The robotic shoe is a light-weighted mechanism that can store the potential energy at heel strike and release it by using an active locking mechanism at the terminal stance phase to provide push-up ankle torque and assist the toe-off. Lower-limb Kinematic time series data are collected for subjects wearing these devices in the passive and active mode. The changes of kinematics with and without these devices on lower-limb motion are first studied. Orbital stability, as one of the commonly used measure to quantify gait stability through calculating Floquet Multipliers (FM), is employed to asses the effects of these wearable devices on gait stability. It is shown that wearing the passive knee exoskeleton causes less orbitally stable gait for users, while the knee joint active assistance improves the orbital stability compared to passive mode. The robotic shoe only affects the targeted joint (right ankle) kinematics, and wearing the passive mechanism significantly increases the ankle joint FM values, which indicates less walking orbital stability. More analysis is done on a mechanically perturbed walking public data set, to show that orbital stability can quantify the effects of external mechanical perturbation on gait dynamic stability. This method can further be used as a control design tool to ensure gait stability for users of lower-limb assistive devices.Dissertation/ThesisMasters Thesis Mechanical Engineering 201