Lower Extremity Exoskeleton Research Articles

Interaction learning control with movement primitives for lower limb exoskeleton.

Research on robotic exoskeletons both in the military and medical fields has rapidly expanded over the previous decade. As a human-robot interaction system, it is a challenge to develop an assistive strategy that makes the exoskeleton supply efficient and natural assistance following the user's intention. This paper proposed a novel interaction learning control strategy for the lower extremity exoskeleton. A powerful representative tool probabilistic movement primitives (ProMPs) is adopted to model the motion and generate the desired trajectory in real-time. To adjust the trajectory by the user's real-time intention, a compensation term based on human-robot interaction force is designed and merged into the ProMPs model. Then, compliant impedance control is adopted as a low-level control where the desired trajectory is put into. Moreover, the model will be dynamically adapted online by penalizing both the interaction force and trajectory mismatch, with all the parameters that can be further learned by learning algorithm PIBB. The experimental results verified the effectiveness of the proposed control framework.

Vision Assisted Control of Lower Extremity Exoskeleton for Obstacle Avoidance With Dynamic Constraint Based Piecewise Nonlinear MPC

This article proposed a humanoid obstacle passability strategy (OPS) to enhance human-exoskeleton integrated system to cross over multi-obstacle in sagittal plane. A hybrid bounding box integrated with closeness regression in L-shape section and synchronous convergence in convex hull search (HBB-LC) is designed to recognize geometric features of staggered obstacles with partial surface occluded. A human-in-the-loop gait pattern planning mechanism based on nonlinear model predictive control (NMPC) is designed for decoupling mapping. To overcome local oscillation and achieve humanoid behavior transition during NMPC optimization, an intermediate switching criterion based on instantaneous capture point (ICP) transfer principle and guiding interpolation inflection is established to trigger the external intervention of piecewise NMPC models. Exploitation of OPS achieves the contributions below: 1) Sectional attitude and dimension features of spatial multi-obstacle are accurately extracted, with size deviation within 3.4–16.1 mm, recognition accuracy within 89.2%–96.8%. 2) Traversal avoidance region of swing toe/heel orbit (SHO/STO) is enhanced by 28.7%–86.8% (with uplift/sidesway range (UR/SR) by 0.071–0.154 m/0.057–0.092 m). 3) Security margin between STO/SHO and obstacle vertex is improved by 0.05–0.19 m (53.3%–161.1%). 4) Elapsed time of crossing obstacles is reduced by 0.62–1.03 s (with gait phase by 36.2%–43.5%). Experimental verification proves significant augmentation for crossing obstacles by OPS.

Review of current developments in lower extremity exoskeleton systems

A lower extremity exoskeleton is device which can enhance the human strength and speed while walking and running. It mimics the human gait motion and enhance the endurance of human with help of actuators to carry heavy loads. It can help to carry heavy loads in rough terrain where a wheeled vehicle can’t reach. The most prominent option for elderly people and people with walking disabilities such as paraplegia is to use a wheelchair, which is not convenient in every environment. A lower extremity exoskeleton with the particular purpose of either power augmentation or gait assistance can solve this problem. This paper discusses an overview on the historical developments in the field of exoskeleton type system along with the biomechanical considerations and the human gait cycle for the development of a lower extremity exoskeleton and analyses various parameters needed for the design of a lower limb exoskeleton system. The kinematic analysis of human lower limb is studied using Denavit-Hartenberg (DH) parameters. Furthermore, a comparison of the degrees of freedom and their ranges along with the actuators used in various exoskeleton developed is discussed.

Electromyography-controlled lower extremity exoskeleton to provide wearers flexibility in walking

Objective:In recent years, there have been significant developments in lower extremity robotic exoskeletons intended for gait rehabilitation. However, wearers may not be sufficiently motivated to participate in traditional rehabilitation robot training as the training pattern is usually predefined and rigid. Enabling wearers to actively control the exoskeleton to assist them in walking may improve rehabilitation treatments. Methods:This paper presents an electromyography (EMG)-based gait pattern adaptation method that allows subjects to control the exoskeleton via EMG signals of thigh muscles (quadriceps femoris and hamstrings muscles). Six healthy subjects participated in the initial experiment on a treadmill based lower extremity rehabilitation robot system. In a single walking routine, six widely used adaptation gait patterns were tested. Results:The results indicate that all of the subjects were able to change the gait pattern of the exoskeleton and achieved the adaptation goals stably within average 16 strides by generating EMG signals. The muscle activation during the adaptation condition is significantly higher than that in fixed normal walking condition (p<0.05). The subjects gave positive evaluation on the designed system. Conclusion:With this method, the subjects were involved in the control loop and actively participated in the training. Significance:The proposed EMG-based two-layer admittance control algorithm is novel, which enabled subjects to adjust gait trajectories continuously and smoothly.

Design, development and model analysis of lower extremity Exo-skeleton.

Over the past few years the growth and development of exo-skeleton has dramatically raised with the development of precise control elements and actuation systems. Many exo-skeleton systems have been designed, developed and tested for performance optimization. In the recent years, the significance of exo-skeleton in medical fields have got increased and are used in providing therapy and rehabilitation to the patients. With this development there comes the importance for analysis and control of the exo-skeleton for precise functioning and to avoid malfunction of the system in the later part. Dynamic analysis of limb joints is essential to better facilitate a deeper understanding of the exo-skeleton limb during various environmental conditions like varied loading. The dynamic model so developed will assist in choosing an apt actuation system based on the torque requirement of the model.This paper focusses on the analysis of a 2DOF lower limb active control exo-skeleton system and makes a torque calculation for actuator selection for the lower limb to provide rehabilitation to the patients as wearable walking aid. The work also makes a trajectory planning for the lower limb to move in sequence for making a walking cycle with angular limitations to avoid damage to the user's limbs. The motion analysis for the developed lower limb Exoskeleton as per the analysis is 52.055 Nm at hip joint 11.677 Nm at knee joint.

Design and analysis of a lightweight lower extremity exoskeleton with novel compliant ankle joints.

The exoskeleton for lower limb rehabilitation is an uprising field of robot technology. However, since it is difficult to achieve all the optimal design values at the same time, each lower extremity exoskeleton has its own focus. This study aims to develop a modular lightweight lower extremity exoskeleton (MOLLEE) with novel compliant ankle joints, and evaluate the movement performance through kinematics analysis. The overall structure of the exoskeleton was proposed and the adjustable frames, active joint modules, and compliant ankle joints were designed. The forward and inverse kinematics models were established based on the geometric method. The theoretical models were validated by numerical simulations in ADAMS, and the kinematic performance was demonstrated through walking experiments. The proposed lower extremity offers six degrees of freedom (DoF). The exoskeleton frame was designed adjustable to fit wearers with a height between 1.55m and 1.80m, and waist width from 37cm to 45cm. The joint modules can provide maximum torque at 107Nm for adequate knee and hip joint motion forces. The compliant ankle can bear large flexible deformation, and the relationship between its angular deformation and the contact force can be fitted with a quadratic polynomial function. The kinematics models were established and verified through numerical simulations, and the walking experiments in different action states have shown the expected kinematic characteristics of the designed exoskeleton. The proposed MOLLEE exoskeleton is adjustable, modular, and compliant. The designed adjustable frame and compliant ankle can ensure comfort and safety for different wearers. In addition, the kinematics characteristics of the exoskeleton can meet the needs of daily rehabilitation activities.

Feasible Wrench Set Computation for Legged Robots

During locomotion, legged robots interact with the ground by sequentially establishing and breaking contact. The interaction wrenches that arise from contact are used to steer the robot Center of Mass (CoM) and reject perturbations that make the system deviate from the desired trajectory and often make them fall. The feasibility of a given control target (desired CoM wrench or acceleration) is conditioned by the contact point distribution, ground friction, and actuation limits. In this work, we develop a method to compute the set of feasible wrenches that a legged robot can exert on its CoM through contact. The presented method can be used with any amount of non-co-planar contacts and takes into account actuation limits and limitations based on an inelastic contact model with Coulomb friction. This is exemplified with a planar biped model standing with the feet at different heights. Exploiting assumptions from the contact model, we explain how to compute the set of wrenches that are feasible on the CoM when the contacts remain in position as well as the ones that are feasible when some of the contacts are broken. Therefore, this method can be used to assess whether a switch in contact configuration is feasible while achieving a given control task. Furthermore, the method can be used to identify the directions in which the system is not actuated (i.e. a wrench cannot be exerted in those directions). We show how having a joint be actuated or passive can change the non-actuated wrench directions of a robot at a given pose using a spatial model of a lower-extremity exoskeleton. Therefore, this method is also a useful tool for the design phase of the system. This work presents a useful tool for the control and design of legged systems that extends on the current state of the art.

Optimal Linear Quadratic Control for Knee-Ankle Orthosis System

The control technique for an exoskeleton system for lower limb rehabilitation is complicated, and numerous internal and external elements must be taken into account, in addition to the uncertainties in the system model. In this paper, through the analysis of the lower extremity exoskeleton is utilized to obtain the corresponding equation and its linearized form. The nonlinear differential equations have been linearized by using Jacobean’s method in order to facilitate the controller design. Considering the interior and external factors of the connecting rod, the uncertain elements are introduced and therefore the optimal control technique is applied to regulate the system. An optimal state feedback control strategy of Linear Quadratic Regulator (LQR), and LQR-Servo have been implemented in this work. Finally, the physical parameters of the Knee-Ankle Orthosis (KAO) exoskeleton are used, and the simulation results show the advantage and applicability of the proposed controller’s design of the Knee-Ankle orthosis system. Index Terms— Rehabilitation, Knee-Ankle orthosis (KAO), Optimal control, LQR-Servo controller.

A CNN–SVM MODEL USING IMU FOR LOCOMOTION MODE RECOGNITION IN LOWER EXTREMITY EXOSKELETON

Human activity intention is indispensable for wearable powered lower extremity exoskeleton such that ensuring the compliant control of the robot. Lots of researches have been done on gait phase detection, which served as a sub-module of locomotion mode recognition to support the follow-on task. Therefore, it is self-evident that locomotion mode recognition is of great importance. Many model-based recognition methods are usually applied in manual extraction of cumbersome features, such as the traditional neural network (NN), support vector machine (SVM), etc. In contrast, the feature mapping layer coming with the convolutional neural network (CNN) can effectively solve the above time-consuming problem. Given that the training of NN is prone to overfitting, SVM with optimal characteristics is considered. A hybrid CNN–SVM model is proposed to identify human locomotion modes by collecting multi-channel inertial measurement unit (IMU) signals and is integrated with the error correction function of the finite state machine (FSM). Therefore, the CNN–SVM model has great influence on the generalization performance and recognition accuracy. The recognition rates of five single locomotion modes and eight mixed locomotion modes reach 97.91% and 98.93%, respectively. The system meets the demand of real-time performance, and the recognition time exceeds 370[Formula: see text]ms on heel strike.

Kinematics study of a 10 degrees-of-freedom lower extremity exoskeleton for crutch-less walking rehabilitation.

Wearable lower extremity exoskeletons can provide walking assistance for the physical rehabilitation of paralyzed individuals. However, most of the existing exoskeletons require crutches to maintain balance, thus a self-balancing type is needed to improve applicability. The purpose of this work is to study the kinematic characteristics of a novel lower extremity exoskeleton for crutch-less walking rehabilitation, and evaluate the movement performance through practical experiments. Based on the human lower limb structure and movement characteristics, a fully actuated 10 degrees-of-freedom (DoF) lower extremity exoskeleton was proposed. The kinematic characteristics of the exoskeleton were analyzed by the D-H method and geometric method, and the model validity was verified through simulations and experiments. The closed-form solutions for both forward and inverse kinematics models were obtained. The consistent results of theoretical calculation and numerical simulation have shown the accuracy of the established models. The practical experiments regarding six trials have demonstrated the movement performance of the proposed exoskeleton, including sit, stance, leg extension/flexion, and left/right swing. The kinematic characteristics of the proposed 10-DoF lower extremity exoskeleton are similar to the human lower limb, and it could meet the motion demands of crutch-less walking rehabilitation.

Exercise and Rehabilitation of Chronically Ill Patients Assisted by Small Digital Bio-Pulse Sensors

With the continuous increase of the ageing population, the number of patients with chronic diseases has increased dramatically. The limited medical resources and the strong demand for high-quality medical services are in stark contradiction. Active rehabilitation training is one of the most effective rehabilitation methods, but it is difficult to achieve through traditional medical equipment. In this paper, aiming at effective clinical rehabilitation, an in-depth study was carried out on the exercise intention recognition experiment of patients, hoping to provide an effective rehabilitation treatment method for the recovery of patients with lower extremity motor function injury. This paper introduces the detection principle of the photoelectric pulse sensor and the design scheme that is used in physical education. In the experiment, through the hardware connection, programming, and the development of the host computer software, the pulse signal of the human body can be presented stably on the computer. The monitoring of the pulse of the students can help the physical education teacher to understand the situation of the students and prevent accidents. In addition, we believe that the acquisition of students’ pulse signals in physical education to build a database is of great significance for research and tracking of students’ health status and the research results will certainly promote the development of pulse diagnosis. In order to make the lower extremity exoskeleton rehabilitation system that can be applied in clinical rehabilitation, according to the Brunnstrom staging of patients with central nervous system injury, the needs of patients in different stages in the rehabilitation process were analysed and active and passive lower extremity rehabilitation strategies were formulated. Aiming at the problem of real-time and accurate identification of human motion intention, and to alleviate the mechanism motion delay caused by the delay of mechanical and control systems in the human-machine integrated system, a differential sEMG real-time feature extraction algorithm is proposed. The results show that the sensor and monitoring system have excellent stability, and the auxiliary system can accurately reflect the changing trend of the human biological pulse, achieve the expected effect, and effectively assist in the monitoring of exercise data for patients with chronic diseases. After treatment, the joint range of motion and muscle strength basically returned to normal levels, and the patient was able to walk independently. Compared with traditional treatment methods, the recovery time is shorter, the recovery of muscle strength is more effective, and the medical staff is more relaxed.

Development of a sliding mode controller with chattering suppressor for human lower extremity exoskeleton robot

Exoskeleton robot-based human upper and lower extremity rehabilitation is an appealing solution for many disabled individuals. To deliver the desired form of physical therapy efficiently, robots with streamlined maneuverability are needed. Sliding mode control is a robust control scheme that works efficiently for a nonlinear system such as a serial link manipulator. The main limitation of the sliding mode control system is the chattering phenomenon. Chattering leads to rough movements and often causes actuator saturation leading to reduced actuator life and an energy-inefficient system. A sliding mode control system without a chattering suppressor needs more powerful actuators that make the manipulator heavy. In this paper, a seven degrees of freedom human lower extremity dynamic model for an exoskeleton robot is presented first. To control the robot dynamics, a sliding mode controller (SMC) is employed. To study the effects of chattering phenomenon, dynamic simulations are done with and without continuous mode chattering suppressor. A continuous mode chattering suppressor is introduced. A realistic friction model is included in the human lower extremity dynamic model to simulate joint friction effects. Simulation results show the effectiveness of the developed Sliding mode controller with continuous mode chattering suppressor while tracking trajectories involving sequential as well as simultaneous joint movements. A performance comparison of the developed Sliding mode controller with linear (PID) and nonlinear control techniques (Computed torque controller) is presented.

Biomechanical design and control of an eight DOF human lower extremity rehabilitation exoskeleton robot

Exoskeleton robot-assisted physical therapy has received a lot of research attention due to its positive impact on human upper and lower extremity rehabilitation. The exoskeleton robot attaches closely to the human limbs and facilitates natural movements at the joints. The effectiveness of the exoskeleton robot-assisted physical therapy is influenced by the mechanical design of the manipulator. An ergonomic design helps the user exercise for an extended period. In this paper, the mechanical design of an 8 degrees of freedom human lower extremity exoskeleton robot for rehabilitation is presented. Among the 8 degrees of freedom, 7 are actively actuated and 1 is passively actuated. A cam follower mechanism is developed to replicate the complex motion at the knee joint. To make the exoskeleton comfortable and easy to use for the users, ergonomic design rules are followed. A dynamic model of the human lower extremity exoskeleton robot is also developed for controlling the robot. A Sliding mode controller with super twisting algorithm is employed to simulate the dynamic model. Results for stability analysis of the developed controller are also presented.

A Survey on Design and Control of Lower Extremity Exoskeletons for Bipedal Walking

Exoskeleton robots are electrically, pneumatically, or hydraulically actuated devices that externally support the bones and cartilage of the human body while trying to mimic the human movement capabilities and augment muscle power. The lower extremity exoskeleton device may support specific human joints such as hip, knee, and ankle, or provide support to carry and balance the weight of the full upper body. Their assistive functionality for physically-abled and disabled humans is demanded in medical, industrial, military, safety applications, and other related fields. The vision of humans walking with an exoskeleton without external support is the prospect of the robotics and artificial intelligence working groups. This paper presents a survey on the design and control of lower extremity exoskeletons for bipedal walking. First, a historical view on the development of walking exoskeletons is presented and various lower body exoskeleton designs are categorized in different application areas. Then, these designs are studied from design, modeling, and control viewpoints. Finally, a discussion on future research directions is provided.

A pace recognition method for exoskeleton wearers based on support vector machine-hidden Markov model

In order to improve the motion fluency and coordination of lower extremity exoskeleton robots and wearers, a pace recognition method of exoskeleton wearer is proposed base on inertial sensors. Firstly, the triaxial acceleration and triaxial angular velocity signals at the thigh and calf were collected by inertial sensors. Then the signal segment of 0.5 seconds before the current time was extracted by the time window method. And the Fourier transform coefficients in the frequency domain signal were used as eigenvalues. Then the support vector machine (SVM) and hidden Markov model (HMM) were combined as a classification model, which was trained and tested for pace recognition. Finally, the pace change rule and the human-machine interaction force were combined in this model and the current pace was predicted by the model. The experimental results showed that the pace intention of the lower extremity exoskeleton wearer could be effectively identified by the method proposed in this article. And the recognition rate of the seven pace patterns could reach 92.14%. It provides a new way for the smooth control of the exoskeleton.

Lower limb exoskeleton robots’ dynamics parameters identification based on improved beetle swarm optimization algorithm

AbstractEfficient and high-precision identification of dynamic parameters is the basis of model-based robot control. Firstly, this paper designed the structure and control system of the developed lower extremity exoskeleton robot. The dynamics modeling of the exoskeleton robot is performed. The minimum parameter set of the identified parameters is determined. The dynamic model is linearized based on the parallel axis theory. Based on the beetle antennae search algorithm (BAS) and particle swarm optimization (PSO), the beetle swarm optimization algorithm (BSO) was designed and applied to the identification of dynamic parameters. The update rule of each particle originates from BAS, and there is an individual’s judgment on the environment space in each iteration. This method does not rely on the historical best solution in the PSO and the current global optimal solution of the individual particle, thereby reducing the number of iterations and improving the search speed and accuracy. Four groups of test functions with different characteristics were used to verify the performance of the proposed algorithm. Experimental results show that the BSO algorithm has a good balance between exploration and exploitation capabilities to promote the beetle to move to the global optimum. Besides, the test was carried out on the exoskeleton dynamics model. This method can obtain independent dynamic parameters and achieve ideal identification accuracy. The prediction result of torque based on the identification method is in good agreement with the ideal torque of the robot control.

Development of a Sliding Mode Controller with Chattering Suppressor for Human Lower Extremity Exoskeleton Robot

Development of a Sliding Mode Controller with Chattering Suppressor for Human Lower Extremity Exoskeleton Robot

Mma-Elm: A Model-Free Motion Error Compensation Method for Self-Balancing Lower Extremity Exoskeleton Robot

Mma-Elm: A Model-Free Motion Error Compensation Method for Self-Balancing Lower Extremity Exoskeleton Robot

Developing a Linear Quadratic Regulator for Human Lower Extremity Exoskeleton Robot

During the last two decades, exoskeleton robot-assisted neurorehabilitation has received a lot of attention. The major reason for active research in robot-assisted rehabilitation is its ability to provide various types of physical therapy at different stages of physical and neurological recovery. The performance of the robot-assisted physical therapy is greatly influenced by the robot motion control system. Robot dynamics are nonlinear, but many linear control schemes can adequately handle the nonlinear dynamics with the help of feedback linearization techniques. In this study, the dynamic model of the human lower extremities was developed. A state-space form of the human lower extremity nonlinear dynamic model is presented. LuGre friction model was used to simulate the robot joint friction. A Linear Quadratic Regulator (LQR) was designed to control the human lower extremity dynamics. Dynamic simulations were carried out in the Matlab-Simulink environment. The designed controller's tracking performance was demonstrated in the presence of joint friction. The developed controller’s tracking performance is assessed by comparing the results obtained using LQR with other linear and nonlinear controllers (PID, Computed torque control, and Sliding mode control). For performance verification, the same robot dynamics, friction model, and trajectories were used. The stability of the developed control system is also analyzed.

Analysis and Evaluation of a Quasi-Passive Lower Limb Exoskeleton for Gait Rehabilitation

Lower extremity exoskeletons can assist with performing particular functions such as gait assistance, and physical therapy support for subjects who have lost the ability to walk. This paper presents the analysis and evaluation of lightweight and adjustable two degrees of freedom, quasi-passive lower limb device to improve gait rehabilitation. The exoskeleton consists of a high torque DC motor mounted on a metal plate above the hip joint, and a link that transmits assistance torque from the motor to the thigh. The knee joint is passively actuated by spring installed parallel with the joint. The action of the passive component (spring) is combined with mechanical output of the motor to provide a good control on the designed exoskeleton while walking. The results show that muscles' efforts on both the front and the back sides of the user's leg were decreased when walking using the exoskeleton with the motor and spring.