Nonlinear compliant actuators are being increasingly used in human-robot interaction scenarios due to their inherent flexibility. However, a limitation is that nonlinear hysteresis exists, which will degrade the force/torque tracking performance if the hysteresis is not modeled accurately. Moreover, the existing methods are difficult to deal with the multi-loop asymmetry hysteresis. In this work, we present a novel modeling method, in which the hysteresis curves are decoupled into nonlinear reference lines and symmetrical hysteresis loops. A hybrid hysteresis model based on power function and Maxwellslip model is then developed to fit the nonlinear reference lines and symmetrical hysteresis loops respectively. Experiments were conducted on a nonlinear compliant actuator and the results show that the root-mean-square-errors (RMSE) of the hysteresis model decreases by 24.4% when compared with the Maxwellslip based hysteresis model.

This article presents a systematic approach to robustly control a lower-limb exoskeleton in real-time using multimodal sensors. The control adopts two sensor bands that combine an array of force-sensitive resistors (FSR) and an inertial measurement unit (IMU) to measure both force myography (FMG) signals and limb motion. A robust sensor fusion algorithm that combines FMG and IMU signals with artificial neural networks is developed to accurately estimate the wearer’s hip and knee rotation angles. Moreover, a mathematical model of the lower-limb exoskeleton is calibrated and validated with real-time experimental data. Finally, a model-based controller is designed to track position references generated from the network through linear matrix inequalities. The biomimetic control algorithm is tested in simulation and a physical setup to show the effectiveness of the novel control method.Note to Practitioners—This work addresses the challenge of real-time control of lower-limb exoskeletons. Our approach to address the trajectory generation problem involves emulating the movements of a healthy limb. This is achieved through a robust sensor fusion algorithm to integrate data from two multimodal sensors, namely, FMG and IMU sensors, enabling a reliable estimation of the hip and knee’s angular positions. Signals are processed and neural networks are trained with exoskeleton encoder measurements as targets. Physical tests show both the accuracy and robustness of the control method. Potential applications of the new method include real-time gait analysis, control of upper limb exoskeletons, and study of the induction of neural plasticity.

The ability to continuously recognize locomotion modes and accurately predict transition intentions is essential for intelligent prosthetic knees. In this study, an innovative framework for locomotion recognition and transition prediction was introduced based on fusing mechanical (inertial measurement unit (IMU)) and biomechanical (force myography (FMG)) signals. This framework integrated an FMG-IMU dual-modal sensing system implemented on a prosthetic knee, enabling simultaneous acquisition of FMG-IMU fusion signals from transfemoral amputees during dynamic walking. A novel feature-driven CNN-BiLSTM model was developed and trained as the classifier, enhancing the accuracy and efficiency of locomotion mode prediction. The RelifF-MI algorithm was employed to optimize FMG-IMU features, ensuring efficient data processing by effectively eliminating feature redundancy. The framework was evaluated using data collected from eight transfemoral amputees. The results demonstrated that the fusion of FMG-IMU dual-modal gait data with the feature-driven classifier significantly improved classification performance, achieving an overall average recognition accuracy of 98.51% and an average prediction time of 274 ms (21.82% of the gait cycle) across five locomotion modes-level walking (LW), stair ascent/descent (SA/SD), and ramp ascent/descent (RA/RD)-and eight transitions between these modes. These promising results highlighted the considerable potential of the proposed method for application in prosthetic knee control.

Real-time motion planning in dynamic environments presents a significant challenge for robotic manipulators. This paper introduces an innovative parallel Model Predictive Path Integral (MPPI) algorithm enabling the robot to navigate swiftly and safely in such environments. Unlike the conventional MPPI methods that rely on a single sequence of Gaussian means for trajectory sampling, the proposed Parallel MPPI (PMPPI) concurrently runs multiple planners with different strategies and adaptively integrates planned paths based on the current state, leveraging the advantages of different strategies and greatly improving the MPPI's exploration capability. Moreover, a Gradient-Velocity Modulated Signed Distance Field (SDF) cost function that dynamically adjusts costs based on the robot's velocity and the SDF gradient is defined, thereby promoting safer and purposeful motion planning. In the implementation, techniques like utilizing inverse kinematics solver for path guidance and Sparse Reward to expedite reaching time are integrated into the MPPI cost function design. Comparative evaluations against the traditional MPPI architecture and standard SDF cost designs demonstrate the superiority of the new method. Real-world experiments, including human-robot interaction, obstacle-crossing, and grasping tasks, validate the robustness and universality of our methodology, with average and maximum end effector speeds of 0.523 m/s and 1.225 m/s respectively.

A major challenge in the lower limb exoskeleton for walking assistance is the adaptive gait control. In this article, a modified dynamic movement primitive (DMP) (MDMP) control is proposed to achieve gait adjustment with different assistance levels. This is achieved by inclusion of interaction forces in the formulation of DMP, which enables learning from physical human–robot interaction. A threshold force is introduced accounting for different levels of walking assistance from the exoskeleton. The MDMP is, thus, capable of generating adjustable gait and reshaping trajectories with data from the interaction force sensors. The experiments on five subjects show that the average differences between the human body and the exoskeleton are 4.13° and 1.92° on the hip and knee, respectively, with average interaction forces of 42.54 N and 26.36 N exerted on the subjects' thigh and shank. The results demonstrated that the MDMP method can effectively provide adjustable gait for walking assistance.

High motion compatibility with the human body is essential for lower limb exoskeletons. However, in most exoskeletons, internal/external rotational degrees of freedom are not provided, which makes accurate alignment between the biological and mechanical joints difficult to achieve. To solve this problem, a novel hip exoskeleton with a parallel structure is developed in this article. The unique parallel structure eliminates the misalignment problem and enables walking free of restrictions. On the other hand, this requires a coordinated control among actuations within the parallel exoskeleton structure. In this light, a model-based controller is proposed in this article. The controller is based on a human–machine integrated dynamic model and can generate coordinated force control references that could increase the closed-loop system's sensitivity to its wearer's movements. The controller requires only kinematic information from the wearer, but not interaction force data that most existing exoskeletons require in their control design, which saves spaces and makes the system compact for use. Experiments were conducted to demonstrate the kinematic compatibility and assistive performance of the proposed hip exoskeleton.

Upper-limb exoskeletons (ULEs) have the potential to assist severely paralyzed individuals. Many rehabilitative upper-limb exoskeletons have been developed in the recent years, where the prominent issue of bulkiness and actuator placement has been effectively resolved by the cable-transmission systems, allowing the actuators to be mounted away from the joints. This results in increasing the compactness and wearability of the exoskeleton. Thus, this article summarizes twenty-five cable-driven upper-limb exoskeletons designed for rehabilitating disabled individuals. This paper also presents a comparison based on usability of ULEs by end users considering the design characteristics and user-based experiments for real-time applications of the arm exoskeletons.

A novel 6-DOF parallel manipulator with two spherical-universal-revolute limbs is proposed in this work. Compared with general 6-DOF parallel manipulators of six kinematic limbs, this new manipulator actuated by spherical motion generators has only two limbs, which brings kinematic advantages such as small footprint and large workspace. The inverse position problem of the manipulator is solved by an analytical approach, upon which velocity equations are formulated. Kinematic performance including workspace and also manipulability are calculated to show the advantages of the new design.

This paper presents a novel portable passive lower limb exoskeleton for walking assistance. The exoskeleton is designed with built-in spring mechanisms at the hip and knee joints to realize gravity balancing of the human leg. A pair of mating gears is used to convert the tension force from the built-in springs into balancing torques at hip and knee joints for overcoming the influence of gravity. Such a design makes the exoskeleton has a compact layout with small protrusion, which improves its safety and user acceptance. In this paper, the design principle of gravity balancing is described. Simulation results show a significant reduction of driving torques at the limb joints. A prototype of single leg exoskeleton has been constructed and preliminary test results show the effectiveness of the exoskeleton.

In this paper, a novel revolute joint of adjustable stiffness with reconfigurability (JASR) is presented. The JASR is designed with zero-length base link four-bar linkage, and allows adjusting its stiffness to achieve soft- and hard-spring behaviour. The new joint has a compact and light-weight structure and can be integrated in robot and transmissions for different applications. In the paper, mathematical models are developed for the JASR, with which influences of design parameters on stiffness performance are analyzed. A prototype of JASR is constructed and preliminary test results demonstrate the compliance properties of the new joint.

This paper presents the design of Impedance Control to a redundantly actuated Parallel Kinematic Manipulator. The proposed control is based on treating each limb as a single system and their connection through the internal interaction forces. The controller introduces a stiffness and damping matrices that decouples the dynamic behaviour of the robot. Control allocation of torques is applied through an optimization that promotes even distribution of torques over actuators. Simulations showed a good compliance behaviour in low frequencies.

In this paper, the static stiffness of cable-driven wrist robotic rehabilitor (CDWRR) in its whole workspace is analyzed. A total stiffness matrix of this device consists of the exoskeleton stiffness model and the human arm stiffness model, which is obtained by using an equivalent stiffness model of exoskeleton and assuming a series of simplification of human arm. The analysis on stiffness was made with the MATLAB simulation results. Herein, the stiffness performance of the device during rehabilitation training was elucidated by the average stiffness index, stabilizability and relative stabilizability. Results show that the exoskeleton just has a great impact on the rotational stiffness. And the stiffness of cables plays a key role in the exoskeleton stiffness model and its relative stabilizability. Since the stiffness is associated with safety and comfort of rehabilitation training, it is essential to analyze the stiffness of rehabilitation device. Also, the analysis process of this paper can be referred by other similar cable-driven rehabilitors.

This paper presents stiffness analysis of a family of three/four degree-of-freedom pick-and-place robots with four identical limbs. The Cartesian stiffness matrix of the robots is modeled with the virtual spring approach. By means of a nondimensionalization of the stiffness matrix, the 6 × 6 inhomogeneous stiffness matrix is decomposed into two homogeneous sub-matrices, of which two performance indices, corresponding to the translational and rotational stiffnesses, are defined to evaluate the stiffness behaviors of the manipulators. A comparative study among the robot counterparts is carried out with respect to the elasto-static performance.

This paper deals with the parametric optimum design of a parallel Schoenflies-motion robot, named "Ragnar", designed for fast and flexible pick-and-place applications. The robot architecture admits a rectangular workspace, which can utilize the shop-floor space efficiently. In this work, the parametric models of the transmission quality, elasto-statics and dynamics are established. By taking into consideration of design requirements and pick-and-place trajectory, a comprehensive multi-objective optimization problem is formulated to optimize both kinematic and dynamic performances. The Pareto-front is obtained, which provides optimal solutions to the robot design. Robot prototyping work based on the optimal results is described.

In this paper, 3-PPR planar parallel manipulators with D- or U-shape base are compared with respect to their workspace size and kinematic sensitivity to joint clearances. First, the singularities and workspace of a general 3-PPR planar parallel manipulator are analyzed. Then, an error prediction model applicable to both serial and parallel manipulators is developed. As a result, two nonconvex quadratically constrained quadratic programs are formulated in order to find the maximum reference-point position error and the maximum orientation error of the moving-platform for given joint clearances. Finally, the contributions of the paper are highlighted by means of a comparative study of two manipulators.

Cam-roller mechanism can be used in transmissions and robotic devices as an alternative to their bevel-gear counterparts. Compared with gear transmissions, cam-roller mechanisms feature low friction, low backlash, and high strength. However, cam mechanisms may end up with a high pressure angle if negative action, a motion in which the cam is driven by rollers, occurs. This paper reports the design of transmissions with multi-lobe-cams (MLC), as a means to reduce the pressure angle. The kinematics problems associated to MLC, including profile generation and transmission errors, are studied. As an example, a pith-roll wrist designed with MLC is described

Robotic wrists are commonly used in manipulators for applications that require a large dexterous workspace. Although bevel-gear wrists are widely used in industrial manipulators due to their simple kinematics and low manufacturing cost, their gear trains function under rolling and sliding, the latter bringing about noise and vibration. Sliding is inherent to the straight teeth of the bevel gears of these trains. To alleviate these drawbacks, a wrist with cam-roller pairs under pure rolling is proposed here. This paper reports the design of an innovative robotic pitch-roll wrist using multi-lobe cams. The wrist has two degrees of freedom and consists of two roller-carriers (RCs) driving a multi-lobe cam to produce roll and pitch motions. The pure rolling between cam and rollers helps reduce the friction. Moreover, the cam transmission works with low backlash and high stiffness. In view of the kinematic performance, the wrist exhibits favorable features, including a singularity-free unlimited workspace.

The application of a parallel manipulator requires kinematic calibration of the base and tool transformation in order to eliminate the errors resulting from manufacturing and assembly of connecting parts. In this paper, a calibration method of base and tool transformation is developed by virtue of optical position sensors. Error model for calibration is constructed and an iterative calibration procedure is introduced. In addition, the pose measurement using a commercial 3D position measurement system, OPTOTRAK 3020, is described. Simulation and experiment results show that the method effectively improves the accuracy of manipulator.

The path planning of legged locomotion is difficult in that not only the body trajectory but also the foot placement need to be considered. A general approach of path planning will fail in generating a feasible path for walking machines when facing the huge searching space of legged locomotion. In this paper, an effective method of path planning for 3D walking is introduced. The basic idea is that a feasible path is generated based on the terrain evaluation which produces an index of terrain complexity with respect to the machine mobility. The terrain evaluation is first carried out in a 2D space by considering the leg placement. The 2D terrain complexity is then modified according to the height's influence on the body motion for the 3D results. As an illustration, a feasible path is generated by applying the potential-guided searching technology to the 3D terrain complexity.

The path generation for legged locomotion systems is difficult in that the legged motion must be considered together with the body movement. Owing to the large searching space resulted from the combination of leg movement and terrain condition, it is quite time consuming to produce an optimal result of body trajectory planning. In this paper, an effective method of body trajectory planning is introduced by virtue of a terrain evaluation that links the terrain condition with the machine mobility. Based on the evaluation, a potential field is constructed for the graph searching. The best first planning (BFP) is adopted to search the optimal path. The path generated with the proposed method could offer the best opportunity to place the machine feet moving with a certain gait over a rough terrain.

A parallel manipulator can be used for medical operations for its high stiffness and motion accuracy. However, the small and oddly distributed workspace of the general manipulator limits its applications. In this paper, a special manipulator which has a redundant degree of freedom (the 7-DOF) is introduced to improve the workspace. The workspace of the manipulator was analyzed with respect to the selected orientations. Two configurations of the 7-DOF were considered. The preliminary result of the analysis shows that the workspace of the manipulator can be effectively improved with the redundant degree of freedom.

When walking on uneven terrain, a walking machine needs to use free gait to deal with the irregularity of terrains. The gait of animals can greatly help the development of free gaits. However, there are few description on the gaits of animal walking on uneven terrain. In the paper, a kinematograph experiment on the moving pattern, of an insect, the cockroach, was introduced. The experiment investigated the behaviour of cockroach walking over different terrain: single obstacle and stairs using high speed cameras. The moving patterns of the cockroach on these terrains are presented and compared in terms of trajectories, displacements, and stick diagrams.

A method of free gait generation for quadrupeds is presented by introducing the primary/secondary gait. The primary gait is a fixed sequence of leg transfers with modified leg-ends kinematic limits according to the obstacle presence, while the secondary gait is a flexible gait which is generated to adjust the leg-end position. Using the proposed method, a machine tends to move with primary gait during the motion. When the primary gait cannot move the vehicle, the secondary gait is adopted to serve as a complement of the primary gait. Four constraints, namely, stability constraint, kinematic constraint, sequential constraint and neighboring constraint are considered when gaits are generated. The influence of the obstacle is taken into account to modify the gait parameters. With the proposed method based on primary/secondary gait, it is expected to improve the efficiency of free gait generation while maintaining the mobility of the vehicle. Simulation results are given to demonstrate the efficiency of the proposed algorithm.