Linear Series Elastic Actuators Research Articles

Design, Control, and Experimental Evaluation of A Novel Robotic Glove System for Patients with Brachial Plexus Injuries.

This paper presents the development of an exoskeleton glove system for people who suffer from brachial plexus injuries, aiming to assist their lost grasping functionality. The robotic system consists of a portable glove system and an embedded controller. The glove system consists of Linear Series Elastic Actuators (LSEA), Rotary Series Elastic Actuators (RSEA), and optimized finger linkages to provide imitated human motion to each finger and a coupled motion of the hand. The design principles and optimization strategies were investigated to balance functionality, portability, and stability. The model-based force control strategy compensated with a backlash model and model-free force control strategy are presented and compared. Results show that our proposed model-free control method achieves the goal of accurate force control. Finally, experiments were conducted with the prototype of the developed integrated exoskeleton glove system. Results from 3 subjects with 150 trials show that our proposed exoskeleton glove system has the potential to be used as a rehabilitation device for patients.

Design of Parallel Exoskeleton System for Wrist Tremor Suppression

This paper introduces the design of a Parallel Exoskeleton for Wrist Tremor Suppression (PEWTS). The design features dual six degree-of-freedom (DOF) subsystems that aim to be compact and suppress tremors in both radial/ulnar deviation (RUD) and flexion/extension (FE) motions of the wrist. A linear Series Elastic Actuator is employed, to provide compactness and back drivability to the system which would make the orthoses more human-compatible. The presented study focuses on providing a fundamental understanding of the working of PEWTS and investigating its feasibility through kinematic workspace analysis.

Research on the Inherent Nonlinearity Calibration of the Potentiometer of a Miniature Linear Series Elastic Actuator

This paper presents a miniature Linear Series Elastic Actuator (LSEA), in which two low-cost linear potentiometers were adopted to achieve a compact design. To improve the measurement accuracy of the linear potentiometer, a novel Bezier-based Calibration Method (BCM) and its optimization method were proposed to calibrate the inherent nonlinearity of the linear potentiometer. Calibration efficiency of the BCM was investigated numerically by making a comparison with the widely used Polynomial Calibration method (PCM), and the effect of the BCM calibration on the control performance of the LSEA was investigated experimentally by displacement and force control. Results reveal that the BCM exhibits an excellent local calibration ability for the nonlinearity with knee points. Due to the above characteristic, the BCM could produce a better calibration accuracy than the PCM under the same model order and improve the control performance of the LSEA. In addition, the BCM could calibrate the inherent nonlinearity of the potentiometer in a continuous form rather than that of piecewise ones. The continuous calibration form could bring more convenience to practical applications.

Design, Analysis, and Experiment of an Underactuated Robotic Gripper Actuated by Linear Series Elastic Actuator

Abstract The primary motivation of this study is to develop a cost-effective, safe, easily controlled, and passively adaptive robotic gripper. A novel linear series elastic actuated robotic gripper (LSEA-RG) is proposed to accomplish the aforementioned goals. The LSEA-RG has the ability to adaptively grasp objects with different shapes and sizes by grasping force control. First, the mechanical design, including the transmission scheme, the finger mechanism, and the actuating mechanism is presented. Subsequently, the kinematic analysis is performed to explore the transmission characteristic of the actuating displacement and force. Then a model-free control method based on the lookup functions of the kinematic transmission characteristics is presented to achieve the grasping range and force control. Finally, several grasping experiments are carried out based on the LSEA-RG prototype to evaluate the grasping performance. The results demonstrate that the presented grasping control method which based on the lookup functions is valid. The LSEA-RG could estimate the grasping force without the help of a fingertip force sensor and grasp objects with different shapes and hardness adaptively.

Design and optimisation of load-adaptive actuator with variable stiffness for compact ankle exoskeleton

To overcome the limitations of linear series elastic actuators (SEAs) with constant stiffness, a novel load-adaptive actuator with variable stiffness is proposed for actuating ankle exoskeletons through an inverted slider-crank mechanism. Using the proposed actuator, the stiffness of the exoskeleton can be adjusted passively based on the external load and joint angle. To achieve compactness, a novel nonlinear spring mechanism with user-defined load–deflection behaviour is designed by combining a cam mechanism with parabolic beams; this proposed mechanism is used as the series elastic element of the proposed actuator. Dimensional optimisation of the ankle exoskeleton, including the actuator, is subsequently performed for maximising the energy efficiency based on gait biomechanics and considering the constraints of the inverted slider-crank mechanism, electric motor, and the nonlinear spring mechanism. Simulation and experimental results show that the energy efficiency is improved by using the proposed load-adaptive actuator-powered ankle exoskeleton compared with using an exoskeleton driven by linear SEAs.

A NOVEL DESIGN OF A ROBOTIC GLOVE SYSTEM FOR PATIENTS WITH BRACHIAL PLEXUS INJURIES.

This paper presents the design of an exoskeleton glove system for people who suffer from the brachial plexus injuries in an effort to restore their lost grasping functionality. The robotic system consists of an embedded controller and a portable glove system. The glove system consists of Linear Series Elastic Actuators (SEA), Rotary SEA and optimized finger linkages to provide motion to each finger and a coupled motion of the hand and the wrist. The design is based on various functionality requirements such as being lightweight and portable for activities of daily living, especially for grasping. The contact force at each fingertip and bending angle of each finger are measured for future implementation of intelligent control algorithms for autonomous grasping. To provide better flexibility and comfort for the users, abduction and adduction of each finger as well as flexion of the thumb were taken into consideration in the design. The glove system is adjustable for different hand sizes. The micro-controllers and batteries are integrated on the forearm in order to provide a completely portable design solution.

External Force Estimation in Linear Series Elastic Actuator Without Load-Side Encoder

This article presents a novel estimation method of the external force in linear series elastic actuators (SEAs). Conventionally, such force is estimated through the deflection of an elastic element, placed between motor and end-effector in SEAs (measured using two encoders). However, in some applications (e.g., wearable devices), it is difficult to mount a load-side encoder, due to cost or manufacturing issue. Moreover, methods based on encoders have limitations in terms of bandwidth, and, in turn, in estimating fast-varying impact forces. Additionally, SEAs may have a nonnegligible end-effector mass, therefore gravity (or accelerations of the actuator support) may add a contribution to the external force, not accounted by deflection-based estimation methods. To address these issues, we propose a method based on the Kalman filter in combination with the use of two MEMS accelerometers, mounted on the load-side and on the support of the SEA, for external force estimation and gravity (or support acceleration) compensation, respectively. We tested the proposed method even in a reduced version, with a single load-side accelerometer. Finally, it is shown that using the acceleration signal also improves the robustness against spring stiffness variations, compared to conventional methods. The experimental results show the effectiveness of the proposed methods.

Stable Grasp Control With a Robotic Exoskeleton Glove.

An exoskeleton robotic glove intended for patients who have suffered paralysis of the hand due to stroke or other factors has been developed and integrated. The robotic glove has the potential to aid patients with grasping objects as part of their daily life activities. Grasp stability was studied and researched by various research groups, but mainly focused on robotic grippers by devising conditions for a stable grasp of objects. Maintaining grasp stability is important so as to reduce the chances of the object slipping and dropping. But there was little focus on the grasp stability of robotic exoskeleton gloves, and most of the research was focused on mechanical design. A robotic exoskeleton glove was developed as well as novel methods to improve the grasp stability. The glove is constructed with rigidly coupled four-bar linkages attached to the finger tips. Each linkage mechanism has one-DOF (degree of freedom) and is actuated by a linear series elastic actuator (SEA). Two methods were developed to satisfy two of the conditions required for a stable grasp. These include deformation prevention of soft objects, and maintaining force and moment equilibrium of the objects being grasped. Simulations were performed to validate the performance of the proposed algorithms. A battery of experiments was performed on the integrated prototype in order to validate the performance of the algorithms developed.

A 5-Degrees-of-Freedom Lightweight Elbow-Wrist Exoskeleton for Forearm Fine-Motion Rehabilitation

Exoskeleton robots have been demonstrated to effectively assist the rehabilitation of patients with upper or lower limb disabilities. To make exoskeletons more accessible to patients, they need to be lightweight and compact without major performance tradeoffs. Existing upper limb exoskeletons focus on assistance with coarse motion of the upper arm, whereas forearm fine-motion rehabilitation is often ignored. This paper presents an elbow-wrist exoskeleton with five degrees of freedom (DoFs). Using geared bearings, slider crank mechanisms, and a spherical mechanism for the wrist and elbow modules, this exoskeleton can provide 5-DoF rotary motion forearm assistance. The optimized exoskeleton dimensions allow sufficient rotation output while the motors are placed parallel to the forearm and elbow joint. Thus, compactness and less inertia loading can be achieved. Linear and rotary series elastic actuators with high torque-to-weight ratios are proposed to accurately measure and control interaction force and impedance between exoskeleton and forearm. The resulting 3-kg exoskeleton can be used alone or easily in combination with other exoskeleton robots to provide various robot-aided upper limb rehabilitation.

Application of Taguchi design in system identification: A simple, generally applicable and powerful method

This paper is a development of the authors’ recently published idea on a simple and generally applicable system identification algorithm based on Taguchi method. In the previous paper, the effectiveness of the algorithm in identifying nonlinear systems was shown. This paper presents the modified algorithm with the capability of identifying Multi-Input Multi-Output (MIMO) systems, and systems with unknown parameters of quite different value-ranges. Regarding the former, a contribution analysis is introduced and utilized to determine that the model between which of the input-output pairs should be identified first. The results proved that this idea significantly results in less time-cost. Moreover, the study develops the identification algorithm by adding a step of breaking down the identification process to multiple steps, each identifying a group of parameters that are in about the same value-range. Utilizing Taguchi method that includes human intelligence in the loop, the presented method effectively saved the process time and led to a more accurate estimation. In the method presented, the algorithm identifies a group of parameters at each step starting with the most contributor ones rather than identifying all of them at once. To group parameters, a contribution analysis is employed. To prove the effectiveness of the method, without loss of generality, the algorithm is applied on a multi-input single-output system. The experimental test-setup used in this study is a custom-made linear series elastic actuator that is designed for lower limb exoskeletons in Ferdowsi University of Mashhad (FUM)-Robotic Laboratory.

A compact wrist rehabilitation robot with accurate force/stiffness control and misalignment adaptation

Robots have been demonstrated to assist the rehabilitation of patients with upper or lower limb disabilities. To make exoskeleton robots more friendly and accessible to patients, they need to be lightweight and compact without major performance tradeoffs. Existing upper-limb exoskeleton robots focus on the assistance of the coarse-motion of the upper arm while the fine-motion rehabilitation of the forearm is often ignored. This paper presents a wrist robot with three degrees-of-freedom. Using a geared bearing, slider crank mechanisms, and a spherical mechanism, this robot can provide the complete motion assistance for the forearm. The optimized robot dimensions allow large torque and rotation output while the motors are placed parallel to the forearm. Thus lightweight, compactness, and better inertia properties can be achieved. Linear and rotary series elastic actuators (SEAs) with high torque-to-weight ratios are proposed to accurately measure and control the interaction force and impedance between the robot and the wrist. The resulting 1.5-kg robot can be used alone or easily in combination with other robots to provide various robot-aided upper limb rehabilitation.

원통형 Linear Series Elastic Actuator 설계 및 제어

원통형 Linear Series Elastic Actuator 설계 및 제어

Wrench Capability of a Stewart Platform With Series Elastic Actuators

This paper proposes a novel method for analyzing linear series elastic actuators (SEAs) in a parallel-actuated Stewart platform, which has full six degrees-of-freedom (DOF) in position and orientation. SEAs can potentially provide a better human–machine interface for the user. However, in the study of parallel-actuated systems with full 6DOF, the effect of compliance in series with actuators has not been adequately studied from the perspective of wrench capabilities. We found that some parameters of the springs and the stroke lengths of the linear actuators play a major role in the actuation limits of the system. This is an important consideration when adding SEAs into a Stewart platform or other parallel-actuated robots to improve their human usage.

Design of a Parallel Actuated Exoskeleton for Adaptive and Safe Robotic Shoulder Rehabilitation

Powered exoskeletons can facilitate after-stroke rehabilitation of patients with shoulder disabilities. Designs using serial mechanisms usually result in complicated and bulky exoskeletons. This paper presents a new parallel actuated shoulder exoskeleton that consists of two spherical mechanisms, two slider crank mechanisms, and a gravity balancing mechanism. The actuators are grounded and placed side-by-side. Thus, better inertia properties can be achieved while lightweight and compactness are maintained. An adaptive mechanism with only passive joints is introduced to compensate for the exoskeleton–limb misalignment and size variation among different subjects. Linear series elastic actuators (SEAs) are proposed to obtain accurate force and impedance control at the exoskeleton–limb interface. The total number of force sensors and actuators is minimized using the adaptive mechanism and SEAs. An exoskeleton prototype is shown to provide bidirectional actuation between the exoskeleton and upper limb, which is required for various rehabilitation processes. We expect this design can provide a means of shoulder rehabilitation.

Series elastic actuation of an elbow rehabilitation exoskeleton with axis misalignment adaptation.

Powered exoskeletons can facilitate rehabilitation of patients with upper limb disabilities. Designs using rotary motors usually result in bulky exoskeletons to reduce the problem of moving inertia. This paper presents a new linearly actuated elbow exoskeleton that consists of a slider crank mechanism and a linear motor. The linear motor is placed beside the upper arm and closer to shoulder joint. Thus better inertia properties can be achieved while lightweight and compactness are maintained. A passive joint is introduced to compensate for the exoskeleton-elbow misalignment and intersubject size variation. A linear series elastic actuator (SEA) is proposed to obtain accurate force and impedance control at the exoskeleton-elbow interface. Bidirectional actuation between exoskeleton and forearm is verified, which is required for various rehabilitation processes. We expect this exoskeleton can provide a means of robot-aided elbow rehabilitation.

Team VALOR's ESCHER: A Novel Electromechanical Biped for the DARPA Robotics Challenge

The Electric Series Compliant Humanoid for Emergency Response (ESCHER) platform represents the culmination of four years of development at Virginia Tech to produce a full‐sized force‐controlled humanoid robot capable of operating in unstructured environments. ESCHER's locomotion capability was demonstrated at the DARPA Robotics Challenge (DRC) Finals when it successfully navigated the 61 m loose dirt course. Team VALOR, a Track A team, developed ESCHER leveraging and improving upon bipedal humanoid technologies implemented in previous research efforts, specifically for traversing uneven terrain and sustained untethered operation. This paper presents the hardware platform, software, and control systems developed to field ESCHER at the DRC Finals. ESCHER's unique features include custom linear series elastic actuators in both single and dual actuator configurations and a whole‐body control framework supporting compliant locomotion across variable and shifting terrain. A high‐level software system designed using the robot operating system integrated various open‐source packages and interfaced with the existing whole‐body motion controller. The paper discusses a detailed analysis of challenges encountered during the competition, along with lessons learned that are critical for transitioning research contributions to a fielded robot. Empirical data collected before, during, and after the DRC Finals validate ESCHER's performance in fielded environments.

Linear Multimodal Actuation Through Discrete Coupling

Due to technological limitations, robot actuators are often designed for specific tasks with narrow performance goals, whereas a wide range of behaviors is necessary for autonomous robots in uncertain complex environments. In an effort to increase the versatility of actuators, we introduce a new concept of multimodal actuation (MMA) that employs dynamic coupling in the form of clutches and brakes to change its mode of operation. The dynamic coupling allows motors and passive elements such as springs to be engaged and disengaged within a single actuator. We apply the concept to a linear series elastic actuator which uses friction brakes controlled online for the dynamic coupling. With this prototype, we are able to demonstrate several modes of operation including stiff position control, series elastic actuation as well as the possibility to store and release energy in a controlled manner for explosive tasks such as jumping. In this paper, we model the proposed concept of actuation and show a systematic performance analysis of the physical prototype that we developed in our laboratory.