Joint Actuators Research Articles

Multi-objective optimization for joint actuator and sensor placement for guided waves based structural health monitoring using fibre Bragg grating sensors

Structural Health Monitoring (SHM) systems have a potential to reduce lifecycle costs of structures. As a result, there is a lot of active research in the area for SHM of civil and mechanical structures. Guided waves (GW) based SHM techniques allow monitoring of large plate-like structures with a few sensors and have been identified as the most promising of techniques for SHM. Fibre Bragg grating (FBG) sensors due to their low weight, and ability to be multiplexed have been long thought to be ideal sensors for SHM. The recent development of the edge filtering approach has increased their sensitivity to GW sensing and made them ideal sensors. Unfortunately the FBG sensors are passive sensors and show directional sensitivity. These operational constraints make extension of the earlier developed GW based SHM techniques for FBG sensors difficult. Recently the authors developed a technique for damage detection specifically designed for a network with FBG sensors. This paper builds on the past work by the authors and develops a methodology for a design of an actuator–sensor (AS) network for improving the damage assessment capability of the previously proposed method. The paper develops a multi objective optimization technique for the joint optimization of actuator and sensor placement for a network with FBG sensors. The joint optimization of the actuators and sensors is necessary due to the passive nature of the FBG sensors and also incorporates the directional nature of the FBG sensors. The paper develops an integer encoded NSGA-II for the optimization of the AS network. The objectives for the optimization are derived from the specific damage detection technique tailored for the use of FBG sensors. The objective are: coverage with at least 2 AS pairs, coverage with at least 1 edge reflected path and the cost of the deployed network. The results indicate that the encoding of the objectives of the optimization is valid and indeed the damage detection capabilities of the AS network are as predicted analytically. The paper for the first time develops a joint optimization of network for FBG sensors. It is also the first attempt at a truly multi-objective optimization of the AS network and promises to have applications on real structures.

Design and experiment of a SMA-based continuous-stiffness-adjustment torsional elastic component for variable stiffness actuators

The variable stiffness actuator (VSA) is widely applied in robotics and human-robot interactions, generating controllable torque with sufficient compliance to ensure safety and robustness. At present, typical VSA design usually adopted pure mechanical structures that integrated an additional motor for stiffness adjustment, making it hard to meet the demand of lightweight joint actuation due to large actuator size and weight. As a new functional material, shape memory alloy (SMA) shows the prospect of lightweight actuation due to its high power-to-weight ratio. In this paper, a compact SMA-based torsional elastic component (SMA-TE) for the VSA is proposed. The SMA-TE with torsional stiffness is implemented by the confrontational arrangement of paired spiral SMA springs. The ceramic heating plate and semiconductor refrigeration plate are installed in the elastic component shell, and the thermally conductive medium is filled between the internal gap. The torsional stiffness of the SMA-TE can be continuously adjusted via a feed-forward cascade controller. Repeated experimental results show that the SMA-TE can precisely maintain constant stiffness and continuously adjust its stiffness between 2.6 Nm rad−1 and 6.8 Nm rad−1 when performing precise square wave stiffness tracking under variable frequencies.

Seismic control performance of a three‐story frame prototype equipped with semi‐active variable stiffness and damping structural joints

AbstractThis paper presents numerical and experimental studies on semi‐active seismic response control of structures equipped with variable stiffness and damping structural joints. Such adaptive joints, which are comprised of a shape memory polymer (SMP) core reinforced by an SMP‐aramid composite skin, function as load‐transfer components as well as semi‐active control devices. The SMP core material can transition from a glassy to a rubbery state through thermal actuation resulting in a shift of the structural natural frequencies and a parallel increase of damping ratio, which enables a new semi‐active control strategy. Control performance has been evaluated on a three‐story frame equipped with 12 adaptive joints and subjected to seismic excitations. Full‐transient analysis has shown that when the joints are thermally actuated to the transition temperature (65°C), acceleration and base shear are reduced by up to 62% and 65%, respectively. Shake‐table tests have been carried out on a 1/10‐scale prototype, confirming that through thermal actuation of the adaptive joints the structural damping ratio increases from 2.6% to 11.3% and the first natural frequency shifts by up to 37%. As the structure becomes more flexible, an increase of displacements and interstory drift might occur. However, depending on the seismic excitation, top‐story acceleration and base shear are significantly reduced in the range 43%–50% and 35%–51%, respectively. These results confirm that semi‐active control through thermal actuation of variable stiffness and damping structural joints is effective to mitigate the structure response under seismic excitation.

Performance Investigations of an Improved Backstepping Operational space Position Tracking Control of a Mobile Manipulator

This article implies an improved backstepping control technique for the operational-space position tracking of a kinematically redundant mobile manipulator. The mobile manipulator thought-out for the analysis has a vehicle base with four mecanum wheels and a serial manipulator arm with three rotary actuated joints. The recommended motion controller provides a safeguard against the system dynamic variations owing to the parameter uncertainties, unmodelled system dynamics and unknown exterior disturbances. The Lyapunov’s direct method assists in designing and authenticating the system’s closed-loop stability and tracking ability of the suggested control strategy. The feasibility, effectiveness and robustness of the recommended controller are demonstrated and investigated numerically with the help of computer based simulations. The mathematical model used for the computer-based simulations is derived based on a real-time mobile manipulator and the derived model is further verified with an inbuilt gazebo model in a robot operating system (ROS) environment. In addition, the proposed scheme is verified on an in-house fabricated mobile manipulator system. Further, the recommended controller performance is correlated with the conventional backstepping control design in both computer-based simulations and in real-time experiments.

A New Approach of Soft Joint Based on a Cable-Driven Parallel Mechanism for Robotic Applications

A soft joint has been designed and modeled to perform as a robotic joint with 2 Degrees of Freedom (DOF) (inclination and orientation). The joint actuation is based on a Cable-Driven Parallel Mechanism (CDPM). To study its performance in more detail, a test platform has been developed using components that can be manufactured in a 3D printer using a flexible polymer. The mathematical model of the kinematics of the soft joint is developed, which includes a blocking mechanism and the morphology workspace. The model is validated using Finite Element Analysis (FEA) (CAD software). Experimental tests are performed to validate the inverse kinematic model and to show the potential use of the prototype in robotic platforms such as manipulators and humanoid robots.

Buffer Compliance Control of Space Robots Capturing a Non-Cooperative Spacecraft Based on Reinforcement Learning

Aiming at addressing the problem that the joints are easily destroyed by the impact torque during the process of space robot on-orbit capturing a non-cooperative spacecraft, a reinforcement learning control algorithm combined with a compliant mechanism is proposed to achieve buffer compliance control. The compliant mechanism can not only absorb the impact energy through the deformation of its internal spring, but also limit the impact torque to a safe range by combining with the compliance control strategy. First of all, the dynamic models of the space robot and the target spacecraft before capture are obtained by using the Lagrange approach and Newton-Euler method. After that, based on the law of conservation of momentum, the constraints of kinematics and velocity, the integrated dynamic model of the post-capture hybrid system is derived. Considering the unstable hybrid system, a buffer compliance control based on reinforcement learning is proposed for the stable control. The associative search network is employed to approximate unknown nonlinear functions, an adaptive critic network is utilized to construct reinforcement signal to tune the associative search network. The numerical simulation shows that the proposed control scheme can reduce the impact torque acting on joints by 76.6% at the maximum and 58.7% at the minimum in the capturing operation phase. And in the stable control phase, the impact torque acting on the joints were limited within the safety threshold, which can avoid overload and damage of the joint actuators.

Study on operational space control of a redundant robot with un-actuated joints: experiments under actuation failure scenarios

This paper analyzes operational space dynamics for redundant robots with un-actuated joints and reveals their highly nonlinear dynamic impacts on operational space control (OSC) tasks. Unlike conventional OSC approaches that partly address the under-actuated system by introducing rigid grasping or contact constraints, we deal with the problem even without such physical constraints which have been overlooked, yet it includes a wide range of applications such as free-floating robots and manipulators with passive joints or unwanted actuation failure. In addition, as an intuitive application example of the drawn result, an OSC is formulated as an optimization problem to alleviate the dynamics disturbance stemmed from the un-actuated joints and to satisfy other inequality constraints. The dynamic analysis and the proposed control method are verified by a number of numerical simulations as well as physical experiments with a 7-degrees-of-freedom robotic arm. In particular, we consider joint actuation failure scenarios that can be occurred at certain joints of a torque-controlled robot and practical case studies are performed with an actual redundant robot arm.

Dynamic Modeling and Control of Antagonistic Variable Stiffness Joint Actuator

This study aims to develop a novel decoupling method for the independent control of the position and stiffness of a variable stiffness joint actuator (VSJA), which has been proven to be able to vary its stiffness in a larger range than other variable stiffness actuators. Using static analysis and the Jacobian matrix, we obtained the model of the stiffness of the robot joint actuator and dynamics. Based on the hybrid dynamic model of position and stiffness, it is possible to compensate for the torque of the variable stiffness joint actuator (VSJA) to enhance position control. Finally, after describing the actuator prototype, the established compliance control method is verified using simulation and experimental analysis.

A gravitational torque-compensated 2-DOF planar robotic arm design and its active control

Serial robot manipulators have their servo motors with reduction gears on the link joints. When it comes to hyper-redundant robots, this kind of joint actuation mechanism cannot be implemented since this makes hyper-redundant robots too heavy. Instead, cable driven mechanisms are preferred. However, the positioning accuracy is negatively affected by the cables. This paper addresses the positioning accuracy problem of cable driven hyper-redundant robots by employing a 2-DOF robotic arm whose modules are counter-balanced. While the actuators connected to the base actively do most of the work using cables and springs, light and compact actuators connected to the links produce precise motion. The method will result in compact, light and precise hyper-redundant robotic arms. The above-mentioned procedure governed by a control software including a 2D simulator developed is experimentally proved to be a feasible method to compensate the gravitational torque successfully.

Design and Modeling of a Bio-Inspired Flexible Joint Actuator

Spiders rely on a hydraulic system to stretch their legs but use muscles to make their legs flex. The compound drive of hydraulics and muscle makes an integrate dexterous structure with powerful locomotion abilities, which perfectly meets the primary requirements of advanced robots. Inspired by this hydraulics-muscle co-drive joint, a novel flexible joint actuator was proposed and its driving characteristics were preliminarily explored. The bio-inspired flexible joint manifested as a double-constrained balloon actuator, which was fabricated by the composite process of 3D printing and casting. To evaluate its performance, the mathematical model was deduced, as well as the finite element analysis (FEA) model. A series of experiments on the rotation angles, driving forces, and efficiencies of the flexible joint were carried out and compared with the mathematical calculations and FEA simulations. The results show that the accuracy of the two theoretical models can be used to assess the joint actuator. The locomotion test of a soft arthropod robot with two flexible joints was also implemented, where the moving speed reached 22 mm/s and the feasibility of the proposed flexible joint applied to a soft robot was demonstrated.

A Muscle-First, Electromechanical Hybrid Gait Restoration System in People With Spinal Cord Injury.

The development of a hybrid system for people with spinal cord injuries is described. The system includes implanted neural stimulation to activate the user's otherwise paralyzed muscles, an exoskeleton with electromechanical actuators at the hips and knees, and a sensory and control system that integrates both components. We are using a muscle-first approach: The person's muscles are the primary motivator for his/her joints and the motors provide power assistance. This design philosophy led to the development of high efficiency, low friction joint actuators, and feed-forward, burst-torque control. The system was tested with two participants with spinal cord injury (SCI) and unique implanted stimulation systems. Torque burst addition was found to increase gait speed. The system was found to satisfy the main design requirements as laid out at the outset.

Novel design method for multi-configuration dexterous hand based on a gesture primitives analysis

Purpose This paper aims to propose a novel method based on a gesture primitives analysis of human daily grasping tasks for designing dexterous hands with various grasping and in-hand manipulation abilities, which simplifies the complex and redundant humanoid five-finger hand system. Design/methodology/approach First, the authors developed the fingers and the joint configuration with a series of gesture primitives configurations and the modular virtual finger scheme, refined from the daily work gesture library by principal component analysis. Then, the authors optimized the joint degree-of-freedom configuration with the bionic design analysis of the anatomy, and the authors optimized the dexterity workspace. Furthermore, the adaptive fingertip and routing structure were designed based on the dexterous manipulation theory. Finally, the effectiveness of the design method was experimentally validated. Findings A novel lightweight three-finger and nine-degree-of-freedom dexterous hand with force/position perception was designed. The proposed routing structure was shown to have the capability of mapping the relationship between the joint space and actuator space. The adaptive fingertip with an embedded force sensor can effectively increase the robustness of the grasping operation. Moreover, the dexterous hand can grasp various objects in different configurations and perform in-hand manipulation dexterously. Originality/value The dexterous hand design developed in this study is less complex and performs better in dexterous manipulation than previous designs.

Long Short Term Memory Model Based Position-Stiffness Control of Antagonistically Driven Twisted-Coiled Polymer Actuators Using Model Predictive Control

Twisted-coiled polymer actuators (TCA) have many interesting properties that show potentials for making high performance bionic devices. This letter presents a long short term memory (LSTM) model to predict the behavior of an antagonistic joint driven by hybrid TCA bundles made from Spandex and nylon fibers. By using automatic differentiation, which can be done with PyTorch, a linearized discrete state space model can be formulated and then be utilized in model predictive control (MPC). The experimental results show that by combining LSTM and MPC in modeling and control of the TCA, both high prediction performance and control performance can be achieved. It is verified that using the LSTM model, the joint angle and actuator temperatures can be predicted accurately with an avarage steady state error less than 0.1 deg and 0.2 Cel. deg., respectively. The MPC control results show the controller's ability to reach the desired joint angle with an avarage steady state error of 0.21 degrees in set-point regulation and track a sinusoidal waveform at composite frequencies of 0.1 Hz to 0.15 Hz with the steady-state error less than 0.38 degrees while changing joint stiffness.

Disturbance rejection sliding mode control for robots and learning design

The control of a robot to achieve both good dynamic and static performance against external disturbances is a challenging task, especially when high speed and a wide range of motion is required. In this paper, a disturbance rejection sliding mode control (SMC) methodology is designed for a robot manipulator. This methodology synthesizes the SMC design with the active disturbance rejection control (ADRC) technique. An extended state observer is employed to estimate unknown disturbances, which is difficult to deal with in a conventional SMC design, and to simplify the SMC law design. A learning-based parameter tuning methodology is presented to autonomously obtain the control parameters offline. To develop a robust and transferring controller, a neural network is used to learn the joint actuation ability for the controller optimizing process. Compared with other state-of-the-art controllers, both numerical simulations and experiments of a 6-DOF robot are provided to demonstrate the proposed control method and design methodology. These results reveal that the proposed control method has a satisfying tracking performance and strong disturbance rejection ability.

Research on High-Precision Attitude Control of Joint Actuator of Three-Axis Air-Bearing Test Bed

Three-axis air-bearing test bed is important semiphysical simulation equipment for spacecraft, which can simulate spacecraft attitude control, rendezvous, and docking with high confidence. When the three-axis air-bearing table is maneuvering at a large angle, if it is only controlled by the flywheel, it will cause the problems of slow maneuvering speed and high energy consumption, and when the external interference torque becomes large, the control accuracy will decline. A combined actuator including flywheel, air-conditioner thruster, and automatic balancing device is designed, and a hierarchical saturation PD control algorithm is proposed to improve the control accuracy and anti-interference ability of the three-axis air-bearing test bed. Finally, the mathematical simulation of the proposed control algorithm is carried out, and the physical verification is carried out on the three-axis air-bearing test bed. The results show that the control algorithm has higher control accuracy than the traditional control algorithm, and the control accuracy is better than 0.1 ∘ and basically meets the attitude control requirements of the ground simulation in-orbit satellite.

Observer-based adaptive sliding mode fault-tolerant control for the underactuated space robot with joint actuator gain faults

An adaptive sliding mode fault-tolerant controller based on fault observer is proposed for the space robots with joint actuator gain faults. Firstly, the dynamic model of the underactuated space robot is deduced combining conservation law of linear momentum with Lagrange method. Then, the dynamic model of the manipulator joints is obtained by using the mathematical operation of the block matrices, hence the measurement of the angular acceleration of the base attitude can be omitted. Subsequently, a fault observer which can accurately estimate the gain faults is designed, and the estimated results are fed back to the adaptive sliding mode fault-tolerant controller. It is proved that the proposed control algorithm can guarantee the global asymptotic stability of the closed-loop system through the Lyapunov theorem. The simulation results authenticate the effectiveness and feasibility of the control strategy and observation scheme.

Preliminary design and development of a low-cost lower-limb exoskeleton system for paediatric rehabilitation.

In this work, the design, modeling, and development of a low-cost lower limb exoskeleton (LLES) system are presented for paediatric rehabilitation (age: 8-12 years, mass: 25-40 kg, height: 115-125 cm). The exoskeleton system, having three degrees-of-freedom (DOFs) for each limb, is designed in the SolidWorks software. A wheel support module is introduced in the design to ensure the user's stability and safety. The finite element analysis of the hip joint connector along with the wheel support module is realized for maximum loading conditions. The holding torque capacity of exoskeleton joints is estimated using an affordable spring-based experimental setup. A working prototype of the LLES is developed with holding torque rated actuators. Thereafter, the dynamic analysis for the human-exoskeleton coupled system is carried out using the Euler-Lagrange principle and SimMechanics model. The simulation results of estimating joint actuator torques are obtained for two paraplegic subjects (Case I: 10 years age, 30 kg mass, 120 cm height and Case II: 12 years age, 40 kg mass, 125 cm height). The details of input parameters such as body mass, link lengths, joint angles, and contact forces are discussed. The simulation results of dynamic analysis have shown the potential of estimating the torques of joint actuators for the developed prototype during motion assistance and gait rehabilitation.

Hopping frequency influences elastic energy reuse with joint series elastic actuators

Robotic limb design struggles to combine energy efficiency with human-like levels of movement versatility. High efficiency and a range of angles and torques are characteristics of human hopping at different frequencies. Humans use muscles in combination with tendons to achieve the required joint actuation. Therefore, we consider whether appropriately tuned series elastic actuators (SEAs) placed at the leg joints can be used to reduce the functional gap between robots and humans. Human hip, knee, and ankle biomechanics were recorded over a range of hopping frequencies to extract joint angles and torques, which were used as an input to a mechanical simulation SEA model. This model was used to optimize the SEA stiffness of each joint to either minimize peak power or energy requirements. This work investigated the relationship between hopping frequency and SEA stiffness, the utility of using SEAs at each joint, and the reasons behind humans’ preferred hopping frequency. Although the constant stiffness values across different hopping frequencies are suitable for the knee and the ankle, a variable serial elastic actuator stiffness could still further reduce energy requirements. Optimal SEA stiffness was found to reduce peak power requirements by up to 73% at the ankle and up to 66% at the knee, with greatest benefits found around the preferred frequency. However, no SEA benefits were found for the hip and above the preferred hopping frequency for the knee. These insights could be used to aid in the design of robotic and assistive devices to achieve versatile and energy efficient human-like movements.

Design and control of a passive compliant actuation with positioning measurement by LED and photodiode detector for medical application

Control of assistive exoskeleton robot recently has to be crucial of development and innovation of medical application. To support daily motions for humans, control application of assistive exoskeleton robot allows for limb movement with increased strength and endurance during patient’s wearable exoskeleton robot application. The interaction between such exoskeleton device and the human body at the connecting joint, especially the knees, is the main interest of this design formation. The assistive device requires to design and to develop into innovation design aspect. This research presents the novel design of an active compliant actuation joint in order to increasing the higher torque of actuation than conventional actuation joint. Control design of the higher torque actuation usually difficult priori to conventional torque control. This will contributed to applying the supervisory control for compliant actuation that verified by experiment method. Then the hybrid Radial Basis Function neural network (RBFNN) and PID were proposed for actuating torque control methods. Experimental results show that the design of supervisory control is get better response, and higher producing torque output than the conventional design. Error of torque control of compliant actuation is not instead of [Formula: see text] N·m for applying supervisory control, RBFNN with PID controller. Indeed, the low electromagnetic interference (EMI) positioning system using LED and photodiode detector is proposed to be usable in medical application.

Analysis of the PU-2UPS antenna parallel mechanism

In order to meet the requirements of the antenna pedestal for supporting and driving antenna to complete specific trajectory movement, the PU-2UPS parallel mechanism is proposed for antenna pedestal, where P, U and S represent the prismatic joint, universal joint and spherical joint, respectively. The screw theory is applied to analyze the mobility of the PU-2UPS antenna parallel mechanism (APM). Meanwhile, kinematic analysis of the mechanism is carried out based on the theory of twist and spatial acceleration. Based on the wrench and force balance relationship, the dynamic model of the PU-2UPS APM is established. Therefore, the actuation forces of each actuation joint are obtained. Afterward, the work-space of the PU-2UPS APM is constructed based on the boundary search method. Finally, the mathematical model of the PU-2UPS APM is established by MATLAB software, and the theoretical analysis of the PU-2UPS APM is verified by ADAMS software. The studies have been performed that the PU-2UPS APM is suitable for antenna pedestal in respect of mobility, kinematics and dynamics properties. This research results serve as a fundamental theoretical basis for design and analysis of the APM.