Actuator Research Articles

Gravity and Impedance Compensation of Body Weight Support System Driven by Two Series Elastic Actuators

Robotic devices that can support human’s motions, such as exoskeletons, have recently drawn attention. Body weight support (BWS) systems are potential robotic devices that support and assist trunk motions with elaborate force control for rehabilitation and training exercises. This article presents a BWS system with 2-D motion assistance controlled by two series elastic actuators, which can perform not only weight compensation but also impedance compensation for trunk motion assistance. As the core technology of the proposed system, a wire-driven mechanism is developed that consists of a compact planetary-geared elastic actuator (cPEA), which is a series elastic actuator. All the dynamic aspects of the cPEA output torque, the tension-generating system using a winding drum, and the dynamics interacting with the BWS system are analyzed and used to design a motion-assistive controller. The functionality and ability of the proposed system are validated through experiments performed under various conditions. The results verify that the dynamic behavior of trunk motion can be assisted by the impedance control of the proposed BWS system. In particular, the inertial force required for the trunk motion can be reduced by the proposed BWS system.

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.

Whole-Body Control of an Autonomous Mobile Manipulator Using Series Elastic Actuators

The flexible-joint robot has superior attractive features because of its high mobility, high load ratio, high torque fidelity, robustness for external disturbance, task adaptability, and safety. In this article, an autonomous mobile manipulator driven by the designed series elastic actuators is developed. A complete dynamic model of a nonholonomic mobile manipulator including a mobile platform and a manipulator with joint flexibility operating simultaneously is proposed. An integrated whole-body trajectory control framework is proposed for such robot to perform mobile manipulation tasks. Considering the nonholonomic and holonomic constraints in the mobile manipulation, the whole-body dynamics is formulated and reduced. To address the highly nonlinear of the dynamics and model uncertainty, a novel integral Lyapunov function based adaptive neural network control for task tracking under uncertainties of the flexible-joint robot model is proposed. Compared with existing methods, the proposed method provides an alternative for controlling flexible-joint robots. The feasibility of the proposed method is verified by the extensive trajectory tracking experiment results in our developed flexible joint manipulator.

Addition of passive-carriage for increasing workspace of cable robots: automated inspection of surfaces of civil infrastructures

Cable-driven robots are parallel manipulators in which rigid links are replaced by actuated cables. The end-effector is then supported by a set of cables commanded by motors that are usually placed in a fixed frame. By varying the cables length, it is possible to change the end-effector position and/or orientation. Among the advantages presented by cable robots are they light-weight structure, high energy efficiency and their ability to cover large workspaces since cables are easy to wind. When high-speed operation is not required, a safer solution is to design cable-driven suspended robots, where all vertical components of cables tension are against gravity direction. Cable-driven suspended robots present limited workspace due to the elevated torque requirements for the higher part of the workspace. In this paper, the addition of a passive carriage in the top of the frame is proposed, allowing to achieve a much greater feasible workspace than the conventional one, i.e., with the same size as the desired inspection area while maintaining the same motor requirements. In the opposite, this new scheme presents non-desired vibration during the end-effector maneuvers. These vibrations can be removed by means of a more complex control strategy. Kinematics and dynamics models are developed in this paper. An analysis of sensor system is carried out and a control scheme is proposed for controlling the end-effector pose. Simulation and experimental results show that the feasible workspace can be notoriously increased while end-effector pose is controlled. This new architecture of cable-driven robot can be easily applied for automated inspection and monitoring of very large vertical surfaces of civil infrastructures, such as facades or dams.

HOSM controller design with an output constraint and its application

This work has developed an approach to a high-order sliding mode (HOSM) controller design for nonlinear systems subject to output constraints. To handle the output constraints, a barrier Lyapunov function (BLF) has been adopted. By inserting the BLF into the revamped adding a power integrator (API) technique, a HOSM controller, which can handle the closed-loop sliding mode dynamics synchronously with and without output constraints, is constructively framed. The strictly mathematical proof shows that the presented strategy can assure the finite-time stability of the closed-loop system and the achievement of a pre-set output constraint. The differences between the proposed HOSM controller and the traditional HOSM controller are discussed via a numerical example. Finally, the theoretical results obtained are successfully applied to a series elastic actuator system. The simulation results clearly confirm the merit of the presented HOSM algorithm.

A Hybrid Impedance Controller for Series Elastic Actuators to Render a Wide Range of Stable Stiffness in Uncertain Environments

Accurate and wide-range stiffness control is important for safe human-robot interaction. Accurate stiffness control can be better achieved using series elastic actuators (SEAs) than conventional rigid actuators. However, the stable range of virtual stiffness rendered by SEAs is limited by the stiffness of the actual spring, which cannot be too high in order to ensure good force control accuracy. Adding a virtual damper or derivative gain can increase the stable range of virtual stiffness, but the stable range would highly depend on the environmental stiffness. To relax the stiffness limitation in uncertain environments and explore more merits of SEAs, this letter proposes a hybrid impedance controller. This new controller linearly combines the spring force feedback and inertia force feedback. The stable range of virtual stiffness can be easily increased to ten times the actual spring stiffness with minimum effect on the force control accuracy. Unlike typical impedance controllers, the environmental stiffness can be used to raise the stable range of stiffness and hence the robustness of the controller can be ensured. Experiments will be provided to verify the hybrid impedance controller. We expect that the hybrid impedance controller can be used for SEAs in unstructured environments to provide a wide range of virtual stiffness.

Numerical and experimental investigation for monitoring and prediction of performance in the soft actuator

Due to various benefits such as unlimited degrees of freedom, environment adaptability, and safety for humans, engineers have used soft materials with hyperelastic behavior in various industrial, medical, rescue, and other sectors. One of the applications of these materials in the fabrication of bending soft actuators (SA) is that they have eliminated many problems in the actuators such as production cost, mechanical complexity, and design algorithm. However, SA has complexities, such as predicting and monitoring behavior despite the many benefits. The first part of this paper deals with the prediction of SA behavior through mathematical models such as Ogden and Darijani, and its comparison with the results of experiments. At first, by examining different geometric models, the cubic structure was selected as the optimal structure in the investigated models. This geometrical structure at the same pressure showed the most significant bending in the simulation. The simulation results were then compared with experimental, and the final gripper model was designed and manufactured using a 3D printer with silicone rubber as for the polymer part. This geometrical structure is capable of bending up to a 90-degree angle at 70 kPa in less than 2 seconds. The second section is dedicated to monitoring the bending behavior created by the strain sensors with different sensitivity and stretchability. In the fabrication of the sensors, silicon is used as a soft material with hyperelastic behavior and carbon fiber as a conductive material in the soft material substrate. The SA designed in this paper is capable of deforming up to 1000 cycles without changing its characteristics and capable of moving objects weigh up to 1200 g. This SA has the capability of being used in soft robots and artificial hand making for high-speed objects harvesting.

Nonlinear Positioning Compensator of a Novel Thin-Disc Ultrasonic Motor Using Fuzzy Sliding-Mode Control

ABSTRACT An approach of controller design based on fuzzy set and sliding-mode control theory for positioning purpose of a thin-disc edge-driving ultrasonic motor (USM) is developed in this study. A commercially available thin-disc buzzer used as a novel actuating component of ultrasonic motor with simple structure has been demonstrated its cost effectiveness and advantage in flexible mechanism design of a stator and rotor separately according to the necessary of applications. The driving mechanism associated with the non-linear phenomena such as dead-zone and hysteresis behaviors were studied too. Through system identification technique, the approximated transfer function of ultrasonic motor was obtained for controller design using fuzzy sliding-mode (FSMC). It has been successfully applied to position tracking to prove the excellent robust performance in noise rejection.

Multi-objective gain optimizer for a multi-input active disturbance rejection controller: Application to series elastic actuators

Series elastic actuators (SEA) have been gaining increasing popularity as a mechanical drive in contemporary force-controlled robotic manipulators thanks to their ability to infer the applied torque from measurements of the elastic element’s deflection. Accurate deflection control is crucial to achieve a desired output torque and, therefore, unmodelled dynamics and dynamic loads can severely compromise force fidelity. Multi-input active disturbance rejection controllers (ADRC) have the ability to estimate such disturbances affecting the plant behaviour and cancel them via an appropriate feedback controller. Thus, they offer a promising control architecture for SEA. ADRC, however, can have upwards of eight tuning parameters for each controlled state. Tuning the controller becomes quite challenging, especially in the context of multi-input, multi-objective control.This paper tackles the problem of ADRC tuning as a multi-parametric and multi-objective optimization approach. An ADRC is developed to regulate the output torque of a multi-input hybrid motor-brake–clutch SEA. The controller has a total of 22 tunable parameters. Point dominance-based nondominated sorting genetic algorithm is used to find the optimal control gains, first considering nine individual control objectives, and then in the context of multi-objective. The algorithm provides a set of potential solutions that highlight the tradeoffs between the control objectives. It is up to the discretion of the designer to select the appropriate solution that best suits a given application. The approach is validated experimentally and the results are compared with a simulated model. Experimental results confirm the suitability of the proposed approach for single and multiple control objectives in a variety of experimental scenarios and show good agreement with the analytical model.

Fuzzy torque trajectory control of a rotary series elastic actuator with nonlinear friction compensation

For humanoid/memetic robots, modeling and accurate torque trajectory control of a rotary series elastic actuator (RSEA) is of great importance. In this study, the fuzzy logic torque controller with nonlinear friction compensation (NLFC) is used to improve the deteriorating trajectory tracking performance caused by these nonlinear elements in RSEA systems. In order to demonstrate the power efficiency and performance of the proposed control system, several experiments have been performed on the experimental setup, including a torque motor with worm gear and torsional flat-double spiral spring (TFDSS). The proposed novel RSEA is designed and tested using different controllers, including PID feedforward controller (PID-FFC), fuzzy logic feedforward controller (FL-FFC), and fuzzy torque controller with friction compensation (FTC-FC). A comparative study among controllers is conducted to show the robustness of FTC-FC against a step and ramp-type disturbances. The simulation and experimental results here strongly confirm that the proposed control method produces better control performance.

Actuated and Performative Architecture: Emerging Forms of Human-Machine Interaction

This Spool [CpA] #3 issue poses and attempts to answer questions on the nature of this intimate human-machine bond, encouraging the discussion of its potentials also in terms of individual and social resilience. This issue of Spool, moreover, attempts to explore the design of bio-cyber-physical systems, which requires integration of natural, physical, and virtual architectures with digital systems and social organizations. In designing interactions between the (augmented) human and cyber-physical environments, the collection and use of personal data, the management of a multi-layered design approach, and the ethics of such design activity require attention from experts in architectural design, interaction and UX design, civil and architectural engineering, mechanical and electrical engineering, computer and information science, sociology, psychology, education, ethics, philosophy, media arts, and science and technology studies.

Cell-Free and User-Centric Massive MIMO Architectures for Reliable Communications in Indoor Factory Environments

Factory automation is one of the use cases for 5G-and-beyond mobile networks where strict requirements in terms of latency, availability and reliability are required. In this paper, we investigate the potentials of massive MIMO in delivering those promises for industrial automation. Namely, communications between actuators (ACs) and Access Points (APs) inside an industrial scenario is considered and different transmission modes are compared: joint transmission (JT) where the distributed antennas are used to communicate with each AC, cell-free transmission (CFT) where all the ACs are served by all APs, single AP transmission (SAT) where each AC is served by only one AP, and user-centric transmission (UCT) where each AC is served by a subset of APs. A power control strategy, aimed at maximizing the minimum signal-to-interference plus noise ratio (SINR), is also introduced. Numerical results, shown in terms of downlink SINR and achievable rate, evaluated using the final block length capacity formula (FBLC), demonstrate that the use of distributed antenna setting and of power control bring substantial performance improvements in terms of reliability and latency.

Design and Experiment of an Anthropomorphic Robot Hand for Variable Grasping Stiffness

This paper introduces an anthropomorphic robot hand that has variable grasping stiffness to unknown objects without damage. The robot hand has four fingers, 13 DoFs (Degrees of Freedom) with three SEA (Series Elastic Actuator) modules in the index, middle, and ring finger. The performance for broader grasping stiffness using variable impedance control based on the parameter update block and the procedure of grasping capability evaluations of the robot hand are explained. The robot finger modules are constructed with a thumb, index, middle, and ring finger to complete the anthropomorphic type robot hand. The robot finger module has three DoFs and functions as one actuator based on an under-actuated mechanism. Additionally, external force measurement is possible in the fingertip force sensor and SEA (Series Elastic Actuator) module. For the variable impedance control implementation, a parameter update block is added to adjust the desired impedance parameter. To implement the parameter update block, we define and evaluate the correlation equation between the fingertip force, shore hardness, and damping ratio.

End-effector Cartesian stiffness shaping - sequential least squares programming approach

Control of robot end-effector (EE) Cartesian stiffness matrix (or the whole mechanical impedance) is still a challenging open issue in physical humanrobot interaction (pHRI). This paper presents an optimization approach for shaping the robot EE Cartesian stiffness. This research targets collaborative robots with intrinsic compliance - serial elastic actuators (SEAs). Although robots with SEAs have constant joint stiffness, task redundancy (null-space) for a specific task could be used for robot reconfiguration and shaping the stiffness matrix while still keeping the EE position unchanged. The method proposed in this paper to investigate null-space reconfiguration's influence on Cartesian robot stiffness is based on the Sequential Least Squares Programming (SLSQP) algorithm, which presents an expansion of the quadratic programming algorithm for nonlinear functions with constraints. The method is tested in simulations for 4 DOF planar robot. Results are presented for control of the EE Cartesian stiffness initially along one axis, and then control of stiffness along both planar axis - shaping the main diagonal of the EE stiffness matrix.

Exploring the Limits of a Redundant Actuation System Through Co-Design

This paper assesses the energy efficiency of a redundant actuation architecture combining Quasi-Direct Drive (QDD) motors and Series Elastic Actuators (SEAs) by comparing its energy consumption to Geared Motors (GMs) and SEAs alone. We consider this comparison for two robotic systems performing different tasks. Our results show that using the redundant actuation we can save up to 99% of energy with respect to SEA for sinusoidal movements. This efficiency is achieved by exploiting the coupled dynamics of the two actuators, resulting in a latching-like control strategy. We also show that these large energy savings are not straightforwardly extendable to nonsinusoidal movements, but smaller savings ( e.g. , 7%) are nonetheless possible. The presented results were obtained thanks to the framework of concurrent design (co-design), namely the simultaneous optimization of hardware parameters and control trajectories. This shows that the combination of complex hardware morphologies and advanced numerical co-design can lead to peak hardware performance that would be unattainable by human intuition alone.

Stability/instability of the graphene reinforced nano-sized shell employing modified couple stress model

The current research deals with, stability/instability and cylindrical composite nano-scaled shell's resonance frequency filled by graphene nanoplatelets (GPLs) under various thermal conditions (linear and nonlinear thermal loadings). The piece-wise GPL-reinforced composites' material properties change through the orientation of cylindrical nano-sized shell's thickness as the temperature changes. Moreover, in order to model all layers' efficient material properties, nanomechanical model of Halpin-Tsai has been applied. A functionally modified couple stress model (FMCS) has been employed to simulate GPLRC nano-sized shell's size dependency. It is firstly investigated that reaching the relative frequency's percentage to 30% would lead to thermal buckling. The current study's originality is in considering the multifarious influences of GPLRC and thermal loading along with FMCS on GPLRC nano-scaled shell's resonance frequencies, relative frequency, dynamic deflection, and thermal buckling. Furthermore, Hamilton's principle is applied to achieve boundary conditions (BCs) and governing motion equations, while the mentioned equations are solved using an analytical approach. The outcomes reveal that a range of distributions in temperature and other mechanical and configurational characteristics have an essential contribution in GPLRC cylindrical nano-scaled shell's relative frequency change, resonance frequency, stability/instability, and dynamic deflection. The current study's outcomes are practical assumptions for materials science designing, nano-mechanical, and micro-mechanical systems such as micro-sized sensors and actuators.

Complementary Stability of Markovian Systems: Series Elastic Actuators and Human-Robot Interaction

Abstract This paper presents an extension of the complementary stability analysis for discrete-time Markovian systems applied in interaction control. The control strategy considers an internal loop force control for Series Elastic Actuators based on the Robust Regulator for Discrete-time Markov Jump Linear systems and extern loop impedance control to regulate the interaction between the human and robot. Two examples show the complementary stability analysis of the human-robot interaction system, where is regarded as different values of the virtual impedances, different levels for the robustness of the force control, uncertainties, and abrupt changes for human parameters. Simulation results show the capacity of control strategy to deal with active interaction models, where the Mean Square Stability guarantees safe for the user. Additionally, from results it is understood the transmissibility of impedance achieved is a consequence of the robust performance given by the force controller.

Study on a Stator Yoke Structure of a Magnetic Screw Motor

Linear actuators mounted on collaborative robots that work in the same space as humans are required to have high thrust density and back drivable characteristics. Therefore, a series elastic actuator with elasticity using a mechanical spring has been proposed, but the problem is that the structure is complicated and the size is increased. Therefore, we proposed a magnetic screw motor that integrates a magnetic screw and a motor, but there was a problem that the thrust was small. This paper describes the influence of a step skew structure in the stator yoke on the motor characteristics. A magnetic screw motor with a spiral stator is proposed. However, it is difficult to achieve the spiral stator in terms on the cost and manufacturing. In order to solve the problem, a step skew structure is proposed, and its torque characteristics are investigated using 3-D finite element analysis.

The SE-AssessWrist for robot-aided assessment of wrist stiffness and range of motion: Development and experimental validation.

IntroductionPhysical human-robot interaction offers a compelling platform for assessing recovery from neurological injury; however, robots currently used for assessment have typically been designed for the requirements of rehabilitation, not assessment. In this work, we present the design, control, and experimental validation of the SE-AssessWrist, which extends the capabilities of prior robotic devices to include complete wrist range of motion assessment in addition to stiffness evaluation.MethodsThe SE-AssessWrist uses a Bowden cable-based transmission in conjunction with series elastic actuation to increase device range of motion while not sacrificing torque output. Experimental validation of robot-aided wrist range of motion and stiffness assessment was carried out with five able-bodied individuals.ResultsThe SE-AssessWrist achieves the desired maximum wrist range of motion, while having sufficient position and zero force control performance for wrist biomechanical assessment. Measurements of two-degree-of-freedom wrist range of motion and stiffness envelopes revealed that the axis of greatest range of motion and least stiffness were oblique to the conventional anatomical axes, and approximately parallel to each other.ConclusionsSuch an assessment could be beneficial in the clinic, where standard clinical measures of recovery after neurological injury are subjective, labor intensive, and graded on an ordinal scale.

Design of gust alleviation active control law considering time-delay of servo actuator

Design of gust alleviation active control law considering time-delay of servo actuator