Actuator Research Articles

Lower limb exoskeleton for gait rehabilitation with adaptive nonsingular sliding mode control

Abstract This paper introduces a lower limb exoskeleton for gait rehabilitation, which has been designed to be adjustable to a wide range of patients by incorporating an extension mechanism and series elastic actuators (SEAs). This configuration adapts better to the user’s anatomy and the natural movements of the user’s joints. However, the inclusion of SEAs increases actuator mass and size, while also introducing nonlinearities and changes in the dynamic response of the exoskeletons. To address the challenges related to the human–exoskeleton dynamic interaction, a nonsingular terminal sliding mode control that integrates an adaptive parameter adjustment strategy is proposed, offering a practical solution for trajectory tracking with uncertain exoskeleton dynamics. Simulation results demonstrate the algorithm’s ability to estimate unknown parameters. Experimental tests analyze the performance of the controller against uncertainties and external disturbances.

A novel design method of a nonlinear elastic mechanism for series elastic actuators

Series elastic actuators with increasing nonlinear stiffness(NlSEA) have promising applications in human-machine interactions for the existence of a nonlinear elastic mechanism that supplies inherent safety and can implement high torque resolution under low load and quick dynamic response under high load. In this study, a novel design method for a nonlinear elastic mechanism that can be applied in NlSEA is proposed. By combining the S-shaped cantilever beam and specialized cam surface, this study proposed an analytical approach to design a nonlinear elastic mechanism satisfying a given stiffening torque-deformation relationship. The geometric parameters constraints were discussed to support parameter determination. Two kinds of cases of nonlinear elastic mechanisms with increasing and constant stiffness characteristics were conducted to verify the effectiveness and generalizability of the proposed design method by simulations. Moreover, a nonlinear elastic mechanism with an increasing stiffness characteristic was designed and a prototype was fabricated. A torque-deformation verification experiment, regulation response experiments, and sinusoidal tracking experiments were conducted to verify the accuracy, response performance, and tracking performance of the prototype. The designed nonlinear elastic mechanism has a large torque-to-mass ratio in a compact and lightweight structure.

Dynamic Contact Force Estimation via Integration of Soft Sensor Based on Fiber Bragg Grating and Series Elastic Actuator

Dynamic Contact Force Estimation via Integration of Soft Sensor Based on Fiber Bragg Grating and Series Elastic Actuator

Force feedback reduces test time and interaction forces in telemanipulated palpation using a robotic endoscope with series elastic actuated joints.

Abstract Force feedback is currently missing in most surgical robot systems, even though it would be essential for many surgical tasks, such as palpating tissue. In this study, we used a previously developed robotic endoscope based on series elastic actuation and tested the feasibility of using it for force feedback with a pilot study. The participants had a lower test time (−27%) and lower applied forces (−23%) with force feedback enabled. While further studies are needed for validation in realistic surgical scenarios, these results suggest improved safety and efficiency in telemanipulated surgeries.

Vibration suppression of series elastic actuator used for robotic grinding based on reconstructed hybrid optimized input shaper

Vibration suppression of the series elastic actuator (SEA) used for robotic compliant grinding is significant for guaranteeing force control accuracy and stability, which is important for material removal accuracy and surface quality improvement. This paper establishes a multi-feedforward control system to achieve vibration suppression for the SEA. The hybrid optimized input shaper (HOIS), instruction reconstructor, and nonlinear friction compensation model are seamlessly integrated into the multi-feedforward control method. Combination optimization principles are employed to formulate the HOIS, ensuring selectable performances on vibration suppression, response speed, and model stability when addressing the residual vibration caused by elastic elements. Base on the inherent delay characteristics of the shaper, a head/tail compression and intermediate shift instruction reconstruction (CSIR) method is introduced to address the tracking error induced by the time-delay of the shaper. Additionally, the friction compensator is established based on the Stribeck model and integrated into the feedforward control system to mitigate the vibration caused by the low-speed viscous friction. The effectiveness of the proposed method is verified by experimental tests. Results show that the HOIS has rich optional performance and can effectively suppress vibration. The tracking error caused by the shaper is effectively mitigated through the reconstruction of instructions. Grinding tests on curved parts are conducted on the robotic grinding system with SEA used as the force-control system. Compared with the benchmark method, the developed reconstructed hybrid optimized input shaper reduces the average force tracking error, the average absolute grinding depth error, and average roughness by 26.6 %, 22.5 %, and 21.5 % respectively. At the natural frequency of the grinding system, the amplitude of the frequency spectrum with the developed multi-feedforward method decreases by 48.13 %.

[formula omitted]adaptive resonance ratio control for series elastic actuator with guaranteed transient performance

Series elastic actuator (SEA) technology is promising for the development of compliant robotic joints. Despite advancements in the realization of precise tracking, challenges persist in controlling the vibration and transient performance. This study enhanced the resonance ratio control (RRC) algorithm by integrating it with the L1 adaptive control (L1AC) method to address overshoot, static error, and vibration in SEA position control. Initially, the resonance between the motor and link sides caused by the elastic transmission structure was analyzed, which can result in overshoots and vibrations that affect the transient performance of the SEA control. Subsequently, a control scheme based on L1AC was introduced to enhance the performance. The stability of the proposed algorithm was demonstrated through a comprehensive exploration of key control parameters. Furthermore, the algorithm was augmented with gravity compensation, effectively reducing the predicted and reference errors. Consequently, the transient performance was improved. The efficacy of this enhanced algorithm was validated through simulations and experimental platforms, and comparisons with the RRC and model reference adaptive control algorithms. In all the experiments, the overshoot did not exceed 1.1%, the maximum jitter amplitude on the link side was within 0.2° , and a larger time constant in the controller could effectively eliminate the overshoot and vibration with a small response time delay. Furthermore, the algorithm exhibited a protective response during link side collisions by moderating link velocity and limiting motor current, to safeguard the contact environment, humans, and the SEA itself, which take advantage of the L1AC’s low-pass filter (LPF) properties in disturbance handling.

Predictor-based Tracking Control of a Class of Series Elastic Actuators With Input Delay

Predictor-based Tracking Control of a Class of Series Elastic Actuators With Input Delay

A Multiaxial Bionic Ankle Based on Series Elastic Actuation With a Parallel Spring

A Multiaxial Bionic Ankle Based on Series Elastic Actuation With a Parallel Spring

Torque control of grasping force feedback using a series elastic actuator with an ultrasonic motor for a teleoperated surgical robot

In the context of teleoperated surgical robots, much research has been conducted into haptic feedback systems. Ultrasonic motors (USMs) are expected to be used as actuators for generating feedback torque because of their fast response and high torque–weight ratio. However, their nonlinear characteristics make control of the output torque difficult, leading to large spike-like torque errors during rotation reversal. One approach to counteracting this effect is to use series elastic actuators (SEAs). In this study, a force feedback system using an SEA with a USM was proposed. The spike-shaped errors in the output torque of the USM were reduced by up to 60 % by attaching an SEA to its output shaft. The dependence of the torque accuracy and the response time of the SEA on its spring constant were also examined. By introducing the SEA with a spring constant of 42.4 mN·m/°, it was indicated that the torque error of 14.8 mN·m and the response time of 9 ms could be simultaneously achieved. Finally, the force feedback system was applied to a teleoperated surgical robot, and the stiffness of the objects grasped by the robot was successfully fed back with the error of 0.65 % when grasping a sponge.

Whole Body Control of Mobile Manipulators With Series Elastic Actuators for Cart Pushing Tasks.

Human-centered environments provide affordance for the use of two-handed mobile manipulators. Yet robots designed to function in and physically interact with such environments are not yet capable of meeting human users' requirements. This work proposes a whole body control framework of a two-handed mobile manipulator driven by series elastic actuators (SEAs) for cart pushing tasks. A whole body dynamic model for an integrated mobile platform and on-board arms is revealed, which takes into account the interaction forces with the cart. Then, the explicit force/position control of the mobile manipulator is performed. It enables the robot to interact dynamically with the environment while providing motion, i.e., the manipulators provide both output force control and motion control for pushing a cart. To cope with the highly nonlinear system dynamics and parameter variation of a SEA-driven mobile manipulator, this work proposes an adaptive robust controller based on a novel integral barrier Lyapunov function for cart pushing tasks by considering model uncertainty. The proposed controller enables the mobile manipulator to complete cart pushing tasks by regulating the position and output force of the mobile base and arms. The experimental results show the effectiveness of this approach in cart pushing tasks.

Modeling of flexible shaft for robotics applications

Reducing moving mass and effective inertia is essential for achieving safe human–robot collaboration. This can be achieved either by employing remote actuation, which moves the mass of actuators away from the moving elements of the robot, or by elastic actuation, which decouples the inertia of the actuator from the inertia of the robot’s link. Flexible shafts, being torsionally compliant slender long shafts, offer a combination of remote and series elastic actuation over an obstacle. Modeling such transmission is complicated due to its three-dimensional deformation in torsion, bending, and helical buckling. This paper proposes an algorithm for estimating the polar moment of inertia and stiffness of the flexible shafts using their physical dimensions of length and diameter. Using an inertia-spring–damper model, the stiffness parameters are estimated experimentally for nine flexible shafts with variable diameters and lengths in straight and bent configurations. The proposed algorithm is validated with experimental stiffness values for variable diameters and lengths in straight configuration. For bent configuration, an empirical formulation is provided to incorporate the bending deformation effect till a bending angle of 45∘.

Selective load control of lumbar muscles in robot-assisted isometric lumbar stabilization exercise

Lumbar stabilization exercises are commonly employed in the rehabilitation of patients with low back pain. However, many patients discontinue these exercises, generally calisthenics using various postures or tools, due to the difficulty of providing an appropriate exercise load intensity. This challenge results in an inability to apply the desired strength to the target lumbar muscles and sometimes leads to an excessive load on unintended areas during calisthenics. Consequently, a method that enables patients to exercise continuously and progressively recover is required, specifically one that can target the lumbar muscles with a desired load. To address this issue, we propose a rehabilitation assistive device that quantitatively controls the lumbar spine load. In isometric lumbar stabilization exercises, our method involves precise compensation for gravity. The device, equipped with a series elastic actuator, is positioned beneath the patient in a lying posture. It applies an assistive force in the direction opposite to gravity, enabling precise control of the load on the lumbar region and reducing the vertical load on the spine. To validate the effectiveness of our proposed method, we conducted experiments with 20 healthy subjects across three exercises and analyzed the electromyography signal using nonparametric statistical methods. Our objective was to determine whether the load on the target lumbar muscles could be precisely and gradually controlled. The statistical results indicate that exercises performed using the proposed device produce statistically significant load changes in the target lumbar muscles.

Robust Elastic Structure Preserving Control for High Impedance Rendering of Series Elastic Actuator

Robust Elastic Structure Preserving Control for High Impedance Rendering of Series Elastic Actuator

A Lightweight Shoulder Exoskeleton With a Series Elastic Actuator for Assisting Overhead Work

A Lightweight Shoulder Exoskeleton With a Series Elastic Actuator for Assisting Overhead Work

Research on Compliance Control of Electro-Hydraulic Loading Experimental System

This article discusses the challenges in preventing workpiece damage due to impacts in electro-hydraulic loading systems, especially in unknown environments. We propose an innovative compliance control strategy, synergizing a series elastic actuator with impedance control to significantly mitigate impact forces between the mechanism and test workpieces. The controller consists of two loops: an internal loop and an outer loop. The internal loop integrates a position loop utilizing a radial basis function observer within a backstepping control framework, effectively countering the nonlinear dynamics of hydraulic actuators and ensuring precise trajectory tracking. The outer loop advances traditional impedance control by adaptively modifying the damping coefficient, resulting in a straightforward and easily implementable damping control law. For the unknown environment parameters, our system employs a parameter estimation law to estimate the unknown environmental stiffness and position parameters. The effectiveness of this strategy has been verified through comparative simulation with traditional impedance control, indicating that the proposed method can not only effectively reduce contact shock in unknown environments, improve response speed, and reduce overshoot, but also improve steady-state accuracy. We provided a feasible control scheme for similar systems to ensure precise and safe operation.

Robust dynamic surface control of series elastic actuators based on reduced-order extended state observer

The compliance of series elastic actuators (SEAs) is significant for ensuring safe human–machine interaction. However, in practical applications, the SEA is inevitably encountered with unknown disturbances, such as parameter uncertainties, unmodeled dynamics, and environmental interferences. In this paper, a robust dynamic surface control scheme is presented to address the high-precision trajectory tracking problem of the SEA. Firstly, a reduced-order extended state observer (RESO) with only link-side position measurements is designed to estimate the unknown lumped disturbance and unmeasurable system states. Based on the estimated values from the RESO, a robust dynamic surface controller is proposed for the trajectory tracking of the SEA. Additionally, the finite-time filter is introduced to overcome the “explosion of complexity” in the backstepping controller, and the filtering error in the dynamic surface controller is reduced compared with the conventional first- and second-order filters. The stability of the closed-loop system is guaranteed by the Lyapunov theory. Finally, the effectiveness and superiority of the proposed control algorithm are verified through simulations and experiments.

DESIGNING OF DYNAMIC SURFACE CONTROL BASED ON BACKSTEPPING TECHNIQUE FOR SERIES ELASTIC ACTUATOR ROBOT

Serial elastic actuators have gained significant attention in robotics research due to their ability to meet safety requirements in physical interactions between humans and robots. However, one problem of series elastic actuator is the oscillation of the robot due to the flexibility of the robotic joints leading to a decline in the accuracy of the robot’s position control. In this paper, a dynamic surface control algorithm based on the backstepping technique for the position control of serial elastic actuator robot is proposed to overcome the oscillation problem. In addition, the proposed control algorithm has been proved to be stable and robust. The simulation results clearly demonstrate the effectiveness of the proposed method.

Development of a compact rotary series elastic actuator with neural network-driven model predictive control implementation

Development of a compact rotary series elastic actuator with neural network-driven model predictive control implementation

Actuators for Improving Robotic Arm Safety While Maintaining Performance: A Comparison Study

Since robotic arms operating close to people are becoming increasingly common, there is a need to better understand how they can be made safe when unintended contact occurs, while still providing the required performance. Several actuators and methods for improving robot safety are studied and compared in this paper. A robotic arm moving its end effector horizontally and colliding with a person’s head is simulated. The use of a conventional electric actuator (CEA), series elastic actuator (SEA), pneumatic actuator (PA) and hybrid pneumatic electric actuator (HPEA) with model-based controllers are studied. The addition of a compliant covering to the arm and the use of collision detection and reaction strategies are also studied. The simulations include sensor noise and modeling error to improve their realism. A systematic method for tuning the controllers fairly is proposed. The motion control performance and safety of the robot are quantified using root mean square error (RMSE) between the desired and actual joint angle trajectories and maximum impact force (MIF), respectively. The results show that the RMSE values are similar when the CEA, SEA, and HPEA drive the robot’s first joint. Regarding safety, using the PA or HPEA with a compliant covering can reduce the MIF below the safety limit established by the International Organization for Standardization (ISO). To satisfy this ISO safety limit with the CEA and SEA, a collision detection and reaction strategy must be used in addition to the compliant covering. The influences of the compliant covering’s stiffness and the detection delay are also studied.

Design and control of a compliant robotic actuator with parallel spring-damping transmission

AbstractPhysically compliant actuator brings significant benefits to robots in terms of environmental adaptability, human–robot interaction, and energy efficiency as the introduction of the inherent compliance. However, this inherent compliance also limits the force and position control performance of the actuator system due to the induced oscillations and decreased mechanical bandwidth. To solve this problem, we first investigate the dynamic effects of implementing variable physical damping into a compliant actuator. Following this, we propose a structural scheme that integrates a variable damping element in parallel to a conventional series elastic actuator. A damping regulation algorithm is then developed for the parallel spring-damping actuator (PSDA) to tune the dynamic performance of the system while remaining sufficient compliance. Experimental results show that the PSDA offers better stability and dynamic capability in the force and position control by generating appropriate damping levels.