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Journal Description

Actuators

Actuators is an international, peer-reviewed, open access journal on the science and technology of actuators and control systems published monthly online by MDPI.

Open Access— free for readers, with article processing charges (APC) paid by authors or their institutions.
High Visibility: indexed within SCIE (Web of Science), Scopus, Inspec, and other databases.
Journal Rank: JCR - Q2 (Engineering, Mechanical) / CiteScore - Q2 (Control and Optimization)
Rapid Publication: manuscripts are peer-reviewed and a first decision is provided to authors approximately 17.7 days after submission; acceptance to publication is undertaken in 1.9 days (median values for papers published in this journal in the second half of 2024).
Recognition of Reviewers: reviewers who provide timely, thorough peer-review reports receive vouchers entitling them to a discount on the APC of their next publication in any MDPI journal, in appreciation of the work done.

Impact Factor: 2.2 (2023); 5-Year Impact Factor: 2.4 (2023)

Imprint Information Journal Flyer Open Access ISSN: 2076-0825

Latest Articles

36 pages, 13393 KiB

Open AccessArticle

An Improved Design of a Continuously Variable Transmission Based on Circumferentially Arranged Disks for Enhanced Efficiency in the Low Torque Region

by Muhammad Bilal, Qidan Zhu, Shafiq R. Qureshi, Ghulam Farid, Ahsan Elahi, Muhammad Kashif Nadeem and Sartaj Khan

Actuators 2025, 14(5), 253; https://doi.org/10.3390/act14050253 - 19 May 2025

A continuously variable transmission can improve the energy efficiency of actuators with rotary output by providing an optimum transmission ratio. A continuously variable transmission based on circumferentially arranged disks (CAD CVT) is a new type of CVT that is highly beneficial for applications [...] Read more.

A continuously variable transmission can improve the energy efficiency of actuators with rotary output by providing an optimum transmission ratio. A continuously variable transmission based on circumferentially arranged disks (CAD CVT) is a new type of CVT that is highly beneficial for applications requiring large torques, like heavy road transport. However, its major drawback is that its efficiency drops in the low torque region. To overcome this problem, the current paper proposes an improved mechanical design in which the force on traction disks is changed according to the instantaneous torque requirement, thus resulting in improved efficiency in low torque regions. Furthermore, a hydraulic-actuation-based control system has been designed to ensure the optimum control of the improved mechanical design. The improved mechanical design of the CAD CVT is named CAD CVT-II, which is highly beneficial for variable torque applications such as road transport and wind turbines. Full article

(This article belongs to the Section Actuators for Surface Vehicles)

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25 pages, 13985 KiB

Open AccessArticle

A Low-Cost Prototype of a Soft–Rigid Hybrid Pneumatic Anthropomorphic Gripper for Testing Tactile Sensor Arrays

by Rafał Andrejczuk, Moritz Scharff, Junhao Ni, Andreas Richter and Ernst-Friedrich Markus Vorrath

Actuators 2025, 14(5), 252; https://doi.org/10.3390/act14050252 - 17 May 2025

Soft anthropomorphic robotic grippers are attractive because of their inherent compliance, allowing them to adapt to the shape of grasped objects and the overload protection needed for safe human–robot interaction or gripping delicate objects with sophisticated control. The anthropomorphic design allows the gripper [...] Read more.

Soft anthropomorphic robotic grippers are attractive because of their inherent compliance, allowing them to adapt to the shape of grasped objects and the overload protection needed for safe human–robot interaction or gripping delicate objects with sophisticated control. The anthropomorphic design allows the gripper to benefit from the biological evolution of the human hand to create a multi-functional robotic end effector. Entirely soft grippers could be more efficient because they yield under high loads. A trending solution is a hybrid gripper combining soft and rigid elements. This work describes a prototype of an anthropomorphic, underactuated five-finger gripper with a direct pneumatic drive from soft bending actuators and an integrated resistive tactile sensor array. It is a hybrid construction with soft robotic structures and rigid skeletal elements, which reinforce the body, focus the direction of the actuator’s movement, and make the finger joints follow the forward kinematics. The hand is equipped with a resistive tactile dielectric elastomer sensor array that directly triggers the hand’s actuation in the sense of reflexes. The hand can execute precision grips with two and three fingers, as well as lateral grip and strong grip types. The softness of the actuation allows the finger to adapt to the shape of the objects. Full article

(This article belongs to the Special Issue Advancement in the Design and Control of Robotic Grippers—Second Edition)

15 pages, 3629 KiB

Open AccessArticle

Dual-Layer Flexible Capacitance Sensor with Wide Range and High Sensitivity

by Benyuan Fu, Zipei Wang, Kun Chen, Zebing Mao, Hao Wang, Benxiang Ju and Yanhong Peng

Actuators 2025, 14(5), 251; https://doi.org/10.3390/act14050251 - 16 May 2025

Flexible pressure sensors have attracted great attention due to their extensive applications in human–computer interaction and health monitoring. So far, the development of flexible pressure sensors that balance high sensitivity and a wide measurement range remains a challenge. Herein, a double-layer dielectric structure [...] Read more.

Flexible pressure sensors have attracted great attention due to their extensive applications in human–computer interaction and health monitoring. So far, the development of flexible pressure sensors that balance high sensitivity and a wide measurement range remains a challenge. Herein, a double-layer dielectric structure with a surface convex structure is reported for the preparation of flexible capacitive pressure sensors. The double-layer dielectric structure, which is composed of a silicone rubber-based conductive elastomer with a surface micro-convex structure and a PVA-H-based conductive elastomer, balances the advantages and disadvantages of the two conductive elastomer dielectrics. It can form a complete micro-capacitive network under relatively large pressures, enabling the sensor to have high sensitivity at different stages (1.7 kPa⁻¹, 0–104 kPa; 19.14 kPa⁻¹, 104–140 kPa), thus achieving a dual enhancement of sensitivity and sensing range. Additionally, the sensor has been successfully applied to scenarios such as monitoring of human breathing, speaking, and movement, as well as mouse clicks, demonstrating its great potential in the fields of health monitoring and human–computer interaction applications. Full article

(This article belongs to the Special Issue Bioinspired Structures for Soft Robots)

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21 pages, 1927 KiB

Open AccessArticle

A Study on a Variable-Gain PID Control for a Pneumatic Servo System Using an Optimized PSO-Type Neural Network

by Shenglin Mu, Satoru Shibata, Daisuke Baba and Rikuto Oshita

Actuators 2025, 14(5), 250; https://doi.org/10.3390/act14050250 - 16 May 2025

This study investigates the application of proportional–integral–derivative (PID) control enhanced with an optimized particle swarm optimization (OPSO)-type neural network (NN) to improve the control performance of a pneumatic servo system. Traditional PID methods struggle with inherent nonlinearities in pneumatic servo systems. To address [...] Read more.

This study investigates the application of proportional–integral–derivative (PID) control enhanced with an optimized particle swarm optimization (OPSO)-type neural network (NN) to improve the control performance of a pneumatic servo system. Traditional PID methods struggle with inherent nonlinearities in pneumatic servo systems. To address this limitation, we integrate two OPSO-type NNs within the PID framework, thereby developing a robust control strategy that compensates for these nonlinear characteristics. The OPSO-type NNs are particularly efficient in solving complex optimization problems without requiring differential information, demonstrating superior simplicity and efficacy compared to traditional methods, such as genetic algorithms. In our approach, one of the OPSO-type NNs is utilized to tune the PID controller gains, while the other adjusts the control output. The experimental results show that the proposed method enhances the position control accuracy of the pneumatic servo system. Furthermore, this approach holds promise for improving the responsiveness, stability, and disturbance suppression capabilities of pneumatic systems, paving the way for advanced control applications in this field. Full article

(This article belongs to the Special Issue Intelligent Control for Pneumatic Servo System)

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25 pages, 7813 KiB

Open AccessArticle

Deep Learning-Based Speech Recognition and LabVIEW Integration for Intelligent Mobile Robot Control

by Kai-Chao Yao, Wei-Tzer Huang, Hsi-Huang Hsieh, Teng-Yu Chen, Wei-Sho Ho, Jiunn-Shiou Fang and Wei-Lun Huang

Actuators 2025, 14(5), 249; https://doi.org/10.3390/act14050249 - 15 May 2025

This study implemented an innovative system that trains a speech recognition model based on the DeepSpeech2 architecture using Python for voice control of a robot on the LabVIEW platform. First, a speech recognition model based on the DeepSpeech2 architecture was trained using a [...] Read more.

This study implemented an innovative system that trains a speech recognition model based on the DeepSpeech2 architecture using Python for voice control of a robot on the LabVIEW platform. First, a speech recognition model based on the DeepSpeech2 architecture was trained using a large speech dataset, enabling it to accurately transcribe voice commands. Then, this model was integrated with the LabVIEW graphical user interface and the myRIO controller. By leveraging LabVIEW’s graphical programming environment, the system processed voice commands, translated them into control signals, and directed the robot’s movements accordingly. Experimental results demonstrate that the system not only accurately recognizes various voice commands, but also controls the robot’s behavior in real time, showing high practicality and reliability. This study addresses the limitations inherent in conventional voice control methods, demonstrates the potential of integrating deep learning technology with industrial control platforms, and presents a novel approach for robotic voice control. Full article

(This article belongs to the Section Actuators for Robotics)

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20 pages, 5416 KiB

Open AccessArticle

Research on a Novel Shape-Memory Alloy Artificial Muscle with Active and Passive Heat Dissipation

by Qin Zhang, Liang Xu, Hao Chen, Zhou Li, Liwu Huang and Sicheng Yi

Actuators 2025, 14(5), 248; https://doi.org/10.3390/act14050248 - 15 May 2025

Due to their high energy density and favorable load-to-weight ratio, shape-memory alloy (SMA) materials are ideal actuation sources for soft robots. However, the relatively long cooling time of SMA wires in soft bodies limits their response speed. In this study, we designed and [...] Read more.

Due to their high energy density and favorable load-to-weight ratio, shape-memory alloy (SMA) materials are ideal actuation sources for soft robots. However, the relatively long cooling time of SMA wires in soft bodies limits their response speed. In this study, we designed and fabricated a novel SMA artificial muscle. When active heat absorption was enabled through thermoelectric modules and the evaporation/dehydration effects of hydrogels, the cooling rate of the SMA wires increased significantly. Simulation and experimental results demonstrate that with the proposed heat-dissipation scheme, the cooling speed of the SMA wires improved notably, with a temperature drop of 9.6 °C within 4 s. Additionally, the designed agar/polyacrylamide hydrogel, which has a porous skeleton structure, achieved a water-absorption expansion rate that was 600% of the previous value. When a PVC elastic substrate was used, the bending angle of the SMA artificial muscle reached 71°, with minimal bending attenuation after 45 consecutive cyclic tests. A soft gripper composed of the novel SMA artificial muscles was capable of manipulating objects of various shapes. Overall, the combination of active and passive heat-dissipation strategies enabled the SMA artificial muscle to achieve excellent durability, rapid heat dissipation, and strong versatility, demonstrating its significant potential for various applications. Full article

(This article belongs to the Section Actuators for Robotics)

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13 pages, 12964 KiB

Open AccessArticle

A Soft Variable Stiffness Actuator with a Chain Mail Structure as a Particle Jamming Interface

by John E. Bermeo, Eduardo Castillo-Castañeda and Med Amine Laribi

Actuators 2025, 14(5), 247; https://doi.org/10.3390/act14050247 - 14 May 2025

Variable stiffness actuators (VSAs) have attracted considerable attention in wearable robotics and soft exoskeletons due to their ability to adapt to various load conditions. This study presents a modular design for VSAs that incorporates a chain mail structure with various link topologies, allowing [...] Read more.

Variable stiffness actuators (VSAs) have attracted considerable attention in wearable robotics and soft exoskeletons due to their ability to adapt to various load conditions. This study presents a modular design for VSAs that incorporates a chain mail structure with various link topologies, allowing for a reconfiguration of stiffness. The proposed VSA consists of three main parts: the vacuum chamber, the VSA actuator, and the chain mail structure. The VSA fabrication process was carried out in five stages: (1) mold fabrication by 3D FDM printing, incorporating a film of oil to facilitate easy demolding; (2) mold preparation using silicone, with a precise ratio of 1:1 weight-based mixture to optimize material utilization; (3) silicone pouring into molds while applying vibration to eliminate air bubbles; (4) curing for four hours to achieve optimal mechanical properties; and (5) careful demolding to prevent damage. Experimental tests were conducted to characterize the stiffness of actuators with different chain mail fabric configurations, using an experimental setup designed to securely fix the actuator and accurately measure the pneumatic pressure and the angle of deformation after applying weights at its end. The European 6-in-1 and rounded square configurations were shown to be the most effective, increasing stiffness up to 382% compared to the chain mail-free configuration, highlighting the positive impact of these structural designs. Full article

(This article belongs to the Special Issue Advanced Technologies in Soft Actuators)

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29 pages, 10730 KiB

Open AccessArticle

Connected and Automated Vehicle Trajectory Control in Stochastic Heterogeneous Traffic Flow with Human-Driven Vehicles Under Communication Delay and Disturbances

by Meiqi Liu, Yang Chen and Ruochen Hao

Actuators 2025, 14(5), 246; https://doi.org/10.3390/act14050246 - 13 May 2025

In this paper, we study the stability of the stochastically heterogeneous traffic flow involving connected and automated vehicles (CAVs) and human-driven vehicles (HDVs). Taking the stochasticity of vehicle arrivals and behaviors into account, a general robust

H_{\infty}

platoon controller is proposed to [...] Read more.

In this paper, we study the stability of the stochastically heterogeneous traffic flow involving connected and automated vehicles (CAVs) and human-driven vehicles (HDVs). Taking the stochasticity of vehicle arrivals and behaviors into account, a general robust

H_{\infty}

platoon controller is proposed to address the communication delay and unexpected disturbances such as prediction or perception errors on HDV motions. To simplify the problem complexity from a stochastically heterogeneous traffic flow to multiple long vehicle control problems, three types of sub-platoons are identified according to the CAV arrivals, and each sub-platoon can be treated as a long vehicle. The car-following behaviors of HDVs and CAVs are simulated using the optimal velocity model (OVM) and the cooperative adaptive cruise control (CACC) system, respectively. Later, the robust

H_{\infty}

platoon controller is designed for a pair of a CAV long vehicle and an HDV long vehicle. The time-lagged system and the closed-loop system are formulated and the

H_{\infty}

state feedback controller is designed. The robust stability and string stability of the heterogeneous platoon system are analyzed using the

H_{\infty}

norm of the closed-loop transfer function and the time-lagged bounded real lemma, respectively. Simulation experiments are conducted considering various settings of platoon sizes, communication delays, disturbances, and CAV penetration rates. The results show that the proposed

H_{\infty}

controller is robust and effective in stabilizing disturbances in the stochastically heterogeneous traffic flow and is scalable to arbitrary sub-platoons in various CAV penetration rates in the heterogeneous traffic flow of road vehicles. The advantages of the proposed method in stabilizing heterogeneous traffic flow are verified in comparison with a typical car-following model and the linear quadratic regulator. Full article

(This article belongs to the Special Issue Motion Planning, Trajectory Prediction, and Control for Robotics)

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19 pages, 1520 KiB

Open AccessArticle

Adaptive Fixed-Time Tracking Control of Cart–Pendulum Robotic Systems with Bias Actuator Dynamics

by Shuo Chen, Xuansen Zhao, Xiaozheng Jin and Hai Wang

Actuators 2025, 14(5), 245; https://doi.org/10.3390/act14050245 - 13 May 2025

This research addresses the challenge of precise trajectory tracking for cart–pendulum robotic systems affected by unknown nonlinear actuator dynamics. We introduce a novel control framework that combines neural network modeling with adaptive parameter estimation to handle these complex dynamics. By characterizing state-dependent actuator [...] Read more.

This research addresses the challenge of precise trajectory tracking for cart–pendulum robotic systems affected by unknown nonlinear actuator dynamics. We introduce a novel control framework that combines neural network modeling with adaptive parameter estimation to handle these complex dynamics. By characterizing state-dependent actuator behavior through custom-designed linear filters and adaptive laws, our approach identifies system parameters with high precision. We then develop an innovative fixed-time adaptive sliding mode controller that guarantees convergence within a predetermined timeframe regardless of initial conditions. Lyapunov stability analysis confirms that tracking errors converge to a small neighborhood around zero within the specified time bounds, with the size of the neighborhood determined by the design parameters. Simulation studies on a watermelon transportation robot validate our approach’s practical effectiveness, demonstrating improved tracking accuracy and robustness against actuator disturbances compared with conventional methods. Full article

(This article belongs to the Section Actuators for Robotics)

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10 pages, 3221 KiB

Open AccessArticle

Research on a Miniature Underwater Vehicle Based on a Multi-Unit Underwater Coupled Jet Drive

by Dong Zhang, Xingming Ma, Xue Zhang, Peng Gao and Kai Li

Actuators 2025, 14(5), 244; https://doi.org/10.3390/act14050244 - 13 May 2025

The underwater unstructured environment poses new challenges for the miniaturization and flexibility of underwater vehicles. This paper proposes a method of using micrometer-scale vibrations of piezoelectric vibrators to drive macroscopic jets. Then, we use two coupled piezoelectric jet driving units to construct a [...] Read more.

The underwater unstructured environment poses new challenges for the miniaturization and flexibility of underwater vehicles. This paper proposes a method of using micrometer-scale vibrations of piezoelectric vibrators to drive macroscopic jets. Then, we use two coupled piezoelectric jet driving units to construct a miniature underwater vehicle. Numerical simulation is used to investigate the flow field characteristics of coupled jets. Finally, the impact of the angle between the two piezoelectric jet drive units on the propulsion force is analyzed. The miniature underwater vehicle measures 77.8 mm in length and 87 mm in width. While achieving miniaturization, it maintains high flexibility, maneuverability, and controllability. By adjusting the input signals to the two piezoelectric jet drive units, the miniature underwater vehicle can move in a straight line, turn, and rotate. Its maximum linear velocity reaches 54.23 mm/s. Its outstanding motion ability and environmental adaptability allow it to perform various tasks in unknown and complex environments. It also has broad application prospects. Full article

(This article belongs to the Special Issue Piezoelectric Ultrasonic Actuators and Motors)

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26 pages, 2703 KiB

Open AccessArticle

Design of Actuators for a Humanoid Robot with Anthropomorphic Characteristics and Running Capability

by Chathura Semasinghe, Drake Taylor and Siavash Rezazadeh

Actuators 2025, 14(5), 243; https://doi.org/10.3390/act14050243 - 13 May 2025

In this paper, we present the details of the actuator design for our humanoid robot, Mithra. Mithra has been designed to match an average adult human in terms of kinematic and kinetic characteristics. This poses various challenges in actuator design that we have [...] Read more.

In this paper, we present the details of the actuator design for our humanoid robot, Mithra. Mithra has been designed to match an average adult human in terms of kinematic and kinetic characteristics. This poses various challenges in actuator design that we have addressed in this work. First, we discuss how the high-level design can help in achieving anthropomorphic traits. Next, the detailed design is verified and finalized using stress and fatigue analyses. Further, we conduct experiments to validate the actuator’s bandwidth and backdrivability, and discuss the outcomes in comparison with human characteristics. The results show that Mithra’s actuators have sufficient structural strength to withstand high running forces, and at the same time, provide human-like traits and capabilities to accommodate human-inspired control paradigms. Full article

(This article belongs to the Special Issue Design and Application of Actuators with Multi-DOF Movement-2nd Edition)

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18 pages, 11369 KiB

Open AccessArticle

Multi-Metric Fusion Hypergraph Neural Network for Rotating Machinery Fault Diagnosis

by Jiaxing Zhu, Junlan Hu and Buyun Sheng

Actuators 2025, 14(5), 242; https://doi.org/10.3390/act14050242 - 13 May 2025

Effective fault diagnosis in rotating machinery means extracting fault features from complex samples. However, traditional data-driven methods often overly rely on labeled samples and struggle with extracting high-order complex features. To address these issues, a novel Multi-Metric Fusion Hypergraph Neural Network (MMF-HGNN) is [...] Read more.

Effective fault diagnosis in rotating machinery means extracting fault features from complex samples. However, traditional data-driven methods often overly rely on labeled samples and struggle with extracting high-order complex features. To address these issues, a novel Multi-Metric Fusion Hypergraph Neural Network (MMF-HGNN) is proposed for fault diagnosis in rotating machinery. The approach involves constructing hypergraphs for sample vertices using three metrics: instance distance, distribution distance, and spatiotemporal distance. An innovative hypergraph fusion strategy is then applied to integrate these normalized hypergraphs, and a dual-layer hypergraph neural network is utilized for fault diagnosis. Experimental results on three different fault datasets demonstrate that the MMF-HGNN method excels in feature extraction, reduces reliance on labeled samples, achieving a classification accuracy of 0.9965 ± 0.0025 even with only 5% of labeled samples, and shows strong robustness to noise across varying signal-to-noise ratios. Full article

(This article belongs to the Section Actuators for Manufacturing Systems)

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21 pages, 5367 KiB

Open AccessArticle

Analysis and Optimization Design of a Brushless Power Feedback PM Adjustable Speed Drive with Bilayer Wound Rotor

by Xinlei Zheng, Heyun Lin, Yibo Li, Jian Wang and Quanwei Wen

Actuators 2025, 14(5), 241; https://doi.org/10.3390/act14050241 - 12 May 2025

A novel brushless power feedback permanent magnet adjustable speed drive (BLPF-PMASD) is developed for the energy-saving of a large power electrical machine drive system in this paper. It can transfer the slip power between the input and output shafts to a stator and [...] Read more.

A novel brushless power feedback permanent magnet adjustable speed drive (BLPF-PMASD) is developed for the energy-saving of a large power electrical machine drive system in this paper. It can transfer the slip power between the input and output shafts to a stator and then transmit it back to the power grid, achieving higher drive efficiency and stability. First, the topology feature, operation principle, and power feedback mechanism of the proposed drive are clearly illustrated. Second, a multi-objective optimization design method suitable for all working conditions is proposed to provide an effective design means for this type of adjustable speed drive with power feedback. Finally, the electromagnetic performance of the optimized drive is analyzed by using the finite element method (FEM) to demonstrate the effectiveness and superiority of the proposed drive. Full article

(This article belongs to the Section High Torque/Power Density Actuators)

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20 pages, 12723 KiB

Open AccessArticle

Line-of-Sight Stabilization and High-Precision Target Tracking Technology of the Risley Prism System on Motion Platforms

by Huayang Xia, Hongfeng Xia, Jinying Li, Yunxia Xia, Yihan Luo, Liangzhu Yuan, Haotong Ma, Piao Wen and Wenna Yuan

Actuators 2025, 14(5), 240; https://doi.org/10.3390/act14050240 - 9 May 2025

The Risley prism system offers advantages such as compact structure and excellent dynamic performance, making it suitable for installation on static and motion platforms for target acquisition, aiming, and tracking. This paper presents a strapdown line-of-sight (LOS) stabilization method for the Risley prism [...] Read more.

The Risley prism system offers advantages such as compact structure and excellent dynamic performance, making it suitable for installation on static and motion platforms for target acquisition, aiming, and tracking. This paper presents a strapdown line-of-sight (LOS) stabilization method for the Risley prism system on motion platforms. The method establishes the coordinate transformation between the Risley prism and the motion platform. Real-time platform attitude angles from an inertial measurement unit (IMU) are used to compute the direction cosine matrix, which, combined with the coordinate transformation, determines the target’s actual guided position in the Risley prism’s coordinate. The Risley prism’s rotational angles are then calculated based on the target’s actual guided position to ensure LOS stability and capture the target. After LOS stabilization, an image-based closed-loop tracking cascade control system that integrates a Risley prism and a fast steering mirror with a single image detector (IBCLTCR-F), is used to enable fast and high-precision target tracking. Experimental results demonstrate that the proposed method achieves disturbance rejection of −32.8 dB, −28.8 dB, and −17.3 dB for platform disturbances at 0.05 Hz, 0.2 Hz, and 0.5 Hz, respectively. Furthermore, compared to the Risley prism system, the IBCLTCR-F system improves the dynamic response capability of target tracking in the nonlinear region by a factor of 10 and reduces the tracking error by 70%. Full article

(This article belongs to the Section Precision Actuators)

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21 pages, 14800 KiB

Open AccessArticle

Robust Continuous Sliding-Mode-Based Assistive Torque Control for Series Elastic Actuator-Driven Hip Exoskeleton

by Rui Wang, Xiaoou Lin, Changwei Yin, Zhongtao Liu, Yang Zhang, Wenping Liu and Fuxin Du

Actuators 2025, 14(5), 239; https://doi.org/10.3390/act14050239 - 9 May 2025

In this paper, a real-time assistive torque controller based on sliding-mode control is proposed for a Series Elastic Actuator (SEA)-driven lower limb assistive exoskeleton. To address the problem of the lack of buffering properties and the uneven torque output in traditional exoskeletons, a [...] Read more.

In this paper, a real-time assistive torque controller based on sliding-mode control is proposed for a Series Elastic Actuator (SEA)-driven lower limb assistive exoskeleton. To address the problem of the lack of buffering properties and the uneven torque output in traditional exoskeletons, a novel SEA is designed for the hip joint lower-limb exoskeleton. This structure features excellent cushioning properties and smooth torque output. On this basis, to enhance the torque tracking performance of the hip joint exoskeleton, in this study, a robust composite control strategy is proposed, which can maintain accuracy in the presence of unknown external disturbances and model parameter inaccuracies. The strategy consists of an adaptive phase oscillator for outputting the phase of the gait, a single-peak curve to provide a reference assistive torque, and a low-level controller to track the torque. The low-level controller employs Continuous Sliding-Mode Control (CSMC) to obtain a continuous control law and utilizes an Extended State Observer (ESO) to estimate the lumped disturbance. It ensures that the tracking error is asymptotically convergent with minimized chatter. The closed-loop stability of the system is theoretically proven by the Lyapunov method. The validity of the proposed algorithm is validated on a designed exoskeleton. Full article

(This article belongs to the Section Actuators for Robotics)

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25 pages, 6671 KiB

Open AccessArticle

An Adaptive BiGRU-ASSA-iTransformer Method for Remaining Useful Life Prediction of Bearing in Aerospace Manufacturing

by Youlong Lyu, Qingpeng Qiu, Ying Chu and Jie Zhang

Actuators 2025, 14(5), 238; https://doi.org/10.3390/act14050238 - 9 May 2025

In aerospace manufacturing, the reliability of machining equipment, particularly spindle bearings, is critical to maintaining productivity, as bearing health significantly constrains operational efficiency. Accurate prediction of the remaining useful life (RUL) of bearings can preempt failures, reduce downtime, and boost productivity. While conventional [...] Read more.

In aerospace manufacturing, the reliability of machining equipment, particularly spindle bearings, is critical to maintaining productivity, as bearing health significantly constrains operational efficiency. Accurate prediction of the remaining useful life (RUL) of bearings can preempt failures, reduce downtime, and boost productivity. While conventional BiGRU-based models for bearing RUL prediction have shown promise, they often overlook handcrafted extracted time-series features that could enhance accuracy. This study introduces a novel model, BiGRU-ASSA-iTransformer, that integrates deep learning and handcrafted feature extraction to improve RUL prediction. The approach employs two parallel processes with a fusion step: First, a bi-directional gated recurrent unit (BiGRU) captures dynamic degradation features from raw vibration signals, with an adaptive sparse self-attention (ASSA) mechanism emphasizing short-term degradation cues. Second, 13 time-domain, frequency-domain, and statistical features, derived from traditional expertise, are processed using iTransformer to encode temporal correlations. These outputs are then fused via an attention mechanism. Experiments on the PHM 2012 and XJTU-SY datasets demonstrate that this model achieves the lowest prediction error and highest accuracy compared to existing methods, highlighting the value of combining handcrafted and deep learning approaches for robust RUL prediction in aerospace applications. Full article

(This article belongs to the Section Actuators for Manufacturing Systems)

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22 pages, 2908 KiB

Open AccessArticle

Composite Adaptive Control of Robot Manipulators with Friction as Additive Disturbance

by Daniel Gamez-Herrera, Juan Sifuentes-Mijares, Victor Santibañez and Isaac Gandarilla

Actuators 2025, 14(5), 237; https://doi.org/10.3390/act14050237 - 8 May 2025

In this paper, an adaptive control scheme composed of an estimated feed-forward compensation and a PD control law with three mutually independent estimators is proposed for the tracking of desired trajectories in joint space for a robotic arm. One of the estimators is [...] Read more.

In this paper, an adaptive control scheme composed of an estimated feed-forward compensation and a PD control law with three mutually independent estimators is proposed for the tracking of desired trajectories in joint space for a robotic arm. One of the estimators is used to identify inertial and geometrical parameters, while the others determine the two principal components of the friction phenomenon: the part whose magnitude is position-dependent but velocity-independent and the part whose magnitude is proportional to velocity. Next, the persistently exciting condition is satisfied for each regression matrix of the estimators in a way that is easier to prove than the classical structure. Then, uniform global asymptotic stability can be concluded for the tracking error, regardless of parametric convergence, by applying the direct Lyapunov theorem. This scheme has been applied experimentally for a robotic arm to verify the theoretical results. The experimental results yielded a better performance in both estimating the parameters and tracking, with a much simpler overall analysis than the alternatives consulted. Full article

(This article belongs to the Special Issue Nonlinear Control of Mechanical and Robotic Systems)

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20 pages, 7137 KiB

Open AccessArticle

Design and Analysis of a Serial Position-Controlled Variable Stiffness Rotating Mechanism Based on Multi-Stage Torsional Compliant Mechanisms

by Kai Wen and Guanglei Wu

Actuators 2025, 14(5), 236; https://doi.org/10.3390/act14050236 - 8 May 2025

This work presents the design and experimental validation of a position-controlled rotating mechanism featuring multi-stage variable stiffness. Before designing the overall mechanism, three different compliant mechanisms, based on flexible beams, are parametrically optimized using a SolidWorks–Ansys co-simulation technique. The flexible beams are composed [...] Read more.

This work presents the design and experimental validation of a position-controlled rotating mechanism featuring multi-stage variable stiffness. Before designing the overall mechanism, three different compliant mechanisms, based on flexible beams, are parametrically optimized using a SolidWorks–Ansys co-simulation technique. The flexible beams are composed of multiple straight segments, Bezier curves, and multiple arc segments. The corresponding torque–deflection curves of the compliant mechanisms are collected and fitted into analytical expressions, from which the stiffness equation varying with the angular position is derived for stiffness regulation. A combination of three-stage compliant mechanisms connected in serial is realized to prototype the physical mechanism, which can have three different stiffness ranges of the output shaft. The maximum stiffness is about nine times higher than the lowest one, leading to a broader bandwidth of varying stiffness, which can make the mechanism more adaptive to the external payloads for safety consideration. Experimental measurements are carried out, and the comparison shows a good agreement between the experimental and simulation results, which experimentally validated the design concept. The compact and simple structure, as well as the multi-stage variable stiffness ranges, implies high adaptability of the designed mechanism. Full article

(This article belongs to the Section Actuators for Robotics)

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18 pages, 4421 KiB

Open AccessArticle

Model-Based Control Allocation During State Transitions of a Variable Recruitment Fluidic Artificial Muscle Bundle

by Jeong Yong Kim and Matthew Bryant

Actuators 2025, 14(5), 235; https://doi.org/10.3390/act14050235 - 8 May 2025

A model-based control scheme for state transitions of a variable recruitment fluidic artificial muscle (FAM) bundle is developed and experimentally validated. FAMs can be bundled together in parallel to exhibit variable recruitment functionality, which is an activation strategy inspired by how motor units [...] Read more.

A model-based control scheme for state transitions of a variable recruitment fluidic artificial muscle (FAM) bundle is developed and experimentally validated. FAMs can be bundled together in parallel to exhibit variable recruitment functionality, which is an activation strategy inspired by how motor units (MUs) in skeletal muscle are recruited. By adapting variable recruitment, an FAM bundle is able to operate efficiently over its entire force-contraction space while increasing control authority and bandwidth at low recruitment states. A variable recruitment bundle poses a hybrid control problem as it operates by controlling pressure as a continuous variable while simultaneously shifting between discrete recruitment states. During such state transitions, the bundle may experience a lag in strain if the shift timing is not properly anticipated. In this study, a model that captures the interaction effects between FAMs and a hydraulic system model is used to inform the controller of when a state transition should be made. The proposed control scheme is compared to a baseline control scheme that uses a percentage of the source pressure as the threshold for when a shift is made. The controller performance is evaluated by tracking a sinusoidal strain trajectory, and the average and maximum strain errors are compared between the baseline and proposed controller. The applied FAM pressures are presented to show that the model-based compensation is able to determine when a transition needs to be made. As a result, the tracking performance of the proposed control scheme is shown to significantly decrease the integrated absolute and maximum errors. Full article

(This article belongs to the Special Issue Analysis and Design of Linear/Nonlinear Control System)

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Open AccessArticle

Inner–Outer Loop Intelligent Morphology Optimization and Pursuit–Evasion Control for Space Modular Robot

by Wenwei Luo, Ling Meng, Fei Feng, Pengyu Guo and Bo Li

Actuators 2025, 14(5), 234; https://doi.org/10.3390/act14050234 - 8 May 2025

This paper proposes an inner–outer loop computational framework to address the morphology optimization and pursuit–evasion control problem for space modular robots. First, a morphological design space considering the functional characteristics of different modules is designed. Then, an elite genetic algorithm is applied to [...] Read more.

This paper proposes an inner–outer loop computational framework to address the morphology optimization and pursuit–evasion control problem for space modular robots. First, a morphological design space considering the functional characteristics of different modules is designed. Then, an elite genetic algorithm is applied to evolve the morphology within this space, and a proximal policy optimization algorithm is applied to control the space modular robot with evolved morphology. Considering symmetry, centrality, module cost, and average cumulative reward, a comprehensive morphological assessment is proposed to evaluate the morphology. And the assessment result serves as the fitness of evolution. In addition, by implementing the algorithm on the JAX framework for parallel computing, the computational efficiency was significantly enhanced, allowing the entire optimization process within 17.3 h. Comparative simulation results verify the effectiveness and superiority of the proposed computational framework. Full article

(This article belongs to the Special Issue Actuators in Robotic Control—3rd Edition)

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