Search Results (75)

Oxidative degradation stands as the principal cause of gear oil failure and transmission system malfunctions in wind turbines. Electrochemical impedance spectroscopy (EIS) offers a novel technical avenue for the condition monitoring of gear oil. This research centers on the evolution mechanism of electrochemical properties during the oxidative degradation process, utilizing high-viscosity gear oil commonly employed in wind turbines as the research subject. Through a combination of accelerated oxidation tests, broadband EIS measurements, and equivalent circuit fitting, the study examines the variations in the electrochemical response of gear oil with respect to oxidation temperature and duration. The findings reveal that oxidative degradation does not modify the single-relaxation dielectric characteristics of the gear oil; however, various electrochemical parameters undergo systematic evolution. Following oxidation at temperatures ranging from 90 to 120 °C, the charge transfer resistance escalates by approximately 5.9-fold; the base resistance diminishes by 10% to 20%; both the admittance constant and dispersion index of the constant phase element (CPE) exhibit changes of less than 5%, indicating that the system retains its capacitive properties. During constant-temperature oxidation at 90 °C for durations spanning 50 to 175 h, the charge transfer resistance increases in an approximately linear fashion with oxidation time, while the base resistance continues to decline, and the CPE parameters remain largely stable. Various electrochemical parameters evolve monotonically with the extent of oxidation, with charge transfer resistance demonstrating the highest sensitivity to thermal oxidation and thus serving as a pivotal indicator for evaluating the degree of thermal oxidative degradation in gear oil. This study lays an experimental foundation for the application of EIS technology in the realm of online gear oil monitoring. Full article

This paper presents a control framework that integrates adaptive sliding mode control (ASMC), time-delay control (TDC), and admittance filtering to achieve robust force and position tracking in robot manipulators. TDC is employed to estimate unmodeled dynamics using delayed measurements, while ASMC enhances robustness by compensating for time-delay estimation (TDE) errors and mitigating chattering effects. An adaptive law incorporating a decline-rate reduction factor is introduced to explicitly regulate the decay of the adaptive gain inside the boundary layer, thereby preserving compensation capability against time-delay estimation errors and external disturbances for a longer duration while improving position tracking performance. In addition, the admittance mechanism converts force-tracking errors into position correction signals, enabling force tracking without modifying the underlying position control structure. The stability of the closed-loop system is analyzed based on Lyapunov theory, ensuring bounded tracking performance in the presence of estimation errors and uncertainties. Simulation results demonstrate that the proposed method improves position tracking accuracy—reducing the root mean square error (RMSE) from 0.0522 mm to 0.019 mm—while maintaining reliable force tracking performance. Full article

Achieving safe physical interaction on the human back is challenging due to respiratory rhythms, complex topography, and varying tissue stiffness. To enable compliant force tracking within commercial closed position-control robot architectures, this paper presents an adaptive variable damping admittance control framework driven by multi-dimensional force sensor feedback. A stiffness-free admittance model is constructed to eliminate steady-state tracking errors, integrated with a nonlinear adaptive damping law that sensitively responds to real-time force sensor measurements. This mechanism rapidly dissipates dynamic impact energy during contacts while maintaining low impedance during steady state. Validated via a high-fidelity MATLAB R2024b-CoppeliaSim co-simulation platform replicating Traditional Chinese Medicine (TCM) manipulations, the proposed sensor-driven strategy significantly improves force tracking fidelity over traditional fixed-parameter control. Quantitative results demonstrate that across all complex therapeutic waveforms, the root mean square error (RMSE) remains below 0.42 N, the mean absolute error (MAE) is within 0.32 N, and the squared correlation coefficient ($r^2$) exceeds 0.97. These findings confirm the high efficiency and clinical potential of the proposed framework. Full article

Slip ring–brush assemblies are widely used in satellite mechanisms to transmit power and signals across rotating interfaces. Under authentic space environments—vacuum, radiation-dominated thermal exchange, and long-duration operation—the coupled effects of mechanical contact dynamics, electrical conduction, intermittent separation, and arcing can accelerate wear and degrade reliability. This paper presents a surface-resolved multiphysics model for multi-track slip rings with staggered brushes. The ring surface is discretized on a circumferential–axial grid and endowed with correlated 3D roughness, enabling interference-based asperity contact. Brush normal dynamics (mass–spring–damper) convert runout and micro-vibration into normal-force ripple and separation events. Electrical conduction is modeled by a parallel admittance network combining pressure-dependent micro-contact conduction and an event-based arc channel activated by separation, opening velocity, and current density with stochastic ignition. A 2D thermal model with ADI integration accounts for Joule/friction heating, radiative cooling, and optional hub conduction. Wear evolves via an Archard-type mechanical term and an arc-energy-driven erosive term. A FAST–MACRO multiscale scheme (20 s FAST, 100 h MACRO with periodic recalibration) enables tractable long-horizon wear prediction while preserving arc statistics. Baseline simulations for a 28 V bus demonstrate rare but nonzero arc activity and predict spatially non-uniform wear at the micrometer scale after 100 h. Full article

In collaborative robot lead-through teaching, fixed admittance parameters impose an inherent trade-off between operational ease and motion stability. This paper proposes a SAC-optimized fuzzy variable admittance control method (SAC-FAC). A fuzzy variable admittance controller (FAC) quantifies the operator’s motion and turning intents using interaction-force and end-effector motion information, and modulates the damping coefficient online via interpretable fuzzy rules. Soft Actor-Critic (SAC) searches offline for a well-balanced membership-function configuration on an episode basis in simulation, and the optimized configuration is then fixed for deployment. A saturation mechanism in the reward function suppresses degeneration of the damping configuration toward its physical lower bound. To counter parameter degradation under high-disturbance training, potential-based reward shaping and performance-gated curriculum learning are jointly introduced to promote stable convergence. Ablation studies and comparisons with four alternative optimizers verify the training design and support the suitability of SAC in this framework. Experiments on a UR10 collaborative robot platform with three trajectory types show that, relative to the hand-tuned FAC, SAC-FAC reduces the mean trajectory tracking error, work per unit path, and root-mean-square interaction force by 19.5%, 11.6%, and 6.8%, respectively, with more evident advantages on the compound and 3D ramp trajectories while preserving the interpretability of the fuzzy rule structure. Full article

To verify the harmonic amplification mechanism of wideband oscillations, this paper constructs a benchmark system. Firstly, the theoretical framework of the harmonic amplification mechanism for wideband oscillations is elaborated. Subsequently, the structure and parameters of the benchmark system are presented. Based on the s-domain nodal admittance matrix method, the resonant characteristics of the benchmark system, including resonance frequency, damping ratio, and nodal voltage mode shape, are calculated. Using electromagnetic transient simulation, the time-domain waveforms of the current, voltage, and power of the benchmark system under typical operating conditions are obtained, and harmonic decomposition of these waveforms is performed. By comparing and analyzing the harmonic components of current, voltage, and active power with the resonant characteristic quantities, the harmonic amplification mechanism of wideband oscillations is verified. The applicability of analyzing the harmonic amplification effect based on the positive-sequence network model is demonstrated. It is shown that the resonance frequency, damping ratio, nodal voltage mode shape, and resonant peak voltage are four key factors determining the harmonic amplification effect. Finally, the relationship between the frequency of the oscillatory power component and the frequency of the harmonic source is revealed. Full article

The tectonic setting of Taiwan and its surrounding regions is characterized by the complex interaction between the northwest-oriented Ryukyu subduction zone and the east-oriented Manila subduction zone. Within this subduction framework, the elastic thickness of the lithosphere (Te) serves as a critical parameter for elucidating the mechanical behavior of the area. In this study, we employed the admittance–correlation method to estimate Te values across Taiwan and adjacent territories. The findings indicate that sedimentary loading results in an overestimation of the maximum Te by approximately 50 km; after adjustment, the Te values range from 0 to 60 km throughout the study area. On Taiwan, Te values predominantly lie between 20 and 30 km, decreasing to 10–20 km near the margins adjacent to the Ryukyu and Manila subduction fronts. The Philippine Sea Plate exhibits comparatively higher Te values, ranging from 40 to 65 km. The spatial distribution of Te broadly corresponds with major tectonic subdivisions. Statistical analyses reveal a weak negative correlation between Te and surface heat flow (r = −0.44) and a weak positive correlation with shear-wave velocity anomalies at a depth of 100 km (r = 0.22), suggesting that the thermal structure exerts only a moderate influence on lithospheric strength in this region. Nonetheless, within oceanic crustal domains, the relationship between Te and oceanic crustal age largely adheres to models of crustal cooling and lithospheric thickening, consistent with isotherm depths of approximately 200–400 °C. Additionally, dynamic topography associated with slab subduction may locally diminish Te by up to 25 km. Cross-sectional profiles through northern Taiwan and the Philippine Sea block reveal pronounced coupling between subduction geometry and Te distribution. The observed spatial patterns of Te reflect the mechanical imprint of prolonged tectonic evolution, with the orientation of Te gradients generally aligned with the direction of maximum principal compressive stress. Collectively, these results suggest that subduction geometry and tectonic processes are important factors influencing the spatial variability and evolutionary trajectory of lithospheric strength in Taiwan and its environs. Full article

This study proposes a model predictive control (MPC)-based Assist-as-Needed (AAN) strategy for upper-limb rehabilitation robots, with particular emphasis on mitigating motor slacking. In conventional error-based AAN approaches, robotic assistance is regulated through a single coefficient tied to the tracking error; thus, a reduction in voluntary effort is absorbed into the assistive channel and remains obscured by a small tracking error. The proposed method decouples this mechanism by introducing a two-channel admittance structure, in which the robotic-assistance gain $A_k$ and the user-participation-reflection gain $B_k$ are jointly optimized within a single convex MPC formulation. The cost function addresses trajectory tracking, participation-aware force alignment, assistance suppression, and passivity, enforced through energy-tank constraints. The controller was validated in two experiments on a mobile upper-limb rehabilitation robot. The first experiment confirmed differential adaptation of $A_k$ and $B_k$ across three instructed contribution levels, with the participation ratio increasing from 0.103 to 0.879 as the contribution shifted from insufficient to appropriate. The second experiment compared the controller with an error-based AAN baseline and a forgetting-factor AAN baseline under an induced motor-slacking condition, in which the task-direction contribution was reduced to 45%. Under an identical synthesized input, the proposed controller yielded a lower aggregate human-contribution ratio of 0.282, compared with 0.595 and 0.535 for the two baselines, respectively. This indicates that the externally imposed reduction in participation was represented more explicitly in the controller allocation, rather than being masked by error-driven assistive compensation. These results suggest that the proposed approach extends AAN control toward a participation-preserving, anti-slacking strategy for robot-assisted rehabilitation. Full article

The synchronous continuum robot (SCR) was developed to emulate biological structures, such as animal tails and elephant trunks, based on continuum robot principles. By synchronizing disk motions, the SCR generates biologically inspired continuous movements while maintaining precise trajectory control. However, its synchronization-based architecture limits adaptability during physical interaction due to rigid trajectory-following characteristics. To address this limitation, this paper proposes a sensorless variable admittance control (VAC)-based compliant motion generation framework for the SCR. A dynamic model-based sensorless disturbance observer is designed to estimate external torques without additional force sensors. To compensate for uncertainties inherent in the cable-driven transmission mechanism, an adaptive term is incorporated into the parameter identification process, improving disturbance estimation accuracy. Based on the estimated external torques, admittance parameters are adaptively modulated according to joint angles, angular velocities, and robot posture, enabling interaction-aware motion speed regulation. Furthermore, the proposed method simultaneously enforces constraints on both joint angles and angular velocities through the adaptive regulation of target positions and velocities, ensuring safe and physically feasible motion. Experimental results under various interaction scenarios demonstrate reliable contact-independent force estimation and effective compliant motion generation. The proposed framework provides an integrated solution for robust force estimation, adaptive compliance control, and simultaneous constraint enforcement in mechanically synchronized continuum robots. Full article

Upper limb motor dysfunction resulting from stroke requires effective rehabilitation solutions; however, current exoskeletons are limited by single-input control, inadequate adaptation to various rehabilitation stages, and restriction to one limb. This study presents the development of a three-degree-of-freedom upper limb rehabilitation exoskeleton with three core innovations: (1) a neuro-fuzzy adaptive admittance control architecture that integrates human–robot interaction force and joint angular velocity as dual inputs for real-time damping adjustment, enabling accurate capture of dynamic movement intentions; (2) a Brunnstrom stage-specific fuzzy rule base that directly links clinical rehabilitation needs to adaptive control parameters; (3) a bilateral adaptable mechanical structure, allowing dual-upper limb training to enhance practical application. By combining radial basis function (RBF) neural network-based adaptive proportional–integral–derivative (PID) control with fuzzy variable-parameter admittance control, the system achieves a maximum trajectory tracking error of less than 1.2° and a root mean square (RMS) error of ≤0.13°. Trajectory tracing experiments confirm an RMS error of 2.99 mm for a circular trajectory at Bd = 2. The proposed strategy, validated through position tracking, admittance interaction, and trajectory tracing experiments, effectively balances tracking accuracy and human–machine compliance, providing valuable technical support for robot-assisted upper limb rehabilitation. Full article

This paper presents a bench-top occupational power-assist robot aimed at reducing biomechanical effort during repetitive material handling. The prototype adopts a SCARA-like structure with three degrees of freedom and provides assistance on the vertical (z) axis through a three-phase brushless DC (BLDC) motor driven in field-oriented control with inner-loop current regulation. The user interacts with the robot through a single handle-mounted load cell. The measured interaction force is converted, via a calibration-based mapping, into a motor current reference that enforces a prescribed force-sharing ratio. In this way, the drive’s embedded current loop acts as the low-level torque regulator, and the system can share gravitational and inertial loads without additional environment force sensing or explicit high-level impedance/admittance dynamics. A coupled electro-mechanical model is derived and used to select the assistance gain and to verify feasibility in simulation. A pilot experimental campaign with eight participants and two payloads (0.5 kg and 1.5 kg) was carried out on sinusoidal and random tracking tasks. With assistance enabled, the operator contribution was reduced to about 15% of the total load, and the mean bicep brachii EMG amplitude decreased by about 60%, while tracking accuracy was generally preserved and often improved. Full article

To elucidate the stability mechanisms of grid-forming (GFM) inverters governed by dispatchable virtual oscillator control (dVOC), this paper develops a comprehensive sequence-impedance modeling and stability analysis framework for dVOC-based GFM inverters with different inner-loop control structures. Three representative configurations are investigated: open-loop dVOC control, dVOC with dual-loop voltage–current control (DLC), and dVOC with virtual admittance control (VAC). For each configuration, unified positive-sequence impedance models are derived and analytically validated. Based on these models, the stability characteristics are first analyzed in a single-inverter grid-connected system under different grid strengths. The analysis is then extended to a mixed inverter system consisting of grid-forming and grid-following (GFL) inverters. Particular attention is paid to the impedance interaction between GFM impedance shaping and the capacitive negative damping introduced by GFL inverters under weak-grid conditions. Quantitative analyses reveal that the dVOC–DLC configuration significantly enhances oscillation damping in mixed systems. Under benchmark scenarios, stable operation can be ensured with approximately a 25% GFM capacity penetration. In contrast, the open-loop and VAC configurations require around 50% and 75% capacities, respectively, to maintain stability. These findings indicate that the DLC-based inner-loop design offers superior stability margins while substantially reducing the required GFM capacity, thereby improving economic efficiency. This study establishes a quantitative impedance-based criterion for inner-loop control selection and provides practical design guidelines for deploying dVOC-based GFM inverters in future converter-dominated power systems. Full article

This paper presents a comprehensive framework for evaluating the robustness and adaptability of human–robot collaboration (HRC) controllers under a spectrum of dynamic and unpredictable human intentions. Building upon variable admittance controller (VAC) frameworks augmented with Radial Basis Function Neural Network (RBFNN) online adaptation, we introduce two key innovations: (1) an intent-aware human force generator capable of simulating aggressive, hesitant, oscillatory, conflicting, and nominal behaviors, through the modulation of force gains and the introduction of stochastic noise, and (2) the extension of VAC to incorporate variable stiffness as an adaptive control parameter alongside damping and inertia. The adaptive parameters are jointly tuned online using a self-supervised learning (SSL) mechanism driven by motion error metrics and interaction dynamics. The framework is simulated in a dual-arm collaborative manipulation scenario involving two 7-DoF Franka Emika Panda robots transporting a shared object in a high-fidelity simulation environment. Simulation results demonstrate the system’s capability to maintain stable behavior and minimize tracking error despite abrupt changes in human intent. This work provides a novel and systematic tool for stress-testing adaptive controllers in HRC, with implications for the design of resilient, safe, and reliable robotic systems in real-world collaborative environments. Full article

Based on the dilemma of analyzing the resonance stability of AC power grids using impedance models, it is demonstrated that the “negative resistance” mechanism of wideband oscillation is untenable. A general method for describing power electronic devices using wideband voltage-source converter models and wideband current-source converter models is proposed, thereby representing the nonlinear characteristics of power electronic devices with harmonic voltage sources and harmonic current sources. This allows the renewable energy power system to still be described by a linear system, and interprets the mechanism of wideband oscillation as a “harmonic amplification” phenomenon caused by network resonance, thus establishing a new framework for explaining the mechanism of wideband oscillation in renewable energy power systems. Through the analysis of two basic resonant circuits, the relationship between the damping ratio of resonant modes and the harmonic amplification factor is derived, laying a theoretical foundation for the analysis and suppression of wideband oscillation based on the s-domain nodal admittance matrix method and C-type damping filters. Based on the maximum damping criterion, a design method for C-type damping filters is proposed. The designed C-type damping filters exhibit strong broadband damping effects. Full article

With the rapid advancement of science and technology, robotics is evolving towards more profound and extensive applications. Nevertheless, the inherent limitations of traditional industrial “caged” robots have significantly impeded the full utilization of their capabilities. Consequently, breaking free from these constraints and realizing human–robot collaboration has emerged as a new developmental trend in the robotics field. The collision-response mechanism, as a crucial safeguard for human–robot collaboration safety, has become a pivotal issue in enhancing the performance of human–robot interaction. To address this, an adaptive admittance control collision-response algorithm is proposed in this paper, grounded in the principle of admittance control. A collision simulation model of the AUBO-i5 collaborative robot is constructed. The effectiveness of the proposed algorithm is verified through simulation experiments focusing on both the end-effector collision and body collision of the robot, and by comparing it with existing admittance control algorithms. Furthermore, a collision-response experimental platform is established based on the AUBO-i5 collaborative robot. Experimental studies on end-effector and body collisions are conducted, providing practical validation of the reliability and utility of the proposed adaptive admittance control collision-response algorithm. Full article