Rotor-stator rub interactions are a significant area of interest in rotordynamics because they can generate backward whirling, often causing malfunctions. This paper extends the analytical solution for rotor-stator rub dynamics to include friction at the rotor-stator contact. A simple two-degrees-of-freedom Jeffcott rotor model is used, and the contact mechanics follows a piecewise-smooth model with Coulomb friction. The rotor can be in the contact or non-contact regime, introducing motion-dependent discontinuities and strong nonlinear behaviour due to contact forces. To obtain an analytical solution, these nonlinear forces are approximated using the first-order Taylor expansion. Eigenvalue and nonlinear dynamics analyses of the model are also undertaken. The analytical solution is validated against fourth/fifth order Runge-Kutta integration. Effects of varying friction, rotor damping, and stator stiffness are investigated. The main contribution is an analytical solution applicable to wide range of problems from rotordynamics including drill-string whirling, offering faster computation while maintaining accuracy, and contributing to improved rotor dynamics modelling.

Downhole stuck tools are a significant source of downtime during drilling, and jarring tools can remedy such problems. In a typical jarring operation, axial impact releases a stuck tool during drilling. Innovative jarring tool designs generate multiple, high-frequency impacts, but existing models of jarring operations focus on conventional jarring with a single impact. In this work, we developed two low-dimensional models to describe a multi-impact jarring system to investigate nonlinear behaviour during multi-impact jarring. A simplified strongly nonlinear one degree-of-freedom model with a realistic dynamic loading scenario was selected for investigation through a direct numerical simulation. Typical nonlinear dynamics characteristics, such as time–history evolutions, phase portraits, and bifurcation diagrams, are presented. In addition, novel steady-state force equilibrium diagrams are constructed. Results of the numerical simulations indicate the system behaviour is sensitive to some operational parameters, namely the excitation frequency. We present examples of the system dynamic responses ranging from periodic to chaotic solutions, strongly affecting the transmitted force onto a stuck pipe. Our analysis shows that the external excitation frequency has a major effect on system operation and can be used to improve tool performance.

We present the design, optimization, and closed-loop pressure control of a three-finger soft gripper composed of dual-cavity pleated soft pneumatic actuators (DCPS-SPAs). Each finger integrates a pleated actuation chamber and a stabilizing cavity to enable large bending with low supply pressure while preserving structural stiffness for load transfer. We derive an Integral-augmented Sliding Mode Control (ISMC) law for valve-based pressure regulation using a previously established DCPS-SPA pressure model and implement a real-time control on a dSPACE MicroLabBox/Simulink platform. Finite element analysis (using Abaqus/CAE) and experimental tests are used to compare two DCPS-SPA lengths, quantify deformation, and measure tip force. The shorter actuator exhibits higher tip force for the same pressure, whereas the longer achieves larger workspace; we therefore select and further optimize the short geometry by varying chamber width, spacing, and pitch. Pressure-controlled gripping tests with diverse objects demonstrate stable tracking, low overshoot, and consistent grasping performance. Key findings include: (i) deformation up to 159 mm for the longer SPA vs. 120 mm for the shorter at 0.5 bar; (ii) higher tip force for the shorter SPA across 0 bar to 0.8 bar; and (iii) controlled gripping with rise times 0.63 s to 1.88 s, steady-state errors <= 0.0256 bar, and RMS errors <= 0.1113 bar. The proposed architecture offers a rigorously validated pathway towards low-pressure, high-stability, multi-object dexterous grasping. Remaining gaps include precise modeling of material viscoelasticity, contact/friction variability, and multi-finger force distribution under object-dependent constraints.

In this article, an integrated model in spatiotemporal domain is developed to study the coupled axial-torsional dynamics of a drill-string. Unlike most models that only consider the interaction between an idealized drill-bit with identical radial blades and rock, this study introduces a drill-bit that aligns to engineering practicability more closely, with a structural configuration of uniformly alternating full and partial blades. According to the cutting geometry at bit-rock interface, the evolution of rock surface morphology (RSM) at well-bottom is formulated through algebraic equations to accurately capture the multiple regenerative effects induced by bit-rock disengagement, i.e., bit-bounce. The entire drill-string structure and rock formation are spatially discretized by using the finite-element (FE) method, and the resultant dynamic model is tested by checking its numerical convergence, showing that the model is robust. Then, a comparative analysis is conducted between the traditional state-dependent-delay (SDD) model and the proposed RSM model. The results demonstrate that the RSM model maintains volume conservation for the total cuttings generated in drilling process while achieving lower computational cost, whereas the SDD model fails to preserve volume conservation. Hence, the proposed RSM model can capture the cutting dynamics of the drill-bit more accurately. Finally, a simulation-driven case study under various drilling conditions is conducted to reveal the nonlinear dynamic characteristics of drill-string system. These findings provide a theoretical framework for subsequent development of model-based active control strategies to suppress detrimental drill-string vibration, specifically targeting stick-slip vibration and bit-bounce.

This study addresses the stabilisation of wheeled self-balancing robots under practical constraints imposed by low-cost hardware, where communication and sensing delays critically degrade control performance. A comprehensive nonlinear electromechanical model is developed by integrating the dynamics of an inverted pendulum with the electrical and mechanical characteristics of DC motors, thereby capturing the full actuator-plant interaction. Based on this, a cascaded sliding mode control scheme is proposed to regulate the pitch angle directly through motor voltages. The controller is designed to enhance robustness against modeling uncertainties and external disturbances, while explicitly accounting for actuator and sensor delays. Extensive numerical simulations are conducted to evaluate closed-loop stability and control torque behaviour under varying delay profiles and hardware limitations such as motors with reduced capacity. The results demonstrate that it maintains stable balancing performance despite significant time delays and degraded hardware quality, which highlights its suitability for cost-efficient robotic design. The modeling framework and delay study presented here provide a general methodology for assessing the robustness of nonlinear electromechanical systems subject to communication and sensor delays, as well as actuation constraints.

This paper presentsPole-placement control a prediction-basedNanopositioning pole-placement controllerPole-placement control for a nanopositioning system, designed to address the challenges posed by input time-delaysInput time-delay. The controller incorporates both feedforward and feedback control laws, ensuring precise tracking of the reference signal while compensating for time-delay effects. SimulationSimulations results demonstrate the controller’s ability to achieve zero steady-state error and stable tracking of unit-step reference signal. The closed-loop system exhibits improved stabilityStability, with a significant increase in 3 dB bandwidth and a reduced settling time, compared to existing controllers. The closed-loop transfer function is formulated as a Butterworth filter to ensure the desired tracking performance. A key design criterion is the constrained control input voltage, ensuring the system operates within its practical limits. The proposed controller is shown to be highly effective in handling time-delays, ensuring smooth and accurate operation within the system’s constraints. These findings confirm the controller’s potential for enhancing the performance of high-precision nanopositioningNanopositioning applications.

Advancements in soft robotic technology have led to the development of various human-like soft designs with enhanced capabilities to replicate targeted functions. A major challenge in this field is developing accurate models to represent the soft materials in alignment with the chosen actuation method. Additionally, creating precise control schemes to manage their inherent nonlinearity while delivering the required control action is critical. In this study, we present a mathematical model of the pressure dynamics for a dual-cavity pleated structure soft pneumatic actuator (DCPS-SPA), actuated via a three-outlet–two-inlet solenoid valve within a pneumatic circuit using an energy-based approach. We propose a fuzzy-based integral augmented sliding mode control combined with an improved enhanced exponential reaching law (FISMC+IEERL) for pressure control in the SPA. The performance of the proposed control strategy is compared with other successive sliding mode control (SMC) schemes. Both experimental and simulation results demonstrate the capability of the designed control scheme to provide precise pressure control in the SPA with minimal error and optimized control action.

Fatigue, driven by long-term stress concentrations arising from continuous dynamic loading, is a key cause of damage and potential failure in offshore structural connections. This issue is further amplified in floating offshore wind turbines (FOWTs) due to their dynamic sensitivity and exposure to coupled aero-and hydrodynamic loading, emphasising the need for reliable fatigue assessment methodologies. This study investigates the influence of global member flexibility and local joint flexibility on the dynamic and fatigue response of the OC4 semi-submersible FOWT, with fatigue assessments focused on representative critical hotspots including the tower base, brace connections, and mooring lines. Local joint flexibility is modelled using the Buitrago parametric formulation, enabling representation of inter-member deformation mechanisms not captured in conventional rigid-body or beam-only flexible models. The fatigue evaluation framework integrates stress concentration factor formulations, rainflow cycle counting, S-N and T-N methodologies, and Palmgren-Miner linear damage accumulation with sensitivity checks for Goodman, Soderberg, and Gerber mean stress corrections. Environmental loading conditions are derived from joint probabilistic wind-wave representations for the North Sea, complemented by Norwegian Sea conditions, with coupled aero-hydro-moor-servo-elastic simulations performed in Orcina's OrcaFlex in accordance with IEC 61400 standards using stochastic time-domain wind and wave loading. Diffraction-based inertia loads combined with Morison-type drag formulations are employed to capture both global and local hydrodynamic effects. Results show that rigid-body floater models can overestimate tower-base fatigue damage, predicting a fatigue life of 62 years compared to over 300 years for flexible configurations, while flexibility-induced axial loading reduces brace fatigue life from thousands of years to only a few hundred. Structural flexibility lowers mooring line tension by approximately 6% and extends fatigue life, with studded chains exhibiting up to 68% longer lifetimes than studless configurations. The findings demonstrate that structural flexibility and local joint compliance can significantly influence fatigue-sensitive regions within semi-submersible floating wind platforms, highlighting the importance of modelling fidelity when assessing long-term structural response.

In this article, a distributed-lumped model is developed for identical pieces of pipes that are welded together, as the main structure of the pipelines. The model can predict the signal amplitude and velocity in high attenuation, where dispersion effects limit the range of the health test. The health test is simulated by sending a torsional-guided wave across the pipe. The wave is attenuated as it passes through the welds, which act as lumped elements, while it becomes significantly dispersed when travelling through the pipe segments, which function as distributed elements. The theoretical shape of the angular displacement and angular velocity signal, in the frequency domain, is calculated and commented upon. The approach herein is the dynamic stiffness matrix method, which is developed for both the velocity and displacement signals. This approach enabled the signal pattern to be found in a 100-m pipe test comprising 8 pipe segments. This range is the current practice in Long Range Ultrasonic Testing (LRUT) of the pipelines. The new approach in this article combines distributed parameters of pipe segments with lumped parameters of welds. Then, it implements the Dynamic Stiffness Matrix Method (DSMM) to pipelines as “Distributed-Lumped” systems. In a numerical example, the results are demonstrated via the frequency response signature of a healthy pipe. This helps the damaged pipe to be designated via comparison with experimental frequency response signatures.

This study investigates the effects of member and local joint flexibilities on the response of a semi-submersible floating offshore wind turbine (FOWT) by carrying out fully coupled nonlinear time-domain aero-hydro-servo-elastic dynamic analysis of the OC4 floater. Rigid body models and flexible models of the OC4 semi-submersible floater, supporting the NREL 5MW wind turbine, are used to perform this study. A total of four unique models of the FOWT system were developed and these include a model with a rigid body floater (M1), a flexible floater with each OC4 column modelled as separate rigid bodies connected by flexible braces (M2), and a flexible floater with local joint flexibilities explicitly introduced at inter-member connections to capture joint-level deformation (M3). The fourth is a parametric sensitivity model, derived from the previous one, where the rotational stiffnesses of the floater joints are systematically increased to assess their influence on the system dynamics as validation for M2. Decay tests were first carried out on all 4 models, to ascertain their characteristic modal behaviour, then fully coupled dynamic simulations were carried out for all the models. Analysis was performed for cases of regular wave only, random wave only, regular wave plus steady-state wind, and random wave plus turbulent wind. The global and local response of the FOWT system is then investigated. Results show that floater flexibility modifies the platform pitch natural period by about 10%. The surge and heave natural periods are, however, minimally affected. Platform flexibility also significantly affects the local response of floater braces (pontoons), the tower base loads and tower top kinematics, with implications for both design optimisation and performance forecasting in next-generation FOWT systems. Fatigue analysis of the tower base and selected brace connections reveal that fatigue damage is overpredicted at the tower base when the platform is modelled as a rigid floater, leading to a conservative (underestimated) fatigue life by approximately 50%, while fatigue damage is substantially underpredicted up to about 80% in the pontoon braces when the platform is modelled as rigid.

The growing deployment of floating offshore wind turbines (FOWTs) presents new challenges in maintenance operations, particularly concerning in-situ component replacement. This study builds upon prior research into the feasibility of using a floating crane for generator exchange on a semi-submersible FOWT, specifically the UMaine VolturnUS-S supporting a 15 MW reference turbine. Utilising a marine simulation environment at NDC, the dynamic responses of the generator, nacelle, and crane barge were evaluated under various sea states. The results highlight that while generator accelerations are a significant operational factor, the primary constraint is the risk of collision between the generator and turbine structure during lifting operations. Parametric studies revealed critical wave periods that exacerbate generator motions and collisions, and while modifications to the lifting methodology proved ineffective, reorienting the crane barge parallel to incoming waves showed a modest reduction in collisions. These findings underline the importance of vessel selection, wave direction, and sea state limitations in ensuring the viability of in-situ maintenance using floating cranes for FOWTs.

The fatigue performance of offshore structures in the oil and gas industry is vital to the reliability and integrity during service life, and there are very few studies on fatigue cracks induced dynamically to tubulars. Capability of a unique experimental fatigue rig designed to test single-edge notched beams [1] was in this work expanded allowing to obtain for new insights into fatigue crack growth in tubular specimens. Fatigue cracks were initiated in pre-cut grooves in aluminium tubular specimens and then under dynamic loading propagated in the direction of the groove. The alternating current potential difference (ACPD) method was adopted to monitor the crack propagation in time histories. Three specimen types featuring different groove depths were made and tested in fifteen individual experiments. And for each specimen type, a three-dimensional finite element model was created to calculate the stress concentration factor (SCF). Experimental measurements of specimen accelerations and forces generated were used to develop a novel calibrated dynamic model, for which dynamic responses were computed. The predictions obtained from a mathematical model were compared with the experimental results and a close correlation was observed.

This study delves into the vibration characteristics of a piezoelectric cylindrical shell situated on an elastic foundation under various physical fields and boundary conditions. The investigation stems from the necessity to comprehend the behavior of nanobuilding blocks on foundations in the realm of nanoelectromechanical systems, which cater to applications in intricate environments. Employing a semi-analytical approach, we combine wave-based methods with Kirchhoff–Love shell theory and nonlocal theory. By applying the Hamilton principle, we derive kinematic relationships and governing equations for the cylindrical shell. Our methodology adopts a wave function approach to establish displacement solutions, integrating scaling parameters to formulate control equations for the piezoelectric shell’s vibration model. This model enables the extraction of natural characteristics and determination of steady-state responses to diverse external loads. The accuracy of our model is verified through numerical examples, comparative analyses, and experimental results. Furthermore, a parametric study is conducted to evaluate the influence of non-local parameters, elastic support stiffness, and external physical fields on the characteristics and response of the cylindrical shell. Ultimately, a prediction model for the piezoelectric cylindrical shell under multi-field conditions is developed, offering a theoretical foundation and references for the advancement of nanoelectromechanical systems.

The utilization of Dielectric Elastomer Actuators (DEAs) in soft robotics is becoming increasingly popular due to their distinctive properties. However, controlling these actuators is challenging due to their nonlinear nature. This study addresses precise control of DEAs while minimizing nonlinear effects over a wide frequency range using a Sliding Mode Control (SMC) scheme. Traditional SMC approaches face two main issues: they do not ensure finite-time convergence during the sliding phase and compromise tracking accuracy due to approximations used to reduce chattering. This work presents a non-singular terminal sliding mode control combined with an enhanced boundary layer switching function to improve precision and finite-time stability. Using stability analysis, extensive MATLAB/SIMULINK simulations, and experimental validation on a conical DEA across diverse reference trajectories and its resilience against uncertainties, the proposed scheme demonstrates superior performance in trajectory tracking compared to PID-based and conventional SMC schemes. The results highlight the scheme’s effectiveness in high-frequency trajectory tracking and its robustness to possible uncertainties and disturbances, offering a robust theoretical framework for dielectric elastomer actuator control and a promising approach for advancing soft robotic technologies.

This paper investigates anchor and fairlead loads under various environmental conditions for both spar and semi-submersible floating wind turbine types. As expected, it is shown that the anchor and fairlead loads for the 15 MW semi-submersible type are higher than for the 5 MW spar type. However, the difference is substantial, and therefore the loads are not proportional to the output power of the turbine. It can be concluded that spar-type floating platforms are more suitable than semi-submersible ones because they experience lower forces on their anchors and fairleads.

This paper aims to develop a robust controller capable of suppressing stick-slip oscillations. To achieve this goal, metaheuristic optimization algorithms, including Particle Swarm Optimization (PSO), Artificial Bee Colony (ABC), and Differential Evolution (DE), were integrated with Modified Integral Resonant Control (MIRC) to create a novel hybrid MIRC. Several simulation runs were conducted to investigate the robustness of the hybrid MIRC in automatically responding to disturbances. The results indicated that PSO outperforms the ABC and DE algorithms in the expeditious determination of optimal controller parameters. Additionally, simulation results demonstrated the robustness of the proposed PSO-MIRC in adeptly identifying key control parameters that suppress stick-slip vibrations within a succinct timeframe. The findings of this paper offer a promising tool for ensuring the stability of the drill-string.

This paper presents an integrated model that considers the spatial inertia of drill-string and the non-uniform distribution (NUD) of drill-bit blades, aiming to research the nonlinear dynamics of rotary drilling. Unlike most studies that overlook the interplay between different modes of vibrations and focus on a limited number of nonlinearities, this model comprehensively addresses these aspects. The proposed model incorporates various nonlinear behaviours resulting from the interaction between NUD drill-bit and rock, including dry friction, regenerative effect, and loss of contact. The spatial discretization of the drill-string is conducted through finite element (FE) method, and the predictive capabilities of the resultant FE model is validated using field data. Linear stability analysis reveals that both NUD of blades and the addition of extra blades can expand stable drilling region. Numerical simulations capture several types of self-excited vibrations, notably stick–slip and bit-bounce. Considering rate-of-penetration as a variable parameter, system states will undergo qualitative changes, evolving from periodic to quasi-periodic dynamics, and further leading to chaos through period-doubling bifurcation. A sliding mode controller is designed to suppress drill-string vibrations, and its effectiveness in nonlinear drilling system, both with and without the presence of stick–slip and bit-bounce, is numerically validated.

Self-balancing robots (SBRs) are increasingly used in domestic and industrial applications due to their compact design and manoeuvrability. However, their dynamic behaviour presents significant challenges, particularly in the presence of actuator delay. This study investigates how such delays affect the stability and control performance of an SBR, with a focus on comparing linear and nonlinear modelling. A nonlinear planar dynamics (PD) model with two degrees of freedom is developed to describe the robot's motion, which is then linearized using a small angle approximation. To regulate the pitch angle, a sliding mode controller (SMC) is designed. The differences between the linear and nonlinear response of the system are analysed. Numerical simulations indicate that the nonlinear PD model performs better under delayed conditions compared to its linearized counterpart.

Rotor–stator whirling is a critical malfunction frequently encountered in rotating machinery, often resulting in severe damages. This study investigates the nonlinear dynamics of a whirling rotor interacting with a snubber ring through numerical simulations that account for the stiffness asymmetries of the snubber ring. A two-degrees-of-freedom (DOF) model is employed to analyse the contact interactions that occurred between the rotor and the snubber ring, assuming a linear elastic contact model. The analysis also incorporates the static offset between the centers of the rotor and the snubber ring. The dynamic behaviour of the whirling system is characterised by pronounced nonlinearity due to transitions between contact and non-contact states. The model is first validated against our prior theoretical and experimental studies. The nonlinear responses of the rotor are analysed to evaluate the effects of stator asymmetry through various techniques, including time-domain waveforms, frequency spectra, rotor orbits, and bifurcation diagrams. Furthermore, the influence of varying system parameters, such as rotational speed and the damping ratio, both with and without stator asymmetry, are systematically analysed. The results demonstrate that the rubbing response is highly sensitive to small variations in system parameters, with stator asymmetry significantly affecting system behaviour, even at low asymmetry levels. Direct stiffness asymmetry is shown to have a more pronounced effect than cross-coupling stiffness. The system exhibits a range of dynamics, including periodic, quasi-periodic, and chaotic responses, with regions of periodic orbits coexisting with chaotic ones. Complex phenomena such as period doubling, period halving, and jump bifurcations are identified, alongside quasi-periodic and period doubling routes to chaos. These findings contribute to a deeper understanding of the nonlinear phenomena associated with rotor–stator whirling and provide valuable insights into the unique characteristics of rubbing faults, which could facilitate fault diagnosis.

The role of time delay in the control processes aimed at mitigating stick-slip vibration in drill-strings, is of paramount importance. Its influence on the controlled system's settling time cannot be understated; indeed, it can lead to significant consequences such as controller instability. In this study, a numerical analysis was conducted, initially a sliding mode control was employed to suppress stick-slip vibration in drill-string. Then we explore a spectrum of time delay values alongside varying reference angular velocities. The results of this investigation suggest a critical finding: the controller's susceptibility to limit cycles is notably heightened in the presence of time delay and its experiences a decline when faced with fluctuating reference angular velocities. Despite this degradation, it is noteworthy that the system does not always reach a state of complete instability. These insights are invaluable, offering essential considerations for the development of robust control strategies. Addressing the challenges posed by time delay-induced issues in suppressing stick-slip vibration during drilling operations requires a nuanced approach, taking into account the intricacies highlighted in this study. Copyright (c) 2024 The Authors.