Actuator Surface Research Articles

SDBD Flexible Plasma Actuator with Ag-Ink Electrodes: Experimental Assessment

In the present work, a single dielectric barrier discharge (SDBD)-based actuator is developed and experimentally tested by means of various diagnostic techniques. Flexible dielectric barriers and conductive paint electrodes are used, making the design concept applicable to surfaces of different aerodynamic profiles. A technical drawing of the actuator is given in detail. The plasma is sustained by audio frequency sinusoidal high voltage, while it is probed electrically and optically. The consumed electric power is measured, and the optical emission spectrum is recorded in the ultraviolet–near infrared (UV–NIR) range. High-resolution spectroscopy provides molecular rotational distributions, which are treated appropriately to evaluate the gas temperature. The plasma-induced flow field is spatiotemporally surveyed with pitot-like tube and schlieren imaging. Briefly, the actuator consumes a mean power less than 10 W and shows a fair stability over one day, the average temperature of the gas above its surface is close to 400 K, and the fluid speed rises to 4.5 m s−1. A long, thin layer (less than 1.5 mm) of laminar flow is unveiled on the actuator surface. This thin layer is interfaced with an outspread turbulent flow field, which occupies a centimeter-scale area. Molecular nitrogen-positive ions appear to be part of the charged heavy species in the generated filamentary discharge, which can transfer energy and momentum to the surrounding air molecules.

Qualitative and quantitative analysis of the reliability of NPC and ANPC power converters for aeronautical applications

In more electrical aircraft, most flight control wing surface actuators are either electro-hydrostatic or electro-mechanical, whether in manual or autopilot mode. Their reliability is essential for flight safety; failure of even one actuator can have catastrophic consequences. According to ARP4761 guidelines for the development and certification of critical aircraft systems, the probability of failure of flight-control actuator components must be less than 10-9 per hour of flight. In this paper, we compare the probability of failure per hour (PFH) of conventional two-level power converters and three-level NPC and ANPC converter topologies designed in our laboratory. In accordance with ARP procedures, PFH was calculated using an approach that includes qualitative analysis based on failure mode and effect analysis (FMEA) methodology as well as quantitative analysis based on truth tables. ANPC converter met all the requirements of the functional hazard assessment in all cases of defects. Finally, we discuss factors that determine the feasibility of implementing power converters in flight control systems.

Computational simulation of ice accretion and shedding trajectory of a rotorcraft in forward flight with strong rotor wakes

To ensure safe flight in icing conditions it is essential to understand ice accretion, its effect on aerodynamic performance and ice shedding in any aircraft certification program. Ice fragments shed from a rotorcraft windshield and other locations can be detrimental if they are ingested into the engine intake or impinge on the tail rotor. Because of the lack of experimental data about the forces and moments acting on ice in such a complex flow field, the computational simulation of ice shedding trajectories becomes essential. This study presents a methodology to predict the location of ice accretion and break-off, ice shape and shedding trajectory in a rotorcraft flow field with strong rotor wakes during forward flight. The methodology includes the creation of an aerodynamic database for different ice shapes (rectangle, disc, ellipse, and glaze ice shape) at various combinations of Euler angles; the analysis of rotorcraft flow field by computational fluid dynamics for different advance ratios; and a six degree-of-freedom trajectory analysis using artificial neural networks and the Monte Carlo method. The actuator surface method was applied to account for the rotor wake effect, which is capable of modeling the tip vortices and inboard sheets emanating from the rotor blades. The main results are a probability map of the ice shedding trajectory footprints on the engine intake and tail rotor planes. Disc-shaped ice fragments with a sharp edge turned out to be most dangerous. The rotation of the main rotors also substantially affected ice accretion and shed trajectory, indicating the importance of integrated simulations of all components when designing ice protection systems for rotorcraft.

From main axes control to flexible body control of large telescope structures

AbstractAstronomers have to point their telescopes with extreme high precision to their targets on the sky. The early craftsmen, which built their telescope mounts, were masters in mechanics and developed sophisticated axis mechanisms based on clockwork motors for the hour angle. The situation changed with the growth of the size of the optical telescopes, the upcoming of large radio telescopes, the related transition to elevation over azimuth mounts and the introduction of electrical main axes drives. The new telescopes were equipped with gears of high transmission ratios, and the dynamics of the motor control was limited by mechanical resonances between the rotors of the motors and the mass of the telescope structure. Early layout tools for the controllers were lumped mass models for the telescope structure including gear stiffnesses and inertias of rotors and brakes, with the free rotor and locked rotor frequencies as critical characteristics influencing the servo loop performance. Nowadays, static and dynamic analysis is performed with the help of finite element models, and the axes controllers are optimized with end‐to‐end models. The emergence of large radio telescopes initiated the development of additional compensation methods for environmental influences as temperature and wind on the shape of the reflectors and their pointing to the sky. Additional active elements on the telescope as surface actuators and subreflector positioners, and additional state sensors on the structure allow the identification and compensation of structural deformations. The method is called flexible body control (FBC). The increasing size of the optical telescopes initiated the development of new mirror technologies as thin meniscus or segmented mirrors in sizes not feasible with the passive supported thick meniscus mirrors. These mirrors need active elements as shape actuators, segment positioners, and wavefront and edge sensors. The method is called active optics (AcO). Astronomical observations in the visible are disturbed by atmospheric blur, and the operability is depending of the active compensation of that blur, based on a fast wavefront sensor analyzing an artificial star and an additional fast wavefront corrector. The method is called adaptive optics (AdO). The following text gives an overview on the historic development of telescope control systems from the viewpoint of the telescope mount and its axes control systems, finally culminating in FBC applications. Actual examples are the large millimeter telescope LMT/GTM in Mexico, the advanced technology solar telescope DKIST in Hawaii, and the airborne telescope of SOFIA, the Stratospheric Observatory for Infrared Astronomy with its base in Palmdale, California.

Dynamic response analysis of aircraft with actuation system failure: open-loop and closed-loop cases

On flight control implementation, the control surface actuator system plays one of the key roles that will significantly affect the closed-loop performance of the aircraft. The actuators must provide the required effort for moving the control surface so that the desired control action can be reached. Any failure on the actuator system will limit or degrade the operation of the control surface, so that it will affect the performance of the system in controlling the aircraft, or even worse, may bring the aircraft into an unsafe condition. In this project, the effect of some control surface problems due to actuator failures are modeled, simulated, and analyzed. To reconstruct the situation, the control device failure model is integrated into the flight dynamic model of an aircraft. The numerical model of the equation then is used for simulating the aircraft response under some failure conditions. It can be seen from the simulation results that control surface failure can significantly affect the trim condition of the aircraft, since the failures can change the balance of forces and moments that work on the aircraft. The simulation results also show that for some control surface failures may initiate an asymmetrical response from the aircraft dynamics. On closed-loop simulations, where some type of flight control systems for stabilization and maneuvering tasks are implemented, control surface failures may affect the performance of the control system. The results of the study suggest that any possibilities of control surface failure must be anticipated, by using a robust control scheme or other reconfigurable schemes that can cope with the change in the dynamic characteristics due to the failures.

Moving Surface Actuation, Effects of Frequency-based Shear Layer Excitation on the Response of a Bluff Body Wake

The effect of actuation frequency, using moving surface actuation, is investigated for a square cylinder bluff body wake. Pressure sensor data are used to optimize actuation characteristics through the implementation of an NSGA-II evolutionary algorithm. Velocity field data are obtained using Particle Image Velocimetry (PIV) for baseline and optimized actuation cases. A Proper Orthogonal Decomposition (POD) analysis shows that the vortex shedding frequency shifts between frequencies associated with the actuation, moving between regions of lock-on and quasi-periodicity. Additionally, the POD shows that the energy contained in the coherent shedding motion is reduced through actuation, while the total energy in the velocity field stays relatively constant. A reconstruction of the first 10 POD modes indicates that the coherent contribution to the Reynolds stresses significantly decreases compared to the non-actuated case. The mechanism for drag reduction is investigated using the shed circulation flux and Kochin’s drag formulation model. The drag obtained using PIV measurements and Kochin’s formulation is consistent with trends observed for the base pressure as a function of actuation frequency.

Numerical modelling of vertical axis turbines using the actuator surface model

This paper presents the development and validation of a lightweight fluid solver based on the Navier–Stokes equations coupled to a special blade element method (BEM) for vertical axis turbines. This method is called Actuator Surface Model (ASM) and leads to a medium fidelity tool designed to bridge the gap between expensive blade resolving computational fluid dynamics (CFD) calculations and simple, often inaccurate momentum models such as the double-multiple streamtube (DMST) model. Since the simulation runs on coarse meshes, it is possible to use an explicit solver with a reasonable time step to significantly reduce the computing time to almost real-time speed. The fluid model is validated for the classical lid-driven cavity flow problem, while the coupled ASM code is compared with three different experimental measurements from the literature. For a more accurate description of blade specific phenomena, several higher order corrections were implemented, such as dynamic stall, parasitic drag and finite aspect ratio. The model is able to calculate the performance of all tested turbines reasonably well and can predict the maximum efficiency up to a few percent. Therefore it is an excellent alternative for a fast and precise initial design of vertical axis turbines compared to traditional BEM methods.

Desalinating microalgal-rich water via thermoresponsive membrane distillation

In this study, composite membranes for direct contact membrane distillation (DCMD) were fabricated by casting a top layer of PVDF that has blended with different dosages of thermoresponsive particles onto PTFE support. The fabricated composite membrane exhibits thermal-cleaning properties using pure water as a cleaning agent. The surface property and thermoresponsiveness of composite membranes were tested underwater at both room and elevated temperature (60 °C). The fouling tendency and cleaning efficiencies of composite membranes were evaluated using model foulants namely bovine serum albumin (BSA) and sodium alginate (SA). Membranes dosed in with poly(N-isopropylacrylamide) (PNIPAM) showed both hydrophobic and oleophobic properties under elevated temperature (60 °C). This makes it suitable to be used for membrane distillation (MD) operation. It was found that composite membranes with 1.0% PNIPAM showed excellent cleaning efficiency with flux recovery of 88.26% after microalgae fouling. Hence, the swelling and shrinking of PNIPAM in composite membrane that create surface actuation force during membrane thermal cleaning was able to repel the algal organic matter (AOM) that fouled on membrane surface. Besides, the rehydrated PNIPAM could repel the foulants that interacted with hydrophobic interaction during cold water washing.

Experimental investigation of an electrostatic adhesion device used for metal/polymer granular mixture sorting

Electrostatic adhesion (EA) technique have been used recently for the electrostatic sorting of metal/polymer granular material mixture. The action seat on the materials is an EA actuator composed of an interposed electrodes network covered by a dielectric barrier and powered by polyphase voltages. The sorting principle consists of attaching the metal particles by EA, and of moving the polymer particles larger than 500 μm, mainly by gravity and vibration. The purpose of this paper is to optimize the EA mechanism for electrostatic sorting. The first part consists of evaluating the behavior of materials (Cu, PE, and PVC) on the EA actuator surface using a 180°-rotating system. The results showed that the metal attaches to the actuator surface while the polymer does not; they also indicate the optimal factors contributing to the adhesion process. In the second part related to sorting application, the granular materials behavior and the influencing factors are explored in an inclined vibrating system to process a Cu/PVC mixture. For an inclination of 10°, Cu recovery on the actuator exceeds 98% with a purity of about 99%.

The study of slide-valve throttle orifice’s geometry influence on control surface actuator’s dynamical capabilities of maneuverable aircraft

The study of slide-valve throttle orifice’s geometry influence on required dynamical capabilities at the low angular deflection of maneuverable aircraft’s actuator has been carried out. To find the correlation dependences of slide-valve throttle control area opening to its transfer, the six configurational forms of fluid flow throttle regulation orifices were considered, the geometrical dependences were obtained and mathematical model were designed. The Bode plots of control surface actuator at six different geometrical forms of orifices have been achieved. Main findings of slide-valve orifice influence on the actuator dynamical sensitivity have been stated. The obtained results have a theoretical benefit for the designers of aircraft control systems and actuators.

An adaptive control design for dynamic positioning of unmanned surface vessels having actuator dynamics

An adaptive control design for dynamic positioning (DP) of unmanned surface vessels is addressed in this study. The control design is realized by taking actuator dynamics into account for obtaining a DP system where distribution of generalized forces to the actuators is considered. Owing to this approach DP system is made suitable for unmanned surface vessels having actuators equal to or more than number of generalized forces. All parameters of the surface vessel's body and actuator dynamics except thrust configuration matrix are assumed to be uncertain for the proposed control design. The parametric uncertainties are coped with adaptive compensation rules while the designed controller is equipped with a term that provides the robustness of the designed controller against disturbances including internal and external unmodeled effects. Backstepping control design approach is preferred for the control design process by considering the appropriateness of this approach for the system model of unmanned surface vessels include actuator dynamics. Theoretical proof of the designed controller is realized via Lyapunov–based arguments. Simulation studies where the efficiency of the designed controller is observed for both of aforementioned cases are conducted for the performance demonstration of the designed controller.

Tollmien–Schlichting Wave Control on an Airfoil Using Dynamic Surface Modification

The present study demonstrates experimental mitigation of Tollmien–Schlichting waves on an unswept natural laminar flow airfoil at a chord-based Reynolds number of . This was facilitated by dynamic surface modifications using piezoelectrically driven oscillating surface actuators. The actuators were located at three streamwise locations on the airfoil’s suction side. The upstream actuator introduced a disturbance to the flow, which phase locked the Tollmien–Schlichting waves. The downstream actuators were used to mitigate the induced Tollmien–Schlichting waves by introducing an antiphase disturbance with the proper amplitude. The disturbances included either a single-frequency or multiple-frequency input signals. The Tollmien–Schlichting waves were mitigated in both open- and closed-loop control schemes. In open-loop control, the results demonstrated that dynamic surface modification can be used to mitigate the waves, even in the presence of a large adverse gradient for varying frequency bandwidths. In addition, the closed-loop scheme, using an iterative learning control algorithm, succeeded in reducing the induced disturbance amplitudes to 11%. In addition, the experiments demonstrated the ability to not only mitigate the wave but also amplify it if transition to turbulence is desired.

Identification and assessment of a nonlinear dynamic actuator model for controlling an experimental flexible wing

In this paper, the effect of nonlinear actuator dynamics on the performance of an active load alleviation system for an experimental flexible wing is studied. Common nonlinearities such as backlash or rate limits are considered for the control surface actuator. An aeroelastic simulation model of a flexible wing with control surface is being used. With this, a parameter study is carried out to quantify the impact of the individual nonlinearities on the overall closed-loop performance by means of describing functions. Finally, the nonlinear actuator model with parameters identified from dedicated tests is experimentally validated allowing for an accurate prediction of the expected gust load alleviation performance.

Actuator Surface Model with Computational-Fluid-Dynamics-Convected Wake Model for Rotorcraft Applications

An actuator surface model has been developed that couples an aerodynamics model to a computational fluid dynamics solver. The model is designed for simulation of rotors in complex operating environments, and it advances a recently developed actuator surface model through the addition of two new elements: a computational fluid dynamics (CFD)-convected wake model, and an improved coupling algorithm. The wake model uses Lagrangian particle tracking to determine the geometry of the rotor wake from the CFD solution. Prior study of the influence of the maximum wake age in the model led to the conclusion that only 45 deg of wake is required for accurate resolution of air loads. The S-76 rotor in hover and the UH-60A rotor in forward flight are used for validation. Thrust coefficients, torque coefficients, and figures of merit over a range of collective settings agree well with experimental data for the S-76 rotor. Sectional normal force coefficients in forward flight are compared with experimental data, results from resolved-blade simulations, and other rotor models: again showing good agreement. These results suggest the new actuator surface model can predict time-accurate rotor loads where accurate resolution of the near-blade flowfield and high-frequency loads, such as blade–vortex interactions, are not critical.

Modeling and Control of a Modular Iron Bird

This paper describes the control architecture and the control laws of a new concept of Modular Iron Bird aimed at reproducing flight loads to test mobile aerodynamic control surface actuators for small and medium size aircraft and Unmanned Aerial Vehicles. The iron bird control system must guarantee the actuation of counteracting forces. On one side, a hydraulic actuator simulates the hinge moments acting on the mobile surface due to aerodynamic and inertial effects during flight; on the other side, the actuator to be tested applies an active hinge moment to control the angular position of the same surface. Reference aerodynamic and inertial loads are generated by a flight simulation module to reproduce more realistic conditions arising during operations. The design of the control action is based on a dynamic model of the hydraulic plant used to generate loads. This system is controlled using a Proportional Integral Derivative control algorithm tuned with an optimization algorithm taking into account the closed loop dynamics of the actuator under testing, uncertainties and disturbances in the controlled plant. Numerical simulations are presented to show the effectiveness of the proposed architecture and control laws.

ALFA: A dataset for UAV fault and anomaly detection

We present a dataset of several fault types in control surfaces of a fixed-wing unmanned aerial vehicle (UAV) for use in fault detection and isolation (FDI) and anomaly detection (AD) research. Currently, the dataset includes processed data for 47 autonomous flights with 23 sudden full engine failure scenarios and 24 scenarios for 7 other types of sudden control surface (actuator) faults, with a total of 66 minutes of flight under normal conditions and 13 minutes of post-fault flight time. It additionally includes many hours of raw data of fully autonomous, autopilot-assisted and manual flights with tens of fault scenarios. The ground truth of the time and type of faults is provided in each scenario to enable evaluation of the methods using the dataset. We have also provided the helper tools in several programming languages to load and work with the data and to help the evaluation of a detection method using the dataset. A set of metrics is proposed to help to compare different methods using the dataset. Most of the current fault detection methods are evaluated in simulation and, as far as we know, this dataset is the only one providing the real flight data with faults in such capacity. We hope it will help advance the state of the art in AD or FDI research for autonomous aerial vehicles and mobile robots to enhance the safety of autonomous and remote flight operations further. The dataset and the provided tools can be accessed from https://doi.org/10.1184/R1/12707963.

A Fault-Tolerant Control Method for Unmanned Aerial Vehicle (UAV) Formation with Topological Faults Considered

A fault-tolerant control method for the UAV formation that has topological fault, control surface fault, actuator fault and uncertainty is proposed. Firstly, the formation motion model and the UAV motion model are established. Then based on the topological fault detection method, the topological fault reconstruction and optimization algorithm is proposed to realize the formation topological fault reconstruction and optimization with minimum communication cost and formation reconstruction cost. In designing the backstepping fault-tolerant control method, the actuator's fault identification module and the auxiliary system module are used to estimate and compensate for the faults of the actuator and the control surface respectively so as to realize the stable flight of the UAV formation under the conditions of actuator fault, control surface fault and uncertainty. The simulation results verify the superiority of the fault-tolerant control method for the UAV formation with topological faults.

Towards Detached-Eddy Simulation of Flow Around Rotorcraft Using Immersed Boundary Method

A coupled immersed boundary method and unsteady actuator surface model (IBM–ASM) is developed and implemented into a finite-volume-based incompressible Navier–Stokes solver within OpenFOAM for efficient full rotorcraft simulations. Given a geometry definition, a simple Cartesian mesh generated through a fully automated process can be used for an immersed boundary method (IBM) since it is a non-body-conformal mesh approach that incorporates geometrical complexities using momentum forcing to enforce its boundary condition onto the flowfield. Detached-eddy simulation is employed for the turbulence modeling, and an appropriate wall function is defined and implemented as needed for efficient high-Reynolds-number computations. Also, it is demonstrated that a total-variation-diminishing reconstruction must be used to produce physically reasonable results. Finally, an actuator surface model is integrated with the IBM solver, concluding the overall IBM–ASM methodology to address rotorcraft problems. Detailed verification and validation of the IBM solver are demonstrated for a low-Reynolds-number cylinder, a turbulent flat plate, and a high-Reynolds-number cylinder. Lastly, the IBM–ASM solver is validated against experimental measurements for the flow around a simplified airframe developed by the Georgia Institute of Technology. Unsteady rotor-wake/fuselage interactions in a forward flight condition are qualitatively and quantitatively analyzed. Results are competitive with existing best methods using a fraction of the setup and computational effort of the body-conforming method.

Surface Actuation and Sensing of a Tensegrity Structure Using Robotic Skins.

Tensegrity robots comprising solid rods connected by tensile cables are of interest due to their flexible and robust nature, which potentially makes them suitable for uneven and unpredictable environments where traditional robots often struggle. Much progress has been made toward attaining locomotion with tensegrity robots. However, measuring the shape of a dynamic tensegrity without the use of external hardware remains a challenge. Here we show how robotic skins may be attached around the exterior of a tensegrity structure, to both control and measure its shape from its surface. The robotic skins are planar, skin-like membranes with integrated actuators and sensors, which we use to transform a passive tensegrity structure into an active tensegrity robot that performs tasks such as locomotion. In addition, sensors placed on the ends of the tensegrity rods are used to directly measure orientation relative to the ground. The hardware and algorithms presented herein thus provide a platform for surface-driven actuation and intrinsic state estimation of tensegrity structures, which we hope will enable future tensegrity robots to execute precise closed-loop motions in real-world environments.

Experimental Comparison of the Wake of a Vertical Axis Wind Turbine and Planar Actuator Surfaces

Wind tunnel experiments on a scaled vertical axis wind turbine (VAWT) and square porous plate with a porosity of 64% are conducted in the W-tunnel of TU-Delft. The VAWT thrusts in axial and lateral directions are measured with an in-house load cell system based on moment conservation. Wake of the VAWT in tip speed ratio of 1.5 and 2.5 and the porous plate is measured with the robotic particle image velocimetry technique, which enables a three-dimensional velocity measurement in a combined volume encompassing from 1 diameter upstream to 3 diameters downstream. Counter-rotating vortex pairs in VAWT wake and the wake shape deformation and deflection are discussed, which are related to the lateral thrust. A square porous plate inducing a similar axial thrust is compared, which has the same shape as the cross-section of the VAWT. Wake of the right porous plate with a yaw angle of 15° is investigated, which produces similar deflection as the VAWT.