Individual Blade Control Research Articles

Reducing aerodynamic vibration of rigid rotors with retreating side active control avoidance

This paper proposes a new approach to eliminate aerodynamic lift oscillation, called the Dominant Sector Individual Blade Control (DS-IBC) method for rigid rotor helicopters. An Advancing Blade Concept (ABC) rotor model for aerodynamic analysis based on the free-wake method is applied. DS-IBC avoids applying active control on the rotor’s retreating side by employing and restricting active control inputs to a sector area of the rotor disc. Outside this sector, only primary collective and cyclic pitch control are used. Each blade takes turns entering the sector, creating a “relay” active control form to ensure continuous control inputs. The method also includes outer-trim and inner-trim iteration modules. Results show that DS-IBC can eliminate aerodynamic lift oscillation using much smaller control inputs than the sine-trim method. By focusing active control on the rotor’s advancing side, DS-IBC improves the effective lift-to-drag ratio and reduces the implementation difficulty of active rotor control for aerodynamic oscillation elimination, especially at a large lift-offset.

Individual Blade Control Approach for Active Vibration Suppression of a Lift-Offset Coaxial Rotorcraft

This study explores the best vibration reduction using an individual blade control (IBC) actuation scheme for a lift-offset coaxial helicopter in high-speed flight. The rotorcraft dynamics analysis model consists of coaxial, three-bladed counter-rotating rotors and a finite element fuselage stick model constructed based on the measured data of the XH-59A helicopter. The XH-59A helicopter is well-known for its severe vibration level encountered during the flight (over 0.5g), leading to the cancellation of the development program. A special focus is given to assessing the accuracy and efficiency of the integrated rotor–body vibration predictions evaluated between the one-way and two-way coupling methods in reference to the flight data of the vehicle. The two-way rotor–body coupled results show excellent correlations with the test data for rotor blade loads and airframe vibrations. The best actuation scenarios are then sought for the minimum vibration at the rotor hub and the pilot seat. The IBC pitch actuation effectively reduces the vibrations at both locations of the rotorcraft. Specifically, a multiple-harmonic IBC actuation enables to suppress the pilot seat vibration by 93% compared to the uncontrolled case, achieving a significantly reduced vibration level (below 0.05g).

Surrogate Modeling and Aeroelastic Analysis of a Wind Turbine with Down-Regulation, Power Boosting, and IBC Capabilities

As the maturity and complexity of wind energy systems increase, the operation of wind turbines in wind farms needs to be adjustable in order to provide flexibility to the grid operators and optimize operations through wind farm control. An important aspect of this is monitoring and managing the structural reliability of the wind turbines in terms of fatigue loading. Additionally, in order to perform optimization, uncertainty analyses, condition monitoring, and other tasks, fast and accurate models of the turbine response are required. To address these challenges, we present the controller tuning and surrogate modeling for a wind turbine that is able to vary its power level in both down-regulation and power-boosting modes, as well as reducing loads with an individual blade control loop. Two methods to derive the setpoints for down-regulation are discussed and implemented. The response of the turbine, in terms of loads, power, and other metrics, for relevant operating conditions and for all control modes is captured by a data-driven surrogate model based on aeroelastic simulations following two regression approaches: a spline-based interpolation and a Gaussian process regression model. The uncertainty of the surrogate models is quantified, showing a good agreement with the simulation with a mean absolute error lower than 4% for all quantities considered. Based on the surrogate model, the aeroelastic response of the entire wind turbine for the different control modes and their combination is analyzed to shed light on the implications of the control strategies on the fatigue loading of the various components.

Fuzzy Neural Network PID Control Used in Individual Blade Control

In order to further reduce the vibration level of helicopters, the active vibration control technology of helicopters has been extensively studied. Among them, individual blade control (IBC) independently applies high-order harmonics to each blade with an actuator, which can improve the aerodynamic environment of the blade and effectively reduce the vibration load of the hub. The rotor structural dynamics model based on the Hamilton energy variation principle and the medium deformation beam theory were established firstly, and the aerodynamic model based on the dynamic inflow model and the Leishman–Beddoes unsteady aerodynamic model were also established. The structural finite element method and the direct numerical integration method were used to calculate the vibration response of the rotor to determine the vibration load of the hub. After these, the steepest descent-golden section combinatorial optimization algorithm was used to find the optimization parameters of IBC. Based on this, the input parameters of fuzzy neural network PID control were determined, and the rotor hub vibration load control simulation was conducted. Under the effect of IBC, the vibration loads of the hub could be reduced by about 60%. The article gives the best control laws of individual harmonic pitch control and their combinations. These results can theoretically be applied to the design of control law to reduce helicopter vibration loads.

중형기동 회전익기 진동 하중 감소를 위한 선형 개별 블레이드 제어 법칙 설계 및 시뮬레이션

In the recent decades, rotorcrafts have been examined and subjected to experimentation for higher harmonic control (HHC) for hub vibratory load reduction through hydraulic swashplate actuation, or individual blade control (IBC). This paper presents a design of a linear quadratic Gaussian (LQG) regulator based on the LTI representation of HART-II rotor and evaluate its controlled stability and sensitivity. Specifically, a linear quadratic control synthesis for vibration reduction using the revised IBC control inputs of N/rev and (N±1)/rev, which will enable a linear time-invariant identification of a rotor, is presented. The following aspects are detailed: (1) a multi-body aeroelasitc rotor analysis of the baseline HART-II and UH-60A rotor; (2) the LTI identification method; and (3) the design and simulation of LQG closed-loop vibratory load control law.

Computational Fluid Dynamics Modeling Analysis of a Martian Rotorcraft With Individual Blade Control

Abstract Computational fluid dynamics (CFD) analysis was conducted on a proposed blade root-actuated individual blade control (IBC) system for future Martian rotorcraft. IBC offers many potential benefits to rotary-winged exploration of Mars, including precision control of rotor blade forces. This study seeks to provide an estimate of rotor blade force and system power as a basis for concept feasibility analysis and experimental prototyping. ansys fluent was used to compute blade pitching moment, lift, and drag under various feathering waveforms, amplitudes, biases, and frequencies. It is determined that the rapid feathering characteristic of IBC has a non-negligible impact on blade forces. It is also found that actuators with power ratings on the order of 101 W are likely sufficient for blade actuation on Martian rotorcraft.

Reducing Helicopter Vibration Loads by Individual Blade Control with Genetic Algorithm

A rotor that can realize individual blade pitch control was designed. This paper focuses on finding the trend of helicopter vibration loads after applying multiple high-order harmonic control. The Glauert inflow model was introduced to calculate the induced velocity of rotor blades in a rotor disk plane, and the Leishman Beddoes (L-B) unsteady dynamic model was employed to calculate the aerodynamic forces of each section of a rotor blade. It was found that the influence of each high-order harmonic control on individual blade vibration load reduction is similar in different advanced ratios. After these calculations, the genetic algorithm was used to calculate the best combination of amplitude and phase of the higher order harmonic under a specific flight state. Under the effect of high harmonic input, the vibration loads of the hub could be reduced by about 65%. These results can be theoretically applied to design control law to reduce helicopter vibration loads.

Bionic Intelligent Algorithms Used in Helicopter Individual Blade Control Optimization

Bionic algorithms are established by imitating human neural structures and animal social behaviors. As an important part of bionic technology, bionic algorithms are often used to solve the control problems of complex nonlinear systems, such as the rotor aeroelasticity dynamics model used in the helicopter individual blade control (IBC) optimization process. Two control methods based on bionic intelligent algorithms are introduced, respectively. The first method is to combine the fuzzy neural network and the classical PID control together. Compared with traditional PID control, the combined one was able to adjust the PID control parameters automatically by using the learning ability of the fuzzy neural network. The second method is to directly search the optimal control parameters by using the particle swarm algorithm. Both two methods demonstrate higher efficiency and accuracy; according to the results obtained by the algorithms, the vibration level was 80% less than without the applied high order harmonics. This indicates great application prospects for bionic intelligent algorithms in solving complex nonlinear system problems.

Effect of active camber morphing on rotor performance and control loads

A computational investigation of a helicopter main rotor with an active-camber morphing mechanism was conducted to identify the capability to simultaneously save rotor power and reduce pitch-link loads using as low as possible camber deflection magnitudes. Comprehensive rotor aeromechanics analysis with elastic blade modeling and a free vortex wake for the aerodynamics model was used to ensure computational efficiency. The investigation was based on a full-scale Bo 105 helicopter main rotor in level flight condition at μ=0.3 and CT/σ=0.089. In addition to a variation of the radial position and length of the active-camber section, the capabilities and system behavior were investigated in an extensive parametric study using practical actuation inputs. The same computational framework was used to obtain optimal control inputs that led to best performance in terms of power savings using two-per-rev (2P) individual blade control (IBC) via pitch-link inputs and 2P active-twist control. The relative potential of the three active mechanisms and the aerodynamic phenomenology was compared. All of the investigated active-rotor mechanisms contributed to rotor power savings via a more uniform distribution of thrust over the rotor disk. Both IBC and active twist yielded maximum performance improvement of about 1.8% over the baseline in terms of power reduction. With active camber, simultaneous rotor power reduction of 3.8% and a peak-to-peak pitch-link load reduction of 20% were obtained using a nonharmonic camber deflection deployment schedule with a half-peak-to-peak magnitude of 2∘. The global maximum of power savings observed with active camber was 4.4%, which resulted in an increase of peak-to-peak pitch-link loads by 35% and a required half-peak-to-peak camber deflection magnitude of 3.6∘.

Investigation of Linear Higher Harmonic Control Algorithm for Rotorcraft Vibration Reduction

Abstract A linear quadratic Gaussian (LQG) controller for active vibratory loads reduction in helicopters is proposed based on a revisited higher harmonic control (HHC) input by active trailing-edge flaps (ATEFs). Conventional individual blade control (IBC) input is redefined using N − 1/rev interblade phase lead, N/rev collective, and N + 1/rev interblade phase lag signals where 1/rev frequency modulation originates from the multiblade coordinate (MBC) transform. A Mach-scaled flap blade is designed and analyzed by the multibody dynamics analysis DYMORE. A linear time-invariant representation is identified from N/rev envelopes of the input and output responses obtained by DYMORE analysis. A matlab/simulink closed-loop control simulation is designed using the identified state-space realization. The N/rev vibratory loads are reduced up to 52% with flap deflections and the linear control results match well with the nonlinear responses obtained from DYMORE. Furthermore, the multivariable closed-loop stability estimated by the loop transfer functions using disk margin analysis reveals sufficient gain and phase margins.

Improved sine-trim2 model for the simultaneous elimination of aerodynamic lift, roll, and pitch moment vibrations of rigid rotor helicopters

An improved sine-trim method called sine-trim2 is proposed from the original sine-trim method, based on the phasor superposition principle, to simultaneously eliminate the aerodynamic lift, roll, and pitch moment vibrations of rigid rotor helicopters. The advancing blade concept rotor aerodynamic analysis model is modified and applied on the basis of the free-wake method associated with the inner-trim and outer-trim modules. A hybrid individual blade control system with the blade pitch and linear twist is applied to simultaneously adjust the blade lift and lift arm at each azimuth angle. In the sine-trim2 method, the number of rotor blades should be greater than or equal to four to successfully apply the phasor superposition principle for all kinds of aerodynamic vibration phasors of each blade. Two kinds of lift arm distributions (constant lift arm and sinusoidal lift arm) in one revolution that can both meet the requirements of vibration elimination are considered. Calculation results show that although the sine-trim2 method can eliminate the aerodynamic lift, roll, and pitch moment vibrations of the rigid rotor with either lift arm distribution. A proper sinusoidal lift arm as the control target will have lower control difficulty and efficiency loss than the constant lift arm. The optimal individual blade pitch and twist control distributions, which are both comprehensive superpositions of many higher harmonic inputs, are obtained in each azimuth in one rotation revolution.

개별 블레이드 제어(IBC) 기법을 이용한 동축반전 회전익기의 진동하중 억제에 관한 연구

본 연구에서는 능동적인 블레이드 제어기법인 개별 블레이드 제어(Individual Blade Control, IBC) 기법을 적용하여 고속비행 시 동축반전 회전익기의 허브 진동하중을 억제하기 위한 최적 제어입력을 탐색하였다. 통합 공탄성 해석 프로그램인 CAMRAD II를 이용하여 동축반전 회전익기인 XH-59A를 모델링하고 다양한 IBC 입력 조건에 대하여 파라미터 연구를 수행하였다. 파라미터 조절 연구를 통하여 허브 진동억제 성능을 구한 결과, 3/rev 가진 주파수의 0.5° 진폭에 300° 위상각을 갖는 IBC 제어 입력을 적용할 경우 기준 대비 진동 수준이 최대 50% 감소하는 것을 확인하였다. 진동 억제 성능은 후류 간섭에서 보다 자유로운 상부로터에서 6% 가량 하부로터보다 크게 나타났다. 로터의 전진면에서만 IBC 입력를 가진하는 경우에는 조화 가진 입력과 동일한 입력을 가할 경우 진동 수준이 최대 17% 정도 추가적으로 감소하는 것을 확인하였다. 이러한 진동 감소는 전진면만을 대상으로 적은 에너지 투입 비용으로 달성한 특징이 있다.

Linear Analysis of Vibration Reduction Using an Active Helicopter Rotor

In the field of active rotor control, the use of individual blade control (IBC) systems for vibration reduction is a major topic of research. However, as most studies are focused on a specific helicopter rotor, the influence of topological rotor parameters, such as the number of rotor blades, is seldom investigated. The present investigation analyzes vibration reduction using an IBC system in a very general manner with particular focus on the influence of the number of rotor blades. To this end, a model for the quasi-steady transfer behavior of an IBC rotor with an arbitrary number of rotor blades is derived as a function of the blade transfer behavior using only a minimal set of assumptions. Using this model, a minimal set of blade-root load components relevant for the rotor hub loads is determined. Based on this set of components, a coordinate frame of effective blade-root loads is introduced. Subsequently, the vibration reduction potential for the -bladed rotor is assessed, showing no direct relation between the number of rotor blades and the vibration reduction potential. Finally, the assumptions for the model, as well as the results of the investigation of the vibration reduction potential, are validated using open-loop flight data.

Down-regulation and individual blade control as lifetime extension enablers

As more and more wind turbines are coming close to the end of their design lifetime, evaluation of end of life strategies is becoming highly relevant. Moreover, as turbine technology matures and wind farms grow larger, lifetime extension becomes a financially attractive option compared to re-powering and decommissioning. Present work suggests control strategies, namely down-regulation and individual blade control, as lifetime extension enablers. The concept of using them as retrofit control implementations is explained. Their individual and combined potential in fatigue load reduction is evaluated, along with their effect on other performance and pitch system metrics. Finally, the possible period of extension, beyond the nominal 20 years, is evaluated in an example case where the retrofit control strategy is applied after 15 years of baseline operation. The aeroelastic simulations are performed with a 10 MW reference wind turbine, according to load certification standards. Results show that the two methods complement each other in load alleviation. The pitch actuator demands are also significantly decreased when the two methods are combined.

The second wind-tunnel test of DLR’s multiple swashplate system

After proving its individual blade control (IBC) capabilities on a four-bladed rotor system in the wind tunnel, the DLR’s multiple swashplate system and its control software were developed further to allow IBC operation on a five-bladed rotor system. After preliminary tests in hover conditions, a second wind-tunnel test was performed in the DNW-LLF wind tunnel in late 2016. The goal of this test on a Mach-scaled model rotor was to reduce vibration, noise, and required rotor power in different flight conditions using proven IBC strategies as well as localized pitch control (LPC) on five blades. In descent flight condition, significant reductions of blade–vortex interaction noise relative to the baseline case were achieved on both sides of the rotor disk through the application of 2/rev higher harmonic control as well as using an LPC schedule. Through the application of 3/rev IBC as well as a vibration controller using a 4–6/rev multi-harmonic IBC signal, 5/rev hub loads were reduced significantly and the 5/rev vertical vibrations were nearly eliminated. In addition, during simulated high-speed-level flight with a wind speed of 76 m/s, the required rotor power was successfully reduced using a 2/rev input with an amplitude of 1 $$^\circ$$ .

Investigation on the potential of individual blade control for lifetime extension

In recent years the focus of wind energy industry is on reducing levelized cost of energy by rotor upscaling. Moreover, a current topic of interest to both industry and academia is the extension of lifetime to existing wind turbines approaching the end of initial design span. Thus, the need for load alleviation technologies integrated in the design process or for retrofit purposes is becoming more relevant. One of these is individual blade pitch control, a recurring topic in research, with known advantages and weaknesses namely the pitch actuator and bearing wear. The present work suggests such a system incorporating three independent controllers with input the root bending moments on the rotating frame. The linear system used for controller design is based on black box identification of non-linear simulations and filters are used both for the input and output. Different setups of the independent blade control scheme are applied on a 10 MW reference turbine, with a large and highly flexible rotor representative of the current industrial status, under wind conditions as defined by relevant certification standards. The investigation aims on evaluating the system’s performance based on the fatigue load alleviation potential for different components as well as identifying the tradeoff for each design choice. Finally, based on basic assumptions the reductions are translated to possible life time extension for each component based on a combined operation where the new controllers are applied for a percentage of the initial 20 year lifetime.

VPM/CFD-Based Research on Rotor Performance and Loads of Individual Blade Control Rotor System

VPM/CFD-Based Research on Rotor Performance and Loads of Individual Blade Control Rotor System

The First Wind Tunnel Test of the Multiple Swashplate System: Test Procedure and Principal Results

In September 2015, the DLR completed the first wind tunnel test of its patented Multiple Swashplate System in the DNW's Large Low-Speed Facility in the Netherlands. During these tests, the potential of this new active rotor control system to effectively reduce noise, vibration, and power consumption using several individual blade control strategies, including 2/rev inputs, was successfully demonstrated on two different model rotors without using actuators in the rotating frame. For a Mach-scaled Bo105 model, rotor power reductions of up to 4% in level flight and reductions of the weighted 4/rev vibratory hub loads of up to 77% in a descent flight condition were measured, and BVI noise levels were reduced by up to 4.5 dB by the application of 3/rev higher harmonic control, confirming findings from the HART II test. For the second model rotor using a contemporary blade geometry, the maximum reductions were measured at 3%, 52%, and 3.9 dB for power, vibration, and BVI noise, respectively.

On-Blade Control of Rotor Vibration, Noise, and Performance: Just Around the Corner? The 33rd Alexander Nikolsky Honorary Lecture

A concise review of the active control approaches for vibration reduction in rotorcraft is presented. Next, the evolution and status of higher harmonic control and pitch link actuated individual blade control is presented since these serve as the foundations of on blade control. Despite the success of these approaches, demonstrated by both full-scale wind tunnel and flight tests, higher harmonic control and pitch link actuated individual blade control have not managed to earn their way onto a production helicopter. An alternative, on blade control, is defined as a special implementation of individual blade control, where the control surfaces are located on the rotating blade and each blade has its own controller. A concise description of four on blade devices: (1) the actively controlled flap, (2) the active twist rotor, (3) the active tip rotor, and the (4) deployable Gurney flap, or microflap, is presented. An outline of an aeroelastic response modeling capability used to simulate active vibration and noise reduction using flaps or microflaps is presented. The simulation is a thread that links the various parts of the paper. Next, selected results from simulations and scale wind tunnel model tests on active flaps are used to provide insight on the operational and modeling aspects of these systems. Full-scale wind tunnel and flight tests are presented as culmination of the research effort invested in active flap rotors. Then, the evolution and application of the active twist rotor, and deployable Gurney flaps, or microflaps, is presented. The paper concludes with lessons learned and speculation about the potential implementation of on blade control on production rotorcraft.

Combined wind turbine fatigue and ultimate load reduction by individual blade control

If each blade of the wind turbine has individual pitch actuator, there is possibility of employing the pitch system to mitigate structural loads through advanced control methods. Previously, considerable reduction of blade lifetime equivalent fatigue loads has been achieved by Individual Blade Control (IBC) and in addition, it has also been shown the potential in blade ultimate loads reduction. However, both fatigue and ultimate loads impact on the design and life of wind turbine blades. In this paper, the design and application of IBC that concurrently reduce both blade fatigue and ultimate loads is investigated. The contributions of blade load spectral components, which are 1P, 2P and edgewise mode from blade in-plane and/or out-of-plane bending moments, are firstly explored. Four different control options for reducing various combinations of these load components are compared. In response to the different spectral peaks of both fatigue and ultimate loads, the controller has been designed so that it can act on different frequency components which vary with wind speed. The performance of the IBC controller on fatigue and ultimate load reduction is assessed by simulating a 5MW exemplar wind turbine. Simulation results show that with a proper selection of controlling inputs at different wind speed, the use of a single combined IBC can achieve satisfactory reduction on both fatigue and ultimate loads.