Global reconstruction of nonlinear systems from families of linear systems

This note concerns a fundamental issue in the modelling and realisation of nonlinear systems; namely, whether it is possible to uniquely reconstruct a nonlinear system from a suitable collection of transfer functions and, if so, under what conditions. It is established that a family of frozen-parameter linearisations may be associated with a class of nonlinear systems to provide an alternative realisation of such systems. Nevertheless, knowledge of only the inputoutput dynamics (transfer functions) of the frozen-parameter linearisations is insufficient to permit unique reconstruction of a nonlinear system. The difficulty with the transfer function family arises from the degree of freedom available in the choice of state-space realisation of each linearisation. Under mild structural conditions, it is shown that knowledge of a family of augmented transfer functions is sufficient to permit a large class of nonlinear systems to be uniquely reconstructed. Essentially, the augmented family embodies the information necessary to select state-space realisations for the linearisations which are compatible with one another and with the underlying nonlinear system. The results are constructive, with a state-space realisation of the nonlinear system associated with a transfer function family being obtained as the solution to a number of linear equations

Control design toolbox for large scale variable speed pitch regulated wind turbines

The trend towards large multi-MW wind turbineshas given new impetus to the development of wind turbine controllers.Additional objectives are being placed on the controllermaking the specification of the control system more complex. A new toolbox, which assists with most of the control design cycle,has been developed. Its purpose is to assist and guide the control system designer through the design cycle, thereby enabling faster design. With the choice of control strategy unrestricted,the toolbox is sufficiently flexible to support the design processfor the aforementioned more complex specifications

Control of sideslip and yaw rate in 4-wheel steering car using partial decoupling and individual channel design

This paper presents a new steering control structure for cars equipped with 4-wheel steering. This control structure is based on a simplified linear model of the lateral dynamics of such cars and aims to decouple the control of sideslip from the control of yaw rate. The control design is based on a linear multivariable plant which incorporates the model of the lateral dynamics mentioned above and whose inputs are linear combinations of the front and rear steering angles. The plant also contains a cross-feedback element. The matrix transfer function of the resulting plant is upper-triangular (partially decoupled). The MIMO design problem can then be recast as two SISO design problems using channel decomposition according to the Individual Channel Design (ICD) paradigm. The proposed control structure has been applied to design sideslip and yaw rate controllers using a more accurate model of the lateral dynamics of 4-wheel steering cars. This model incorporates the tyre force dynamics and the steering actuators. Simulations are used to illustrate the performance and robustness of the designed controllers

Analysis of tower/blade interaction in the cancellation of the tower fore-aft mode via control

With the increase in size of wind turbines, there is increasing interest in exploiting the pitch control capability of variable speed turbines to alleviate tower fatigue loads. The most direct method is to modify the blade pitch angle in response to a measurement of tower acceleration. It is shown that the ap mode has a central role in determining whether this approach is effective since there is a strong interaction between the blade ap-wise mode and the tower fore-aft mode. Several different approaches to the design of the controller for the tower speed feedback loop are investigated. It is concluded that a reduction in the tower loads of up to 18% is possible for multi-megawatt sized wind turbines

Gyrotorque transmission system for wind turbines

The GyroTorqueTM transmission system employs gyroscopic torque reaction to transmit power offering an alternative to the gearbox and electrical variable speed drive of a conventional wind turbine. The power transmission is fundamentally oscillatory and is rectified by mechanical elements. A precessing gyro maps speed to torque and, since the wind turbine rotor inertia strongly filters rotor speed variation, output power is insensitive to wind turbulence because it reflects wind turbine rotor speed variability rather than rotor torque variability. The GyroTorqueTM system has only bearing losses and potentially a high efficiency. Mechanical control of the input to the GyroTorqueTM system enables wide range variable speed operation of the wind turbine rotor using a conventional synchronous generator. At present, a 6 gyro system driven by an axial cam and connected to a conventional synchronous generator is the preferred system. Loads and power quality have been addressed with computer simulation models of the GyroTorqueTM system. Outline assessment of system mass and cost gives encouragement that it may be less than for conventional transmission systems

Alleviation of unbalanced rotor loads by single blade controllers

A novel approach to reducing the unbalance rotor loads by pitch control is presented in this paper. Each blade has its own actuator, sensors and controller. These localised blade control systems operate in isolation without need of communication with each other. This single blade control approach to regulation of unbalanced rotor loads has several advantages including being straightforward to design and easy to tune. Furthermore, it does not affect the operation of the central controller and the latter need not be re-designed when used in conjunction with the single blade controllers. Their performance is assessed using BLADED simulations

Wind turbine rotor acceleration : identification using Gaussian regression

Gaussian processes prior model methods for data analysis are applied to wind turbine time series data to identify both rotor speed and rotor acceleration from a poor measurement of rotor speed. In so doing, two issues are addressed. Firstly, the rotor speed is extracted from a combined rotor speed and generator speed measurement. A novel adaptation of Gaussian process regression based on two independent processes ratherthan a single process is presented. Secondly, efficient algorithms for the manipulation of large matrices are required. The Toeplitz nature of the matrices is exploited to derive novel fast algorithms for the Gaussian process methodology that are memory efficient

Application of velocity-based gain-scheduling to lateral auto-pilot design for an agile missile

In this paper a modern gain-scheduling methodology is proposed which exploits recently developed velocity-based techniques to resolve many of the deficiencies of classical gain-scheduling approaches (restriction to near equilibrium operation, to slow rate of variation). This is achieved while maintaining continuity with linear methods and providing an open design framework (any linear synthesis approach may be used) which supports divide and conquer design strategies. The application of velocity-based gain-scheduling techniques is demonstrated in application to a demanding, highly nonlinear, missile control design task. Scheduling on instantaneous incidence (a rapidly varying quantity) is well-known to lead to considerable difficulties with classical gain-scheduling methods. It is shown that the methods proposed here can, however, be used to successfully design an effective and robust gain-scheduled controller

Collective control strategy for a cluster of stall-regulated offshore wind turbines

The power converter is one of the most vulnerable components of a wind turbine. When the converter of an offshore wind turbine malfunctions, it could be difficult to resolve due to poor accessibility. A turbine generally has a dedicated controller that regulates its operation. In this paper, a collective control approach that allows a cluster of turbines to share a single converter, hence a single controller, that could be placed in a more accessible location. The resulting simplified turbines are constant-speed stall-regulated with standard asynchronous generators. Each cluster is connected by a mini-AC network, whose frequency can be varied through a centralised AC-DC-AC power converter. Potential benefits include improved reliability of each turbine due to simplification of the turbines and enhanced profit owing to improved accessibility. A cluster of 5 turbines is assessed compared to the situation with each turbine having its own converter. A collective control strategy that acts in response to the poorest control is proposed, as opposed to acting in response to the average control. The strategy is applied to a cluster model, and simulation results demonstrate that the control strategy could be more cost-effective than each turbine having its own converter, especially with optimal rotor design

Database management for high resolution condition monitoring of wind turbines

Wind turbine condition monitoring (CM) is an area of research which has been receiving a large amount of attention in the recent years. This has been influenced mainly by the recent uptake of wind farms being installed around the country. Operational and maintenance expertise in the area of wind turbine CM is therefore seen to be of growing importance but is yet to be well established in the industry due to its unverified economic benefits. The majority of the research which can be found in the literature has been based on simulation or test rig data, often due to the lack of availability of extensive historical data sets containing 'interesting' events, as well as the difficulties associated in gaining access to such data, due to its commercially sensitive nature. It can not be readily claimed, nor shown that laboratory based testing or simulations actually reflect real turbine operation, due to scaling, control and dynamic considerations. In order that different patterns of machine deterioration can be determined and detected in their incipient stages, precise high resolution data, of existing monitored parameters, should be sampled at frequencies higher than that is typically available in the integrated SCADA systems installed in most modern turbines today. This paper reports the design of a data acquisition platform which will be mounted on a 660 kW VESTAS V47 wind turbine. Details of the monitoring equipment used, the installation requirements as well as the system architecture will be presented and discussed