The balance sheet of agriculture, 1954

Agriculture - Economic aspects

Selecting Windows

Covers double-hung, horizontal sliding, casement, awning, jalousie, top-hinged, and fixed windows. Includes patio doors and skylights

The balance sheet of agriculture, 1957

Agriculture ; Agricultural productivity

Predictive control design on an embedded robust output-feedback compensator for wind turbine blade-pitch preview control

The use of upstream wind measurements has motivated the development of blade-pitch preview controllers to improve rotor speed tracking and structural load reduction beyond that achievable via conventional feedback design. Such preview controllers, typically based upon model predictive control (MPC) for its constraint handling properties, alter the closed-loop dynamics of the existing blade-pitch feedback control system. This can result in the robustness properties of the original closed-loop system being no longer preserved. As a consequence, the aim of this work is to formulate an MPC layer on top of a given output-feedback controller, with a view to retaining the closed-loop robustness and frequency- domain performance of the latter. The separate nature of the proposed controller structure enables clear and transparent qualifications of the benefits gained by using preview and predictive control. This is illustrated by results obtained from closed-loop simulations upon a high-fidelity turbine, showing the performance comparison between a nominal feedback compensator and the proposed MPC-based preview controller

Analysis and design of tower motion estimator for wind turbines

The use of blade individual pitch control (IPC) provides a means of alleviating the harmful turbine loads that arise from the uneven and unsteady forcing from the oncoming wind. Such IPC algorithms, which mainly target the blade loads at specific frequencies, are designed to avoid excitations of other turbine dynamics such as the tower. Nonetheless, these blade and tower interactions can be exploited to estimate the tower movement from the blade load sensors. As a consequence, the aim of this paper is to analyse the observability properties of the blade and tower model and based on these insights, an estimator design is proposed to reconstruct the tower motion from the measurements of the flap-wise blade loads, that are typically available to the IPC. The proposed estimation strategy offers many immediate benefits, for example, the estimator obviates the need for hardware sensor redundancy, and the estimated signals can be used for control or fault monitoring purposes. We further show results obtained from high-fidelity turbine simulations to demonstrate the performance of the proposed estimator

Modular model predictive control upon an existing controller

The availability of predictions of future system inputs has motivated research into preview control to improve set-point tracking and disturbance rejection beyond that achievable via conventional feedback control. The design of preview controllers, typically based upon model predictive control (MPC) for its constraint handling properties, is often performed in a monolithic nature, coupling the feedback and feed-forward problems. This can create problems, such as: (i) an additional feedback loop is introduced by MPC, which alters the closed-loop dynamics of the existing feedback compensator, potentially resulting in a deterioration of the nominal sensitivities and robustness properties of an existing closed-loop and (ii) the default preview action from MPC can be poor, degrading the original feedback control performance. In our previous work, the former problem is addressed by presenting a modular MPC design on top of a given output-feedback controller, which retains the nominal closed-loop robustness and frequency-domain properties of the latter, despite the addition of the preview design. In this paper, we address the second problem; the preview compensator design in the modular MPC formulation. Specifically, we derive the key conditions that ensure, under a given closed-loop tuning, the preview compensator within the modular MPC formulation is systematic and well-designed in a sense that the preview control actions complement the existing feedback control law rather than opposing it. In addition, we also derive some important results, showing that the modular MPC can be implemented in a cascade over any given linear controllers and the proposed conditions hold, regardless of the observer design for the modular MPC. The key benefit of the modular MPC is that the preview control with constraint handling can be implemented without replacing the existing feedback controller. This is illustrated through some numerical examples

Phase sensitive amplification in a highly nonlinear lead-silicate fibre

We experimentally demonstrate phase-sensitive amplification in a highly nonlinear lead-silicate W-type fibre. A phase-sensitive gain swing of 6dB was observed in a 1.56m sample of the fibre for a total launched power of 33dBm

Preview predictive control layer design based upon known wind turbine blade-pitch controllers

The use of upstream wind measurements has motivated the development of blade-pitch preview controllers for improving rotor speed tracking and structural load reduction beyond that achievable via conventional feedback control. Such preview controllers, typically based upon model predictive control (MPC) for its constraint handling properties, alter the closed-loop dynamics of the existing blade-pitch feedback control system. This can result in a deterioration of the robustness properties and performance of the existing feedback control system. Furthermore, performance gains from utilising the upcoming real-time measurements cannot be easily distinguished from the feedback control, making it difficult to formulate a clear business case for the use of preview control. Therefore, the aim of this work is to formulate a modular MPC layer on top of a given output-feedback blade-pitch controller, with a view to retaining the closed-loop robustness and frequency-domain performance of the latter. The separate nature of the proposed controller structure enables clear and transparent quantification of the benefits gained by using preview control, beyond that of the underlying feedback controller. This is illustrated by results obtained from high-fidelity closed-loop turbine simulations, showing the proposed control scheme incorporating knowledge of the oncoming wind and constraints achieved significant 43% and 30% reductions in the rotor speed and flap-wise blade moment standard deviations, respectively. Additionally, the chance of constraint violations on the rotor speed decreased remarkably from 2.15% to 0.01%, compared to the nominal controller. Copyright © 2017 John Wiley & Sons, Ltd

Estimation and control of wind turbine tower vibrations based on individual blade-pitch strategies

In this paper, we present a method to estimate the tower fore-aft velocity based upon measurements from blade load sensors. In addition, a tower dampening control strategy is proposed, based upon an individual blade pitch control architecture that employs this estimate. The observer design presented in this paper exploits the Coleman transformations that convert a time-varying turbine model into one that is linear and time-invariant, greatly simplifying the observability analysis and subsequent observer design. The proposed individual pitch-based tower controller is decoupled from the rotor speed regulation loop and hence does not interfere with the nominal turbine power regulation. Closed-loop results, obtained from high fidelity turbine simulations, show close agreement between the tower estimates and the actual tower velocity. Furthermore, the individual pitchbased tower damping controller achieves 45% reductions in the variance of the tower vibrations for a modest 5% increase in the variance of the pitch rates, with negligible impact upon the nominal turbine power output