A. W. Hulskamp

Two-Degree-of-Freedom Active Vibration Control of a Prototyped “Smart” Rotor

This paper studies the load reduction potential of a prototyped “smart” rotor. This is, a rotor where the blades are equipped with a number of control devices that locally change the lift profile on the blade, combined with appropriate sensors and controllers. Experimental models, using dedicated system identification techniques, are developed of a scaled rotating two-bladed “smart” rotor of which each blade is equipped with trailing-edge flaps and strain sensors. A feedback controller based on H∞-loop shaping combined with a fixed-structure feedforward control are designed that minimizes the root bending moment in the flapping direction of the two blades. We evaluated the performance using a number of different realistic load scenarios. We show that with appropriate control techniques the variance of the load signals can be reduced up to 90%.

Closed-loop control wind tunnel tests on an adaptive wind turbine blade for load reduction

Wind tunnel tests on a non-rotating, dynamically scaled wind turbine blade equipped with variable trailing edge geometry were carried out. The effectiveness of the system for active load reduction purposes, with the interaction between structural dynamics, aerodynamics and control was tested. The actuation of the adaptive trailing edge was based on a piezoelectric bender actuator. The full aeroservoelastic system was identified based on input and output measurement signals. A feedback controller, using strain signals on the blade root, was designed, tuned and applied on the system in order to minimize root bending moments. The results show remarkable performance in reduction of blade root strains for both open-loop and closed-loop tests. The sensitivity of various design and control parameters are analyzed both in the prescribed cases and in the feedback-controlled system.

A high-rate shape memory alloy actuator for aerodynamic load control on wind turbines

This article discusses the development of a high-rate shape memory alloy–driven actuator. The concept of the actuator was developed to act as aerodynamic load control surface on wind turbines. It was designed as a plate or beam-like structure with prestrained shape memory alloy wires embedded off its neutral axis. Moreover, the shape memory alloy material was embedded in channels through which air was forced to actively cool the wires when the recovery load was to be released. Wires were implemented on both sides of the neutral axis to deflect the beam in both directions. Thermal analysis of the cooling channels showed that they increased the cooling rate up to 10-fold in comparison to the same set-up without forced convection. Subsequently, a fuzzy logic controller was designed to control the thermo-mechanical system. The inputs were the error between the deflection and the set point, the value of the set point and the time derivative of the set point. The output consisted of two signals to the valves that controlled the flow through the channels and a signal heating signal that was split into both sets of wires, depending on its sign. The controller was tested on an antagonistic set-up, through which a similar thermo-mechanical behaviour as with the actuator was obtained, but eliminating the beam dynamics. The results were satisfactory; an actuation bandwidth of 1 Hz was attained. Subsequently, the controller was tested on the actuator. With increasing actuation frequency, until 0.6 Hz, a relatively small error between the set point and the actual deflection was observed. Above that frequency, the error increased, but the sinusoidal response was lost. This is believed to be due to snap-through behaviour around the neutral position of the actuator. This was substantiated by the apparent inability of the actuator to track the set point around the neutral position in tracking a composite sinusoidal set point.

Implementation Of The ‘smart’ Rotor Concept

Fatigue is one of the biggest issues in wind turbine design. Most of the sources for fatigue loads are related to changes in the airfl ow around the blades due to, for instance, turbulence, tower shadow or jaw misalignment. Therefore systems are proposed that counteract these load fl uctuations, which are both deterministic and stochastic in nature. The systems aim at infl uencing the lift and drag at different stations along the blades length. This way, the aerodynamics along the blade can be controlled and the dynamic loads and modes can be dampened. This is called the ‘smart’ rotor concept. Such systems are already used in aerospace, both with airplanes and helicopters, although in the latter case mostly experimental. They aim at reconfi guration of the wing or rotor, fl ight control or vibration reduction. The concepts are often implemented using adaptive materials such as piezoelectric materials and shape memory alloys (SMAs). Piezoelectric materials are mostly implemented for high frequent actuation, in which low strains but high actuation forces are required. SMAs can exert very high forces and can recover very large strains, but within a limited bandwidth. At the Delft University of Technology a series of experiments is conducted in which the feasibility of the concept is proven and in which several control issues are being addressed. For instance, it has been shown that the control concept can be based on the structural response of the blade to the fl ow disturbance and that the presence of natural modes has a large infl uence on the performance of the system.