H. De Battista

Sliding mode control of wind energy systems with DOIG-power efficiency and torsional dynamics optimization

Wind turbines with double output induction generators can operate at variable speed permitting conversion efficiency maximization over a wide range of wind velocities. However, random wind fluctuations, wind shear and tower shadow, may excite the oscillation mode of the mechanical system, producing large torque ripple. Consequently, damage to drive train components and power quality problems may occur. In this paper, a sliding mode control is developed which provides a suitable compromise between conversion efficiency and torque oscillation smoothing. The resultant sliding dynamics is completely robust to uncertainties in the electrical variables and parameters. In addition, to prevent windup problems due to saturation, a combined sliding surface that incorporates such limitations is proposed.

Dynamical variable structure controller for power regulation of wind energy conversion systems

The paper addresses the problem of output power regulation of fixed-pitch variable-speed wind energy conversion systems. Operation is constrained by practical reasons to the low-speed side of the turbine power-speed curve. Unfortunately, this region is characterized by a nonminimum phase dynamics which is an obstacle to perform the regulation task. A dynamical variable structure controller is developed that accomplishes the control objective despite this limitation. The proposed control strategy presents attractive features such as robustness to parametric uncertainties of the turbine and generator as well as to electric grid disturbances.

Sliding mode scheme for adaptive specific growth rate control in biotechnological fed-batch processes

This paper addresses the control of biomass growth rate in fed-batch bioreactors. The main difficulty when designing controllers for these processes is the lack of accurate on-line knowledge of the controlled variable as well as the strong parameter and model uncertainties. A completely novel approach to the control design is introduced in this paper which allows us to overcome these problems. In fact, the proposed controller, which is applicable to a large class of fermentation processes, requires minimal knowledge of the process parameters and only uses on-line measurement of volume and biomass concentration. First, a reference model is proposed and a goal manifold in the state space is derived where the control objective is satisfied. A partial state feedback law is then proved to be an invariant control for the goal manifold. Then, the feedback gain is dynamically adjusted via a discontinuous action that enforces a sliding regime such that all state trajectories are steered towards the goal manifold. This sliding mode controller presents very attractive robustness properties. The performance of the controller is evaluated through numerical analysis and experimental results.

Stability preserving maps for finite-time convergence: Super-twisting sliding-mode algorithm

The super-twisting algorithm (STA) has become the prototype of second-order sliding mode algorithm. It achieves finite time convergence by means of a continuous action, without using information about derivatives of the sliding constraint. Thus, chattering associated to traditional sliding-mode observers and controllers is reduced. The stability and finite-time convergence analysis have been jointly addressed from different points of view, most of them based on the use of scaling symmetries (homogeneity), or non-smooth Lyapunov functions. Departing from these approaches, in this contribution we decouple the stability analysis problem from that of finite-time convergence. A nonlinear change of coordinates and a time-scaling are used. In the new coordinates and time-space, the transformed system is stabilized using any appropriate standard design method. Conditions under which the combination of the nonlinear coordinates transformation and the time-scaling is a stability preserving map are given. Provided convergence in the transformed space is faster than O(1/@t)-where @t is the transformed time-convergence of the original system takes place in finite-time. The method is illustrated by designing a generalized super-twisting observer able to cope with a broad class of perturbations.

Harmonic series compensators in power systems: their control via sliding mode

This paper deals with harmonic current compensation in power systems using shunt passive and series active filters. It is shown how constraints of the active filters can notoriously degrade the performance of the compensating system. The use of variable structure system theory is suggested for analysis and design of series active filters. Sliding mode control strategies to improve harmonic rejection are proposed, and active filters with different switching-ripple filters are considered. These control strategies provide robustness to active filter modeling errors and external disturbances.

Dynamical sliding mode power control of wind driven induction generators

Summary form only given as follows. This paper concerns power regulation of variable-speed wind energy conversion systems. These systems have two regions of operation, depending on the tip speed ratio of the wind turbines. They are distinguished by a minimum phase behavior in one of these regions and a nonminimum phase one in the other. A sliding mode control strategy is proposed that assures stability in both regions of operation and imposes the ideally designed feedback control solution in spite of model uncertainties. Moreover, power regulation by the proposed sliding control in the minimum phase region is completely robust to wind disturbances and parameter uncertainties.

Safety Auxiliary Feedback Element for the Artificial Pancreas in Type 1 Diabetes

The artificial pancreas aims at the automatic delivery of insulin for glycemic control in patients with type 1 diabetes, i.e., closed-loop glucose control. One of the challenges of the artificial pancreas is to avoid controller overreaction leading to hypoglycemia, especially in the late postprandial period. In this study, an original proposal based on sliding mode reference conditioning ideas is presented as a way to reduce hypoglycemia events induced by a closed-loop glucose controller. The method is inspired in the intuitive advantages of two-step constrained control algorithms. It acts on the glucose reference sent to the main controller shaping it so as to avoid violating given constraints on the insulin-on-board. Some distinctive features of the proposed strategy are that 1) it provides a safety layer which can be adjusted according to medical criteria; 2) it can be added to closed-loop controllers of any nature; 3) it is robust against sensor failures and overestimated prandial insulin doses; and 4) it can handle nonlinear models. The method is evaluated in silico with the ten adult patients available in the FDA-accepted UVA simulator.

Energy-based approach to the output feedback control of wind energy systems

Wind energy systems can be classified into constant speed and variable speed ones. In constant speed schemes, the generator is directly connected to the electric grid. On the other hand, variable speed operation can be accomplished interposing a static converter in the energy flow between the generator and the grid, permitting a high control flexibility. The main control objectives are the maximization of the conversion efficiency and the elimination of torque oscillations propagated through the drive train. It is assumed in this paper that the most flexible part of the system lies on the turbine, constraining the control solutions to generator speed feedback. The control task is addressed from a passivity-based control viewpoint. The drive train dynamics is modelled as a port-controlled Hamiltonian system with dissipation. Then, stabilization of the desired operating point is achieved through energy shaping and damping injection. Depending on the damping matrix assignment, different control solutions are recovered. Finally, a dynamic feedback controller which preserves the system structure is proposed to improve the system performance without measuring the wind velocity.

Sliding mode control of torque ripple in wind energy conversion systems with slip power recovery

This paper deals with wind energy conversion systems (WECS) with slip power recovery. These systems use a double-output induction generator, which is connected directly to grid by stator and through a static converter by rotor. An appropriate control of the converter allows the WECS to operate at the optimum tip speed ratio, maximizing the efficiency of the energy conversion. Wind turbine generators are characterized by a low frequency mode of oscillation. Random wind fluctuations, wind shear and tower shadow effects, may excite this mode, producing large ripple on drive train torque as well as on the generated electric power. Torque ripple may damage drive train components, and fluctuation of the generated electric power may cause flicker in weak grids. In this paper, a sliding mode control strategy of the static converter is proposed. It forces the system to follow wind speed variations and provides damping of the oscillation mode, resulting in a large reduction of torque ripple and electric power fluctuation. The closed-loop system dynamics are completely robust to uncertainties in electrical parameters of the generator and grid voltages disturbances.

Comments on "variable-structure PID control to prevent integrator windup"

In the paper by Hodel and Hall, the authors introduce a variable-structure proportional-integral-derivative controller with antiwindup features and compare it with other simple antiwindup methods. It is pointed out here that the conclusions reached from the comparative example are incorrect as the performance of the controllers is not appropriately compared.