Germán A. Ramos

Generalized proportional integral control for periodic signals under active disturbance rejection approach.

Conventional repetitive control has proven to be an effective strategy to reject/track periodic signals with constant frequency; however, it shows poor performance in varying frequency applications. This paper proposes an active disturbance rejection methodology applied to a large class of uncertain flat systems for the tracking and rejection of periodic signals, in which the possibilities of the generalized proportional integral (GPI) observer-based control to address repetitive control problems are studied. In the proposed scheme, model uncertainties and external disturbances are lumped together in a general additive disturbance input that is estimated and rejected on-line. An illustrative case study of mechatronic nature is considered. Experimental results show that the proposed GPI observer-based control successfully rejects periodic disturbances even under varying speed conditions.

Robust Active Disturbance Rejection Control Approach to Maximize Energy Capture in Variable-Speed Wind Turbines

This paper proposes an alternative robust observer-based linear control technique to maximize energy capture in a 4.8 MW horizontal-axis variable-speed wind turbine. The proposed strategy uses a generalized proportional integral (GPI) observer to reconstruct the aerodynamic torque in order to obtain a generator speed optimal trajectory. Then, a robust GPI observer-based controller supported by an active disturbance rejection (ADR) approach allows asymptotic tracking of the generator speed optimal trajectory. The proposed methodology controls the power coefficient, via the generator angular speed, towards an optimum point at which power coefficient is maximum. Several simulations (including an actuator fault) are performed on a 4.8 MW wind turbine benchmark model in order to validate the proposed control strategy and to compare it to a classical controller. Simulation and validation results show that the proposed control strategy is effective in terms of power capture and robustness.

Design and analysis strategies for digital repetitive control systems with time-varying reference/disturbance period

This article analyses stability and performance features of different design schemes for digital repetitive control systems subject to references/disturbances that exhibit non-uniform frequency. Aiming at maintaining a constant value for the ratio T p /T s , T p being the period of the reference/disturbance signal and T s being the sampling period, two approaches are proposed. The first one deals with the real-time adaptation of T s to the actual changes of T p ; stability is studied by means of an LMI gridding method and also using robust control techniques. The second one propounds the introduction of an additional compensator that annihilates the effect of the time-varying sampling in the closed-loop system and forces its behaviour to coincide with that of an a priori selected nominal sampling period; the internal stability of the compensator-plant subsystem is checked by means of LMI gridding. The theoretical results are experimentally tested and compared through a mechatronic plant model.

Digital repetitive control under time-varying sampling period: An LMI stability analysis

Digital repetitive control is a technique which allows to track periodic references and/or reject periodic disturbances. Repetitive controllers are usually designed assuming a fixed frequency for the signals to be tracked/rejected, its main drawback being a dramatic performance decay when this frequency varies. A usual approach to overcome the problem consists of an adaptive change of the sampling time according to the reference/disturbance period variation. This article presents a stability analysis of a digital repetitive controller working under time-varying sampling period by means of an LMI gridding approach. Theoretical developments are illustrated with experimental results.

Stability Analysis Methods

Repetitive control is a widely used strategy applied in the tracking/rejection of periodic signals, however, the performance of this controller can be seriously affected when the frequency of the reference/disturbance signal varies or is uncertain. One approach that overcomes this problem is the adaptation of the controller sampling period, nevertheless, the system framework changes from a Linear Time Invariant to Linear Time-Varying and the closed-loop stability can be compromised. Indeed, the proposals applying this scheme in repetitive control do not provide formal stability proofs. This work presents two different methodologies aimed at analyse the system stability under these conditions. The first one uses a Linear Matrix Inequality gridding approach which provides necessary conditions for the closed-loop Bounded Input Bounded Output stability of the system. The second one applies robust control techniques in order to analyse the stability and yields sufficient stability conditions. Both methodologies, entails a frequency variation interval for which the system stability can be assured.

An optimal anti-windup strategy for repetitive control systems

Repetitive Control includes an Internal Model with high gain and slow time response characteristics which make it prone to the windup effect. A solution to this problem is the inclusion of an Anti-Windup compensator. Although there exist many general Anti-Windup synthesis methods in literature, some problems can arise as a result of a straightforward application of them in Repetitive Control. This paper presents the analysis and adaptation of the Model Recovery Anti-Windup strategy in the Repetitive Control frame. Thus, an optimal LQ design is proposed that looks for a deadbeat recover behaviour after saturation and a global asymptotic stability for the closed loop system. Through simulation it is shown that the propounded scheme achieves better tracking performance than other similar LQ designs.

Robust high-order repetitive control of an active filter using an odd-harmonic internal model

Shunt active power filters have proven to be an efficient means to compensate for the negative effects of nonlinear and reactive loads on the power quality of the electrical distribution network. In this context, the control objective is to achieve a power factor close to 1, as well as load current harmonics and reactive power compensation. A useful control strategy for this purpose is repetitive control. However, the performance of repetitive controllers is strongly affected by frequency variations of the involved signals. This work analyzes the effect of such variations and describes the architecture of an odd-harmonic, high-order repetitive controller specifically designed to obtain robust closed-loop performance against frequency variations that may occur in the electrical network.

Second-order odd-harmonic repetitive control and its application to active filter control

High order repetitive control has been introduced to overcome performance decay of repetitive control systems under varying frequency of the signals to be tracked/rejected or improving the interhamonic behavior. However, most high order repetitive internal models used to improve frequency uncertainty are unstable, as a consequence practical implementations are more difficult. In this work a stable, second order odd-harmonic repetitive control system is presented and studied. The proposed internal model has been implemented and validated in a shunt active filter current controller. This high order controller allows dealing with the grid frequency variations without using adaptive schemes.

Odd-harmonic repetitive control of an active filter under varying network frequency: Control design and stability analysis

This work deals with the design and analysis of a controller for a shunt active power filter. The design is based on combined feedforward and feedback actions, the last using repetitive control, and aims at the obtention of a good closed-loop performance in spite of the possible frequency variations that may occur in the electrical network. As these changes affect the performance of the controller, the proposal includes a compensation technique consisting of an adaptive change of the digital controllers sampling time according to the network frequency variation. However, this implies structural changes in the closed-loop system that may destabilize the overall system. Hence, this article is also concerned with closed-loop stability of the resulting system, which is analyzed using a robust control approach through the small gain theorem. Experimental results that indicate good performance of the closed-loop system are provided.

A Varying Frequency LPV-Based Control Strategy for Three-Phase Inverters

Grid-connected inverters have drawn a lot of attention in the integration of distributed generation systems and microgrids, as they are an effective interface for renewable and sustainable energy sources. Several strategies, including repetitive and resonant controllers, have been implemented in order to achieve low distortion and high-quality power. However, it has been proved that their performance decreases substantially when the grid frequency varies. This paper proposes a resonant control strategy based on a linear parameter varying (LPV) design, which is able to deal with changes in the network frequency. Controller aim is associated with injecting a clean sinusoidal current to the grid, even in the presence of nonlinear/unbalanced loads and/or grid-voltage distortions. Main emphasis is focused on presenting an applied LPV design procedure that covers plant modeling, controller synthesis, stability analysis, and experimental results that show the feasibility and effectiveness of the proposed scheme.