Pablo Millán

Formation Control of Autonomous Underwater Vehicles Subject to Communication Delays

This paper addresses the formation control problem for fleets of autonomous underwater vehicles (AUVs). The solution is based on the virtual leader approach, with the main goal of designing a control system to cope with the inter-vehicle communication problems, especially significant in underwater environments. The use of kinematic relations allows the linearization of the AUV dynamics maintaining its turning capacities. The control strategy consists of a feedback H2/H∞ controller in combination with a feedforward controller, which makes it possible to deal with delays and packets dropouts while ensuring a good formation control performance.

Distributed estimation in networked systems under periodic and event-based communication policies

This papers aim is to present a novel design technique for distributed estimation in networked systems. The problem assumes a network of interconnected agents each one having partial access to measurements from a linear plant and broadcasting their estimations to their neighbours. The objective is to reach a reliable estimation of the plant state from every agent location. The observers structure implemented in each agent is based on local Luenberger-like observers in combination with consensus strategies. The paper focuses on the following network related issues: delays, packet dropouts and communication policy (time and event-driven). The design problem is solved via linear matrix inequalities and stability proofs are provided. The technique is of application for sensor networks and large scale systems where centralized estimation schemes are not advisable and energy-aware implementations are of interest. Simulation examples are provided to show the performance of the proposed methodologies.

Design and Application of Suboptimal Mixed

This brief tackles the problem of designing suboptimal H2/H∞ controllers for linear networked control systems (NCS) subject to time-varying delays and packet dropouts. The formulation provides state feedback NCS controllers allowing to tradeoff performance and disturbance rejection. The control objective consists in designing an H2 suboptimal control minimizing a quadratic performance index, with a disturbance rejection constraint (H∞ constraint). To characterize the network, only the lower and upper bounds for the delay, as well as the maximum number of consecutive dropouts are required. The approach relies on the formulation of the problem in terms of the minimization of a single scalar parameter, that can be cast as a standard linear matrix inequality (LMI) problem, yielding a suboptimal cost-guaranteed solution. As a difference from previous works, the solution provided is independent of initial conditions. Stability and robustness properties of the proposed controller are theoretically demonstrated and tested on an experimental testbed consisting in the stabilization of a robot arm in the proximities of the unstable upright position. The application shows good performance and disturbance rejection capabilities even for stringent network conditions.

H_{2}/H_{\infty}

This paper presents a procedure to construct distributed observers for a system consisting in a perturbed linear time-invariant plant being observed by a network of sensor nodes. Based on local measurements of the plant and the information of its neighbours, every node builds an estimation of the plant states. Remarkably, each node can be designed to estimate a subspace of the plant, either because the observation attributes of the node impose a physical restriction or because of a free-design criterion. This relaxes the usual requirement of collective observability. The method presents features of suboptimal H2/H∞ performance, as the upper bound of a cost index is minimised, given a prescribed disturbance attenuation level. The computational cost as well as energy and bandwidth usage are reduced compared to existing solutions. Simulation results are presented to show the applicability of the results.

Controllers for Networked Control Systems

Abstract This paper proposes a model-based control strategy for networked control systems subject to norm bounded disturbances. The communication channel is supposed to be shared with several processes and, therefore, the access to the network needs to be minimized to avoid collisions and packet losses. We propose to use a variable sample rate scheme in which the controller operates in open-loop between successive state measurements. The sampling time is decided on-line solving a sequence of quadratic optimization problems in order to minimize the access to the common network while guaranteeing closed-loop practical stability. Both discrete and continuous time schemes are considered.

Reduced-order H2/H∞ distributed observer for sensor networks

This paper is concerned with event-based H2/H∞ control design for networked systems with interval time-varying delays. The contributions are twofold. First, conditions for uniform ultimately bounded stability are provided in the H2/H∞ event-based context. The relation between the boundedness of the stability region and the threshold that triggers the events is studied. Second, a practical design procedure for event-based H2/H∞ control is provided. The method makes use of Lyapunov–Krasovskii functionals (LKFs) and it is characterised by its generality, as only mild assumptions are imposed on the structures of the LKF and the cost functional. The robustness and performance of the proposed technique is showed through numerical simulations.

Self-triggered sampling selection based on quadratic programming*

This paper proposes an optimal control solution for the L2-gain disturbance rejection problem for networked control systems. The problem, usually referred as mixed H2/H∞, aims at designing a linear stabilizing controller minimizing a given cost function subject to a L2-gain disturbance rejection constraint. The problem is formulated assuming network induced time-varying delays and/or packet losses. An LMI-based minimization problem is derived based on a Lyapunov-Krasovskii approach, considering a polytopic covering of the time-delay range. Simulation results are provided to verify the performance of the methodology.

Event-based H2/H∞ controllers for networked control systems

This paper is concerned with the stability analysis for uncertain systems with interval time-varying delays. An appropriate class of Lyapunov-Krasovskii functionals is proposed and, splitting the known bounds of the delay interval in two subintervals, a new delay-dependent criterion is derived. In addition, we resort to the use of a polytope to introduce a novel treatment of the time-varying delays. A number of different examples are given to demonstrate the reduced conservatism of this method.

An optimal control L 2 -gain disturbance rejection design for networked control systems

This paper presents a practical algorithm to design networked control systems able to cope with high data dropout rates. The algorithm is intended for application in packet based networks protocols (Ethernet-like) where data packets typically content large data fields. The key concept is using such packets to transmit not only the current control signal, but predictions on a finite horizon without significantly increasing traffic load. Thus, predictive control is used together with buffered actuators and a state estimator to compensate for eventual packet dropouts. Additionally, some ideas are proposed to decrease traffic load, limiting packet size and media access frequency. Simulation results on the control of a three-tank system are given to illustrate the effectiveness of the method.

Delay-dependent robust stability analysis for systems with interval delays

The paper proposes an innovative estimation and control scheme that enables the distributed monitoring and control of large-scale processes. The proposed approach considers a discrete linear time-invariant process controlled by a network of agents that may both collect information about the evolution of the plant and apply control actions to drive its behaviour. The problem makes full sense when local observability/controllability is not assumed and the communication between agents can be exploited to reach system-wide goals. Additionally, to reduce agents bandwidth requirements and power consumption, an event-based communication policy is studied. The design procedure guarantees system stability, allowing the designer to trade-off performance, control effort and communication requirements. The obtained controllers and observers are implemented in a fully distributed fashion. To illustrate the performance of the proposed technique, experimental results on a quadruple-tank process are provided.