George Nikolakopoulos

Power Conservation through Energy Efficient Routing in Wireless Sensor Networks

The power awareness issue is the primary concern within the domain of Wireless Sensor Networks (WSNs). Most power dissipation ocurrs during communication, thus routing protocols in WSNs mainly aim at power conservation. Moreover, a routing protocol should be scalable, so that its effectiveness does not degrade as the network size increases. In response to these issues, this work describes the development of an efficient routing protocol, named SHPER (Scaling Hierarchical Power Efficient Routing).

Development and Experimental Verification of a Mobile Client-Centric Networked Controlled System

In this paper the development and experimental verification of mobile client-centric networked controlled system is presented. The feedback control loop is closed over a General Purpose Radio Service (GPRS) communication channel, between the plant and the controller. The inserted delays from the communication network, are time-varying, degrade the system dynamic performance, while forcing the system to instabilities. The designed controller has primitive characteristics and ensures the stability of the overall discrete time-delayed system. LMI-theory based theoretical results provide the worst-case scenario of the latency time that this controller can tolerate.

Model predictive quadrotor indoor position control

This article addresses the control problem of quadrotors in environments where absolute-localization data (GPS, positioning from external cameras) is inadequate. Based on an attached IMU and an optical flow sensor the quadrotors translational velocity is estimated using an Extended Kalman Filter. Subject to the velocity measurements, the roll, pitch and yaw (RPY) angles, the angular rates and the translational accelerations a switching Model Predictive Controller is designed. The quadrotor dynamics is linearized at various operating points according to the angular rates and the RP-angles. The switching is inferred according to the various linearized models of the quadrotor. The controller is applied on a quadrotor prototype in low-altitude position hold maneuvers at very constrained environments. The experimental results indicate the overall systems efficiency in position/altitude set-point maneuvers.

Principal Component Analysis of the start-up transient and Hidden Markov Modeling for broken rotor bar fault diagnosis in asynchronous machines

This article presents a novel computational method for the diagnosis of broken rotor bars in three phase asynchronous machines. The proposed method is based on Principal Component Analysis (PCA) and is applied to the stators three phase start-up current. The fault detection is easier in the start-up transient because of the increased current in the rotor circuit, which amplifies the effects of the fault in the stators current independently of the motors load. In the proposed fault detection methodology, PCA is initially utilized to extract a characteristic component, which reflects the rotor asymmetry caused by the broken bars. This component can be subsequently processed using Hidden Markov Models (HMMs). Two schemes, a multiclass and a one-class approach are proposed. The efficiency of the novel proposed schemes is evaluated by multiple experimental test cases. The results obtained indicate that the suggested approaches based on the combination of PCA and HMMs, can be successfully utilized not only for identifying the presence of a broken bar but also for estimating the severity (number of broken bars) of the fault.

A state-of-the-art review of structural control systems:

The utilization of structural control systems for alleviating the responses of civil engineering structures, under the effects of different kinds of dynamics loadings, has become a standard technology, although there are still numerous research approaches for advancing the effectiveness of these methodologies. The aim of this article is to review the state-of-the-art technologies in structural control systems by introducing a general literature review for all types of vibrations control systems that have appeared up to now. These systems can be classified into four main groups: (a) passive; (b) semi-active; (c) active; and (d) hybrid systems, based on their operational mechanisms. A brief description of each of these main groups and their subgroups, with their corresponding advantages and disadvantages, is also given. This article will conclude by providing an overview of some innovative practical implementations of devices that are able to demonstrate the potential and future direction of structural control systems in civil engineering.

Piecewise Affine Modeling and Constrained Optimal Control for a Pneumatic Artificial Muscle

In this paper, the modeling and control problem of a pneumatic artificial muscle (PAM) is being considered. The PAM is an actuator characterized by a decrease in the actuating length when pressurized. Its nonlinear nature and time-varying parameters cause difficulties in modeling their characteristics, as well as in designing controllers for high-performance positioning systems. A constrained linear and piecewise affine system model approximation is formulated, and a control scheme composed of the following is being synthesized: 1) a feedforward term regulating the control input at specific set points and 2) a constrained finite-time optimal controller handling any deviations from the systems equilibrium points. Extended experimental studies are utilized to prove the efficacy of the suggested controller.

Experimental model predictive attitude tracking control of a quadrotor helicopter subject to wind-gusts

Accurate navigation and control of unmanned quadrotor helicopters in harsh environmental conditions due to wind disturbances is an open challenge. The aim of the work presented in this article is the design of a Model Predictive Control scheme based on Piecewise Affine (PWA) modeling of an Unmanned quadrotor Helicopter (UqH) in attitude maneuvers. The suggested algorithm is verified in experimental studies in the execution of sudden maneuvers subject to forcible wind disturbances. The UqH manages to reject the induced wind-disturbances while performing accurate attitude tracking.

Design and experimental verification of a Constrained Finite Time Optimal control scheme for the attitude control of a Quadrotor Helicopter subject to wind gusts

In this paper the design and the experimental verification of a Constrained Finite Time Optimal (CFTO) control scheme for the attitude control of an Unmanned Quadrotor Helicopter (UqH) subject to wind gusts is being presented. In the proposed design the UqH has been modeled by a set of Piecewise Affine (PWA) linear equations while the wind gusts effects are embedded in the system model description as the affine terms. In this approach the switching among the PWA model descriptions are ruled by the rate of the rotation angles. In the design of the stabilizing CFTO-controller both the magnitude of external disturbances (worst case applied wind gust), and the mechanical constraints of the UqH such as maximum thrust in the rotors and UqHs angles rate are taken under consideration in order to design an off-line controller that could rapidly be applied to a UqH in a form of a look-up table. The proposed control scheme is applied in experimental studies and multiple test-cases are presented that prove the efficiency of the proposed scheme.

Constrained optimal attitude control of a quadrotor helicopter subject to wind-gusts: Experimental studies

A Constrained Finite Time Optimal Controller (CFTOC) for attitude set-point maneuvers of an Unmanned Quadrotor Helicopter (UqH) operating under severe wind conditions is the subject of this article. The UqHs nonlinear dynamics is linearized in various operating points resulting in a set of piecewise models. The CFTOC is designed for set-point maneuvers taking into account the switching between the linear models and the constraints of the actuators. The control scheme is applied in experimental studies in a prototype UqH operating in harsh weather conditions resulting from wind gusts. The UqH rejects the induced wind-disturbances while performing attitude set-point maneuvers.

Model predictive attitude control of an unmanned Tilt-Rotor aircraft

The design of a Model Predictive Controller (MPC) for attitude maneuvers of an Unmanned Vertical Take Off and Landing (VTOL) Tilt-Rotor vehicle, flying in its helicopter mode is the subject of this article. Tilt-Rotor air vehicles combine the flight envelopes of: a)fixed wing aircrafts, and b) rotorcrafts, leading to systems with advanced flight characteristics. However, the highly nonlinear and underactuated dynamics of these systems, necessitate the design of advanced control schemes to achieve a satisfactory control performance. In the proposed control design approach, the Tilt-Rotors nonlinear dynamics are linearized in various operating points, resulting in a set of multiple piecewise affine models that describe its helicopters flying mode. The corresponding novel MP-controller is designed for set-point maneuvers taking into account the arbitrary switchings between the linear models as well as the constraints on the actuators, while extended simulation studies indicate the high overall efficiency of the proposed MPC scheme.