Siamak Tafazoli

A Recurrent Neural-Network-Based Sensor and Actuator Fault Detection and Isolation for Nonlinear Systems With Application to the Satellite's Attitude Control Subsystem

This paper presents a robust fault detection and isolation (FDI) scheme for a general class of nonlinear systems using a neural-network-based observer strategy. Both actuator and sensor faults are considered. The nonlinear system considered is subject to both state and sensor uncertainties and disturbances. Two recurrent neural networks are employed to identify general unknown actuator and sensor faults, respectively. The neural network weights are updated according to a modified backpropagation scheme. Unlike many previous methods developed in the literature, our proposed FDI scheme does not rely on availability of full state measurements. The stability of the overall FDI scheme in presence of unknown sensor and actuator faults as well as plant and sensor noise and uncertainties is shown by using the Lyapunovs direct method. The stability analysis developed requires no restrictive assumptions on the system and/or the FDI algorithm. Magnetorquer-type actuators and magnetometer-type sensors that are commonly employed in the attitude control subsystem (ACS) of low-Earth orbit (LEO) satellites for attitude determination and control are considered in our case studies. The effectiveness and capabilities of our proposed fault diagnosis strategy are demonstrated and validated through extensive simulation studies.

Parameter Estimation-Based Fault Detection, Isolation and Recovery for Nonlinear Satellite Models

This brief is concerned with the problem of nonlinear fault detection, isolation, and recovery (FDIR) for the satellites orbital and attitude models through construction of residual generators that are based on least-squares parameter estimation techniques. By viewing system anomalies caused by faults and/or malfunctions as changes of certain parameters in the system, our goal is to detect, isolate, and recover from faults through estimating these parameters and adaptively redesigning and reconfiguring the controllers. The convergence and robustness properties of the residual generators are analytically and experimentally investigated. Furthermore, the corresponding decision logic and thresholds for fault diagnosis are properly selected and specified. Numerical simulation results for the proposed technique as applied to nonlinear satellite models are presented to demonstrate its performance capabilities.

Hybrid System State Tracking and Fault Detection Using Particle Filters

When particle filters are used for fault detection, they have the problem of sample impoverishment, which means there are not enough particles that can transition to a rare-occurring faulty mode. The consequence is that the fault cannot be properly detected. This paper proposes a method to overcome this problem. Essentially, we develop an algorithm for tracking the states of hybrid systems where fault detection is modeled as a special case of the state tracking of a hybrid system. Extensive simulations are carried out to analyze the effects of various parameters on the performance of the algorithm. It is shown that our algorithm can detect both known and unknown faults using a very small number of particles

On the structural controllability of multi-agent systems subject to failure: A graph-theoretic approach

This paper considers the structural controllability of a leader-follower multi-agent system. Graphical conditions for structural controllability based on the information flow graph of the system are provided. Then, the notions of p-link and q-agent controllability are introduced as quantitative measures for the controllability of the system subject to failure in communication links or agents. Necessary and sufficient conditions for the system to remain structurally controllable in the case of the failure of some of the communication links or loss of some agents are derived in terms of the topology of the information flow graph. Moreover, a polynomial-time algorithm for determining the maximum number of failed communication links under which the system remains structurally controllable is presented (which can be analogously developed for the case of agents loss). Finally, the proposed algorithm is extended to the case of loss of agents.

Cooperative tracking control of Euler-Lagrange systems with switching communication network topologies

Formation control of multiple agents has been studied extensively in the past few years. In most of these studies the structure of the communication network was considered to be fixed. However, in some applications this assumption may not hold, specially when the communication among the agents depends on their relative distance or attitude. In this paper, the problem of formation control of multiple Euler-Lagrange (EL) systems under switching network topologies is considered. Our proposed distributed control algorithm guarantees state synchronization and trajectory tracking under arbitrary switching among a family of connected communication topologies. Our proposed control algorithm is extended to cooperative state regulation of multiple EL systems. Simulation results show the effectiveness of our developed control algorithms.

NEOSSat: a Canadian small space telescope for near Earth asteroid detection

Although there is some success in finding Near Earth asteroids from ground-based telescopes, there is a marked advantage in performing the search from space. The ability to search at closer elongations from the sun and being able to observe continuously, allowing quick revisits of new asteroids, are some of the unique benefits of a space platform. The Canadian Space Agency (CSA) together with Defense Research and Development Canada (DRDC) are planning a micro-satellite platform with a 15 cm telescope dedicated for near space surveillance. The NEOSSat (Near Earth Object Surveillance) spacecraft is expected to be able to detect 20 v magnitude objects with a 100 sec exposure, with a 0.85 deg FOV, on a 1024x1024 CCD, and sub arcsec pointing stability. For detection of NEO small bodies, it will be able to search an area from 45 degrees solar elongation and approximately 40 degrees north to south degrees in elevation. The observation strategy will be optimized to find as many asteroids as possible, based on recent models of asteroid population. Ground based telescopes will also be used to complement follow-ups for orbit determination when possible. The microsatellite is based on the CSA very successful MOST micro-satellite, operating since 2003. Baselined for launch in 2010, the NEOSSat is a shared project with DRDC to demonstrate the technology of an inexpensive space platform to detect High Earth Orbit (HEOSS) earth-orbiting satellites and debris.

State Synchronization of Networked Euler-Lagrange Systems with Switching Communication Topologies Subject to Actuator Faults

Abstract Reconfigurable control of networked heterogeneous Euler-Lagrange (EL) systems subject to actuator faults is considered in this paper. It is assumed that the communication network is time-varying (switching). We first introduce a distributed control strategy for state synchronization of multiple EL systems. This controller is denoted as the “nominal” controller. To guarantee state synchronization of the switching communication network topologies we require existence of a non-vanishing dwell-time between any two sequential switches. Next, we consider two types of actuator faults namely (1) an additive actuator fault, and (2) a loss of effectiveness actuator fault. By employing the nominal control algorithm developed for state synchronization, we introduce two other control algorithms for state synchronization in presence of the faults. Simulation results illustrate and demonstrate the effectiveness of our proposed control algorithms.

Coordinated Attitude Control of Spacecraft Formation without Angular Velocity Feedback: A Decentralized Approach ∗

In this paper, motivated by recent developments of velocity-free spacecraft (SC) attitude control techniques and behavioral-based SC formation control a decentralized control algorithm for attitude coordination of SC formation without angular velocity feedback is presented. Asymptotic stability of the SC formation is guaranteed by using Lyapunov analysis and LaSalle’s theorem. The advantage of our proposed algorithm is that it requires limited information exchange among the SC in the formation (only attitude information exchange is necessary). Additionally, the proposed algorithm can be extremely useful when angular velocity information of the SC in the formation in not available due to sensor failures or communication constraints. Unlike other popular methods in the robotics area which tend to assume simple dynamics such as linear systems and single or double integrator dynamic models, in this paper, the full nonlinear attitude dynamics of the SC is considered to track fast time-varying reference trajectories.

Velocity synchronization of networked Euler-Lagrange systems with switching network topologies subject to actuator faults

In this paper, we first introduce a distributed control strategy for velocity synchronization (or velocity consensus seeking) of multiple heterogeneous Euler-Lagrange (EL) systems with switching communication network topologies. This controller is denoted as the “nominal” controller. To guarantee velocity synchronization for switching communication network topologies we require existence of non-vanishing dwell-time between any two sequential switches. Next, we consider two types of actuator faults namely (1) additive actuator fault, and (2) loss of effectiveness actuator fault. By employing the nominal control algorithm developed for velocity synchronization, we introduce two control algorithms for velocity synchronization in presence of the two types of faults. Simulation results illustrate and demonstrate the effectiveness of our proposed control algorithms.

Quaternion-Based Attitude Synchronization and Tracking for Spacecraft Formation Subject to Sensor and Actuator Constraints ¤Constraints

Two constrained algorithms for attitude synchronization and tracking of multiple SC in a formation are proposed in this work. The flrst algorithm requires feedback and exchange of angular velocity measurements among the SC in the formation. However, the second algorithm does not require measurement and exchange of angular velocities (or their estimates) among the SC in the formation. We have employed unit-quaternion, which is a singularity free attitude representation, to describe the SC attitude so that large attitude maneuvers can be executed. It is important to note that when the desired SC formation angular velocity is zero, our proposed control algorithms do not require knowledge of the SC moment of inertia. Several simulations are provided to demonstrate advantages of our proposed algorithms.