Showing papers by "Zhiwei Gao published in 2018"

Robust Fault Tolerant Control for Discrete-Time Dynamic Systems With Applications to Aero Engineering Systems

Abstract: Unexpected faults in actuators and sensors may degrade the reliability and safety of aero engineering systems. Therefore, there is motivation to develop integrated fault tolerant control techniques with applications to aero engineering systems. In this paper, discrete-time dynamic systems, in the presence of simultaneous actuator/sensor faults, partially decoupled unknown input disturbances, and sensor noises, are investigated. A jointly state/fault estimator is formulated by integrating an unknown input observer, augmented system approach, and optimization algorithm. Unknown input disturbances can be either decoupled by an unknown input observer, or attenuated by a linear matrix inequality optimization, enabling the estimation error to be input-to-state stable. Estimator-based signal compensation is then implemented to mitigate adverse effects from the unanticipated actuator and sensor faults. A pre-designed controller, which maintains normal system behaviors under a fault-free scenario, is allowed to work along with the presented fault tolerant mechanism of the signal compensations. The fault-tolerant closed-loop system can be ensured to mitigate the effects from the faults, guarantee the input-to-state stability, and satisfy the required robustness performance. The proposed fault estimation and fault tolerant control methods are developed for both discrete-time linear and discrete-time Lipschitz nonlinear systems. Finally, the proposed techniques are applied to a jet engine system and a flight control system for simulation validation.

A Novel Fault Detection with Minimizing the Noise-Signal Ratio Using Reinforcement Learning.

Abstract: In this paper, a reinforcement learning approach is proposed to detect unexpected faults, where the noise-signal ratio of the data series is minimized to achieve robustness. Based on the information of fault free data series, fault detection is promptly implemented by comparing with the model forecast and real-time process. The fault severity degrees are also discussed by measuring the distance between the healthy parameters and faulty parameters. The effectiveness of the algorithm is demonstrated by an example of a DC-motor system.

Integrated fault estimation and fault‐tolerant control for stochastic systems with Brownian motions

Abstract: This paper presents an integrated robust fault estimation and fault‐tolerant control technique for stochastic systems subjected to Brownian parameter perturbations. The augmented system approach, unknown input observer method, and optimization technique are integrated to achieve robust simultaneous estimates of the system states and the means of faults concerned. Meanwhile, a robust fault‐tolerant control strategy is developed by using actuator and sensor signal compensation techniques. Stochastic linear time‐invariant systems, stochastic systems with Lipschitz nonlinear constraint, and stochastic systems with quadratic inner‐bounded nonlinear constraint are respectively investigated, and the corresponding fault‐tolerant control algorithms are addressed. Finally, the effectiveness of the proposed fault‐tolerant control techniques is demonstrated via the drivetrain system of a 4.8 MW benchmark wind turbine, a 3‐tank system, and a numerical nonlinear model.

Grey-box Model Identification and Fault Detection of Wind Turbines Using Artificial Neural Networks

Abstract: In this paper, a model identification method based on artificial neural networks (ANN) for wind turbine dynamics is studied. Due to the fact that wind turbine has a nonlinear dynamics with partially measured states, ANN cannot be applied directly. To cope with this problem, first a Luenberger observer is designed to estimate the states (both measured and unmeasured ones) and then, for the nonlinear part, a multi-input multi-output (MIMO) back propagation neural-network based observer is proposed. By having an ANN model as the reference, a fault detection method is studied based on the residual of the system. This algorithm is evaluated in simulation on a 4.8 MW wind turbine benchmark and the results approve satisfactory performance of the proposed approach.

Synchronization of Pulse-Coupled Oscillators for IEEE 802.15.4 Multi-Hop Wireless Sensor Networks

Abstract: As a key enabling technology in mission-critical Wireless Sensor Networks (WSNs), time synchronization provides a common timescale for distributed sensor nodes in many wireless applications, such as coordinated control and underwater navigation and tactical surveillance. Inspired by the behaviour of fireflies, along with mathematical model, Pulse-Coupled Oscillators (PCO), has been proposed to enable synchronization in complex networks, where all the PCO's firing signal Pulses are broadcasted simultaneously when synchronization is achieved. The requirement of zero-drift clock oscillators, fully-connected network and concurrent transmission of Pulses are, in reality, impossible to achieve. To avoid transmission collision and enable the PCO extension in the multi-hop WSNs, the desynchronization mechanism is adopted to enable the Pulse packets to be transmitted to the wireless channel in a uniformly distributed fashion. Due to the contention-free period's feature of low-latency, thereby avoiding the need to wait for a random and potentially long period until the channel is available, the PCO's Pulse packets are transmitted in the contention-free period of IEEE 802.15.4-2015 superframe. Thus, a novel state-space model for desynchronization-based pulse-coupled nonidentical oscillators is proposed to model a realistic drifting clock oscillator. Moreover, the timestamped Pulse packets are transmitted to determine the offset of connected sensor nodes, and an attenuated clock correction scheme is adopted to correct the local drifting clocks by using measured offset and skew. The intensive simulations of the three-hop three-cluster wireless network and the seven-hop linear network have been carried out to evaluate performance of timestamped PCO with desyn-chronization method.

Reinforcement learning–based fault-tolerant control with application to flux cored wire system

Abstract: Background: Processes and systems are always subjected to faults or malfunctions due to age or unexpected events, which would degrade the operation performance and even lead to operation failure. Therefore, it is motivated to develop fault-tolerant control strategy so that the system can operate with tolerated performance degradation. Methods: In this paper, a reinforcement learning-based fault-tolerant control method is proposed without need of the system model and the information of faults. Results and Conclusions: Under the real-time tolerant control, the dynamic system can achieve performance tolerance against unexpected actuator or sensor faults. The effectiveness of the algorithm is demonstrated and validated by the rolling system in a test bed of the flux cored wire.

Simulation and evaluation of pulse-coupled oscillators in wireless sensor networks

Abstract: The clock in embedded systems usually is driven by a crystal oscillator and implemented via a counter register, such a crystal clock is non-identical and drifting due to the manufacturing tolerance and variation of working conditions. Thus, a common time among distributed wireless sensor nodes, also referred to as Time Synchronization, is required for many time-sensitive wireless applications, such as collaborative condition monitoring, coordinated control and localization. Inspired by fireflies’ behaviour, the Pulse-Coupled Oscillators (PCO) has been proposed for synchronization in complex networks. Since the concurrent transmission of PCO’s Pulses is impossible in Wireless Sensor Networks (WSNs), the desynchronization mechanism is adopted to ensure the implementation of PCO in WSNs. Moreover, due to the uncertainties in radio channels and the complexities of communication protocols and packet-exchange behaviours in wireless networks, it is challenging to have a closed-form solution to the performance of PCO synchronization in WSNs. The realistic software simulation, in particular, the discrete event simulator has been a powerful tool to exam the performance of communication protocols in various scenarios, since an order sequence of well-defined event in time is to represent the behaviour of a complex system. This paper presents the development of a pulse-coupled oscillators time synchronization simulator on the OMNeT++ platform for simulating and studying its behaviour and performance in sensor networks. A clock module with configurable phase and frequency noises, and adjustable and higher resolution is developed to mimic various crystal oscillators in embedded systems, for example, the real-time clock. The developed simulator also supports the full functions devices defined by ZigBee protocol, which allows realistic simulation of multi-hop IEEE 802.15.4 wireless networks. Finally, the intensive simulations of classical PCO with the refractory period in IEEE 802.15.4-based WSNs have been carried out to demonstrate the features and benefits of the developed simulator. It is shown that for the non-identical and time-varying PCO clocks in the WSNs, the achieved synchronization will lose gradually, and the time that maintained synchronization depends on the length of refractory period.

Chattering-free discrete-time sliding mode control with event-trigger strategy

Abstract: In this paper, a novel discrete-time sliding mode control (DSMC) method based on event-trigger strategy is proposed. The DSMC method here used is a chattering-free method. With the introduction of event-trigger strategy, the system performance is improved in terms of the control updating times. Hence, less resource is required in control execution. It is shown that the proposed control techniques ensure the reachability of the sliding surface with a small band. Finally, simulations are presented to verify the effectiveness of the proposed methods.

Modelling and Synchronization of Pulse-Coupled Non-identical Oscillators for Wireless Sensor Networks

Abstract: Time synchronization in wireless sensor networks,aiming to provide a common sense of timing among distributed sensor nodes, is a key enabling technology for many applications, such as collaborative condition monitoring, time-of-flight localization and underwater navigation and tactical surveillance. In order to solve the challenges of the manufacturing tolerance and working condition variations in any real-world environments, a novel state-space model for pulse-coupled non-identical oscillators is proposed to model a realistic clock oscillator with nonidentical and time-varying frequency. A state feedback correction, referred to as hybrid coupling mechanism, is also proposed to ensure the system move into steady state, thus achieving time synchronization in wireless sensor networks. Furthermore, the intensive simulations of single-hop wireless sensor networks have been carried out to evaluate the performance of proposed pulsecoupled non-identical oscillators. It is shown that a partially connected wireless network consisting of 50 non-identical pulsecoupled oscillators can achieve the synchronization with the precision of $40us.$

Time Synchronization of Pulse-Coupled Oscillators for Smart Grids

Abstract: Wireless sensor networks has been widely recognized as a promising technology in smart grids. The sensors are installed on the critical power grid equipments to acquire the essential grid datas such as voltage, current and system frequency. And the data acquisition in smart grids is particularly time sensitive: the accurate timestamp of information is required from the sensors so that the absolute time of acquisition can be determined. Due to the crystal manufacturing tolerance and working conditions, the clocks of sensor nodes oscillators are non-identical and drifting. Thus, the key enabling technology time synchronization, aiming to provide a common sense of timing, is required in distributed smart grids sensor networks. Inspired by Peskin's model for self-synchronization of cardiac pacemaker, the pulse-coupled oscillators is implemented to the wireless sensor nodes to ensure the wireless networks of smart grids be able to achieve a common timescale, it is also suitable for distributed deployment in large-scale smart grids. In addition, the intensive simulations of three kinds of single-hop wireless sensor networks have been carried out to evaluate the performance of linear pulse-coupled non-identical oscillators.

Implementation of Timestamped Pulse-Coupled Oscillators in IEEE 802.15.4 Networks

Abstract: Time Synchronization (TS) is a key enabling technology of mission-critical Wireless Sensor Networks (WSNs) to provide a common timescale for distributed sensor nodes. Inspired by synchronous flashing of fireflies, a bio-inspired model, Pulse-Coupled Oscillators (PCO), has been intensively studied. The most studies on PCOs are theoretical work, and the assumption is given that oscillators broadcast and receive the Pulses simultaneously when synchronization of a network is achieved. This is not true when it comes to any real-world environments. From the viewpoint of WSNs, the clock of a sensor node driven by crystal oscillators can be modelled as an oscillator, and Pulse firing can be implemented by transmitting a packet. However, the concurrent transmission of Pulse packets is impossible due to the packet collision in the single wireless channel. To avoid this issue in WSNs, this paper adopts a desynchronization mechanism, in which the Pulse packets are transmitted to the wireless channel in a uniformly distributed fashion and in accordance with standard IEEE 802.15.4. A hardware testbed is developed to implement the desynchronized pulse-coupled oscillators, and it can also be extended to the large-scale wireless sensor networks.

Condition monitoring for the quadruple water tank system using H-infinity Kalman Filtering

Abstract: The problem of statistical fault diagnosis for the quadruple watertanks system is examined. The solution of the fault diagnosis problem for the dynamic model of the four-water tanks system is a non-trivial case, due to nonlinearities and the system's multivariable structure. In the article's approach, the system's dynamic model undergoes first approximate linearization around a temporary operating point which is recomputed at each sampling period. The linearization procedure relies on Taylor series expansion and on the computation of the Jacobian matrices of the state-space description of the system. The H-infinity Kalman Filter is used as a robust state estimator for the approximately linearized model of the quadruple water tanks system. By comparing the outputs of the H-infinity Kalman Filter against the outputs measured from the real water tanks system the residuals sequence is generated. It is concluded that the sum of the squares of the residuals' vectors, being weighted by the inverse of the associated covariance matrix, stands for a stochastic variable that follows the χ2 distribution. As a consequence, a statistical method for condition monitoring of the quadruple water tanks system is drawn, by using the properties of the χ2 distribution and the related confidence intervals. Actually, normal functioning can be ensured as long as the value of the aforementioned stochastic variable stays within the previously noted confidence intervals. On the other side, one can infer the malfunctioning of the quadruple water tanks system with a high level of certainty (e.g. of the order of 96% to 98%), when these confidence intervals are exceeded. The article's method allows also for fault isolation, that is for identifying the specific component of the quadruple water tanks system that has been subject to fault or cyber-attack.