Event-Triggered Control Strategy Research For Networked System Under Denial Of Service Attacks

With the rapid development of communication technology and automatic control technology,the traditional control technology implemented through wiring is gradually being updated and replaced.Networked control systems(NCSs),which deeply integrate the physical system and the information space,emerg as the time requirement.Its popularization and application are involved in many aspects such as urban construction,military technology and social life.The open shared communication network environment exposes the networked system to potential security risk s and threats.Denial of Service(DoS)attacks,as a cyber-attack with the highest frequency and strong destructive power,are the most commonly used attack strategy by the opponents.In networked systems,DoS attacks prevent the real-time transmission of communication data,resulting in the intermittent operation of physical devices,which has a serious impact on control performance and even leads to system instability.Under the impact of DoS attack,how to design security control strategy to ensure the security and stability of networked system has important theoretical and practical significance.Great progresses have been made in the research on security control of networked systems,but there are still urgent scientific research problems to be solved.For example,the efficient use of resources cannot be considered while ensuring the security and stability of the system;DoS attacks,network-induced delays and modeling uncertainties have not been fully considered;there’s no more DoS attack modeling methods in different scenarios;security control strategies are not resilient to the effects of DoS attacks;in complex network environments,it is unavoidable to be affected by the dual effects of attacks and external disturbances;it is expensive to implement feedback using all states;it is urgent to design novel and efficient security control strategies,and so on.In view of the above problems,this thesis takes the networked systems as the research object,takes ensuring the system stability and meeting the performance level under DoS attacks as the main control goal,and uses event-triggered state feedback control,event-triggered output feedback control,event-triggered cascade control and event-triggered tracking control as the main control strategies.This thesis comprehensively considers modeling uncertainty,DoS attacks model,network-induced problem,actuator saturation problem,incomplete measurable states,event-triggered controller design problem and other factors.It focuses on the construction of the system model under the event-triggered mechanism,the design and improvement of the resilient event-triggered control algorithm,and the criteria for stability analysis and synthesis under the condition that the DoS attacks intermittently blocks the communication channel(s),the network communication bandwidth is limited and the system is affected by external disturbances.Based on the above problems,this thesis will study the design of event-triggered controllers for networked systems under DoS attacks from the following aspects:1.For periodic DoS attacks model,the resilient event-triggered control problem for networked systems with network-induced delays,modeling uncertainty and external disturbances is studied.Firstly,in the absence of attacks,considering the limited network bandwidth and full utilization of communication resources,a sampling-based dynamic event-triggering mechanism is established to regulate the necessary data transmission.In order to resist the impact of the attack resiliently,an improved trigger mechanism is designed to ensure that the control command acts on the system immediately at the end of the attack.Then,a state feedback controller is designed based on the transmtted data.Under the DoS attacks and event-triggered control strategy,a new switched time-delay system model is constructed.Finally,the stability of the system is analyzed by using the constructed piecewise Lyapunov-Krasovskii functional,the maximum allowable attack duration of the system is derived,and the co-design scheme of event-triggered weight matrix and control gain is provided.2.For aperiodic DoS attack methods constrained by frequency and duration,the resilient event-triggered output feedback control problem for continuous-time networked stochastic systems is investiagted.First,considering that the state of the system is not completely measurable,the unknown state of the system is estimated by constructing an observer.Then,an adaptive event-triggered output feedback control strategy is designed by using the observed state to resist the adverse effects of attacks and optimize data transmission,so as to achieve security control objectives and improve resource utilization efficiency.Under this framework,a switched stochastic system with time-varying delay is constructed,and the sufficient conditions for the system’s mean square exponential stability and the weighted gain performance are given by using the time-delay analysis method and the the switched system method.Finally,while ensuring the safety and stability of the system,the controller parameters,observer parameters and event-triggered gain matrix are jointly designed.3.In the presence of aperiodic DoS attacks and external disturbances,the security control problem for networked cascaded control systems with discrete-time delay and actuator saturation is addressed.First of all,considering that the system needs a huge amount of control to stabilize the system at the end of the attack,it is inevitable that the actuator will reach saturation.A nonlinear function is used to describe the relationsh ip between the real input and the saturated input,and then the relationship between the nonlinear function and the real input is established by using the inequality technique for stability analysis.Secondly,in order to reduce the update frequency of the controller and resist the damage of attacks,an adaptive event-triggered cascade control strategy is designed.Further,under the DoS attacks,a new switching system model is established.Finally,by using Lyapunov stability theory combined with linear matrix inequality techniques,sufficient conditions for the system to be exponentially stable and satisfying the performance level and a co-design scheme of controller gain and event-triggered parameter are established.4.For the random DoS attack mode satisfying Bernoulli distribution,the event-triggered output feedback control problem for discrete-time networked random system is studied.First,considering that the data is only transmitted when the trigger conditions are met,the attacks only affects the trigger data.According to the random influence of the attack on the transmitted data,a random packet loss model satisfying Bernoulli distribution is established.By introducing a new random variable,the Bernoulli distribution can be equivalently transformed into a new random distribution,which is convenient to show the indirect impact of the attack on the data at each moment.Secondly,by designing an observer to estimate the unknown state of the system,the estimation error is introduced and an augmented system model is established in combination with the original system.Finally,the controller is designed based on the state of the observer to resist random DoS attacks,so that the augmented system is stochastic stable and meets the performance index.5.For the random DoS attack mode satisfying the packet loss process of Markov chain,the event-triggered tracking control problem for discrete-time nonlinear networked system is solved.Firstly,an adaptive event-triggered fuzzy control strategy is designed to resist the impact of DoS and discard unnecessary data packets to save communication resources.The problem caused by the event-triggered mechanism is that the controller cannot receive system modal information during the trigger interval.To solve this problem,a modal estimator is designed to compensate for the unavailable modal signals.Then,an asynchronous tracking fuzzy controller related to modal estimation is designed.Finally,the conservativeness of the stability criterion is reduced by constructing the membership function-dependent Lyapunov function.