Research On Analysis And Design For Networked Control Systems With Random Delays And Packet Dropouts

Since networked control systems (NCSs) offer many advantages such as low cost, simple system diagnosis and maintenance, and increased system agility, recently, they have found successful applications in manufacturing plants, industrial automation, aircrafts and spacecrafts and so on. However, the introduction of the communication networks leads to time delay, data packet dropout and disordering inevitably, which makes the NCS analysis and synthesis complex. In this dissertation, we focus on the problems of network-induced delay and data packet dropout which are induced by the introduction of network. Considering the NCSs with random delays and packet dropouts, the modelling, stability analysis and controller design are investigated by using linear matrix inequality (LMI) method, Lyapunov theorem and Nyquist stability criterion. The effectiveness and applicability of the theoretical results proposed are demonstrated by the simulation examples. This dissertation mainly includes the following five contents:(1) The problem of stochastic optimal control is considered for NCSs with control packet dropout. A new strategy based on forgetting factor method is proposed for packet dropout compensation. The actuator will construct a packet for use in the current sampling period by past control signals stored in the butter when the control signal is lost. The control packet dropout is described as a Bernoulli process. Then, in terms of the strategy based on forgetting factor method, a new NCS model is proposed. Based on the NCS model, the stochastic optimal control law is derived according to the dynamic programming approach.(2) Based on the hold—input compensation strategy for packet dropouts, two methods are proposed to stabilize the NCSs where both the sensor signals and the control signals may be lost.ⅰ) The T-S fuzzy model is obtained by using the probability of the packet dropouts. The random system is transformed into the nonrandom one. Then, a state observer is designed which can cancel the disturbance due to the calculation. Moreover, the condition for the stability of the closed-loop system is obtained.ⅱ) Based on the random model, the sufficient condition for the globally mean-square asymptotically stability of the closed-loop systems is given based on the Lyapunov theorem. The gain matrix of the state feedback control law is derived in terms of LMI approach.(3) The problem of output feedback stabilization is discussed for NCSs with both sensor-to-controller (S/C) and controller-to-actuator (C/A) network-induced delays. By adding a buffer to the actuator, the S/C delay and the C/A delay can be lumped to a new delay, and its history behavior can be described by a Markov chain. Based on this, the necessary and sufficient conditions for the mean-square stability of the closed-loop system are given, and the one-mode-dependent output feedback controller is designed. The controller gains can be obtained by solving a set of LMIs.(4) The stabilization for a class of high order unstable NCSs by PID controllers is investigated. According to the Nyquist stability criterion, the upper bound of the network-induced delay is established by using P/PI and PD/PID control laws respectively. For each control law, the controller parameter region, in which the stability of the closed-loop system can be guaranteed, is obtained. The results for second-order unstable NCSs are also obtained as the special case.(5) By using the new idea"time delay can stabilize oscillatory systems, and it can improve the performance of the systems", the problem of the delayed positive feedback control is considered for the second-order mass-spring-damping system controlled over the network. However, in the existing literature, the delays are widely known to degrade the performance of the NCSs, and they even make the NCS unstable. The delayed positive feedback control law is proposed which can make the close-loop system have a special stability margin. Furthermore, the theoretical results proposed are used to the simulation examples on the background of the vibration systems of the offshore platforms. The simulation results demonstrate that the response of the platform is effectively reduced by using the delayed positive feedback control law, even when the offshore structure undergoes the irregular wave loads.