This study proposes a novel sampling and control strategy to find a suboptimal sampling period sequence and control input sequence, such that a quadratic cost function of state and control input of a networked control system (NCS) with packet disordering is minimised. First, a discrete-time system model of the NCS with packet disordering, transmission delay and packet loss in terms of displacement values of packets is put forward. Second, a linear quadratic regulation (LQR) problem of the NCS is formulated, showing that the optimal controller depends on sampling period and quality of services (QoS) of networks. Interactive effects between sampling period and QoS of networks pose a challenge in solving the LQR problem of the NCS. To overcome this difficulty, different from traditional transmission-delay-based or packet-loss-based sampling scheme, a novel packet-disordering-based sampling period selection scheme is proposed. Furthermore, an algorithm is presented to find a suboptimal solution to the LQR problem in this study. Finally, simulation results demonstrate the effectiveness of the proposed approach.
IntroductionFacing the increasing demand of control networks in industrial processes, commercial systems and traffic systems, a rapidly increasing focus on networked control systems (NCSs) has been witnessed [1][2][3]. NCSs have been hotspots of research in the past decades. The introduction of control networks can improve the efficiency, flexibility and reliability of these applications, reducing installation, maintenance, reconfiguration and costs. However, some unexpected phenomena potentially occur in network channels including transmission delay, packet loss and packet disordering and so on [4,5]. Therefore, different from traditional control systems, optimal control problems of NCSs require not only control system design, but also network system design. In this paper, we investigate the effects of sampling rate and packet disordering on solution of linear quadratic regulation (LQR) problem. We first formulate the LQR problem for the following continuous linear systemẋwhere x ∈ R n and u ∈ R m , respectively, are state and control input of the plant, and A ∈ R n×n and B ∈ R n×m are matrices with appropriate dimensions. The term t 0 is the initial time instant and x 0 is the initial state. Consider the following quadratic performance index for system (1)where Q ∈ R n×n and R ∈ R m×m are positive semi-definite matrix and positive definite matrix, respectively, and the term t f is the terminal time instant.The control input is assumed to be piecewise constantwhere u k = u(t k ), t k is the kth sampling time instant with 0 ≤ t 0 < t 1 < · · · < t k < · · · ≤ t f , and h k = t k+1 − t k is called sampling period. We assume h min ≤ h k ≤ h max , where h max and h min are the maximum and minimum values of the sampling period, respectively. For given sampling periods, the optimal control input u k that minimises the cost function (2) for system (1) can be analytically determined through solving the disc...