PIM-based Controller Design for Systems with Network-Induced Time Delays

Transmission of data within a control system over a communications network has many advantages in terms of cost, ease of implementation and space requirement. However, network-induced delays in the feedback path degrades the performance and may lead to unstable systems. In order to reduce the effects of network-induced delays in digital control systems, a modified PIM discretization technique that aims to achieve stable closed-loop characteristics in the presence of varying time delays is being proposed. In this paper, a continuous control system is discretized using different values of maximum anticipated delay number in its feedback path, particularly those larger than the sampling period. Results show that zero steady-state error and stability conditions for the digital control system are fulfilled for delays up to 9 times larger than the sampling period but with varying degrees of transient responses. However with larger delays, computational errors due to a nearly singular eliminant matrix in the design process yield erroneous unstable closed-loop poles. Therefore, an adjustment to the stable design polynomial is proposed and it has shown improvements in terms of transient response and eliminating unstable poles. The performance of the proposed digital control system is then tested on a real-time Ethernet network simulator with small varying time delays induced by an interference node of higher priority.