Time synchronization for real-time control systems without PTP-enabled switches

IEEE 1588 is a promising way of time synchronization in control systems that include IT. Intermediate devices such as network switches are needed when the control systems are built on a multi-hop network for wide area operations. However, the introduction of switches causes conflicts between the reply packets from the devices in a switch, and thus, the delays on the backward and forward paths would be different and the synchronization accuracy is degraded as a result. On the other hand, it is difficult to build low cost control systems when using dedicated switches for the precision time protocol (i.e., boundary clock or transparent clock). Therefore, the authors propose a method that estimates the conflicts between synchronization packets based on the observable information by using the synchronization master device and applies the estimation to the synchronization method. We used communication evaluation board to create a prototype for the proposed method and then evaluated it. The results showed that the level of accuracy was ±1μs accuracy under five slaves and that there was a 2-msec cyclic time synchronization.