精確時間同步協定下以軟體模擬振盪器之行為

Abstract

時間同步的應用非常廣泛，例如精密量測與控制、工業自動化、通訊系統、飛航控制與金融貿易等，而IEEE 1588標準定義了精確時間協定(Precision Time Protocol, PTP)，主要的目標即為達成區域網路中各節點之時間同步，其精確度可達次微秒(sub-microsecond)等級。 時間伺服(clock servo)為PTP實現時間同步之核心，通常為一比例積分控制器(proportional-integral controller, PI controller)，利用主時鐘(master clock)與從時鐘(slave clock)的時間差作為回授訊號，經由控制器來調整從時鐘之輸出頻率，以達成主從時鐘之同步。 由於PI控制器之暫態收斂時間過長，本論文使用精確時間協定程序(Precision Time Protocol daemon, PTPd)開放性原始碼，設計其中的控制器，利用交換式控制器之概念，提出修正型PI控制器(modified PI controller)。此控制器在主從時鐘達同步穩態前，利用遞迴濾波器(recursive filter)估測主從時鐘時間漂移量之差，使主從時鐘達穩態時，PI控制器可快速收斂，達到縮短同步暫態收斂時間之目的。另外，由於時間同步之過程非常耗時，故以軟體製造兩時鐘之時間訊號，包括其中所含之雜訊訊號，模擬其時間同步之行為，並以亞倫方差(Allan deviation)作為穩態穩定度之指標，使模擬之訊號符合硬體時鐘之特性，包括時間同步暫態收斂時間與時間達同步穩態時之穩定度。

Time synchronization is widely used in many applications, such as precise measurement and control, industrial automation, communication systems, air traffic control and commercial systems. IEEE 1588 standard defined Precision Time Protocol (PTP), which is designed for each node in a local network to synchronize their clocks to sub-microsecond accuracy. The clock servo is the core of PTP time synchronization, usually a PI controller. It uses the time offset between the master clock and the slave clock as the feedback signal to compute the clock tick rate adjustment, and then adjust the output frequency of the slave clock to synchronize the master clock and the slave clock. Because of the long transient time of the PI controller, we used the Precision Time Protocol daemon (PTPd) and designed the controller. Using the concept of switching controller, we proposed modified PI controller. Before the synchronization reaches the steady state, modified PI controller uses recursive filter to estimate the time drift of the master clock and the slave clock, so that when the synchronization reaches the steady state, the PI controller will converge quickly to reduce the synchronization transient time. In addition, due to the process of time synchronization is very time-wasting, we used software to create the clock signals, including clock noises, to simulate the process of time synchronization, and then used Allan deviation to analyze the steady state stability. The characteristics of the simulated clock signal are in line with the characteristics of the hardware clock, such as the synchronization transient time and the steady state stability.