基於喚醒無線電之雙射頻協同架構於無線感測網路節能與快速啟動之研究

Abstract

由地震激勵所產生的結構動力歷時分析可提供較詳細之耐震性能評估與系統識別結果，因此經常被使用於特殊、高層與不規則結構分析。然而，透過無線感測網路系統量測由地震激勵的結構震動數據往往由於感測系統的節能機制，造成其網路啟動的延遲而錯失量測地震歷時的時機。因此本研究提出一個基於喚醒無線電技術下之雙射頻協同感測結點架構，此架構係由次射頻(Secondary Radio)喚醒無線電(Wake-On-Radio)與低功耗傾聽(Low Power Listening)技術實現一個無線感測網路的雙射頻協同節能機制；在此機制下無線感測網路節點或其主射頻在平時可處於深度睡眠(Hibernation)狀態，並且當特定事件發生時，可透過次射頻傾聽到喚醒指令，並啟動由主射頻、微處理器與感測裝置之主要單元。本研究於IEEE 802.15.4無線感測網路平台上開發與驗證此架構，並於單層星狀、多層長鏈狀網路拓樸下實驗其節能機制的喚醒延遲程度；研究結果顯示，感測結點可於跨網路平均單層219 ms的喚醒延遲，此延遲包括從發送出喚醒指令、結點從睡眠模式中啟動、初始化、加入至網路系統、父節點回報其節點加入訊息，網路協調者接著可由回報訊息中確認各層節點的喚醒情況。本研究也參考了地震預警系統的發展現況，參考其提供的預警時間(lead time)作為無線感測網路系統的喚醒期限(Wakeup deadline)。在節能成效的部分，本研究於開發平台上量測此雙射頻協同架構的節能機制，其無線感測網路節點於節能模式時平均功耗為350μA，可維持長時間的系統監測生命週期。

Nowadays, wireless sensor network (WSN) has been available for over a decade. The stability and durability of the energy supply for sensing nodes in a WSN is an important research issue and receive considerable attention in literature. Control sensors switching active/sleep mode by duty cycling the radio is commonly used to reduce energy consumption. However, for structural health monitoring (SHM) application, especially in higher levels of excitation measurements (e.g. earthquake), the time required to active the network-wide system and performed the response measurements were critical. Transient events such as an earthquake might be missed due to the latency caused by duty cycling scheme. Since the low latency needed in the field demand of high level vibration-based SHM approaches, this work propose a dual radio cooperation scheme based on wake-on-radio (WOR) technique and IEEE 802.15.4 standard. WOR provide as a secondary radio and aimed to monitor the wakeup event. The main radio is based on IEEE 802.15.4 standard and aimed to manager the whole wireless sensor nodes in WSNs. In this scheme, the main radio can be staged in hibernation mode and cooperated with secondary WOR to achieve low power listening. The software combined dual radio advantages to enhance power management with WOR-based long preamble detection for triggering network-wide sensor nodes. The experiments shown the test results for both of current and wakeup-latency measurement. The average current with WOR in power saving mode was measured about 350μA and the latency was 219 ms in 1-hop wakeup procedure. In this scheme, a sensor node has been improved in measuring structure responses after an earthquake, saving energy and extend the lifetime in WSNs.