無線系統的控制回路挑戰

　　本文討論了用無線傳感器網絡實現控制環路的挑戰。它著眼于不同的封閉和開放控制回路架構和能夠支持它們的無線網絡協議的延遲要求。

　　雖然可以使用Wi-Fi與嵌入式模塊如Microchip的mrf24wb0mb，最近的協議如isa100a和WirelessHART?目的無線網絡內提供集中的支持控制回路。設備如Atmel的at86rf231，恩智浦JN5148和Silicon Labs的em357已經支持IEEE802.15.4 Zigbee標準可以用來支持這些協議和嚴格控制回路定制版本。

　　不同于傳統的控制系統，在傳感器、執行器和集中控制器之間有專用的同軸電纜，采用無線通信的控制系統需要傳感器和控制器共享用于通信的公共無線介質。當閉環控制系統使用公共通信系統實現時，網絡的質量是整個系統性能的重要部分。許多系統中不重要的時間延遲可能成為實時閉環系統中的關鍵。

　　The problem is that traditional short-range wireless network standards such as IEEE 802.11 Wi-Fi and IEEE 802.15.4 ZigBee do not consider packets according to their delay deadlines and treat the packets all the same regardless of these requirements. This presents a major issue for a closed loop control system where the data from the sensors is used to control the actuators that influence the process. Delays in the data can lead to negative reinforcement so that a process can run away rather than be kept within close limits.

　　This is different from an open loop system where the data is fed back to a central controller， analyzed with changes made in the order of seconds or even minutes. This is used for slowly changing processes or situations where the process falls outside its limits and can easily be brought back or the process terminated.