Industrial control systems, process control systems, distributed control systems and the like in process industries often comprise both hard-wired data networks and wireless data networks. Wireless sensor networks are used to communicate measurements and some control data between wireless field devices such as sensors, and/or actuators, and the industrial control system. Wireless sensors are often battery powered.
It is known that the environment in industrial plants and installations is harsh which can make reliable wireless communication a challenge. In this description industrial plants may include oil and gas (petroleum) exploration, production or processing installations in oil and gas production/extraction installations; pulp and paper mills, metal rolling mills, cement works, electrical power generation, transmission and distribution installations, processes or equipment on a ship or on an oil platform, mines both above ground and underground, and so on. In some cases the signal strength in an industrial plant may fluctuate by around 40-50 dB. And in addition, in some cases the signal strength may drop even further due to shadow fading (for instance when a truck is parked in front of the wireless device). Despite the challenges posed by a harsh wireless environment, the use of industrial wireless sensor networks is predicted to increase exponentially within the industrial automation domain and they will be deployed in very harsh industrial environments which will make it difficult to ensure reliable communication.
Such a harsh wireless environment liable to temporary changes in wireless transmission/reception conditions makes radio communication in industrial plants a challenge, especially for real-time communication with strict delay requirements and deterministic behavior. Available Industrial Wireless Sensor Network standards such as IEEE 802.15.4 [1], ISA100 [3], WIA-PA [4] and Industrial WLAN [5] provide guidance for normal transmission and reception. However these standards do not provide information on how to improve the reliability in wireless sensor networks used in industrial installations.
From another technical field, the field of mobile wireless communication networks, a patent U.S. Pat. No. 6,982,987, entitled Wireless communication network including data prioritization and packet reception error determination features and related methods, assigned to Harris Corp., describes a method to prioritize communications in a mobile communication network. It further discloses a mobile ad-hoc network (MANET) wherein data is assigned a priority level with a respective Quality of Service (QoS) requirement. A QoS metric, which is said to be e.g. the signal-to-interference ratio (SIR), is determined for different transmission time slots and is used to map prioritised data to the best time slot. Similarly another patent, U.S. Pat. No. 7,142,866, entitled Load leveling in mobile ad-hoc networks to support end-to-end delay reduction, QoS and energy leveling; and also assigned to Harris Corp., also discloses a mobile ad-hoc network (MANET) where data is prioritised according to type-of-service (ToS) and transmitted on routes that are ranked according to their quality-of-service (QoS). The QoS is said to be based on e.g. link reliability, the meaning of which term is, however, not further explained.
In U.S. Pat. No. 7,864,682, entitled Method for routing data in networks, assigned to Samsung Electronics, a wireless mesh network is disclosed, wherein different types of traffic have different requirements and are therefore routed differently. In this disclosure, the metrics used are link latency or link bandwidth. In another disclosure relying on route identification, US2012099587, entitled “Forwarding and routing in sensor networks” and assigned to Toshiba K.K., a wireless mesh sensor network and a method is disclosed wherein traffic is classified into two different levels of priority (standard or critical). The critical priority data is routed in the most reliable route, which is determined by measuring a signal-to-interference-plus-noise (SINR) value of each route. The disclosure describes that the most reliable route is identifiable by the node on the basis of previous successful transmissions, success being determined on the receipt of an acknowledgement from a packet recipient node.
However although there are some known methods to make a static determination of an identified best route there is room in the field of industrial wireless sensor networks for improvement in the reliability of wireless communication.
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