Routing and Unreliable Communication Link Compensation for Wireless Sensor Networks

Wireless communication is widely used in modern devices but has not yet conquered the industry. Introducing wireless control in the industry could result in an economical benefit since installing cables is time consuming and expensive. Wireless control brings other problems to the table such as delays and packet loss.

The foundation for this thesis is the following scenario. In an industry a control loop uses wireless links between the sensors, control computer and actuator. A forklift moving heavy equipment happens to park in a position where it is blocking the communications. The thesis suggests a solution to this problem through the implementation of two separate measures. First a Collection Tree routing Protocol (CTP) is responsible for detecting broken links and finding an interruption free way for the packet through the network. Secondly a Kalman filter based Predictive Outage Compensator (POC) estimates the missing data when the real information is delayed or missing.

In the implementation the Moteiv Tmote Sky was used for the network nodes and the double tank process as test bed for the POC. The results show that the CTP network is capable of routing the data through multiple hops without too much delay and that between 1 and 8 packets are lost during route changes. The POC performs on average 8% better than the more common and naive solution to simply hold the last value in case of outage.

The thesis was carried out in the WiComPi project, lead by Karl-Henrik Johansson, by Ivar Cornell and Jakob Wallander.