Wireless communication systems have been a part of everyday life for many decades but implemented in an industrial control system they are yet to make their big breakthrough. This is despite of the fact that wireless control systems show great promise in reducing installation costs and down time.
The biggest reason why they have not already revolutionised the industry is reliability, as they are much more susceptible to delays and communication breakdowns compared to their wired counterpart.
The foundation for this study is the following scenario. In an industry a control loop uses a wireless link between the control computer and the actuator. A forklift moving heavy equipment happens to park in a position where it is blocking the communications.
This study 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 nding a disturbance free path for the packet through the network. Secondly a Kalman lter based Predictive Outage Compensator (POC) estimates the missing data when the real control signal 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 most recently received value in case of outage.
Author: Wallander, Jakob