Reducing wireless control communication for a water irrigation system

Abstract

For this thesis we have compared communication times of different control schemes on a wireless control network for a water irrigation system. Therefore a control application which can run different protocols was created. We have shown that a significant reduction of communication time can be achieved by using event-triggered control (ETC), and by using self-triggering techniques we could reduce this even further. Wireless control networks (WCN) are networks of one or multiple controllers, sensors and actuators which share a digital communication network. Sensors send their measurements to the controller, and the controller sends a control signal to the actuators. Because the nodes are digital devices, periodic control is the standard approach. Communication therefore is not needed all the time, and nodes can sleep in between updates to save energy. Event-triggered control is a technique to further reduce communication of resource constrained nodes in the network by only updating control when an event, typically a significant change since the last control update, occurs. Water irrigation systems (WIS) are networks of channels transporting water from main rivers, to smaller rivers which need to be controlled to avoid losses. At TU Delft a model of a water irrigation system with three pools has been connected to a WCN to simulate this. To create an application for this testbed, supporting different control schemes, a low power embedded OS capable of doing multitasking with a replaceable network stack is needed. The Contiki Operation System is capable to do this and has been used by the D3S Research Group of the University of Trento to build wireless control bus (WCB), and has also been used for the WIS control application. WCB uses network floods from a protocol called Glossy and a schedule to synchronise communication with low power, and supports both periodic and event-triggered control. Experiments with WCB show periodic control is possible with only 7% communication time for this setup, and event-triggered control with only 2% without much loss of performance. Because the testbed unit has some technical problems, a hardware-in-the-loop simulation was created to connect to the WCN to do the experiments. A further reduction in communication has been achieved by combining a predictive control method called PSTC with ETC, which we call ETC+. Advantages over normal ETC are not large in our experiments, and there are still practical problems, but they show this is a promising research area.