Wireless Sensor and Actuator Networks: Algorithms and Protocols for Scalable Coordination and Data Communication

9.7 FAULT-TOLERANT SEMIPASSIVE COORDINATION AMONG ACTUATORS

A multiactuator/multisensor model was studied in Ozaki et al. (2007) to provide fault tolerance for WSANs. Each sensor node sends a sensed value to multiple actuators and each actuator receives sensed values from multiple sensor nodes. Such reporting model provides fault tolerance if any sensor or any actuator are faulty. A semipassive coordination protocol was proposed in Ozaki et al. (2007).

One actuator plays a role of a coordinator, called primary actuator, while other actuators are backup actuators in an event area. The protocol (Ozaki et al., 2007) consists of three phases: broadcast phase, decision phase, and update phase. In the broadcast phase, if an event occurs, a sensor node sends a measured value to multiple actuators in the area. Due to collision and noise in wireless channels, some actuators may not receive the value correctly. Each receiving backup actuator sends a message with the sensed value and the ID of the corresponding sensor node to the primary actuator. Once the primary actuator receives a certain number of messages (greater than a predetermined threshold), it makes a decision on the measured value from the value set. It can be a majority value from a discrete set, or, in the case of continuous values, a function filter could be used to remove extreme values and compute the average of the remaining ones. The estimated value and other updates are then broadcasted to all the backup actuators. Upon ...