Elevator-Assisted Sensor Data Collection for Structural Health Monitoring

Sensor networks nowadays are widely used for structural health monitoring; for example, the sensor monitoring system deployed on the Guangzhou New TV Tower, China. While wired systems still dominate, it is commonly believed that wireless sensors will play a key role in the near future. One key difficulty for such systems is the data transmission from the sensor nodes to the base station. Given the long span of the civil structures, neither a strategy of long-range one-hop data transmission nor short-range hop-by-hop communication is cost-efficient. In this paper, we propose a novel scheme of using the elevators to assist data collection. A base station is attached to an elevator. A representative node on each floor collects and transmits the data to the base station using short range communication when the elevator stops at or passes by this floor. As such, communication distance can be minimized. To validate the feasibility of the idea, we first conduct an experiment in an elevator of the Guangzhou New TV Tower. We observe steady transmission when elevator is in movement. To maximize the gain, we formulate the problem as an optimization problem where the data traffic should be transmitted on time and the lifetime of the sensors should be maximized. We show that if we know the movement pattern of the elevator in advance, this problem can be solved optimally. We then study the online version of the problem and show that no online algorithm has a constant competitive ratio against the offline algorithm. We show that knowledge of the future elevator movement will intrinsically improve the data collection performance. We discuss how the information could be collected and develop online algorithms based on different level of knowledge of the elevator movement patterns. Theoretically, given that the links capacity assumptions we made, we can prove that our online algorithm can guarantee data delivery on time. In practice, we may set a buffer zone to minimize the possible data delivery violation. A comprehensive set of simulations and MicaZ testbed experiments have demonstrated that our algorithm substantially outperforms conventional multihop routing and naive waiting for elevator scheme. The performance of our online algorithm is close to the optimal offline solution.