A Synchronous TDMA Ultrasonic TOF Measurement System for Low-Power Wireless Sensor Networks

This paper presents the design and evaluation of an ultrasonic time-of-flight (TOF) measurement system in the context of a smart sensor wireless network. In particular, the ZigBee protocol is used for data transmission and synchronization purposes. Low-cost and low-power restrictions are taken into account in the design. A synchronous measurement scheduling approach is used to minimize the network traffic and, therefore, the power consumption, while avoiding the need of wired connections between the nodes or the use of specific radio link to provide synchronization. A theoretical model that describes the accuracy of the proposed system is derived. This model takes into account both clock drift effects and finite clock resolution of the network nodes. According to the model, the estimation of the TOF is biased due to the clock drifts, and a solution is proposed to compensate this bias. The compensation is based on an accurate estimation that each node performs for its own clock drift. An error analysis of this estimation procedure is also developed, and its effects on the TOF accuracy are presented. A theoretical model of the system that predicts the system performance in terms of TOF accuracies is proposed. An implementation of the TOF measurement system is presented, from which experimental results that validate the theoretical derivations and the effect of the clock drift compensation are obtained. Experimental evaluation of the system also demonstrates that TOF accuracies better than 2 μs are achievable, which will be more than adequate for achieving subcentimetric or even submillimetric precisions in ultrasound-TOF-based distance measurement systems. Even though a particular approach for TOF estimation is considered in this work, most of the derived results are also applicable to other systems involving time synchronous measurements.