Low-Latency Data Acquisition Hardware for Real-Time Wireless Sensor Applications

Wireless sensor networks (WSNs) are an attractive alternative to traditional tethered systems for monitoring and feedback control of civil structures. In civil engineering, research has focused on the application of WSN to structural health monitoring (SHM); as a result, hardware has been tailored to SHM applications. However, the real-time performance requirements of WSNs for control are more stringent than for monitoring applications. Wireless communication, processing time, and data-acquisition hardware are a few of the many sources of time-delay in wireless control systems; this paper will focus on the latency due to the acquisition and actuation hardware in the control loop, i.e., the time between capturing a measurement and its availability on the processor. Previous work on smart sensor hardware focuses either on resolution for SHM applications or the actuation interface for control applications. Overall, an analysis of latency due to the data-acquisition hardware and an understanding of the inherent limitations have been lacking. This paper illustrates the limitations of a common analog-to-digital converter (ADC) architecture for SHM applications and presents a low-latency hardware solution for wireless control nodes. The performance of the two different data-acquisition techniques emphasizes the implication of ADC architecture on the latency and resolution of the data. Ultimately, through the use of an successive-approximation-register-type ADC and careful design of the corresponding driver, the latency due to the hardware is almost negligible.