Highly synchronized, simultaneous, high-speed 24-bit data acquisition of triaxial MEMS accelerometers for monitoring a real world civil structure

A hardware system has been developed to obtain data from multiple MEMS triaxial accelerometers for structural health monitoring research in a laboratory setting. The system can be easily configured for a single accelerometer or up to as many as 120 triaxial accelerometers with 24-bit data sampled up to a 2 kHz rate simultaneously. The system is modular where each Aggregator Module consists of a circuit board that supports up to eight accelerometers. Up to 15 modules can be synchronized to within 0.1 μs for simultaneous data acquisition and timestamping by connecting a central clock module that distributes a 10 MHz clock to each Aggregator Module. This represents (4 channels per ADC) × (8 ADCs) × (15 Aggregator Modules) = 480 channels that are all simultaneously sampled and synchronized down to 0.1 μs. The data from each Aggregator Module is transmitted to a single host computer or multiple host computers using serial communication. The advantages of this custom system over a wireless solution are its very tight synchronization, high sample rate, high resolution and ability to process large quantities of data. COTS wired solutions do offer these features; however, to accommodate 480 sensor channels, numerous bulky and expensive pieces of equipment are needed. We decided that a custom design allowed us the greatest performance in a scalable, lower power, portable and affordable package. The original intended deployment of this custom system was in a laboratory setting, i.e. distances of one to three meters. However, we are extending the utility of the system to real world civil structures such as a parking garage or a pedestrian bridge with characteristic length scales of 10´s of meters. The primary constraint on the physical size of the deployment is the cable length from the central clock module to the various Aggregator Modules which directly impacts the signal integrity of the central clock. Laboratory testing has shown the clock - odule can drive 15.25m (50ft) of RG58 coaxial cable without signal degradation. Using this limit we will deploy an accelerometer network on a real world civil structure and report the data from that network. We have also developed a strategy for surpassing the 15.25m limit by daisy chaining modules and using the clock/synchronization module´s FPGA to regenerate the clock for the next Aggregator Module in the daisy chain. Although the clock regeneration at each module will create a clock delay, this delay is deterministic and can be accounted for when postprocessing the data. We will report on laboratory test results showing the clock signal integrity over distances greater than 15.25 meters. Our installation will demonstrate the scalability and portability of our custom, highly synchronous, simultaneously sampled data collection system in a real world deployment. A wireless system would not be capable of maintaining the tight synchronization or handling the large amounts of data, and a COTS wired solution would be less portable and more costly.