Wireless Monitoring and Low-Cost Accelerometers for Structures and Urban Sites

Abstract

New technologies in wireless telemetry and low-cost micromachined silicon accelerometers enable lower cost and denser instrumentation of both structures and the urban environment. Applications include any site where robustness, capital or maintenance costs, spatial density, or ease of communication are controlling issues. This paper reviews these new implementing technologies and gives two practical examples.

Recently introduced techniques have vastly expanded the palette of practical telemetry options. These include unlicensed spread-spectrum, cell-phones in several variants, two-way paging, and a rapidly growing set of satellite services. Many products are available for each technique, so off-the-shelf solutions are routinely achievable. Relevant issues when selecting telemetry for a particular project include whether the service is offered in that region and how complete their coverage is, access from inside structures and urban canyons, capital and recurring costs, ease of installation, and viability after an earthquake. These issues vary greatly from one technique to another. However, an attraction shared by most is the elimination of most hard wiring, which can greatly ease installation and reduce the number of failure modes, both in routine maintenance and during an earthquake.

Low-cost micromachined accelerometers have reached price and performance levels of interest for structural and free-field monitoring, particularly to augment the spatial density of more expensive instruments in urban areas. For example, one can find dynamic ranges reaching from typical urban background noise levels to ±2g for under US$100 per channel, or better the resolution of an SMA-l for a few tens of dollars per channel. This paper includes performance details from USGS tests of the high-dynamic-range accelerometer. Most micromachined accelerometers exhibit small physical size, low cost, and great robustness and stability. In combination, these characteristics yield low capital and maintenance costs.

The new wireless technologies and micromachined accelerometers can contribute separately to structural monitoring, and they may be used separately in many situations. Additionally, the two technologies can be combined to sharply reduce the cost of arrays, and to permit arrays of unprecedented spatial density. The two examples presented here combine both technologies. In the first, spread-spectrum transceivers and micromachined accelerometers were used to monitor a New Mexico highway bridge for structural integrity. In the second, micromachined accelerometers and cell-phone Internet links (CDPD) are being used for a regional urban freefield array in Oakland, California.