Wireless Strain Sensing Networks

Our goal was to demonstrate a wireless communications system capable of simultaneous, high speed data communications, particularly adapted for use with conventional foil strain gauges. We have previously described data logging transceiver nodes capable of 2K samples/second/channel data rates. However, only one node may stream data at a time, since all nodes on the network share the communications channel. To overcome these limitations, 2 generation data logging transceivers were developed with software programmable radio frequency (RF) communications. Each node contains on-board memory (2 Mbytes), regulated sensor excitation, instrumentation amplifiers with programmable gains & offsets, multiplexer, 12 bit A/D converter, microcontroller, and frequency agile, bi-directional, frequency shift keyed (FSK) RF serial data link. The nodes are capable of continuous RF transmission from over 26 nodes, over frequency bands of 402-470 MHz & 804-940 MHz bands, at data rates of 76.8 kbaud. An aluminum beam with half bridge, 1000 ohm bonded foil strain gauges was used to determine sensitivity (through shunt calibration) and to assess gain stability over temperature. With full scale range of 4000 microstrain, and anti-aliasing filter bandwidth of DC to 500 Hz, resolution was +/2.5 microstrain. Environmental chamber tests from +40 to 0 degrees C yielded gain coefficients of +.004%/deg C to +.06%/deg C and offset coefficients of -.007%/deg C to -.09%/deg C. The system has been demonstrated for efficient quality control on Ford Motor Company’s automotive production line. SHM with these wireless strain sensing networks on large civil structures are currently underway.