Characterizing Variability in Strain Measurements Resulting from Temperature Changes Affecting Data Acquisition Equipment

Abstract

This article reports the results of experiments conducted on the National Instruments Compact-RIO (cRIO) data acquisition equipment (DAQ) aimed at characterizing the contributions to strain measurements originating from the DAQ instrument, separate from those of the strain gauges and ultimately the structure being monitored. In most laboratory based measurement situations, the DAQ is at a fixed environmental temperature, and thus does not contribute to the measured strain. In SHM applications, however, this is not necessarily the case. To better understand the relationship between temperature and instrument induced strain, a series of environmental chamber tests were conducted. These tests measured strain from twelve metal-foil strain gauges bonded to an unloaded steel beam which was subjected to temperature variations in the range of –20°C to +20°C. Three separate test cases were explored. One with the cRIO at room temperature while a test beam and gauges were subjected to temperature variations. A second with the test beam at room temperature and the cRIO undergoing temperature variations. And a third with both the cRIO and test beam in the same variable temperature environment. These experiments clearly show that a significant contribution to apparent strain comes directly from the DAQ instrument itself. Strains reported by the cRIO when only the temperature of the test beam was changing indicate a difference of as much as 30με when compared to the case where the instrument and beam were both at the same temperature. Furthermore, the extent of the variation is different from input channel to input channel, and must therefore be characterized for each channel separately. Strains measured by the cRIO when only the instrument itself was undergoing a temperature change showed an approximately linear relationship.