Structural Health Monitoring Using Smart Sensors - ideals ...

Figure 4.8. Eight-pole AA filter board.
Magnitude (dB)
0
-20
-40
-60
0 10 20 30 40 50 60 70
Frequency (Hz)
Figure 4.9. Eight-pole AA filter board transfer function.
Design
Measured
a larger deterministic time lag. This filter is well-suited for measurements in which no
signal of interest is above 25 Hz and the time lag of the filter, 34 ms, is acceptable. (see
Figures 4.8 and 4.9)
4.3 Experimental verification of strain sensor
The strain sensor/AA filter boards must be calibrated before use. These boards are
stacked on the Mica2, and shunt calibration is conducted to determine the sensitivity of the
strain sensor. Shunt calibration simulates a resistance change in the strain gage by
shunting the Wheatstone bridge with a known resistor. The output can then be calibrated.
The data acquired by the Mica2 is first stored on on-board flash memory and then
wirelessly transmitted to the base station (Mechitov et al., 2004). For convenience of
calibration, the strain sensor board is equipped with four switches to shunt the bridge with
different resistors that are wired in parallel with the strain gage. The 4-pole AA filter
board is employed for this calibration as well as the experiments described subsequently.
The sensitivity and offset of the strain sensor board is thus calibrated.
The sensor noise level must also be characterized. Based on the RMS of the measured
signal, the resolution of the strain gage is estimated to be 3.0 , which is slightly larger
than the target noise level. By using a more precise amplifier, as well as electromagnetic
shielding, further reduction in the noise level of the analog circuit is considered possible.
In addition, the flash memory writing process is a noise source. The writing process needs
a relatively large current and destabilizes the on-board power supply. Analog-to-digital
47