The sensing elements in PR accelerometers are comprised of flexures on a middle wafer sandwiched between an upper and lower wafer (Figure 5). The bending of these flexures causes a measurable change in resistance that is proportional to the applied acceleration. A selection of full scale measurement ranges are attained by modifying the stiffness of the flexures or the seismic mass.
The upper and lower wafers are laminated to a middle wafer using a glass bond. This provides a hermetic enclosure for the flexures as well as mechanical stops for over-range protection. Gas damping lowers resonant amplification when PR accelerometers are used in high shock applications. Damping reduces the response to high frequency energy. Air is used rather than a liquid to reduce thermal effects on damping.
Figure 5. MEMS piezoresistive DC accelerometer construction
The sensing elements are arranged in a fully active Wheatstone bridge configuration. A fully active bridge (Figure 6) uses two resistors that increase with the input acceleration or force, and two that decrease. These are called tension and compression gages, respectively. The difference in voltage of these output lines will be proportional to the excitation voltage applied. The excitation voltage used during application should be the same as was used during the calibration process.
Figure 6. Wheatstone bridge
The sensing elements are typically mounted on circuit boards that are placed inside titanium or aluminum housings. Surface mount packages are also available. Surface mount MEMS sensors are typically soldered or epoxy attached at the next level of assembly. Figures 7, 8 and 9 are examples of packaging styles.
Figure 7. PR MEMS sensors in titanium housings
Figure 8. PR MEMS sensor in aluminum housing
Figure 9. Surface mount PR accelerometer packaged in leadless chip carrier