Seismic mass

Figures 13.7 (a) and (b) show an acceleration sensor also made using an HOD film of 15- im thickness, where the seismic mass was Si 422]. Its resonance frequency was 4.7 kHz, the fracture strength was 4.5 GPa, and the estimated shock tolerance was 7000 G. The decay time of the seismic mass oscillation was within seconds.

Fig. 5.3.12 Working design of a discrete accelerometer made by silicon surface micromachining the seismic mass forms the movable inner electrode and is suspended on two springs the movable capacitive fingers are interdi-gitated between the outer electrodes 1 and 2, which have fixed (immovable) capacitive fingers...

Fig. 5.3.13 Micromachined acceleration sensor with seismic mass (a), movable comb fingers (b), two fixed-finger outer electrodes (c), two supporting springs (d), and two over-range stops (e)...

Fig. 5.3.14 Detailed view of seismic mass (left) and comb finger structure (right)...

First, a bias voltage applied between the fixed comb drive electrodes and the movable comb drive electrodes connected to a seismic mass suspended by a... 

Defects include (dust) particles on or within suspended interdigitated seismic masses, leading to electrical shorts and to deviations in moments of inertia. Another example relates to the appearance of crystallographic defects such as dislocations or stacking faults in epitaxial superstructures, causing leakage currents that might impair sensor reliability in the field. Defect densities may of course vary within a wafer, from wafer to wafer, and from lot to lot. Unfortunately, at present not many models are available that link tolerance bands of functional parameters to specific defects and their tolerable density distributions. 

Quartz and piezoelectric ceramic crystals have more temperature independent constants than PVDF, so they are used for force and acceleration transducers. However, PVDF films can be used for large area flexible transducers. Their sensitivity to stress or strain allows the construction of pressure sensors (using the J33 coefficient), and accelerometers by mounting a seismic mass on the film. PVDF electrets are particularly suited for large area hydrophones (Fig. 12.21) that detect underwater signals. Their... 

Acceleration is the time derivative of velocity and the second derivative with respect to time of displacement. Thus, sensors of displacement and velocity can be used to determine acceleration when their signals are appropriately processed. In addition, there are direct sensors of acceleration based on Newton s second law and Hooke s law. The fundamental structure of an accelerometer is shown in Figure 2.3. A known seismic mass is attached to the housing by an elastic element. As the structure is accelerated in the sensitive direction of the elastic element, a force is applied to that element according to Newton s... 

FIGURE 2.4 Example of a silicon chip accelerometer fabricated using MEMS technology. The lower figure shows the upward deflection of the seismic mass with a downward acceleration. Typical dimensions of the silicon chip are 1 and 2 mm length and width, respectively and less than a mm thick. This means that the packaged chip can be very small. 

Acceleration measurement seismic-mass, strain-gauge, potentiometric, piezoelectric and servo accelerometers. 

Inertial sensors, such as seismometers, measure accelerations due to the motion of a seismic mass. However, inertial sensors are also sensitive to static accelerations such as gravity. Normally, accelerations due to gravity are counterbalanced by the mainspring in the suspension of a seismometer, but when a seismometer is tilted (rotated around a horizontal axis), gravity... 

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