5 - Measuring parameters

Si bulk-micromachined acceleration sensor

In bulk micromachined capacitive acceleration sensors, the cantilever structure etch out from Si is bonded between an upper and lower wafer or metallized glass substrates. An acceleration perpendicular to the wafer surface moves the seismic mass out of its center position. The displacement is detected by the top and bottom capacitors between the wafers. A capacitive half bridge is generally used for signal pickup.

Piezoresistive bulk-micromachined acceleration sensor

Bulk micromachined piezoresistive acceleration sensors use a structure in which the seismic mass is suspended on four bridges. The deformation of the bridges due to the acceleration is followed by four piezorestors integrated into them.

Surface micromachined acceleration sensor

In surface micromachined Si sensors movable polysilicon structures are etched out using sacrificial layers. The seismic mass is suspended at the four corners by springs. The fingers to both sides of the seismic mass as well as fingers attached to the wafer surface form a capacitance for signal pickup. This capacitance is changed during acceleration. In contrast to bulk micromachining, surface micomachining uses movements parallel to the wafer surface.

The rotation of a multipole-magnet causes the magnetic field to change. Each rotation of the ring magnet produces multiple cycles of alternating voltage from the Hall sensor. The comparator changes the AC waveform to a pulsed digital signal that can be processed by a frequency meter or a counter circuit.

Hinged vane flow sensing

The flow acts on a spring-loaded hinged vane that responds to flow-rate changes with changes in the deflection.

Differential-pressure flow sensing

Flow is very commonly measured by forcing the fluid to flow through a restriction in a pipe (like a Venturi tube shown in the example) so that its velocity changes at that point and a pressure difference is created.

Rotating type anemometers

In rotating type mechanical anemometers the rotation speed of a propeller or a turbine wheel responds to the flow velocity.

Thermal flow sensing (Thomas flow-meter)

The amount of heat transferred between two points in a moving fluid or gas is proportional to the mass flow rate thus, the a temperature difference can be measured with the temperature sensors located upstream and downstream of a heater element.