ELECTRICAL SENSING

Turning motion into a signal

There are several ways to electrically measure the proof mass's position:

Capacitance change

The most common method in MEMS accelerometers

Inductance change

Based on magnetic coupling

Optical method

Uses light to detect displacement

Scanning probe tips

Physical tip-based position sensing

PolyMUMPs-style devices almost always use capacitance — it's cheap, robust, and easy to build with polysilicon.

CAPACITOR SHAPES

Parallel plate vs. interdigitated

Parallel Plate

Two flat plates facing each other; capacitance changes as the gap changes.

Interdigitated (Comb)

Interleaved finger electrodes; capacitance changes as the fingers slide past each other.

Comb structures are the shape you'll see in most real capacitive MEMS accelerometers, including the ADXL150.

FROM CAPACITANCE TO VOLTAGE

Reading the capacitor

Q = C(x)V Charge on the sensing capacitor

i_C = dQ/dt Current through the capacitor

V_o = −R_F · i_C Output of a transimpedance amplifier

With a DC source, that output ends up proportional to velocity, not position. Using an AC excitation source and a feedback capacitor instead makes the output voltage proportional to C(x) — and therefore to position.

The circuit design choice — DC vs. AC excitation — determines whether you're measuring velocity or position.

UNIT 19 STUDY COMPLETE

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You've covered Capacitive Position Sensing.