A MEMS (Micro-Electro-Mechanical Systems) accelerometer is a chip that measures acceleration by watching a tiny silicon proof mass wiggle. The mass — often just a few micrograms, etched from the same silicon wafer as the surrounding structure — is suspended on flexible silicon springs. When the chip accelerates, the mass lags behind (Newton's first law), deflecting the springs. That deflection is measured, converted to voltage, and read out digitally.

How the sensing works: The proof mass carries a comb of interdigitated fingers that sit between two fixed comb electrodes. This forms a differential capacitor: when the mass moves toward one electrode and away from the other, one capacitance grows and the other shrinks. A charge amplifier senses the difference — typical capacitance changes are femtofarads (10⁻¹⁵ F), which is why the sensing electronics sit on the same die as the mechanical structure.

Governing physics: The proof mass behaves as a damped spring-mass system: ma = -kx - cẋ + F_ext. Under steady acceleration, the displacement settles to x = ma/k. So displacement is directly proportional to acceleration — that's why the reading is linear.

Rule of thumb: Sensitivity scales with proof mass and inversely with spring stiffness. Doubling the mass or halving stiffness doubles sensitivity but halves the resonant frequency (f = (1/2π)√(k/m)) — which halves the usable bandwidth. You always trade sensitivity for bandwidth.

Real-world example — smartphone screen rotation: A ±2g, three-axis MEMS accelerometer (like the STMicro LIS3DH, ~$1) sits on the phone's PCB. Gravity is always 1g pulling toward Earth's center. When you tilt the phone, the g-vector projects differently onto the three axes. The OS reads X, Y, Z at ~50 Hz, computes tilt angle via atan2(x, y), and rotates the display when the angle crosses a threshold for long enough to reject transients. The whole loop takes about 200 ms — deliberately slow, so the screen doesn't flip every time you shake your hand.

Other applications:

• Airbag deployment: ±50g sensors detect crash deceleration in under 20 ms.
• Hard drive protection: Laptop drives park heads within 300 ms of detecting free-fall (all three axes near zero simultaneously).
• Structural monitoring: Low-g (±2g), low-noise sensors on bridges measure vibration modes to detect fatigue cracks.
• Inertial navigation: Integrated with gyroscopes and magnetometers in a 9-DOF IMU for drones and AR headsets.

Key limits: MEMS accelerometers drift with temperature, have offset errors (~20 mg typical), and can't distinguish gravity from linear acceleration — that's why standalone dead-reckoning fails within seconds and why real navigation systems fuse accelerometer data with gyros, GPS, or vision.