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How does an accelerometer work?

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Quick answer

Inside an accelerometer, a tiny suspended mass lags behind when the device speeds up, bends or shifts. Sensors measure that movement as a change in capacitance or resistance and turn it into an electrical signal. Since gravity pulls the same mass, a still accelerometer reads 1 g, letting it also detect tilt and orientation.

An accelerometer measures acceleration by detecting how a small proof mass inside the device moves relative to its fixed frame. That displacement is converted into an electrical signal proportional to the acceleration, which electronics then read as motion, tilt or vibration.

Step by step

  1. A small proof mass is suspended on flexible silicon beams so it can move along one or more axes.
  2. When the device accelerates, inertia makes the mass lag, deflecting it relative to the fixed frame.
  3. The deflection alters capacitance, piezoresistance or another electrical property of the sensor.
  4. Interface circuits convert that change into a voltage or digital reading proportional to acceleration.
  5. Software combines readings from several axes, and often a gyroscope, into motion and orientation data.

Key things to understand

  1. A tiny proof mass is suspended on flexible beams or springs inside the sensor, free to shift when the device accelerates.
  2. When the sensor accelerates, inertia keeps the proof mass briefly behind, so it deflects relative to the surrounding frame.
  3. The deflection changes an electrical property such as capacitance between the mass and fixed plates, or the resistance of a strain gauge.
  4. On-chip circuits convert that tiny change into a voltage or digital number proportional to the acceleration.
  5. Because gravity also pulls on the proof mass, a stationary accelerometer reads about 1 g, which is how it senses tilt.

Labelled diagram

  1. Proof mass

    A tiny silicon weight suspended on flexible beams inside the chip.

  2. Acceleration

    The frame moves, but inertia makes the mass lag behind briefly.

  3. Deflection

    The mass shifts relative to fixed plates, changing capacitance or resistance.

  4. Signal

    Circuits convert the change into a voltage or digital number.

  5. Reading

    Software interprets the value as motion, tilt or vibration.

How an accelerometer senses motion

Real-world examples

  • Smartphones rotate the screen and count steps using a three-axis accelerometer chip.
  • Car airbag systems trigger deployment when an accelerometer detects a sudden, large deceleration.
  • Drones and cameras use accelerometers with gyroscopes to hold steady flight and stabilise footage.
  • Seismometers and structural monitors use sensitive accelerometers to record ground shaking and building sway.

Common mistake

✗ Myth: An accelerometer measures speed.

✓ Fact: It measures acceleration, the rate of change of velocity, not velocity itself. A device moving at constant speed in a straight line reads zero acceleration apart from gravity, which is why accelerometers alone cannot track position for long.

People also ask

What does an accelerometer measure?
It measures proper acceleration: the acceleration of the sensor relative to free fall. That includes the constant upward push from whatever surface it rests on, so a still device reads about 1 g. It does not directly measure speed or position, though software can estimate those by integrating the readings over time.
How does a phone accelerometer work?
A micrometre-scale proof mass sits on silicon springs inside the chip. When you move the phone, the mass shifts, changing capacitance between it and fixed plates. The chip converts that into digital acceleration values for each axis, which apps use for screen rotation, step counting and games.
Why does an accelerometer read 1g when still?
Gravity pulls the proof mass downward, but the surface it rests on pushes the sensor upward, so the mass deflects as if accelerating upward at about 9.8 metres per second squared. That constant reading, called 1 g, is what lets the device sense which way is down and detect tilt.
Why does my phone know which way is up?
A stationary accelerometer still feels gravity pulling the proof mass, so it registers about 1 g along whichever axis points down. The phone reads that direction and rotates the screen to match, which is why tilting the device flips the display.
What is the difference between an accelerometer and a gyroscope?
An accelerometer measures linear acceleration, including the constant pull of gravity. A gyroscope measures angular velocity, how fast the device is rotating. Phones combine both, often with a magnetometer, so software can track full orientation and motion without either sensor drifting badly.
Why do fitness trackers sometimes count steps that did not happen?
Step counting looks for a repeating up-and-down acceleration pattern near walking pace. Hand movements, driving over bumps or gestures can produce a similar rhythm, so the algorithm registers false steps. Makers tune thresholds to trade missed steps against phantom ones.
How small can accelerometers be?
Modern micro-electromechanical accelerometers are built on silicon chips with moving parts only micrometres across, smaller than a grain of salt. They are mass-produced for phones, cars and drones, which is why motion sensing is cheap and everywhere.

Part of the guide: How the Internet Works →