An accelerometer calculates acceleration by measuring the force acting upon a tiny mass inside the device. This force is then translated into an acceleration value.
Here's a breakdown of the process, focusing on a common type of accelerometer – the capacitive accelerometer:
Capacitive Accelerometers: A Detailed Look
Most modern accelerometers use a micro-electromechanical system (MEMS) that relies on changes in capacitance to measure acceleration.
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The Core Component: A Suspended Mass (Proof Mass): Inside the accelerometer is a small mass, often called a proof mass, suspended by tiny springs or beams. This mass is free to move in one or more directions.
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Capacitive Plates: The proof mass is positioned between or surrounded by fixed capacitive plates. These plates form capacitors, which store electrical charge.
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Acceleration Causes Displacement: When the accelerometer experiences acceleration, the proof mass is displaced due to inertia. According to Newton's Second Law (F=ma), the acceleration causes a force on the mass proportional to the acceleration. The springs exert a restoring force, leading to a displacement that is proportional to the acceleration.
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Capacitance Changes: As the proof mass moves, the distance between the mass and the capacitive plates changes. This change in distance alters the capacitance of the capacitors formed by the plates and the proof mass. Specifically, capacitance (C) is inversely proportional to the distance (d) between the plates (C ≈ 1/d).
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Signal Processing: An electronic circuit measures the change in capacitance. This change is a direct measure of the displacement of the proof mass, which is directly proportional to the acceleration experienced by the accelerometer. The circuit converts the change in capacitance into a voltage or digital signal.
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Calibration: The accelerometer is calibrated to accurately translate the measured voltage or digital signal into acceleration units (e.g., g's or m/s²). Calibration involves comparing the accelerometer's output to known accelerations and adjusting the conversion formula to ensure accuracy.
Different Types of Accelerometers
While capacitive accelerometers are common, other types exist:
- Piezoelectric Accelerometers: These use piezoelectric crystals that generate an electrical charge when subjected to stress (caused by acceleration).
- Piezoresistive Accelerometers: These use piezoresistive materials whose resistance changes when subjected to stress.
Summary Table: Capacitive Accelerometer Operation
| Step | Description |
|---|---|
| 1. Acceleration | Accelerometer experiences acceleration. |
| 2. Mass Displacement | Proof mass is displaced due to inertia, proportional to acceleration. |
| 3. Capacitance Change | The distance between the proof mass and capacitive plates changes, altering capacitance. |
| 4. Signal Measurement | Electronic circuit measures the capacitance change. |
| 5. Conversion | Capacitance change is converted into a voltage or digital signal. |
| 6. Calibration | The signal is calibrated to output accurate acceleration values. |
Conclusion
In essence, accelerometers utilize the principle of inertia and precise measurement techniques to determine acceleration by sensing the force required to resist movement of a small mass. The most common method involves measuring changes in capacitance caused by the movement of this mass.
