UPSC Prelims 2023 Q54: Accelerometer Uses in Everyday Devices
Consider the following actions:
1. Detection of car crash/collision which results in the deployment of airbags almost instantaneously
2. Detection of accidental free fall of a laptop towards the ground which results in the immediate turning off of the hard drive
3. Detection of the tilt of the smartphone which results in the rotation of display between portrait and landscape mode
In how many of the above actions is the function of accelerometer required?
- Only one
- Only two
- All three
- None
Answer: (c) All three
Statement-wise explanation
- 1: correct — In a crash, a car slows down very sharply in a fraction of a second. Accelerometers in the airbag control unit measure this deceleration. When it goes past a set threshold, the control unit fires the inflator and the airbag fills in milliseconds. The crash is detected by sensing acceleration, so the accelerometer is the core sensor.
- 2: correct — A body in free fall has nearly zero net acceleration along all three axes, because the sensor and its housing fall together. A three-axis accelerometer in the laptop or hard drive notices this near-zero-g reading. The drive then parks its read/write head in a safe zone before impact, which protects the disk platters.
- 3: correct — A phone at rest always feels Earth's gravity, about 1 g. The accelerometer shows how that 1 g is split across the phone's x, y and z axes, and this tells the software which way the phone is tilted. When most of gravity moves from the long axis to the short axis, the display rotates between portrait and landscape.
How to eliminate
This question is solved by knowing the fact, not by option tricks. The fact to remember is this: an accelerometer measures acceleration, including the constant pull of gravity. So it senses sudden deceleration (a crash), near-zero g (free fall) and how gravity is split across the device's axes (tilt). Statement 3 is the one most likely to cause doubt, because many candidates link screen rotation to the gyroscope. Basic orientation in phones comes from the accelerometer; the gyroscope adds precision for rotation and motion. Statement 2 sounds technical, but 'free fall' is simply a state of almost no measured acceleration. Once you see that all three actions detect a change in acceleration or gravity, the count is complete.
Know more
What an accelerometer does
An accelerometer measures proper acceleration: the acceleration a device feels compared with free fall. Most consumer devices use MEMS (Micro-Electro-Mechanical Systems) accelerometers. In these, a tiny mass on springs, etched in silicon, shifts when the device accelerates. The shift changes an electrical capacitance, and a chip turns that change into a reading in units of g.
Three readings matter for questions like this one:
- At rest: about 1 g, pointing along gravity. Its direction gives tilt and orientation.
- Free fall: close to 0 g on all axes.
- Impact or crash: a large spike, often many g, lasting milliseconds.
Sensors in a smartphone (often tested together)
| Sensor | Measures | Typical use |
|---|---|---|
| Accelerometer | Linear acceleration and gravity | Screen rotation, step counting, fall and shake detection |
| Gyroscope | Rate of rotation (angular velocity) | Gaming, image stabilisation, AR, smoother orientation |
| Magnetometer | Earth's magnetic field | Digital compass, map heading |
| Proximity sensor | Nearby object (usually infrared) | Turns off the screen during calls |
| Ambient light sensor | Light intensity | Auto-brightness |
| Barometer | Air pressure | Altitude, floor-level detection |
An IMU (Inertial Measurement Unit) combines an accelerometer and a gyroscope, and often a magnetometer. IMUs are used in drones, missiles, aircraft and spacecraft for inertial navigation, which works without GPS. Other accelerometer uses include seismometers that record ground shaking, vibration monitoring of machines and bridges, fall-detection alerts in smartwatches, and crash detection in phones.