Accelerometer Applications

An accelerometer senses acceleration. Gravity counts as one kind of acceleration it can sense.

1. What does an accelerometer do?

An accelerometer measures acceleration. The Smithsonian’s How Things Fly site says g-force is measured with an accelerometer. In an aircraft it takes the acceleration the plane feels and converts it into g.

NASA’s microgravity instruments, SAMS and OARE, also measure accelerations on the Space Shuttle.

2. What is "1 g"?

One g is 9.8 metres per second squared. This is the acceleration at which Earth’s gravity pulls things down.

Two g is twice that, or 19.6 metres per second squared. The g unit is easier to read than raw acceleration.

3. Why do astronauts and falling objects feel weightless?

NASA says microgravity, a state of near weightlessness, can only be reached on Earth by putting an object in free fall.

NASA’s Zero-G facility gives 5.18 seconds of low gravity. The drop vehicle falls inside a vacuum chamber 470 ft (about 143 m) long.

The Shuttle is also in free fall. It is continuously falling around Earth. This is why astronauts seem weightless.

• Free fall: the object just falls, with nothing holding it up.
• Orbit: a continuous fall around Earth.
• Sensors on the Shuttle still measure small accelerations from onboard activity.

4. How can gravity be used to test an accelerometer?

Gravity gives a known push at any tilt. The USGS test of a MEMS seismic accelerometer used this idea.

The team rotated the sensor in steps on a motorised stage. Each step applied a different part of gravity along the sensing axis. They first set the output to zero at a zero-g position.

The USGS says tilt calibration showed a flat response all the way down to 0 Hz. So the sensor feels steady gravity as well as shaking.

Most modern accelerometers are tiny chips. They have a small moving mass that reacts to motion.

5. What does MEMS mean and how small is the sensor?

MEMS means micro-electro-mechanical systems. A MEMS accelerometer is a tiny machine built from silicon.

The USGS describes one prototype made from three silicon wafers bonded together. The die measures 6.8 mm by 6.8 mm. The sensing mass is sealed in a vacuum.

NASA Glenn funded a MEMS accelerometer in 2005. NASA says it shrank a device the size of a desk telephone to a chip the size of a dime.

6. How does a capacitive MEMS accelerometer sense motion?

The USGS calls its test sensor a capacitive sensor. It uses surface electrodes to sense the mass.

A peer-reviewed vehicle study uses a MEMS capacitive, single-mass device. Capacitive sensors turn a change in electrical capacitance into a reading.

7. What other ways can a MEMS chip sense acceleration?

A 2023 peer-reviewed paper lists three ways. MEMS accelerometers detect acceleration through a change in capacitance, resistance or charge.

• Capacitive: a change in capacitance.
• Piezoresistive: a change in resistance.
• Piezoelectric: a change in charge.

The same paper says piezoelectric designs have several advantages. These include better temperature stability, lower power use and a wider dynamic range.

8. How common are MEMS accelerometers?

The same paper says they rank second in use among MEMS devices. Only pressure sensors are used more.

It names vibration monitoring, automotive testing and inertial navigation as traditional uses. It adds health-monitoring devices and implantable hearing aids as recent research areas.

9. What is a force-balance accelerometer?

Some sensors hold the mass still instead of letting it move. NASA’s 2013 Spinoff describes how a traditional accelerometer works.

When the proof mass tries to move, magnetic forces that suspend it increase or decrease. The current needed to hold it in place gives the force. That force gives the acceleration.

A phone knows which way is down because gravity is always pulling on the sensor.

10. How does a smartphone rotate its screen?

Google’s Android Developers Blog says most Android devices use the accelerometer to detect when the device is held sideways. The system then rotates the screen.

The blog warns that the sensor axes do not always match the screen axes after a rotation. Developers should read the screen rotation directly.

11. What other uses do researchers see for MEMS accelerometers?

The 2023 paper above names health-monitoring devices and implantable hearing aids as research areas.

In a crash, speed drops very fast. A sensor can feel that sudden change.

12. What do car accelerometers do?

A peer-reviewed study says MEMS accelerometers can coordinate with electronic stability control. This gives a quick and accurate airbag response.

• Airbags and crash detection.
• Electronic stability control.
• Anti-lock braking systems.

13. What else can a car accelerometer measure?

The same study used MEMS sensors to check vibration on a moving vehicle. It measured amplitudes between 0.5 g and 2.5 g above the engine and radiator fan.

The sensors in the test had ranges of plus or minus 3 g and plus or minus 8 g. A piezoelectric reference sensor had plus or minus 100 g.

The paper says dashboard data in driving mode reflects the road condition. It names diagnostics, safety and comfort as aims.

If you add up acceleration over time, you can track movement. This needs no outside signal.

14. What is the role of ISRO's Inertial Systems Unit?

The ISRO Inertial Systems Unit (IISU) is in Thiruvananthapuram. ISRO says it designs and develops inertial systems for launch vehicles and spacecraft.

ISRO lists inertial navigation systems, attitude reference systems and rate gyro packages. It also lists accelerometer packages. It says these are built in India and used in various ISRO missions.

ISRO’s page does not name individual missions.

15. What accelerometer-based systems does DRDO list?

DRDO’s technology page on guidance and navigation lists several items.

• An aircraft navigation system of 1 nautical mile per hour using a ring laser gyro and accelerometers.
• A submarine navigation system of 1 nautical mile per day.
• A MEMS inertial measurement unit chip for inertial and tactical grade uses.
• A 3-axis MEMS gyro and a MEMS accelerometer.

The page does not say these are in service.

16. What was the DARPA TIMU chip?

DARPA announced on 10 April 2013 a timing and inertial measurement unit (TIMU). It was made by researchers at the University of Michigan.

It holds three gyroscopes, three accelerometers and a master clock. DARPA says the package is 10 cubic millimetres and smaller than a penny. Each of its six layers is only 50 microns thick.

DARPA says it contains everything needed to aid navigation when GPS is temporarily unavailable.

17. How are accelerometers combined with gyroscopes?

The DARPA chip pairs three accelerometers with three gyroscopes. This is a six-axis inertial measurement unit.

DRDO also lists accelerometers with ring laser gyros and MEMS gyros. A gyroscope senses turning. An accelerometer senses straight-line change.

18. How do accelerometers help machines and spacecraft?

NASA says products built on its micro-accelerometer track vibration, temperature, humidity and shock. They monitor machine health.

NASA also says the sensors can help improve navigation of air and space vehicles and robotic devices. Devices were produced for under $1,000 each, against hundreds of thousands of dollars for older ones.

Seismologists use accelerometers to record how hard the ground shakes.

19. What is strong-motion recording?

The USGS says strong-motion accelerometers record potentially damaging ground motion. They also monitor how critical structures respond during strong local earthquakes.

The US programme began in 1931 when Congress funded the Coast and Geodetic Survey. The first instruments went out in summer 1932. In 1973 the programme moved to the USGS.

20. Who monitors earthquakes in India?

The National Center for Seismology (NCS) is the nodal agency of the Government of India for monitoring earthquake activity. The NCS page gives its network as more than 170 stations in one place and 160 in another. It monitors earthquakes round the clock.

The NCS page does not say how many of these use accelerometers.

21. How does the Uttarakhand early warning system use accelerometers?

The state launched its earthquake early warning system (UEEWS) on 4 August 2021. A peer-reviewed paper says 170 sensors were deployed. They are low-cost MEMS-based accelerometers (pAlert and pAlert+).

Warnings go out through sirens and the BhuDEV smartphone app. For the November 2022 Tehri Garhwal earthquake, one station had a lead time of about 38 seconds before destructive waves arrived.

The first network covered about 150 by 50 km across five districts. The area later spanned about 280 km east-west and 130 km north-south.

22. Can a MEMS chip match a lab seismometer?

The USGS tested a Hewlett-Packard MEMS seismic accelerometer in October 2010. Its self-noise was 9.8 ng per root hertz above 5 Hz.

Against a Geotech GS-13 seismometer, it showed a flat response from 0.0125 Hz to at least 62.5 Hz. This was a prototype, not a finished product.

Every structure shakes at its own natural frequencies. A change in these can warn of damage.

23. Why use MEMS accelerometers in structural health monitoring?

A 2024 peer-reviewed study says MEMS sensors have lower cost, easy installation and lower power use. These allow wide, battery-less monitoring systems.

It says they can correctly detect modal frequencies. These are useful parameters for damage detection. The ADXL354 sensor in the study has a range of plus or minus 2 g.

In the case study, a hall’s first mode was near 3 Hz. The second was near 3.5 Hz and the third near 4.8 Hz.

Very precise accelerometers set standards. They also test ideas for space telescopes.

24. What was LISA Pathfinder?

ESA’s LISA Pathfinder tested technology for detecting gravitational waves in the range 0.0001 to 0.1 Hz. It carried two freely falling test masses 38 cm apart.

The aim was to put two masses in near-perfect gravitational free fall. ESA calls this the most challenging condition for detecting gravitational waves. It orbited the Sun-Earth point L1, 1.5 million km from Earth.

ESA’s page does not call the test masses accelerometers. In this lesson they are read as very sensitive motion sensors.

25. How does NIST check that an accelerometer is accurate?

A 2001 NIST paper says its primary calibration of accelerometers covers 50 Hz to 5 kHz. It uses three independent methods: fringe counting, minimum point and reciprocity.

NIST’s “supershaker” lets one setup cover the whole range.