# What is accelerometer ?

Hi
i wanna know what is accelerometer ? what can i do with it ?
recently i wanted to make a quadcopter and i saw that accelerometer is used in quadcopterts .so what is the role of accelerometer in quadcopter ?

An accelerometer measures acceleration, which can be due to any forces applied, including gravity.

An accelerometer is a micro-machined pendulum on a chip. With 0 acceleration, the pendulum swings the same about in both directions. During acceleration the pendulum spends more time in one direction than the other. the chip reports the difference in time. So if the chip is at rest on the ground, or traveling very fast, with no change in speed, the acceleration is still zero.

Paul

So if the chip is at rest on the ground,

It will measure the acceleration due to gravity.

jremington:
It will measure the acceleration due to gravity.

Yes, but you have to subtract the acceleration due to resting on the ground.

Paul

Yes, but you have to subtract the acceleration due to resting on the ground.

For what purpose?

You really need to be clear. Beginners are very confused by accelerometers, especially by the facts that they measure "g" when at rest, and zero acceleration when in free fall.

jremington:
For what purpose?

You really need to be clear. Beginners are very confused by accelerometers, especially by the facts that they measure "g" when at rest, and zero acceleration when in free fall.

Perhaps accelerometer is a misnomer. Since acceleration is the second derivative of speed, both are time functions. If time is 0, there is no acceleration. If the device is measuring something at rest, perhaps it is measuring gravity, itself.

Not having used such a device, Does the device display a vector sum of acceleration? Or does it show two values, one for gravity and one for acceleration at an angle to the pull of gravity?

Paul

The "misnomer" is that the accelerometer actually measures the difference between the forces applied to the framework of the sensor element (the chip) and the forces on the sensor element itself, which can be thought of as a pendulum.

In free fall, both the chip and the sensor element are accelerating at the same rate, so the difference is zero. At rest on level ground, the accelerometer registers the force due to the ground pushing up on the chip, which counterbalances the force of gravity on both the chip and the sensing element.

That creates a real problem for the use of these devices to measure the acceleration due to motors etc. (as in a quadcopter), because the effect of gravity has to be subtracted off, and to do that you have to know which direction is "down".

The subtraction process introduces serious errors, which are discussed here.

jremington:
The "misnomer" is that the accelerometer actually measures the difference between the forces applied to the framework of the sensor element (the chip) and the forces on the sensor element itself, which can be thought of as a pendulum.

In free fall, both the chip and the sensor element are accelerating at the same rate, so the difference is zero. At rest on level ground, the accelerometer registers the force due to the ground pushing up on the chip, which counterbalances the force of gravity on both the chip and the sensing element.

That creates a real problem for the use of these devices to measure the acceleration due to motors etc. (as in a quadcopter), because the effect of gravity has to be subtracted off, and to do that you have to know which direction is "down".

The subtraction process introduces serious errors, which are discussed here.

Excellent article! Required reading for anyone thinking about using an accelerometer. I will study it further when time permits. Thanks much.

Paul