# Accelerometer vs. acceleration

1. May 24, 2007

### dhs

Accelerometer vs. "acceleration"

I'm struggling with the basic semantics of accelerometers. An accelerometer that is at rest will produce an "acceleration" reading for gravity (-9.8m/s^2 on some axis). Based on my intuitive (albeit likely naive) understanding of the principals of physics, an object with a constant acceleration should be in some sort of motion.

But the accelerometer is at rest, it is not moving, let alone accelerating. So technically speaking, the reading is not "acceleration", per se, correct? So from a physics standpoint, what does the accelerometer measure, what is the technical term that should be used here? Inertia? Static acceleration? Dynamic? What about "relative acceleration", but relative to what?

Thanks!

DS

2. May 24, 2007

### D H

Staff Emeritus
An accelerometer cannot sense the acceleration due to gravity. Nothing can. An accelerometer at rest on the surface of the Earth is sensing the force exerted on the ground that keeps the accelerometer from falling toward the center of the Earth.

The gravitational acceleration is directed toward the center of the Earth (more or less). The acceleration vector reported by a stationary accelerometeris directed upward.

3. May 24, 2007

### cristo

Staff Emeritus
You can think of an accelerometer as a device which measures the deviation from the acceleration due to gravity. If an accelerometer is in freefall, then it will have a reading of zero acceleration. So, when the accelerometer is at rest on a surface, it gives a reading of +g; the acceleration that the force countering the graviational force gives to the accelerometer (as stated by DH above.)

4. May 24, 2007

### D H

Staff Emeritus
Not quite. The Earth is rotating. Geodesists distinguish between gravity and gravitation. The former is essentially what an Earth-fixed accelerometer senses; the latter is the acceleration due to gravity.

5. Jun 17, 2007

### katchum

A little question about the working of it. I know it's with a piezo crystal, but when you apply a constant force to a piezo crystal (like gravity g), it won't give a signal because you need to variate the force in time to get a charge variating in time resulting in an electrical stream. i = dq/dt

When this accelerometer isn't moving, how can it give a signal then?

Last edited: Jun 17, 2007
6. Jun 17, 2007

### Staff: Mentor

A piezo crystal with a constant force on it will give a constant voltage, provided nothing is dissipating the voltage.

7. Jun 18, 2007

### katchum

Ok, but when you want to measure the voltage with a resistance you need to have a current going through that resistance. And when F = cte then i = 0.

I just don't really get it: in my book it says you can't use a piezo crystal/ accellerometer in d.c. and with slow varying forces. What does that actually mean?

I thought you just can't measure the accelleration when it is constant over time. But Cristo stated when the car is at rest you can measure g. (F = m.g and Cte.F = q and i = dq/dt = 0 => unmeasurable)