# Accelerometer -proof mass

1. Aug 23, 2006

### chandran

correct me if wrong
an accelerometer consists of a mass(proof mass). say the proof mass is
x kg. The accelerometer sensed the acceleration of the proof mass.

suppose the accelerometer reading is y can i say the force acting on the proof mass is x.y?

Some theory says that the accelerometer measures the acceleration of the surface on which it is attached.

Which is true. The accelerometer measure the acceleration of the proof mass or

The accelerometer measure the acceleration of the surface on which the instrument is attached.

2. Aug 23, 2006

### Danger

The only ones that I know of monitor the proof mass, whose relative acceleration is opposed to that of the surface (ie: it's inertia makes it lag behind the accelerometer housing). It doesn't have to be any kg-class weight, either. There are micro-machined units where the proof mass is a cantelever beam etched on a silicon chip. I think that in those ones the electrical resistance of the beam changes as it bends.

3. Aug 23, 2006

### WhyIsItSo

For clarity, let us say a is acceleration (instead of your y), and m is the proof mass (instead of x).
Let:
F = tensional force on spring
k = spring constant
x = displacement of proof mass

Spring equation:

$$(1)\quad F = -kx$$

Acceleration:

$$(2)\quad F = ma\quad \mbox{OR} \quad a=\frac{F}{m}$$

(2) shows you are correct with regards to force being the product of acceleration and mass.
(1) is saying acceleration is measured by this instrument on the proof mass. Under normal circumstances, this will be the same as the surface upon which the instrument rests, but consider the situation where the instrument slides on that surface. It is still registering acceleration on the proof mass, but no longer is that the same as the surface upon which it rests.