# What is the difference between an accelerometer and an inclinometer?

When looking through papers for a controls project I found one that seemed to be referencing the fusing of sensor data from an inclinometer and an accelerometer.

I always had thought that it was impossible to separate acceleration due to gravity and acceleration due to other forces. So wouldn't an inclinometer and accelerometer basically be the same thing? Or does an inclinometer typically measure tilt in some other fashion that does not involve gravitational forces?

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Solid state gyroscopes are normally tiny dumbells - a small mass on a flexible arm. The gadget measures the strain on the arm when the weight moves.

If the weight hangs down (like a pendulum) then any tilt causes it to move to the side and one side of the arm to shrink while the other side stretches.

Any accelration can also make the mass move, so an inclinometer is also an accelerometer.

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Okay, so it would make no sense to "fuse" inclinomter and accelerometer data then? Any idea why they are marketed under the different names? Are inclinometers perhaps better optimized for measuring tilt when at rest? – llakais May 8 '12 at 2:14
An inclinometer cannot tell you if your elevator is accelerating up or down because the direction of the net acceleration does not change. An accelerometer can tell you because it can give you the magnitude of the vector. – BowlOfRed Aug 7 '14 at 23:28

You are correct, acceleration due to movement cannot be separated from acceleration due to gravity. Inclinometers and accelerometers both measure acceleration the difference is in how they do so and what they are used for.

Typically an accelerometer measures acceleration using small proof masses on springs. One mass for each axis. The masses are constrained so that they can only move along their axis. They are typically built to measure a few G's of acceleration. The masses and springs are small and have very high resonant frequencies so that under normal conditions movement of the mass corresponds directly to acceleration along that mass's axis of movement. If you have a 3 axis accelerometer that isn't moving you can do some math to find the direction of the gravitational force acting on it and get inclination.

Inclinometers are essentially a directional accelerometer. They are built to measure the direction of their own acceleration very precisely. Generally they have large settling times and in order to measure inclination accurately must be stationary. Some are measuring inclination usually with a cavity filled with two fluids or an offset mass. Some are just accelerometers in disguise like this one.

In short you're right. These are two different types of accelerometer. One which can tell you at what angle it is accelerating (IE: direction of gravity) very accurately. The other one gives you its acceleration vector from which you can derive inclination imprecisely. No reason to use them together. The alternative is that this is the second kind of inclinometer. In that case it's just a matter of putting together a highly sensitive low range accelerometer with a lower sensitivity high range accelerometer.

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## protected by Qmechanic♦Aug 7 '14 at 23:43

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