[Russ Beavis]

This is a very common problem with integration - what happens when the average value is NOT zero? Well, the integrator output just keeps growing and growing...

There's an excellent chance that your accelerometer has a small amount of offset error. It's unavoidable.

The only way to eliminate the growing error is to "de-drift" the offset error by folding in and "subtracting" a true heading/position sensor somewhere in the calculations (eg a magnetic compass or horizon sensor).

There is one other way - use a highpass filter on your accelerometer output. Unfortunately, you'll never reliably measure DC accelerations (eg measurement of pitch/roll tilt) without that DC measurement.

Russ