ironbot:
I combine output of accelerometer and gyroscope to get a clean angle, as the gyro is fast but has a drift and accelerometer is slow on changes. This is the theory I learned: use not one of them (gyro or angle) but both of them and call it IMU, then filter output to get the angle.gyro gives me the rate of change, but from accelerometer I receive the angle, knowing that it changes from -g to g.
I also submit the datasheet of both of my sensors so you have full information about them,
I'm not convinced that approach will work. The description in the datasheet of using the accelerometer as a tilt sensor only works when the sensor itself is stationary. Attached to your bot, all you'll see is the the accelerations of the sensor itself, the tiny signal of the direction of gravity changing will be lost. You can test it yourself if you believe otherwise, but I think you're barking up the wrong tree there. Don't lose hope, though; there may well be another use for that accelerometer later on.
I suggest you focus on the gyro to start with. If I read it correctly the data sheet shows that the output is proportional to angular velocity and the output sensitivity is about 15mV per degree-per-second. By the time it's gone through the Arduino's ADC what is the smallest angular rate you can read, and what latency (delay) is there between the movement and you being able to see the input change?