Sensing acceleration w.r.t. absolute orientation

I am trying to make a circuit which can sense acceleration in the X, Y, Z axis. (e.g. movement straight down should be in, say, Z axis - not X and Y)

However, the circuit may not always be mounted straight - so if I want the data to be in line with the axis (i.e. compensate for being wonky), what do I need - can it be done easily? Does it need a magnetometer/gyroscope?

I have als开发者_StackOverflowo just found inertial measurement units - are these just a combination of gyroscope + accelerometer? Would this do the job?

This would need

1. A Controller calibration Environment, where you can calibrate output of your circut in known environment.
2. And Error Corrector which can apply the error estimation or offset estimation from the calibration data to your actual output.
3. Would recommend to use Gyroscope along with accelerometer if you want to measure tilts. This is because Accelerometer data is typically noisy, and too smoothen it you need low pas filter but that takes away accuracy. Here combining with gyroscope data can help.

This link give more insight on how to use more then one sensor to get usable data