CODEWITHSUNDEEP
iphonenormalizationaccelerometergyroscope
Andre Senra
Andre Senra

Reputation: 1

Manipulating Gyroscope / Accelerometer Values obtained from iPhone 4

I'm developing a project for my university, to manipulate gyroscope/accelerometer values obtained from the iPhone 4. But i'm stuck with a mathmatical issue and I hope you guys can help me out.

I'll give you an example of what's about:

  1. Your iPhone is face up and you move it UP, on Y axis.
  2. Your iPhone is face right and you move it UP, on X axis this time (since you rotated the iphone 90 degrees).

On the second time, the computer interprets that i've moved the iphone to the RIGHT, but it's wrong. I've moved it up, but my axis were rotated since the iphone was face right.

What do I need? I need a way to VIRTUALY position back the iphone face up (where the 3 axis are correct) and give each axis his correct movement value.

If the iphone is turned 90 degrees, then I can easily switch X-Z axis and its correct. But I want to make it work for any angle of rotation.

I would be really thankfull if anyone can help me with some sort of pseudo-algorithm or mathmatical description of what to do.

NOTE: I only need a way to compensate all three axis acording with the iPhone rotation.

Update:

I don't actually need the precise values, since I'm making a graph coparition between all the records I get from the gyroscope. I'll make it clearer.

-> You draw a LETTER just by moving the iphone in the air and my application will recognize the letter you just drew. The method I use for recognition is based on TFT algorithm, and recording to a database with sample values originated from letters I've previously drawed.

My point is: Don't really matter the values I get, or what they represent. All I need is that all graphs be equal even if the iPhone is on different position. Quite hard to explain, but if you draw the letter 'P' with the iphone turned UP, the graph originated will be different if you draw the 'P' with the iPhone turned RIGHT.

So I need to compensate the axis to their original orientation, that way I'll get always similar graphs.

Upvotes: 0

Views: 2636

Answers (2)

t9mike
t9mike

Reputation: 1626

This post was before iOS5 was released. FYI to anyone coming here, DeviceMotion relative to world - multiplyByInverseOfAttitude shows how to transform device-relative acceleration values to earth-relative for iOS5 and above.

Upvotes: 1

unexpectedvalue
unexpectedvalue

Reputation: 6139

So, what you want is to convert from iPhone's coordinate system (or object/tangent system) to world coordinate system (or vice versa, as you see it, doesn't matter). You know the iPhone coordinate system as you have gyroscope data. So what you want is to create the object->world transformation matrix and multiple each of the velocity vectors (from accelerometer) by it.
Take a look here for a good explanation of tangent space and how to create tangent space -> world transformation matrix. If you aren't familiar with 3D/linear math it might be a bit tricky, but worth the trouble.

Upvotes: 0

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