difference between gyro and accelerometer

Question

According to apple documentation , the accelerometer measures acceleration along the axis, and gyro measure the rotation rate around the axis .

In reality , things are very strange .

When using the accelerometer function:

- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration

it seems that the gyro and accelerometer are working together . When fast moving along the x axis, i get values bigger then 1 like 1.5-2 ,which is ok cause i get acceleration. but when rotating the iphone AROUND its x axis, the values also changing between 0-1, for example , if i hold it and the value is 0.3 ,and the i rotate it a little bit, i see 0.6, if i rotate more its 0.8. but this is a behavior of a gyro , because when velocity is 0 (i stop the rotation) i should see acceleration -0 and not 0.6 . it seems to show me the gyro angle position .

Whats happen here anyway ? the values should be 0, till i move it fast ALONG the axis not around it .

I couldnt find any answer and it seems that many people dont really understand that .

Answer 1

Before All we must agree on terms:

In my opinion APPLE is wrong when it calls acceleration the values you get from sensor. Acceleration in the second derivative of translation, so d(dx)/d(dt), so when an iphone is steady, should be zero. in my opinion these values are simply angles. See apple samples, the use a "2nd order filter" to get acceleration.. "2nd" order filter means 2nd derivate, if I remember correctly what I studied.

Hope this can help.

Answer 2

The accelerometer reports the difference between the acceleration that the device is experiencing and that with it would be experiencing were it in freefall. So if the accelerometer is returning a zero vector then the only force acting on the phone is gravity (ie, you've dropped it). Normally it'll be some other value. If the phone is at rest on a table, it'll reveal the direction of gravity relative to the phone. The accelerometer is therefore most often used to figure out which way is down - so that the UI can rotate, and in games so that you can do things like use the device as a steering wheel.

The gyroscope can observe the rate at which the device is rotating. The CoreMotion library can automatically integrate that to get current angles.

Devices with a gyroscope also have a three-axis accelerometer (in addition to the traditional one) that measures acceleration individually along three axes.