Mobile phone sensors

Question

After some hours of searching I'm so confused, so I'll tell now what I learned, so please correct me if I was wrong :

Light Sensor: surrounding light

Magnetic Sensor: I can get the north location.

Accelerometer Sensor: the gravity X Y Z , earth gravity or what ever acceleration .

Proximity Sensor: just like the parking car sensors.

Orientation Sensor: Tells the X Y Z degrees form their axis.

I've tried those sensors using some apps from android market like "My sensors", and I can confirm that accelerometer Sensor can't feel if you rotate your phone in position over a table. While orientation Sensor seems to catch all the moves. Now I can conclude that compass app uses the magnetic sensor to see where is the north, then orientation sensor to know where are you heading with your device, right?

switching between portrait and landscape modes use "Accelerometer Sensor" and checks the earth gravity on which axis.right?

Q1:so if everything is ok, what is "Gyroscope"? is it same as "Orientation Sensor"?

Q2:is Orientation Sensor avalible on most of the devices?

Q3:what other uses of Orientation Sensor?

Q4:why most of the websites even wikipedia says thet Orientation Sensor == Accelerometer Sensor?

-Rami

Answer 1

Ok, first the easy ones... About Q1, Gyroscope measures the Angular velocity (radians/second) and the Orientation is a different magnitude, telling us how is "placed" the device (I don't really know how to explain something so basic in English).

And about Q2, I would say yes, 3-Axis orientation is avaiable on most of smartphones, at least those running Android.

Q3: Compass actually uses the Orientarion values, Magnetic Field sensor measures that magnitude, (not in degrees) though probably you can calculate the compass values with the magnetic field. Another use... well, you can tell wether the device lies upside or downside, for example.

About Q4, this is more difficult, I'm not that expert in accelerometers, but I think most of these "Sensors" use the same hardware sensor, which measures the magnecit field and makes the necessary calculations, but I insist, maybe it's better to read more detailed and technical information.

If you notice, now OrientarionSensor is deprecated, and this is written in the docs:

Note: This sensor type exists for legacy reasons, please use getRotationMatrix() in conjunction with remapCoordinateSystem() and getOrientation() to compute these values instead.

So yes, it seems it calculates Orientation values trough the Accelerometer, but still, Orientation is given in degrees and Acceleration in (m/s^2), different magnitudes. As I told you, I think they measure different magnitudes with the same sensor, that's why they present different kind of Events in the API.

I hope I haven't written many huge mistakes, because well, I would also like to clarify some concepts regarding to these Sensors.

Answer 2

mdelolmo is perfectly right.

I would like to add the following:

• About Q4. Everyone addresses the Orientation sensor as accelerometer
  because the smartphones use it for the "Auto-Rotate" feature.
  The switching between portrait & landscape modes
  (often called orientation in layman terms) is done
  by determining that the gravity is along which of axis of the phone. This is done by the accelerometer-IC.

• The orientation sensor (in Android) uses the accelerometer data
  and the magnetic data to determine the exact positioning of the
  device. ie. the angles it makes with all the 3axes. These are
  azimuth(or yaw), pitch & roll.

• The gyroscope provides the angular velocity of the device.
  It is NOT the orientation sensor.

Answer 3

I haven't worked on android phones but may in the future, however accelerometers detect acceleration usually through the motions of a proof mass. So they can be used to orient a device roughly because they sense the g-vector so any orientation is totally unconstrained in angle about the g-vector. Now accelerometers can be utilized as gyroscopes but they are not used that way because they would need to be better than any accelerometer currently made to sense rotation via centripetal acceleration.

Gyroscopes directly measure either angular rate or angle directly. Most measure angular rate and the rate is integrated to get angle, so they can be used to measure orientation but since they are inertial sensors they drift and so do not provide an absolute orientation but are excellent sensors for relative rotations or relative orientation with respect to a very recent orientation. I hope this helps.

The magnetic sensor measures the direction to true north via the earth's magnetic field. The magnetic sensor supplied North with the g-vector via the accelerometeres give full orientation information because it breaks the symmetry of the orientation about the g-vector. This only really true when the phone/sensors are not moving. Since I do not know how this is implemented in the phone I can't say much else, but the fact that you need accelerometers and another orientation to get full orientation may be the reason why there is confusion about this subject.

Barometers measure pressure and can be good sensors to measure changes in height but can be fooled by active air moving systems such as as found in air conditioned homes and in forced hot air systems.

If you are not moving and you have sufficient sensitivity to measure earth rate with your gyroscopes you can do something called gyrocompassing where the gyroscopes and accelerometers become analytically aligned or physically aligned with with the local level coordinate system. This is how much better gyroscopes and accelerometers measure orientation in systems like aircraft, spacecraft and ships/submarines. There are many complications but this is the basic idea.