Does gravity sensor return right values if device is in motion? I thought that gravity sensor use accelerometer to recognize gravitation direction. Are these two sensors different piece of hardware?
The Gravity sensor is what Android calls a 'software sensor' and calculates its values using more than one hardware sensor.
The software Gravity sensor is only available if the device has a gyroscope.
By combining accelerometer data with gyroscope data, the acceleration due to moving the device can be filtered out to leave the pure gravity signal. So yes, it will return the right value under motion.
Thus, the Gravity sensor gives a much better signal for device orientation than just the accelerometer on its own.
Combining sensor values is called sensor fusion and important for high quality measurement values.
The Android Documentation describes the Gravity Sensor.
Unfortunately, many Android devices lack a Gyroscope, and thus, will have no Gravity sensor either. This leaves you with a sub optimal signal from just the accelerometer alone, giving a lower quality user experience compared to Android devices with both sensors, and lower quality experience compared to iOS devices.
You can block installations on incompatible devices by using a Google Play requirement specification as follows:
<uses-feature android:name="android.hardware.sensor.gyroscope" />
Albert Einstein answered this question in 1911
A little reflection will show that the law of the equality of the inertial and gravitational mass is equivalent to the assertion that the acceleration imparted to a body by a gravitational field is independent of the nature of the body. For Newton's equation of motion in a gravitational field, written out in full, it is:
(Inertial mass) \cdot (Acceleration) = (Intensity of the gravitational field) \cdot (Gravitational mass).
It is only when there is numerical equality between the inertial and gravitational mass that the acceleration is independent of the nature of the body.
— Albert Einstein
We cannot differentiate the measurement of acceleration from the measurement of gravity since they are equivalent to the observer. Even if the Android had two sensors they would both be measuring the same thing.
Related
I'm developing an app that measures car's accelerations.
Instead of using the accelerometer, I have read about linear accelerometer and implemented it.
I have two devices to test, It's working fine on a Sony Xperia Z1 but I don't get any value on a old Alcatel onetouch.
Is it because of its android version (5.1 vs 4.2), because my code is wrong or is it a hardware limitation?
Taken from Android's documentation:
TYPE_LINEAR_ACCELERATION
Added in API level 9
int TYPE_LINEAR_ACCELERATION
A constant describing a linear acceleration sensor type.
See SensorEvent.values for more details.
Constant Value: 10 (0x0000000a)
This means it should work on any device post API 9
Please take in consideration Lork's response in this post. It states which type of sensor you should use in order to measure certain types of movements.
Update: I'll add the information of Lork's response as it might help future readers:
TYPE_ACCELEROMETER uses the accelerometer and only the accelerometer. It returns raw accelerometer events, with minimal or no processing at all.
TYPE_GYROSCOPE (if present) uses the gyroscope and only the gyroscope. Like above, it returns raw events (angular speed un rad/s) with no processing at all (no offset / scale compensation).
TYPE_ORIENTATION is deprecated. It returns the orientation as yaw/ pitch/roll in degres. It's not very well defined and can only be relied upon when the device has no "roll". This sensor uses a combination of the accelerometer and the magnetometer. Marginally better results can be obtained using SensorManager's helpers. This sensor is heavily "processed".
TYPE_LINEAR_ACCELERATION, TYPE_GRAVITY, TYPE_ROTATION_VECTOR are "fused" sensors which return respectively the linear acceleration, gravity and rotation vector (a quaternion). It is not defined how these are implemented. On some devices they are implemented in h/w, on some devices they use the accelerometer + the magnetometer, on some other devices they use the gyro.
On Nexus S and Xoom, the gyroscope is currently NOT used. They behave as if there was no gyro available, like on Nexus One or Droid. We are planing to improve this situation in a future release.
Currently, the only way to take advantage of the gyro is to use TYPE_GYROSCOPE and integrate the output by hand.
I hope this helps,
Mathias
I've been trying to find up (or down, which wouldn't be much harder to find, just multiply up by -1) on android with no good solution. I need a vector pointing up in the same coordinates system than the one used by the accelerometer. This way, I will be able to:
remove the force added by gravity to the output of the accelerometer
determinate if the device has shaken in a vertical direction or not.
Gravity Sensor:
First, I thought of using the gravity sensor, which would've been the simplest solution. But! my device did not have a gravity sensor...
Magnetic Field Sensor: So, I thought of using the magnetic field sensor to find two vectors pointing north at two different positions but with the same orientation, after what I calculated the cross product of both the vectors to try and find a vector pointing up or down. It didn't work (or it seemed like it didn't).
Magnetic Field with Accelerometer: Then, I thought of doing the cross product but, this time, with a vector provided by the magnetic field sensor and one provided by accelerometer. I couldn't find how to, because the accelerometer points a bit downward when the user is accelerating, because of the gravitational pull.
So, I came to the conclusion that I would need to use the orientation sensor to do so, or only to rely on a low-pass filter....
Put simply, how do I determinate what is the vector pointing up in the coordinates system used by the accelerometer if I have these sensors:
orientation
accelerometer
magnetic field
Unfortunately your problem isn't particularly simple. I'm not familiar enough with the Android API and perhaps if you look around you'll find something which attempts to isolate the gravity acceleration from the external device acceleration (I know there are sensor fusion algorithms that do this with some degree of success).
The core of the problem is that the acceleration due to gravity and the local acceleration on the device are indistinguishable (there is no such thing a a gravity sensor). An accelerometer simply records the total acceleration acting on the device, which is the sum of ALL accelerations. It is not trivial to isolate the gravity vector, but the methods I have seen mainly involve using the gyroscope as the magnetometer is simply too inaccurate and contains too much latency.
Instead I suggest that you either assume that the local forces on the device are negligible and therefore your acceleration IS the gravity vector. Or alternatively as you mentioned, use a low pass filter which for most purposes will suffice (and is a common way of defining DOWN for the purposes of sensor fusion and augmented reality).
All the best!
The magnetic field is not horizontal, which is why your solutions with the magnetometer didn't work. This is called the magnetic dip. Generally speaking if you want to determine the up/down vector you should not rely on the magnetic field because other solutions are easier to implement and more precise.
The two main solutions to find the up/down vector are:
Low-pass on the accelerometer. This may not be precise enough for your purposes.
Sensor fusion on the gyroscope and accelerometer. This will be precise enough, but requires a good head for math to implement (i.e.: matrices or quaternions)
Good luck!
Reading here: Android Sensors - Which of them get direct input? ,
I am wondering if anyone has experience or a technically detailed link about the accuracy of the linear acceleration versus just manual processing of the acceleration raw data. E.g., do the new phones have dedicated hardware chips for filtering out gravity, or are most devices just going to filter the same raw source?
Update, proposed answer for someone to confirm if they have such a device (Xoom,Nexus S,?):
"If the device has gyro, or possibly multiple accelerometers, then the returned values for gravity (G) versus external linear acceleration (L) can be fundamentally more accurate than any processing on accelerometer data alone. Without extra sensors, e.g. as on most phones, one could in principle post-process the Acceleration (A) to attempt separation as accurately as what the device is returning for A = G+L"
It seems, the gravity/lin. acceleration can be calculated by a low-pass-filter - just as described in the Android-Documentation.
However only filtering the last value will not do it. I get acceptable results by averaging the accelerometer values of ca. 200ms (for moderate movement, this will still screw up, e.g. when you flip your phone fast between your fingers).
Your proposed answer is most likely correct.
You can check the statistics of several smartphone models on Android fragmentation.
For many models the power consumption of the lin. acceleration and gravity sensor is just the sum of accelerometer, gyroscope and magnetometer.
The gyroscope lets you recognize fast angular movement and it can be used to improve the gravity value, which is not possible with just the low-pass-filter. For the magnetometer im not sure if it really gets you more information.
On my phone (HTC One S) the gravity sensor uses just as much power as the accelerometer, but is still better than my simple filter. So either it is another hardware sensor or probably they use different weights on it. I tried to weight acc-data stronger, if their absolute value is closer to gravity, which is nice but was still not as good as the actual gravity sensor.
For compatibility reasons I would suggest to use a low-pass-filter for gravity if possible, as still not every smartphone has a gyroscope or mentioned sensors.
the Android SDK actually offers a nice interface to access the sensors.
But e.g. the linear acceleration-sensor can be evaluated as the documentation describes from gravity and acceleration - so there is no real physical counterpart for this Sensor, it is rather a - let's call it - "virtual sensor".
For the proximity-sensor things are rather clear, i can't imagine it is influenced by some other values.
But the GPS-sensor could be influenced by the accleremoter sensor when the GPS-signal is rather weaks I think values are somehow estimated supported by other sensors.
So basically my question is: which sensors do get direct input from physical sensors and which are somehow altered or totally calculated by the Android-SDK?
And how do I get raw input from the sensors?
I appended a list of the sensors available through the Sensor-class. GPS, W-LAN, Camera, etc. missing
//API-Level: 3
TYPE_ACCELEROMETER
TYPE_GYROSCOPE
TYPE_LIGHT
TYPE_MAGNETIC_FIELD
TYPE_PRESSURE
TYPE_PROXIMITY
TYPE_TEMPERATURE
//API-Level: 9 (2.3)
TYPE_GRAVITY
TYPE_LINEAR_ACCELERATION // can be calculated via acc. and grav. (link above)
TYPE_ROTATION_VECTOR
I am pretty sure the GPS at the moment is a stand alone or give raw data output.
The orientation sensor is one that I know that is not a raw from a single sensor but is actually the fusion of 2 sensors and in the future possibly more (gyro). As of now the orientation is a combination of the magnetic field sensor (compass) and the accelerometer. Any modern day compass will use both the compass and accelerometer to calculate its final direction and to compensate for drift, noise and other interference. If you notice when calculating the orientation with get rotation matrix and get orientation it requires you to listen for both magnetic field and accelerometer sensors.
I would say the gravity, linear acceleration and rotation vector sensors are not actual sensors and just parts of data from other sensors separated out, mostly from accelerometer and compass.
Lastly the pressure and temperature sensor are actually calculated through a single sensor.
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
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.
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.
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.