I have a very basic question about Sensors:
Do magnetic sensors return readings w.r.t the phone's initial orientation or w.r.t the world coordinates?
What about accelerometers? Do they return values w.r.t their previous readings or is each value an independent acceleration relative to the world coordinate system?
I know that gyros return readings relative to the phone's initial orientation. So, how do I convert the yaw, pitch and roll readings from a gyro into the azimuth, pitch and roll readings from a magnetic sensor of a smartphone (I'm using HTC hero)
Thanks!
As mentioned, the gyroscope measures the angular velocity.
The third value returned (values[2]) is the angular velocity regarding the
z axis. You can use this value together with the initial value from the magnetometer to
calculate current heading: Theta(i+1) = Theta(i) + Wgyro*deltaT
You can receive initial heading orientation from 'Orientation' measurement (values[0])
This measurement is dependent only on the magnetometer. (you can put a magnet or a second phone close to the Smartphone and watch the output going crazy)
The second and third values of the 'Orientation' are dependent on the readings of the
Accelerometer. Since the Accelerometer measures gravity, it is possible to calculate
the pitch and roll angles from the Accelerometer readings in Axis Y and X.
Hope this helps
Ariel
Android Sensors (upto FroYo) provide the application with "raw" data.
There is bare minimum of "cooking" (ie processing) involved.
The accel & compass device provide absolute accel & magnetic data respectively.
The gyroscope provides relative angular velocity.
Gyroscopes do NOT provide relative data wrt any specific state/position.
What you need to understand is that gyroscopic data is angular-velocity.
Angular velocity is simply, how fast the phone is rotating (in degrees-per-second).
So once you hold it still, gyro says (0,0,0) &
as you rotate, you get how fast it is rotating.
This continues until u again hold it back still
when the gyro reading again becomes (0,0,0).
Theoretically the gyro can be used in "callibrate" the compass.
But to do so would require a lot of experimentation on your part.
The ideal place to fiddle around would be the sensor-HAL.
NOTE: You would need to turn-ON all the sensor h/w even if
ONLY compass data is reqd. As you will be cross-referencing
the gyro/accel data for that. This will mean larger power consumption &
extremely poor battery life. All the sensors turned on continuously can
drain the battery of a standard Android phone in 4-5hrs.
You can read more Android Sensors here.
Related
I am creating app that will measure acceleration of vehicle in each axis using accelerometer in Android smartphone. I need somehow rotate phone measurement coordinations system to coordination system of vehicle - to allow driver to put phone in holder - and so phone will have some different rotation to vehicle.
I started with calibration process where I tell user to hold phone to match vehicle coordination system - so I save TYPE_GRAVITY sensor X, Y, and Z gravity acceleration value. Then I tell user to put phone in holder and again save TYPE_GRAVITY sensor X, Y, and Z gravity acceleration value.
Now I need to find some relation between those two vectors so I can use it to correct (rotate) TYPE_LINEAR_ACCELERATION X,Y,Z data to match vehicle coordinations system.
Using just a gravity sensor you'll lack one rotational axis to compute what you want.
Imagine your phone being held vertically, the accelerometer will show you direction to the bottom of the phone. If you now rotate it around vertical axis - you'll still get the same result from the accelerometer. This means you can't get the rotation around vertical axis this way.
One solution would be to use a gyroscope - this gives you entire rotation of the phone, but increases a hardware requirements of your app.
But the better solution IMHO would be to get rid of the entire calibration process. Cars move mostly straight, only sometimes you get the side way acceleration so you could scan your readings for few seconds and find a 'main' axis and a 'side' one.
Besides, your calibration process still depends on user precision during placing the phone, so it may not work as you've expected.
I need to create an app that Calculates the device's velocity, with x/y/z speed.
My idea is using device's accelerometer and gyroscope,
like this pipeline
I wanted to know that whether accelerometer and gyroscope right sensor choice for this ?
(in the pipeline).
What Rotation table should i use for this?
So, right now I'm grabbing the accelerometer data and converting them to a decently rough estimate of the angle at which the phone is being held. For right now I'm just focused on the yaw axis.
My area of interest is between 0 and 45 degrees on the yaw axis, so I made a limited queue of the past 5 to 10 readings and compared the numbers to determine if it's going up or down, which kind of works, but it is slow and not really as precise or reliable as I'd want it to be.
Is there a way you can kind of just determine which direction your phone is rotating with just the accelerometer and the magnetic field sensor I guess, without keeping a history of past readings, or something like that? I'm really new to sensor manipulation and Android in general. Any help understanding would be great.
It's not clear exactly what you're looking for here, position or velocity. Generally speaking, you don't want to get a position measurement by using integration on the accelerometer data. There's a lot of error associated with that calculation.
If you literally want the "direction your phone is rotating," rather than angular position, you can actually get that directly from the gyroscope sensor, which provides rotational velocities. That would let you get the direction it's rotating from the velocity without storing data. You should be aware that not every phone has a gyroscope sensor, but it does seem like the newer ones do.
If you want the absolute orientation of the phone (position), you can use the Rotation Vector sensor. This is a combined sensor that automatically integrates data from several of the sensors in one go, and provides additional accuracy. From this, you can get roll-pitch-yaw with a single measurement. Basically, you first want to get your data from the Rotation_vector sensor. Then you use the sensor data with getRotationMatrixFromVector. You can use the output from that in getOrientation (see the same page as the previous link), which will spit out roll-pitch-yaw measurements for you. You might need to rotate the axes around a bit to get the angles measured positive in the direction you want.
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.