I know by combining accelerometer and gyroscope, we can access the gravity and device motion separately. But I need to know how does Xcode calculate it when I simply using devicemotion.gravity. Any algorithm available?
Related
Does anyone know if Unity's API Gyroscope.userAcceleration (https://docs.unity3d.com/ScriptReference/Gyroscope-userAcceleration.html) is using gyroscope sensor to work or accelerometer?
Edit my answer:
Gyroscope.userAcceleration uses the acceleration data from the gyroscope and the accelerometer sensor of your device. (refers to linear acceleration, not rotational)
So it seems the Gyroscope class performs some sort of sensor fusion.
AccelerometerInput uses the build in accelerometer sensor of your device.
I believe it must use the accelerometer, since gyroscope cannot measure accelleration (while accelerometer can be used to extract rotation to a certain degree), userAcceleration just takes out the constant part (gravity aka earth acceleration) out, so when the device is not being moved user acceleration is zero even if actual data from the accelerometer contains gravity
I want compute the current iphone motion velocity anytime based on accelerometer, the accelerometer is variable. Anyone can give any idea?
It's basically impossible. The only way is to integrate the acceleration, but that magnifies the inaccuracy of the iPhone's not very accurate accelerometer, and because you don't have an independent orientation sensor (the iPhone uses gravity to figure that out!), you can't distinguish lateral acceleration from tilting the phone.
How people do this in the real world is to measure velocity using something else like GPS, and use the accelerometer to interpolate.
I am about to use iPhone 4's gyroscope/GPS on a game, to detect rotation and translation. As far as I know, the gyroscope can be used to detect rotations in all 3 axis.
But rotations, at least on the horizontal plane can be detected with the compass, tilts can be detected with the accelerometer and positions with the GPS.
Can a combination of compass/accelerometer/GPS create the same level of detection of gyroscope/GPS? (I am thinking of allowing this combination for people without iPhone 4).
Will this work perfectly?
The precision of the gyroscope and accelerometer sensors is much greater than the precision of the compass and GPS. The compass and GPS are for finding out where the device is on the globe, and the gyroscope and accelerometer are good for finding out where the device has moved in the last few milliseconds.
Therefore it depends upon what you're trying to control with the device's movement. Trying to simulate a gyroscope input to control a 3D simulation (like the Jenga game Jobs showed in the keynote that introduced the iPhone 4) will not work perfectly with just the compass/accelerometer/GPS. Figuring out if the device is pointed at the grocery store on the west side of the street instead of the furniture store on the east side of the street in an augmented reality game will work perfectly with just the compass/accelerometer/GPS.
I'd like to transform the yaw, pitch and roll of the iPhone from the body frame to the world frame, i.e. azimuth, pitch and roll. On Android this is easily done with the
SensorManager.remapCoordinateSystem(), SensorManager.getOrientation methods as detailed here: http://blog.mysticlakesoftware.com/2009/07/sensor-accelerometer-magnetics.html
Are similar methods available for the iPhone or can someone point me in the right direction how to do this transformation?
Thanks
The accelerometer is good enough to get gravity direction vector in device coordinate system. That is in case when device calms down.
The next step for full device orientation is to use CLLocationManager and get the true north vector in device coordinate system.
With the normalized true north vector and gravity vector you can easily get all other directions using the dot and cross vectors product.
The accelerometer (UIAccelerometer) will give you a vector from the device's accelerometer chip straight down. If you can assume that the device is being held fairly steady (i.e., that you're not reading acceleration from actual movement), then you can use simple trig (acos(), asin()) to determine the device's orientation.
If you're worried that the device might be moving, you can wait for several accelerometer readings in a row that are nearly the same. You can also filter out any vector with a length that's ± TOLERANCE (as you define it) from 1.0
In more general terms, the device has no way of knowing its orientation, other than by "feeling gravity", which is done via the accelerometer. The challenges you'll have center around the fact that the accelerometer feels all acceleration, of which gravity is only one possible source.
If you're targeting a device with a gyroscope (iPhone 4 at the time of writing), the CoreMotion framework's CMMotionManager can supply you with CMDeviceMotion updates. The framework does a good job of processing the raw sensor data and separating gravity and userAcceleration for you. You're interested in the gravity vector, which can define the pitch and roll with a little trig. To add yaw, (device rotation around the gravity vector) you'll also need to use the CoreLocation framework's CLLocationManager to get compass heading updates.
Is there a way to detect if an iphone lying down in a table face up is rotating?. I do realize that this kind of movement is not reported by the accelerometer and neither is it reported to the - (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation method.
Is there a way to detect angle variations for the phone rotating this way?. Thank you.
The gravity vector will be constant as it rotates on a flat table so you won't see anything on the accelerometers. You could follow compass heading changes to detect this rotation but only on an iPhone 3G S. See the CLLocationManager for details, look at the heading methods.
EDIT - With an iPhone 4 you can detect the rotation using the gyros. There is a new class in iOS 4 called CMMotionManager for getting rotation rate from the gyros.
When the phone is stationary the sum of the acceleration vectors should be +1. When the phone is rotating (assuming the sensor is off-center) the sum of the vectors should be more than 1 and (hopefully) somewhat constant.
If you look at the decay of that curve, I wouldn't be surprised if that shape is distinctive enough to be used to determine whether the phone is rotating or not.
This is the AccelerometerGraph sample app from Apple.
I guess you could do it if the iPhone has a compass. Other than that I don't think it will be possible or reliable.
This would really depend on the location of the accelerometer on the device, i just tested this using the accelerometergraph sample application on a 2g itouch and you can see the initial acceleration on the x and y axis(the 2g does not have the accelerometer in the center of the device I guess). So in a sense you could detect the rotation, however I think the challenge would be differentiating that acceleration from directional acceleration. And I'm sure the values would change if apple placed the accelerometer in different locations on different models. There would definitally not be any way of doing it via shouldAutorotateToInterfaceOrientation, I recommend you load the accelerometergraph sample application in the sdk and experiment with the acceleration vectors to see if you can isolate a rotation vector reliably on multiple devices.