I'm going to create an app which uses sensors in a smartphone. Before doing this job, I also want to test the sensors of the smartphone so that I can evaluate their correctness. What are the common ways to achieve that? I'm going to use phone's accelerometer sensor. But I also wonder about other sensors too.
Using a level, verify that a surface is flat. Place the phone on the surface, and ensure that there is a 1g reading in one direction and zero in the others. This can be done in the other two directions as well to ensure they are reading 1g from gravity, but you will need a holder that keeps it at exactly a right angle with the surface (or alternatively, set on the floor with the back of the phone against a wall once you know the wall and floor are level).
Related
I'm trying to use an android phone's internal sensors as a means of defining a 3D object for use in a video game. I therefore need to create a map of the phone's orientation, and relative location over time. (relative to a starting position) I tried creating a test app which kept a float[] (x,y,z) of a 'current location' and kept adding the data from the sensors to it. But the noise from the sensors just made the current location slowly wobble off the grid I was using to visualize the feedback. I've looked through filters online, and all explanations are just confusing. Is there anyway to get a cleaner output from the sensors? Is there a better way of tracking the phone's exact relative location?
You will not be able to accurately track a device's position with a typical phone or tablet. In theory, you can use a device's internal sensors to measure its acceleration in each axis. In theory, you integrate acceleration once to obtain speed and once more to obtain change in position. Thus, in theory, you should be able to calculate a device's position relative to its starting location. HOWEVER, in practice, things don't work out so well.
A device's sensors are not perfect and contain small errors. Integrating these small errors twice turns them into large errors. These large errors accumulate and end up making your calculations useless.
Take a look at Project Tango, which contains a bunch of fancy sensors and algorithms to implement 6 DOF tracking. This device was built by a research team and is still a work in progress, so keep in mind that it's a tough problem.
I'm trying to write a small android game where the phone is placed on the table.
On the screen there is a ball, which the user control its movement by moving the phone.
Along all the game the user won't lift the phone from the table.
At the beginning the ball will placed in the middle of the screen:
Pushing the phone from the user:
should move the ball toward the top of the smartphone screen:
And from the current position of the ball, moving the phone back to the user, and to the right:
will move the ball accordingly:
I read the Android Motion Sensors Guide carefully but I didn't even realize what Sensor \ Sensors should I use.
I would love to get any directions.
First of all TYPE_LINEAR_ACCELERATION, TYPE_ROTATION_VECTOR, TYPE_GRAVITY are not physical sensors but made from sensor fusion.
Secondly from Android 4+ these fused sensors make use of device Gyroscope, so they WON'T work if the mobile device doesn't has a gyroscope.
So if you want to make a generic app for all phones prefer using only Accelerometer (TYPE_ACCELEROMETER).
Now for your case since user won't lift the mobile from table, and if you want you can easily subtract the Gravity component from accelerometer. See http://developer.android.com/reference/android/hardware/SensorEvent.html
under section Sensor.TYPE_ACCELEROMETER. (Code is given too).
Now you can see How can I find distance traveled with a gyroscope and accelerometer? to find the linear displacement & the 1st answer states their is NO use of Gyroscope. (Or you can just google for finding the displacement/Linear velocity from acceleromter readings)
Hope this all would give you quite lot an idea.
It's really difficult to do this type of linear position sensing using the types of sensors that smartphones have. Acceleration is the second derivative of position with respect to time. So in theory, you could use the accelerometer data, integrating twice in time to achieve the absolute position. In practice, noise makes that calculation inaccurate.
On the other hand, if you're really talking about putting the camera facedown on the table, maybe you can come up with some clever way of using OpenCV's optical flow features to detect motion using the camera, almost like an optical mouse. I don't know how you would light the surface (the flash would probably burn through the battery) but it might be possible to work something out.
In my project, I want to detect if a user is moving or not by using either wifi rssi or accelerometer sensor.
What I should do to achieve that?
It actually all depends on what kind of movement you want to detect.
WiFi RSSIs : From a starting position and scan results (initial RSSIs for newly discovered access points), you can check through time their evolution in term of signal quality. A short displacement of the user will not be easy to find as RSSI values are tweaked by a large amount of parameters (orientation, obstacles, setup of the room, atmospheric conditions, people around). Thus you would need averaged values (scans must then be performed quickly to have enough data) and leaving an access point perimeter would make you lose the information.
Accelerometer : Depends on what quality of sensor you are using. If you're using embedded sensors within smartphones, it will be tough. Their accuracy is bad, and as you'll need to integrate its values (m/s² to get m/s) the error will grow subsequently. Plus it might be hard to discern real user movement from the device's tilt if you're using a mobile phone or tablet.
Without really knowing the details of your projet, I believe that RSSIs should be easier to use if you actually need to detect not so tiny motion. If you want something more precise, you'll need some way bigger research work.
See Android accelerometer accuracy (Inertial navigation) for RSSI-based indoor localization.
I'm working with android sensor data. My application use
SensorManager.getRotationMatrixFromVector(
mRotationMatrix , event.values);
and it has been working well until this morning, when the rotation matrix started to send a lot of noise data (Change N to W in a second).
It's not a problem with my code, because on friday was working and no changes have been done. I have used a compass app from the market, and the compass is giving random data.
I have tested my app on another tablet, and it is working well.
Does someone know why is this happening? A problem with the sensor? Does it need a calibration?
I've worked quite a lot with these electronic compasses on mobile phones and its quite possible that there is nothing wrong with your code or sensor.
Instead it could very well be a problem with your environment. There are magnetic fields interfering with the earth's magnetic fields all the time. From electrical equipment interference to the metal structure holding up a building. At the end of the day a compass is just a magnet. If you stand beside a large lump of metal the compass will be attracted to it and point to it rather than the magnetic north pole.
Try this:
Install GPS status
then turn off all filtering (settings... gps & sensors...sensor filtering... no filtering).
Do the calibration (figure of 8 wavy stuff) and then move the phone around your desk.. near monitors, cables, etc. You'll see it go crazy. The information is completely unreliable. I found in the past that moving the phone a few inches to the right completely changed its reading. The same happens with a real compass. Strictly speaking there is no "problem". The device's compass is assigning itself with the strongest magnetic field. Even the magnetic content of nearby rocks can interfere with the compass.
As a further test I've just placed a real (orienteering) compass over my phone which has a compass app installed. The real compass is now pointing everywhere but magnetic North. The two devices are interfering with each other.
So my advice is.. go somewhere in the open, like a park or field, away from any potential interference and power lines, (if you have one bring a real compass to check that the GPS status app is pointing the right way), and see if your compass works as you'd expect.
Extra: The answer from #resus is also important when calibrating. Rotate the phone a few times in each axis. Looks silly but it does calibrate it properly.
Extra 2: Would it be possible/practical to use the compass bearing of your GPS? It would require that the device be moving (walking speed should be fine) but you would not need to worry about any interference. It should give an accurate reading provided your GPS signal is good.
Extra 3: Another thought just occurred to me.. You could try apply a low pass filter to the sensor. This means that the sudden changes in the sensor reading are filtered out .. have a look at this answer. And if that doesn't do a good job there are lots of algorithms on the web for you to choose from.
If you definitely haven't changed anything in your code, and it still works fine on other devices, it would suggest a problem with that particular device.
While your app is running (i.e. the compass is in use), you should be able to wave it in a figure of 8 in order to automatically recalibrate the compass. You should also make sure you aren't standing next to any large lumps of metal etc. that might interfere with readings.
You can override the onAccuracyChanged() method of SensorEventListener to flash up a message to the user when the compass requires recalibration (probably when accuracy drops to SENSOR_STATUS_ACCURACY_LOW).
In my experience of playing with the compass on android phones, they can be pretty unreliable...
If your application work on another tablet and other compass application do not work on your device, this is probably due to a bad calibration.
As said in the post above, to make the calibration, wave your device in a figure of 8. I just want to add that you should do it for EACH axis. This should fix your problem.
If it is not a calibration error, as some people have already answered, it is possible that the compass had gone through a magnetic field and now it is desmagnetized, so it is not working properly.
Where do you usually keep the tablet? Could it be that it was near big servers or magnets?
You should check the compass just in case, talk to to android's tech support.
Hope it helps.
I think the question was if calibration could be done without sending any data to compass. Because not everybody says that the compass is calibrated as shown in this video: https://support.google.com/maps/answer/6145351?hl=en and obviously you can not do anything else than advise the user to calibrate before using the program or when you get too much changes.
For example going left and right 90 degrees in about 25 ms.
Anyway I think it's good to give some seconds to the app before start taking data, because it gives some unstable values (too high and low in short time without movement) at the app loading moment.
Just let the handler onSensorChanged() coded with a conditional, and start a thread on the onCreate() handler, which will set a boolean to true, after some seconds.
Then you start to capture data on the onSensorChanged() handler.
Also this thread can help to detect the sensor accuracy, and then you can popup: In Android can I programmatically detect that the compass is not yet calibrated?
I know because I am building a robot using the compass of the smartphone, and I'm having this experience. So, if you are making a robot, make sure to give an spaced place between electronics and hardware to the smartphone, but remember that it's on any compass: electromagnetic fields can be altered by metals so heavily.
Nowadays I have the luck of developing a robot with an HMC-5983 and an MPU-6050, which can be calibrated by using its libraries with Arduino.
That code is compatible/portable to other uController but for not also so easy for smartphones, I guess that the offsets needed for calibrating the compass, the gyro and the accelerometer are inside some internals of Android, not available in the SDK.
I answered before thinking that maybe calibration was only for some devices, but realized that must be as I said before.
So, if playing with robots its possible, I mean it's also easy, but when using an smartphone maybe some custom firmware as CyanogenMod would bring the possibility of investigating the way of setting that offsets, but more important to run some program ported from sketch (following its concept only) to get them first ...
So, good luck ! What is also true, is that in both devices (smartphone and my robot) it's need to move them for them to get working well, as I showed you in the video of latest answer, also helpful on robots.
Good luck and a lot of fun with those things, are very powerful.
I'm making an application that works as a compass..
I'm using the accelerometer and the magnetic field sensors to compute the azimuth angle through, sensor.getOrientation().
I'm searching for something that can improve the magnetic field sensor accuracy, since I'm getting it state of accuracy as UNRELIABLE!
Any one knows anything about this?I'm looking for something that can be either hardcoded or for instance just physically moving the phone until it gets calibrated!
This is not a final answer (I don't know anything for sure), but my understanding from online posts is that waving the phone around in a figure of 8 a few times while the compass is in use is supposed to trigger automatic recalibration. This is what the google maps app suggests, for example. I don't know whether this is dependent on application functionality (something in maps that detects the waving by accelerometer and triggers a recalibration), or something in the android stack, or something specific to per-phone implementations. Try it and see!
Eg discussion: http://androidforums.com/epic-4g-support-troubleshooting/217317-cant-get-compass-calibrate.html
This reference appears to suggest this per-axis / figure-8 rotation process is built-in functionality: http://m.eclipsim.com/gpsstatus/
And here another article that claims this is built-in functionality, and that you don't even need to be running a compass-consuming app for the recalibration to work: http://www.ichimusai.org/2009/06/20/how-to-calibrate-the-htc-magic-compass/
Just a few points
The figure 8 motion works sometimes and not others, I have no idea why, they really need to have some kind of code based way to check if the 8 motion worked (Assuming that the physical motion is actually required)
They also need a way to detect that calibration is required, I looked at the code for the accuracy output (the unreliable constant) and once they send it to you they will not send it again, so for instance if you calibrate but then come within a strong magnetic field it will not resend (not sure why they did that)
One not completely reliable way to detect ongoing issues is that you can also use the magnetic sensor output and do something like field=sqrt(x*x+y*y+z*z) and check that field falls between say 25 and 65 and then ask the user to calibrate if it does not.
The bottom line after testing 18 phones is that I would never depend on a Android based compass with the current crop of phones, accuracy would always be in question.
I have also found even if you are lucky and have a fairly reliable phone you can never be sure that it's calibrated without checking it against a real compass, which kind of defeats the purpose.
NOTE: On a lot of the mis-behaving phones we have found that the sensor writes a calibration file and a tmp file with the same name. If you delete those files and re-boot the phones the calibration file is recreated with zero'd values and the cold start and general calibration problems resolve themselves.
The bad news is that they are stored in /data/misc and require root privileges to get at (thanks Google & Sensor mfg!) so even though I suspect this would solve a lot of problems for a lot of developers it just is not viable from a marketplace app perspective.
I am developing for Android. I'm using Titanium Alloy as development tool with the Titanium Geolocation module.
I have only tested 2 devices [Galaxy Note and S4] against a commercial magnetic compass. Following a calibration process [tilt along the 3 axis] and using 2 different compass apps and the app I'm working on, the Android compass seems accurate enough for basic use ... correlation was good enough for my purpose anyway. I also found the device compass reading to be very sensitive to other magnetic and electrical field interference ... initial mistake I made was to use the compass feature whilst device was in a device protector with a magnetic closure facility [quite common on tabs] ... this interference is particularly strong. I thus need to suggest to users of my app to remove device protectors, keep device free of other electronics and then do standard calibration before initializing the app.
Another option is:
Go To sensors menu: #*0#*
Then if you see a red line in Magnetic Sensor section and a Need for Calibration you should recalibrate your compass.
How;
According those guys;
Turn the Samsung Galaxy Mini S5 around all of its axes until the red
line in the black circle changes color from red to blue. You can also
run through a motion that follows the shape of an 8. It may be that
several attempts are needed to calibrate the compass...