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I've been experimenting with the compass and gyroscope on iPhone 4 and would like some help with an issue I'm having. I want to compensate for the slowness of the compass by using data from the gyroscope.

Using CMMotionManager and its CMDeviceMotion object (motionManager.deviceMotion), I get the CMAttitude object. Correct me if I'm wrong (please), but here is what I've deduced from the CMAttitude object's yaw property (I don't need pitch nor roll for my purposes):

  • yaw ranges from 0 to PI when the phone is pointing downwards (as indicated by deviceMotion.gravity.z) and swinging counterclockwise and 0 to -PI when swung clockwise
  • when the device is pointing upwards, yaw ranges from -PI to 0 and PI to 0, respectively
  • and from the compass data (I'm using locationManager.heading.magneticHeading), I see that the compass gives values from 0 to 360, with the value increasing when swinging clockwise

All right, so using all of this information together, I'm able to get a value I call horizontal that, regardless of whether the device is pointing up or down, will give values from 0 to 360 and increase when the device is swung clockwise (though I am still having trouble when deviceManager.gravity.z is around 0 -- the yaw value freaks out at this gravity.z value).

It seems to me that I could "synchronize" the horizontal and magneticHeading values, using a calculated horizontal value that maps to magneticHeading, and "synchronize" the horizontal value to magneticHeading when I feel the compass has "caught up."

So my questions:

  • Am I on the right track with this?
  • Am I using the gyro data from CMDeviceMotion properly and the assumptions I listed above correct?
  • Why might yaw freak out when gravity.z is around 0?

Thank you very much. I look forward to hearing your answers!

donkim
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4 Answers4

22

Just trying to answer... correct me if i'm wrong..

1.Yes you are on the right track

2.gravity in CM is already "isolated" from user gravity (gravity value caused by user acceleration) thats why there is two gravity, the "gravity" and "userAcceleration" its on apple CM documentation // Note : not entirely isolated //

3. if you have a gravity 0 it mean that the coresponding axis is perpendicular with gravity. gravity.z is the iPhone screen thats why it -9.82m/s2 if you put on the desk with screen upright, actualy it hard to get 0 or maximum value of the gravity due to the sensor noise (it's normal, all sensor has a noise expecially cheap sensor).

what i do on my apps is I will switch my reference axis to other axis (in your case may be x or y) for certain limits, how the strategy is depend on the purpose or which side is your reference.

the other thing is, gyro is fast but its not stable, you need to re-calibrate the value for several interval. In my case every 5 second. I've experiment with gyro for calculating angle between two plane, i try with exacly 90 degree ruler and it will give an error about 0.5 degree every second try and keep increasing, but thats is mine, maybe others have a better method for avoid the error.

below is my steps "

  1. Init
  2. Read gravity XYZ -> Xg Yg Zg
  3. Check if Xg < 0.25 If TRUE try Yg then Zg // Note 1 = 1g = 9.82 m/s^2
  4. Read the compass and gyro
  5. Configure and calibrate the gyro using the compass and calulate based on which axis i use in point 3.
  6. If 5 second is pass then recalibrate, read the compass
  7. If the the difference with gyro reading is > 5 degree skip recalibartion the gyro.
  8. If the the difference with gyro reading is < 5 degree calibrate the gyro using compass value

Note: for number 7 : is to check if the phone affected with magnetic field or near huge steel such or high voltage electrical line or in noisy and heavy equipment in factory plant.

Thats all... Hope this could help you... And sorry for my english..

DoogyHtw
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  • for initialization step, make sure prompt the user to stop a while. skip step 7 if your apps will not encounter steel or electricity, in my case i need it because it used on factory with a high voltage equipment and pipeline which is steel. – DoogyHtw Dec 13 '10 at 09:26
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Here is an example of an iPhone app where the compass get compensated with the gyroscope. Code and project can be seen here: http://www.sundh.com/blog/2011/09/stabalize-compass-of-iphone-with-gyroscope/

Ellen S
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The direction of the yaw axis vector is undefined when in zero gravity (or free fall, or close enough).

In order to do synchronization while in motion, you need to create a filter for your "horizontal" value that has the same lag/delay response characteristics as the magnetic compass. Either that, or wait until motion stops long enough for both values to settle before recalculating the offset.

hotpaw2
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Answer to question 1 is Yes, question 2 you are on the right track but you could use a variable name that is not 'horizontal', question 3 is answered by hotpaw2 and also a yaw in a chopper or helicopter at near zero altitude would alert the pilot with an alarm. There is a time lag because part of the software is local while there are other factors which can slow it down including access to a sensor for detecting magnetic waves, the device position and direction, preparing the graphic output for the compass display, computing and outputting data from the gyro and sensors through a relatively slow interface, using a general purpose handheld device not custom designed for the type of task being asked of it.

Developer
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