Methods and devices for detecting magnetic interference affecting the operation of a magnetometer
Methods and apparatus employing non-magnetometer navigational sensor data to assist in determining whether a change in a magnetic field detected by a magnetometer is likely due to a source of internal or external magnetic interference, and more generally, whether such interference is likely to be persistent or transient. If the magnetic field data detected by the magnetometer indicates a large change in magnetic field, but the non-magnetometer navigational sensor (e.g., gyroscope) data does not indicate a corresponding change in orientation of the mobile device contemporaneous with the change in magnetic field, then the cause of the magnetic field change may be determined as likely originating from a localized external interference source and the device may prompt the user to move away from the interference source, rather than initiating a recalibration of the magnetometer.
1. A method for detecting magnetic interference in a mobile device, the method comprising:
monitoring magnetometer sensor data generated by a magnetometer for a first time interval;
detecting a change in the magnetometer sensor data for the first time interval;
computing expected motion data that characterizes an expected motion of the mobile device based on the magnetometer sensor data;
comparing the expected motion data with reference data; and
deferring recalibration of the magnetometer when the expected motion data does not correspond to the reference data;
wherein the deferring comprises displaying an indication of a corrective action in a user interface of the mobile device.
2. The method of claim 1 , further comprising, prior to computing the expected motion data, computing the reference data of the mobile device based on non-magnetometer sensor data.
3. The method of claim 2 , wherein the non-magnetometer sensor data is generated at the mobile device.
4. The method of claim 2 , wherein the non-magnetometer sensor data is generated by one or more non-magnetometer sensors in the first time interval.
5. The method of claim 4 , wherein the one or more non-magnetometer sensors is selected from the group consisting of: an accelerometer, a gyroscope and a GPS unit.
6. The method of claim 1 , wherein the indication identifies that magnetic interference is present near the mobile device.
7. The method of claim 1 , wherein the corrective action comprises moving the mobile device.
8. The method of claim 1 , wherein the corrective action comprises removing a possible source of the magnetic interference.
9. The method of claim 1 , wherein the deferring comprises monitoring the magnetometer sensor data for a second time interval and computing new expected motion data of the mobile device based on the magnetometer sensor data for the second time interval, and wherein, when the new expected motion data does not correspond to new reference data, the method further comprises recalibrating the magnetometer.
10. The method of claim 9 , further comprising, when the new expected motion data corresponds to the new reference data, cancelling recalibration of the magnetometer.
11. The method of claim 1 , wherein the magnetic field change exceeds a predetermined change threshold.
12. A non-transitory computer readable medium storing instructions executable by a processor of a mobile device, the instructions for carrying out a method for detecting magnetic interference in the mobile device, the method comprising:
monitoring magnetometer sensor data generated by a magnetometer for a first time interval;
detecting a change in the magnetometer sensor data for the first time interval;
computing expected motion data that characterizes an expected motion of the mobile device based on the magnetometer sensor data;
comparing the expected motion data with reference data; and
deferring recalibration of the magnetometer when the expected motion data does not correspond to the reference data;
wherein the deferring comprises displaying an indication of a corrective action in a user interface of the mobile device.
13. A mobile device comprising:
a magnetometer;
one or more sensors;
a processor coupled to the magnetometer and the one or more sensors, the processor configured to:
monitor magnetometer data generated by the magnetometer for a first time interval;
detect a change in the magnetometer data for the first time interval;
compute expected motion data that characterizes an expected motion of the mobile device based on the magnetometer data;
compare the expected motion data with reference data;
defer recalibration of the magnetometer when the expected motion data does not correspond to the reference data; and
display an indication of a corrective action in a user interface of the mobile device when deferring recalibration.
14. The mobile device of claim 13 , wherein the processor is further configured to, prior to computing the expected motion data, compute the reference data of the mobile device based on sensor data generated by the one or more sensors.
15. The mobile device of claim 14 , wherein the sensor data is generated by the one or more sensors in the first time interval.
16. The mobile device of claim 13 , wherein the indication identifies that magnetic interference is present near the mobile device.
17. The mobile device of claim 13 , wherein the corrective action comprises moving the mobile device.
18. The mobile device of claim 13 , wherein the corrective action comprises removing a possible source of the magnetic interference.
19. The mobile device of claim 13 , wherein, when deferring recalibration, the processor is further configured to monitor the magnetometer data for a second time interval and compute new expected motion data of the mobile device based on the magnetometer data for the second time interval, and wherein, when the new expected motion data does not correspond to new reference data, the processor is further configured to recalibrate the magnetometer.
20. The mobile device of claim 19 , wherein, when the new expected motion data corresponds to the new reference data, the processor is configured to cancel recalibration of the magnetometer.
21. The mobile device of claim 13 , wherein the magnetic field change exceeds a predetermined change threshold.
22. The mobile device of claim 13 , wherein the one or more sensors is selected from the group consisting of: an accelerometer, a gyroscope and a GPS unit.