Sensor fusion
Accurate and reliable techniques for determining a current status of an accessory device in relation to an electronic device are described.
1. A method of continuously monitoring by a tablet device pivotally attached to a cover, a pivot angle of the cover in relation to the tablet device, the cover comprising a magnet, the tablet device comprising a processor coupled to which is a display having a top protective layer, a magnetic sensor configured to detect a magnetic field from the magnet through the top protective layer, and an inertial system configured to determine a spatial orientation of the tablet with respect to an inertial reference frame, in a closed configuration the pivot angle is 0 radians and the magnetic sensor is substantially co-planar with the magnet and separated by a first distance R 0 and detects a first magnetic field value propagating through the top protective layer from the magnet, and in an open configuration, the pivot angle is it radians and the magnetic sensor is substantially coplanar with the sensor and separated by a second distance R 1 , and detects a second magnetic field value propagating from the magnet, the second distance R 1 being greater than first distance R 0 , the method comprising: sensing a current magnetic field value from the magnet by the magnetic sensor; determining a relative magnetic field value by comparing the current magnetic field value to the first and second magnetic field values; determining a current spatial orientation of the tablet device with respect to the inertial reference frame by the inertial system; adjusting the relative magnetic field value in accordance with the current spatial orientation; and providing a current pivot angle in accordance with the adjusted relative magnetic field value.
2. The method as recited in claim 1 , wherein the magnetic sensor is a magnetometer.
3. The method as recited in claim 2 , the cover further comprising a first magnetic attachment unit.
4. The method as recited in claim 3 , the tablet device further comprising a second magnetic attachment unit, the first and second magnetic attachment units configured to generate a mutual magnetic attachment force of sufficient strength to pivotally attach the tablet device and the cover.
5. The method as recited in claim 4 , wherein the magnetometer detects a magnetic offset value in accordance with the mutual magnetic attachment force.
6. The method as recited in claim 5 , wherein the processor determines that the cover and the tablet device are magnetically attached to each other when the processor receives an indication that the magnetometer has detected the magnetic offset value.
7. The method as recited in claim 2 , wherein the magnetometer is a compass.
8. The method as recited in claim 7 , further comprising compensating for a moving cover magnetic offset at the compass induced by the magnet as the cover moves.
9. The method as recited in claim 8 , wherein the compensating for the moving cover magnetic offset comprises: determining a horizontal component of an external magnetic field; and using the horizontal component to estimate a maximum change in a magnetic compass heading induced by the relative change in position of the cover in relation to the tablet device.
10. The method as recited in claim 1 , wherein the inertial system is an accelerometer coupled to the processor.
11. The method as recited in claim 1 , wherein the inertial system is a gyroscope coupled to the processor.
12. The method as recited in claim 1 , wherein the inertial system comprises an accelerometer and a gyroscope each coupled to the processor.