CALIBRATION FOR THREE DIMENSIONAL MOTION SENSOR

• US 20110301901A1
• Filed: 06/04/2010
• Published: 12/08/2011
• Est. Priority Date: 06/04/2010
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Panagas, Peter
• Granted Patent: US 8,527,228 B2
• Time in Patent Office: Days
• US Class Current: 702/104
• CPC Class Codes: G01C 25/005 initial alignment, calibration or starting-up of inertial devices
• CPC Class Codes: G01P 21/00 Testing or calibrating of apparatus or devices covered by the preceding groups

Abstract

An electronic device can include an inertial measurement unit (IMU) operative to monitor the movement of the electronic device. The IMU used in the device can be inaccurate due to the manufacturing process used to construct the IMU and to incorporate the IMU in the electronic device. To correct the IMU output, the electronic device in which the IMU is incorporated can be placed in a testing apparatus that moves the device to known orientations. The IMU output at the known orientations can be compared to an expected true IMU output, and correction factors (e.g., sensitivity and offset matrices) can be calculated. The correction factors can be stored in the device, and applied to the IMU output to provide a true output. The testing apparatus can include a fixture placed in a gimbal movable around three axes.

Claims

1. A method for simultaneously calibrating several electronic devices having inertial measurement units (IMUs), comprising:

placing several electronic devices having embedded IMUs in a testing apparatus;

moving the testing apparatus in a sequence of testing positions, where a true IMU output corresponding to each testing position is known;

retrieving, at each testing position, an IMU output from each of the several devices;

calculating, for each device, sensitivity and offset factors for adjusting the IMU output to approach the known true IMU output; and

storing, in storage of each device, the calculated sensitivity and offset factors corresponding to the device.

2. The method of claim 1, wherein:

moving further comprises moving the testing apparatus through at least four different positions.

3. The method of claim 2, wherein:

the at least four different positions are uncorrelated.

4. The method of claim 2, wherein:

moving further comprises moving the testing apparatus through six different positions.

5. The method of claim 2, further comprising:

rotating the testing apparatus around an axis for each of the different positions.

6. The method of claim 1, further comprising:

retrieving a true IMU output from an IMU integrated in the testing apparatus.

7. The method of claim 6, wherein:

the IMU integrated in the testing apparatus is more accurate than the IMUs embedded in each of the several electronic devices.

8. The method of claim 1, wherein:

the IMUs embedded in each of the several electronic devices comprise commercially available IMUs having limited accuracy.

9. A testing apparatus for calibrating an inertial measurement unit embedded in an electronic device, the testing apparatus comprising a processor operative to:

direct the electronic device to move in a sequence of positions;

receive, from the inertial measurement unit embedded in the electronic device, an output corresponding to each of the sequence of positions in a reference frame associated with the inertial measurement unit;

compare the received output to a known inertial measurement unit output in a reference frame associated with the electronic device for each of the sequence of positions;

calculate correction factors for the inertial measurement unit, wherein applying the correction factors to the received inertial measurement unit output converts the reference frame of the received output from the reference frame associated with the inertial measurement unit to the reference frame associated with the electronic device; and

direct the electronic device to store the correction factors.

10. The testing apparatus of claim 9, wherein the processor is further operative to:

receive, from a second inertial measurement unit, an output in a reference frame associated with the electronic device.

11. The testing apparatus of claim 9, wherein the processor is further operative to:

move a gimbal in which the electronic device is placed in a sequence of uncorrelated positions.

12. The testing apparatus of claim 11, wherein the processor is further operative to:

move the gimbal to a first position;

rotate the gimbal around a single axis while in the first position; and

retrieve an inertial measurement unit output while the gimbal rotates.

13. The testing apparatus of claim 9, wherein the processor is further operative to:

receive a plurality of outputs from the inertial measurement unit over a duration for a single position; and

extract a single value from the plurality of outputs for the single position.

14. The testing apparatus of claim 13, wherein the processor is further operative to:

average the plurality of outputs over the duration.

15. The testing apparatus of claim 9, wherein the processor is further operative to:

receive more outputs than required to calculate the correction factors; and

apply a least square regression to calculate the correction factors.

16. A method for calibrating a plurality of electronic devices, comprising:

placing a plurality of electronic devices in a testing apparatus, wherein each electronic device includes an inertial measurement unit (IMU) oriented in a IMU reference frame;

moving the testing apparatus through a plurality of different orientations;

receiving an output from an IMU in each of the plurality of electronic devices as the testing apparatus is moved;

receiving, from a testing device IMU oriented in a testing apparatus reference frame, an output as the testing apparatus is moved;

comparing the outputs received at a same moment in time from the device IMUs and the testing apparatus IMU to calculate a correction factor for transforming the device IMU output to an IMU output in the testing apparatus reference frame.

17. The method of claim 16, wherein:

each of the plurality of electronic devices is positioned in a same electronic device reference frame.

18. The method of claim 17, wherein:

the testing apparatus reference frame is substantially the same as the electronic device reference frame.

19. The method of claim 16, further comprising:

applying the correction factor to the device IMU outputs; and

comparing the corrected device IMU outputs to a corresponding testing apparatus IMU output.

20. The method of claim 19, further comprising:

determining whether the difference between the corrected IMU outputs and the testing apparatus IMU output is less than a threshold.