Magnetometer and accelerometer calibration for cart navigation system

• US 10,001,541 B2
• Filed: 09/02/2016
• Issued: 06/19/2018
• Est. Priority Date: 09/04/2015
• Status: Active Grant

• Current Assignee: Gatekeeper Systems, Inc.
• Original Assignee: Gatekeeper Systems, Inc.
• Inventors: Hannah, Stephen E., Carter, Scott J.
• Primary Examiner(s): Astacio-Oquendo, Giovanni
• Application Number:
  US15/255,530
• Publication Number:
  US 20170067981A1
• Time in Patent Office: 655 Days
• Field of Search: 324 7111, 324 731, 324 7611
• CPC Class Codes: B62B 5/0096 Identification of the cart or merchandise, e.g. by barcodes or radio frequency identification [RFID]
• CPC Class Codes: G01C 17/38 Testing, calibrating, or compensating of compasses
• CPC Class Codes: G01C 25/00 Manufacturing, calibrating, cleaning, or repairing instruments or devices referred to in the other groups of this subclass testing, calibrating or compensating compasses G01C17/38
• CPC Class Codes: G01P 15/18 in two or more dimensions
• CPC Class Codes: G01P 21/00 Testing or calibrating of apparatus or devices covered by the preceding groups
• CPC Class Codes: G01R 35/005 Calibrating; Standards or reference devices, e.g. voltage or resistance standards, "golden" references G01R33/0035, G01R35/002 take precedence

Abstract

A wheeled object (e.g., a human-propelled cart such as a shopping cart) can include a navigation system that uses dead reckoning to determine the position of the wheeled object. The navigation system can include a magnetometer and an accelerometer and may be disposed on a handle of the wheeled object. Examples of a system for calibrating the magnetometer and accelerometer during manufacture as well as during installation on the wheeled object are disclosed. The calibration includes calibrating for the soft iron distortion induced at the navigation system by the ferromagnetic shopping cart handle (or other ferromagnetic portions of the cart such as the frame) and hard iron distortions induced by nearby magnetic sources (e.g., a permanent magnet in an audio speaker). The calibration system can include a plurality of Helmholtz or Maxwell coils.

Claims

1. A method for calibrating a magnetometer in a navigation system for a human-propelled wheeled cart, the method comprising:

calibrating the magnetometer prior to installation of the magnetometer in the navigation system, the calibrating comprising;

disposing the magnetometer at a position surrounded by a first magnetic coil;

applying a time-varying current to the first magnetic coil to produce an applied magnetic field at the position of the magnetometer;

measuring a magnetometer response to the applied magnetic field; and

generating, based at least partly on the magnetometer response, a pre-installation calibration model for the magnetometer; and

calibrating the magnetometer after installation of the magnetometer in the navigation system and after attachment of the navigation system to a handle of the human-propelled wheeled cart, the calibration comprising;

disposing a second magnetic coil around the magnetometer;

applying a time-varying current to the second magnetic coil to produce an applied magnetic field at the position of the magnetometer;

measuring a magnetometer response to the applied magnetic field;

generating, based at least partly on the magnetometer response, a post-installation calibration model for the magnetometer; and

storing the pre-installation calibration model or the post-installation calibration model in a non-transitory memory in communication with the navigation system.

2. The method of claim 1, further comprising installing the magnetometer in the navigation system.

3. The method of claim 1, wherein the first magnetic coil or the second magnetic coil comprises one or both of a Helmholtz coil or a Maxwell coil.

4. The method of claim 1, wherein the first magnetic coil or the second magnetic coil comprise at least three magnetic coils arranged to provide magnetic fields along three different directions, respectively.

5. The method of claim 1, further comprising determining an orientation of a coordinate system associated with the navigation system relative to an orientation of a body frame of the human-propelled, wheeled cart.

6. The method of claim 5, wherein the human-propelled, wheeled cart comprises a pair of wheels, the navigation system comprises a first accelerometer, and determining the orientation of the coordinate system associated with the navigation system relative to the orientation of the body frame of the human-propelled, wheeled cart comprises:

attaching an alignment fixture between the pair of wheels, the alignment fixture comprising a second accelerometer; and

comparing a first measurement from the first accelerometer with a second measurement from the second accelerometer.

7. The method of claim 1, wherein applying the time-varying current to the first magnetic coil or the second magnetic coil comprises using a feedback loop, a current sense amplifier, and an H-bridge for directional switching.

8. A method for calibrating a magnetometer in a navigation system attached to a handle of a human-propelled wheeled cart, the method comprising:

disposing a 3-axis magnetic coil system around the navigation system attached to the handle of the human-propelled wheeled cart;

applying time-varying currents to the 3-axis magnetic coil system to produce an applied 3-axis magnetic field at the position of the magnetometer;

measuring a magnetometer response to the applied 3-axis magnetic field;

generating, based at least partly on the magnetometer response, a calibration model for the magnetometer; and

storing the calibration model in a non-transitory memory in communication with the navigation system attached to the handle of the human-propelled wheeled cart.

9. The method of claim 8, wherein the 3-axis magnetic coil system comprises a magnetic coil having a break-apart that permits a portion of the magnetic coil to be separated, and wherein disposing the 3-axis magnetic coil system around the navigation system attached to the handle of the human-propelled wheeled cart comprises:

opening the break-apart of the magnetic coil to permit the coil to surround the handle; and

closing the break-apart prior to applying the time-varying currents to the 3-axis magnetic coil system.

10. A method for calibrating a magnetometer associated with a wheeled object, the method comprising:

disposing a magnetometer in a region surrounded by a first plurality of coils configured to generate a first magnetic field in a first direction, a second plurality of coils configured to generate a second magnetic field in a second direction, and a third plurality of coils configured to generate a third magnetic field in a third direction, each of the first, second, and third pluralities of coils configured to generate the respective first, second, and third magnetic fields in response to a first current, a second current, and a third current, respectively;

varying, by a controllable current source, each of the first current, the second current, and the third current;

measuring, by a hardware controller, the response of the magnetometer in response to the varied first, second, and third currents; and

generating, by the hardware controller, a calibration model for the response of the magnetometer.

11. The method of claim 10, wherein the first plurality of coils, the second plurality of coils, or the third plurality of coils comprises a Helmholtz coil.

12. The method of claim 10, wherein the first plurality of coils, the second plurality of coils, or the third plurality of coils comprises a Maxwell coil.

13. The method of claim 10, wherein the second direction is perpendicular to the first direction, and the third direction is perpendicular to a plane containing the first direction and the second direction.

14. The method of claim 10, wherein disposing the magnetometer comprises disposing the magnetometer in a fixture designed such that the fixture can be placed in at least six different attitudes, such that each of three orthogonal axes of the magnetometer can be oriented in an upward direction and in a downward direction in one of the at least six attitudes.

15. The method of claim 10, wherein the magnetometer is disposed in an enclosure mounted to a handle of the wheeled object, the handle of the wheeled object defining a handle axis.

16. The method of claim 15, wherein the first direction is parallel to the handle axis, and wherein at least one coil in the first plurality of coils is designed to break apart so as to surround the handle of the wheeled object.

17. The method of claim 15, further comprising:

establishing an X-direction in a direction of motion of the wheeled object;

establishing a Y-direction perpendicular to the X-direction and in a horizontal plane of the wheeled object; and

determining an orientation of the magnetometer relative to the X-direction and the Y-direction.

18. The method of claim 15, wherein generating the calibration model comprises generating a magnetic response model for the handle of the wheeled object.

19. The method of claim 10, wherein the wheeled object comprises a shopping cart.

20. The method of claim 10, further comprising providing the first, second, and third pluralities of coils in a fixed orientation in a clamshell that is designed to close around the magnetometer, such that the first, second, and third pluralities of coils are locked into a desired position relative to the magnetometer when the clamshell is closed.

21. The method of claim 10, wherein varying each of the first current, the second current, and the third current comprises using a feedback loop, a current sense amplifier, and an H-bridge for directional switching.