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Saying goodbyes to rotating your phone: Magnetometer calibration during SLAM (2409.01242v1)

Published 2 Sep 2024 in cs.RO

Abstract: While Wi-Fi positioning is still more common indoors, using magnetic field features has become widely known and utilized as an alternative or supporting source of information. Magnetometer bias presents significant challenge in magnetic field navigation and SLAM. Traditionally, magnetometers have been calibrated using standard sphere or ellipsoid fitting methods and by requiring manual user procedures, such as rotating a smartphone in a figure-eight shape. This is not always feasible, particularly when the magnetometer is attached to heavy or fast-moving platforms, or when user behavior cannot be reliably controlled. Recent research has proposed using map data for calibration during positioning. This paper takes a step further and verifies that a pre-collected map is not needed; instead, calibration can be done as part of a SLAM process. The presented solution uses a factorized particle filter that factors out calibration in addition to the magnetic field map. The method is validated using smartphone data from a shopping mall and mobile robotics data from an office environment. Results support the claim that magnetometer calibration can be achieved during SLAM with comparable accuracy to manual calibration. Furthermore, the method seems to slightly improve manual calibration when used on top of it, suggesting potential for integrating various calibration approaches.

Summary

  • The paper demonstrates that magnetometer calibration can be integrated into SLAM using a factorized particle filter and Kalman Filter, eliminating the need for manual device rotation.
  • The method improves positioning accuracy by effectively mitigating magnetometer bias during natural movement, as validated on both smartphone and robot navigation data.
  • The approach enables robust, real-time calibration in dynamic indoor environments, simplifying sensor fusion for enhanced positioning performance.

Magnetometer Calibration During SLAM: A New Approach to Indoor Positioning

The paper "Saying Goodbyes to Rotating Your Phone: Magnetometer Calibration During SLAM" explores the possibility of eliminating manual calibration procedures traditionally required for magnetometer calibration in Simultaneous Localization and Mapping (SLAM) processes. This research is particularly significant for applications in indoor positioning where magnetic field (MF) features are used alongside Wi-Fi positioning.

Motivation and Context

In indoor environments, traditional Global Positioning Systems (GPS) are ineffective due to signal unavailability. Hence, alternative methods such as Wi-Fi-based positioning and utilizing local MFs for positioning have gained attention. However, MF-based techniques face the critical challenge of magnetometer bias, which can severely degrade positioning accuracy. Standard calibration methods, such as rotating devices in specific patterns, are often impractical, especially for large or dynamically moving devices.

Methodology

This paper introduces a novel approach to magnetometer calibration within an ongoing SLAM operation, circumventing the necessity of a pre-collected MF map. The proposed method leverages a factorized particle filter that concurrently estimates positional information and calibrates the device by factoring out calibration aspects from the acquired magnetic data. Calibration is facilitated using a Kalman Filter (KF) that adjusts the device's bias estimate dynamically as SLAM processes unfold.

The authors validate their approach using two distinct data sets. The first involves smartphone data collected in a shopping mall, presenting high pedestrian dead reckoning (PDR) and orientation uncertainty. The second data set utilizes robot navigation data from controlled office and apartment environments, where the robot's path provides more stable odometric data compared to smartphone user motion.

Key Findings

  1. Calibration Without Manual Movement: The paper demonstrates that magnetometer calibration can be effectively achieved during the SLAM process, using natural walking data. This is an advancement over existing models that require explicit calibration maneuvers or pre-compiled maps.
  2. Improved Accuracy: The method's accuracy was compared to operating system (OS)-reported bias and was found to either match or slightly enhance the calibration, suggesting a potential improvement over automated OS calibrations.
  3. Consistency Across Different Platforms: Experiments show that SLAM with magnetometer calibration is effective for both smartphones and mobile robots, suggesting versatility across different platforms.
  4. Application for Dynamic Systems: The flexibility in calibration makes this method particularly advantageous for dynamically moving systems where manual calibration is infeasible.

Theoretical and Practical Implications

Theoretically, this work extends the utility of particle filtering methods in SLAM by integrating real-time calibration adjustments, potentially influencing future research on multi-sensor fusion techniques. Practically, it simplifies the deployment of positioning systems in environments where pre-calibration is impossible or impractical, such as high-footfall shopping areas or industrial spaces with moving machinery.

Future Directions

While the current approach assumes a constant bias, future research can explore bias variations due to changes in the device environment or physical properties. Additionally, the integration of more advanced filtering techniques like Unscented Kalman Filters (UKF) or cross-validation with other sensory data could further enhance calibration accuracy. A potential area of exploration is the development of crowd-sourced mapping techniques that can utilize aggregate data from multiple devices to improve individual calibration models.

In conclusion, this paper lays important groundwork for more adaptive and less intrusive magnetometer calibration methods in indoor positioning systems, addressing current limitations in practical deployment and opening pathways for more robust sensor fusion techniques in dynamic environments.

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