Helmholtz Coil Calibration for AR Sensor Accuracy
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Solution Overview
Problem
Magnetic sensors in Electromagnetic Tracking (EMT) systems, used in Augmented Reality (AR) and Virtual Reality (VR), often require accurate calibration to prevent incorrect position and orientation readings, which is challenging due to interference from materials in head-mounted displays (HMDs) and the need for precise magnetic field control.
Innovation Solution
A calibration system utilizing a Helmholtz device generates uniform magnetic fields to characterize the sensor, allowing for the determination of calibration correction factors, which are then applied to ensure accurate position and orientation data, while minimizing interference by using low-frequency magnetic fields to reduce eddy currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high-frequency magnetic fields are used for sensor calibration, then calibration speed is improved, but interference from HMD materials (eddy currents) increases causing measurement errors
Solution Approach 1:
The patent changes the frequency parameter of the magnetic field from high-frequency to low-frequency (e.g., below 100 Hz) during calibration to minimize eddy current interference from HMD materials, thereby maintaining measurement precision while accepting a trade-off in calibration speed that is compensated by the efficiency of the automated mount system
2Adaptability or versatility
If a translation system is used to move the transmitter to multiple locations, then calibration coverage is improved, but device complexity increases
Solution Approach 1:
The patent replaces the mechanical translation system with a fixed Helmholtz device that generates magnetic fields at multiple predetermined locations. This substitution eliminates complex mechanical moving parts while maintaining calibration coverage through the use of multiple fixed coil pairs that can be independently activated
Solution Approach 2:
The calibration space is segmented into multiple predetermined locations, each served by a dedicated pair of Helmholtz coils. This segmentation allows the system to achieve comprehensive calibration coverage without requiring a single complex translation mechanism, as each coil pair independently serves its specific spatial zone
3Measurement precision
If the sensor is removed from the HMD for calibration, then calibration accuracy is improved by eliminating material interference, but ease of operation deteriorates
Solution Approach 1:
The patent introduces an intermediary low-frequency magnetic field generation mechanism that enables accurate calibration to be performed with the sensor remaining in the HMD. The low-frequency fields act as an intermediary that penetrates the HMD materials without generating significant eddy currents, thus maintaining measurement precision while allowing the sensor to remain in its operational context
Solution Approach 2:
The system performs preliminary calibration actions by characterizing the sensor at multiple predetermined locations within the HMD using low-frequency fields before actual operation. This preliminary characterization captures the interaction between the sensor and HMD materials, allowing accurate calibration without requiring removal of the sensor
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method enables quick and accurate calibration of multiple sensors without moving parts, simplifying the calibration process and improving accuracy by eliminating the need for separate translation systems and allowing sensors to be calibrated within their operational devices.
Implementation Method 1
a Helmholtz device that includes three pairs of coils defining an inner volume. Each of the three pairs of coils is configured to generate a magnetic field that is uniform throughout the inner volume
Implementation Method 2
The generated magnetic fields are received by the sensor and converted into one or more electrical signals indicative of one or more characteristics of the sensor
Implementation Method 3
the Helmholtz devices can generate the magnetic fields at a relative low frequency (e.g., 90 Hz) during calibration to minimize or eliminate interference in the generated magnetic fields that may otherwise be caused by materials of the HMD
Data Source
AI summary
A calibration system comprising: a Helmholtz device comprising thee pairs of coils defining an inner volume, wherein each of the three pairs of coils is configured to generate a magnetic field that is uniform throughout the inner volume; a mount configured to accept a device that includes a magnetic sensor, wherein at least a portion of the mount is positioned within the inner volume such that the magnetic sensor is positioned at or near a center of the inner volume when the device is positioned on the mount; and a computer system configured to communicate with the Helmholtz device and the magnetic sensor, wherein the computer system is configured to: provide instructions to cause each of the three pairs of coils to generate a magnetic field; receive signals from the magnetic sensor that are based on characteristics of the magnetic fields received at the magnetic sensor; measure, based on the signals received from the magnetic sensor, one or more characteristics of the magnetic sensor; and determine, using a calibration algorithm, one or more calibration correction factors for the magnetic sensor based on the one or more characteristics of the magnetic sensor and the provided instructions.