Spherical Calibration Device for Center of Rotation Accuracy
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Solution Overview
Problem
Current methods for determining the center of rotation of a spherical object in computer-assisted surgery are prone to high error margins, are time-consuming, and require awkward patient positioning, especially when only three points are captured, and can result in imprecise calculations due to linear point capture or surface digitization errors.
Innovation Solution
A calibration device with a tubular tip portion and tracking member is used to define an annular contact region on a spherical object, allowing for precise determination of the center of rotation by capturing spatial coordinates of points on the object's surface and calculating the center using a computer system, which can also handle unknown diameters through additional positional readings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If only three points are captured on the spherical surface, then the calibration process is faster, but the error margin remains relatively high
Solution Approach 1:
The patent transitions from capturing discrete points on the spherical surface to capturing the entire spherical surface through rotation. By rotating the spherical object and capturing images at multiple angular positions, the system gathers comprehensive surface data in three-dimensional space, enabling accurate center calculation without requiring excessive discrete point measurements.
Solution Approach 2:
The patent employs preliminary actions by first rotating the spherical object to capture its complete surface geometry before calculating the center of rotation. This pre-capture of full surface data eliminates the need for multiple separate measurement sessions and provides all necessary information for precise center determination in a single calibration process.
2Measurement precision
If a plurality of points are captured on the spherical surface, then accuracy improves, but the process becomes overly time consuming
Solution Approach 1:
The patent implements periodic action by rotating the spherical object at regular angular intervals and capturing images at each position. This systematic rotation through discrete angular steps efficiently captures the complete spherical surface in a structured sequence, achieving high accuracy without requiring continuous or excessive measurement time.
Solution Approach 2:
The complete spherical surface is captured in advance through rotation, providing all necessary measurement data in one preliminary action. This eliminates the need for multiple separate measurement sessions and enables accurate center calculation from comprehensive pre-gathered data.
3Ease of operation
If points are captured too close to each other (linearly or quasi-linearly), then the measurement process is simpler, but the resulting cone calculated by the CAS system will be skewed and not representative of the true COR
Solution Approach 1:
The patent employs dynamic measurement by rotating the spherical object through multiple angular positions rather than capturing static points in a fixed linear arrangement. This dynamic approach ensures points are distributed across the spherical surface in three-dimensional space, preventing collinearity and producing an accurate representative cone for center calculation.
4Measurement precision
If the surgeon holds and rotates the limb through a relatively large region above the operating table, then sufficient points can be captured, but the procedure becomes awkward
Solution Approach 1:
The patent introduces an intermediary device (the spherical calibration object with reflective markers) that mediates between the surgeon and the CAS system. The surgeon simply holds and rotates this calibrated spherical object rather than manipulating the patient's limb through large regions, achieving sufficient point distribution while maintaining ease of operation and patient safety.
Data Source
AI summary
A method of determining a center of curvature of the spherical outer surface of an object using a computer system is provided. The method includes defining at least one contact region on the spherical outer surface in a plane substantially tangential to a circumference thereof and a first reference axis normal to said plane. Spatial coordinates of at least one of a first and a second geometric parameter are determined, the first geometric parameter including at least two points located on the spherical outer surface and the second geometric parameter including a second reference axis normal to the spherical outer surface. The center of curvature of the spherical outer surface is then calculated using the first reference axis and at least one of the first and second geometric parameters. An associated system and calibration device is also provided.