Fiber Optic Shape Sensor for Kinematic Chain Pose
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Solution Overview
Problem
Current optical fiber shape sensing methods are limited by resolution, accuracy, and data processing challenges, particularly in determining the pose of kinematic chains, especially in real-time applications like telerobotically controlled surgical instruments, due to assumptions about constant bend radius and friction influences.
Innovation Solution
A shape sensing system using an optical fiber coupled to multiple links in a kinematic chain, with defined shape sensing segments and reference frames, senses strain to output Cartesian position and orientation, facilitating accurate pose determination through high-resolution strain measurement and integration techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If fiber Bragg gratings are used to sense strain for shape determination, then position and orientation information can be obtained, but measurement resolution and accuracy are limited by the tether segment length and FBG spacing
Solution Approach 1:
The fiber is divided into multiple discrete tether segments with FBG's at specific locations. By increasing the number of segments and FBG's along the fiber length, the system achieves higher spatial resolution for shape sensing while maintaining manageable segment lengths for each measurement point.
Solution Approach 2:
Multiple cores are used within the fiber, with FBG's positioned at the same location along different cores. This multi-dimensional approach allows simultaneous measurement of strain in multiple directions, improving shape sensing resolution without requiring shorter tether segments.
2Measurement precision
If strain information from FBG's is used to determine fiber shape through forward kinematic calculations, then position of distal end can be calculated, but accuracy is affected by assumptions about constant bend radius and friction
Solution Approach 1:
The mechanical kinematic chain model with its assumptions about constant bend radius and friction is replaced with a direct optical measurement approach. Phase-sensitive interferometry directly measures fiber shape and orientation without requiring complex mechanical models, eliminating the sources of error associated with those assumptions.
Solution Approach 2:
An interferometric measurement system serves as an intermediary between the fiber and the measurement process. The phase-sensitive interferometer directly measures the optical path differences caused by fiber deformation, providing accurate pose information without requiring complex kinematic calculations or assumptions about mechanical behavior.
3Adaptability or versatility
If multiple cores with FBG's at the same location are used, then bend direction and axial twist can be determined, but device complexity and data processing requirements increase
Solution Approach 1:
Multiple cores with FBG's at the same location are merged into a single fiber structure. The interferometric interrogation system processes signals from all cores simultaneously, extracting bend direction and axial twist information through combined analysis of phase differences across cores, thereby achieving versatile sensing without proportionally increasing system complexity.
Solution Approach 2:
The multi-core fiber structure with co-located FBG's provides universal sensing capability for multiple parameters (bend direction, axial twist, and position) using a single integrated sensor element. The interferometric system universally processes all these measurements through phase analysis, eliminating the need for separate sensing systems for each parameter.
4Productivity
If FBG's are interrogated using traditional techniques, then strain information can be obtained, but measurement speed and immunity to disturbances are limited
Solution Approach 1:
The interferometric interrogation uses periodic modulation of the optical path to encode strain information as phase differences. This periodic measurement approach enables high-speed data acquisition while the phase-sensitive detection method maintains high measurement precision by measuring relative phase changes that are immune to many environmental disturbances.
Solution Approach 2:
Instead of directly measuring absolute strain values that are sensitive to disturbances, the system measures copies of the strain information encoded as phase differences between reference and measurement paths. This copying approach through interferometry preserves the strain information while eliminating sensitivity to many external disturbances such as temperature variations and light source fluctuations.
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 approach provides precise, high-resolution shape information for kinematic chain pose estimation, independent of fiber path, enhancing accuracy and applicability in real-time surgical instrument control.
Implementation Method 1
Two fiber Bragg gratings (FBG's) 102a,102b are shown formed in fiber core portion 100, which are illustrative of many such FBG's typically formed along the full length of a core
Implementation Method 2
Among the interferometric techniques that are based on detection of the phase of the reflected light are Optical Time Domain Reflectometry and Optical Frequency Domain Reflectometry (OFDR)
Data Source
AI summary
A shape sensing system to determine the position and orientation of one link with respect to another link in a kinematic chain. An optical fiber is coupled to two or more links in a kinematic chain. A shape sensing segment is defined to start at a proximal link and to end at a distal link, crossing one or more joints. A reference frame is defined at the start of the shape sensing segment. As the joints move, an interrogator senses strain in the shape sensing segment. The sensed strain is used to output a Cartesian position and orientation of the end of the shape sensing segment with respect to the reference frame defined at the start of the shape sensing segment. The pose of the kinematic chain is determined from the Cartesian positions and orientations of one or more shape sensing segments defined for the kinematic chain and from an a priori model and constraints of the kinematic chain.