Flexure Lens Mounting for Stress-Free Polymer Optics
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Solution Overview
Problem
Head-mounted devices face challenges in mounting lenses due to stress sensitivity, particularly with polymer lenses that exhibit stress-induced birefringence, exacerbated by thermal expansion mismatches between polymer lenses and metal supports.
Innovation Solution
The use of lens mounts with flexures, such as ring-shaped or discrete flexures, to isolate lenses from stress sources, utilizing materials with matched thermal expansion coefficients and high yield strength to accommodate temperature-induced expansion and contraction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If rigid support structures are used to hold the lens, then structural stability is improved, but stress-induced birefringence increases due to thermal expansion mismatches
Solution Approach 1:
The patent changes the mechanical parameters of the support structure by introducing flexures with specific elastic properties. The flexures are designed with controlled stiffness and compliance to accommodate thermal expansion while maintaining structural stability. This allows the support structure to transition from a purely rigid state to a dynamically compliant state that adapts to thermal changes.
Solution Approach 2:
The patent employs composite construction by combining rigid materials (for structural stability) with flexible flexure elements (for stress accommodation). The lens mount assembly integrates rigid housing materials with flexible flexure materials having different elastic moduli, creating a composite structure that simultaneously provides stability and thermal compliance.
2Manufacturing precision
If the lens is tightly secured in the mount, then positioning precision is improved, but stress on the lens increases due to thermal contraction
Solution Approach 1:
The patent introduces dynamic compliance to the lens mounting system through flexures that can deform under thermal loading. The flexures are designed to maintain precise positioning during normal operation but can dynamically deform to accommodate thermal contraction, preventing stress accumulation while preserving positioning accuracy.
Solution Approach 2:
The flexures are pre-designed and pre-computed to provide cushioning against thermal stress. The elastic properties of the flexures are selected to absorb the expected thermal contraction forces before they can damage the lens or compromise positioning precision.
3Device complexity
If discrete flexures are used instead of continuous ring flexure, then device complexity is reduced, but stress distribution uniformity worsens
Solution Approach 1:
The patent segments the continuous ring flexure into discrete flexure elements positioned at strategic locations around the lens periphery. This segmentation reduces manufacturing complexity and assembly difficulty while maintaining adequate stress distribution through proper positioning of the discrete flexures.
Solution Approach 2:
The discrete flexures are positioned at specific locations around the lens periphery where stress concentration is most likely to occur. This local placement strategy provides targeted stress relief at critical positions while maintaining overall structural integrity and simplicity of the mount design.
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
The flexure-based lens mounts effectively minimize stress-induced birefringence and maintain lens positioning, ensuring stable image presentation across varying temperatures.
Implementation Method 1
allowing them to expand and contract without applying pressure to the support structures
Implementation Method 2
polymer lens elements that exhibit stress-induced birefringence when subjected to excessive stress
Data Source
AI summary
A head-mounted device may include optical assemblies for presenting images to a user. The optical assemblies may each have a display for generating an image and a lens that presents the image to a corresponding eye box for viewing by a user. The optical assemblies may have lens barrels. Each lens barrel may have a first end to which a display is mounted and may have an opposing second end. A mount may be used to attach the lens to the second end of the lens barrel. The mount may have at least one flexure. The flexure may have a U-shaped cross-sectional shape. The mount may have a ring-shaped flexure that extends around a peripheral edge of the lens, may have flexure segments, or may have a set of three discrete flexures located at different positions along the edge of the lens.