MEMS Motion Limiter With Two-Threshold Variable Stiffness
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Solution Overview
Problem
Microelectromechanical (MEMS) devices such as accelerometers and gyroscopes face issues with direct physical contact between the mass element and fixed structures, leading to structural damage, stiction, and electrical short-circuits during exceptional external shocks.
Innovation Solution
A multi-stage motion limiter is implemented where the second motion limiter is introduced, with the first stopper element contacting the fixed structure at a first threshold and the second stopper element at a second threshold, increasing the spring constant to minimize impact and reduce damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the mass element and suspension structure are dimensioned to prevent direct contact during regular operation, then the device operates reliably under normal conditions, but exceptional external shocks can still cause direct contact leading to structural damage and electrical short-circuits
Solution Approach 1:
The motion limiter is pre-positioned between the mass element and fixed structures before any shock occurs. The flexible element is pre-configured with a specific spring constant to provide immediate resistance upon contact, preventing the mass element from directly impacting fixed structures during exceptional shocks.
Solution Approach 2:
The flexible element acts as a cushioning mechanism that absorbs impact energy before it can reach the fixed structures. By designing the flexible element with appropriate material properties and geometric dimensions, the system prepares in advance to soften any potential impact from external shocks, protecting the mass element and fixed structures from damage.
2Device complexity
If a single-stage motion limiter is used with a fixed spring constant, then the structure is simple, but it cannot effectively handle varying impact forces across different shock intensities
Solution Approach 1:
The motion limiter transitions from a static, single-stage design to a dynamic, multi-stage system. The spring constant is no longer fixed but changes based on the displacement of the mass element. As the mass element moves further toward the fixed structures, the spring constant increases, providing progressively stronger resistance that matches the intensity of the applied shock force.
Solution Approach 2:
The key parameter that changes is the spring constant of the flexible element. By designing the flexible element with a variable cross-sectional area or thickness along its length, the spring constant becomes a function of displacement rather than a fixed value. This allows the motion limiter to adapt its stiffness characteristic to the magnitude of the applied force, improving protection effectiveness across different shock scenarios.
3Object-affected harmful factors
If the gap between the motion limiter and fixed structure is made narrow to ensure early contact, then impact protection is improved, but the risk of contact during regular operation increases
Solution Approach 1:
The motion limiter uses a variable spring constant that compensates for the reduced gap distance. Even though the gap is narrower to ensure early contact during shocks, the flexible element is designed with a low initial spring constant that allows normal operational movements without triggering contact. Only when exceptional forces are applied does the spring constant increase sufficiently to prevent contact with fixed structures.
Solution Approach 2:
The flexible element's spring constant parameter changes as a function of displacement. In the initial region corresponding to normal operation, the spring constant is low enough to permit free movement. As displacement increases toward the fixed structures, the spring constant increases dramatically, creating a progressive resistance that prevents contact even with the reduced gap 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 multi-stage motion limiter reduces the risk of structural damage and electrical faults by absorbing impact forces through flexible elements, providing a softer impact and minimizing contact with the fixed structure.
Implementation Method 1
The motion limiter may comprise a flexible element... configured to resist further movement of the mass element toward the fixed structure with a first spring constant... The first spring constant may be smaller than the second spring constant
Implementation Method 2
The motion limiter may be configured to resist further movement of the mass element toward the fixed structure with a first spring constant when the first threshold has been crossed but the second threshold has not yet been crossed... The first spring constant may be smaller than the second spring constant
Data Source
Figure 1a~1c
Figure 2a~2c
Figure 3a~3c
AI summary
A microelectromechanical device comprising a mobile mass element, a fixed structure which is adjacent to the mass element and a motion limiter comprising a first stopper element and a second stopper element. The motion limiter is configured to resist further movement of the main body of the mass element toward the fixed structure with a first spring constant when a first threshold has been crossed but a second threshold has not yet been crossed. The motion limiter is configured to resist further movement of the main body of the mass element toward the fixed structure with a second spring constant after a second threshold has been crossed.