MEMS Accelerometer Stiction Release via Dual Mass Dynamics
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Solution Overview
Problem
MEMS accelerometers face challenges in overcoming stiction forces, which can prevent the inertial mass from returning to its rest position and accurately detecting further accelerations.
Innovation Solution
The design incorporates a second inertial mass coupled with a stiffer spring element, which creates a dynamic system that helps detach the first inertial mass from stop elements by exerting an elastic thrust during the return motion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a stop element is used to limit the displacement of the inertial mass, then the measurement range is improved, but stiction forces prevent the inertial mass from returning to its rest position
Solution Approach 1:
The patent introduces a second inertial mass that can move dynamically to generate a release impulse. This dynamic element interacts with the first inertial mass to provide the necessary force to overcome stiction and restore the system to its initial state after measurement, resolving the contradiction between maintaining a stopped position for measurement and enabling return for repeated measurements.
Solution Approach 2:
The second inertial mass acts as an intermediary that transfers energy to the first inertial mass. Through their interaction, the second mass provides the impulse needed to detach the first mass from the stop element, mediating between the measurement function and the reset function.
2Reliability
If a compliant element is used to couple the inertial mass to the constraint element, then the ability to return to rest position is improved, but the spring constant must be increased to overcome stiction forces
Solution Approach 1:
The patent combines two inertial masses into a single system where they interact through their relative motion. The second inertial mass is coupled to the constraint element and interacts with the first inertial mass to provide the necessary force to overcome stiction, merging the functions of displacement limitation and reset capability into one integrated system.
Solution Approach 2:
The second inertial mass acts as a counterweight that provides the opposing force needed to overcome the stiction holding the first inertial mass against the stop element. By utilizing the inertial properties of the second mass, the system generates the necessary release impulse without requiring an excessively stiff spring.
3Measurement precision
If the inertial mass is stopped against a stop element, then the full-scale acceleration measurement is improved, but stiction forces increase and prevent accurate detection of further accelerations
Solution Approach 1:
The system employs periodic action through the interaction between the two inertial masses. The second mass periodically imparts impulses to the first mass, creating a rhythm of stopping against the limit stop and subsequent release. This periodic cycle allows the system to achieve full-scale measurement while periodically eliminating stiction effects.
Solution Approach 2:
The patent converts the harmful stiction effect into a beneficial mechanism. By designing the system so that the second inertial mass interacts with the first mass at the moment of contact with the stop element, the stiction event triggers the release mechanism. The harmful adhesion force becomes the activation point for the corrective impulse that restores measurement accuracy.
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 effectively overcomes stiction forces, allowing the MEMS accelerometer to return to its initial position and maintain sensitivity, resonance frequency, linearity, and accuracy similar to traditional designs.
Implementation Method 1
a spring element (7) configured to allow a displacement, here a translation, of the inertial mass (3) along the sensing axis (S) in response to an external acceleration (aext)
Implementation Method 2
The first and second surfaces 3A, 3B and the respective first and second electrodes 13, 15 are capacitively coupled to each other and form the plates of corresponding capacitors, having capacitances C1, C2
Data Source
AI summary
An inertial structure is elastically coupled through a first elastic structure to a supporting structure so as to move along a sensing axis as a function of a quantity to be detected. The inertial structure includes first and second inertial masses which are elastically coupled together by a second elastic structure to enable movement of the second inertial mass along the sensing axis. The first elastic structure has a lower elastic constant than the second elastic structure so that, in presence of the quantity to be detected, the inertial structure moves in a sensing direction until the first inertial mass stops against a stop structure and the second elastic mass can move further in the sensing direction. Once the quantity to be detected ends, the second inertial mass moves in a direction opposite to the sensing direction and detaches the first inertial mass from the stop structure.