MEMS Sensor Contact Sensing for Electrostatic Stiction Control
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Solution Overview
Problem
MEMS sensors, particularly accelerometers, suffer from stiction phenomena due to adhesion between the movable mass and stoppers, leading to malfunction and reduced reliability, with existing solutions either increasing noise and cost or being ineffective.
Innovation Solution
A MEMS sensor design incorporating a contact sensing structure and control circuitry that senses contact between the movable mass and a deformable bumper portion, adjusting the electrostatic force to prevent stiction by reducing the voltage difference and enhancing elastic return forces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the coupling elastic elements are designed to have high stiffness, then the risk of stiction phenomenon is decreased, but sensing noise increases and sensor stability decreases
Solution Approach 1:
A deformable bumper portion is introduced as an intermediary element between the movable mass and the stopper. This bumper deforms upon contact, preventing direct rigid contact and reducing adhesion forces while maintaining the mechanical return path through the coupling elastic elements, thus avoiding the need for high stiffness that would increase noise
2Reliability
If the stoppers are provided with an anti-stiction layer, then the adhesion force is reduced, but manufacturing complexity and cost increase
Solution Approach 1:
The deformable bumper portion acts as a sacrificial, replaceable element that absorbs the wear and adhesion effects through its deformability. Instead of requiring complex anti-stiction coatings on permanent components, the simple deformable bumper can be designed with basic materials that deform elastically, reducing manufacturing complexity while maintaining anti-stiction functionality
3Reliability
If dedicated electrodes are configured to exert electrostatic force on the movable mass, then the movable mass can be detached from stoppers, but the movable mass may bump with further stoppers causing further stiction
Solution Approach 1:
The deformable bumper portion provides beforehand cushioning by being positioned between the movable mass and the stopper. When the movable mass moves toward the stopper, the bumper deforms first, absorbing the impact and preventing direct contact. This cushioning effect works preemptively to avoid stiction before it occurs, eliminating the need for electrostatic detachment that could cause further bouncing
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 design achieves high robustness against stiction, maintaining sensor reliability by minimizing adhesion forces and ensuring the movable mass returns to its equilibrium position, thus enhancing sensor stability and reducing sensing noise.
Implementation Method 1
The movable mass is coupled to the substrate through coupling elastic elements
Implementation Method 2
In response to the contact between the movable mass and the stoppers, an attractive adhesion force may establish between the movable mass and the stoppers
Implementation Method 3
the sensor may have dedicated electrodes configured to exert an electrostatic force on the movable mass, in the opposite direction to that of the adhesion force
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
A MEMS sensor (1) is configured to measure a physical quantity and has a substrate (10) and a movable mass (12) suspended at a distance from the substrate along a direction (Z), wherein the movable mass is coupled to the substrate so as to undergo a movement (M) along a sensing direction (S), with respect to the substrate, as a function of the physical quantity to be measured. The MEMS sensor also has a contact sensing structure (30) coupled to the substrate and which extends, at rest, at a distance (gsw) from the movable mass along the sensing direction; and at least one stator electrode (18A, 18B) coupled to the substrate and configured to form with the movable mass at least one capacitor having a capacitance variable as a function of the movement of the movable mass. A control circuit (5) is configured to: induce a voltage difference between the movable mass and the stator electrode, for sensing a capacitance variation between the movable mass and the stator electrode; sense a contact between movable mass and contact sensing structure; and in response to sensing the contact between movable mass and contact sensing structure, modify the induced voltage difference, so as to reduce an electrostatic force exerted by the at least one stator electrode on the movable mass.