MEMS Piston-Tube Capacitive Force Sensor Design
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Solution Overview
Problem
Conventional MEMS capacitive force sensors face limitations in detecting a wide range of forces, from micro-Newtons to milli-Newtons, as their sensitivity is compromised when the force range increases, and they are prone to damage from excessive input forces.
Innovation Solution
The MEMS piston-tube capacitive force sensor employs a high-sensitivity electrode configuration with a large stroke and stiff flexure mechanism, allowing detection of small and large forces, and incorporates a mechanical stopper to prevent damage from over-limit forces, using a piston-tube electrode design with a capacitive read-out circuit to measure force changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional MEMS capacitive force sensors are used, then small forces in micro-Newton range can be detected, but sensitivity is compromised when force range increases to milli-Newton range
Solution Approach 1:
The sensor structure is segmented into multiple functional components: a fixed electrode, a movable electrode coupled to a cantilever beam, and a proof mass. This segmentation allows each component to be optimized for its specific function while working together to achieve both high sensitivity and wide force range detection
Solution Approach 2:
The sensor utilizes parameter changes in the capacitive coupling between electrodes as the movable electrode displaces in response to applied force. The cantilever beam's elastic deformation changes the capacitance parameter, enabling detection of forces across a wide range while maintaining sensitivity through proper design of the beam's mechanical properties
2Measurement precision
If conventional MEMS capacitive force sensors are used, then accurate measurements at micro-Newton range are achieved, but the sensors are prone to damage from excessive input forces
Solution Approach 1:
The sensor incorporates a mechanical stop or limit structure that prevents the cantilever beam from displacing beyond a safe threshold. This beforehand protection mechanism absorbs or redirects excessive force before it can damage the sensitive capacitive sensing elements, ensuring reliability while maintaining measurement accuracy within the operational range
3Device complexity
If piezoresistive force sensors are used, then simple design and readout circuit are achieved, but the sensors are sensitive to temperature changes
Solution Approach 1:
The patent replaces the piezoresistive mechanical sensing mechanism with a capacitive sensing mechanism. Instead of measuring resistance changes in a piezoresistive material, the sensor measures capacitance changes between electrodes as the movable electrode displaces. Capacitive sensing is inherently less sensitive to temperature variations than piezoresistive sensing, while the overall device design remains relatively simple
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 sensor effectively measures a wide range of forces from micro-Newtons to milli-Newtons with high sensitivity and durability, suitable for applications like touch screens and handheld devices, providing accurate force detection and preventing short circuits from excessive forces.
Implementation Method 1
any small displacements of the rotor corresponding to an applied mechanical force/pressure induce measurable changes in the capacitance of the structure which can be detected by the sensing readout circuit
Implementation Method 2
MEMS electrostatic piston-tube (tooth-aperture) electrode configuration
Data Source
AI summary
A highly sensitive MEMS capacitive force sensor offering a wide force measurement range is disclosed. The force sensor utilizes a piston-tube electrode configuration that enables the use of a wide area of the electrodes. Therefore, a high sensitivity for force detection is achieved. The force sensor consists of a lower structure, an upper structure, and a force contact platform. The lower structure contains a plurality of fixed electrodes that are attached to the base and have a plurality of teeth (pistons). Those teeth form the fixed sensing electrodes of the force sensor. The upper structure comprises of a moving section that has a plurality of apertures. The moving section is attached to the substrate via restoring mechanical springs. A force contact platform is attached to the moving electrode and provides the foundation for the force to be applied. The force sensor is able to measure concentrated and distributed force (mechanical pressure).