MEMS Accelerometer with Variable Geometry for Signal Discrimination
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Solution Overview
Problem
Existing accelerometric sensors in MEMS technology face challenges in distinguishing external acceleration signals from spurious signals caused by temperature variations, humidity, structural deformations, and aging, as these phenomena induce low-frequency or DC capacitive variations that cannot be electronically differentiated from the useful signal.
Innovation Solution
The sensor employs a geometrically variable configuration by dividing the suspended region into a mobile and a translatable part, allowing modulation of the sensing arm to differentiate the signal due to external accelerations from stress-induced signals, enabling frequency modulation of the useful signal to distinguish it from DC signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a standard MEMS accelerometer structure with fixed suspended mass is used, then the device is simple to manufacture and operate, but it cannot distinguish external acceleration signals from stress-induced spurious signals
Solution Approach 1:
The suspended mass structure is made dynamically variable through a modulation mechanism that periodically changes the position of a movable portion relative to the fixed portion. This dynamic modulation allows the sensing arm length to vary with time, enabling frequency modulation of the acceleration signal to distinguish it from DC stress-induced signals.
Solution Approach 2:
The suspended mass is divided into a fixed portion and a movable portion that can be independently positioned. This segmentation allows the movable portion to be modulated separately from the fixed portion, creating a variable sensing arm configuration that enables signal differentiation without requiring complete structural redesign.
2Reliability
If the suspended mass structure is made variable to modulate the sensing arm, then external acceleration signals can be differentiated from stress signals, but the device complexity increases
Solution Approach 1:
A periodic modulation signal is applied to the movable portion of the suspended mass, causing it to oscillate at a specific modulation frequency. This periodic action creates a frequency-modulated acceleration signal that can be easily distinguished from DC stress-induced signals through frequency filtering, improving reliability while keeping the modulation mechanism simple.
Solution Approach 2:
An intermediary movable portion is introduced between the fixed suspended mass and the proof mass. This intermediary element acts as a mediator that can be independently modulated without affecting the overall structural integrity, allowing complex modulation functionality to be achieved through a relatively simple additional component.
3Measurement precision
If frequency modulation is applied to the acceleration signal, then stress-induced DC signals are eliminated, but the sensor structure becomes more complex
Solution Approach 1:
The sensing arm length is made dynamically variable by modulating the position of the movable portion during operation. This dynamic adjustment enables frequency modulation of the acceleration signal, allowing precise separation of acceleration signals from DC stress-induced signals through frequency-domain filtering.
Solution Approach 2:
The movable portion is pre-positioned at a defined initial position relative to the fixed portion, establishing a baseline sensing arm length. This preliminary positioning allows subsequent modulation to be applied systematically, simplifying the control mechanism while achieving the desired frequency modulation effect for signal separation.
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 eliminates the effects of stress-induced signals, enhancing the accuracy of acceleration detection by allowing the separation of external acceleration signals from spurious stress-related signals, thereby improving the sensor's precision and stability.
Implementation Method 1
The sensor 1 comprises a suspended region 2, typically of monocrystalline or polycrystalline silicon, eccentrically anchored to a fixed region 3 via an anchorage 13 and elastic suspension elements, also referred to as 'springs 14'
Implementation Method 2
Mobile electrodes 7 are formed by, or fixed with respect to, the suspended region 2 and face fixed electrodes 8, in turn fixed to, or formed by, the fixed region 3. Each electrode pair 10a-10d defines a capacitive element, the capacitance C whereof depends upon the distance between the respective fixed and mobile electrodes 8, 7
Data Source
AI summary
The accelerometric sensor has a suspended region, mobile with respect to a supporting structure, and a sensing assembly coupled to the suspended region and configured to detect a movement of the suspended region with respect to the supporting structure. The suspended region has a geometry variable between at least two configurations associated with respective centroids, different from each other. The suspended region is formed by a first region rotatably anchored to the supporting structure and by a second region coupled to the first region through elastic connection elements configured to allow a relative movement of the second region with respect to the first region. A driving assembly is coupled to the second region so as to control the relative movement of the latter with respect to the first region.