Accelerometer Offset Compensation via Stress Capacitor Segmentation
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Solution Overview
Problem
Accelerometers face errors due to thermally induced and mechanically applied stresses, which cause substrate bending and affect the accuracy of acceleration measurements.
Innovation Solution
An accelerometer design that incorporates a variable stress capacitor integrated with the acceleration capacitor to measure and mitigate stress-induced errors, using a movable mass suspended above a substrate with a mass anchor and stress fingers to detect relative displacements, and a spring mechanism to minimize noise in the output signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the die is bonded to the package interior with adhesive or solder, then the die is securely attached to the package, but thermal expansion differences cause mechanical stress that bends the substrate and causes measurement errors
Solution Approach 1:
The invention segments the measurement function by introducing a separate stress sensing capacitor that independently measures substrate deformation. This allows the bonding structure to remain intact for mechanical strength while the segmentation of measurement functions enables separate compensation of stress effects from actual acceleration signals
Solution Approach 2:
The stress sensing capacitor acts as an intermediary element that indirectly measures the substrate bending caused by thermal stress. By measuring the relative displacement between the mass anchor and substrate through this intermediary capacitor, the system can detect and compensate for stress-induced errors without changing the bonding structure
2Temperature
If temperature changes occur, then thermal energy is transferred through the system, but different coefficients of thermal expansion cause package stress on the die substrate
Solution Approach 1:
The invention changes the parameter being measured from raw acceleration to stress-compensated acceleration. By continuously monitoring the capacitance change in the stress sensing capacitor and using this information to compensate the output signal, the system maintains measurement accuracy across a wide temperature range despite thermal stress variations
Solution Approach 2:
The stress sensing capacitor provides real-time feedback about substrate deformation due to thermal stress. This feedback signal is used to compensate the acceleration measurement in real-time, allowing the system to maintain precision despite temperature-induced stress changes
3Force
If mechanical or torsional stress is applied to the package, then the package structure experiences load, but this stress translates to the die causing substrate bending and measurement errors
Solution Approach 1:
The invention segments the measurement capability into two independent capacitors: one for acceleration and one for stress. This segmentation allows the system to separately measure and compensate for mechanical stress effects, maintaining measurement precision even when the package experiences external mechanical loads
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 solution produces a noise-mitigated output acceleration signal that is less affected by die stress, resulting in a more stable and accurate measurement of acceleration, reducing errors caused by substrate distortion.
Implementation Method 1
a variable stress capacitor, which also includes the stress finger, for determining movement of the mass anchor relative to the acceleration capacitor stationary finger or substrate
Implementation Method 2
the accelerometer also may have a spring coupled between the anchor and the main portion of the movable mass
Implementation Method 3
a movable mass suspended above a substrate, and a variable acceleration capacitor supported by the substrate
Data Source
AI summary
An accelerometer has a movable mass suspended above a substrate, and a variable acceleration capacitor supported by the substrate. The movable mass has a mass anchor securing the mass to the substrate, while the acceleration capacitor has both a stationary finger extending from the substrate, and a movable finger extending from the movable mass. The accelerometer also has a variable stress capacitor, which also includes the stress finger, for determining movement of the mass anchor relative to the substrate.