Accelerometer Proof Mass Thermal Isolation
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Solution Overview
Problem
Accelerometers exhibit bias and thermal model errors due to factors like temperature changes and non-ideal construction conditions, leading to inaccurate acceleration measurements, particularly in aerospace applications where precise navigation and payload delivery are critical.
Innovation Solution
The configuration of a proof mass assembly with raised pads and flexures that isolate the proof mass from thermal strains and construction forces, along with a dimensionally compensated suspension system that matches the coefficient of thermal expansion of materials, helps maintain a stable capacitance gap and reduces hysteresis, thereby improving the accuracy of acceleration measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional accelerometer construction methods are used, then manufacturing is simpler, but thermal model errors and hysteresis increase due to temperature changes and construction forces
Solution Approach 1:
The proof mass assembly is segmented into distinct components: an inner hoop holding the proof mass, an outer hoop, and flexible connections between them. This segmentation allows the inner hoop and proof mass to be isolated from thermal strains and construction forces applied to the outer hoop, reducing thermal model errors and hysteresis while maintaining manufacturing feasibility through modular assembly.
Solution Approach 2:
Flexible hoop connections serve as intermediaries between the inner hoop (holding the proof mass) and the outer hoop (subject to thermal and construction forces). These flexures transmit necessary mechanical support while isolating the proof mass from harmful thermal strains and construction forces, thereby improving measurement accuracy without requiring complete redesign of the support structure.
2Device complexity
If the proof mass is directly connected to the support structure, then the structure is simpler, but hysteresis increases due to transmission of thermal strains and construction forces
Solution Approach 1:
Flexible hoop connections and center pad flexures are used instead of rigid direct connections. These flexible elements allow the support structure to remain simple while providing thermal isolation and force decoupling. The flexures transmit mechanical support necessary for structural integrity while blocking the transmission of harmful thermal strains and construction forces to the proof mass, improving measurement stability.
3Reliability
If materials with different thermal expansion coefficients are used in the support structure, then thermal isolation is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent applies parameter changes by selecting specific materials with appropriate thermal expansion coefficients for different components. The inner hoop and proof mass use materials with low thermal expansion (e.g., fused silica), while the outer hoop and support structure may use materials with higher thermal expansion (e.g., Invar). This parameter optimization improves thermal isolation effectiveness while keeping manufacturing precision requirements within practical limits through careful material selection rather than extreme dimensional control.
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 configuration reduces thermal model errors and hysteresis, enhancing the stability and accuracy of accelerometer readings over time and temperature changes, leading to more precise navigation and control in applications like aircraft and satellite systems.
Implementation Method 1
An acceleration or force applied along the sensitive axis of the accelerometer causes the proof mass to deflect either upwardly or downwardly causing the distance (e.g., a capacitive gap) between the pick-off capacitance plates and upper and lower non-moving members to vary. This variance in the capacitive gap causes a change in the capacitance of the capacitive elements, which is representative of the displacement of the proof mass along the sensitive axis.
Implementation Method 2
The change in the capacitance may be used as a displacement signal, which may be applied to a servo system that includes one or more electromagnets (e.g., a force-rebalancing coil) to return the proof mass to a null or at-rest position.
Implementation Method 3
one or more hoop flexures, wherein the one or more hoop flexures flexibly connect the inner hoop to the outer hoop; a center pad flexure comprising a center raised pad at a distal end of the center pad flexure, wherein the center pad flexure flexibly connects the center raised pad to the inner hoop; and two or more proof mass flexures, wherein the two or more proof mass flexures flexibly connect the proof mass to the inner hoop
Data Source
AI summary
Accelerometers as disclosed herein include a proof mass assembly and an accelerometer support. In some examples, a combined height and a combined coefficient of thermal expansion (CTE) of the materials of the accelerometer support is configured to substantially match a CTE of material of the non-moving member with a height substantially similar to the combined height of the accelerometer support. In some examples, the accelerometer support is configured to connect to a center raised pad of the proof mass assembly and maintain a capacitance gap between a capacitance plate on a proof mass of the proof mass assembly and a portion of the non-moving member.