Athermal Hung Mass Accelerometer Thermal Gradient Compensation
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Solution Overview
Problem
The original open-loop hung mass accelerometer requires tight thermal control to achieve desired bias stability and scale factor error performance, as temperature gradients along the longitudinal axis can produce false acceleration readings.
Innovation Solution
The athermal open-loop hung mass accelerometer is designed with sensor heads having effective coefficients of thermal expansion (CTEs) that are equal and opposite to the body's expansion, or different to offset body growth, ensuring that the effects of temperature gradients are minimized, and the proof mass is thermally isolated to maintain constant temperature.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If tight thermal control is implemented to achieve desired bias stability and scale factor error performance, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent applies thermal expansion compensation by designing the sensor head with a specific coefficient of thermal expansion (CTE) that counteracts the body's thermal expansion. The sensor head CTE is selected to be equal and opposite to the body's CTE, causing the sensor head to expand/contract in opposition to the body's thermal growth, thereby nulling the net displacement effect of temperature gradients and achieving athermal operation without active thermal control
Solution Approach 2:
The patent changes the material parameter (CTE) of the sensor head to achieve thermal compensation. By selecting a material with a specific CTE value that is equal and opposite to the body's CTE, the system transforms the thermal expansion problem into a beneficial compensatory mechanism, allowing the accelerometer to operate without tight thermal control while maintaining high measurement precision
2Stability of the object's composition
If sensor heads are designed with CTEs equal and opposite to the body's expansion, then bias stability is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent utilizes thermal expansion compensation by selecting sensor head materials with CTEs that are equal and opposite to the body's CTE. This design causes the sensor head to expand and contract in opposition to the body's thermal growth, nulling the net displacement effect of temperature gradients and achieving athermal operation that improves bias stability
3Device complexity
If the accelerometer operates without tight thermal control, then device complexity is reduced, but temperature gradients produce false acceleration readings
Solution Approach 1:
The patent applies thermal expansion compensation by designing the sensor head with a specific coefficient of thermal expansion (CTE) that counteracts the body's thermal expansion. The sensor head CTE is selected to be equal and opposite to the body's CTE, causing the sensor head to expand/contract in opposition to the body's thermal growth, thereby nulling the net displacement effect of temperature gradients and achieving athermal operation without active thermal 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 design achieves bias stability and scale factor error performance without the need for tight thermal control, with bias less than 10 micro-g's and scale factor error less than 10 ppm, significantly reducing false acceleration readings.
Implementation Method 1
differential Eddy current sensing
Implementation Method 2
top and bottom flexures and a proof mass suspended between the flexures inside an internal cavity to deflect along a longitudinal axis
Implementation Method 3
the sensor heads (formed of one or more materials) exhibit effective coefficients of thermal expansion (CTEs) such that the growth of the sensor heads along the longitudinal axis is approximately equal and opposite the growth of the body along the longitudinal axis in response to a predicted body temperature gradient
Data Source
Figure 1
Figure 2a
Figure 2b
AI summary
An athermal open-loop hung mass accelerometer configures the CTE of the sensor heads such that any growth by the body in response to a body temperature gradient along the longitudinal axis is offset by the growth of the sensor heads in the equal and opposite direction to null the effects of the temperature gradient. In many configurations, the sensor head CTE is strictly less than the body CTE and typically between 60-80% of the body CTE to null the effects of the predicted body temperature gradient.