Capacitive Transducer Linearity via On-Chip Neutralization
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Solution Overview
Problem
Capacitive accelerometers in harsh environments suffer from non-linearity due to feed-through capacitance and core mismatch, which affects their accuracy and linearity, particularly in self-balanced designs where feed-through capacitance is the dominant source of non-linearity.
Innovation Solution
An automatic calibration method is introduced to neutralize feed-through capacitance by applying electrostatic forces and measuring output changes to determine the optimal neutralization capacitance, using a binary search algorithm to adjust the neutralization capacitance until the desired linearity threshold is met, thereby canceling unwanted parasitic capacitances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If feed-through capacitance is present in capacitive transducers, then the transducer can be manufactured with standard processes, but non-linearity occurs in the output signal
Solution Approach 1:
The patent introduces neutralization capacitors as intermediary elements that are coupled between the sense electrodes and the proof mass. These neutralization capacitors act as mediators to cancel the unwanted feed-through capacitance effects. By adding these intermediate capacitive elements, the harmful feed-through path is compensated, allowing the transducer to maintain both manufacturability and high linearity performance.
Solution Approach 2:
The patent employs binary search algorithm to automatically determine the optimal neutralization capacitance values. This involves systematically varying the capacitance parameters during calibration to find the precise values that maximize linearity. The parameter change approach allows the system to adapt to process variations and achieve optimal performance without requiring manual adjustment.
2Manufacturing precision
If manual calibration methods are used to adjust neutralization capacitance, then linearity can be improved, but the calibration process becomes time-consuming and complex
Solution Approach 1:
The patent implements an automatic calibration system that performs the neutralization capacitance adjustment without requiring external manual intervention. The system uses an on-chip binary search algorithm that automatically measures the output signal, determines the optimal capacitance values, and configures the neutralization capacitors. This self-calibrating approach eliminates time-consuming manual procedures while maintaining high linearity performance.
Solution Approach 2:
The automatic calibration method incorporates feedback mechanisms where the system continuously monitors the output signal linearity and uses this information to iteratively adjust the neutralization capacitance values. The binary search algorithm uses the measured output as feedback to converge on the optimal capacitance settings, enabling fast and accurate calibration without manual intervention.
3Manufacturing precision
If on-chip neutralization capacitors are added to cancel feed-through capacitance, then linearity is improved, but device complexity increases
Solution Approach 1:
The patent integrates the neutralization capacitors directly into the capacitive transducer structure, merging them with the existing sense electrodes and proof mass. Rather than adding separate external calibration circuits, the neutralization capacitors are combined with the sensor elements themselves, sharing common structures and reducing overall device complexity despite the added functionality.
Solution Approach 2:
The neutralization capacitors serve multiple functions: they cancel feed-through capacitance effects, enable automatic calibration through binary search, and can be used for both sensing and calibration operations. This multi-functionality reduces the need for separate dedicated calibration circuits, thereby limiting the increase in device complexity while achieving improved linearity.
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 method effectively reduces non-linearity caused by feed-through capacitance, improving the linearity and accuracy of capacitive transducers and accelerometers by precisely determining the necessary neutralization capacitance, ensuring a linear output voltage response to displacement.
Implementation Method 1
applying an electrostatic actuation force to a proof mass of the capacitive transducer where the magnitude of the electrostatic actuation force is proportional to a duty cycle of a linear actuation signal
Implementation Method 2
capacitive transducers produce a change of capacitance, corresponding to the magnitude of the measured input signal
Data Source
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AI summary
A system and method are disclosed for automatically calibrating capacitive transducers to neutralize feed-through capacitance using linear actuation. The method includes starting with an initial neutralization capacitance, applying no electrostatic force and two known electrostatic forces to a proof mass of the transducer, recording the transducer output changes due to the applied forces; and determining how to revise neutralization capacitance based on the changes. The method can use a binary search to find a final neutralization capacitance providing the best linearity. The method can include comparing the final linearity to a threshold linearity. The electrostatic forces can be applied using a charge control method where the electrostatic force is a linear function of the actuation duration. The linear actuation can be used for continuous self-test of capacitive sensors.