Capacitance Measurement Circuit for MEMS Actuator Positioning
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Solution Overview
Problem
MicroElectroMechanical System (MEMS) actuators in translational systems, such as those used in digital cameras and mobile phones, exhibit non-linear relationships between current/voltage and displacement, leading to inaccurate positioning due to manufacturing variations and environmental factors, making direct position measurement challenging.
Innovation Solution
A measurement circuit that includes a current source, a reference capacitor, a current mirror, and an analog-to-digital converter (ADC) to derive the capacitance of a capacitive device by driving a test signal and measuring the resulting voltage, allowing for the calculation of the device's unknown capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If open-loop control is used to drive MEMS actuator, then device complexity is reduced, but positioning accuracy deteriorates due to non-linear relationships and manufacturing variations
Solution Approach 1:
The patent implements closed-loop control by measuring the actual capacitance of the MEMS actuator and comparing it with expected capacitance values to generate feedback signals. This feedback mechanism corrects positioning errors caused by non-linear relationships and manufacturing variations, resolving the contradiction between simplified control and positioning accuracy.
Solution Approach 2:
The patent replaces direct mechanical position measurement with electrical capacitance measurement. By measuring the capacitance of the MEMS actuator, which varies with position, the system indirectly determines position without complex mechanical sensors, maintaining simplicity while improving accuracy.
2Measurement precision
If closed-loop control is implemented to improve positioning accuracy, then measurement precision is improved, but device complexity increases due to feedback requirements
Solution Approach 1:
The patent makes the MEMS actuator serve dual functions: both actuation and sensing. The same capacitive structure used for positioning also provides the sensing mechanism through capacitance measurement, eliminating the need for separate position sensors and reducing overall system complexity while maintaining high positioning accuracy.
Solution Approach 2:
The MEMS actuator measures its own position through its inherent capacitance property. The actuator's electrical characteristic (capacitance) directly reflects its mechanical state (position), allowing it to self-report position information without external sensing components, thereby improving accuracy without proportionally increasing complexity.
3Measurement precision
If direct position measurement of MEMS actuator is attempted, then measurement precision is improved, but device complexity increases due to additional sensing components
Solution Approach 1:
The patent replaces direct mechanical position sensing with electrical capacitance measurement. Instead of using mechanical position sensors that would add complexity, the system measures the electrical capacitance of the MEMS actuator, which naturally varies with position, achieving accurate position measurement without additional mechanical sensing components.
Solution Approach 2:
The patent uses capacitance as an intermediary parameter to indirectly measure position. Rather than measuring position directly, the system measures capacitance (which is easier to measure accurately) and converts it to position information, achieving high measurement precision without complex direct position sensing.
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
Enables accurate measurement of MEMS actuator position, improving positioning accuracy by accounting for manufacturing and environmental variations, thereby enhancing the performance of translational systems.
Implementation Method 1
mirroring a current developed in the reference capacitor to a second circuit system that includes a measurement impedance
Implementation Method 2
measuring an unknown capacitance of a device
Data Source
AI summary
Embodiments of the present invention may provide a method of measuring an unknown capacitance of a device. The method may comprise the steps of driving a test signal to a circuit system that includes a current divider formed by the device with unknown capacitance and a reference capacitor; mirroring a current developed in the reference capacitor to a second circuit system that includes a measurement impedance; measuring a voltage within the second circuit system; and deriving a capacitance of the unknown capacitance based on the measured voltage with reference to a capacitance of the reference capacitor and the measurement impedance.