Field Mill Control Circuit Using Subharmonic Drive to Cut Parasitic Coupling
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Solution Overview
Problem
Existing MEMS-based field mills face issues with parasitic coupling between the shutter's control signal and sensor electrodes, leading to inaccurate measurements due to overcoupling, which cannot be effectively eliminated.
Innovation Solution
A circuit and method that uses an alternating voltage component with a frequency corresponding to an even fraction of the shutter's resonant frequency to drive the actuator, reducing parasitic coupling and enabling precise determination of electric field strength by separating the measurement signal frequency from the drive voltage frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the shutter is oscillated at its resonant frequency, then the oscillation efficiency is improved, but parasitic coupling between the control signal and sensor electrodes increases causing measurement inaccuracy
Solution Approach 1:
The patent segments the frequency domain by using different frequencies for drive and measurement signals. The shutter is driven at a frequency that is an even fraction (e.g., half) of the resonant frequency, while the measurement signal operates at the full resonant frequency. This frequency segmentation prevents overlap between drive signal harmonics and measurement signal, eliminating parasitic coupling effects from the measurement.
2Reliability
If the excitation frequency is set to the resonant frequency, then the measurement signal strength is improved, but overcoupling cannot be eliminated
Solution Approach 1:
The patent introduces an intermediary frequency relationship between the drive signal and measurement signal. By driving the shutter at an even fraction of the resonant frequency, the drive signal acts as an intermediary that does not directly interfere with the measurement signal at the full resonant frequency. This intermediary frequency approach allows both strong measurement signals and elimination of overcoupling to coexist.
3Ease of operation
If a feedback loop is implemented, then the oscillation control is improved, but the complexity of the circuit increases
Solution Approach 1:
The patent implements a feedback loop where the measurement signal from the sensor electrodes is fed back to the actuator that drives the shutter. This feedback mechanism automatically maintains the shutter oscillation at the correct amplitude and frequency, improving ease of operation. The feedback loop leverages the natural resonance of the shutter system, reducing the need for complex external control circuitry.
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 approach reduces overcoupling, allowing for more accurate measurement of electric field strength by using electronic filters to filter out noise and crosstalk, thereby improving measurement precision.
Implementation Method 1
an electrostatic actuator for setting a shutter of the field measuring device into oscillation
Implementation Method 2
internal parasitic coupling capacitances, which cause overcoupling between the shutter's control signal and the sensor electrodes
Implementation Method 3
the sensor electrodes can be alternately discharged and charged by induction of the electric field to be measured
Implementation Method 4
the excitation and the measurement signal oscillate electrically at the same frequency
Data Source
Figure 1
Figure 2
Figure 3a~3b
AI summary
The invention relates to a circuit (10) for controlling a field measuring device, in particular a micromechanical field mill, wherein the circuit comprises an electrostatic actuator (12) for setting a shutter (14) of the field measuring device into oscillation and a sensor (16) for detecting the oscillation of the shutter (14), wherein a signal output (18) of the sensor (16) is coupled to a signal input (20) of the actuator (12) for feedback, and wherein the circuit (10) is designed such that a control voltage (uDRIVE) that can be applied to the actuator (12) via the signal input (20) comprises at least one alternating voltage component (uAC·sin(ωt)) with a frequency (ω) that corresponds to an even fraction of a resonant frequency (ω0) of the oscillation of the shutter (14). Furthermore, the invention relates to a method for controlling a field measuring device with the above circuit (10), as well as a field measuring device.