MEMS Vibrating Beam Accelerometer with Pressure-Damped Proof Mass
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Solution Overview
Problem
Vibrating beam accelerometers face challenges in achieving underdamped resonators while effectively damping the proof mass, leading to increased noise and reduced bias stability due to parasitic feedthrough capacitance and environmental vibrations.
Innovation Solution
The implementation of damping combs with movable and anchored comb fingers provides air damping for the proof mass, while the resonator geometry is configured to avoid air damping, and capacitive comb fingers are designed into discreet electrodes with equal parasitic feedthrough capacitances to cancel out feedthrough currents using differential amplification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If air damping is applied to the proof mass to reduce environmental vibration effects, then proof mass stability is improved, but resonator quality factor decreases leading to increased noise
Solution Approach 1:
The device is segmented into two distinct functional regions: a first cavity for the resonator and a second cavity for the proof mass. This physical segmentation allows independent optimization of each component's environmental conditions, enabling the proof mass to experience pressure damping while the resonator maintains vacuum conditions for high Q-factor
Solution Approach 2:
A vacuum cavity serves as an intermediary environment for the resonator, isolating it from atmospheric pressure effects that would otherwise dampen its oscillation. This vacuum barrier allows the resonator to maintain high quality factor while the proof mass in the atmospheric cavity experiences beneficial pressure damping
2Device complexity
If parasitic feedthrough capacitance is present in the accelerometer structure, then device complexity is reduced, but noise performance deteriorates and bias stability decreases
Solution Approach 1:
The harmful parasitic feedthrough capacitance is extracted and isolated into a separate measurement channel. By measuring the feedthrough capacitance independently and subtracting it from the total capacitance signal, the useful acceleration measurement is separated from the parasitic effect, improving noise performance and bias stability
Solution Approach 2:
The system uses feedback by measuring the parasitic feedthrough capacitance and using this information to compensate for its effects in the final measurement. The feedthrough capacitance measurement is subtracted from the total capacitance change to isolate the true acceleration signal
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 enables navigation-grade accelerometers with reduced cost and size, weight, and power (SWaP) while maintaining bias repeatability and improving noise performance by achieving underdamped resonators and critically damped proof masses.
Implementation Method 1
The movable comb fingers may be interdigitated with anchored comb fingers that are attached to fixed geometry. These damping comb fingers may provide air damping for the proof mass when the MEMS die is placed into a pressure cavity of a package containing a pressure above a vacuum.
Implementation Method 2
Accelerometers function by detecting a displacement of a proof mass under inertial forces. A resonator may be designed to change frequency proportional to the load applied to the resonator by the proof mass under acceleration.
Implementation Method 3
The resonator may be electrically coupled to signal generation circuitry forming an oscillator, which causes the resonator to vibrate, and in some examples at the resonant frequency of the resonator.
Data Source
AI summary
The disclosure describes techniques to damp the proof mass motion of an accelerometer while achieving an underdamped resonator. In an example of an in-plane micro-electromechanical systems (MEMS) VBA, the proof mass may contain one or more damping combs that include one or more banks of rotor comb fingers attached to the proof mass. The rotor comb fingers may be interdigitated with stator comb fingers that are attached to fixed geometry. These damping comb fingers may provide air damping for the proof mass when the MEMS die is placed into a package containing a pressure above a vacuum. The geometry of the damping combs with a reduced air gap and large overlap area between the rotor comb fingers and stator comb fingers. The geometry of resonator of the VBA of this disclosure may be configured to avoid air damping.