MEMS Accelerometer Noise Rejection for Flat Frequency Response

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Solution Overview

Problem

MEMS sensors, such as accelerometers, face challenges in achieving low thermomechanical noise while maintaining a flat frequency response, as increasing the quality factor to reduce thermomechanical noise leads to increased electromechanical noise due to electrostatic damping.

Innovation Solution

The implementation of a sensor apparatus with a resonator, transducer, damping resistor, and noise rejection stage, where the damping resistor electrostatically actuates the transducer to convert thermomechanical noise into electromechanical noise, which is then filtered and reduced using a Sallen-Key topology and analog front end to achieve low noise output.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If the quality factor Q of the mechanical system is increased to reduce thermomechanical noise, then thermomechanical noise is reduced, but electromechanical noise increases due to electrostatic damping

Engineering Contradiction:

Improvethermomechanical noiseVSAvoidelectromechanical noise

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

The patent replaces mechanical damping with electrostatic damping by introducing a damping resistor in the electrical circuit. This substitution allows the mechanical system to maintain high Q for low thermomechanical noise while the electrical damping provides the necessary frequency response control without adding significant electromechanical noise

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent changes the damping parameter from mechanical to electrical domain by using a damping resistor with specific resistance value. This parameter change enables independent optimization of mechanical Q and electrical damping characteristics, resolving the noise trade-off

Inventive Principle:

Principle #35Parameter changes

2Stability of the object's composition

If electrostatic damping is applied to achieve flat frequency response, then frequency response is flattened, but electromechanical noise increases

Engineering Contradiction:

Improvefrequency responseVSAvoidelectromechanical noise

Core Design Contradiction:

Stability of the object's compositionVSObject-generated harmful factors

Solution Approach 1:

The patent substitutes electrical damping components (damping resistor) for mechanical damping mechanisms. This substitution achieves flat frequency response through electrical circuit characteristics while avoiding the electromechanical noise that would result from increased mechanical damping

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The damping resistor acts as an intermediary element that couples the mechanical resonator to the electrical readout circuit. It provides the necessary damping for flat frequency response while its thermal noise can be managed through circuit design and switching techniques

Inventive Principle:

Principle #24Intermediary (Mediator)

3Measurement precision

If a high-Q mechanical system is used to reduce thermomechanical noise, then thermomechanical noise is reduced, but the system requires low-Q electrical damping for flat frequency response

Engineering Contradiction:

Improvethermomechanical noiseVSAvoiddamping control

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent merges the mechanical resonator with the electrical readout circuit into a unified electromechanical system. The damping resistor is integrated into the readout circuit, combining mechanical high-Q and electrical damping functions into a single device structure

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The damping resistor serves multiple functions: it provides electrical damping for flat frequency response, acts as a noise source that can be switched out, and interfaces the mechanical resonator with the electrical readout circuit. This multi-functionality reduces overall device complexity

Inventive Principle:

Principle #6Universality (Multi-functionality)

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 effectively reduces electromechanical noise while maintaining a flat frequency response, allowing for a high-quality factor mechanical system with low thermomechanical noise and low-Q electrical damping.

Implementation Method 1

The damping resistor is configured to electrostatically actuate the transducer and convert a thermomechanical noise of the resonator to an electromechanical noise

Methodology Applied

Scientific EffectElectrostatic actuation: Electrostatic Induction

Data Source

PatentUS11885647B2Accelerometer apparatuses and systems for noise rejection

Publication Date: 2024.01.30 ROHM CO LTD

AI summary

A sensor apparatus includes a resonator, a transducer, a damping resistor, a first switch, a filter stage, a second switch, and a noise rejection stage. The transducer is configured to detect a position of the resonator. The damping resistor is configured to electrostatically actuate the transducer and convert a thermomechanical noise of the resonator to an electromechanical noise. The first switch is configured to receive a first signal from the transducer. The filter stage is configured to receive the first signal and adjust a phase and a gain of the first signal and output a filtered first signal. The second switch is configured to receive a second signal from the transducer. The noise rejection stage is configured to receive the filtered first signal and the second signal and reduce the filtered first signal from an output signal.