Digital Charge Amplifier for Piezoelectric Sensor Drift

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Solution Overview

Problem

Piezoelectric sensors face challenges in accurately measuring quasi-static forces due to drift issues in analog charge amplifiers, which lead to inaccurate force measurements over time.

Innovation Solution

A digital charge amplifier system that samples the charge signal, converts it to digital values, and uses bias estimation and drift compensation methods, such as low pass filter, switched low pass filter, and state machine bias estimators, to eliminate drift and improve accuracy in force measurement.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If analog charge amplifiers are used to convert piezoelectric sensor output to force measurements, then the sensor can measure force directly, but drift occurs leading to measurement errors over time

Engineering Contradiction:

Improveforce measurement accuracyVSAvoidmeasurement stability over time

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent replaces the mechanical/analog charge amplifier system with a digital signal processing system. The piezoelectric sensor output is amplified and filtered using digital algorithms (low-pass filter, moving average filter) rather than analog circuits, eliminating drift while maintaining measurement accuracy. This substitution of digital processing for analog processing resolves the contradiction between measurement precision and reliability over time.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent changes the operational parameters of the signal processing system by introducing multiple time constants (fast time constant for transient response, slow time constant for drift compensation) and dynamically adjusting filter characteristics. This allows the system to maintain both accuracy during dynamic events and stability during static periods, resolving the contradiction between responsive measurement and long-term reliability.

Inventive Principle:

Principle #35Parameter changes

2Ease of operation

If piezoelectric sensors are used for quasi-static force measurement, then direct force measurement is achieved, but drift in analog circuits causes large sensor errors

Engineering Contradiction:

Improvedirect force measurement capabilityVSAvoidforce measurement accuracy

Core Design Contradiction:

Ease of operationVSMeasurement precision

Solution Approach 1:

The patent replaces the problematic analog charge amplifier with a digital signal processing system that includes programmable amplification and filtering stages. This digital approach maintains the ease of direct force measurement while eliminating the drift-induced precision errors through software-based signal conditioning and drift compensation algorithms.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent implements feedback mechanisms where the processed signal is continuously monitored and used to adjust the measurement parameters. The system uses feedback from the sensor output to dynamically compensate for drift and maintain measurement precision, allowing direct force measurement to remain both easy to operate and highly accurate.

Inventive Principle:

Principle #23Feedback

3Adaptability or versatility

If analog charge amplifiers with integration are used, then voltage from sensor can be converted to force values, but drift leads to large sensor errors

Engineering Contradiction:

Improvesignal conversion capabilityVSAvoidforce measurement accuracy

Core Design Contradiction:

Adaptability or versatilityVSMeasurement precision

Solution Approach 1:

The patent substitutes digital signal processing for analog integration circuits. The conversion from voltage to force values is performed using digital algorithms that include programmable gain amplification, multi-stage filtering with adjustable time constants, and digital integration. This maintains the versatility of signal conversion while eliminating drift through software-based processing.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent introduces dynamic adaptability by allowing the filter time constants and gain parameters to be adjusted based on the measurement conditions. The system can switch between fast response mode (for transient events) and slow response mode (for static measurements), providing versatile signal conversion while maintaining precision in both dynamic and static conditions through adaptive parameter adjustment.

Inventive Principle:

Principle #15Dynamics

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

The digital charge amplifier system significantly reduces drift, providing more accurate and stable force measurements by compensating for bias errors, thus enhancing the reliability of quasi-static force measurement using piezoelectric sensors.

Implementation Method 1

In a piezoelectric material, as a first approximation, the charge produced is linearly proportional to the applied stress or force

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Data Source

PatentUS8789426B1System and method of measuring quasi-static force with a piezoelectric sensor

Publication Date: 2014.07.29 PURDUE RES FOUND

AI summary

A system and method employing a piezoelectric sensor for quasi-static force measurement substantially free of drift and with improved low-frequency response. The output signal from the sensor is sampled and integrated using digital techniques that include a drift compensation algorithm. The algorithm continually monitors the sensor output and estimates bias errors that will cause the output to drift.