Tuning Fork Resonator Grooves for Motional Capacitance Control

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Solution Overview

Problem

Miniaturization of piezoelectric resonators leads to an increase in equivalent resistance, and existing designs struggle to maximize motional capacitance for fundamental frequency while minimizing it for partial frequency modes, affecting mechanical stress distribution and shock resistance.

Innovation Solution

The design features tapered vibrating arms with grooves extending beyond the widening point of fins, optimized groove depth, and specific base-to-arm length ratios to enhance mechanical stress distribution, reduce equivalent resistance, and adjust frequency, incorporating fins to couple torsion modes and reduce partial frequency motional capacity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Volume of moving object

If the resonator is miniaturized to reduce dimensions, then the size is reduced, but the equivalent resistance increases

Engineering Contradiction:

Improveresonator dimensionsVSAvoidequivalent resistance

Core Design Contradiction:

Volume of moving objectVSReliability

Solution Approach 1:

The patent applies local quality by creating grooves only in specific regions of the vibrating arms rather than uniformly throughout. The grooves are positioned to maximize piezoelectric coupling in critical areas while preserving mechanical strength in other regions, achieving enhanced performance without proportional increases in equivalent resistance

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The grooves are pre-positioned in optimal locations during manufacturing to maximize piezoelectric coupling before the resonator is miniaturized. This preliminary optimization of material distribution allows the resonator to maintain lower equivalent resistance at smaller dimensions compared to conventional designs

Inventive Principle:

Principle #10Preliminary action

2Reliability

If grooves are made deeper to increase piezoelectric coupling, then the coupling increases, but the mechanical strength decreases

Engineering Contradiction:

Improvepiezoelectric couplingVSAvoidmechanical strength

Core Design Contradiction:

ReliabilityVSStrength

Solution Approach 1:

The grooves are designed with specific depth and positioning characteristics that concentrate piezoelectric coupling enhancement in localized regions where mechanical stress is lowest, rather than uniformly deep grooves throughout the entire arm structure

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The grooves extend partially along the length of the vibrating arms rather than throughout the entire structure. This partial action provides sufficient piezoelectric coupling enhancement while leaving portions of the arm structure intact to maintain mechanical strength and shock resistance

Inventive Principle:

Principle #16Partial or excessive action

3Volume of moving object

If the resonator is miniaturized further, then the volume decreases, but the quality factor decreases and equivalent resistance increases

Engineering Contradiction:

Improveresonator volumeVSAvoidquality factor

Core Design Contradiction:

Volume of moving objectVSReliability

Solution Approach 1:

The patent optimizes the geometric parameters of the grooves (depth, width, spacing, and positioning) to maximize piezoelectric coupling efficiency at miniaturized dimensions. By carefully adjusting these parameters, the resonator maintains high quality factor and low equivalent resistance despite reduced volume

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The resonator structure combines regions with grooves (enhanced piezoelectric coupling) and regions without grooves (maintained mechanical strength) in a composite-like configuration, optimizing the balance between electrical performance and mechanical integrity at miniaturized scales

Inventive Principle:

Principle #40Composite materials

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 configuration increases the quality factor, adjusts frequency, and enhances shock resistance by better distributing mechanical stresses, maintaining low equivalent resistance and maximizing motional capacitance for fundamental frequency while minimizing it for partial frequencies.

Implementation Method 1

Piezoelectric tuning fork resonator comprising two vibrating arms joined by a base, the assembly being made in a single piece of a piezoelectric material

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

They also make it possible to ensure a better distribution of the mechanical stresses along the vibrating arms and to increase the resistance to shocks

Methodology Applied

Scientific EffectMechanical stress distribution: Stress Relaxation

Implementation Method 3

The arrangement of the central electrodes inside the grooves, in the thickness of the arms, increases the piezoelectric coupling

Methodology Applied

Scientific EffectPiezoelectric coupling: Piezoelectric Effect

Data Source

PatentEP2017960B1Piezoelectric resonator with optimised motional capacity

Publication Date: 2009.12.30 ETA SA MFG HORLOGERE SUISSE

Figure 1~1A
Figure 2~3

AI summary

The tuning fork resonator has parallel vibrating arms (2,3) extended along base (4), each arm comprises set of excitation electrodes (5,6) for vibrating arm at various frequencies. The flipper-shaped portions (9,10) are formed at free end of arms. The grooves (7,8) are extended in direction of free end of each arm beyond the start of flipper-shaped portions to increase or decrease motional capacitance value of resonator for fundamental frequency or partial frequency respectively.