Resonator Development for Beauty Devices: Customer-Specific Piezoelectric Elements and SUS HEAD Modularization

GPN is working with specialist piezoelectric partners to develop customer-specific ultrasonic piezoelectric resonators for beauty devices and advance modularization with SUS HEAD. Skin-contact heads must maintain the required vibration and transmitted output as contact changes, while controlling temperature during repeated use. Electrical, mechanical and thermal behavior must therefore be evaluated together. The specifications below describe existing reference components, not verified ratings or demonstrated performance of the new design.

1. Development Scope and System Boundaries

Retaining the existing board requires compatible frequency, waveform, voltage, current and protection limits. Retaining the head requires compatible mounting, contact geometry and load paths. Driver redesign extends development to power supply, control and packaging. During use, head inclination, contact force and area can change. Tests must represent moving or stationary heads and continuous or intermittent operation, rather than only a rigidly fixed condition.

Resonance describes structural response, not the method of electrical-to-mechanical conversion. Measure electrical input at the assembly’s defined terminals; evaluate a structural resonant component within its excitation assembly. These boundaries support performance agreements without disclosing the internal operating structure.

2. Reference Disc and Assembled Resonator Specifications

The reference piezoelectric disc specifies thickness-mode resonance at 3,000 kHz ±5% (2.85–3.15 MHz) and resonance impedance of 30 Ω maximum, tested at 1 Vrms. Capacitance is 0.65 nF ±20% (520–780 pF), measured at 1 kHz, 1 Vrms and 25°C. Diameter is Ø8.10 ±0.15 mm; thickness is 0.70 ±0.05 mm, including the external electrodes. The combined flatness/parallelism entry is 30 μm maximum; applicable surfaces and measurement procedures require drawing-level definition.

A separate radial-mode requirement is (f_a − f_r)/f_r ≥ 0.130 at 1 Vrms, where f_a and f_r denote antiresonance and resonance frequencies. This ratio is not an electromechanical coupling coefficient or conversion efficiency and belongs to a different mode from the 3 MHz thickness resonance. The 1 Vrms test level is neither a rated operating voltage nor a maximum permissible drive voltage.

The reference assembled resonator specifies f_r1 = 1.0–1.4 MHz, f_r2 = 2.7–3.3 MHz and f_r3 = 9.5–11.0 MHz; the entry designated “resonance resistance 2 / Zr2” is 25 Ω maximum. Capacitance is 550 pF ±20% at 1 kHz, 1 Vrms and 25°C. These bands do not establish particular vibration modes or harmonic relationships.

Assembly inspection includes removing the protective film from the central Ø9 area of the radiating face, which is distinct from the disc diameter. The 650/550 pF and 30/25 Ω limits are separate specifications, not evidence of improvement through assembly. Operating and storage ranges of 5–55°C and −20–85°C, with storage humidity ≤85% RH, apply to the reference assembly. They are not new-product ratings or permissible skin-contact temperatures.

3. Resonance and Contact-Surface Motion

Base thickness, sidewalls, reinforcement, fastening and curvature influence mass and stiffness distribution. Mounting can change frequency and motion without changing overall dimensions. For a linearized single mode, the undamped natural frequency follows f_n ≈ (1/2π)√(k_eff/m_eff). These modal quantities differ from total component mass and static stiffness. Damping, excitation, observation position and loading affect the measured response peak.

A large central displacement does not establish adequate motion across the working surface. Assess spatial amplitude, phase and motion direction. Noncontact vibrometry with suitable bandwidth and resolution supports mode identification and simulation validation. Narrow resonance can increase sensitivity to temperature or contact changes; evaluate quality factor alongside useful bandwidth, output and frequency control.

4. Compatibility with the Existing Drive Circuit

Similar resonance frequencies can coexist with different impedance and phase, changing current demand and terminal voltage. Include actual cables, connectors and matching networks. Near piezoelectric resonance, mechanical motion contributes to electrical response; capacitance measured at 1 kHz cannot alone predict MHz current or output.

Record impedance magnitude and phase, or resistance and reactance, across frequency. In lossy devices, minimum impedance, zero phase and maximum conductance need not coincide; use a consistent resonance definition. Apply instrument-specific fixture compensation and define the measurement plane. Distinguish component characterization from system tests including cables and matching circuits.

Real input power is the average of v(t)i(t). For sinusoidal steady-state operation, it is V_rmsI_rmscosφ. Pulsed or burst operation requires a defined averaging interval and duty cycle; voltage/current timing and phase errors also matter. Apparent power or an impedance limit cannot establish acoustic output. Measure transmitted output independently and assess driver and matching-component heating.

5. Skin Contact, Coupling Media and Output Distribution

Coupling medium, contact force, area and support conditions change load and damping. Air-test results require comparison with representative use conditions. Air gaps, coupling-layer variation and contact geometry affect transmission; polish alone does not establish good coupling. For gel-coupled devices, use the specified medium and control quantity, layer thickness, temperature and changes through repeated contact. Moving-head tests should represent surface curvature and contact-force variation.

Use equivalent film, medium, mounting and temperature conditions for reference and prototype. Surface displacement, local pressure and total acoustic power are distinct quantities. Calibrated hydrophones can characterize the ultrasonic field; a suitable radiation-force balance measures total acoustic power radiated into the test medium. Define position, direction, medium, reflection control and calibration. Water-tank output and transmission under skin-contact conditions have different load boundaries and require separate evaluation. Neither establishes cosmetic efficacy or human-use performance.

6. Contact Temperature and Repeated Operation

Low-signal compliance does not establish behavior at working amplitude or during continuous operation. Losses in the assembly and electronics generate heat, which can alter response and challenge fixed-frequency operation or tracking. Compare temperature rise at the same transmitted output and load: reducing voltage and output together is not evidence of improved efficiency.

Track input power, output, resonance and temperature from startup through operating cycles and restart. Contact-face, internal, joint and circuit temperatures can differ. Check local temperature distribution as well as averages, including maintained and interrupted contact, and relate findings to output control and protection. Sensor attachment must not disturb vibration; infrared measurements of polished surfaces require emissivity correction. For battery-powered devices, compare input power and intended runtime at equal transmitted output. Prototype temperature limits require separate verification.

7. Assembly Variation and Production Repeatability

A successful specimen does not establish production consistency. Thickness, shape, fastening and support tolerances should reflect resonance and output sensitivity. Where adhesive bonding is used, thickness, stiffness, loss and cure condition affect transmission; static strength alone is insufficient. This does not imply that a customer-specific design uses an adhesive joint.

Control fastening sequence, tightening conditions and contact repeatability. Removal and remounting tests help separate fixture variation from manufacturing variation. Final polishing can change local thickness, edges and contact geometry, so appearance acceptance and functional dimensions require separate controls. Verify response after final finishing rather than imposing uniformly tight tolerances without identifying sensitive features.

Where applicable, assess exposure to specified gels, cosmetic residues and repeated cleaning. Compare surfaces, joints, seals, resonance and output before and after testing, including electrical insulation where relevant. Small visual changes do not establish functional durability; waterproofing and chemical resistance require their own criteria and verification.

8. Comparative Validation and Acceptance

Compare reference and prototype under common input, mounting and load conditions. Report unchanged-board tests separately from tests optimized for each design. The sequence should cover assembly fit, low-signal response, existing-board operation, loaded output, sustained and cyclic operation, and variation between specimens and manufacturing histories. Include remounting repeatability and pre-/post-endurance measurements.

Define acceptance ranges from device requirements before examining results. Instrument calibration, repeatability, uncertainty and sample count determine whether a small difference supports an improvement claim. Link specimen identity, drawing revision, assembly conditions, circuit version and test history. Approval should reflect repeatedly achieved performance rather than the best result from a selected specimen.

9. Development Inputs and Manufacturing Scope

Development discussions require the reference specification and samples, application requirements, retained hardware and permitted changes. Supply drive waveforms, frequency range, duty cycle and protection conditions; mounting drawings and coupling conditions; output metrics, operating duration and temperature-monitoring locations; and production volume, approval and traceability requirements.

Cost comparison should include board changes, matching components, assembly and inspection time, functional finishing and final yield. Agreed interfaces and reproducible performance tests make the required changes and remaining validation work concrete without revealing confidential construction details.