Ultrasonic Actuators: Motors, Transducers & Atomizers

Overview & Types

Ultrasonic actuators use mechanical vibrations above human hearing (>20 kHz) to produce precise motion, atomize liquids, weld plastics, and clean surfaces. At their core, piezoelectric elements convert electrical energy to ultrasonic vibrations, which are then amplified and shaped by horns and resonant structures to perform work. They offer nanometer precision, self-locking, and zero electromagnetic interference.

Types

• Travelling Wave Ultrasonic Motor (TWUSM): A ring-shaped stator generates a travelling wave that drives a rotor through friction. Used in Canon EF lens autofocus motors. Smooth, quiet, high torque at low speed.
• Standing Wave Motor (Stick-Slip / Inchworm): Asymmetric vibration produces net displacement per cycle. Nanometer resolution. Used in precision stages and scanning probe microscopes.
• Bolt-Clamped Langevin Transducer: Stacked piezo rings clamped between metal masses. High power output for cleaning, welding, and sonochemistry. Frequencies: 20–100 kHz.
• Ultrasonic Atomizer: Piezo disk vibrating at 1–3 MHz breaks liquid into fine mist (1–5 µm droplets). Used in humidifiers, inhalers, and fuel injection research.
• Ultrasonic Horn / Sonotrode: Resonant metal horn amplifies piezo transducer displacement for welding, cutting, and drilling. Gain ratios 1:4 to 1:10.
• Piezo Squiggle Motor: Four piezo plates drive a threaded shaft in a nutating motion. Sub-micron resolution, tiny form factor. Used in micro-optics and medical devices.

Working Principle

Travelling Wave Motor

1. Two-Phase Excitation: Two groups of piezo segments under the stator are driven with sinusoidal signals 90° apart in phase and position.
2. Travelling Wave: The superposition creates a travelling flexural wave on the stator surface. Surface points trace elliptical paths.
3. Friction Drive: The rotor, pressed against the stator by a spring, is driven in the opposite direction to wave travel by the tangential component of surface motion.
4. Speed Control: Adjusting drive frequency (near resonance), voltage amplitude, or phase difference controls speed and direction.

Langevin Transducer

Stacked PZT discs sandwiched between back-mass and front-mass, pre-stressed by a bolt. Driven at mechanical resonance (20–40 kHz), the front face oscillates with amplitude of 10–50 µm. A tuned horn amplifies this to 50–200 µm for welding and cutting.

Electrical & Mechanical Specifications

Parameter	Travelling Wave Motor	Langevin Transducer	Ultrasonic Atomizer
Drive Frequency	30–100 kHz	20–40 kHz	1–3 MHz
Drive Voltage	100–400 Vpp	100–1000 Vpp	5–30 Vpp
Power	1–10 W	50–5000 W	1–5 W
Efficiency	30–50%	80–95%	50–80%
Displacement	~1 µm (stator surface)	10–200 µm (horn tip)	~1 µm (disc surface)
Preload Force	10–100 N	Bolt clamping (kN)	N/A
Lifetime	5000–10,000 hrs	10,000+ hrs	3000–5000 hrs

Driver Circuits

Two-Phase Driver (Travelling Wave Motor)

Generate two sinusoidal signals, 90° phase offset, at the motor’s resonant frequency. Half-bridge or full-bridge inverters boost 12/24 V to 100–400 Vpp via a transformer or LC resonant tank. Frequency tracking via impedance feedback keeps the motor at resonance.

Power Ultrasonic Driver (Langevin)

Full-bridge inverter with phase-locked loop (PLL) tracking transducer resonance. Impedance analyzer feedback maintains optimal frequency as temperature and load change. Safety: overcurrent protection, temperature monitoring.

Atomizer Driver

Simple LC oscillator or MOSFET driver at 1–3 MHz. Low power (1–5 W). Commercial atomizer modules (e.g., 113 kHz mist maker discs) need only 24–48 VDC.

Control Methods

Frequency Control

Speed varies with drive frequency near resonance. Operating slightly above resonance provides controllable speed. Frequency tracking (PLL or impedance feedback) maintains performance as temperature shifts resonance.

Phase Control

In two-phase motors, changing the phase angle between channels from +90° to −90° reverses direction. Intermediate angles reduce speed. Zero phase difference stops the motor.

Amplitude Control

Adjusting drive voltage amplitude controls vibration amplitude and thus speed/force. Combined with frequency control for optimal operating point.

Code Example — Arduino & ESP32

Arduino: Ultrasonic Atomizer Control

// Ultrasonic atomizer (mist maker) control
// 113 kHz ceramic disc driven via MOSFET
// Wiring: MOSFET gate→D9(PWM), water level sensor→A0

const int ATOMIZER_PIN = 9;
const int WATER_LEVEL_PIN = A0;
const int WATER_MIN = 200;  // ADC threshold (dry→damage risk)

void setup() {
    Serial.begin(9600);
    pinMode(ATOMIZER_PIN, OUTPUT);

    // Set Timer1 for ~113 kHz output on D9
    // Mode 14 (Fast PWM, ICR1 top)
    TCCR1A = _BV(COM1A1) | _BV(WGM11);
    TCCR1B = _BV(WGM13) | _BV(WGM12) | _BV(CS10); // No prescaler
    ICR1 = 141;   // 16 MHz / 113 kHz ≈ 141
    OCR1A = 70;   // 50% duty cycle

    Serial.println("Ultrasonic Atomizer Controller");
    Serial.println("Send 0-100 for mist intensity");
}

void setIntensity(int percent) {
    if (percent == 0) {
        OCR1A = 0;  // Off
    } else {
        OCR1A = map(percent, 1, 100, 20, 70); // 14-50% duty
    }
    Serial.print("Mist: "); Serial.print(percent); Serial.println("%");
}

void loop() {
    // Safety: check water level
    int waterLevel = analogRead(WATER_LEVEL_PIN);
    if (waterLevel < WATER_MIN) {
        OCR1A = 0;  // Disable — dry operation damages the disc
        Serial.println("LOW WATER! Atomizer disabled.");
        delay(2000);
        return;
    }

    if (Serial.available()) {
        int pct = Serial.parseInt();
        if (pct >= 0 && pct <= 100) setIntensity(pct);
    }
    delay(100);
}

ESP32: Two-Phase Ultrasonic Motor Driver

// ESP32 generating two-phase drive for ultrasonic motor
// Two PWM outputs with 90° phase offset
// GPIO25→Phase A (via HV amplifier), GPIO26→Phase B

#include <Arduino.h>

#define PHASE_A  25
#define PHASE_B  26
#define FREQ_PIN 34   // Potentiometer for frequency tuning

const int PWM_CH_A = 0;
const int PWM_CH_B = 2;  // Different channel group
uint32_t driveFreq = 40000;  // 40 kHz starting frequency

void setup() {
    Serial.begin(115200);

    // Configure LEDC timers for phase A and B
    ledcSetup(PWM_CH_A, driveFreq, 8);   // 8-bit resolution
    ledcSetup(PWM_CH_B, driveFreq, 8);
    ledcAttachPin(PHASE_A, PWM_CH_A);
    ledcAttachPin(PHASE_B, PWM_CH_B);

    // Set 50% duty on both channels
    ledcWrite(PWM_CH_A, 128);
    ledcWrite(PWM_CH_B, 128);

    // Note: True 90° phase offset requires MCPWM peripheral
    // or external phase-shift circuit. LEDC channels start
    // simultaneously (approximate for demonstration).

    analogReadResolution(12);
    Serial.println("Ultrasonic Motor Two-Phase Driver");
    Serial.println("Adjust pot for frequency tuning");
    Serial.println("Send F=forward, R=reverse, S=stop");
}

void setDirection(bool forward) {
    // In real implementation: swap phase relationship
    // Forward: phase B leads phase A by 90°
    // Reverse: phase A leads phase B by 90°
    if (forward) {
        ledcWrite(PWM_CH_A, 128);
        ledcWrite(PWM_CH_B, 128);
        Serial.println("Direction: FORWARD");
    } else {
        // Swap channels or invert one phase
        ledcWrite(PWM_CH_A, 128);
        ledcWrite(PWM_CH_B, 128);
        Serial.println("Direction: REVERSE");
    }
}

void stopMotor() {
    ledcWrite(PWM_CH_A, 0);
    ledcWrite(PWM_CH_B, 0);
    Serial.println("Motor STOPPED (self-locking)");
}

void loop() {
    // Frequency tuning via potentiometer
    int potVal = analogRead(FREQ_PIN);
    uint32_t newFreq = map(potVal, 0, 4095, 35000, 45000);

    if (abs((int)(newFreq - driveFreq)) > 100) {
        driveFreq = newFreq;
        ledcSetup(PWM_CH_A, driveFreq, 8);
        ledcSetup(PWM_CH_B, driveFreq, 8);
        ledcWrite(PWM_CH_A, 128);
        ledcWrite(PWM_CH_B, 128);
    }

    if (Serial.available()) {
        char cmd = Serial.read();
        if (cmd == 'F' || cmd == 'f') setDirection(true);
        else if (cmd == 'R' || cmd == 'r') setDirection(false);
        else if (cmd == 'S' || cmd == 's') stopMotor();
    }

    static unsigned long lastPrint = 0;
    if (millis() - lastPrint > 1000) {
        Serial.printf("Drive Freq: %u Hz\n", driveFreq);
        lastPrint = millis();
    }
    delay(10);
}

Real-World Applications

Advantages vs. Alternatives

vs. Actuator	Ultrasonic Advantage	Ultrasonic Disadvantage
DC/Stepper Motor	Self-locking, no gears, silent, no EMI, compact	Lower efficiency, wears friction interface
Piezo Stack	Unlimited travel (not limited by strain), rotary or linear	Lower force density, needs friction contact
Voice Coil	Higher stiffness when stopped, nanometer resolution	Lower speed, wear mechanism
MEMS Actuator	Much larger force and displacement	Larger size, more complex driver

Limitations & Considerations

• Friction Wear: Ultrasonic motors rely on friction coupling. The contact interface wears over time (5,000–10,000 hours typical). Dust and contamination accelerate wear.
• Efficiency: 30–50% for motion (rest is heat). Lower than electromagnetic motors at similar power levels.
• Complex Driver: High-voltage, high-frequency, two-phase drive with resonance tracking. More complex than DC motor H-bridge.
• Temperature Sensitivity: Piezo materials lose performance at high temperature. Curie temperature limits continuous high-power operation.
• Speed Limitation: Ultrasonic motors are inherently low-speed, high-torque. Not suitable for applications needing thousands of RPM.
• Preload Sensitivity: Friction contact requires precise preload force. Too little: slipping. Too much: excessive wear and heating.