Piezo motors and actuators

Introduction

Piezo motors and actuators are widely used in precision applications. This page explains the working principles, advantage, disadvantages and applications of ultrasonic piezo motors, stick-slip piezo motors, piezo stepper motors and piezo actuators.

Overview

Piezo actuator

Ultrasonic piezo motor

Stick-Slip piezo motor

Stepper piezo motor

Resolution

< 1 nm

~ 1 nm

< 1 nm

Stroke/Travel range

1/1000 of its length

Unlimited

Force

> 100 N

1 - 10 N

~ 50 N

Speed/Frequency

~ 100 kHz

> 100 mm/s

< 20 mm/s

10 - 50 mm/s

Speed profile

Nearly constant

Saw tooth

Stepping

Lifetime

> 1E6 cycles

> 100 km

1-10 km

~ 10 km

Noise

Yes (~ 5 kHz)

Yes (~ 2 kHz)

More info

Piezo actuator

A piezo actuator converts electrical energy into mechanical motion through the inverse piezoelectric effect. When a voltage is applied to a piezoelectric material (e.g. PZT), it deforms, producing extremely small, precise linear displacements. This makes them ideal for ultra-fine positioning on a nanometer scale.

Piezo actuators are often made from a multilayer stack structure, where many thin layers of piezo material are stacked and electrically connected. This structure amplifies the displacement while reducing the required voltage. However, the stroke of a piezo actuator is relatively short, typically only about 1/1000 of its length. For applications that require greater displacement, mechanical amplification systems such as flexures can be used, though this can reduce the actuator’s stiffness.

Key features:

High precision: sub-nanometer positioning accuracy.
Fast response: sub-millisecond response times.
High force: can generate significant force despite small size.
Non-magnetic and vacuum-compatible: suitable for ultra-high vacuum and non-magnetic environments.
Challenges: susceptible to hysteresis and non-linearity, requiring closed-loop control.

Applications:

Piezo actuators are widely used in fields such as semiconductor manufacturing, nano-positioning, precision machining, and biotechnology. They are commonly found in devices like microscopy stages, optical alignment systems, and laser beam steering mechanisms, where precise, wear-free motion is essential for achieving high-performance results.

Ultrasonic piezo motors

An ultrasonic piezo motor operates on the principle of high-frequency vibrations generated by a piezo material. Unlike other piezo motors, which rely on direct expansion and contraction, ultrasonic piezo motors use alternating electrical field to induce ultrasonic vibrations in a piezoelectric element or its housing. These high-frequency vibrations (typically between 100-200 kHz) are converted into mechanical motion.

The motor consists of a stator (a piezoelectric block, sometimes with housing) and a slider or rotor. When an AC voltage is applied to the piezoelectric element, it vibrates in an elliptical trajectory at its contact point. This elliptical motion is transferred to the rotor or slider, generating continuous movement. The key to the motor’s operation lies in controlling the shape of the elliptical trajectory, ensuring smooth, precise movement in the desired direction.

Key features:

High precision: fine positioning accuracy.
Silent operation: operates at ultrasonic frequencies beyond human hearing.
High speed: achieves speeds up to 100 mm/s.
Self-locking: holds position when powered off, enhancing energy efficiency.
Low power consumption: operates at resonance, generating minimal heat.
Durability: minimal wear due to low-impact of the contact point.

Applications:

Ultrasonic piezo motors are commonly used in precision-driven applications such as microscopy stages, camera manipulation systems, theodolites, and various high-tech fields requiring quiet, high-speed motion with minimal power consumption. Their compact size, silent operation, and high resolution make them ideal for use in sensitive environments like medical devices and laboratory equipment.

Stick-Slip piezo motors

A stick-slip piezo motor, also known as an inertia drive, is a type of piezo motor that utilizes the frictional interaction between a piezo element with contact point and a moving part to generate motion. The motor operates through a two-phase cycle: the “stick” phase and the “slip” phase, driven by the rapid expansion and contraction of the piezoelectric element with contact point when a voltage is applied.

Working principle:

Stick phase: a saw tooth voltage signal is applied to the piezo element, causing it to expand slowly. During this phase, the motor’s moving part (often called the slider or stage) is pushed forward due to the frictional force holding the piezo element with contact point and the slider together.
Slip phase: in the rapid retraction phase, the piezo element with contact point contracts quickly, causing the slider to momentarily lose frictional contact with the contact point due to the inertia of the slider or stage, allowing it to stay in place while the piezo resets its position.

By repeating this cycle in quick succession, the motor can move the stage in small, incremental steps. Reversing the saw tooth signal changes the direction of motion.

Key features:

Compact design: simple mechanical structure with low-voltage electronics.
Fine positioning: high-resolution control down to nanometer scale.
Self-locking: maintains position without power.
Challenges: non-deterministic motion can lead to unpredictable step sizes and noise from the slipping phase.

Drawbacks:

Vibrations and noise: the impact from the rapid retraction phase can cause vibrations in the system, which may be undesirable in sensitive applications. The noise generated from this impact can be irritating, particularly in environments where operators work near the motor.
Wear and limited lifetime: the repeated impact of the contact point on the slider creates frictional wear over time, limiting the motor’s lifetime.
Limited speed: most stick-slip motors have slower speeds, typically around 20 mm/s, making them unsuitable for applications requiring high-speed movement.

Applications:

Stick-slip piezo motors are commonly used in areas where compactness, simplicity, and fine positioning are required. They are widely implemented in optical beam path manipulation, research setups, and compact scanning systems, such as those used in biotechnology.

Piezo stepper motors (piezo walkers)

A piezo stepper motor, also known as a piezo walker, is a type of piezoelectric motor that achieves motion by utilizing a series of coordinated, step-like movements of piezoelectric actuators. This mechanism mimics the walking motion of a legged creature, where each piezo actuator moves incrementally to generate precise, controlled linear or rotational motion.

Working principle:

A piezo stepper motor consists of multiple piezoelectric actuators arranged in a configuration that allows them to “walk” the motor’s slider or stage. Each actuator expands and contracts in a specific sequence, gripping and releasing the load to move it incrementally. This stepping mechanism can be compared to how legs take small, precise steps to move an object forward or backward.

Typically, a piezo stepper motor involves multiple actuators (often four “legs”), and the motion is generated by alternating the expansion and contraction of these actuators. This walking motion results in extremely fine positioning accuracy, as each step can be as small as a few nanometers.

Key features:

High precision: nanometer-level accuracy.
Strong holding and drive forces: friction locks the motor in place without power, while the drive force is higher than other types of piezo motors.
Challenges: high complexity due to the coordination of the different actuators.

Drawbacks:

Lower speed: due to the step-by-step motion, piezo stepper motors are slower than ultrasonic piezo motors.
Wear and lifetime limitations: although these motors are generally robust, the friction involved in the stepping process can lead to wear over time, especially if the tolerances between the actuators and the stage are not carefully maintained.
Complex electronics: controlling the precise sequence of steps requires sophisticated electronics, making the control system more complex than other piezo motor technologies.
Higher cost: the complex structure, which involves multiple piezo actuators, and the need for precise manufacturing tolerances make piezo stepper motors more expensive compared to simpler piezo motor designs like stick-slip or ultrasonic motors.

Applications:

Piezo stepper motors are used in applications that require extreme precision, stability and higher forces, such as semiconductor manufacturing, biotechnology, optical systems, astronomy and aerospace technology.

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