Piezoelectric Actuator transducer : Principle & Its Applications

The piezoelectric actuator is a vital component of numerous smart and electromechanical systems, along with signal processing units and various types of sensors. In many smart systems, actuators function as responding units. Currently, there has been significant advancement in non-resonance piezoelectric actuators such as step motors, multilayer ceramic actuators, resonance ultrasonic motors, and inertial motors. These actuators generate a small displacement with a high force capacity when voltage is applied. They are employed in numerous applications, including handling and generating high pressures or forces in both dynamic and static conditions, as well as ultra-precise positioning. This article provides an overview of a piezoelectric actuator, its operation, and applications.

 

What is a Piezoelectric Actuator?

 

A piezoelectric actuator is a transducer that converts an electrical signal into a precisely controlled physical displacement or stroke using the piezoelectric effect based on electromechanical coupling rather than electromagnetic induction. If the stroke is prevented, usable energy will develop. The precise movement control offered by these actuators is primarily utilized to finely adjust machining tools, mirrors, lenses, or other equipment.

 

A piezo actuator is used to control hydraulic valves, functioning as a special-purpose motor, small-volume pump, and in other applications requiring force or movement. The piezoelectric actuator diagram is shown below.

Compared to electromagnetic actuators, piezoelectric actuators offer features such as flexible design, compact size, and sub-micrometer or nanometer positioning.

 

Piezoelectric Actuator Working Principle & Operation

 

The working principle of a piezoelectric actuator is that when voltage is applied, it generates a small displacement with a high force capacity. This makes them suitable for many applications, including ultra-precise positioning and handling and generating high forces in both stationary and dynamic conditions.

 

The configuration of the piezoelectric actuator can vary greatly depending on the application. These devices are ultrasonic actuators designed to generate strokes of several micrometers at ultrasonic frequencies above 20 kHz. They are particularly used for positioning, controlling vibration, and quick switching applications.

The important specifications of piezoelectric actuators mainly include force, displacement, and operating voltage. The factors to consider while using these actuators are capacitance, stiffness, and resonant frequency.

 

Stiffness refers to the energy required to achieve a certain deformation of a structure. For piezoelectric actuators, stiffness is the energy needed to extend the device by a specific amount, usually expressed in terms of Newtons per micrometer. Capacitance depends on the excitation voltage frequency, and resonance is the frequency at which the piezoelectric actuators respond with the highest output amplitude.

 

Types of Piezo Actuators

 

Piezo actuators come in various types, such as stack, stripe, longitudinal, shear, tube, and contracting.

Piezo Stack Actuators

 

A Piezo stack actuator delivers a low stroke with high blocking energy. Depending on your design needs, this actuator can be either discrete or co-fired. If discrete, its complex structures are created by stacking finished piezoelectric ceramic rings and metal electrode foil using an adhesive. The typical operating voltage ranges from 500V to 1,000V.

 

Co-fired stack actuators, also known as monolithic stacks, do not use adhesives. Instead, they employ high-temperature sintering of the ceramic pile and complete electrode, operating at around 200 volts. Whether co-fired or discrete, these actuators can be protectively insulated from mechanical stresses and environmental impacts, usually by coating materials, bare stacks, or encasing them in stainless steel.

 

Stripe Actuators

 

The Stripe Actuator, also referred to as a bending actuator, generates a significant mechanical deflection in response to an electrical signal. This deflection results in a large stroke with minimal blocking force compared to the stack actuator.

 

Piezoelectric Actuator

 

This type of actuator utilizes two thin piezoelectric ceramic layers connected in an electrically parallel configuration with coinciding polarization directions. When an electrical input is applied, one ceramic layer contracts while the other expands, causing the actuator to bend.

 

Longitudinal Actuators

 

Longitudinal actuators, also known as piezo stacks, are created by stacking several piezo elements on top of each other. Each element's expansion generates a useful force and movement. These actuators leverage the piezoelectric effect to produce linear displacements ranging from 0.1 to 0.15% of the actuator's length. They have a high force density, typically around 30 N/mm^2, and high resonant frequencies, making them suitable for dynamic applications.

 

Shear Actuators

 

Shear actuators are similar to longitudinal actuators in that they consist of multiple layers of piezo elements. However, they differ in the application of voltage and the type of motion produced. In shear piezo actuators, the elements are horizontally polarized, and the electrical field is applied orthogonally.

 

This results in displacement within the horizontal plane, creating a shear-type motion. The height of these actuators is limited by bending and shear stresses, but they are often combined with longitudinal actuators in multi-axis systems.

 

Tube Actuators

 

Tube actuators feature radial polarization and utilize the transverse piezoelectric effect to achieve displacement. Depending on the voltage applied relative to the electrodes, these actuators can experience radial, axial, or lateral motion. While they are not suitable for generating significant forces, they provide micron-level travel, making them ideal for applications such as pumping, nanoliter dosing, and scanning microscopes.

 

Contracting Actuators

 

Flat actuators consisting of two piezo elements can generate contracting motion when both elements operate together. These actuators use the transverse piezoelectric effect to produce motion in a single direction. Contracting piezo actuators have a small displacement, up to 20 microns, but can generate forces in the hundreds of Newtons.

 

When mounted to a substrate or base, these actuators can function as bending actuators. In this configuration, applying voltage causes one piezo element to expand while the other contracts.

 

Advantages

 

The advantages of the piezoelectric actuator include:

 

• Simple design
• Few moving parts
• High reliability
• Easily optimized for specific applications, such as non-magnetic, cryogenic, ultra-high vacuum, and high stiffness environments
• High force per unit area
• Unlimited resolution
• Compatibility with vacuum and clean room conditions
• High force generation
• No wear and tear
• Low energy consumption
• No magnetic fields
• Quick response time
• Operates at cryogenic temperatures

 

Where are Piezoelectric Actuators used?

 

The applications of piezoelectric actuators include:

 

• Adjusting various equipment such as lenses, machining tools, and mirrors.
• Precision motion control-based applications.
• Controlling hydraulic valves, functioning as special-purpose motors or small-volume pumps.
• Applications requiring force or movement.
• Industries like aviation, automotive, consumer electronics, medical, and aerospace, due to their numerous benefits.
• Braille and precision knitting machines.
• Mobile phones and video cameras equipped with microphones, owing to their silent drive characteristics.
• Environments that do not require lubrication, such as vacuum and cryogenic settings.

 

What are piezoelectric actuator materials?

 

The most commonly used piezo actuator materials are modified lead zirconate titanate ceramics.

 

What is a piezo stack actuator?

 

A piezo stack actuator is formed by stacking multiple piezo elements on top of each other. These actuators operate in various environments, including cryogenic conditions, vacuums, and the presence of magnetic fields. They are used in the aerospace, consumer electronics, semiconductor, and automotive industries.

 

What are the two parameters considered in piezo actuators?

 

Two parameters are usually considered in piezo actuators: blocking force (Fb) and free deflection (Xf). Blocking force is the maximum force that can be produced when the maximum voltage is applied and the actuator is not allowed to move. Similarly, free deflection is the movement achieved when the maximum voltage is applied and no force is generated.

 

What does a piezoelectric actuator do?

 

A piezoelectric actuator converts an electrical signal into a precisely controlled physical displacement. It is used to control hydraulic valves, function as a special-purpose motor or small-volume pump, and in applications requiring force or movement.

 

Thus, this provides an overview of the piezoelectric actuator and its working. The primary function of this actuator is based on the inverse piezoelectric effect: when voltage is applied, the actuator expands or contracts. However, if the piezo actuator is prevented from moving, it produces a force. Here is a question for you: what are the disadvantages of a piezoelectric actuator?