To effectively utilize piezoelectric actuators, one must possess a comprehension of both their mechanical and electrical performance. Familiarity with the fundamental operational principles of piezoelectric actuators can also aid in their control and operation. When comparing these actuators to conventional electromagnetic alternatives (such as solenoids, linear motors, and voice-coils, motor-screw driven combinations) and other materials that induce strain, like electrostrictive ceramic, magnetostrictive, and shape memory alloys, it can be useful to refer to the following information regarding piezo-actuator design and performance issues.

Multilayer Stack Actuators (PICMA)

Why Piezo Ceramic Motion Devices are Different

Piezoelectric ceramic-based motion devices, also known as piezoelectric actuators, are increasingly popular in fields that demand high-frequency operation and precise motion control. Unlike descrete stepper motor actuators, these actuators have the ability to generate smooth and uninterrupted motion with resolutions at the nanometer and sub-nanometer level, making them ideal for nanoliter dosing, precision positioning and scanning systems such as super resolution microscope stages. Additionally, their extremely fast response times, wide operating bandwidth, and high specific force make them suitable for various other applications, including fluid valve control, optical scanning, fast steering mirrors for free space optical communication, vibration isolation, and precision machining.

Video: Principle Operation of Piezo Actuators, Piezo Flexure Guided Motion and Piezo Motors

The Piezoelectric Effect in Motion Applications

Piezoelectric actuators are commonly utilized to create fast and / or precise movement or generate forces by using electro-active materials, like lead zirconate titanate (PZT), as the primary driving force. These materials exhibit an induced strain effect, where they expand or contract in the presence of an applied electric field. This change in displacement or length occurs as the electrical dipoles within the material rotate and align with the direction of the electric field. The magnitude of the length change is generally proportional to the strength of the applied electric field as determined by the actuation voltage of the device. Typically, the length change is about 0.1 percent of the total material length in the direction of the applied field. For example, a 1mm thick layer of PZT will increase in thickness by one micron when actuated. Modern piezoelectric actuators use a monolithic stack of PZT layers and electrodes, known as a PZT stack.

Displacement modes of piezoelectric stack actuators

Piezoelectric material manufacturers provide multilayer piezo materials stacked in thin layers to maximize the piezoelectric effect at lower operating voltages.  Various sizes and shapes of PZT stacks are available, which can be combined to produce extended motion. Even at low voltage levels of around 10 volts, useful expansions can be achieved, although typically maximum performance is achieved at operation around 100 volts.  Classical discrete piezo stacks are also available, for higher force applications and with higher operating voltages. PZT stacks cross sections range from 1mm to 60mm,  the larger the diameter the higher the produced force and load capacity. In addition to this, PZT actuators can also be produced in the form of tubes and thin patches or strips bonded to one or both sides of a substrate material. Here bending actuators need to be mentioned, because they provide large displacements at minimum volumes.  

Closed-loop piezoelectric bender actuators

Piezo Bending Mode

Another way to increase the motion output of a piezo actuator is the use of a mechanical amplifier.  Amplified piezo actuators can provide more than 1mm of motion with still very fast response times and output forces, if designed properly.

Amplified flexure actuator

High Speed Operation

When operated at high frequency, pezoelectric actuators generate heat that may have to be dissipated. Recent progress in various cooling measures is described in this paper. 
In addition, an improved hermetically sealed piezoceramic multilayer actuator suitable for liquid cooling is proposed. With a newly developed amplifier, nominal displacement of 35µm at 3,300Hz was demonstrated with actuator temperatures not rising above 80°C.

Self-heating tests on encapsulated 5x5x36mm³ PICMA® stack actuators filled with heat conducting media for different cooling measures; Drive signal: 0 to 100V sine, E-617KDYN amplifier.

Force Generation / Blocked Force

PZT stacks are known for their ability to generate force, but in order to generate force, the actuator needs to be restricted in its motion.  The force output is proportional to the applied electric field or actuation voltage. When an external load resists the motion of the PZT expansion, the PZT stacks apply a force that depends on the stiffness of the external load. If the stiffness of the external load is high enough to prevent expansion of the PZT, the PZT stacks can generate a very high level of pressure against the load. Typical blocked pressure levels range up to 50 MPa.

Force Generation vs Displacement of a PICMA Multilayer Actuator

Advantages of Piezoelectric Actuators

Piezoelectric actuators require unique design considerations due to the relatively small displacement they can develop. However, they excel in precision positioning applications where small, high-force moves are desirable. In applications such as fabricating, measuring, or testing extremely small structures or features, piezoelectric actuators can provide very smooth and continuous motion over a range of a few microns to a few millimeters. With proper system design, piezoelectric actuators hold the potential for high-speed operation. Typically, the response time of a piezo-stack is limited by the speed of sound in the material, resulting in a natural frequency of several kilohertz. Even with the added mass and lower stiffness of an amplification mechanism, the natural frequency of an amplified piezo-actuator may still be a few kilohertz.

Nanopositioning Stages Based on Piezo Actuators

By combining piezo actuators,  flexure guiding systems, motion amplifiers and high resolution position sensors, sub-nanometer precise motion systems with single or multiple degrees of freedom can be designed. The find applications in fields such as microscopy, surface metrology,  semiconductor test and production, photonics alignment to name a few.

3-Axis Piezo Nanopositioning Stage for Super Resolution Microscopy  

Objective Focus Nanopositioning Stage, based on Piezo Actuators

Piezoelectric actuators and nanopositioning systems can offer many benefits when designed and constructed correctly. These include a solid-state construction with no backlash, stiction, or cogging, as well as the ability to hold positions with low or zero power consumption. They also have a high frequency response with a wide bandwidth, and can generate a high force per unit area, with force and stroke scaling directly with size. Furthermore, flexure-based designs used in piezoelectric actuators typically have little or no friction, meaning they generate little to no outgassing or particle generation, and require no lubrication. Additionally, piezoelectric actuators produce relatively low levels of heat and are highly scalable and reliable.

Modern piezoelectric actuators are capable of utilizing the precise expansion generated by the piezoelectric effect to produce a wide range of actuator solutions. Piezoelectric actuators can offer valuable performance attributes and properties that are useful in precision positioning, scanning, and vibration control applications. Solid-state actuation mechanisms can provide smooth and precise motion ranging from sub-nanometer to multiple-millimeter levels.

Piezo Motors for Longer Travel Motion

When long travel motion is required, so-called piezoelectric motors such as ultrasonic motors can be used.

Ultrasonic motors make use of ultrasonic vibrations to produce rotary or linear motion. They typically consist of a piezo transducer, the stator, which is a stationary component, and a moving part the slider or rotor. The stator contains one or more piezoelectric transducers, which are driven by an HF voltage source to produce high-frequency ultrasonic vibrations. The rotor or slider is in contact with the stator and is moved by the ultrasonic vibrations.  Ultrasonic motors have some unique advantages over traditional electromagnetic motors, such as very fast response time, lack of electromagnetic interference and the intrinsic ability to hold a position without drift and energy consumption at rest.  

Low Profile Microscope stages based on ultrasonic piezoelectric motor

Video: Operation of Different Types of Piezo Motors