piezoelectric actuator (multilayer piezoelectric ceramic)
Piezoelectric actuators are an integral part of many industrial devices and sensors in a wide variety of applications. Among them, piezoelectric stack actuator (multilayer piezoelectric ceramic) has made great progress. This article will introduce the knowledge related to piezoelectric actuators in detail, and tell users how to choose a suitable piezoelectric actuator.- What is a piezoelectric actuator
- Classification of piezoelectric actuators
- What is a piezoelectric stack actuator(multilayer piezoelectric ceramic)
- What is a stripe actuator
- What is a longitudinal actuator
- What is a Shear Actuator
- What is a Tube Actuator
- What is a contract actuator
- Working principle of piezoelectric stack actuators
- How to choose piezoelectric stack actuators for displacement
- Applications of piezoelectric stack actuators
- Case Pictures of piezoelectric stack actuators from He-Shuai Company
- What are the trends in piezoelectric actuators
2、Classification of piezoelectric actuators
Piezo actuators are available in different types like stack, stripe, longitudinal, shear, tube, and contracting.
Piezo stacks (multi-layer piezoelectric ceramics)
Stripe actuators (bending actuators)
Longitudinal actuators
Shear actuators
Tube actuators
Contracting actuators
3、What is a piezo stack actuator?
Piezo stack actuators provide low stroke with high blocking force. Depending on your design requirements, piezo stack actuators can be co-fired or discrete. If it is indeed discrete (high voltage stack actuator), its composite structure is formed by stacking finished piezoelectric ceramic rings or discs with metal electrode foils respectively with adhesive. Typically, the operating voltage range is 500 to 1,000 volts.
Now, co-fired stack actuators (multi-layer) also known as monolithic stacks, do not use adhesives. However, they do use a complete electrode and ceramic stack that is high-temperature sintered, with operating voltages up to 200 volts. Both discrete and co-fired types can be isolated from extreme environments and mechanical stresses protectively by using bare stacks, coating materials or encasing them in stainless steel.
4、What is a stripe actuator?
A stripe actuator, also known as a bending actuator, is used to produce significant mechanical deflection in response to electrical signals. Therefore, compared to stack actuators, this deflection provides large travel with minimal blocking force.
This type of actuator uses two connected thin piezoelectric ceramic layers. They are typically electrically connected in parallel and in polarized direction in coincidence. Once we apply an electrical input, one ceramic layer combines, while the other ceramic layer expands, causing the actuator to bend.
5、What is a longitudinal actuator?
Longitudinal actuators, also called piezo stacks, are formed by stacking multiple piezoelectric elements on top of each other, so that the expansion effect of each element produces useful force and motion. These types of actuators use the piezoelectric effect to produce linear displacement, which ranges from 0.1-0.15% of the actuator length. These actuators have a high power density, typically in the range of 30 N/mm^2, and also have a high resonance frequency, so they can be used in dynamic applications.
6、What is a shear actuator?
These types of actuators are related to the longitudinal-type actuators, as they have multiple layers of piezoelectric elements, but they differ in the way the voltage is applied and the type of motion produced. In shear piezoelectric actuators, different elements are horizontally polarized, and the electric field can be applied orthogonally.
The displacement produced occurs in the horizontal plane, so a shear-type motion can be generated. These actuators are limited in height by bending and shear stress, although they are often combined with longitudinal actuators in multi-axis systems.
7、What is a tubular actuator?
A tubular actuator includes radial polarization and utilizes transverse piezoelectric effect to generate displacement. These types of actuators experience radial, axial, or transverse motion depending on the voltage applied relative to the electrodes. These actuators are not suitable for generating force; however, they provide micron-level travel for applications such as pumping, nanopositioning, and scanning microscopy.
8、What is a contraction actuator?
Once two components interact with each other, a planar actuator including two piezoelectric elements can produce contraction motion. These types of actuators utilize transverse piezoelectric effect to generate motion in only one direction. The displacement of the contraction piezoelectric actuator is small, up to a maximum of 20 micrometers, but it can produce a force of 100 Newtons. Once this type of actuator is mounted on a substrate or base, a bending actuator can be created. In a bending actuator, the applied voltage causes one piezoelectric element to expand while the other contracts.
9、Working principle of piezoelectric stack actuators
When piezoelectric elements are stacked, the amount of movement is amplified based on the number of individual elements in the stack. The displacement is proportional to the stack height. The most common stack actuator is 150 volts. Higher voltages result in larger displacement. However, high voltages require a lot of insulation and using a voltage higher than the specified voltage will not achieve higher performance and may damage the equipment.
Piezoelectric stack actuators are particularly useful because they can generate large forces, but the displacement is almost linear depending on the voltage. Therefore, the high force capacity of stack actuators does not affect accuracy. The large force and small displacement allow for very precise and accurate motion with nanometer-level accuracy. Force is a function of stack size and proportional to the cross-sectional area of the piezoelectric stack.
10、How to choose a piezoelectric stack actuator
Displacement
Each piezoelectric element in the stack actuator has alternating polarity and the electric field is applied parallel to the polarization direction. When voltage is applied, strain or displacement occurs in the polarization direction. The displacement of the piezoelectric element is equal to the applied voltage multiplied by the piezoelectric coefficient. (The piezoelectric coefficient d33 is related to the efficiency of the material to convert electrical energy into mechanical energy.) Since they are connected in series mechanically, the total displacement of the stack actuator is the movement of a single element multiplied by the number of elements in the stack.
The total displacement of the stack actuator is typically between 0.1% and 0.15% of the actuator length.
Voltage and electric field
Stacked piezoelectric actuators are typically categorized as low voltage (less than 200 V) or high voltage (up to 1000 V), referring to the maximum input voltage for maximum stroke. The allowable voltage size depends on the material and thickness of each element.
Therefore, the electric field increases as the layer thickness decreases. Similarly, for a given electric field, the driving voltage must decrease as the layer thickness decreases.
Force and stiffness
The stiffness of the actuator has a significant impact on force generation and is determined by the elastic modulus of the piezoelectric ceramic material, the cross-sectional area of the actuator, and its length. Although small in size, the force density of a stacked piezoelectric actuator is within the range of 30 N/mm2, allowing them to generate tens of thousands of Newtons of useful force. It is important to note that there is no current flow or power supply required for steady-state operation (no movement, constant force). Stack actuators can also maintain their position when the power supply is off and do not generate heat.
Tensile strength and preload
Stacked piezoelectric actuators typically experience both compressive and tensile forces, especially during high dynamic motion. The tensile strength of the stack actuator depends largely on the method of stacking the individual elements together and is typically lower by an order of magnitude than the compressive strength. Preloading the actuator to a value greater than the applied tensile load will ensure that the actuator always remains in compression and can operate in high dynamic, bidirectional applications. Successful integration of a stacked piezoelectric actuator requires that any force applied is only axial force and compression force. Manufacturers typically offer multiple mounting options to help prevent bending, shearing, or twisting forces.
11、What is the application of piezoelectric stack actuators?
The application of piezoelectric stack actuators is extremely wide-ranging, almost limitless. Stack actuators are widely used and preferred for mechanical operations that operate at the nanoscale to achieve high forces and extremely precise small movements. Consumer electronics, industry, automotive, medical, and aerospace industries use piezoelectric actuators to achieve precise and durable results.
Microscopic devices such as microscopes and telescopes use piezoelectric stack actuators for very fine adjustments. Many medical tools, such as ultrasound devices and optical readers, also use piezoelectric stack actuators in their manufacture.
Piezoelectric stack actuators for cars are used in a wide variety of vehicle components. Micro-adjustments are used for positioning reflectors and lenses. The force generated by piezoelectric stacks is also used to trigger hydraulic valves, fuel injectors, and pumps to open and close faster than traditional methods, enabling more precise control of the desired amount of oil or released fuel.
Industrial applications of piezoelectric stacks include industrial processing tools that require precise movement. Laser adjustment, laboratory equipment (pipettes and pumps), and semiconductor machinery all use piezoelectric stack actuators for nanometer positioning. Another common industrial application is active vibration control for wind turbines, motors, and other vibration-sensitive equipment. Piezoelectric stacks suppress adverse vibrations in mechanical structures by generating opposing motion. Inkjet printers also rely on the piezoelectric stack effect to initiate printing. When a voltage is applied, the piezoelectric stack generates enough energy to print.
12、Case Pictures of piezoelectric stack actuators from He-Shuai Company
13、What are the trends in piezoelectric actuators?
Piezoelectric actuators are forming a new field between electronics and structural ceramics. The application areas are divided into three categories: positioners, motors, and vibration suppressors. The manufacturing accuracy of optical instruments such as lasers and cameras, as well as the positioning accuracy of semiconductor chip manufacturing, is regulated by solid-state actuators, achieving an accuracy of 0.1μm. For traditional electromagnetic motors, miniaturized motors smaller than 1cm are typically required for office or factory automation equipment, and it is difficult to produce motors with sufficient efficiency. Ultrasonic motors that are insensitive to size and have good energy efficiency are performing well in the field of micro-motors. Using piezoelectric actuators to suppress vibration in space structures and military vehicles is also a promising technology.
