Basic Properties of Piezo Stack Actuators

Piezo stack actuator is a linear motor based on electrically controllable deformation of a solid body. The piezoelectric effect is understood as the linear electromechanical interaction between the mechanical and the electrical state in certain crystalline. The piezoelectric effect is a reversible electro-mechanical mechanism:

The reverse piezoelectric is deployed for actuators and is used for static positioning as well as dynamic actuation up to the production of ultrasonic sound waves.

The Key Advantages of a Piezo Stack Actuator

are to generate with elements of small size:

• —  extraordinary strong force,
• —  immediate fast reaction, and
• —  precise stroke behaviour.

The displacement of a piezo actuator is in first instance proportional to the voltage input. Forces are much higher than that of any other electrical device of comparable size such as a linear voice coil motor. It is extremely important to notice, that the force capability is associated with fairly small stroke.

Piezo Materials - From Detection of the Effect in 1880 to Intensive Material Development from the 1960s

Materials such as quartz generate a voltage when they are deformed. This sensor effect was discovered by Pierre and Jacques Curie in 1880 and is called piezo effect. They detected the effect in crystals such as quartz, tourmaline and Rochelle salt. The mechano-electric coupling mechanism is reversible: When a force is applied to a piezo the deformation of the body is associated with displacement of internal charges and a voltage is generated at the electrodes and vice versa. The inverse piezoelectric effect describes actuation, the deformation of a body in the presence of an electrical field.

Intensive material development results in the invention of ceramic piezoelectric materials in 1960s like BaTiO3 and many others. Today, a broad range of materials are available with specialized characteristics.

In terms of ceramic materials, PZT dominates economically and is the most commonly produced piezoelectric ceramics. PZT is the abbreviation for Lead zirconate titanate. The NAVY has created a widely used classification system to describe the different areas of application. In the MIL-STD-1376B standard, material groups NAVY TYPE I to NAVY VI are classified. Another classification is soft and hard PZT. Soft PZT material is most suitable for many actuator applications. Soft PZT has a larger piezoelectric constant d33, high electric permittivity. But in comparison to a hard PZT material, the soft material have some disadvantages like, higher dissipation (losses) and higher hysteresis of strain-electric field relation.   A soft PZT is the ideal material for a piezo stack actuator.

The internal electrical field exerts electrostrictive forces on the crystal structure of the piezo material and the body deforms and generate displacement. For the practical application materials with strong a strong effect is required. The right material class PZT (lead zirconate titanate) ceramics. Piezo actuators are solid-state components which are made of a special class of ceramics material. These ceramics have a perovskite structure. The actuators from Piezotechnik are high performance compositions bases on formulations of PZT, Pb (lead) -Zr (zirconium) -Ti (Titanium) mixed-oxides. The properties of the PZT material for piezo technology actuators are optimized by dopants and manufacturing process. The materials were engineered for optimized electrical behavior, high displacement, force generation, low losses and stability, life and reliability. Actuators of Piezotechnik demonstrated long life and reliability and proved to perform more than 1010 cycles with full voltage amplitude at high speed.

PZT crystal structure. Left: beyond the critical Curie temperature the crystal is symmetric. Right: The crystal is non-symmetric at temperatures lower than the the Curie temperature Tc

Piezo actuators are electrically polarized components. The analogy to piezo is a permanent magnet, which has also the property of being polarized when an external magnet field is applied. During the polarisation process the internal structure (the so-called domains) were oriented and the material obtain a predominant direction. The polarization process increases significantly the strain capability of the component. Piezo materials have physical coupling effects and such as electro-striction (deformation in electric fields) is analogue to the magneto-striction effects present in magnetic materials. 

A Plate - The Basic Piezoelectric Actuator Element

Plate made of piezo material expanded in the presence of an electric field E. The plate is covered on top and bottom faces with conductive material to form electrodes.

Piezo Actuator Stack Design

The piezo stack actuator technology is based on the expansion of piezo ceramics plates, which expands when a electrical voltage is applied. An internal electrical field is generated by the electrodes on top and bottom of the plate.  

The generation of strain for actuation requires field strength in the order of 1.000 - 2.000 Volt per mm layer thickness.  As generally low voltage is required, many thin piezo layers are stacked to create actuators. This result in a body of piezo material which is furnished with an electrode structure. A design for the electrode structure is the well-known pattern of a MLCC (multi layer ceramics capacitor). Electrodes are insulated against anti-pole electrodes inside the stack. The next figure indicates, that such insulation method result in inhomogeneous field distribution, which cause inhomogeneous field which is associated with mechanical stress concentration (see circle).

Stack design of a ceramics multilayer device. Field generation is indicated by red arrows. The electric field generation is disturbed in the insulation areas.

Optimization of the Inner And Outer Electrode Structure

One key design aspect of Piezotechnik actuators is the homogeneity of the internal field and an adequately optimized electrode structure. The applied electrical field directly produce internal mechanical stress in the ceramics. For best performance and efficiency uniform deformation of the stack is most desirable. Also, inhomogeneous field distribution cause high tensile stress components in a piezoelectric stack, which is detrimental to the mechanical integrity of ceramics body. Thus, homogeneous field distribution is of fundamental importance for performance and life of a piezo stack actuator. Piezotechnik stacks have full stack-through electrodes which cover the whole area of the active layers. The stacks have external insulation on the sides to insulate the connector which links the electrodes of the same polarity. This geometry is the only solution for an ideal homogeneous field distribution.

The preferred design is presented in the next figure. The internal  electrodes of same polarity are connected by external electrodes placed on the face of the actuator. The internal electrodes are insulated from antipode external electrodes with a thin glass layer placed between the faces of the ceramics material and the external electrodes. The insulation alternates for each pole. Thus, each electrode of common polarity are connected by a metallic strip. This result in a perfect internal electric field pattern.

Piezo actuator stack design with external insulation. Homogeneous electrical field.

The Standard Ceramic Multilayer Actuator - CMA

The benchmark solution for piezoelectric actuators is the co-fired Multilayer Actuator (CMA) technology. In the CMA production process the whole actuator stack structure including internal electrodes and piezo ceramics material is produced in one step. Electrode and ceramics raw materials are formed to a block and sintered at high temperatures to a monolithic block. This process is named co-firing. This design he best design of a piezo plate with regard to deformation and associated mechanical stress distribution is a perfect electrostatic capacitor, where two parallel electrode plates fully cover the piezo material and generate a homogenous electric field.

The advanced piezo stack comprises a pile of those piezo layers to increase the displacement performance. The electrodes cover all the layers and are open at side faces of the stack. The layers are contacted with metallic stripes at the side face where antipole electrodes are insulated.