Piezo actuators are unique in terms of dynamics, force and precision. They are the ideal solution for the construction of unique scientific instruments and high-precision positioning systems for optics and machines.
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Piezo actuators are linear motors based on the electrically controllable deformation of a solid. The piezoelectric effect is the linear electromechanical interaction between the mechanical and electrical states in certain crystals. The piezoelectric effect is a reversible electromechanical mechanism. The direct piezoelectric effect describes the generation of electrical charge due to an applied mechanical force.
The reverse piezoelectric effect is the generation of an internal mechanical voltage due to the effect of an electric field that is impressed on the piezo material. A piezo material is always an electrical insulator. The inverse effect is used for piezoelectric actuators. Piezo actuators offer decisive advantages: they can generate ultrasonic waves, position objects and even actuate them dynamically.
The displacement of a piezo actuator is primarily proportional to the applied electrical voltage. The usable actuating forces are much greater than those of any other electrical actuator of comparable size. A decisive feature is the comparatively small stroke, which goes hand in hand with an enormously high force capacity.
The most important advantages of a piezo actuator are the ability to generate:
exceptionally high forces
an immediate reaction
precise, high-resolution positioning movements
Piezoeletric Materials
Piezo actuators are linear motors based on the electrically controllable deformation of a solid. The piezoelectric effect is the linear electromechanical interaction between the mechanical and electrical states in certain crystals. The piezoelectric effect is a reversible electromechanical mechanism. The direct piezoelectric effect describes the generation of electrical charge due to an applied mechanical force.
The pair of electrodes on the plates generates an electric field that exerts electrostrictive forces on the crystal structure inside the piezo material. The body deforms and generates a displacement that can be used as a stroke movement for technical applications. Materials with a particularly strong effect have been developed and selected for practical applications. The preferred material class for piezoelectric actuator materials is PZT (lead zirconate titanate) ceramics. The development of piezo materials for actuators focuses on high displacement, force generation, low losses, stability and service life.Piezo actuators are electrically polarized components.
The analogy to a piezo is the well-known permanent magnet. Both have the property that the material can be polarized in a field. During the polarization process, the inner magnetic microstructure (the domains) is aligned in the magnet and the electrical microstructure in the piezo. This gives the materials a preferred direction. The polarization process establishes the piezoelectric function of the component. The physical coupling effects inherent in piezo materials (electrostriction, deformation in the electric field) are identical to the magnetostriction effects present in magnetic materials. As is known for magnets, the piezoelectric property disappears when heated above the Curie temperature, a characteristic of the respective material.
Piezo Actuator Stack Design
Piezo stack actuators are manufactured by stacking piezoceramic plates with interposed electrodes. The stack of plates is joined by gluing or, in the case of modern stacks, by sintering the plates in the green state. As soon as an electrical voltage is applied, an electric field is created in all the plates. The stroke of the actuator is the sum of the deformations of all the individual plates.
A field strength of 1000 to 2000 volts per mm of layer thickness is required for the full expansion of the material. Since a low voltage in the range of 100 volts is generally preferred, plates thinner than 100 micrometers are used. A common concept for the electrode structure is the multi-layer ceramic capacitor (MLCC). The individual electrodes are electrically insulated from the opposing electrodes within the stack. The illustration clearly shows that this insulation structure leads to an inhomogeneous field distribution. The piezo effect leads to mechanical stress concentrations resulting from the inhomogeneous field (see circle).
Optimised Electrode Structure
The homogeneity of the internal electric field and the resulting mechanical stresses is the most important design aspect of piezo actuators. The design of the electrode structure plays a decisive role here. An ideal electrode covers the entire surface of the active layers. In this concept, the isolation of opposite-pole electrode surfaces from the collecting electrodes takes place on the outer surfaces of the stack. The ideal electron geometry is the perfect solution, as it generates an ideally homogeneous field distribution within the piezoelectric layers. The following illustration shows how the electrodes of common polarity (red, black) are connected by a metal strip on the outer surfaces of the actuator and are electrically insulated from the opposite-polarity electrode.
Die Homogenität des inneren elektrischen Feldes und der daraus folgenden mechanischen Spannungen ist der wichtigste Designaspekt von Piezoaktuatoren. Die Gestaltung der Elektrodenstruktur spielt dabei eine entscheidende Rolle. Eine ideale Elektrode bedeckt vollflächig die aktiven Schichten. Die Isolation gegenpoliger Elektrodenflächen gegenüber den Sammelelektroden erfolgt bei diesem Konzept an den Außenflächen des Stapels. Die idealtypische Elektronengeometrie ist die perfekte Lösung, denn sie erzeugt innerhalb der piezoelektrischen Schichten eine ideal homogene Feldverteilung. Die nachfolgende Abbildung zeigt, wie die Elektroden gemeinsamer Polarität (rot, schwarz) durch einen Metallstreifen an den Außenflächen des Aktuators verbunden sind und gegenüber der gegenpoligen Elektrode elektrisch isoliert sind.
Co-Fired Multilayer Actuator (CMA) with external Insulation.
The benchmark solution for piezo actuators is the technology of the multilayer "co-firing" process (CMA). In the CMA production process, a stack of "green" ceramic foils and thin layers of electrode material are sintered in a furnace at high temperatures to form a monolithic block (co-firing). The longevity and reliability of the actuators is exceptionally high. We have demonstrated more than 10 billion cycles at full voltage amplitude.
Read about other topics:
Constitutive Equations of Piezoelectricity
In the linear theory of the piezoelectric effect, a description of the small-signal behavior was developed (for further information see * 1 * 2). This theory does not include...
The real piezo actuator deviates significantly from a linear behavior in large-signal operation and the expansion shows a pronounced hysteresis behavior. There...
Piezoelectric actuators exhibit electrical behavior similar to capacitors. This behavior is overlaid with the piezoelectric effect, which couples mechanical and electrical...
There are two fundamental designs of piezoelectric actuators: I) Direct actuation with stacks. The most advancedadvanced type of piezo stacks is the...
The linear constitutive equation of piezoelectricity describes a mechanically linear-elastic material with the superimposed effect of an electrically induced...
This class of materials allows for the straightforward implementation of functions within load-bearing mechanical structures. While in artifacts such as machines or...
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