CLASSIFICATION OF PIEZOELECTRIC STACKS
1.1. Working Principle
Stacking piezoelectric actuators are using a shape deformation of piezoelectric ceramics under the influence of an electrical field: When a voltage of proper polarity is applied to a simple piezoceramic disc or plate, the thickness of the disc increases slightly. To get a greater expansion, a number of discs are stacked together, thereby adding the effects of the individual components. Such piezoelectric actuators (piezoelectric stacks, piezoelectric translators) make use of the axial movement of this arrangement, the maximum travel is (nearly) proportionally related to the stacklength (max. strain approx. 1-1.5%).
Common stacks are generally made from lead-zirconium-titanate (PZT) and its derivatives. The properties of piezoelectric actuators are determined by the dimensions of the stack, the layer structure and type of PZT material.
1.2.1. Discrete Stacking
The individual discs are sintered and metalized separately. The contacting of each disc during pile up is done by introducing thin metal sheets between the layers. The package is joined using high quality adhesives. The discs cannot be made too thin, so that elevated driving voltages are needed to get reasonable actuator properties: the 500 V and 1000 V actuators described below are made by discrete stacking. The main advantage of this design is the wide choice of the type of PZT-material and of shape and dimensions of the stack. At present, actuators with larger crossections are only available as discretely stacked elements.
1.2.2. Multilayer Structure
For many applications, low driving voltages of actuators are required. It is not possible to handle the necessarily thin layers (typical 0.1 mm) in the above described way. Low voltage stacks are therefore manufactured by "co-firing" technology, meaning that the complete PZT-layer/electrode structure is built up before sintering the ceramic. This compound structure is pressed and sintered as a whole (so called "monolithics"). The technological considerations of this rather complex procedure restrict at the moment the stacks to "soft" PZT material with low Curie-temperature and to maximum diameters of the stacks to about 15-20 mm.
1.3. Actuator designs
1.3.1. Bare stacks (without casing, w.o.c.)
The ceramic body of the stack is contaced by simple pigtails/leads and then coated with insulting polymers. This simplest version is often used for OEM-purposes. Because of the lack of mechanical protection, following operating conditions are important: acceptable air humidity, protection against shock/tensile stress/side impact, sufficiently high mechanical prestress etc.
1.3.2. Stacks with casing (with/without mechanical prestress)
For a lot of applications, the ceramic stacks are mounted in stainless steel casings for protections against unwanted mechanical impacts, environments influences and easier handling. Further more one type of casing includes internal mechanical prestress which is recommended for prevention of any tensile stress from the ceramic itself. Mechanical prestress allows the manufacturing of stacks with higher length/diameter ratios. The prestress force compensates externally applied tensile forces generated in a static or dynamic way, e.g. pulsed operation of actuators (avoids overshooting of the accelerated PZT masses).
For some applications, no internal prestress is necessary e.g., when no critical forces are acting on the system or when mechanical prestress is applied externally by the complete system. An example for such an application is optomechanic, where pieozactuators are mostly operated with no or only low dynamics and the mechanic is preloaded as a whole by reset springs to eliminate backlash.
1.4. Electrostrictive actuators
Beside the common PZT ceramics, there exists another type of electroactive ceramic, showing electrostriction. This material can be used in the same way for electromechanical converters as PZT ceramic, and actuators based on such a ceramic are built up and operated in the same way as PZT actuators. The main difference in application is, that electrostrictive actuators show a strongly reduced hysteresis of 1-2% compared to piezoelectric actuators. But this is only valid within a very limited temperature range of about 10o K (e.g. around room temperature) and the electrical capacitance is much higher than for PZT ceramics. Electrostrictive material is only used for very limited applications, but actuators can be supplied on request.
| Classification of piezoelectric stacks | Mounting of Piezoelectric actuators | Operating Performance of Piezoelectric actuators
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