Basic Introduction... (cont.)
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Different Piezo Actuator Designs to Suit Various Applications
Stack Actuators (Translators)
The most common form of piezo actuator is a stack of ceramic layers with two electrical leads. To protect the ceramic against external influences, a metal case is often placed around it. This case may also contain built-in springs to compress the ceramic to allow both push and pull operation.
The P-845 closed-loop LVPZT translator (Fig. 2) is one example of a low voltage translator with internal spring preload and integrated high-resolution strain gauge position sensor. This translator is available with displacements up to 90 microns. It can handle loads up to 300 kilograms and withstand pulling forces to 700 N (see the "PZT Actuators" section for details). Applications include vibration cancellation, shock wave generation and machine tool positioning for fabrication of non-spherical contact lens surfaces.
PI offers PZT stack translators with travel ranges from a few microns for small designs to as much as 200 microns for 200 mm long units. In some applications, space restrictions do not allow for such long stacks. In these cases, it is possible to use mechanical lever amplifiers to decrease the length of the ceramic stack. The increase in travel gained with a mechanical amplifier reduces the actuator's stiffness and maximum operating frequency.
Other Basic Actuators
Apart from stack translators, a number of other basic PZT actuators are available: bender actuators providing long travel (millimeter range), contraction actuators, tube actuators, shear actuators etc. See "Basic Designs," see link ff for more information.
Actuators with Motion Amplifiers & Trajectory Control
In some applications a stack actuator alone is not enough to perform complex tasks. For example, when straight motion is needed and only nanometer deviation from the ideal trajectory can be tolerated, a stack translator cannot be used because it may tilt as much as a few tens of arcseconds while expanding. If the stack and the part to be moved are decoupled and a precision guiding system is employed, exceptional trajectory control can be achieved. The best guiding precision can be achieved with flexures.
Fig. 3 shows one example of a piezoelectrically driven miniature flexure stage with integrated flexure guiding system and motion amplifier. The stage is made of stainless steel and all flexures are wire EDM (electrical discharge machining) cut. The flexures are computer designed by an FEA (Finite Element Analysis) program. The central part of the stage can move +/- 40 micrometers along one axis. The movement is accomplished by an integrated 3:1 lever, driven by a PZT stack pushing a spherical tip built into to the lever. The resonant frequency of the unloaded stage is 1 kHz (high when the lever amplification is considered).
The lever is connected to the platform by a flat spring which is very stiff in the push/pull direction but flexible in the lateral direction. This flexibility ensures straight stage motion with minimum tilt and lateraldeviation. The system runout and flatness are in the nanometer realm and even this low figure can be reduced with a larger flexure base.Sub-nanometer, sub-microradian flatness can be achieved with multi-axis systems using active error compensation (see the "PZT Flexure NanoPositioners" section for details). The flexure design is not limited to single-axis stages; systems with up to six degrees of freedom are available.
Single- and multi-axis flexure positioners are used in research, laboratory and industrial applications. Examples are disk drive testing, mask aligners for X-Ray steppers, adaptive optics, precision machining, fiber aligners, scanning microscopy, autofocus systems for surface profilers and hydraulic servo valves.
Piezo Actuators Combined with Motorized Long-Travel Positioning Systems
Piezo actuators can be combined with other actuators to form long-travel, high-resolution systems. A good example is the combination of a P-250 piezo actuator with a closed-loop motor-driven leadscrew (Fig. 4). This combination provides 25 mm coarse range (versions with 50 mm are available) but preserves the high-resolution characteristics intrinsic to PZTs. Coarse motion is provided by a micrometer with a non-rotating tip driven by a DC motor/encoder/gearhead unit capable of < 0.1 Ám resolution. A short PZT stack providing sub-nanometer resolution is mounted inside the micrometer tip. Both piezo and DC motor can be computer controlled.
Fig. 3. P-780 PZT Flexure NanoPositioner and scanner with integrated motion amplifier.
Fig. 2. P-845 Closed-loop LVPZT translator