Basic Designs of Piezoelectric Positioning Elements

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Basic Designs ... (cont.)

PZT Flexure NanoPositioners

For applications where extremely straight motion in one or more axes is needed and only nanometer deviation from the ideal trajectory can be tolerated, simple stacks, tubes and other basic actuators are not ideal because they may exhibit too much off-axis error. Furthermore, they may not provide sufficient motion in a small enough package. PI PZT Flexure NanoPositioners with passive or active trajectory control provide an excellent solution in such situations.

A flexure is a frictionless, stictionless device based on the elastic deformation (flexing) of a solid material. Sliding and rolling are entirely eliminated. In addition to absence of internal friction, flexure devices exhibit high stiffness and high load capacity. Flexures are also less sensitive to shock and vibration than other guiding systems.

Basic parallelogram flexure actuators show a second-order cross-coupling (parasitic motion) between two axes due to arcuate motion (travel is in an arc). This can lead to out-of-plane errors on the order of 0.1% of the distance traveled (see Fig. 48). The error can be estimated by the following equation:

(Equation 28)

where:

DH = Lateral runout (out-of-plane error) [m]

DL = Distance traveled [m]

H = Length of the flexures [m]

For applications where this error is intolerable, PI has designed a zero-arcuate-error multi-flexure guiding system. This special design, employed in most PI flexure stages, eliminates the cross-coupling inherent in common parallelogram guiding systems and provides flatness and straightness in the nanometer and micro-radian ranges respectively (see Fig. 49).

For applications requiring sub-nanometer and sub-µrad flatness and straightness, PI offers a system with integrated multi-axis error compensation (active trajectory control). It measures and actively controls motion in all six degrees of freedom to sub-nanometer and sub-microradian tolerances. The exceptional flatness provided by this system is shown in Fig. 50.

Examples:

P-527, P-734, see link ff. in the "PZT Flexure NanoPositioners" section.

Fig. 48. Basic parallelogram flexure guiding system with motion amplification. The amplification r (transmission ratio) is given by (a+b)/a.

Fig. 49.Zero-arcuate-error flexure guiding system.

Fig. 50. Flatness (Z) of an actively error compensated flexure stage over a 100 x 100 µm scanning range.