ff. Notes The smooth motion in the sub-nanometer range shown in Fig. 8 can only be attained by frictionless and stictionless solid state actuators and guid-ance such as piezo actuatorsand flexures. “Traditional” technologies used in motion positioners (stepper or DC servo-motor drives in combina- tion with dovetail slides, ballbearings, and roller bearings)all have excessive amounts of friction and stiction. This fun- Important: There are no inter-national standards for defining these specifications. This means that claims of different manufacturers can not neces- sarily be compared directlywith one another. Since the displacement of apiezo actuator is based on ionicshift and orientation of the PZT unit cells, the resolution depends on the electrical field applied. Resolution is theoreti- cally unlimited. Because thereare no threshold voltages, thestability of the voltage source is critical; noise even in the µV range causes position changes. When driven with a low-noise amplifier, piezo actuators can be used in tunneling and atom-ic force microscopes providingsmooth, continuous motion with sub-atomic resolution (see Fig. 8). Amplifier Noise One factor determining theposition stability (resolution) of a piezo actuator is noise in the drive voltage. Specifying the noise value of the piezo driverelectronics in millivolts, how-ever, is of little practical use without spectral information. If the noise occurs in a frequency band far beyond the resonant frequency of the mechanical system, its influence onmechanical resolution and sta- bility can be neglected.If itcoincides with the resonant fre- quency, it will have a far more significant influence on the sys- tem stability.Therefore, meaningful infor-mation about the stability and resolution of a piezo position- ing system can only be acquired if the resolution of the complete system—piezo actua- tor and drive electronics—ismeasured in terms of nanome-ters rather than millivolts. For further information see p. 2-8 and p. 4-31

Piezo

•

Nano

•

Positioning

Notes The piezoelectric coefficientsdescribed here are often pre- sented as constants. It should be clearly understood that their values are not invariable. The coefficients describe material properties under small-signalconditions only. They vary withtemperature, pressure, electric field, form factor, mechanical and electrical boundary condi- tions, etc. Compound compo- nents, such as piezo stack actu- ators, let alone preloaded actu-ators or lever-amplified sys-tems, cannot be described suf- ficiently by these material parameters alone. This is why each component or systemmanufactured by PI is accom- panied by specific data such as stiffness, load capacity, dis- placement, resonant frequen- cy, etc., determined by individ- ual measurements. The param- eters describing these systemsare to be found in the technicaldata table for the product.

Piezo Actuators Piezo Actuators Nanopositioning & Scanning Systems Nanopositioning & Scanning Systems
Polarisation Active Optics / Steering Mirrors Active Optics / Steering Mirrors
Tutorial: Piezo- electrics in Positioning Tutorial: Piezo- electrics in Positioning Capacitive Position sensors Capacitive Position Sensors Piezo Drivers & Nano- positioning controllers Piezo Drivers & Nano- positioning Controllers
Fig. 7. Orthogonal system describing theproperties of a poled piezoelectric ceramic.Axis 3 is the poling direction. Hexapods / Micropositioning Hexapods / Micropositioning Photonics Alignment Solutions Photonics Alignment Solutions Motion controllers Motion Controllers Ceramic Linear motors & Stages Ceramic Linear Motors & Stages

Resolution

Index Index

damental property limits reso-lution, causes wobble, hystere-sis, backlash, and an uncertain-ty in position repeatability. Their practical usefulness is thus limited to a precision of several orders of magnitude below that obtainable with PIpiezo nanopositioners.

Fig. 8. Smooth response of a P-170 HVPZT translator to a 1 V, 200 Hz triangu-lar drive signal. Note that one division is only 2 nanometers.

4-15