Piezo Act u ator Electrical F u n d a m entals

Electrical Requirements for Piezo Operation

General When operated well below theresonant frequency, a piezoactuator behaves as a capaci- tor: The actuator displacement is proportional to stored charge (first order estimate). The capacitance of the actuator depends on the area and thick- ness of the ceramic, as well as on its material properties. Forpiezo stack actuators, whichare assembled with thin, lami- nar wafers of electroactive ceramic material electrically connected in parallel, the capacitance also depends on the number of layers.The small-signal capacitance ofa stack actuator can be estimat- ed by:(Equation 14)

Piezo actuators Piezo Actuators

Where:C= capacitance [F (As/V)]n = number of layers =

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Every change in the charge (and therefore in displacement)of the PZT ceramics requires acurrent i:(Equation 15)Relationship of current andvoltage for the piezo actuator

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Where:i= current [A]Q = charge [coulomb (As)] C = capacitance [F] U = voltage [V]t = time [s] For static operation, only theleakage current need be sup- plied. The high internal resist- ance reduces leakage currents to the micro-amp range or less.Even when suddenly discon-nected from the electrical source, the charged actuator will not make a sudden move, but return to its uncharged dimensions very slowly. For slow position changes,only very low current isrequired. Example: An amplifier with anoutput current of 20 µA can fully expand a 20 nF actuator in one second. Suitable amplifierscan be found using the “Con- trol Electronics Selection Guide” on p. 6-8.

33T

= dielectric constant[As/Vm]A = industrial electrode surface areaof a single layer [m

]

d

= distance between theindividual electrodes (layer-thickness) [m] I

= actuator length The equation shows that for agiven actuator length, thecapacitance increases with thesquare of the number of layers. Therefore, the capacitance of a piezo actuator constructed of 100 µm thick layers is 100 times the capacitance of an actuator with 1 mm layers, if the twoactuators have the samedimensions. Although the actu- ator with thinner layers draws 100 times as much current, thepower requirements of the twoactuators in this example areabout the same. The PI high- voltage and low-voltage ampli- fiers in this catalog are de- signed to meet the require- ments of the respective actua- tor types. Fig. 25. Design of a piezo stack actuator.

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