Piezo Act u ator Electrical F

PZT ceramics are (reactive)capacitive loads and therefore require charge and discharge currents that increase with operating frequency. The ther- mal active power, P (apparent power x power factor, cos

u n d a m entals

follow. Rapid actuation ofnanomechanisms can causerecoil-generated ringing of the actuator and any adjacent com- ponents. The time required for this ringing to damp out can be many times longer than the move itself. In time-critical industrial nanopositioning applications, this problemobviously grows more seriousas motion throughputs increase and resolution requirements tighten.Classical servo-control tech-niques cannot solve this prob-lem, especially when reso-nances occur outside the servo-loop such as when ring- ing is excited in a sample on a fast piezo scanning stage as it reverses direction. A solution isoften sought in reducing thescanning rate, thereby sacrific- ing part of the advantage of a piezo drive. A patented real-time feedfor-ward technology calledInputShaping ),generated in the actuator dur- ing harmonic excitation can be estimated with the following equation:(Equation 23)Heat generation in a piezo actu-ator. Where:P = power converted toheat [W]tan = dielectric factor ( power factor, cos , for smallangles
® and ) f = operating frequency[Hz] C = actuator capacitance[F]U nullifies reso-nances both inside and outsidethe servo-loop and thus elimi- nates the settling phase. For more information see p. 4-33or visit www.Convolve.com.
® actu- ators are also quite well suited.With their high Curie tempera- ture of 320 °C, they can be operated with internal temper-atures of up to 150 °C.
p-p = voltage (peak-to-peak) For the description of the losspower, we use the loss factor tan instead of the power fac-tor cos , because it is themore common parameter forcharacterizing dielectric materi-als. For standard actuator piezoceramics under small-sig- nal conditions the loss factor is on the order of 0. 1 to 0. 2. Thismeans that up to 2 % of theelectrical “power” flowingthrough the actuator is convert- ed into heat. In large-signal conditions however, 8 to 12 % of the electrical power pumped into the actuator is converted to heat (varies with frequency, temperature, amplitude etc.).Therefore, maximum operatingtemperature can limit the piezoactuator dynamics. For large amplitudes and high frequen- cies, cooling measures may be necessary. A temperature sen- sor mounted on the ceramics is suggested for monitoring pur- poses. For higher frequency operationof high-load actuators with high capacitance (such as PICA™-Power actuators, see p. 1-20), a special amplifiers employing energy recovery technology has been devel- oped. Instead of dissipating thereactive power at the heatsinks, only the active power used by the piezo actuator has to be delivered. The energy not used in theactuator is returned to theamplifier and reused, as shown in the block diagram in Fig. 26. The combination of low-loss, high-energy piezoceramics and amplifiers with energy recov- ery are the key to new high-level dynamic piezo actuatorapplications. For dynamic applications withlow to medium loads, the newly developed PICMA
Fig. 26. Block diagram of an amplifier with energy recovery forhigher frequency applications. © PI 1998-2005. Subject to change w/o notice. Cat 118 05/09.17 4-30