Text version of the page 
Text version of the page 
Where:
U Where:
t = time to charge piezoto U Piezo • Nano • Positioning Dynamic Operation(Switched) Example: To determinewhether a selected amplifier
can drive a given piezo actua-
tor at 50 Hz with 30 µm peak-to-peak displacement, multiplythe actuator’s DOCC by 50 x 30
and compare the result with
the average output current of
the selected amplifier. If the
current required is less than orequal to the amplifier output,then the amplifier has suffi-
cient capacity for the applica-
tion. For applications such as shockwave generation or valve con-
trol, switched operation (on/
off) may be sufficient. Piezo actuators can provide motion
with rapid rise and fall times
with accelerations in the thou-sands of g’s. For informationon estimating the forces
involved, see “DynamicForces,” p. 4-24).The simplest form of binarydrive electronics for piezo
applications would consist of a
large capacitor that is slowlycharged and rapidly dis-charged across the PZT ceram-
ics.The following equation relatesapplied voltage (which corre-
sponds to displacement) to
time.(Equation 21)Voltage on the piezo afterswitching event. Dynamic Operating CurrentCoefficient (DOCC) Piezo Actuators Piezo Actuators Instead of calculating therequired drive power for a
given application, it is easier to
calculate the drive current,
because it increases linearly
with both frequency and volt-
age (displacement). For this
purpose, the DynamicOperating Current Coefficient(DOCC) has been introduced.
The DOCC is the current that
must be supplied by the ampli-
fier to drive the piezo actuator
per unit frequency (Hz) and
unit displacement. DOCC val-ues are valid for sinewaveoperation in open-loop mode.
In closed-loop operation the
current requirement can be up
to 50% higher.The peak and long-term aver-age current capacities of the
different piezo amplifiers canbe found in the technical datatables for the electronics, the
DOCC values in the tables for
the piezo actuators. Nanopositioning &
Scanning Systems Nanopositioning &
Scanning Systems 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 Hexapods /
Micropositioning Hexapods /
Micropositioning Photonics Alignment
Solutions Photonics Alignment
Solutions Motion controllers Motion Controllers Ceramic Linear motors & Stages Ceramic Linear Motors & Stages Index Index = start voltage [V]U p-p [s]C = piezo actuator capaci-tance [F]U p-p = source output voltage(peak-to-peak) [V]R = source output resist-ance [ohm]C = piezo actuator capaci-tance [F]t = time [s] p-p = voltage change(peak-to-peak) [V]i max = peak amplifiersource/sink current [A] For fastest settling, switched
operation is not the best solu-tion because of the resultingovershoot. Modern techniques The voltage rises or falls expo-nentially with the RC time con-
stant. Under quasi-static condi-
tions, the expansion of the PZT
ceramics is proportional to the
voltage. In reality, dynamicpiezo processes cannot bedescribed by a simple equa-
tion. If the drive voltage rises
too quickly, resonance occurs,
causing ringing and overshoot.
Furthermore, whenever thepiezo actuator expands or con-tracts, dynamic forces act on
the ceramic material. These
forces generate a (positive or
negative) voltage in the piezo
element which is superim-posed on the drive voltage. A
piezo actuator can reach itsnominal displacement in
approximately 30 % of the peri-
od of the resonant frequency,
provided the controller can
deliver the necessary current.
(see p. 4-26). The following equation appliesfor constant-current charging
(as with a linear amplifier):(Equation 22)Time to charge a piezoceramicwith constant current. With
lower-capacity electronics,amplifier slew rate can be alimiting factor. like InputShaping ® (see p. 4-33) solve the problem of reso-
nances in and around the actu-ator with complex signal pro-cessing algorithms. Note Piezo drives are becomingmore and more popular
because they can deliver
extremely high accelerations.
This property is very important
in applications such as beam
steering and optics stabiliza-tion. Often, however, the actua-tors can accelerate faster than
the mechanics they drive can 4-29
|