ff. ). Example: Piezoelectrically driv-en fiber aligners and trackingsystems derive the control sig-
nal from an optical industrial power meter in the system. There, the
goal is to maximize the optical
signal level as quickly as possi-
ble, not to attain a predeter-
mined position value. An open-loop piezo system is sufficientfor such applications. Ad-
vantages like unlimited resolu-
tion, fast response, zero back-
lash and zero stick/slip effect
are most welcome, even with-out position control.
Piezo
•
Nano
•
Positioning
Note: PI piezo actuators and stagesare designed for high reliability
in industrial applications. The
travel, voltage and load ranges
in the technical data tables can
actually be used in practice.
They have been collected over
many years of experience in
piezo actuator production andin numerous industrial applica-tions.In contrast to many other piezosuppliers, PI has its own piezo ceramic development and pro-duction facilities together withthe necessary equipment and
knowhow. The goal is always
reliability and practical useful-
ness. Maximizing isolated
parameters, such as expansionor stiffness, at the cost of piezolifetime might be interesting to
an experimenter, but has no
place in practical application. When selecting a suitablepiezo actuator or stage, consid-er carefully the fact that “maxi-
mum travel” may not be the
only critical design parameter.
Piezo Actuators Piezo Actuators
Hysteresis (Open-Loop Piezo Operation)
Nanopositioning &
Scanning Systems Nanopositioning &
Scanning Systems
Hysteresis is observable inopen-loop operation; it can be
reduced by charge control andvirtually eliminated by closed-
15 mode.For example, if the drive volt-age of a 50 µm piezo actuatoris changed by 10 %, (equiva-
lent to about 5 µm displace-
ment) the position repeatabili-
ty is industrial still on the order of 1 % of
full travel or better than 1 µm. The smaller the move, thesmaller the uncertainty.Hysteresis must not be con-
fused with the backlash of con-
ventional mechanics. Backlash
is virtually independent of trav-
el, so its relative importanceincreases for smaller moves. For tasks where it is not theabsolute position that counts,
hysteresis is of secondary
importance and open-loop
actuators can be used, even ifhigh resolution is required. In closed-loop piezo actuatorsystems hysteresis is fully
compensated. PI offers these
systems for applications re-
quiring absolute position infor-mation, as well as motion with
high linearity, repeatability and
accuracy in the nanometer and
sub-nanometer range (see pp.
4-31
Active Optics / Steering Mirrors Active Optics / Steering Mirrors
Tutorial: Piezo-
electrics in Positioning Tutorial: Piezo-
electrics in Positioning Capacitive
Industrial 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
Industrial motion controllers Motion Controllers Ceramic
Linear motors & Stages Ceramic Linear Motors & Stages Index Index
L loop operation (see pp. 4-31 ff. ). Open-loop piezo actuatorsexhibit hysteresis in their
dielectric and electromagnetic
large-signal behavior. Hys-
teresis is based on crystalline
polarization effects and molec-ular effects within the piezo-electric material.
The amount of hysteresis
increases with increasing volt-
age (field strength) applied to
the actuator. The “gap” in the
voltage/displacement curve(see Fig. 11) typically beginsaround 2 % (small-signal) and
widens to a maximum of 10 %to 15 % under large-signal con-
ditions. The highest values are
attainable with shear actuators
in d
V Fig. 11. Hysteresis curves of an open-loop piezo actuator for variouspeak voltages. The hysteresis is related to the distance moved, not
to the nominal travel range. 4-17