Parallel and Serial
Metrology In multi-axis positioning sys-
Piezo • Nano • Positioning Act u ators an d Sensors
Metrology for Nanopositioning Systems
ff. There are two basic techniquesfor determining the position of
piezoelectric motion systems:
Direct metrology and indirect
metrology. Indirect (Inferred) Metrology Indirect metrology involvesinferring the position of the
platform by measuring posi-
tion or deformation at the actu-
ator or other component in the
drive train. Motion inaccuracieswhich arise between the driveand the platform can not be
accounted for. Direct Metrology
Piezo actuators With direct metrology, howev-er, motion is measured at the
point of interest; this can bedone, for example, with aninterferometer or
capacitive sensor.
Direct metrology is more accu-
rate and thus better suited to
applications which need
absolute position measure-
ments. Direct metrology also
eliminates phase shiftsbetween the measuring pointand the point of interest. This
difference is apparent in high-
er-load, multi-axis dynamic
applications.
Nanopositioning &
Scanning Systems Active Optics / Steering Mirrors Tutorial: Piezo-
electrics in Positioning Capacitive Industrial position sensors Piezo Drivers & Nano-
positioning controllers Hexapods /
Micropositioning Photonics Alignment
Solutions tems parallel and serial metrolo-gy
Industrial motion controllers must also be distinguished.
With parallel metrology, allsensors measure the position
of the same moving platformagainst the same stationary ref-erence. This means that all
motion is inside the servo-loop,
no matter which actuator
caused it (see Active Trajectory
Control). Parallel metrology
and parallel kinematics can be
easily integrated.With serial metrology the refer-ence plane of one or more sen-
sors is moved by one or more
actuators. Because the off-axis
motion of any moving refer-
ence plane is never measured,it can not be compensated. See also p. 2-5
Ceramic Linear motors & Stages Index High-Resolution Sensors
Bandwidth: to 5 kHz Strain Gauge Sensors Advantages SGS sensors are an implemen-tation of inferred metrology
and are typically chosen for
cost-sensitive applications. An
SGS sensor consists of a resis-tive film bonded to the piezostack or a
guidance element;
the film resistance changes
when strain occurs. Up to four
strain gauges (the actual con-
figuration varies with the actu-ator construction) form aWheatstone bridge driven by a
DC voltage (5 to 10 V). When
the bridge resistance changes,
the sensor electronics converts
the resulting voltage change
into a signal proportional to thedisplacement.A special type of SGS is knownas a piezoresistive sensor. Ithas good sensitivity, butmediocre linearity and temper- High Bandwidth Vacuum Compatible Highly Compact Other characteristics: Low heat generation (0.01 to0.05 W sensor excitationpower) Long-term position stabilitydepends on adhesive quality Indirect metrology ff. ). Examples
Fig. 13. Strain gauge sensors. Paperclip for size comparison. Most PI LVPZT and HVPZT actu-ators are available with strain
gauge sensors for closed-loop
control (see the “Piezo
Actuators” section p. 1-8 Note The sensor bandwidth for thesensors described here shouldnot be confused with the band-
width of the piezo mechanics
servo-control loop, which is
further limited by the electronic
and mechanical properties of
the system. ature stability. See also p. 2-5 ff . Resolution: better than 1 nm(for short travel ranges, up to
about 15 µm)
4-19 4-19