Sensor Technology for
Nanopositioning Technology

The PIOne linear encoder is used in PI's high-resolution
nanopositioning system, such as N-664. This linear
positioner is driven by NEXACT® piezo linear motors
and, depending on the motion controller, can achieve
a position resolution of less than one nanometer at
30 mm travel range

PI offers the widest range of high-dynamics
and high-resolution nanopositioning systems
worldwide. Their linearity and repeatability
would not be possible without highest-resolution measuring devices.
Accuracies in the range of a few nanometers
and below require a position measurement
method that can also detect motion in this range. The most important specifications for selecting a suitable method are linearity, resolution(sensitivity), stability, bandwidth and, last but
not least, the costs. Another important factor isthe ability to directly record the motion of theplatform. The contact with the movable parts
also affects the measuring result; therefore, PI
uses noncontact measurement methods as far
as possible. Furthermore, the sensors need tobe small and may not heat up.
PI nanopositioning systems use three different
types of sensors: Capacitive sensors and linear
encoders for direct metrology as well as strain
gauges for indirect position measurement.

Measurements have confirmed the excellent repeatability of the piezo positioning system with capacitive
sensors with 1.4 nm (1 σ value) of standard deviation

Maximum Accuracy through Direct Metrology
When directly measuring positions with noncontact sensors, each change in position of
the moving platform is directly captured by the controller relatively to the base body.
There are no drive or guiding elements, which would affect the measurement, between
measured point and moving platform. This method allows a bandwidth in kilohertz range,
resolution in sub-nanometer range and excellent stability.

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■ Vacuum compatible down to 10-9 hPa■ Maintenance-free, no wear
■ High bandwidth up to 10 kHz■ High temperature and long-term stability(<0.1 nm/3 h)■ Invar versions for highest temperature stabi-lity (5 × 10-6/K)■ Compact one and two-electrode sensors,custom designs
Capacitive Sensors
Nanopositioning systems from PI are driven
by translation piezo actuators and have travel
ranges of a few hundred micrometers up to
one millimeter. Capacitive sensors achieve
a position resolution in the sub-nanometer
range, high stability and bandwidth, as well as
the best linearity and accuracy.
Capacitive sensors from PI determine thedistance between two plate electrodes without
contact. An active guard ring electrode generates a homogenous field in the measurement
area. This and the very precise parallel adjustment of the two electrodes guarantees the best
possible linearity of the sensor signals overthe entire measuring range. These sensors areintegrated in the nanopositioning system insuch a way that no effects on size and mass(inertia) are to be expected. With a corresponding arrangement, they directly detect themotion of the platform (direct metrology).

■ Processing electronics in various configura-tion levels, from analog OEM versions to amodular digital controller system that can beexpanded at any timec = ε0 · εr ·Ad

Operating principle of a capacitive sensor. The capacitance C is proportional to the active
sensor area A, d is the measuring range (distance from sensor to target surface), ε0 isconstant, εr is the dielectric constant of the material between the plates, generally air.
The measured quantity is the change in capacitance of the electric field. An active guard
ring electrode generates a homogenous field in the measurement area

PISeca single-plate capacitive sensors measure against all kinds of conductive surfaces
and are easier to handle mechanically, forexample during installation or cable routing.
Their employment is also more versatile, e.g.
for detecting motions perpendicular to thedirection of measurement. The quality of thesensor signals, however, strongly depends on
the parallelism and condition of the surfacemeasured.
■ Noncontact absolute measurement ofdistance, motion and vibration■ Measuring ranges from a few 10 μm to2 mm feasible■ Only minimum heating, no scattered light■ Direct metrology: Direct position measure-ment of moving objects

PISeca single-electrode capacitive sensors measure against all kinds of conductive surfaces and are easier to handle mechanically, for example during installation or cablerouting. Their employment is also more versatile, e.g. for detecting motions perpendicular to the direction of measurement. The E-852 stand-alone processing electronics forPISeca only shows minimum noise and integrates a linearization system. All systems
are calibrated at PI and optimized for the intended bandwidth and the measuring range

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PIOne Linear Encoders:
Small with Picometer Resolution
The capacitive measuring systems reach theirlimits with larger travel ranges: Either sensor
areas become larger or resolution and linearity deteriorate. Nanopositioning systemwith piezomotors, which have travel rangesof several 10 millimeters, use linear encoders
as posi tion sensors. These are incremental
measuring systems that consist of a scale andsensor head. The high-resolution linear sensor
PIOne ensures a position resolution of far lessthan a nanometer with adequate processing ofthe measurement.

The sensor head of the PIOne contains a MachZehnder interferometer, which is moved along
a linear scale. Sine and cosine signals aregenerated from the signals of the reflections
at the grid. Additional interpolation accounts
for the demonstrably small resolution of thesystem. The sensor head also generates adirection-sensing reference. The sensor headhere measures 23 x 12 x 9.5 mm³. PIOne uses
a patented technology.

■ Resolution to 20 picometer RMS;

0.12 nanometer peak-to-peak■ Velocities up to 0.5 m/s at maximum

resolution■ Compact dimensions

23 mm × 12 mm × 9.5 mm■ Sine, cosine or quadrature output signals■ Low power consumption and low heatdissipation■ Bakeout temperature up to 80°C

Noise measurement of a positioning system with the PIOne at 1 MHz bandwidth and 18-bit resolution of the sensor input: 16 picometer RMS and 100 picometer peak-to-peak

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Indirect Position Measurementwith Strain Gauge Sensors
Strain gauge sensors consist of a thin metal(SGS) or semiconductor foil (piezo-resistant,PRS), which is attached to the piezoceramics
or, for improved precision, to the guiding
system of a flexure stage. This type of position measurement is done with contact andindirectly, since the position of the moving

platform is derived from a measurement on
the lever, guide or piezo stack. Strain gauge
sensors derive the position information fromtheir expansion. Full-bridge circuits with several strain gauge sensors per axis improve thermal stability.

Sensor type

Sensitivity/
Resolution*

Linearity*

Stability/
Repeatability

Bandwidth*

Measurement
methodMeasurement
range

Capacitive

excellent

excellent

excellent

excellent

direct / noncontact<2 mm

Strain gauge
sensors made ofmetal foil (SGS)very goodvery good

goodvery goodindirect / withcontact<2 mm

Piezoresistive
strain gauge sensors (PRS)

excellent

goodaveragevery goodindirect / withcontact<1 mm

Linear encoders

excellentvery good

excellentvery good

direct / noncontact

up to >100 mm* The classifications refer to the characteristics of the nanopositioning system. The information on resolution, linearity and repeatability in therespective data sheet reflects the specifications of the overall system, including controller, mechanical system and sensor. They are tested using
external measuring instruments (Zygo interferometer)

Direct Parallel Metrology: Multi-Axis
Measurements using a Fixed Reference
A multi-axis stage design with parallel kinematics allows you to use direct parallel metrology, measuring all degrees of freedom of themoving platform in relation to a fixed reference. Unintended crosstalk of the motion intoa different axis, e.g. as a result of an externalforce, can thus be detected and actively corrected in real time. This so-called active guiding can keep the deviation from the trajectory
down to a few nanometers, even in dynamicoperation.

Parallel-kinematic nanopositioning system with capacitive sensors, parallel-metrology
arrangement and reduced inertia. The arrows show the signal flow from the sensor to theclosed-loop control. Red: X axis, blue: Y axis

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|POSITIONING

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TEC67 Sensor Technology for Nanopositioning Technology 10/2017 Subject to change without notice. © Physik Instrumente (PI) GmbH & Co. KG 2017

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