WHITEPAPER – Hybrid Nanopositioning Systems with Piezo Actuators-- Oliver Dietzel --Hybrid Nanopositioning Systems with Piezo Actuators
Long Travel Ranges, Heavy Loads, and Exact Positioning

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

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WHITEPAPER – Hybrid Nanopositioning Systems with Piezo Actuators-- Oliver Dietzel --1 Introduction
Hybrid drives combine two different drive concepts into ahigh-performance and precision positioning system. They
take advantage of both drives. These types of solutions arealways in demand when one drive alone is not in a position tomeet all of the requirements of applications. An example ofthis is nanometer-precision positioning of heavy loads overlong travel ranges. Nanopositioning systems that combinepiezo drives with classical drive screws therefore offer apractical solution, but also other drive concepts are possible.

Linear motors offer long travel ranges with high positioning
accuracy and repeatability, but do have the disadvantage ofhigh friction in the linear guides used for taking up the load.

The examples of applications for hybrid drives range fromsemiconductor manufacturing through quality assurance andoptical inspection to biomedical engineering.

3 Hybrid Concept with Piezo
Walking Drive and Piezo
Actuator

2 Hybrid Concept with Electric
Motor and Voice CoilNevertheless, voice coil actuators do provide precision
positioning as well as good force control and smooth motion.
However, their travel range is limited to 100 mm and they can
only hold loads at rest when they are powered on.

PI has already successfully developed and established afurther hybrid drive concept for other applications such as
those for inspecting semiconductors.

Various concepts have already been developed andimplemented for hybrid positioning systems. One solution for
example, is a combination of linear motors and voice coil
actuators (see Fig. 1).

Fig. 2

Fig. 1Hybrid concept with electric motor and voice coil actuator(image: PI)

Hybrid concept with piezo walking drive and piezo actuator(image: PI)

A PiezoWalk® drive that offers long travel ranges with highstiffness, is combined with a PICMA® actuator for highdynamics applications (see Fig. 2). The advantage in this caseis, depending on the design of the PiezoWalk® drive, thepossibility of realizing high holding forces. In this case, thehigh dynamics of the PICMA® drive is important for fast
focusing of the object to be scanned.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

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WHITEPAPER – Hybrid Nanopositioning Systems with Piezo Actuators-- Oliver Dietzel --4 Hybrid Concept with Electric
Motor and Piezo Actuator
PI is working on a project together with engineers andastronomers from the European Southern Observatory (ESO),
which will result in the world's largest terrestrial telescope on
the 3000-meter-high Cerro Amazones in the Atacama desert
in Chile (see Fig. 3).

Fig. 5

Conceptual design of the M1 Segment Subunit (Image: ESO)

Considerable masses must be moved for this: Each mirror
segment including its support weighs approximately 250 kg.
Due to the different alignments of the telescope, a total of
2,394 actuators need to be able to move and hold loads withpush/pull forces between 463 N and 1050 N (see Fig. 5).

Fig. 3The European Extremely Large Telescope (ELT) will be the largest
terrestrial telescope for scientific evaluation of electromagnetic
radiation in the visible and near-infrared wavelength range(image: ESO)One of the most important tasks of the telescope will be tosupply the sharpest possible images of the universe, which
researchers can use to search for Exoplanets, i.e., planets thatexist beyond the solar system. Therefore, the greatest
technical challenge will be to move the mirror segments overthe entire range with a maximum position deviation of 2 nm.

The telescope's revolutionary design consists of a main mirror(M1) with a diameter of 39 meters, which is divided into 798
independent mirror segments. Each mirror segment has adiameter of 1.4 m and is positioned by three independent
hybrid drives with a nominal travel range of ±5 mm (see Fig.
4).

Fig. 6

Fig. 4The electric motor is suitable for heavy loads and long travel
ranges. In conjunction with a piezo drive, the hybrid system also
provides additional positioning accuracy in the subnanometer
range (image: PI)

Depiction of a main mirror segment, the three hybrid drives areshown here in blue (image: ESO)

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

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WHITEPAPER – Hybrid Nanopositioning Systems with Piezo Actuators-- Oliver Dietzel --In order to achieve this high path accuracy, PI developed ahybrid concept where a motor drive screw that is suitable forheavy loads and long travel ranges, is combined with a piezo
actuator (see Fig. 6). Serial combination of both very different
drives results in a powerful and high-precision positioning
system (see Fig. 7).

Fig. 8Fig. 7

Very stiff hybrid linear actuator with a diameter of approx.
200 mm with an overall length of approx. 285 mm (image: PI)

4.1 Piezo Actuators Position with
Subnanometer Accuracy

All-ceramic insulated, high-power piezo actuators: Durable evenunder difficult operating conditions in industry, life science, andmicroscopy as well as in medical technology and research(image: PI)

The piezo actuators used in the hybrid drive for the telescope
segments are also encapsulated in sealed metal bellows filledwith nitrogen (see Fig. 9) in order to reach the 30-years
lifetime necessary in the adverse ambient conditions in
Atacama desert at an altitude of 3,000 m.

Precision motion that results when an electrical voltage isapplied to a piezoelectric material is of particular importancefor nanopositioning. The electrical power is converted intomechanical energy directly inside the crystalline solid state,
which means that there are no rotating or frictional parts.
The piezo actuators not only work with high precision but arealso maintenance and wear free. They can move large loads
with weights up to several tons. Electrically, they act ascapacitive loads and need virtually no power in staticoperation. The behavior in the power circuit is very much likean electrical capacitor. Similar to capacitors, they don'tgenerate any heat in a static condition.
The lifetime of piezo actuators are also convincing: In the caseof PICMA® multilayer actuators (see Fig. 8), the active layers
consist of thin ceramic films and are surrounded by an allceramic insulating layer that protects the actuators against air
humidity and failure from leakage current. The monolithic
piezoceramic block of such an actuator is very reliable evenunder extreme ambient conditions and high temperature
differences.

Fig. 9An encapsulated PICMA® linear actuator of part of the hybriddrive concept (image: PI)

4.2 A High-Resolution Sensor for Both
Drive Systems
A further feature of the hybrid drives is the common highresolution sensor, which helps to control both drives
simultaneously and continuously. This is the only way toimplement the high resolution of the piezo actuators over theentire travel range.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

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WHITEPAPER – Hybrid Nanopositioning Systems with Piezo Actuators-- Oliver Dietzel --The high-resolution sensor is an incremental optical encoderthat is placed near to the drive axis (Fig. 10). It operates at aresolution of 0.1 nanometers and is also not sensitive to thechanging environmental conditions prevailing at thetelescope's location in the Atacama desert.

The drive screw is driven by a brushless high-torque motor via
a high-ratio reduction gearhead.
The gearhead ensures zero-play operation and guarantees aconstant transmission ratio. The motor can therefore be verysmall even though large masses have to be moved. The hightransmission also supports self-locking of the motor when atrest.
A dedicated controller controls both drives simultaneously
and also controls the high-resolution position measuringsystem. The servo algorithms consider the motor and thepiezo system as a single drive unit and compare the actual
motion with a calculated trajectory.

Fig. 10

Schematic diagram of the hybrid drive. The common control with
one single high-resolution linear encoder allows an extremely
constant velocity with high positioning accuracy (image: PI)

The motor drive screw is suitable for heavy loads and long
travel ranges starting at a few millimeters and going up to one
meter. The piezo actuator provides a nominal displacement ofapproximately 0.1 to 0.15 % of the actuator length but
nevertheless achieves a positioning accuracy in thesubnanometer range with one high-resolution sensor and cantherefore compensate for the inaccuracies of the motor drive
screw.

Fig. 11

The control principle of the hybrid drive is easy to understand(Fig. 11): The motor voltage is derived from the control
voltage of the piezo. The greater this voltage, the faster themotor runs. When the piezo expands, the motor drives thedrive screw in the same direction. In this way, the roughpositioning of the drive screw is supplemented by the finepositioning of the piezo. At the same time, the drive screw
always moves the piezo near to its zero position
automatically. This gives it the best chance of correcting theposition in both directions. In this way, relatively long travel
ranges can be combined with an extremely high positioning
accuracy.
The characteristics of these types of hybrid drives are not only
useful for telescopes but are always a practical solution whena position needs to be detected with high precision andmoved repeatedly over long travel ranges or when a target
position needs to be reached with nanometer precision.
Other typical applications areas include for example,
measuring technology or surface inspection, semiconductor
manufacturing, microscopy, and laser technology.

The controller structure (image: PI)

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

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WHITEPAPER – Hybrid Nanopositioning Systems with Piezo Actuators-- Oliver Dietzel --5 Author

Oliver Dietzel, Team Leader for Project Management
Corporate Platform Development at Physik Instrumente (PI)GmbH & Co. KG

6 About PI
WP4014E Hybrid Nanopositioning Systems with Piezo 10/2018.0 subject to change. © Physik Instrumente (PI) GmbH & Co. KG 2018

Well known for the high quality of its products, PI (PhysikInstrumente) has been one of the leading players in the global
market for precision positioning technology for many years. PI
has been developing and manufacturing standard and OEM
products with piezo or motor drives for more than 40 years.
Continuous development of innovative drive concepts,products, and system solutions and more than 200
technology patents distinguish the company history today.
PI develops, manufactures, and qualifies all core technology
itself: From piezo components, -actuators, and motors as wellas magnetic direct drives through air bearings, magnetic andflexure guides to nanometrological sensors, control
technology, and software. PI is therefore not dependent oncomponents available on the market to offer its customers
the most advanced solutions. The high vertical range ofmanufacturing allows complete control over processes andthis allows flexible reaction to market developments and newrequirements.
By acquiring the majority shares in ACS Motion Control, aworldwide leading developer and manufacturer of modular
motion controllers for multi-axis drive systems, PI can also
supply customized complete systems for industrial
applications that make the highest demand on precision anddynamics. In addition to four locations in Germany, the PI
Group is represented internationally by fifteen sales andservice subsidiaries.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

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