WHITE PAPER – Reliability of PILine® Components– Dr. Christian Benz –Reliability of PILine® Components
Lifetime, Wear and Reliability of PILine® Systems

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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WHITE PAPER – Reliability of PILine® Components– Dr. Christian Benz –1

Overview

PILine® positioning systems are based on a direct drive using
ultrasonic piezo motors. Therefore, very low response times aswell as accurate positioning can be accomplished. Due to theparticular operating principle, PILine® motors are self-locking
even when switched off. However, the drive is prone to acertain degree of wear.
This white paper is intended to provide a better understandingof what this wear is and what it means in terms of productlifetime. Furthermore, it focuses on showing why and underwhich circumstances neither lifetime nor general reliability is
an issue when using PILine® motors.

The contact between the coupling element and the runneron the other hand, is subject to friction. Since this contact
can account for up to 40 % of the system’s total energy loss,
great effort is made to determine the optimum composition ofmaterials used.
Runners and coupling elements made of alumina (Al2O3) orzirconia-toughened alumina (Al2O3/ZrO2, ZTA) have proven to
be most effective.

3

Implications of Wear

Due to the friction-related power transfer between the couplingelement and the runner, abraded particles accumulate in thehousing of the motor (see Fig. 2).

2	Operating Principle ofPILine® Positioning Systems
At the core of every PILine® system, a piezoelectric actuatoris preloaded against a runner using a coupling element (seeFig. 1).
Scale

Sensor
RunnerLateral Guiding

Coupling
Element
Piezoelectric
Actuator
PreloadFig. 1	Schematic design of a PILine® motor: A piezoelectric actuatoris preloaded against a runner. Exciting the actuator electricallycauses it to oscillate. This oscillation is transformed into forward
motion, which is transmitted to the runner via a couplingelement (Image: PI)

The actuator is excited electrically to generate highfrequency
oscillations. In conjunction with preloading, continuous fastfeed motion results. The oscillation of the piezoelectric actuatoris based on crystalline effects and is therefore completely freeof wear.

Fig. 2	Motor of a PILine® M-663.5U linear stage after 19,000 hoursof operation. Abraded particles accumulate on the couplingelement and piezoelectric actuator due to electrostatic effects.
Scale bar: 2 mm (Image: PI)

However, the use of ceramic coupling elements keeps thedegree of abrasion to a minimum. Furthermore, the couplingelements are dimensioned to prevent critical abrasion levels
from being reached within the specified lifetime. Due toelectrostatic effects, a large quantity of abraded particles
remains in the housing because they are attracted directly tothe piezoelectric actuator and coupling element.

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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WHITE PAPER – Reliability of PILine® Components– Dr. Christian Benz –3.1 Wear over Time
During the run-in period (approximately 100 h), unevenness
caused by minor imperfections in the alignment between thecoupling element and the runner are smoothed. After thisinitial process, the volume loss decreases to a low, constantvalue as visualized exemplarily in Fig. 3.

Fig. 3	Volume loss of a coupling element and corresponding
gradient versus operating time. The gradient decreases
considerably after the initial run-in period. In this exemplary
test (described in more detail in chapter 4.1) the amount ofabraded material per hour levels off below 10-4 mm3/h(0.5 μg/h) (Image: PI)

Fig. 4	Top view of two coupling elements: As manufactured (left)
and after 19,000 h of operation (right). The asymmetric wear
(cf. width top to width bottom) results from slightly unevenalignment between the coupling element and the runner,caused by nonuniform preload and/or inhomogeneous load on
the stage. On average, the height of the coupling element wasreduced by approx. 60 μm during the test cycle, as determinedby laser microscopy. Scale bar: 0.5 mm (Image: PI)3.2 Surfaces
On delivery, the surfaces of the ceramic coupling element andrunner are smoothly polished. During the run-in procedure,asymmetry of the alignment is compensated by materialabrasion. Over the course of a lifetime, signs of wear as well as
effects of asymmetric alignment are visible on the surfaces of
the coupling element and the runner.

µm
2.6

The outcome of a 19,000 h lifecycle test of a PILine® M-663.5U
linear stage test is discussed below, where these impacts
can be readily observed. For more details on the test, referto chapter 4.2. Wear and degree of abrasion on the couplingelement and the runner are analyzed in Fig. 4 and Fig. 5,
respectively. During the lifecycle test, the average heightof the coupling element decreased by about 60 μm, withmore than 85 % of the original height (400 μm) remaining.
The removed volume amounts to approximately 0.05 mm³ (0.2mg) of the abraded material.
The mean abrasion depth of the runner was estimated tobe approximately 1 μm in the contact area of the couplingelement, as described in Fig. 5. From profile scans, an abrasion
volume of 0.03 mm³ (0.11 mg) can be estimated.
In total, this particular PILine® linear stage has produced
approximately 0.08 mm³ (0.31 mg) abrasion. Therefore,
after an initial run-in period, an abrasion volume of less than
0.02 μg/h can be expected, assuming constant wear.

1.8
0.9
0-0.9
-1.7Fig. 5	Top: Optical microscope image of the runner, scale bars
1 mm. In the contact area of the coupling element, signs ofwear can be identified. The trajectory of the runner versus
coupling element is indicated by the black arrow. Bottom: Further
analysis with a laser microscope provides information on thesurface profile of the runner. Accumulation of abraded material
is visible on the left-hand side. The mean abrasion depth within
the contact area of the coupling element can be estimated to be≈ 1 μm (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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WHITE PAPER – Reliability of PILine® Components– Dr. Christian Benz –3.3 Grain Size and Composition of Abrasion
The grain size of the abraded material was determined to be
below 1 μm, as shown in Fig. 6.

Fig. 7	Leica Nova TS60 total station withPILine® direct drive (Image: Leica Geosystems)

Fig. 6	Scanning electron microscope (SEM) images of the runnersurface. Scale bars: 2 μm (left), and 10 μm (right). Individual
abraded particles can be identified (the small bright particles< 1 μm). The vast majority of particles however, form tightly
stacked clusters, as can be seen in the image on the right-handside (Image: PI)

However, the appearance of single, isolated particles is rare;most particles agglomerate to large clusters.
The composition of the abraded particles was analyzed using
electron diffraction x-ray (EDX). It corresponds to that of theceramic coupling element and runner; therefore these can beidentified as the sole source materials. As such, the abradedparticles are electrically nonconductive, chemically inert andshould not pose any danger to your electronic components.
Furthermore, they have no impact on the performance of thePILine® motors.

4Case Studies

During product development, numerous tests are conductedwhich are designed to meet customer requirements as well
as attain the anticipated lifetime. The following representative
case studies highlight some of our products.

Total stations are used for angle and distance measurement ingeodesy. Customer requirements included an extensive increaseof the rotation speed and the angular resolution compared tothe previously used conventional drive. To verify the fulfillmentof these requirements, a test recreating the typical usage of thedevice was conducted. It consisted of rotating a mass of 6 kg
(15,000 kg/m²) in alternating directions. After completion of thetest with a total of 7,000 h or 2,500 km of operation, all customer
specifications continued to be fulfilled. The corresponding testwith the analysis of the wear on the motor is shown in Fig. 3.

4.2 PILine® M-663.5U Translation Stage
The PILine® M-663.5U is a very fast and versatile linear
translation stage. Due to its versatility, customers are expected to use it in unconventional situations or, more specific,suboptimal mounting orientations.
However, the mounting orientation has a significant impact on
product wear due to the varying load on the coupling element
and bearings. Fulfillment of the requirements on productlifetime was validated during a long-term test, where the stage
was mounted at the least-favorable mounting orientation as
depicted in Fig. 8.

g

4.1 Total Stations – Leica Geosystems
Leica Geosystems, a leading manufacturer of high-precision
surveying instruments, relies on PILine® direct drive technology
in their new total station product line (see Fig. 7).

Fig. 8	M-663.5U linear stage, mounted in vertical orientation of themotion axis. The load on the stage was varied from 0.5 N to 2 N(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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WHITE PAPER – Reliability of PILine® Components– Dr. Christian Benz –During this test, the stage travelled continuously across thefull available travel range of 18 mm at a speed of 20 mm/s.
A maximum applicable load of 2 N was determined as one of
the test results. All stages remained in good condition during
the trial period of over 19,000 h or 600 km, fulfilling thespecifications as stated in the data sheet. Further analysis of
the coupling elements and runners in these stages is discussed
in chapters 3.2 and 3.3.

Condition

Wear

CommentVacuum↑↑

Max. power transfer

Dry environment↑

High power transfer

Humid environment↓Lower power transfer

4.3 Customized PILine® M-663 Stage

Temperature >40 °C

Frequency slightly higher
than optimumHigh acceleration ornumber of occurancesThis case study deals with a specifically customized PILine®
M-663 stage. The customer requested a maximum permissible
target deviation of ±1.2 μm and a settling time less than
250 ms. Both requirements were verified in longterm tests aspresented in Fig. 9.
107

10

10610

1051010

4

10

10

3

10102

10110deceleration
High load↓↑

Slightly slower
positioning
Risk of overheating
increases↑↑↑

Tab. 1	Impact of different ambient conditions on the lifetime of PILine®
positioning systems

75.1 Humidity

6The degree of abrasion depends on the power transmitted via
the coupling element, which in turn depends on environmentalconditions. Dry environments enable higher power transmission
at the cost of increased wear. This is especially true undervacuum conditions, where maximum power transmission
can be achieved. However, in humid environments, powertransmission between the coupling element and the runner isreduced due to lower friction.

5432

10-1.8 -1.2 -0.6 0.0 0.6 1.2 1.8

10target deviation / µm50

100 150 200 250 300

settlingtime / msFig. 9	Test results of a customized PILine® M-663 stage. Both target
deviation (failure: none) and settling time (failure: 0.33 permillion) are well within customer requirements (visualized by
the light blue background). Note the logarithmic scaling of they-axis (Image: PI)

After performing 45.6 million positioning cycles (380 km),the stage only failed to reach the target 15 times within therequested settling time, which is equivalent to 0.33 failures
per million. The target deviation always remained below thethreshold of ±1.2 μm.

5Factors Influencing Wear

Depending on the ambient conditions, under which PILine®
motors are operated, impact on wear may vary (see Tab. 1).
This chapter intends to give you advice on how to maximize thelifetime of your PILine® positioning systems.

Fig. 10	Recommended duty cycle and motor power depending on theambient temperature. In order to prevent overheating andincreased wear, the motor power and the duty cycle should notexceed the limits specified in this graph (Image: PI)

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

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WHITE PAPER – Reliability of PILine® Components– Dr. Christian Benz –5.2 Temperature

6The ideal operating temperature for PILine® motors is between
10 °C to 40 °C. At lower temperatures, power transmission andmaximum achievable speed are reduced due to the decreased
inverse piezoelectric effect. For operation at temperatures
above 40 °C, a reduction of motor power is advised to prevent
overheating (see Fig. 10).

Despite the wear-related drive principle, PILine® piezo motors
are perfectly fit for applications where continuous operation isrequired.

5.3 LoadConclusion

As a general rule of thumb, PILine® systems typically reach
2,000 h of operation, 2,000 km or 20 million cycles, whicheveroccurs first. The operating time of 2,000 h corresponds to theactual duty time of the stage.

Extended operation at high motor power leads to overheating
and causes wear that could be prevented.
 It is recommended to keep the motor output below

60 %. The remaining 40 % is dedicated to acceleration,deceleration and use as a control reserve.
 PILine® motors undergo the heaviest load during
acceleration and deceleration, while overcoming
stiction. Try keeping them as low as possible, especiallywhen moving heavy masses. Generally, 1,000 mm/s²
is a good value, although a PILine® motor can easily
achieve 10,000 mm/s².
 When targeting the same destinations repetitively,
bumps may form on the runner. From time-to-time,
try targeting a destination further away to promotesmoothening of the runner surface.

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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WHITE PAPER – Reliability of PILine® Components– Dr. Christian Benz –Author
Dr. Christian Benz is development engineer for piezomotor
products at Physik Instrumente (PI) GmbH & Co. KG.

About PI
In the past four decades, PI (Physik Instrumente) with
headquarters in Karlsruhe, Germany has become the leading
manufacturer of nanopositioning systems with accuracies in
the nanometer range. With four company sites in Germany andeleven sales and service offices abroad, the privately managed
company operates globally.
Over 850 highly qualified employees around the world
enable the PI Group to meet almost any requirement in thefield of innovative precision positioning technology. All key
technologies are developed in-house. This allows the company
to control every step of the process, from design right down
to shipment: precision mechanics and electronics as well as
position sensors.
The required piezoceramic elements are manufactured by itssubsidiary PI Ceramic in Lederhose, Germany, one of the global
leaders for piezo actuator and sensor products.
PI miCos GmbH in Eschbach near Freiburg, Germany, isa specialist for positioning systems for ultrahigh vacuum
applications as well as parallel-kinematic positioning systems
with six degrees of freedom and custom-made designs.

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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WP4002 R1 PILine® Lifetime, Wear and Reliability 11/2018 .0 Subject to changes. © Physik Instrumente (PI) GmbH & Co. KG 2018

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