Technical Information

Encoders for Linear Motors in the Electronics Industry
The semiconductor industry and automation technology increasingly require more precise and faster machines in order to satisfy growing
demands on miniaturization, quality, and manufacturing cost reduction. Linear motors are gradually becoming more important in suchhighly-dynamic applications with one or more feed axes. The benefits of this direct drive technology are low wear, low maintenance, andhigher productivity.
However, this increase in productivity is possible only if the control, the motor, the machine frame, and the position encoder are optimally
adjusted to one another. Direct drives place rigorous demands on the quality of the measuring signals.
Optimum measuring signals• reduce vibration in the machine frame,• stop excessive noise exposure from velocity-dependent motor sounds, and• prevent additional heat generation,• so allowing the motor to realize its maximum mechanical power rating.
The efficiency of a linear motor is therefore greatly influenced by the selection of the position encoder. Encoders with optical scanning
methods provide benefits in the accuracy, speed stability, and thermal behavior of a direct drive. HEIDENHAIN offers a wide range of
linear encoders with technical characteristics specifically designed for linear motors in the electronics industry.

Design of direct drives
The decisive advantage of direct drive
technology is the very stiff coupling of thedrive to the feed component without anyother mechanical transfer elements. This
allows significantly higher gain in thecontrol loop than with a conventional drive.
Velocity measurement ondirect drives
On direct drives there is no additional
encoder for measuring the speed. Both theposition and speed are measured by theposition encoder: linear encoders for linear
motors, angle encoders for rotating axes.
Since there is no mechanical transmission
between the speed encoder and the feed
unit, the position encoder must have acorrespondingly high resolution in order toenable exact velocity control at slow
traversing speeds. The velocity is calculated
from the distance traversed per unit oftime. This method—which is also applied
to conventional axes—represents aApril 2006

numerical differentiation that amplifies
periodic disturbances or noise in the signal.
The combination of significantly higher
control loop gain, as is used particularly

with direct drives, and insufficient encoder
signal quality can result in a dramatic
decline in drive performance.

Encoders for Linear Motors in the Electronics Industry
Requirements and Effects

A comparison of the effects of linear
encoders with low and high interpolation
error on a linear motor illustrates thesignificance of high-quality position signals.
The LIDA linear encoder used here
generates only barely noticeable
disturbances in the motor current: themotor operates normally and develops littleheat.
If at the same controller setting, theinterpolation errors of the same encoder
are increased through poor adjustment,
significant noise arises in the motor
current. This causes an increased amountof noise and heat generated in the motor.

Signal level f

Position error u within one signal period

Motor current [A] f

Effects of interpolation error
Generation of heat and noise
If the frequency of the interpolation error
increases, the feed drive can no longer
follow the error curve. However, thecurrent components generated by theinterpolation error cause increased motor
noises and additional heating of the motor.

Position error f

Signal quality of position encoders
Modern encoders feature either anincremental, which means counting, or an
absolute method of position measurement.
The path information is transformed in theencoder into two sinusoidal signals with
90° phase shift. Both methods require thatthe sinusoidal scanning signals beinterpolated in order to attain a sufficiently
high resolution. Inadequate scanning,
contamination of the measuring standard,
and insufficient signal processing can leadto a deviation from the ideal sinusoidal
shape. As a consequence, during
interpolation periodic position error occurs
within one signal period of the encoder’soutput signals. These position errors within
one signal period are referred to as“interpolation error.” On high-quality
encoders it is typically 1% to 2% of thesignal period.

Signal period
360 °elec.

40200
20
400.4

A
B
0.6

0.8

1.0

1.2

1.4

1.8

2.0

Time [s] fMotor current of a direct drive with position encoder
A: With low interpolation error
B: With high interpolation error

Dynamic behaviorDigital filters are often used with directdrives to smooth the position signals.
However, the additional phase delaycaused by filtering in the speed-control
loop must be kept to a minimum,
otherwise the dynamic accuracy
decreases.
Position encoders with optimum signal
quality help to reduce the use of filters,
meaning that the control bandwidth ismaintained.

1.6AB

Heat generation of a linear motor controlled with an encoder
A: With low interpolation error
B: With high interpolation error

Position Encoders for direct drives
Linear encoders that generate a highquality position signal with low interpolation errors are essential for optimal
operation of direct drives in the electronics
industry. Encoders that use photoelectricscanning are ideally suited for this task,
since very fine graduations can be scannedby this method.
Encoders with optical scanning therefore
play a significant role in exploiting thepotential of direct drives.
Exact graduations
HEIDENHAIN encoders with opticalscanning incorporate measuring standardsof periodic structures known asgraduations. The substrate material is
glass, steel, or—for large measuring
lengths—steel strips. These finegraduations—graduation periods from40 µm to under 1 µm are typical—aremanufactured in a photolithographic
process. They are characterized by highedge definition and excellent homogeneity—a fundamental prerequisite forlow interpolation error, and therefore forsmooth operating performance and highcontrol loop gain.
Durable measuring standards
By the nature of their design, themeasuring standards of exposed linear
encoders are less protected from theirenvironment. HEIDENHAIN therefore
always uses tough gratings manufactured
in special processes.
In the DIADUR process, hard chrome
structures are applied to a glass or steel
carrier. The AURODUR process applies
gold to a steel strip to produce a scale tapewith a hard gold graduation.
In the SUPRADUR process, a transparent
layer is applied first over the reflective
primary layer. Then an extremely thin, hard
chrome layer is applied to produce agrating. Scales with SUPRADUR
graduations have proven to be particularly
insensitive to contamination because thelow height of the structure leaves
practically no surface for dust, dirt or waterparticles to accumulate.
In this way, HEIDENHAIN production
technologies ensure an enduringly highsignal quality that promotes the use of
direct drives for particularly demanding
applications.

DIADUR phase grating with approx. 0.25 µm grating height

Reflective
layerTransparent
layer
Reflective
base layer

Substrate

SUPRADUR process: Optical three-dimensional graduation
structureOptimized scanning methods
The scanning method and the high qualityof the grating share responsibility for low
interpolation error. An especially beneficial
feature is the single-field scanning withwhich the exposed linear encoders fromHEIDENHAIN operate: The output signals
are generated from only one scanningfield. This large scanning field, and thespecial optical filtering through thestructure of the scanning reticle andphotosensor, generate scanning signals
with constant signal quality over the entire
range of traverse. This is the prerequisitefor:• Low signal noise• Low interpolation error• High traversing speed• Good control loop performance for directdrives• Low heat generation of the motor
Signal generation based on the imaging
scanning principle (LIDA 400)
To put it simply, the imaging scanningprinciple functions by means of projectedlight signal generation: two scale gratings
with equal grating periods are moved
relative to each other—the scale and thescanning reticle. The carrier material of thescanning reticle is transparent, whereas
the graduation on the measuring standard
may be applied to a transparent orreflective surface.
When parallel light passes through agrating, light and dark surfaces areprojected at a certain distance. An index
grating with the same grating period islocated here. When the two gratings movein relation to each other, the incident light ismodulated: if the gaps are aligned, lightpasses through. If the lines of one grating
coincide with the gaps of the other, no lightpasses through. Photovoltaic cells convert
these variations in light intensity into
electrical signals. The specially structured
grating of the scanning reticle filters thelight current to generate nearly sinusoidal
output signals.
In the XY representation on an oscilloscope
the signals form a Lissajous figure. Ideal
output signals appear as a concentric innercircle. Deviations in the circular form andposition are caused by position error within
one signal period and therefore go directly
into the result of measurement. The size ofthe circle, which corresponds to theamplitude of the output signal, can vary
within certain limits without influencing themeasuring accuracy.
On direct drives, deviations from the circularform cause acoustic noise, reduce control
quality and increase heat generation.

Linear scale

Structured
detector

Window

Scanning
reticleCondenser
lens

Index grating

LED light source

Photoelectric scanning in accordance with the imaging principle with steel scale tape andsingle-field scanning (LIDA 400)

Signal period
360° elec.

0090°

Phase shift

Lower sensitivity to contamination
Production facilities and handling devices
for the semiconductor industry demandhigh acceleration and compact designs.
Such requirements call specifically for
exposed measuring systems that operate
without friction and, because they canoperate without their own housing, can be
designed to be very small and therefore
low in mass. Special scanning methods
and production techniques are used toprovide tough protection against
contamination even without sealing theencoder.

An essential prerequisite for opticalencoders with low sensitivity tocontamination is therefore an optimized
scanning method, the large scanning field,
and the contamination-tolerant graduation.
Application-oriented mounting
tolerances
Very small signal periods usually comewith very narrow distance tolerances
between the scanning head and scaletape. Thanks to the interferential scanningprinciple of the LIF 400 and innovativeindex gratings, such as in the LIDA 400, it
has become possible to provide ample
mounting tolerances in spite of the small
signal periods. Within the mounting
tolerances, therefore, changes in the signal
amplitude remain negligible.
This behavior is substantially responsible
for the high reliability of exposed linear
encoders from HEIDENHAIN.

WaterToner

DustFingerprint

2Position error [µm]The large scanning field additionallyreduces sensitivity to contamination. In
many cases this can prevent encoder
failure. This is particularly clear with theLIDA 400 and LIF 400, which in relation
to the grating period have a very large
scanning surface of 14.5 mm2. Even withcontamination up to 3 mm in diameter, theencoders continue to provide high-quality
signals. The position error remains far
below the values specified for the accuracy
grade of the scale.

Oil

1010.2

0.402

0.2

04060

80

100120140160180

Position [mm]Reaction of the LIF 400 to contamination

120%
Signal amplitude [%]Exposed linear encoders fromHEIDENHAIN operate with single-fieldscanning. Only one scanning field is used
to generate the scanning signals. Local
contamination on the measuring standard(e.g., fingerprints from mounting or oil
accumulation from guideways) influences
the light intensity of the signal components,
and therefore of the scanning signals, inequal measure. The output signals dochange in their amplitude, but not in theiroffset and phase position. They stay highlyinterpolable, and the interpolation error
remains small.

100%
80%
60%
Mounting tolerance
LIF 400

40%
20%
0%
0.8

0.9

1

1.1
Scale-to-reticle gap [mm]1.2Position Encoders for Linear Motors

Exposed linear encoders fromHEIDENHAIN are optimized for use on
fast, precise machines as sought by thesemiconductor industry and automation
technology. In spite of the exposedmechanical design they are highly tolerant
to contamination, ensure high long-term
stability, and are fast and simple to mount.
Their low weight and compact design suit
encoders of the LIF 1, LIP 2 and LIDA 3
series particularly for linear motors.

Application

SignalperiodMax. interInterface
polation error

Model

For very high accuracy

0,128 µm

• 0,001 µm« TTL

LIP 372» 1 VPPLIP 382« TTL

LIP 471» 1 VPPLIP 481« TTL

LIP 571» 1 VPPLIP 581« TTL

LIF 471» 1 VPPLIF 481« TTL

LIDA 47x» 1 VPPLIDA 48x

2 µm

4 µm

• 0,02 µm• 0,04 µm• For simple installation 4 µm
• With limit switches
and homing track• 0,04 µm• For high traversing
speed• Limit switches• 0,2 µm

20 µm

HEIDENHAIN position encoders for direct drives (selection): Maximum values ofinterpolation error with respect to the signal period

123DR. JOHANNES HEIDENHAIN GmbH
Dr.-Johannes-Heidenhain-Straße 583301 Traunreut, Germany{ +49 (86 69) 31-0
| +49 (86 69) 50 61
E-Mail: info@heidenhain.dewww.heidenhain.de528 683-22 · 10 · 11/2007 · F&W · Printed in Germany · Subject to change without notice

For more information:• Brochure: Exposed Linear Encoders• Technical Information:
Single-Field Scanning