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|
 David
Hopp
| Rotary
encoders are used for angular measurements in numerous applications
like rotating machine parts, electrical motors or for example to detect
the steering angle in cars. There are different encoding principles
available such as potentiometric, capacitive, inductive or optical,
with the optical systems reaching the highest angular accuracy. As a
disadvantage the resolution
enhancement of optical encoder systems up to now is associated with
increasing cost. |
|
cost
effective: diffractive solid measure on a
plastic disc
|
|
Considering
the need for competitive sensors, we have developed a new concept for
low cost
optical rotary encoders. The basic idea is to use a micro patterned
plastic disc with a metal coating, as it is used for a Compact Disc or
DVD.
This encoder disc can be manufactured
by a conventional DVD injection
compression
moulding process. With this well known technique it is
possible
to
create
highly precise micro patterns while running a cost effective process
for high
numbers of parts.
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The
mechanical and electronic design and the realization of this new kind
of
optical rotary encoders was performed by our project partner, HSG-IMAT.
We
focused on the optical design as well as the fabrication of the
testmaster
discs. The
testmasters were directly written into photo resist on the
circular
laser writing system CLWS 300M.
diffractive
solid
measure
The solid measure
consists of a micro pattern of diffracting gratings placed in a circle
on the
outer radius of the encoder disc. An incremental code is generated by a
periodic arrangement of patterned and unpatterned fields, which
diffract the
incident coherent beam into different diffraction orders.
diffractive
sensor principle
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generation of a sinusoidal signal
Having
an alternating pattern
of fields without and with a diffractive
grating, the intensity
of the first
diffraction order of the reflected beam results in a sinusoidal
signal, which can be detected by a photo diode. A
sinusoidal signal is the desired signal for most
applications.
offset
compensation & detection of rotation direction
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To not only
achieve an incremental measurement but also a detection of the rotation
direction,
it is necessary to implement a second incremental track that is
90°
phase
shifted towards the first. The usage of a different
grating
leads to a
second set of diffraction orders where again the first order is
detected by a
photo diode. To obtain
an offset compensated
output
signal this setup is used twice in a nested
configuration having four different gratings per period.
generation of four
sinusoidal signals & reference by spatial seperation
The spatial
separation of
the resulting four first diffraction order spots is achieved by using different angles for each set of
gratings
per signal. To meet the common request for a reference mark, a fifth
grating is implemented to mark the zero position once on the
circumference. This generates a reference
signal on a fifth photo diode.
signal detection from
first order spots on four photodiodes
alignement-free
sensor assembly
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The
dimensions of the patterns of the gratings and therefore the angular
resolution
of the encoder depend directly on the illumination spot size and the
geometry. In
turn this defines the fabrication tolerances of the system. To cope
with the mechanical
tolerances, the optical components of the sensor are assembled on a MID
(Moulded
interconnect Device). The photo diodes and the light source are
directly bonded onto the MID. The assembly of the polymer lens and
aperture is
done without any
alignment procedure.
MID optic module
In the
current concept a disc of 27 mm diameter is chosen to fit into
a
housing
with about 37 mm outer diameter. With an incremental
tangential
period of
40 µm the
hardware
resolution results in 2048 periods per revolution, what
can be easily interpolated electronically with a standard circuit in
order to
obtain a higher resolution output if necessary.
The
MID optic module is attached to an integrated printed board where the
signal
processing is performed.
assembled sensor
| [1] |
D. Hopp, CH. Pruss,
W. Osten, J. Seybold, V. Mayer, H. Kück: Optischer inkrementaler Drehgeber in
Low-Cost-Bauweise, tm - Technisches Messen, Jahrgang 77 (2010)
Heft 6 S.358-363, Oldenbourg Verlag, 2010, München |
| [2] |
D. Hopp, Ch. Pruss,
W. Osten, J. Seybold, V. Mayer, H. Kück: Optical incremental
rotary
encoder in low cost design, Sensor & Test Conference
Proceedings
Opto 2009, 3.4, Nürnberg |
| [3] |
Hopp, D.; Pruss,
Ch.; Osten, W.,
Seybold, J., Mayer, V., Kück, H.: Untersuchungen
zu einem hochauflösenden optischen Drehwinkelsensor in
Low-Cost-Bauweise; Projektabschlussbericht, AiF ZN219,
Universität Stuttgart, Stuttgart 2008
|
| [4] |
Hopp, D.; Pruss,
Ch.; Osten, W.,
Seybold, J., Mayer, V., Kück, H.: Hochauflösender
optischer Drehgeber in Low-Cost-Bauweise;
DGaO-proceeding, Esslingen, 2008. |
| [5] |
Seybold, J., Mayer,
V., Kück, Hopp, D.; Pruss, Ch.; Osten, W.:
Hochauflösender
optischer Drehgeber mit MID-Optikmodul; 6. Paderborner Workshop
"Entwurf mechatronischer Systeme", Paderborn 2009
|
| [6] |
Mayer, V.: A new
concept for an absolutely encoded angular resolver; Presentation on 4M
2007 Conference on Multi-Material Micro Manufacture, Borovets,
Bulgaria, 2007
|
[7]
|
Alexander G.
Poleshchuk, Evgeny G. Churin, Voldemar P. Koronkevich, Victor P.
Korolkov, Andrei A. Kharissov, Vadim V. Cherkashin, Valerii P.
Kiryanov, Aleksei V. Kiryanov, Sergei A. Kokarev, and Alexander G.
Verhoglyad: Polar Coordinate Laser Pattern Generator for Fabrication of
Diffractive Optical Elements With Arbitrary Structure; Appl. Opt. 38,
1295-1301 (1999)
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links
 HSG-IMAT
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