e engineering.
Running at increasingly higher
speeds, the electric motors re-
quire angle measuring devices
so-called encoders which are
low-cost and small and yet provi-
de high and absolute resolution.
At the same time, the control of
lowest speeds demands high-
precision and signal stability of
the encoders.
Conventional encoders with peri-
pheral scanning encounter funda-
code disc and glass aperture as well
as extensive, sometimes manual ad-
justments are not needed for this
novel MiDi encoder. The spatial di-
mensions and the susceptibility to
interference such as contamination
and breakage of the glass parts,
particularly at high speeds, are dra-
stically reduced and eliminated, res-
pectively. Due to the small number
of components and the assembly
due to be automated, the MiDi en-
coder with the advantages listed
can also open up price sensitive
markets with strict reliability require-
ments for optical encoders. Areas of
application for the MiDi encoder/po-
sition sensor are in controllable ser-
vo motors with feedback, in automa-
tion technology and in motor ve-
hicles, where classic optical encoder
concepts are too big, too sensitive
and too expensive. MiDi encoders
with centric angle sensors, however,
offer the system approach for an
existing mass market which, in turn,
is the prerequisite for a highly inte-
grated solution. 2
S ICK- STEGMAN N
Optical MiDi Encoder with Centric Scanning
The Concept of the
MiDi Encoder
The MiDi encoder consists of a
light source centrally integrated in
the rotation axis and a monolithic
photoelectric scanning unit centri-
cally arranged opposite the light
source. The code disc is then
multi-scanned, to compensate for
any eccentric running. Picture 1
shows the schematic arrange-
ment. The opto ASIC contains, as
the micro system solution, not
only the sensors but also the sig-
nal amplification, signal proces-
sing including digital signal adjust-
ment and various interfaces. The
degree of miniaturisation of the
encoder is based on the motors
defaults.
Picture 1: Schematic Arrangement
The centric arrangement of the
code disc requires a light source
directly in the rotating shaft. Spe-
cial methods for transmission of
the energy into the shaft as well
as measures for ensuring even
lighting were developed for this.
A high degree of automation in
micro-assembly/adjustment in
the µm range will be essential to
the economical production of lar-
ge numbers of MiDi encoders
and to ensure that the require-
ments regarding precision of the
angle transmitter are achieved.
This includes the code disc/shaft,
optics/shaft and housing/scan-
ning unit connections. In doing so,
the requirements and boundary
conditions of micro system en-
gineering were converted to a
design suitable for production.
The advantages of the centric ar-
rangement are an improved resis-
tance to mechanical stresses.
Above all, an ever greater shock
and vibration resistance is expec-
ted of the encoders, e.g. for
shock values (100 g) and vibra-
tions (50 g).
Centric scanning permits very
small code discs secured on the
entire circumference; accordingly,
the MiDi encoders built according
to the Centro principle withstand
high speeds and accelerations,
as shown in Picture 2.
Picture 2: Increased vibration resistance
through a small, circumferentially held
code disc
In case of single scanning appli-
cations, assembly-related eccen-
tricity for code discs with small
diameters affects the absolute
precision much more than for lar-
ge code discs. Picture 3 shows
this connection.
The centric arrangement, how-
ever, allows full (holistic) scanning.
This makes it possible to com-
pensate for code disc run-out.
Holistic scanning achieves accu-
racies which cannot be taken for
granted with the usual discs 10 ti-
mes the size. With multiple scan-
ning it is thus possible, despite
the small size, to produce an en-
coder with high precision.
Picture 4 shows the trace of the
eccentricity error for single scan-
ning compared with quadruple
scanning.
Opto ASIC
At the heart of the encoder is an
opto-electronic encoder ASIC
(see Picture 5) for absolute and
incremental angle measuring sys-
tems. Monolithically integrated
are photo diodes, photo ampli-
fiers, signal conditioning, trans-
mission current control, divider, in-
cremental, RST and SSI interface.
For absolute position indication,
photo diodes are tangentially ar-
ranged, to securely scan a se-
quential code on the code disc.
Further photo diodes are formed
such that they supply sine and
cosine signals with low distortion
factor.
Picture 3: Connection between eccentricity of the code disc and size
Picture 4: Increasing the precision through multiple scanning
Picture 5: Opto-electronic encoder ASIC
MiDi encoder
Opto-IC
Code disc
Lens
Energy transmission
Light source
Eccentricity error
Quadruple scanning
Single scanning
Err
or in angular se
conds
absolute precision ~ 1/size Error = arctan(e/r)
Run
-
out
e
32-bit
scan
128-bit
scan
3-bit
interpolator
6-bit
interpolator
Reference
and bias
Justage
Adjustment
Transmission
current control
Random
scan
128-bit
amplifier
128-bit
adjustment
Digital section
128-bit signal
processing 3
Optical MiDi Encoder with Centric Scanning
S ICK- STEGMAN N
Using an integrated 6-bit sine di-
gital converter achieves an overall
resolution of 13 bits.
The resolution can be freely set to
between 1 and 8192 steps, via a
divider. The direction of rotation
can be inverted through a high le-
vel, at RNV. The programming of
the zero position is triggered via
an input and enables the zero po-
sition to be determined once the
encoder has been assembled.
The position data can be read out
absolutely via an SSI interface or
via an incremental interface. The
RST interface enables the output
of commutation signals for three-
phase machines of 1 to 32 pole
pairs. The commutation data can
be additionally output, in coded
form, on the K0 signal.
Quadrature control is available for
transmission current control. The
quadrature control adds the
squared sine signals and cosine
signals and effects that the ampli-
tudes of the reception currents
are kept constant independent of
temperature, speed and ageing
of the LED. With the signal condi-
tioning also integrated, offset and
amplitude of the sine/cosine sig-
nals can be adjusted to exacting
requirements.
The driver stage of the transmissi-
on current control allows the di-
rect connection of an LED with
currents of up to 50 mA.
For adjustment purposes, the sig-
nals of an adjustment track are
switched to the analogue outputs.
Several test modes permit a full
functional test even without opti-
cal coupling-in. All inputs and out-
puts are ESD-protected against
destruction.
To develop the geometry and ar-
rangement of the light sensitive
photo sensors a simulation envi-
ronment was created, enabling
the optical scanning to be optimi-
sed. For this, the rotation of the
code disc is simulated over the
diodes, and the phase relations-
hip and signal amplitude of the in-
dividual signals is analysed. For
an optimised code disc, the beha-
viour in case of incorrect adjust-
ment and pendulum impact can
be predicted. Picture 6 shows a
selection of diode signals for ideal
assembly.
Packaging technology developed
by iC-Haus was selected for the
opto ASIC, with a glass disc glued
directly onto the semiconductor
surface. Picture 7 shows the
schematic arrangement. The bon-
ding wires are encased by the ca-
sting compound. The benefits of
this technology are very good op-
tical properties, low build height,
high reliability and insensitivity to
further processing.
Picture 7: Glass-On-Chip
The Dam+Fill method shown in
Picture 8 was used for the LED
module of the lighting unit opposi-
te the opto ASIC. The reliability of
the build was proven in stress
tests.
Picture 8: Dam+Fill
Code disc
Precision material measures are
usually made in glass with a code
pattern lithographically produced
on a chrome layer. The mechani-
cal processing of a round glass
disc is a major cost factor in this.
The centric scanning allows for a
code disc without hole with very
small dimensions (code track dia-
meter
4 mm)
This made it possible to select a
square geometry for the code
disc, allowing the cost-effective
separation of the discs using a
wafer saw.
Picture 9 shows a Centro code
disc in the inner bore of a stan-
dard code disc, for comparison.
Picture 9: Comparing the code discs
Lighting Unit
To be able to project the code
pattern of the code disc (which is
centrically arranged on the shaft)
onto the opto ASIC it will be ne-
cessary to fit a light source in the
shaft of the encoder. This must
be supplied with energy as effi-
ciently as possible and indepen-
dent of the shaft position and
speed. For this, a transformer ar-
rangement was developed, which
can be integrated into a small
build space. Picture 10 shows the
principle as well as the construc-
tion of this arrangement.
Furthermore, the code pattern of
the material measure is to be illu-
minated evenly. Picture 11 shows
a simulation result of the light dis-
tribution of a version of the colli-
mator lens.
Picture 11: Simulation result of the light
distribution of the collimator lens
Further requirements of the
lighting unit are:
High reliability
Wide temperature range
( 40 + 125 °C)
Res