### Application Bulletin 229.pub

```Application Bulletin 229
Designing Encoder Elements
for Two Channel Optical Interrupters
GENERAL DISCUSSION
Rotational direction of a shaft can be readily
determined by utilizing the two channels of a dual
optical interrupter, an encoder disc with a number of
openings around the circumference, and some simple
electronics. The speed and relative shaft location
information is available as a by-product and requires
PERFORMANCE CHARACTERISTICS
OPB822SD
Opening
Aperture
(0.010" X 0.080")
OPB950
OPB950
Figure 1 is a pictorial definition of terms used in this
bulletin and should be referred to for clarification. A
period is defined as a fraction of 360 electrical
degrees or the mechanical width of one opening plus
one closure at the central point of the slot near the
circumference of the encoder disc. In shaft encoding
terminology, quadrature is the term defining the ability
to determine the direction of movement by the phase
relationship between the outputs of two channels.
System design normally uses 90° for this phase shift.
Speed can be determined by accumulating the
number of signal pulses for a fixed period of time,
dividing by the number of periods per revolution thus
obtaining the revolutions for this time period. Relative
location is determined by dividing 360 by the number
of periods around the circumference. A pulse is
generated for each of these rotational periods.
Counters may be used to count the periods or to
count the quadrature transitions (4 per period). This
bulletin will describe the method of obtaining the
information (rotation direction, speed, and relative
location) rather than what is done with the
information.
Aperture
(0.010" X 0.030")
Opening
First we should determine the number of periods on
the disc.
This can be an arbitrary number or Slot
referenced to the resolution of the steps on the disc.
Web
As an example: if 0.7° steps are required, you would
Period
count the 4 quadrature transitions per period, the
number of periods per revolution would be 128
(Periods = 360° / [Degrees per transition * number of
transitions] = 360/[128*4]). We will next calculate the
period size, opening and closure width, centerline
Figure 1:
Example of Period Width encoder wheel slots
logical sequence for different off-multiples showing
referenced to the OPB950 & OPB822SD Dual
quadrature transitions will be covered last.
Channel Encoder
OPTEK reserves the right to make changes at any time in order to improve design and to supply the best product possible.
OPTEK Technology Inc. — 1645 Wallace Drive, Carrollton, Texas 75006
Phone: (972) 323-2200 or (800) 341-4747
FAX: (972) 323-2396 [email protected] www.optekinc.com
Issue A
06/06
Page 1 of 4
Application Bulletin 229
Designing Encoder Elements
for Two Channel Optical Interrupters
An off-multiple of periods between the center line of the sensor apertures is required for the 90° phase shift.
The off-multiple can be 1/4, 3/4, 1-1/4, 1-3/4, 2-1/4, etc. periods. The Period Width is calculated by taking
the Spacing between centerlines of the sensor divided by the off-multiple (Period Width = Distance between
Aperture Centers / Off-Multiple).
Period Width = (Distance between Aperture Centers / Off-Multiple)
Example: (0.212” / 4.25) = 0.050 (Reference OPB822SD)
(0.212” / 0.75) = 0.283 (Reference OPB822SD)
(0.040” / 1.25) = 0.032 (Reference OPB950)
(0.040” / 0.75) = 0.053 (Reference OPB950)
The opening in the disc
should be greater than the
width of the aperture (0.010”
for both the OPB822SD and
OPB950) otherwise this
would
decrease
the
guaranteed output signal.
The period width will be
divided into an opening for
the light (slot) as well as an
area for closure (web),
blocking the light. In order
to have a 50% duty cycle,
large Period Widths can be
separated into two equal
sections while small Period
Widths may require the slot
to be smaller then the web.
OPB822SD Package
OPB950 Package
CL
LED
CL
0.212 [5.38]
CH-A
0.040 [1.02]
CL
CL
CH-B
CL
0.090 [2.29]
0.125 [3.17]
0.100 [2.54]
0.195 [4.95]
CL
CL
0.175 [4.45]
0.250 [6.35]
For a small Period Width with respect to the device aperture size, the aperture width should be taken into
consideration. The opening (slot) width for the Period Width is calculated by taking 50% of the Period
Width and subtracting 50% of the device aperture width (Opening Width = [0.5 * Period Width] - [0.5 *
Aperture Width]). The closure (web) width is calculated by taking the Period Width and subtracting the
Opening Width (Slot Width = Period Width - Web Width).
Slot Width = ([0.5 * Period Width] - [0.5 * Aperture Width])
Example: ([0.5 * 0.050”] - [0.5 * 0.010”]) = 0.020” (Reference OPB822SD)
([0.5 * 0.283”] - [0.5 * 0.010”]) = 0.137” (Reference OPB822SD)
([0.5 * 0.032”] - [0.5 * 0.010”]) = 0.011” (Reference OPB950)
([0.5 * 0.053”] - [0.5 * 0.010”]) = 0.022” (Reference OPB950)
Web Width = (Period Width - Opening Width)
Example: (0.050” - 0.020”) = 0.030” (Reference OPB822SD)
(0.283” - 0.137”) = 0.146” (Reference OPB822SD)
(0.032” - 0.011”) = 0.021” (Reference OPB950)
(0.053” - 0.021”) = 0.031” (Reference OPB950)
OPTEK reserves the right to make changes at any time in order to improve design and to supply the best product possible.
Issue A
06/06
Page 2 of 4
OPTEK Technology Inc. — 1645 Wallace Drive, Carrollton, Texas 75006
Phone: (972) 323-2200 or (800) 341-4747
FAX: (972) 323-2396 [email protected] www.optekinc.com
Application Bulletin 229
Designing Encoder Elements
for Two Channel Optical Interrupters
The Centerline Pitch Radius, or the optical centerline of the slots, of the encoder disc is determined by
multiplying the Period Width times the number of Periods per Revolution and divided by two times Pi (Pitch
Radius = [Period Width X Periods per Revolution] / [2π]).
Centerline Pitch Radius = ([Period Width X Periods per Revolution] / 2π)
Example: ([0.050” * 128] / 6.283) = 1.016” (Reference OPB822SD)
([0.283” * 32] / 6.283) = 1.440” (Reference OPB822SD)
([0.032” * 128] / 6.283) = 0.652” (Reference OPB950)
([0.053” * 128] / 6.283) = 1.087” (Reference OPB950)
The slot length should be larger than the sensor aperture opening length. As a good rule of thumb, you can
use 2 times the disc eccentricity of the wheel plus the sensor aperture length. While the maximum slot
length would depend on the mechanical design of the disk. Remember the slot should totally fit inside
housing of the encoder.
Minimum Slot Length =
(Aperture Length + [ 2 * Eccentricity]) (assume 0.010” eccentricity for these calculations)
Example: (0.080” + [2 * 0.010”]) = 0.100” (Reference OPB822SD)
(0.030” + [2 * 0.010”]) = 0.050” (Reference OPB950)
Disc material and thickness will vary depending on the application. The minimum width is determined by
the material stiffness, opacity to near Infrared, and eccentricity expected. The maximum width is
determined by the width of the slot and eccentricity expected.
The minimum Disc Radius is equal to the Centerline Pitch Radius plus one half the minimum slot length
plus mechanically suitable support distance. The Maximum Disc Radius should take Centerline Pitch
Radius plus Optical Centerline of the Aperture to the bottom of the encoder slot minus two times the Disc
Eccentricity.
Minimum Disc Radius = (Centerline Pitch Radius + [0.5 * minimum slot length] + support distance
Example: (1.016” + [0.5 * 0.100”] + X” = 1.066” + X” (Reference OPB822SD)
(1.440” + [0.5 * 0.100”] + X” = 1.490” + X” (Reference OPB822SD)
(0.652” + [0.5 * .050”] + X” = 0.702” + X” (Reference OPB950)
(1.087” + [0.5 * .050”] + X” = 1.137” + X” (Reference OPB950)
Maximum Disc Radius = (Centerline Pitch Radius + Optical Center Line of aperture to Bottom of the
encoder slot - [2 * Disc Eccentricity])
Example: (1.019” + 0.155” - [2 * 0.010”]) = 1.149” (Reference OPB822SD)
(1.441” + 0.155” - [2 * 0.010”]) = 1.571” (Reference OPB822SD)
(0.652” + 0.235 - [2 * 0.010”]) = 0.867” (Reference OPB950)
(1.080” + 0.235 - [2 * 0.010”]) = 1.295” (Reference OPB950)
The Disc should be made from a material that is compatible with the application. Types of material could be
Plastic, Metal, or Glass with openings and or a pattern applied to it. The encoder wheel should have the
pattern as close to the sensor side of the encoder as possible taking into account end play of the encoder.
OPTEK reserves the right to make changes at any time in order to improve design and to supply the best product possible.
OPTEK Technology Inc. — 1645 Wallace Drive, Carrollton, Texas 75006
Phone: (972) 323-2200 or (800) 341-4747
FAX: (972) 323-2396 [email protected] www.optekinc.com
Issue A
06/06
Page 3 of 4
Application Bulletin 229
Designing Encoder Elements
for Two Channel Optical Interrupters
The digital sequence
expected from the
each configuration is
dependent on the
position
of
the
apertures
of
the
device in reference to 0.212 / 4.25
the slots in the
encoder wheel. As
can be seen, the 0.212 / 3.75
sequence of where
channel
A
and
channel B cross the 0.212 / 3.25
opening is dependent
upon the off-multiple
and the spacing of
the channels.
OPB822SD Spacing
OPB950 Spacing
Channel-A
Channel-A
Channel-B
Channel-B
0.040 / 1.25
0.032 [0.81]
0.050 [1.27]
0.040 / 0.75
0.057 [1.44]
0.053 [1.35]
0.040 / 0.25
0.065 [1.66]
0.160 [4.06]
As the channel A of
OPB822SD Spacing
OPB950 Spacing
the OPB822SD is in
an opening for the
A 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
0.212
/
4.25 A 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
B
B 01 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 0 1 0 1 0 1 0 1 0 1 0 1 0 10
configuration with the
pulse width of 0.050”,
0.030 [0.76] Typ.
0.031 [0.79] Typ.
channel B is in the
middle of an opening.
On the other hand
when channel A of the
0.020 [0.51] Typ.
0.022 [0.56] Typ.
OPB950 is in an
opening for the .040 /
Example: Digital Pattern for OPB950
0.75
configuration, Example: Digital Pattern for OPB822SD
with
0.050”
Period
Width
with 0.053” Period Width
channel B is in the
middle of a closed
part of the encoder wheel. This changes the logical
sequence as seen below. By knowing the expected
logical sequence, the direction as well as the speed
of the wheel can be identified.
As in the
OPB822SD example the expected sequence is
1-1, 1-0, 0-0, 0-1 where the expected sequence for
the OPB950 is 1-0, 1-1, 0-1, 0-0. As can be seen
the 1-0 and 0-1 logical level sequences are
OPB950
reversed therefore you need to know the expected
sequence to identify the direction of the encoder
wheel.
Off-multiples ending in “.25” will output a
OPB822SD
quadrature direction sequence opposite to the
sequence with off-multiples ending in “.75”.
See Application Notes 206 for additional information.
OPTEK reserves the right to make changes at any time in order to improve design and to supply the best product possible.
Issue A
06/06
Page 4 of 4
OPTEK Technology Inc. — 1645 Wallace Drive, Carrollton, Texas 75006
Phone: (972) 323-2200 or (800) 341-4747
FAX: (972) 323-2396 [email protected] www.optekinc.com
```