2
Block Diagram of AEDR-8000 Encoder
Phase Error (Df): The deviation of phase, in electrical de-
grees, from its ideal value of 90°e.
Pulse Width (P): The duration of high state of the output,
in electrical degrees, within one cycle. Nominally 180°e
or half a cycle.
Pulse Width Error (DP): The deviation of pulse width, in
electrical degrees, from its ideal value of 180°e.
Count (N): The number of window and bar pairs per revo-
lution (CPR) of codewheel. For linear codestrip, defined
as the number of window and bar pairs per unit length
(lines per inch [LPI] or lines per mm [LPmm]).
One Cycle (C): 360 electrical degrees (°e). Equivalent to
one window and bar pair.
One Shaft Rotation: 360 mechanical degrees. Also equiv-
alent to N counts (codewheel only).
Line Density: The number of window and bar pairs per
unit length, expressed in either lines per inch (LPI) or
lines per mm (LPmm).
Optical Radius (Rop): The distance between the code-
wheel center and the centerline between the two domes
of the encoder.
Gap (G): The distance from the surface of the encoder to
the surface of codewheel or codestrip.
Specular Reflectance (R
f
): The amount of incident light
reflected by a surface. Quantified in terms of the per-
centage of incident light. A spectrometer can be used to
measure specular reflectance of a surface (contact fac-
tory for more information).
Radial and Tangential Misalignment Error (E
R
, E
T
): For ro-
tary motion, mechanical displacement in the radial and
tangential directions relative to the nominal alignment.
Angular Misalignment Error (E
A
): Angular displacement
of the encoder relative to the tangential line.
Theory of Operation
The AEDR 8000 Encoder series combines an emitter and
a detector in a single surface mount leadless package.
When used with a codewheel or linear codestrip, the en-
coder translates rotary or linear motion into digital out-
puts. As seen in the block diagram, the AEDR-8000 En-
coder series consists of three major components: a light
emitting diode (LED) light source, a detector IC consist-
ing photodiodes, and lens to focus light beam from the
emitter as well as light falling on the detector.
The operation of the encoder is based on the principle of
optics where the detector photodiodes sense the absence
and presence of light. In this case, the rotary/linear mo-
tion of an object being monitored is converted to equiv-
alent light pattern via the use of codewheel/codestrip.
As shown in the above diagram, the reflective area (win-
dow) of the codewheel (or codestrip) reflects light back
to the photodetector IC, whereas no light is reflected by
the non-reflective area (bar). An alternating light and
dark pattern, corresponding to the window and bar, falls
on the photodiodes as the codewheel rotates. The mov-
ing light pattern is exploited by the detector circuitry to
produce digital outputs representing the rotation of the
codewheel. When the codewheel is coupled to a motor,
the encoder outputs are then a direct representation of
the motor rotation. The same concept applies to the use
of a codestrip to detect linear motion.
Definitions
State Width (S): The number of electrical degrees be-
tween a transition in Channel A and the neighboring
transition in Channel B. There are 4 states per cycle, each
nominally 90°e.
State Width Error (DS): The deviation of state width, in
electrical degrees, from its ideal value of 90°e.
Phase (f): The number of electrical degrees between the
center of high state of Channel A and the center of high
state of Channel B. Nominally 90°e.
SIGNAL
PROCESSING
CIRCUITRY
V
CC
CH A
V
LED
GND
CH B
GND
R
CODEWHEEL
or
CODESTRIP
