4
Theory of Operation
This encoder translates rotary motion of a shaft into a two
or three-channel digital output.
As shown in the block diagram, these encoders contain
a single Light Emitting Diode (LED) as its light source.
The light is collimated into a parallel beam by means of
a single polycarbonate lens located directly over the LED.
Opposite the emitter is the integrated detector circuit.
This IC consists of multiple sets of photodetectors and
the signal processing circuitry necessary to produce the
digital waveforms.
The codewheel rotates between the emitter and detector,
causing the light beam to be interrupted by the pattern of
spaces and bars on the codewheel. The photodiodes which
detect these interruptions are arranged in a pattern that
corresponds to the radius and design of the codewheel.
The photodiode outputs are then fed through the signal
processing circuitry resulting in A, A-Bar, B and B-Bar
(also I and I-Bar for 3-channel units). Comparators receive
these signals and produce the nal outputs for channels
A and B. Due to this integrated phasing technique, the
digital output of channel A is in quadrature with that of
channel B (90 degrees out of phase).
The output of the comparator for I and I-Bar is sent to
the index processing circuitry along with the outputs of
channels A and B.
The nal output of channel I is an index pulse PO which
is generated once for each full rotation of the codewheel.
This output PO is a one state width (nominally 90 electri-
cal degrees), high state true pulse which is coincident with
the low states of channels A and B.
Block Diagram
Denitions (see phase diagram)
Count (N): The number of bar and window pairs or counts
per revolution (CPR) of the codewheel.
One Cycle (C): 360 electrical degrees (°e), 1 bar and window
pair.
One Shaft Rotation: 360 mechanical degrees, N cycles.
Position Error (∆Θ): The normalized angular dierence
between the actual shaft position and the position
indicated by the encoder cycle count.
Cycle Error (∆C): An indication of cycle uniformity. The dif-
ference between an observed shaft angle which gives rise
to one electrical cycle, and the nominal angular increment
of 1/N of a revolution.
Pulse Width (P): The number of electrical degrees that an
output is high during 1 cycle. This value is nominally 180°e
or ½ cycle.
Pulse Width Error (∆P): The deviation, in electrical degrees,
of the pulse width from its ideal value of 180°e.
State Width (S): The number of electrical degrees between
a transition in the output of channel A and the neighbour-
ing transition in the output of channel B. There are 4 states
per cycle, each nominally 90°e.
State Width Error (∆S): The deviation, in electrical degrees,
of each state width from its ideal value of 90°e.
Phase (φ): The number of electrical degrees between the
center of the high state of channel A and the center of the
high state of channel B. This value is nominally 90°e for
quadrature output
Phase Error (∆φ): The deviation of the phase from its ideal
value of 90°e.
Index Pulse Width (P
O
): The number of electrical degrees
that an index output is high during one full shaft rotation.
This value is nominally 90°e or ¼ cycle.
RESISTOR
LENS
LED
3
EMITTER SECTION CODE
WHEEL
DETECTOR SECTION
5
GND
1
V
CC
4
CHANNEL A
CHANNEL B
COMPARATORS
PHOTO-
DIODES
SIGNAL
PROCESSING
CIRCUITRY
+
-
A
A
+
-
B
B
+
-
I
I
2
CHANNEL I
INDEX
PROCESSING
CIRCUITRY
