3
Theory of Operation
The HEDR-542X translates rotary motion of a shaft into a
two channel digital output. As seen in the block diagram,
the HEDR-542X series has three key parts: a single Light
Emitting Diode (LED) light source, a photodetector IC
with a set of uniquely con gured photodiodes, and a pair
of lenses. The lens over the LED focuses light onto the
codewheel. Light is either re ected or not re ected back
to the lens over the photodetector IC. As the codewheel
rotates an alternating pattern of light and dark corre-
sponding to the pattern of the codewheel falls upon the
photodiodes. This light is used to produce internal signals
A and A', and B and B'. As part of this "push-pull" detector
system, these signals are fed through comparators that
are part of the signal processing circuitry to produce the
nal outputs for channels A and B.
Defi nitions
Count (N): For rotary motion, the number of bar and window
pairs or Counts Per Revolution (CPR) of the codewheel. For
linear motion, the number of bar and window pairs per
unit length (Lines Per Inch [LPI] or Lines Per mm [LPmm]).
One Cycle (C): 360 electrical degree (°e), 1 bar and window
pair.
One Shaft Rotation: 360 mechanical degrees, N cycles
(rotary motion only).
Line Density: The number of re ective and non-re ective
pairs per unit length, expressed as either Lines Per Inch
(LPI) or Lines Per mm (LPmm).
Pulse Width (P): The number of electrical degrees that an
output is high during one cycle, nominally 180°e or 1/2
a 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 the electrical degrees
between a transition in the output of the 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 (): The number of electrical degrees between the
center of the high state on the channel A and the center
of the high state of channel B. This value is nominally 90°e.
Phase Error (): The deviation in electrical degrees of the
phase from its ideal value of 90°e.
SIGNAL
PROCESSING
CIRCUITRY
R
V
LED
GND
V
CC
CH A
CH B
GND
CODEWHEEL
Output Waveforms
Direction of Motor Rotation
When the codewheel rotates in the
clockwise direction (as viewed from the
encoder end of the motor), channel A will
lead channel B. If the codewheel rotates in
the counterclockwise direction, channel B
will lead channel A.
C
P
S1 S2 S3 S4
Ø
CH. I
CH. A
CH. B
CODEWHEEL ROTATION
AMPLITUDE
ALL FOUR STATES (S1 TO S4)
ARE MAINTAINED
