
3
Theory of Operation
The HEDR-542X translates rotary
motion of a shaft into a two chan-
nel 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 photodetec-
tor IC with a set of uniquely con-
figured photodiodes, and a pair
of lenses. The lens over the LED
focuses light onto the codewheel.
Light is either reflected or not
reflected 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 photo-
diodes. This light is used to pro-
duce internal signals A and A',
and B and B'. As part of this
"push-pull" detector system, these
signals are fed through compara-
tors that are part of the signal
processing circuitry to produce
the final outputs for channels A
and B.
Definitions
Count (N):
For rotary motion,
the number of bar and window
pairs or Counts Per Revolution
(CPR) of the codewheel. For lin-
ear 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 me-
chanical degrees, N cycles (rotary
motion only).
Line Density:
The number of
reflective and non-reflective 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, nomi-
nally 180e or 1/2 a cycle.
Pulse Width Error (
P):
The
deviation in electrical degrees of
the pulse width from its ideal
value of 180e.
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 90e.
State Width Error (
S):
The
deviation in electrical degrees of
each state width from its ideal
value of 90e.
Phase (
φ
):
The number of electri-
cal 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 nomi-
nally 90e.
Phase Error (
φ
):
The deviation
in electrical degrees of the phase
from its ideal value of 90e.
SIGNAL
PROCESSING
CIRCUITRY
R
VLED
GND
VCC
CH A
CH B
GND
CODEWHEEL