Theory of Operation
Definitions
Pulse Width (P): The number of
the electrical degrees that an
output is high during one cycle,
nominally 180°e or 1/2 a cycle.
The HEDL-64XX translates
Count (N): For rotary motion,
the number of bar and window
pairs or counts per
rotary motion of a shaft into a
two or three channel digital
output. The HEDL-64XX series
has four key parts: a single light
emitting diode (LED) light
source, a photodetector IC with
a set of uniquely configured
photodiodes , a line drive IC
and a pair of lenses. The lens
over the LED focuses light onto
the codewheel. As the codewheel
rotates, an alternating pattern of
light and dark corresponding to
the pattern of the codewheel
falls upon the photodiodes.
revolution(CPR) of the
Pulse Width Error(∆P): The
deviation in electrical degrees of
the pulse width from its ideal
value of 180°e.
codewheel. For linear motion,
the number of bar and window
pairs per unit length (lines per
inch[LPI] or lines per mm
[LPmm]).
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.
One Cycle(C): 360 electrical
degree(°e), 1 bar and window
pair.
One Shaft Rotation: 360
mechanical degrees, N cycles
(rotary motion only).
State Width Error (∆S): The
deviation in electrical degrees of
each state width from its ideal
value of 90°e.
This light is used to produce
internal signals A and A’, and B
and B’.
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).
Phase (f): 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.
As part of this “push-pull”
detector system, these signals
are fed through comparators and
line driver that are part of the
signal processing circuitry to
produce the final outputs for
channels A and B and index.
Cycle error(∆C):: An indication
of cycle uniformity. The
difference between an observed
shaft angle which gives rise to
one electrical cycle and the
nominal angular increment of 1/
N of a revolution
Phase error (∆f): The deviation
in electrical degrees of the phase
from its ideal value of 90°e.
Position error (∆q):: The
normalized angular difference
between the actual shaft position
and the position indicated by
the encoder cycle count.
RESISTOR
Vcc
PHOTODIODE
Index Pulse Width (Po): The
number of electrical degrees that
an index is high during one full
shaft rotation. This value is
nominally 90°e or ¼ cycle.
Ch A
Ch A
Ch B
LED
SIGNAL
PROCESSING
CIRCUITY
LENS
Ch B
I
I
CODEWHEEL
Gnd
3