2
Theory of Operation
The HEDB-9040 and 9140 is emitter/detector modules.
Coupled with a codewheel, these modules translate
the rotary motion of a shaft into a three-channel digital
output.
As seen in Figure 1, the modules contain a single Light
Emitting Diode (LED) as its light source. The light is col-
limated into a parallel beam by means of a single poly-
carbonate 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 process-
ing 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 code-wheel. These detectors are also spaced
such that a light period on one pair of detectors corre-
sponds to a dark period on the adjacent pair of detectors.
The photodiode outputs are then fed through the signal
processing circuitry resulting in A, Abar, B, Bbar, I and
Ibar. Comparators receive these signals and produce
the nal outputs for channels A and B. Due to this inte-
grated phasing technique, the digital output of channel
A is in quadrature with that of channel B (90 degrees out
of phase).
De nitions
Note: Refer to Figure 2
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 (): The normalized angular di erence
between the actual shaft position and the position
indicated by the encoder cycle count.
Cycle Error (C): An indication of cycle uniformity. The di er-
ence 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 1/2 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 neighboring
transition in the output of channel B. There are 4 states per
cycle, each nominally 90°e.
Phase (): 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 (): The deviation of the phase from its ideal
value of 90°e.
Direction of Rotation: When the codewheel rotates in the
clockwise direction viewing from top of the module
(direction from V to G), channel A will lead channel B. If
the codewheel rotates in the opposite direction, channel
B will lead channel A.
Optical Radius (Rop): The distance from the codewheel’s
center of rotation to the optical center (O.C) of the encoder
module.
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 1/4 cycle.
Speci cation
For encoder electrical, mechanical speci cations, code-
wheel technical speci cations and additional informations
please refer to :
HEDS-9040 /9140 datasheet.
HEDS/HEDG/HEDM – 51xx/61xx Codewheel Datasheet
