4
Theory of Operation
The HEDS-65xx / HEDL-65xx
translate the rotary motion of a
shaft into either a two or three
channel digital output.
The HEDS-65xx uses one of
the standard HEDS-9000 or
HEDS-9040 modules for
encoding purposes. The
HEDL-654x uses the standard
HEDL-9040 for encoding
purposes.
As seen in the block diagram,
these modules contain a single
Light Emitting Diode (LED) as
their light source (emitter). The
light is collimated into a single
parallel beam by means of a
plastic lens located directly over
the LED. Opposite the emitter is
the integrated detector circuit
(detector). This circuit 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 a 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. These
detectors are also spaced such
that a light period on one pair of
detectors corresponds to a dark
period on the adjacent pair of
detectors. The photodiode outputs
are then fed into the signal
processing circuitry resulting
–
–
–
in A, A, B, and B (I and I also in
the three channel encoders).
Comparators receive these
signals and produce the final
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).
In the HEDS-6540 / HEDL-6540
the output of the comparator for
the index pulse is combined with
that of the outputs of channel A
and channel B to produce the final
index pulse. The index pulse is
generated once every rotation of
the codewheel and is a one state
width (nominally 90 electrical
degrees), true high index pulse.
It is coincident with the low states
on channels A and B.
Pulse Width (P): The number of
electrical degrees that an output
is high during one 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.
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
high state on channel A and
the center of the high state on
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.
Direction of Rotation: When the
codewheel rotates in a counter-
clockwise direction (when viewed
from the encoder end of the motor)
channel A will lead channel B.
If the codewheel rotates in the
clockwise direction channel B
will lead channel A.
Index Pulse Width (P0): The
number of electrical degrees that
an index output is high during
one full shaft rotation. This value
is nominally 90 e or 1/4 cycle.
Definitions
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 normal-
ized angular difference between
the actual shaft position and the
position indicated by the encoder
cycle count.
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.
HP [ AGILENT(HEWLETT-PACKARD) ]
