Note: Descriptions are shown in the official language in which they were submitted.
Bac~round of the Invent;on
This invention relates to analog-to-digita]
converters, and more particularly~ to an op-tical encoder
apparatus for indicating revolutions of a shaft.
Optical encoders are commonly used to convert
shaft positional information into digital signals. The
information provided by the encoder may represent the
angular position of the shaft or the number of revolutions
made by the shaft. In the former case, the encoder
includes a complex code member which designates or defines
a number of discrete shaft positions, commonly referred
to as digit positions, and t~e encoder provides a different
multibit binary-coded word for each digit position. The
presence of a complex code member permits shaft position
encoders to take into account interdigital positions so
as to provide an unambiguous coding of shaft position
information.
Shaft position encoders, on the other hand, are
of simple construction, including a single sensor, a light
source and a shutter rotated by the shaft. The shutter
is adapted to normally interrupt or block the illumination
of the sensor by light from the light source, but to permit
illumination of the sensor during a short time for each
revolution of the shutter. Upon illumination, the sensor
provides a change in its output state. The number of times
that the output state changes over a given period is a
representation of the number of complete revolutions of
the shaft during that period.
Ideally, for single sensor encoders, the output
state of the sensor will change only once each revolution,
and under such ideal condition, the encoder would provide
an unambiguous indiction at the completion of each
revolution of the shaft. However, various conditions,
either mechanical or electrical in nature, may cause
multiple false counts in single sensor encoders. For
example, in a multi-dial utility meter register which
measures usage of a commodity such as gas, water or
electricity, the shafts are rotated at a very slow speed
through the use of reduction gear train, and the rotational
speed of the shafts varies with changes in demand of the
commodity. Under such conditions, shutter backlash may
cause multiple outputs to be provided during one
revolution. Other factors contributing -to multiple false
counts are ambient light leakage and device tolerances,
particularly when semiconductor light sensing devices are
usedO
S a-~ ol t~ c~
It is an object of this invention to provide
an encoder apparatus for indicating revolutions of a shaft
and which also provides an unambiguous indication at the
completion of each revolution of the shaft.
Another object of the invention is to provide
an encoder apparatus characterized by low power usa~e.
These and other objects are achieved by the
presen-t invention which provides an encoder appara-tus for
indicating revolutions of a shaft. The encoder apparatus
comprises a source of light, first and second light sensing
means, and a light barrier means driven by the shaft to
shade said first and second light sensing means from said
source of light during a portion of each revolution of
the shaft. The first and second light sensing means are
located in a spaced-apart relationship to be illuminated
in sequence as said light barrier means is driven by the
shaft, and said l;ght barrier means is configured to enable
concurrent illumination of said first and second light
sensing means. A signal processing means responds to both
of said sensing means~ providing the same output
concurrently to provide an indication of the completion
of a revolution of the shaft. Thereafter, the signal
processing means is nonresponsive to outputs provided by
the light sensing means, until both of the outputs have
terminated concurrently, so that the signal processing
means provides an unambiguous indication at the completion
of each revolution of the shaft.
In accordance with a disclosed embodiment, the
signal processing means provides its indication in response
--3--
to outputs indicative of se~uential and concurrent
illumination of the two light sensing means; i.e., the
light sensing means are illuminated in sequence, and the
second light sensing means is illuminated while the first
light sensing means is still illuminated. Thereafter,
the signal processing means does nok again respond to an
output, indicative of illumination, whether provided by
either one or by both of the sensing means until after
such time as the outputs provided by both sensing means
have indicatecl that both sensing means have been shaded
from the source of light.
The location of -the firs-t and second light
sensing means and the configuration of the light barrier
means enable the sensing means to be illuminated in
sequence and concurrently. To this end, the light barrier
means comprises a planar member having a light impervious
portion which is semicircular in shape. The member is
carried by the shaft and rotatable therewith to have its
light impervious portion interposed between the source
of light and the first and second light sensing means as
the shaft rotates through a portion of a revolution, and
to permit illumination of at least one of the sensing means
during the remainder of the revolution of the shaft as
the light impervious portion is rotated out from between
the sensing means and the source of light. Moreover, the
first and second light sensing means are located in a
spaced relationship on the same circumference, i.e., at
the same radial distance from the axis of the shaft.
In the disclosed embodiment, the signal
processing means includes a bistable circuit which is
operated to one stable state in response to outputs
provided when both light sensing means are illuminated.
The bistable circuit is maintained in such state as long
as either one of the light sensing means continues to be
illuminated and provides its output. The bistable circuit
is operated to its other stable state when both light
sensing means become shaded from the light source and both
outputs are terminated. In this arrangement, the bistable
--4~
circuit provides a binary pulse output at a first level
for that portion oE revolution oE the shaft for which the
light barrier means shades the two light sensing means
and provides an output at a second level for the remainder
of the shaft revolution cycle.
In accordance with a feature of the invention,
the ligh-t source means comprises an infrared light emitting
diode which is pulse driven to minimize power consumption.
This results in a low power usage liyht source which
renders the encoder apparatus suitable for use in
aplications where the power source is a battery.
De~ tion of the D~
FIG. 1 is a perspective view of an optical
encoder apparatus provided by the present invention;
FIG. 2 diagramatically illustrates in plan
arrangement the light source and the photosensors;
FIG. 3 is a schematic circuit diagram of a signal
processing circui-t of the optical encoder apparatus
illustrating the light source and the photosensors, and;
FIG. 4 is a schematic representation illustrating
positions of the light barrier relative to the
photosensors.
Descri~tion of a_Preferred Embodiment
Referring to FIG. 1, the optical encoder
apparatus 10 includes a light source 11, a light detecting
circuit 12, and a shutter or light barrier 13, carried
by a shaft 14. The optical encoder apparatus 10 generates
an output indicative of the number of revolutions of the
shaft 14 and provides an output suitable for transmission
to a data processing center using remote interrogation
techniques known in the art.
By way of example, the shaft 14 may be associated
with a register dial (not shown) of a utility meter which
measures the consumption of a commodity such as gas, water,
electricity, and the like. The shaft carries a pointer
(not shown) which provides a visual indication of the
angular portion of the shaft to thereby indicate a measured
quantity. Such meter registers generally have several
--5--
dials and associated shaEt mounted pointers, each dial
indicating a differen~ digit of measurement, such as
hundreds, tens, units, tenths, etc. The encoder apparatus
of the present invention may count the number of
revolutions made by any one of these dial shafts. The
selection of which shaft for which rotations are to be
counted is a function of application, fre~uency of
interrogation, etc.
The light source 11 includes an infrared light
emitting diode 15 and a drive circuit 16 which provides
a pulsed drive for energizing the light emitting diode
15. The light emitting diode 15 is located above the plane
of the barrier 13 and directs infrared radiation
downwardly.
The light detecting circuit 12 comprises a pair
of light sensing devices 17 and 18 and a signal processing
circuit 20. The light sensing devices are located beneath
the plane of the barrier 13.
The barrier 13 is a planar member generally
semicircular in shape and defining a hub 13a at its center
by which it is attached to the shaft 14 in a suitable
manner. The barrier is of a lightweight material and is
light impervious so as to shade the two phototransistors
17 and 18 from the light source when the barrier 13 is
interposed therebetween. While illustrated in FIG. 1 as
being generally semicircular, the barrier rnay be
disc-shaped and having a light impervious position to
permit selective illumination of the light sensing devices
as the barrier is rotated.
Referring to FIG. 2, the encoder lO includes
a housing 19, which may be generally C-shaped, for
example. The housing has an upper horizontally extending
portion l9a which mounts the light emitting diode 15 with
its radiation surface oriented downward towards the
phototransistors 17 and 18 which are mounted in a lower
portion l9b of the housing with their light sensitive
surfaces directed upwardly, opposing the light emitting
diode 15. The vertical spacing between the light emitting
--6--
diode 15 and the phototransistors is suEficient to enable
the harrier 13 to pa.ss therebetweenO The housing also
shades the phototransistors 17 and 18 from direct
illumination by ambient light.
The phototransistors 17 and 18 are spaced apart
on the same circumference as indicated in the simplified
layout shown in FIG. 4, iOe., the same radial distance
from -the axis of the shaft. As the barrier 13 is rotated
to move between the ligh-t emitting diode and the
phototransistors, its leading edge 13a will pass over the
two phototransistors 17 and 18 in succession. The light
emitting diode 15 (FIG. 2), which is located centrally
between and above the two phototransistors 17 and 18, is
chosen for its half-intensity angle which should be no
less than 0 to insure illumination of both
phototransistors. One light emitting diode suitable for
this encoder is the type CQX-47, commercially available
from AEG-Telefunken.
Referring to FIG. 3, the drive circuit 16 for
the light emitting diode 15 includes a pulse generator
21, the output of which is connected through a resistor
Rl to the base of a transistor Ql. The light emitting
diode 15 is connected in series with a resistor R2 in the
collector circuit of transistor Ql which has its emitter
connected to ground.
The pulse generator 21 provides pulses ~t a ra-te
at least twice the maximum speed expected for the shaft
and of a duration of about 40 microseconds. Each pulse
applied to the base of transistor Ql turns on the
transistor Ql, energizing the light emitting diode 15 to
generate a light pulse for approximately ~0 microseconds.
The peak current for the diode is determined by resistor
R2 and the supply voltage ~V, obtained from a 1.5 volt
battery, or from an AC to DC converter when AC power is
available. By way of example 7 the peak current for the
diode is established to be in the range of 10 to 20
milliamps to provide sufficient radiation intensity to
drive the phototransistors to saturation. At this light
~ ;~,P~ ,r~;~?~
level, the rise time of the waveform of the signal at the
emitter of the phototransistor will be in the range of
10 to 20 microseconds. Although the all time is much
longer than the waveform rise time, in the exemplary
embodiment, system response is predicated on the rise time.
The phototransistors 17 and 18 may be the type
B~W-42, commercially available ~rom AEG-Telefunken. Each
of khe phototransistors 17 and 18 has its collector
connected to the supply voltage +V and its emit-ter
connected to an associated resistor R3 to ground, and to
inputs of the signal processing circuit 12.
The signal processing circuit 20 comprises a
pair o~ ~ND gates 22 and 23, a latch circuit 2~, and a
pair of inverters 25 and 260 The outputs of both
phototransistors 17 and 18 at the emitters thereof are
connected to respecti~7e inputs 22a and 22b of AND gate
22, wh ich has a third input 22c connected to an output
of a strobe pulse generator which provides a strobe pulse
at a rate corresponding to that of the pulse generator
21. The outputs of transistors 17 and 18 are also
connected through inverters 25 and 26 to respective inputs
23a and 231~ of AND gate 23, which has its third input 23c
connected to the output of the strobe pulse generator 27.
The output 22d of ~ND gate 22 is connected to the set input
of the latch circuit 24 which has its reset input connected
to the output 23d of AND gate 23. The true output of latch
circuit 24 is at a logic one level whenever the latch is
set and is at a logic zero level when the latch circuit
2~ is in its reset state.
The AND gate 22 is primed to be enabled whenever
both phototransistors 17 and 18 are conducting. When
primed, the AND gate 22 is enabled by the next strobe pulse
and generates a set pulse for the latch circuit 24. The
outputs of the phototransistors as extended through
inverters 25 and 26, maintain AND gate 23 disabled when
gate 22 is primed.
When either of the two phototransistors 17 or
18 is shaded by the barrier 13, neither A~D gate 22 or
J~9~
~8--
23 can be enabled, but when both phototransistors 17 and
1~ are shaded, AND gate 23 is primed and is enabled by
the next strobe pulse to generate a reset pulse for the
latch circuit 24.
Once the latch circuit 24 is set in response
to illumination and conduction of both phototransistors
17 and 18, the latch circuit will remain set even though
the output of one phototransistor may fluctuate.
Subsequently, when both transistors are shaded, AND gate
23 is enabled to cause the latch circuit to reset. Again,
although the output of one of the phototransistors may
fluctuate, the latch circuit 24 will remain reset until
both phototransistors are subsequently illuminated at the
same time.
FIGo 4 illustrates the manner in which the
phototransistors 17 and 18 are arranged to be selectively
and successively illuminated as a function of angular
position of the barrier 13. The state of the latch circuit
24 as well as the state of the two AND gates 22 and 23
(when strobed), are summari~ed in Table I. The legends
S and R represent the condition of the outputs of
respective gates 22 and 23, and legend Q represents the
true output of latch circuit 24. The designation "x"
indicates that the output may fluctuate between logic one
and logic 0.
TABEE I
BARRIER POSITION S R Q
A 1 0
B x 0
C O 0
D 0 x 0
E 0 1 0
F 0 x 0
G 0 0 0
H x 0
When the barrier is in the position illustrated,
designated position A, both phototransistors 17 and 18
. ...................................................................... .
p ~ L'q`l~
_g_
are uncovered and thus fully illuminated by the infrared
beam from the light emitting diode 15~ At position B,
phototransistor 17 will be partially covered and its output
may fluctuate between logic one and logic zero. As the
light level decreases, the rise time of the signal at the
emitter of phototransistor 17 will increase, and at some
point the signal will not be of sufficient amplitude when
the strobe signal occurs to register a logic one~ At this
point, any slight change in conditions, including ambient
light leakage may cause the output to fluctuate. Output
fluctuation may also be due -to backlash of the gear train
which drives the shaft. At position C, transistor 17 is
fully covered.
~t position D, phototransistor 1~ ~ill be
partially covered and its output may fluctuate. At
position E, both phototransistors are Eully covered and
shaded from the light source, and at position F,
phototransistor 17 will be partially uncovered and its
output may fluctuate.
Transistor 17 is uncovered completely at position
G and at position H transistor 18 will be partially
unco~Jered and its output may fluctuate.
In summary, as the barrier approaches position
H, the latch circuit 24 is set the first time AND gate
22 is enabled, i.e., in response to a strobe signal
provided for the first time phototransistor 18 is
illuminated while phototransistor 17 is still illuminated.
The latch circuit 24 is reset as the barrier approaches
position D, and phototransistor 18 first becomes shaded
by the barrier while phototransistor 17 is also shaded
3~ by the barrier.
The signals generated by latch circuit 24 may
be counted and the count stored and used to indicate the
number of revolutions completed by the shaft.
Operation
Referring to FIG~. 3 and 4, it is assumed that
initially the barrier 13 is moving away from the reEerence
point H towards position A so that when the light emitting
--1.0--
diode 15 i~ energ.ized, both phototransist~rs 17 and 18
are .illuminated, ancl that the latch circuit 24 is set in
its true state providin~ a .logic one level output,
indicating completion of the last revolution of the shaft.
~s the barrier 13 rotates and approaches position
Br phototransistor 17 becomes partially covered, and i-ts
output will fluctuate as shown in the Table I. Each time
the output of phototrans.istor 17 is high, AND ~ate 22,
when strobed, will produce a set pulse, but such pulse
will have no effect on the previously latched state of
10 the latch circuit 24. The output of transistor 17 will
eventually stabilize and remain low as the leading edge
of the barrier 13 approaches position C.
With continued rotation of the shutter, when
position D is reached, phototransistor 18 becomes partially
15 covered and its output may fluctuate between zero and one.
The output of phototransistor Q3 coupled with the steady
zero output from phototransistor 17 generates a reset pulse
via AND gate 23, resetting the latch circuit 24, and its
Q output is latched in the zero state. Although the output
20 of phototransistor 18 may briefly fluctuate between logic
zero and logic one, genera-ting occasional reset pulses
for the latch circuit 2~, these pulses have no effect on
the state of the latch circuit 24 wh.ich rernains latched
in the zero stateO
With continued rotation of the barrier 13 towards
position E, transistor 18 becomes covered by the barrier
so that both phototransistors 17 and 18 provide a steady
zero output.
When the barrier reaches position F,
phototransistor 17 becomes partially uncovered and its
output will begin to fluctuate as shown in the Truth Table
I. When the barrier has moved to position G,
phototransistor 17 is fully uncovered; but, because
phototransistor 18 remains covered, no set pulses are
generated. When the barrier moves to position H,
phototransistor 18 becomes partially uncovered and its
output begins to fluctuate. With the first logic one level
~ J~ r~
output coupled with the steacly state output of
phototransistor 17, gate 22 is again enabled, generating
a set pulse for the latch circuit 24, providing an
indication that the shaft has reached the reference point
indicating completion of the present revolution. The latch
5 circuit ~4 then remains set as -the barrier continues in
its movement towards the initial position A~
Software Im~lementation
The signal processing circuit ~0 of the encoder
apparatus 10 may be implemented by software. In such
arrangement, the outputs of the two photo-transistors 17
and 1~ are scanned periodically to detect eoincidence of
first signals indicating that both phototransistors are
eonduc-ting and second signals indicating that both
phototransistors are non-eondueting. The first oceurrenee
of coineidence of the first signals in a given cycle of
revolution of the shaft defines a reference causing a
status flag to be set and a revolution count reg;ster to
be stepped. Once the status flag is set, deteetion of
coincidence o~ first signals is ignored. The first
oecurrence of coincidenee of the second signals eauses
the reset of the status flag so that the system is prepared
to deteet eoineidence of the first signals, indieative
of eo~pletion of another revolution of the shaft.
