Abstrict An improved resolution positive displacement flow meter of the
type having a pair of meshed gears. A plurality of sensors are spaced
to detect movement of the gear teeth as liquid flows through the
flow meter. Where the gears each have n teeth and s sensors are
used, the possible locations around the gear for each sensor relative
to a reference point are determined by the formula ##EQU1## where
K=012 . . . ,(s-1) and L=012 . . . ,(n-1). The outputs from
the sensors are logically combined, for example, by an exclusive
OR circuit or by an OR circuit, to produce s pulses each time the
gear rotates through the angle between two adjacent gear teeth.
Claims We claim:
1. In a positive displacement liquid flow meter having two gears
mounted to rotate in overlapping cylinders, said gears having teeth
which mesh in the region where said cylinders overlap, input and
output chambers located on opposite sides of said meshed gear teeth,
means for delivering liquid to said input chamber and means for
receiving liquid from said output chamber, and means for sensing
movement of said gear teeth for measuring the flow of liquid through
said flow meter, the improvement wherein said sensing means includes
a plurality of sensor spaced for detecting movement of said gear
teeth, said sensors having angular spacings around at least one
of said gears from a predetermined reference location for sequentially
sensing different ones of said teeth as said gears rotate through
an increment equal to 360.degree. n where n= the number of teeth
on each gear.
2. An improved positive displacement liquid flow meter, as set
forth in claim 1 wherein said sensors are spaced to sequentially
sense a different one of said teeth each time said gear rotates
by an increment determined by the formula ##EQU5## where s=the number
of sensors.
3. An improved positive displacement liquid flow meter, as set
forth in claim 2 and including means for combining the outputs
of said sensors for establishing a pulse signal having s pulses
each time said gears rotate through an increment equal to 360.degree./n.
4. An improved positive displacement liquid flow meter, as set
forth in claim 1 and including means for combining the outputs
of said sensors for establishing a pulse signal having s pulses
each time said gears rotate through an increment equal to 360.degree./n.
5. In a positive displacement liquid flow meter having two gears
mounted to rotate in overlapping cylinders, said gears having teeth
which mesh in the region where said cylinders overlap, input and
output chambers located on opposite sides of said meshed gear teeth,
means for delivering liquid to said input chamber and means for
receiving liquid from said output chamber, and means for sensing
movement of said gear teeth for measuring the flow of liquid through
said flow meter, the improvement wherein said sensing means includes
a plurality of sensor spaced for detecting movement of said gear
teeth, said sensors having spacings around at least one of said
gears from a predetermined reference location determined by the
formula ##EQU6## where n=the number of teeth on each gear, s=the
number of sensors, K=012 . . . ,(s-1) and L=012 . . . ,(n-1).
6. An improved positive displacement liquid flow meter, as set
forth in claim 5 wherein said sensing means further includes logic
means for combining outputs from said sensors for producing a pulse
signal having s pulses each time the gears rotate through an increment
equal to 360.degree./n.
7. An improved positive displacement liquid flow meter, as set
forth in claim 6 wherein said logic means combines the outputs
from said sensors through XOR logic.
8. An improved positive displacement liquid flow meter, as set
forth in claim 6 wherein said logic means combines the outputs
from said sensors through OR logic.
Description TECHNICAL FIELD
The invention relates to liquid flow meters and more particularly
to an improved resolution positive displacement liquid flow meter
of the type having two meshed gears rotated by fluid flowing through
the flow meter.
BACKGROUND ART
For many industrial purposes, it is desirable to accurately measure
the flow of liquid. For example, many industrial coatings are formed
from mixing a resin and a hardener just prior to application. When
applying a coating composed of a two component mixture, it is desirable
to accurately measure liquid flow to maintain an accurate mixing
ratio. The coating quality may be significantly affected by relatively
small deviations in the mixing ratio. For many industrial applications
a positive displacement flow meter is used to measure liquid flow.
A typical positive displacement flow meter consists of two meshed
gears mounted to rotate in overlapping cylinders formed in a housing.
Liquid enters an inlet chamber formed between the cylinder walls
and the teeth on the two gears on one side of the location where
the gear teeth mesh. The gears are rotated by the fluid flow until
the fluid trapped between the teeth and the cylinder walls enters
an outlet chamber on the opposite side of the location where the
gear teeth mesh. Each time the gears rotate through an increment
equal to the spacing between two adjacent gear teeth, a volume of
liquid substantially equal to twice the volume trapped between two
adjacent gear teeth on one gear and the cylinder walls is delivered
to the outlet chamber. The flow meter includes a sensor which is
responsive to the movement of the teeth on one gear past a predetermined
location swept by the moving gear teeth. The sensor may be of various
known designs. For example, the sensor may be an electromagnetic
sensor which senses the presence or absence of a gear tooth at the
predetermined location. Each time a gear tooth passes the location,
an electric pulse is generated. By multiplying the number of pulses
over a period of time times the volume of paint delivered through
the flow meter each time the gears rotate through an increment equal
to the gear tooth spacing, paint flow over the period of time is
measured. For certain applications and especially at low flow rates,
a higher resolution is desirable than that available from conventional
positive displacement flow meters.
DISCLOSURE OF INVENTION
According to the invention, the resolution of a positive displacement
flow meter is increased by providing multiple gear sensors positioned
to generate pulses when the flow meter gears rotate through increments
less than the spacing between the gear teeth. The outputs of the
sensors are combined, for example, by exclusive OR (XOR) logic or
by OR logic, to provide a pulse stream with a pulse each time the
gears move through an increment equal to the gear spacing divided
by the number of sensors. For example, if the flow meter gears are
provided with ten teeth, then the leading edge of each tooth is
spaced 36.degree. from the leading edges of the two adjacent gear
teeth. The prior art flow meters generate a pulse stream having
a single pulse each time the gears rotate through 36.degree.. If
the exemplary flow meter delivers 1.2 cc of liquid each time the
gears rotate through 36.degree., then a single pulse is produced
for each 1.2 cc flowing through the flow meter. If two sensors are
used in the same flow meter in accordance with the invention, a
pulse is generated each time the gears rotate through 18.degree.,
or each time 0.6 cc of liquid is delivered. If three sensors are
used, a pulse is generated each time the gears rotate through 12.degree.,
or each time 0.4 cc of liquid is delivered. Thus, the resolution
of the positive displacement flow meter is improved over similar
prior art flow meters having a single gear tooth sensor.
Accordingly, it is an object of the invention to provide a positive
displacement liquid flow meter having a greater resolution than
prior art liquid flow meters.
Other objects and advantages of the invention will be apparent
from the following description and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a positive displacement liquid
flow meter incorporating the invention;
FIG. 2 is a top plan view of the liquid housing for the flow meter,
of FIG. 1 with the sensor housing removed, showing the gears;
FIG. 3 is an enlarged fragmentary top plan view of the two meshed
gears and the adjacent cylinder walls for the flow meter of FIG.
1;
FIG. 4 is a diagrammatic view of a gear with having ten teeth and
five adjacent sensors;
FIG. 5 is a graph showing the relative outputs from the five sensors
in FIG. 4 as the gear is rotated through the angle of one gear tooth
and the XOR logic combination of the five sensor outputs; and
FIG. 6 is a graph showing the relative outputs from the five sensors
arranged for sensing the tips of the teeth on one of the gears in
FIGS. 2 and 3 as the gear is rotated through the angle of one gear
tooth and the OR logic combination of the five sensor outputs.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring to FIG. 1 of the drawings, a positive displacement liquid
flow meter 10 is illustrated. The flow meter 10 generally includes
a liquid housing 11 and a sensor housing 12 which includes a sensor
circuit housing 13. The sensor housing 12 is attached to the liquid
housing 11 by a plurality of screws 14. The liquid housing 11 has
a fluid inlet 15 and a fluid outlet 16 (see FIG. 2). During operation
of the flow meter 10 liquid enters the inlet 15 and after flowing
through the liquid housing 11 is discharged from the outlet 16.
An electric pulse output is produced on a cable 17 from the sensor
circuit housing 13 each time a predetermined quantity of liquid
flows through the flow meter 10.
FIG. 2 is a top plan view of the liquid housing 11 with the sensor
housing 12 removed. The housing defines two overlapping cylinders
18 and 19. A seal 20 is located in a groove on an upper face 21
of the housing 11 to form a liquid tight seal between the liquid
housing 11 and the sensor housing 12 surrounding the cylinders 18
and 19. A gear 22 is mounted on a shaft 23 to rotate in the cylinder
18 and a gear 24 is mounted on a shaft 25 to rotate in the cylinder
19. The gear 22 has a plurality of teeth 26 and the gear 24 has
a like number of teeth 27. The gear teeth 26 and 27 mesh at a common
point 28 between the overlapping cylinders 18 and 19.
In operation, liquid is delivered to the inlet 15 and flows through
a passage 29 to a chamber 30. The chamber 30 is formed between the
gears 22 and 24 on one side of the point 28 where they mesh to the
point where they abut the walls of the cylinders 18 and 19 respectively.
As liquid is forced into the chamber 30 the fluid pressure causes
the gear 22 to rotate in a clockwise direction and causes the gear
24 to rotate in a counter-clockwise direction. Liquid from the chamber
30 is trapped in voids 31 between the teeth 26 on the gear 22 and
the wall of the cylinder 18 and in voids 32 between the teeth 27
on the gear 24 and the wall of the cylinder 19. As the gears 22
and 24 are rotated, liquid in the voids is transferred to a chamber
33. As the teeth approach the point 28 and begin to mesh, the voids
31 and 32 diminish and the fluid which was trapped therein is discharged
through a passage 34 to the liquid outlet 16.
According to the prior art, a single sensor (represented by the
dashed line circle 35 in FIG. 2) was mounted in the sensor housing
12 (FIG. 1) to generate a signal each time one of the gear teeth
26 or 27 passed a predetermined point on the path swept by the gear
teeth as the gears 22 and 24 are rotated. The sensor can be of various
known types and can be located at any convenient location adjacent
the gear teeth path. For example, the sensor 35 may be of the proximity
type which electromagnetically senses the presence or the absence
of a gear tooth at the swept point. Proximity sensors typically
operate in response to changes in capacitance or changes in impedance
in a tuned circuit. The sensor may be, for example, a Hall effect
device.
For the following description of the invention, the flow meter
10 will be considered as having a pair of 10 tooth gears 22 and
24. However, it should be understood that the gears 22 and 24 may
have any desired number of teeth 26 and 27 although they both will
have the same number of teeth. As shown in the enlarged fragmentary
view in FIG. 3 any individual tooth 26' on the gear 22 and any
individual tooth 27' on the gear 24 may be selected as a 0.degree.
reference point. For convenience, the reference point is set at
the leading edge of the tooth 27'. The gear 22 rotates about an
axis 36 and the gear 24 rotates about an axis 37. The angle .alpha.
between the teeth on the gears 22 and 24 is determined by the following
formula ##EQU2## where n equals the number of teeth on each gear
22 and 24. If n=10 as assumed above, than .alpha.=36.degree. between
adjacent gear teeth. Thus, the prior art flow meter will generate
an output pulse each time the gears 22 and 24 rotates through a
36.degree. increment. In other words, a single pulse is generated
for each volume of fluid flow through the flow meter 10 as determined
by the sum volume of the void 31 and the void 32.
According to the invention, several sensors 35 are located to sense
the gear teeth. The multiple sensors 35 either are located to be
sequentially triggered by a single gear tooth as the gears 22 and
24 are rotated through the angle .alpha., or the sensors 35 are
spaced around the area swept by the moving gear teeth to be sequentially
triggered by different gear teeth as the gears 22 and 24 are rotated
through the angle .alpha.. The multiple sensors 35 may be spaced
around only one of the gears 22 or 24 or, if desired, some of the
multiple sensors 35 may be located to be triggered by the teeth
26 on the gear 22 and others of the sensors 35 may be located to
be triggered by the teeth 27 on the gear 24.
In order to provide volume flow pulses representing uniform volume
increments, the minimum angular spacing .beta. between adjacent
sensors is determined by dividing the angle .alpha. between the
n gear teeth by the number of sensors s, as follows: ##EQU3## Typically,
the sensors will be too large to fit within the spacing of a single
gear tooth. This problem is solved by offsetting the sensors around
the gear from the adjacent sensors by multiples of .alpha.. Thus,
the sensor spacings around the gear are determined by the formula
##EQU4## where K=012 . . . ,(s-1) and L=012 . . . ,(n-1).
It will be appreciated that since the gears 22 and 24 must have
the same number of teeth 26 and 27 and the teeth on each gear have
the same spacings, some of the sensors may be located around the
path swept by the teeth 26 on the gear 22 and others of the sensors
may be located around the path swept by the teeth 27 on the gear
24. In each case, the angular location for the sensors are measured
from the reference location for the gear tooth 26' or 27'.
The diagrammatic view of FIG. 4 illustrates a gear 40 having 10
teeth 41 and 5 sensors A-E located to sense the gear teeth 41. Thus,
the gear teeth 41 are spaced apart by .alpha.=36.degree.. The gear
40 is rotated in a counterclockwise direction about an axis 42 as
fluid flows through the flow meter. The Sensor A is located at the
leading edge of a tooth 41' which is arbitrarily selected as the
0.degree. reference. The remaining sensors have primary locations
which are spaced apart from the adjacent sensors by a .beta. of
7.2.degree.. However, due to the symmetry of the gear 40 each sensor
can be placed at the same offset in relation to any gear tooth.
In other words, each sensor can be offset from its primary location
by multiples of .alpha.. Thus, sensor A can be located at 0.degree.,
36.degree., 72.degree., 108.degree., etc. from the reference point,
sensor B can be located at 7.2.degree., 43.2.degree., 79.2.degree.,
etc. from the reference point, sensor C can be located at 14.4.degree.,
50.4.degree., 86.4.degree., etc. from the reference point and similarly
for the remaining sensors. In the embodiment illustrated in FIG.
4 the sensor A is located at the 0.degree. reference, sensor B
is located at 79.2.degree., sensor C is located at 158.4.degree.,
sensor D is located at 237.6.degree. and sensor E is located at
316.8.degree.. In practice, a sensor should not be located too close
to the location where the teeth on the two gears mesh, or the sensor
may erroneously sense the teeth from both gears.
FIG. 5 is a graph of the outputs from the sensors A-E as the gear
40 is rotated in a counter clockwise direction through the increment
.alpha. from the illustrated reference position. At times t.sub.0
and t.sub.5 the output from the sensor A changes, at times t.sub.3
and t.sub.8 the output from sensor B changes, at times t.sub.1 and
t.sub.6 the output from sensor C changes, at times t.sub.4 and t.sub.9
the output from sensor D changes, and at times t.sub.2 and t.sub.7
the output from sensor E changes. Since more than one sensor may
have an output at the same time, it is necessary to logically combine
the outputs from the sensors A-E through exclusive OR or XOR logic
43. Table I shows the XOR logic for combining two digital signals
X and Y. The XOR logic combines the outputs from the 5 sensors A-E
according to the following formula:
Table II shows the outputs from the five sensors A-E and the resulting
pulse output for an XOR logic combination of the outputs from the
sensors A-E as the gear 40 is rotated through the increment .alpha.
over the time t.sub.0 through t.sub.9. As will be seen from Table
II, the output will consist of a pulse signal having 5 pulses occurring
at times t.sub.0 t.sub.2 t.sub.4 t.sub.6 and t.sub.8 when the
gear is rotated through the angle .alpha., or 36.degree.. Thus,
a pulse is produced for each one fifth of the volume of liquid flowing
through the flow meter when the gear rotates through the angle .alpha..
It should be appreciated that the diagram in FIG. 4 illustrates
squared gear teeth 41 for simplicity. The actual gear teeth will
be shaped similar to the gear teeth 26 and 27 shown in FIGS. 2 and
3 to allow the gear teeth to mesh. In order for the sensors A-E
to have the pulse forms shown in FIG. 5 the sensors A-E must be
located in FIGS. 2 and 3 at a point on the path swept by the gear
teeth 26 or 27 wherein the width of the teeth and the width of the
spaces between the teeth are equal. If the sensors A-E are located
to sense only the tips of the gear teeth 26 and 27 then the sensors
A-E may produce sequential pulses as shown in the graph of FIG.
6. Only one of the sensors A-E will produce a pulse at any given
time. The outputs from the five sensors A-E can be logically combined
by an OR gate to produce a train of pulses with one pulse each time
the gear moves through an increment .beta..
The number of divisions for each volume of liquid delivered through
the flow meter is merely a function of the number of sensors. Thus,
2 sensors will divide the volume in half, 3 sensors will divide
the volume in thirds, etc. Conventional circuitry (not shown) including
commercially available integrated circuits may be used for combining
the outputs from the sensors.
The logic 43 may be replaced by known circuitry which is responsive
to only the leading edge or only to the trailing edge of the output
from each of the sensors. Such circuitry will function even though
different combinations of sensors have outputs at different times
since the sensors are spaced so that only one sensor will see a
leading tooth edge or a trailing tooth edge at any given time. This
circuitry will function where the sensors each generate outputs
for less than or more than an increment of .alpha./2. It will be
appreciated that various other modifications and changes may be
made to the improved flow meter of the invention without departing
from the spirit and the scope of the following claims. |