Abstrict The invention is a liquid volume-flow meter suitable for measuring
extremely turbulent flow. The meter comprises a measuring chamber
formed as a pipe having a wall of transparent material fitted inline
with an existing pipe of similar diameter, whereby the detector
has minimal effect upon the operation of an existing installation.
The meter further comprises an optical velocity array for measuring
the speed of liquid flowing within the measuring chamber and an
optical area sensor for measuring the area within the measuring
chamber occupied by liquid flowing. The optical area sensor comprises
a plurality of arrays including a backscatter reflection array to
estimate the height of liquid within the measuring chamber and at
least one further correction array to correct the estimation made
by the reflection array. Each array comprises an optical emitter
and an optical detector that operate through the transparent wall
of the measuring chamber.
Claims What we claim is:
1. A liquid volume-flow meter comprising a. a measuring chamber
formed as a pipe having a wall of transparent material; b. an optical
velocity array for measuring the speed of liquid flowing within
the measuring chamber; and c. an optical area sensor for measuring
the area within the measuring chamber occupied by liquid flowing
within the measuring chamber; in which the optical area sensor comprises
a plurality of arrays including a backscatter reflection array to
estimate the height of liquid within the measuring chamber and at
least one further correction array to correct the estimation made
by the reflection array; and in which each array comprises an optical
emitter and an optical detector that operate through the transparent
wall of the measuring chamber.
2. A liquid volume-flow meter according to claim 1 in which the
emitter of the backscatter reflection array causes radiation to
impinge upon an upper surface of liquid in the measuring chamber
and the detector detects radiation reflected back from that surface.
3. A liquid volume-flow meter according to claim 1 in which the
backscatter reflection array generates an output signal that varies
continuously as the level of liquid in the pipe varies from the
pipe being empty of liquid and the pipe being full of liquid.
4. A liquid volume-flow meter according to claim 1 in which the
emitter of the backscatter reflection array is a narrow-angle emitter.
5. A liquid volume-flow meter according to claim 4 in which the
emitter of the backscatter reflection array has an angle of emission
of approximately 8.degree..
6. A liquid volume-flow meter according to claim 1 in which the
detector of the backscatter reflection array is a wide-angle detector.
7. A liquid volume-flow meter according to claim 6 in which the
detector of the backscatter reflection array has an angle of detection
of approximately 120.degree..
8. A liquid volume-flow meter according to claim 1 in which the
correction array or each of the correction arrays includes a multi-layer
reflection array.
9. A liquid volume-flow meter according to claim 8 in which the
emitter of the multi-layer reflection array directs radiation into
a body of liquid in the measurement chamber and the detector detects
radiation reflected from within the body of the liquid.
10. A liquid volume-flow meter according to claim 8 in which the
multi-layer reflection array generates a signal that varies continuously
as the depth of liquid within the chamber varies from zero to a
fraction of the total depth of the chamber.
11. A liquid volume-flow meter according to claim 8 in which each
of the emitter and the detector of the multi-layer reflection array
are narrow-angle devices.
12. A liquid volume-flow meter according to claim 11 in which the
angles of emission and detection of the emitter and detector of
the multi-layer reflection array is approximately 8.degree..
13. A liquid volume-flow meter according to claim 1 in which the
correction array or each of the correction arrays includes a transmission
array.
14. A liquid volume-flow meter according to claim 13 in which the
emitter of the transmission array directs radiation towards the
detector of the transmission array.
15. A liquid volume-flow meter according to claim 14 in which the
transmission array generates an output signal that is indicative
of the presence or absence of liquid in the path of radiation between
the emitter and the detector.
16. A liquid volume-flow meter according to claim 1 in which the
velocity array comprises a plurality of optical detectors spaced
along the flow path of the measuring chamber that can detect the
presence or absence of liquid at spaced positions within the chamber.
17. A liquid volume-flow meter according to claim 16 in which outputs
from the respective optical detectors are identified as being caused
by successive arrival at the detectors of liquid, and the speed
of that liquid is determined by measurement of the time taken for
it to pass between the detectors and of knowledge of the distance
between the detectors.
18. A liquid volume-flow meter according to claim 1 in which the
output of each emitter is sampled by a respective calibration detector
and the power delivered to the emitter is adjusted to cause the
emitter to emit energy with a predetermined intensity.
19. A liquid volume-flow meter according to claim 1 in which the
radiation emitted by some or all of the emitters is visible light.
20. A liquid volume-flow meter according to claim 1 in which the
radiation emitted by some or all of the emitters is infra-red light.
21. A liquid volume-flow meter according to claim 1 further including
a density detector for measuring the optical density of the liquid
being measured.
22. A liquid volume-flow meter according to claim 21 in which an
output from the density detector is used to modify the output of
the optical area sensor.
23. A liquid volume-flow meter according to claim 1 further comprising
a controller programmed to control the emitters and to analyse signals
received from the detectors to generate an output indicative of
volume-flow or volume-flow rate.
24. A milking installation including a volume-flow meter according
to claim 1 for measuring the volume-flow of milk.
25. A method of measuring liquid volume-flow in a volume-flow meter
comprising: at a plurality of measurement arrays a. directing radiation
from an optical emitter through a transparent wall into a measuring
chamber and b. detecting a proportion of that radiation emerging
from the measuring chamber i. at one array, by backscattering, measuring
the depth of liquid in the measuring chamber to arrive at a first
estimate if the area of the chamber occupied by flowing liquid;
and ii. at one or more other array making a further optical measurement
to derive a corrected area measurement from the first estimate;
c. at an optical velocity array measuring the speed of liquid flowing
within the measuring chamber; and d. calculating volume flow from
the velocity and the corrected area measurement.
26. A method according to claim 25 in which the further optical
measurement includes one or more of a multi-layer scattering measurement
and a transmission measurement.
27. A method according to claim 25 further comprising making a
measurement of the optical density of the liquid and modifying the
estimate of the area of liquid.
Description This application claims foreign priority benefits of United Kingdom
patent application no. 0216490.3 filed Jul. 16 2002.
BACKGROUND TO THE INVENTION
1. Field of the Invention
This invention relates to a flow meter. It has particular application
to a flow meter that can measure volume flow of liquid in a pipe
where that flow is turbulent and where the flow does not always
fill the pipe. It has particular, but not exclusive, application
to measuring volume flow of milk in a milking installation.
Flow of milk the pipes of a milking parlour is notoriously difficult
to measure. The flow is turbulent, rapidly varying in direction,
speed and volume, and the liquid that flows in the pipe is accompanied
by a large amount of air, such that the milk fills a rapidly varying
fraction of the cross-section of the pipe.
2. Summary of the Prior Art
Conventionally, the volume flow of milk in such a situation has
been measured by volumetric means. Most generally, the milk is captured
in a container of known volume that is repeatedly filled. The total
volume flow can be determined by multiplying the number of times
that the container by its volume. An example of such a system is
disclosed in EP-A-0081049. Such systems can be reasonably accurate,
but necessarily have a large number of moving parts. This makes
them bulky, complex and expensive to manufacture. Moreover, the
moving parts are difficult to clean which can lead to a build-up
of contaminants. One solution to this problem is to measure electrical
properties, such as conductivity, of the milk. However, such systems
are of questionable accuracy, and even slight electrical leakage
can cause irritation to an animal being milked. Moreover, installation
of known sensors within an existing milking installation typically
causes significant change to the pattern of milk flow within the
installation, possibly interfering with operation of the system.
A principal aim of this invention is to provide a flow meter that
is suitable for use with milk or other turbulently flowing liquids,
that requires low maintenance and does not interfere with other
processes, such as milking.
SUMMARY OF THE INVENTION
From a first aspect, the invention provides a liquid volume-flow
meter comprising a. a measuring chamber formed as a pipe having
a wall of transparent material; b. an optical velocity array for
measuring the speed of liquid flowing within the measuring chamber;
and c. an optical area sensor for measuring the area within the
measuring chamber occupied by liquid flowing within the measuring
chamber; in which the optical area sensor comprises a plurality
of arrays including a backscatter reflection array to estimate the
height of liquid within the measuring chamber and at least one further
correction array to correct the estimation made by the reflection
array; and in which each array comprises an optical emitter and
an optical detector that operate through the transparent wall of
the measuring chamber.
The measuring chamber can be an entirely smooth pipe fitted inline
with an existing pipe of similar diameter, whereby the detector
has minimal effect upon the operation of an existing installation.
The emitter of the backscatter reflection array typically causes
radiation to impinge upon an upper surface of liquid in the measuring
chamber and the detector detects radiation reflected back from that
surface. That may provide an array that generates an output signal
that varies continuously as the level of liquid in the pipe varies
from the pipe being empty of liquid and the pipe being full of liquid.
This can, therefore, be used as a principal detector for determining
the area of the chamber that is occupied by liquid. Advantageously,
the emitter of the backscatter reflection array is a narrow-angle
emitter. For example, the emitter of the backscatter reflection
array may have an angle of emission of approximately 8.degree..
Moreover, the detector of the backscatter reflection array may be
wide-angle detector, for example having an angle of detection of
approximately 120.degree..
The correction array or each of the correction arrays may be of
various different types. For example, they may include one or more
of a multi-layer reflection array and a transmission array.
The emitter of the multi-layer reflection array can direct radiation
into a body of liquid in the measurement chamber and the detector
detects radiation reflected from within the body of the liquid.
Typically, the radiation is directed upwardly into the liquid from
below and is reflected back form within the body of the liquid.
The multi-layer reflection array generates a signal that varies
continuously as the depth of liquid within the chamber varies from
zero to a fraction of the total depth of the chamber. It has been
found that the signal may cease to change significantly once the
liquid has exceeded a threshold depth. In the multi-layer reflection
array, each of the emitter and detector are narrow-angle devices,
for example, operating with an angle of approximately 8.degree..
A transmission array within this invention most usually has an
emitter that directs radiation towards its detector. The transmission
array may generate an output signal that is indicative of the presence
or absence of liquid in the path of radiation between the emitter
and the detector. The output signal may not vary significantly with
the quantity of the liquid; and as such is largely binary in nature.
The velocity array may comprise a plurality of optical detectors
spaced along the flow path of the measuring chamber that can detect
the presence or absence of liquid at spaced positions within the
chamber. Outputs from the respective optical detectors can therefore
be identified as being caused by successive arrival at the detectors
of liquid, and the speed of that liquid is determined by measurement
of the time taken for it to pass between the detectors and of knowledge
of the distance between the detectors.
Accommodation must be made in variation in the performance of the
emitters due to manufacturing tolerances, age, and other factors.
To achieve this, in preferred embodiments, the output of each emitter
is sampled by a respective calibration detector and the power delivered
to the emitter is adjusted to cause the emitter to emit energy with
a predetermined intensity. The output is best sampled in a manner
such that the sample is unaffected by the contents of the measuring
chamber.
It is envisaged that the radiation emitted by the emitters may
be visible light. However, other radiation, typically close to the
visible range (e.g. infra-red) might be used instead.
A liquid volume-flow meter embodying the invention may further
include a density detector for measuring the optical density of
the liquid being measured. The output from the density detector
is used to modify the output of the optical area sensor to correct
variations in the output that might arise from variation of the
optical density of the liquid.
A liquid volume-flow meter embodying the invention may further
comprise a controller programmed to control the emitters and to
analyse signals received from the receivers to generate an output
indicative of volume-flow or volume-flow rate.
Most typically, embodiments of the invention provide a volume-flow
meter for measuring the volume-flow of milk in a milking installation
being in accordance with any preceding claim.
From a second aspect, this invention provides a method of measuring
liquid volume-flow in a volume-flow meter comprising: at a plurality
of measurement arrays a. directing radiation from an optical emitter
through a transparent wall into a measuring chamber and b. detecting
a proportion of that radiation emerging from the measuring chamber
I. at one array, by backscattering, measuring the depth of liquid
in the measuring chamber to arrive at a first estimate if the area
of the chamber occupied by flowing liquid; and II. at one or more
other array making a further optical measurement to derive a corrected
area measurement from the first estimate; c. at an optical velocity
array measuring the speed of liquid flowing within the measuring
chamber; and d. calculating volume flow from the velocity and the
corrected area measurement.
Typically, the further optical measurement includes one or more
of a multi-layer scattering measurement and a transmission measurement.
Alternatively or additionally, the method may further include making
a measurement of the optical density of the liquid and modifying
the estimate of the area of liquid.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a and 1b are cross-sectional views to illustrate the turbulent
nature of milk flow in a pipe carrying milk from a milking machine;
FIG. 2 illustrates a backscatter reflection array used in the development
of the invention;
FIG. 2a is an enlarged view of the emitters and detectors of the
array of FIG. 2;
FIG. 3 is a graph showing the output of the array of FIG. 2;
FIG. 4 illustrates a multi-layer reflection array used in the development
of the invention;
FIG. 5 is a graph showing the output of the array of FIG. 4;
FIG. 6 illustrates a transmission array used in the development
of the invention;
FIG. 7 illustrates a combined array within a volume-flow measuring
device embodying the invention;
FIG. 7a is a cross-sectional view of the array of FIG. 7;
FIGS. 8a, 8b and 8c are diagrams showing a cross-comparison measuring
arrangement in reflection mode in a measuring chamber of the embodiment.
FIG. 9 is a diagram of a calibration system of the embodiment;
and
FIG. 10 is a block diagram of hardware components of an embodiment
of the invention;
FIG. 11 is a flowchart that summarises the steps performed in making
a flow measurement in apparatus embodying the invention;
FIG. 12 is a flowchart that summarises the steps performed in making
area measurement using horizontal detection arrays in an embodiment
of the invention;
FIG. 13 is a flowchart that summarises the steps performed in making
area measurement using vertical detection arrays in an embodiment
of the invention;
FIG. 14 is a flowchart that that summarises the steps performed
in calibrating emitters in an embodiment of the invention;
FIG. 15 is a diagram of light reflecting from an air bubble within
a measuring chamber;
FIG. 16 is a graph of reflection data showing spikes caused by
a bubble, as shown in FIG. 15;
FIG. 17 is a diagram showing a cross-comparison measuring arrangement
between horizontal and vertical arrays in a measuring chamber of
the embodiment; and
FIG. 18 is a diagram showing a cross-comparison measuring arrangement
in reflection mode in a measuring chamber of the embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of the invention will now be described in detail,
by way of example, and with reference to the accompanying drawings.
The embodiments will be described in relation to a volume-flow
measuring device for use in a milking installation. However, it
should be understood that embodiments can be configured for metering
other non-clear liquids that are flowing in a turbulent manner.
As will be seen from FIG. 1 the type of flow that is to be measured
is potentially highly complex, liquid filling a partial and variable
proportion of the area of the pipe.
Introduction to the Detection Systems
To calculate the volume of a moving mass of milk flowing through
a pipe in any given time period it is necessary to know the cross-sectional
area and the length of the mass. The following equation and then
be applied: Volume=Cross-Sectional Area.times.Length.
Research was carried out to investigate the relationship between
selected optical devices and milk flowing through a measuring chamber
in formed from a glass pipe. Numerous test arrays were constructed
with a view to measuring the level of the milk in the glass pipe
from which a cross-sectional area could be calculated. Research
into milk flow characteristics during transport in the application
revealed the following difficulties: Milk density changed due to
oxygenation and variations in butterfat levels; Milk travelled chaotically
through a typical system and exhibited centrifugal flow characteristics,
and consequently travelled through the glass pipe in one of many
formations.
An array was developed which measures cross-sectional area. It
is includes the following components: A backscatter reflection array
A multi-layer reflection array A transmission array
The backscatter reflection array, three of which are shown in FIG.
2 comprises a narrow-angle emitter 210 and a wide-angle detector
212 both directed downwardly onto milk within a length of pipe 214
that constitutes a measuring chamber. The pipe 214 is formed of
transparent material, preferably optical glass, having, in this
embodiment, a circular cross-section. Glass has proven to be advantageous
because it is durable, easily cleaned, and there is a lesser tendency
for milk splashes to stick to it than there is with plastic.
The angles of the emitter and the detector are typically 8.degree.
and 120.degree. respectively. This produces a sensor that can be
used to detect the level of milk 216 in the pipe up to and including
a full pipe. The angle of the detector must be sufficiently wide
to detect light reflected from the milk even when it is almost or
entirely to the top of the pipe, as shown in FIG. 2a. If the detector's
angle is too narrow, a reflection from the surface of the milk,
when very close to it, might fall outside the field of the detector.
Operation of the array can be enhanced by positioning the emitter
with a slight angle towards the detector. Consequently, if the pipe
is full, or the top quadrant of the pipe is populated, the signal
is reinforced the by reflection on the wall of the pipe, so the
integrity of the reflection characteristic is sustained.
FIG. 2 shows multiple arrays in application illustrating how the
surface can be profiled in the measurement of cross-sectional area.
FIG. 3 shows the shows the increase in signal level against incremental
Increases in the level of milk 216 in the sensor chamber.
The output of the detectors varies in relation to the height of
milk in the pipe. The relationship for any given sensor and pipe,
the output can be determined readily by experiment. The relationship
between the output and the height of the milk for this embodiment
is shown in FIG. 3 the height of the milk on the x-axis being expressed
in units of 1/16 of the height of the pipe 214.
As will be seen, the backscatter reflection array produces an output
that varies as the level of milk in the pipe varies from none at
all to full. However, this array is insufficient to give rise to
a determination of the area of milk in the pipe because of the possibility
of air spaces below the surface of the milk. Moreover, the graph
presented is based on the assumption that the surface of the milk
is approximately level; an approximation that may be highly inaccurate.
Therefore, further measurements must be made by way of the other
arrays in order to accommodate this.
Each multi-layer reflection array, three being shown in FIG. 4
uses a narrow angle emitter 410 and a narrow angle detector 412.
Research has revealed that if an array is positioned with a constant
interface; i.e. the array is covered by the milk 216 the reflection
characteristics will alter with a predictable relationship to the
depth of the mass at the measurement point. FIG. 4 shows multiple
multi-layer reflection arrays to illustrate how the surface can
be profiled in the measurement of cross-sectional area. FIG. 5 shows
the relationship between the signal level with respect to incremental
increases in the level of milk in the pipe in the region of the
array.
As will be seen, this array provides a signal that can give an
indication of the depth of milk in the pipe from zero to about 6/16
of the depth of the pipe.
The transmission array, as shown in FIG. 6 is constructed by positioning
an optical emitter 610 at one side of the pipe 214 and a detector
612 at the opposite side, such that the beam of the emitter takes
the path of a cord of the cross-section of the pipe 214. When the
emitter 610 is switched on, in the absence of milk 216 in the pipe
214 light travels unobstructed to the detector and a relatively
large electrical signal will be generated by the detector 612. When
a mass of milk passes into the pipe 214 the signal will be greatly
attenuated, as only a very small percentage of light will pass through
the mass of milk. This array has an output that is substantially
binary in nature, in contrast with the variable output of the arrays
described above. This characteristic enables the transmission array
to be used in the detection of a moving mass of milk 216 within
the pipe 214.
Application in the Embodiment
Cross-sectional area measurement is achieved in embodiments of
the invention by combination of the above-described optical arrays,
as outlined in the flow charts of FIGS. 11 to 13. The resulting
combined array is shown in FIGS. 7 and 7a. In FIG. 7 the detectors
the emitters `E`; and the other devices are detectors. All of the
devices shown in FIG. 7a are detectors; emitters are omitted in
the interest of clarity. Each emitter can cooperate with one or
more detectors; in the latter case, the detector can either operate
in backscatter reflection or multi-layer reflection mode. Detectors
V1 V2 and V3 are used in backscatter reflection mode. Detectors
H1 H8 and V4 V5 and V6 are used in multi-layer reflection mode.
When an emitter is activated, it is possible to obtain a reading
in both multi-layer and backscatter mode simultaneously. The emitters
and detectors are carried in a mounting block 710 formed in this
embodiment from a machined block of plastic, that surrounds the
pipe 214. All of the emitters and detectors are controlled by a
controller (to be described below) that performs the calculations
necessary to determine volume flow.
In FIG. 7 the middle detectors are the ones shown in FIG. 7a.
On the side arrays, each emitter directly faces a detector and vice
versa. On the sides of the block 710 there are eight transmission
arrays in total, divided into two blocks of four (each side of the
cross-sectional area arrays) separated by 10 mm.
One of the transmission arrays has an emitter and a detector that
operates in the infra-red frequency band. This array normally operates
as described above. However, it can also operate in a density-measuring
mode to make optical density measurements of the milk flowing in
the pipe. (Factors such as fat level or oxygen content, among others,
can affect the optical density.) In this mode, the emitter is supplied
with a short pulse of current that is significantly higher than
the current normally used to drive the device. This produces an
output of magnitude that will pass through the milk to reach the
associated detector. The amount of light received by the detector
is directly related to the optical density of the milk through which
it is passing.
When making an optical density measurement it is necessary to ensure
that the section of the pipe in which the measurement is taken always
contains the same mass per unit area of milk. The array measurements
must confirm that the bottom 3/16 of the pipe is populated before
an optical density measurement is taken. Otherwise, the density
figure would be underestimated because is less milk in the measuring
cell rather than because the actual optical density. If the mass
of milk is fixed then a variation in the signal level generated
at the detector will be result from an effective optical density
of the mass of milk.
The internal area of the pipe 214 is divided up into notional segments,
as shown in FIG. 7 used in calculation of the milk flow. The segments
are arranged as a matrix. The number of these segments that is occupied
by milk in the central column of the pipe can be determined by the
values from detectors V2 and V5 which each operate as multi-layer
reflector arrays. The side smaller segments, which are mainly square,
are plotted according to several different detector readings and
error checked against each other. For example, segment S5 in the
lower-left corner is populated subject to the reading from multi-layer
reflector array H5 and cross-checked against detectors V1 and V6.
The multi-layer reflection technology being used to do this can
be taken further and the whole area could be divided into, a square
matrix of area segments. The present embodiment is limited to useable
measurements in multi-layer reflection up to 6/16th of the diameter
or approx 8.88 mm (6.38 mm if glass excluded).
The general formula for the cross-sectional area is as follows:
Area=A[H8]+C[H7]+B+N[H1]+L[H2]+M+E[H6]+G[H5]+F[V6]+J[H3]+H[H4]+I[V4]+D+K+-
P[V2]+O[V5]
For each area segment, there is an associated look-up table (A
. . . O), which has been populated with values that correspond to
the calibrated reading from the respective detector. These arrays
are populated during manufacture of the device. The value of each
element in the look-up table is closely dependent upon the precise
configuration of an embodiment, and these can be determined by experiment.
The relevant detector is reading is subsequently used as an index
pointer address into the associated look-up table (as expressed
in the above formula) to retrieve the appropriate area value. The
population of segments; B, M, D & K are dependent on the population
of other segments and are best described with conditional statements.
IF (C[H7]>C_THRESHOLD) THEN B=C[H7] ELSE B=B[V1]
The above statement checks area C and compares it against a preset
threshold to determine if there is a relevant quantity of milk in
this segment. If the statement is true, area B will be populated
with the same quantity of milk. If the statement is false, array
B will be accessed using the vertical array V1 as an index. This
allows an estimation to be made of the population of area segments
that cannot be observed directly by any of the arrays.
In embodiments that make optical density measurements, the measured
optical density is used to generate an offset that is added to the
index used to access the look-up tables, thereby adjusting the calculated
area value to compensate for variations in the detector outputs
arising from variation in the optical density.
The above formulae enable measurement of the area of the pipe that
is occupied by milk. As discussed, to measure volume flow, it is
also necessary to measure the speed of flow within the pipe.
As shown in FIGS. 8a to 8c, the system has upstream and downstream
horizontal transmission arrays 810 812 spaced by a distance d
along the pipe. Milk flowing in the pipe 214 will first be detected
by the upstream array, as shown in FIG. 8b, and a short time later,
by the downstream array, as shown in FIG. 8c. By correlation of
the signals received from the detectors of these two arrays, the
processor can track a feature between the two arrays, and measure
the transit time t between detection at the two arrays 810 812.
Having measured the value t it is possible to calculate the velocity
of the feature using the formula V=distance/time
Where t is in ms and d is in mm, he result in millimetres/millisecond
(mm/ms). Using a fixed sample time the length parameter, being the
length of a body of milk within the pipe, is calculated using the
equation: Length=Velocity/Sample time
This measuring system cannot accurately track velocity at a single
measuring point. Therefore, a practical embodiment has several of
the velocity arrays described above, four in this embodiment. The
controller array integrates multiple velocity measurement points
which enables the system to both track an accelerating flow and
accurately track velocities over a broad range of flow rates.
In order that velocity measurements can be made as the height of
milk in the pipe varies, the velocity array is
Finally, the controller can calculate volume from the parameters
determined above. Having derived the cross-sectional area from the
level measurement and the length of the milk sample over, say, a
1 ms period from the velocity measurement the volume can be calculated
by: V=Area.times.Length.
In order to provide an accurate output, the system requires that
the optical arrays provide a stable repeatable measuring system.
Semiconductor optical components, by their nature, are subject to
variations in their performance due to manufacturing variation,
temperature and ageing effects. Therefore, it is highly preferable
that the embodiment includes a calibration system. This will be
described with reference to FIG. 9. The calibration system operates
by taking controlled samples of light radiated from each of the
emitters independently from the effects of other components and
comparing it against a stored vector collected from the array during
manufacture variations in the radiated power may be identified.
The calibration algorithm then adjusts the power delivered to the
emitter that is being calibrated until the calibration sample conforms
to the reference value. The calibration procedure is summarised
in the flowchart of FIG. 14.
FIG. 9 illustrates the calibration array structure that is constructed
in the mounting block 710. A channel 712 is machined within the
block 710 to provide an optical pathway between an emitter 210 and
a corresponding calibration detector 718. A translucent optical
fibre waveguide 720 is inserted into this channel, such that: 1.
the waveguide 720 collects light emitted by the periphery of the
emitter 210 such that it does not interfere with operation of the
emitter in a detector array. The fibre ensures that sufficient light
is channelled along the optical pathway; 2. the fibre establishes
a constant, stable interface between the emitter 210 and the corresponding
calibration detector 718; and 3. the properties of the fibre are
selected to transmit a controlled level of light to prevent the
calibration detector becoming saturated.
Hardware Implementation
The hardware within which the controller of this embodiment is
implemented is illustrated diagrammatically in FIG. 10.
The controller is constructed around a RISC processor 1010. Outputs
of the processor 1010 are connected through a digital-to-analogue
converter (DAC) 1012 the output of which is processed by a high-powered
amplifier 1014 to drive the; emitters. The output of the amplifier
1014 is connected to the emitters through a multiplexer 1016 whereby
the processor 1010 can select which of the emitter will receive
the output signal. Before a signal is applied to an emitter, the
appropriate calibration values for that particular emitter are loaded
into the DAC 1012 to ensure that the optical output of the emitter
is at the correct level.
Signals from the detectors 1018 are fed through a multiplexer 1020
to the input of an instrumentation amplifier 1022. As with the emitters,
the multiplexer 1020 enables the processor 1010 to select which
one of the detectors it will sample. The output of the instrumentation
amplifier 1022 is sampled by an analogue-to-digital converter (ADC)
1024. The processor 1010 receives the data from the ADC 1024 and
uses that data as input to its measurement algorithm. The results
of the algorithm, representative of volume flow, are output for
processing by external systems.
The cross-sectional area is calculated by a series of reflection
and backscatter measurements were certain assumptions are made.
The multi-layer reflection mode works on the principle that with
a constant interface of milk on the sensor wall the depth of the
liquid at that point can be determined and consequently certain
areas within the cross-section area map are populated.
The reality of the situation is that due to the extreme variability
of the flow, scenarios occur where there is no such interface, the
surface of the milk sample being positioned away from the glass.
It is important to detect these and make alternative measurements
to ensure the accuracy of the cross-sectional area is preserved.
As a result of the design if the scenario shown in FIG. 15 occurs,
the signal generated at the multi-layer detector increases out of
its operational band, drawing attention to the fact that the nature
of the sensor wall/measuring medium interface has changed. This
characteristic can be seen on any of the multi-layer reflection
arrays as spikes in the output, as shown in FIG. 16. This allows
them to be correspondingly corrected.
FIG. 17 shows how it is generally possible to use vertical arrays
to cross-check horizontal measurements to determine whether they
are reflection or multi-layer reflection measurements. However,
if a scenario occurs such as illustrated in FIG. 15 the vertical
data will be out of range in respect of cross checking the horizontal
array. FIGS. 8A to 8C show how it is possible to do a cross comparison
in reflection mode using secondary detectors to determine the distance
to the medium being measured and assess whether the relevant defined
area is to be populated.
In this embodiment, the emitters are light-emitting diodes that
radiate visible red light, and the detectors are suitably sensitive
photodiodes. Alternative embodiments may operate in other frequency
ranges, for example in infra-red. Further embodiments may receive
inputs from other devices to improve upon the accuracy of the area
and velocity measurements. |