Note: Descriptions are shown in the official language in which they were submitted.
37;2
This invention relates to a detector suitable for
detecting the presence oE an object with desired
characteristics, or for detecting one or more desired
characteristics of an object, and i5 well suited for
incorporation in a sorting sy~tem wherein desired objects
are detected amongst a plurality of objects and separated
theref roln .
In many sorting applications the objects to be sorted are
illuminated with radiation at a suitable frequency and
radiation reflected by the objects or emitted due to the
stimulating effect of the irradiating radiation is
monitored to detect a desired characteristic. Due inter
alia to high sorting throughputs and to the small sizes of
certain objects it is essential to be able to scan the
radiation emitted by a large number of objects
simultaneously so that desired objects can be identified
in the mass of objects. Scanning may for example be
achieved by causing the illuminating or irradiating source
of radiation to irradiate comparatively small group of
objects, rapidly in succession~ or alternatively by
arranging a mechanical handling facility to separate the
objects sufficiently before they are presented to the
detection system.
In certain instances a device such as a solid state camera
is employed. For example the specification of South
African Patent No. 82/2904 describes the use of an
intensified line scan camera which monitors an image
produced on a fluorescent screen by ore which is
irradiated by X-rays. Such a device can scan and can be
controlled electrically and under certain conditions
offers adequate discrimination. However some devices of
this type have a poor signal to noise characteristic and
it is therefore necessary to irradiate the objects with a
high intensity source of radiation. This may be difficult
at certain frequencies. For example if the irradiating
source is in the ultra violet region it is difficult to
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obtain the required intensity oE radiation. The tubes
which are available are expensive and have a short
operating life and can be hazardous to personnel.
It may also be necessary, particularly in sorting
applications, to make use oE a polarized irradiating
source so that specular reflections from the irradiated
objects are avoided. The provision of a high intensity
polarized irradiating source can be difficult and
expensive due particularly inter alia to heat dissipation
problems.
In some applications use has been made of lenses to view
the material, or radiation rom the material, being
sorted. Arrangements of this general kind are disclosed
in the specifications of U.K. Patent No. 1283902, and
Swiss Patent No. 585588. Other patent specifications
which relate generally to the present invention are those
of U.S.A. Patent No. 3341010, UK Patents Nos. 1176747 and
2067753, German Patent No. 2105914, and French Patents
Nos. 1575850 and 2400396.
It is an object of the present invention to provide an
improved method of detecting a characteristic of an
object.
The invention provides a method of detecting a
characteristic of an object which includes the steps of
irradiating the object with radiation at least at first
and second frequencies, at least one of the frequencies
being associated with the characteristic~ the radiation of
the first Erequency causing a signal to emanate from the
object, intensifying the signal emanating from the object
by means of at least one image intensifier, and monitoring
an output signal of the image intensifier which is
dependent at least on the characteristic.
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The characteristic may be any physical property of the
object including its size, shape, mass, material content,
surface features, or similar data, or combinations of such
data.
`~ The signal from the object may arise from the object
itself or may be a signal which is stimulated by some form
of excitation. For example the signal may arise from
natural fluorescence of the object or from any other non-
stimulated natural physical phenomenon or may be caused by
irradiating the object with radiation at a desired
frequency. When the object is so irradiated the signal
which is image intensified may simply be radiation
reflected by the object or parts of the object, i.e. be
dependent on the reflectance properties of the object, or
it may consist of a signal which is excited or stimulated
by the irradiation process. An example of this phenomenon
is the excitation oE fluorescence in a diamond by means of
X-ray irradiation.
The signal which is intensified may be at a known
frequency or frequencies, or be within a given frequency
band.
The output signal of the image intensiEier may be
monitored by means of a device suited to the wavelength
and amplitude of the signal and depending on these
characteristics use could be made of a photo diode array,
an anode array, a charge coupled device, a camera, a
char~e injection device which may be an intensified
device, or any similar or suitable device. Preferably
those devices are able to scan the image. If a relatively
high resolution is not required use could be made of a
photo multiplier tube, or a similar low resolution device,
or an assembly of such devices.
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The object may be irradiated intermittentl'~ or
continuously, and by one or more sources simultaneously
or successively and the output signals of one or more
image intensifiers may be monitored. Intermittent
irradiation may be employed to reduce energy consumption
or to improve the cooling of the source, or for any other
reason.
The method may include the step of separating or sorting
objects with predetermined output signals from objects
which do not display such signals.
In accordance with the method of the invention a
1~ plurality of objects may be irradiated simultaneously.
The objects may be caused to move relatively to the image
intensifier or intensifiers. The objects may be caused
to move in a random stream or in a plurality of rows, or
with the objects in one or more rows spaced from one
another.
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The output signal of the image intensifier may be
processed to detect or obtain a measure of physical data
relating to an object. Thus the signal may be scanned to
identify the position of an object relatively to a
reference frame, or alternatively or additionally to
detect a surface or other characteristic.
The irradiating frequency may vary according to the
application and may be in the ultra violet, infra red,
optical, or X-ray frequency ranges, or in any other
desired or suitable frequency range.
The method of the invention may include the step o;f
selectively adapting the spectral response of the image
intensifier to detect any predetermined frequency or
frequencies in the signal which emanates from the object.
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mhis may be done by interposing a band pass or band stop
filter between the object and the image intensifier.
The method may also include the step of focussing
radiation from at least the object on to the image
intensifier or intensifiers.
The invention also extends to apparatus for detecting a
characteristic of an object which includes image
intensifier means for intensifying a signal emanating
from the object, and means for monitoring an output
signal of the image i~tensifier means which is dependent
at least on the said characteristic.
The apparatus may include means for focussing the signal
emanating from the object or a plurality of objects on to
the image intensifier means.
The apparatus may include means for selectively adapting
the spectral response of the image intensifier means to a
predetermined frequency or frequencies or frequency
range. This may be achieved through the use of one or
more filters.
The monitoring means may include a visual display such as
a fluorescent screen or similar device and additionally
or alternatively any other suitable monitoring aid.
Preferably the monitoring aid can be scanned. Suitable
devices for use in this regard are photo diode arays,
anode arrays, charge coupled devices, cameras or charge
injection devices.
The appar~tus may include means for irradiating the
object at a desired frequency. The irradiating means may
comprise a source of suitable intensity at any desired
frequency range for example in the ultra violet, optical
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or X-ray frequency ranges. The signal which emanates from
the object may be a stimulated signal and lie in the same
or a different frequency range, or be a reflected signal.
The irradiating source may operate continuously or
intermittently.
In accordance with the invention more than one set of
apparatus of the kind described may be employed,
simultaneously or in succession, for detecting an object.
Each apparatus may be responsive to the same
characteristic or characteristics, or to different
characteristics.
The lnvention also extends to a sorting system which
includes apparatus of the kind described and means for
causing a plurality of objects to be exposed to the
detection apparatus. Means for separating selected
objects Erom the remaining objects may also be provided.
An embodiment oE the invention is further described by way
of example with reference to the accompanying drawing
which schematically illustrates a sorting system which
includes detection apparatus operated in accordance with
the principles of the invention.
The accompanying drawing illustrates apparatus embodying
the invention which includes a conveyor belt lO which is
supplied from a feed chute 12 with objects 14 to be
sortedl a detection device 16, a source 18 of radiation at
a selected frequency, a circuit 20 for controlling the
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operation of the detection device 16, logic and control
circuitry 22, and a~n array 24 of blast no~zles~
The detection device 16 includes a lens 26, a
photocathode plate 28, a micro channel plate 30, and
optionally, depending on the application, a fluorescent
screen 32 which is coupled to a camera 34 by means of an
array 36 of dptical fibres.
The assembly of components 30, 32 and 36 constitutes a
device generally referred to as an image intensifier.
Briefly the operation of the image intensifier is as
follows : photons impinging on the photocathode 28 cause
electrons to be emitted from the plate and these are
drawn through the micro channel plate 30 by means of a
voltage bias applied to the plate. The electrons in
their passage through the micro channel plate 30 initiate
an amplifying effect whereby increased numbers of
electrons are emitted by the plate 3G in dependence on
the number of incident electrons. The electrons emerging
from the plate 30 impinge on the screen 32 which emits
photons, or fluoresces, thereby creating an enhanced or
intensified image of the primary image. This process
should be constrasted to the use of a device such as a
lens which magnifies the image, or concentrates the
image. A lens is a passive device, however, and it does
not enhance the image, in an amplifying or active manner~
as does the micro channel plate.
The frequency of the radiating source 18 is chosen in
accordance with the characteristic or characteristics to
be detected or identified. For example in the detection
of diamonds the source 18 could be an X-ray source for it
is known that diamonds fluoresce when excited by an X-ray
source and at least the bulk of the accompanying gravel
or gangue does not fluoresce. In photometric sorting on
the other hand where a surface characteristic is being
sought the source could have a frequency in the visible
region so that visually contrasting colours on the
objects 14 can be identified. The source l8 could also
for example operate at an ultra violet frequency for this
is useful in detecting scheelite which fluoresces with a
blue colour.
The screen 32 is used to convert electrons emerging from
the micro channel plate 30 into an optical signal. The
screen is a phosphor screen and the spectral
characteristics of the screen are chosen to suit the
camera 34O If the camera is able to detect the electrons
directly the screen 32 may be dispensed with~ The use of
the fibre optics 36 to couple the optical signal to the
camera is also optional and will in practice be
determined by the actual characteristics of the camera
and the screen and the spacing between the two
components.
The camera 34 is ideally a scanning device which is
controlled by the circuitry 20. This enables one to
detect the characteristic in question and to identify the
spatial position of a detected characteristic or object
or the size or mass of the object, and to keep track of a
particular characteristic or object.
In use of the apparatus of the invention the objec~s 14
are deposited on the belt 10 which moves at a speed which
is sufficiently high so as to cause the objects to be
spaced from one another on the belt. At a forward end o
the belt the objects are projected into free space and
pass on a predetermined trajectory past the detection
apparatus 16 and the nozzle array 24. The objects in
space are illuminated by the source 18 and radiation
reflected by the objects or excited in the objects by the
incident source is focused by the lens 26 onto the
photocathode place 2B. The electrons emerging from the
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I photocathode plate 23 are amplified by the micro channel
plate 30 and converted into an optical signal 32 which is
transferred to the camera 34 by the optical fibres 36~
The camera scans the field of vision under the control of
the circuitry 20 and its output signal is applied to the
logic analyser 22 where it i5 compared to data which is
determined from prior measurements on objects of known
characteristics and which is stored in a suitable memory,
for example of the read only type based on the desired
characteristics. Alternatively, or in addition, the
camera is used to detect the shape or outline of each
object and thus to provide a measure of the size or mass
of each object.
A comparison process of this type is naturally dependent
on the nature of the objects being sorted, but it is
readily accomplished with the aid of conventional
techniques and lies within the ability of one skilled in
the art.
The speed of the belt 10 is continuously monitored by a
tachometer 38 and since the objects 14 travel on a
predetermined trajectory with their positions being
monitored by the dètection device 16 it is possible to
predict when the objects pass the array 24. Thus when a
selected object passes the array 24 an appropriate no~zle
is actuated in accordance with conventional techniques by
means of an initiating signal from the analyser 22 and
the object is deflected from its normal trajectory to a
collecting bin or belt.
As an alternative to illuminating the objects in free
space they could be irradiated while still on!the belt.
Desired objects could similarly be detected while still
on the belt. Depending upon the speed of the belt the
sorting step could take place as the objects leave the
belt with the aid of a gate or flap or similar device.
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A significant advantage of the invention lies in the fact
that the source 18 may be low powered for the lack of
intensity in the incident radiation is compensated for by
the use of the image intensifier. Power consumption is
therefore lower. This is a significant feature when the
source 18 is at X-ray frequencies for the cooling of a
high intensity X-ray tube can present difficulties in the
field. The low power requirements may be further
decreased, where applicable, by pulsing the similar benefits
are obtained when the source 18 is a polarised light source.
The image intensifier could be made frequency selective
through a design approach which makes the micro channel
plate frequency dependent. Alternatively a band stop, or
band pass, filter, as desired, for example an optical
filter 40, may be inserted at a suitable location e.g.
interposed between the lens 26 and photocathode plate 28,
or be placed in front of the lens 26.
If, for example, scheelite is irradiated with ultra
violet radiation then a filter is used to prevent
reflected ultra violet radiation from reaching the
scanning camera. The filter is chosen to allow the
fluorescent radiation to reach the camera. The size of
the object may be measured by the camera, simultaneously
during the detection of the fluorescence, by irradiating
the object with a second source of radiation, at an
intensity well below the expected fluorescence level, and
which is at the pass frequency of the filter i.e. at the
fluorescence frequency. Radiation from the second source
which is reflected by the object is detected by the
camera and a size assessment of the object is made, and
fluorescence emitted by the obj~ct is simultaneously
detected.
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A similar result could be achieved by the use of two ~ets
of the apparatus described, arranged in series or any
other suitable way. Contrasting surface features vr
other different characteristics can be detected by
irradiating the object at two or more frequencies,
appropriately chosen to excite the object or to bring out
the contrast. Two or more image intensifiers with
appropriate filters are then used to detect the different
features. The source 18 could for example be a
multi-frequency source.
Applications of t`he principles of the invention in these
and similar manners are all intended to fall within the
scope of the invention.
