Note: Descriptions are shown in the official language in which they were submitted.
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Tnis invention relates to apparatus for sensing
or aetectiny small moving particles or small moving objects,
and is particularly though not exclusively applicable to
apparatus for sensing or detecting cereal grain. The invention
may be used to sense the grain lost with the straw or chaf~
from a combine harvester.
The invention is concerned with particle detectors
of the type having a target on which the particles impinge,
and an acoustic transducer for detecting consequent vibrations of
the target. An earlier example is described in the Specification
of our British Patent No. 13~4882 in which the target is an
elongated cylindrical drum, filled with liquid and with micro-
phones at one or each end. This has give~ good results, but
it does have a drawback in that its sensitivity varies across
its width.
It would be better to have a sensor in the form
of a flat strip of uniform sensitivity across its width, for
then the whole area would give a useful response and sample
a larger proportion of the straw stream for a given obstructlon; ~ ~ -
-It is an object of this invention to provide such a sensor.
According to the present invention there is
provided apparatus for sensing moving particles or small moving
objects comprising an elongated substantially flat plate one side
of which is a target for said objects and which detectably
vibrates in response to impacts by said objects, a base,
relatively massive in relation to the plate, parallel regularly
spaced resilient strips interposed between the base and the other
side of the platle and extending longitudinally of the plate to
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mount the latter on the base, tne spacing of the strips being
sucn that they coincide with nodes of a selected harmonic of
transverse bending vibrations of the plate, and transducer
means mounted on said other side of the plate between resilient
strips to respond to said vibrations.
Conveniently the second harmonic is selected,
and then there are three resilient mounting stri~s with two
transducers mounted between respective adjacent pairs of strips.
For reasons to be explained it is advantageous to mount the
transducers with opposite polarities.
For a better understanding of the invention one
constructional form will now be described, by way of example,
with reference to the remaining figures of the accompanying
drawings, in wnich;
Fig. 1 is a diagrammatic cross section of a prior
art sensor;
Figure 2 is a diagrammatic side elevation of a
combine harvester with two grain detector elements; and ~ ~
Figure 3 is a cross-section of a detector according
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2~ to the invention, showing a related response characteristic.
~ eferring to Fig. 1, showing a prior art sensor,
it will be seen ~at for particles moving in the direction of the~
- arrow, those impinging on the cylinder over the central zone A
strike almost normally against the surface and therefore provlde
firm impacts giving full sensitivity. Particles impinging to-
wards the edge zones 13 only strike glanciny blows and do not
produce such a good response.
Combine harvesters are well known and will not
be described in detail. In Figure 2 the harvester has a driving
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position 1 at the front of the machine and conventional
mechanism for cutting the grain and delivering it to the :
straw and chaff separators. There is a straw walker 2 arranged
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to discharge straw from beneath a hood 3 at the rear of
the machine. Below the straw walker is a grain sieve
system 4 designed to separate grain from the chaff and
to discharge the chaff at a zone 5, also within the hood
3. Particle detecting elements 6 are positioned within
the hood immediately below the output end of the straw
walker 3 and immediately below the discharge end of the
grain sieve 4. Each detector extends horizontally across
the full width of the respective straw wal~er or grain
sieve, so that a proportion or constant fraction of the
grain that is lost from either of these devices will impinge
on one of the target elements and produce an output signal.
The detectors are connected by cables 7 and 8 to a metering
and display unit 9 positioned in front of the driving
position:so as to be readily visible. ~he unit 9 may
include counting or integrating devices to provide an
output indication corresponding to the rate at which grain
is being lost. Another output from the metering unit 9
is coupled to an automatic engine speed regulator 10 so
that the speed of the combine harvester can be controlled
in response to the sensed grain loss. `
The function of the detectors is to distinguish
between grain and the straw and chaff. In the case of the
walker sensor, for instance, the weight of the straw and
the number of individual straws deflected by the sensor
far outweighs the weight and numbers of grain. However
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the grains are denser and harder, so that individual grain
impacts give impulses of shorter duration. The frequency
spectrum of the short duration impulses contains a greater
proportion of high frequenoies and these can be isolated
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by electrical filters. However, vibration resonances of
the sensor also serve to emphasise certain frequencies.
Broadly, the larger the sensor, the lower the frequency
of these mechanical vibration resonances. Thus for a
large area sensor, further electrical filtering would have
to be applied to reduce the effect of straw impact.
There is also the problem that, in practice, energy is
absorbed by various modes of resonant vibrations and the ~ -
energy available in the higher frequencies is reduced.
Mechanical damping of the sensor can reduce
the predominance of low frequency vibrations, but it also
tends to absorb vibrational energy before it can spread
from the point of impact to the point at which the
vibration is sensed. Therefore the sensitivity to impact
is low in parts of the sensor remote from the transducer.
In order to achieve more uniform resonance, a high 'Q'
factor of the resonant vibrations is required, for the
higher that factor is the greater the number of cycles
of vibration executed for a given factor of decay in
amplitude. However, if there are a large number of impacts
per unit time to be resolved, a low Q factor is desirable,
namely one that is associated with a rapid decay in the
resonance. Alternatively and preferably a high resonant
frequency F can be adopted so that although Q may be high, ;
the ratio Q/~ is also high.
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As~well as these requirements of a high 'Q'
! factor, a high Q/F ratio and a high resonant frequency F
to resolve adequately the impacts of grain from those of
chaff and straw, a large area sensor is needed to respond
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to grain loss across the width of the combine. However,
a high resonant frequency is not normally compatible
with a large size sensor.
Referring now to Figure 3, there is shown a
sensor or detector which, as wel:L as offering a good
area facing the direction of straw or chaff flow, is mounted
- in such a way that the high frequencies can be effectively
isolated.
The detector comprises a wooden base in the form
of a beam 11 which extends across the discharge stream
and has on the side facing that s~ream an elongated aluminum
plate 12 mounted by three hard rubber strips 13 extending long-
itudinally of the detector and situated at the nodes of the
second harrnonic of transverse vibrational bending of the
plate 12. Centrally disposed between adjacent pairs of strips
13 and mounted on the underside of the plate 12 are transducers
14 of opposite polarities, and with leads 15 to an amplifier
and further circuitry (not shown) but which is preferably of
the kind described in our co-pending Canadlan Application l~o.
~ 20 257,634 filed 23 July 1976. As an example of dimensions,
- the size of the cross-section of base 11 may be of the
- order of 100 x 25 mm while the superimposed plate 12 may
be of the order of 6 mrn thick. This mounting of the sensing
plate 12 results in a very selective transmission of
frequencies to the amplifier. The resilient mounting at
the three nodal lines and the relatively massive wooden
base wiil severely damp many other transverse modes,
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especially odd harmonic modes, for example the fundamental,
third fifth and so on. ~urthermore, with the transducers
mounted with opposite polarities, similar and equal bending
movements due to vibrations in the fundamental and odd
harmonic modes cancel out. However, the opposite bending
forces experienced by the transducers at the second harmonic
mode (and at odd multiples thereof) are additive.
Longitudinal bending modes of vibration perpendicular
to the plane of the sensor are completely cancelled.
~hese modes are of low frequency and large amplitude
due to combine engine and suspension vibration. Their
cancellation will greatly reduce the isolation required.
With such a detector of the size indicated, a
high resonant frequency has been observed of approx.
12KHz. ~his shows a good 'Q' factor of the order of 20
to 50, whioh has been found to give a substantially uniform
response over a length of about a metre. It permits the
resolution of impulses with less than 5mS separation.
; A similar device but with just two strip supports has
been compared with the sensor described, ànd this vibrates
at about 5KHz. While it shows a slightly higher 'Q' factor,~
the impulse separation interval is now of the order of 10mS.`
It will be appreciated that the sensitivity of
such a sensor is not uniform, varying across the width
of the sensor as indicated by the graph at the top of
Figuxe 3. The sensitivity reduces virtually to zero at the
nodal lines. However, this is not a disadvantage in practice
for between the nodes the response is reasonably uniform
and there are no areas of exceptionally high sensitivity.
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Along its length the sensor is very uniformly sensitive.
The absence of liquid will allow easier mounting
and use of such sensors, and low susceptibility to
external vibration will allow a simple mounting, even to
moving parts; for example to the shoe of a sieve or a
walker.
It will be understood that these sensors can
be employed in other applications where the detecting of
hard particles is required and where particle flow rates
are to be detected.
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