Apparatus for sensing moving particles or small moving objects

A detector for sensing the presence of grain in straw or chaff discharged from a combine has an elongated flat target plate extending across the machine. It is mounted on a relatively massive base through the intermediary of longitudinal resilient strips, which allow transverse bending. Transducers mounted on the reverse side of the plate provide an output from which sharp, grain impact signals can be filtered. The resilient strips are best arranged along nodes of a selected harmonic, preferably the second, of the transverse bending vibrations of the plate. The detector can be used generally for sensing small particles.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention relates to apparatus for sensing or detecting small moving 
particles or small moving objects, and is particularly though not 
exclusively applicable to apparatus for sensing or detecting cereal grain. 
SUMMARY OF THE PRESENT INVENTION 
The invention may be used to sense the grain lost with the straw or chaff 
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 U.K. Pat. No. 1,384,882 in which the 
target is an elongated cylindrical drum, filled with liquid, and with 
microphones at one or each end. This has given good results, but it does 
have a drawback in that its sensitivity varies across its width. Referring 
to FIG. 1 of the accompanying drawing, which shows diagrammatically such a 
sensor in cross-section, it will be seen that for particles moving in the 
direction of the arrow, those impinging on the cylinder over the central 
zone A will be striking almost normally against the surface, and therefore 
providing firm impacts giving full sensitivity, while particles impinging 
towards the edge zones B will strike only glancing blows and will not 
produce such a good response. 
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 
obstruction. 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 plate 
one side of which is a target for said objects, a relatively massive base, 
resilient strips interposed between the base and the other side of the 
plate, and transducer means mounted on said other side of the plate 
between resilient strips. 
Preferably the strips are arranged along nodes of a selected harmonic of 
transverse bending vibrations of the plate. Conveniently the second 
harmonic is selected, and then there are three resilient mounting strips 
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. 
Brief Description of the Drawing 
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 drawing, in which;

DETAILED DESCRIPTIN OF THE PREFERRED EMBODIMENT 
Combine harvesters are well known and will not be described in detail. In 
FIG. 2 the harvester has a driving 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 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 walker 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. The 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 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 frequencies and these can be isolated 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/F is also high. 
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 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 FIG. 3, there is shown a sensor or detector which, as well 
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 stream an 
elongated aluminum plate 12 mounted by three hard rubber strips 13 
extending longitudinally of the detector and situated at the nodes of the 
second harmonic of transverse vibrational bending of the plate 12. 
Centrally disposed between adjacent pairs of strips 13 and mounted on the 
underside of the plate 2 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 application Ser. No. 
31,876/75. As an example of dimensions, the size of the cross-section of 
base 11 may be of the order of 100.times. 25 mm while the superimposed 
plate 12 may be of the order of 6 mm 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 will severely damp many other transverse 
modes, especially odd harmonic modes, for example the fundamental, third 
fifth and so on. Furthermore, 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. These 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. 12 KHz. This shows a good `Q` factor of the order 
of 20 to 50, which has been found to give a substantially uniform response 
over a length of about a meter. It permits the resolution of impulses with 
less than 5 mS separation. A similar device but with just two strip 
supports has been compared with the sensor described, and this vibrates at 
about 5 KHz. While it shows a slightly higher `Q` factor, the impulse 
separation interval is now of the order of 10 mS. 
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 FIG. 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. 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.