Metering method and metering device for powder materials

A metering device which includes a storage reservoir 3, a volumetric extror 30 introducing roughly gauged batches of material into an intermediate hopper 4, which empties periodically into a weighing hopper 1. Under the latter, rapid stabilization scales 2 record the weight of the batch contained in the weighing hopper 1. The latter is emptied onto a belt 5 at the end of a time interval proportional to the weight of the batch or of one of the preceding batches.

BACKGROUND OF THE INVENTION 
The present invention relates to the quantitative metering of materials 
capable of flowing naturally, such as grained products or pulverulent 
products. The metering of such products, whether it be volumetric 
metering, or weight-based metering, has a large number of industrial 
applications. In some cases it is highly desirable for the metering to be 
carried out continuously in order to be able to incorporate the metered 
material in a continuous production process. 
Although quantitative metering has been completely mastered for liquid 
phase products, the same cannot be said for products in powder form. 
Systems which operate by weighing are already known, such as the system 
described in the U.S. Pat. No. 4,320,855. Such a system comprises a 
reservoir containing the material to be metered, the total weight of which 
is continuously evaluated. A flow of material is extracted continuously 
from this reservoir, and knowledge of the progression in the lowering of 
the total weight makes it possible to regulate the output of material 
withdrawn from this reservoir. 
Unfortunately, such a system is not sufficiently accurate. Indeed, the 
scales used must be capable of measuring the total weight of material 
contained in the reservoir. Furthermore, it is desirable to be able to 
measure accurately the instantaneous quantities of material withdrawn from 
the reservoir. Now, the quantities of materials withdrawn per unit of time 
represent typically a very small percentage of the total mass of material 
which the reservoir is capable of containing. It is known that, in 
metrology, it is not generally possible to achieve an accuracy of the 
order of one percent on a deviation which is itself only a very small 
percentage of the measurement range of an apparatus. 
It is therefore very difficult to perform a very accurate metering of 
material on the basis of a method such as that described in the 
aforementioned U.S. Pat. No. 4,320,855. In addition, it may be noted that 
a high degree of accuracy will be all the more difficult to achieve in a 
continuous method where the output is very small: since a continuous 
output is desired, the flow will be divided into as small as possible unit 
quantities; in dynamic operation, achieving an accuracy of better than a 
few percent on a quantity of a few milligrams every second is problematic. 
Metering devices are also known which operate according to a principle of 
volume measurement. Now, most of the time, the metering which must be 
carried out is in reality a weight-based metering, because clearly defined 
masses of material are mixed in order to form a product of given 
composition. Accordingly, volumetric metering is only an indirect approach 
to the weight-based metering which must in fact be performed. 
Consequently, it is often necessary to carry out prior packing of the 
material to be metered, in order to have a strictly constant density of 
material upstream of the volumetric metering system. 
SUMMARY OF THE INVENTION 
The aim of the present invention is to propose a metering method and 
metering device of very great accuracy, which can function with extremely 
small metered unit quantities, very small mean outputs, and which can 
function at a variable nominal output. 
According to the invention, the metering method consists in continually 
repeating the following cycle: 
(a) introducing a unit quantity of material into a weighing hopper, 
(b) recording the weight of the unit quantity in order to characterize one 
weighing, 
(c) removing the quantity of material contained in the weighing hopper at 
the end of a time interval proportional to the weight of a weighing, and 
as a function of the desired output. 
Thus, it is seen that the invention proposes starting from a unit quantity 
which can be quite roughly estimated, then measuring with great accuracy 
said unit quantity, and restoring a flow which is continuous on average by 
releasing said successive unit quantities according to a sequence which 
depends on the measurements actually observed. 
A metering device allowing this method to be carried out comprises: 
a reservoir containing the material to be metered, 
an extractor for withdrawing in a controlled manner from the reservoir the 
material to be metered, 
an intermediate hopper receiving the material which is introduced therein 
by the extractor, 
a weighing hopper receiving the material which is introduced therein when 
the intermediate hopper is opened in order to empty it, 
weighing means giving the weight of the material contained in the weighing 
hopper when the intermediate hopper is closed, 
means triggering the following sequence of operations: opening the weighing 
hopper in order to empty it, then closing it after a given delay, then 
opening the intermediate hopper in order to transfer the contents thereof 
into the weighing hopper, then closing it after another given delay, 
means for adjusting the time interval elapsing between a given sequence and 
the following sequence, on the basis of the measurements from the weighing 
means, as a function of the desired nominal weight output. 
The invention proposes using a weight-based metering technique in order to 
derive advantage from the greater intrinsic accuracy of this technique. In 
contrast to what is known in a weight-based metering system capable of 
operating continuously, the present invention requires the use of scales 
whose operational range corresponds substantially to the metered unit 
quantity. The invention thus makes it possible to profit fully from the 
great accuracy which can be achieved with scales weighing small masses. In 
order to achieve as continuous an operation as possible, it is important 
for the cycle time to be very short. In this case use is preferably made 
of a type of scale whose stabilization time, following a loading, is 
extremely short. Scales capable of weighing several grams with a weighing 
time of less than one second can be found on the market, the weighing time 
including the stabilization time of the scales prior to obtaining the 
measurement. 
The general principle of the invention thus consists in separating the 
nominal flow of material which is to be supplied into successive unit 
quantities which are called "weighings". By virtue of the principle of 
regulating the periods of time separating two successive weighings, or 
more precisely two successive emptyings of the weighing hopper, it is 
possible to achieve a very great accuracy in the overall output delivered 
by the metering system. This very great accuracy can be achieved without 
difficulty even if the system for continuously supplying the weighing 
hopper is itself very inaccurate. The time between the removal of one 
weighing and that of the preceding weighing is proportional to the weight 
of the last weighing or, alternatively, proportional to the weight 
recorded in memory during the weighing carried out on one of the cycles 
preceding the cycle in progress. Said time is, for example, proportional 
to the weight of the preceding weighing, as is the case in the example 
discussed in detail hereinbelow.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The diagrammatic view in FIG. 1 provides an overall impression permitting a 
full understanding of the principle of the invention. The heart of the 
metering installation consists of the weighing hopper 1 and rapid scales 
2. The weighing hopper comprises a fixed wall 10 and a movable wall 11 
articulated about the axis 12. The material to be metered is stored in a 
reservoir 3. 
An extractor 30 allows material to be brought into the intermediate hopper 
4, designed in the same way as the weighing hopper 1 with a fixed wall 40 
forming an inclined plane edged with flanges, on which the material can 
slide, and with a movable wall 41, articulated about the axis 42, capable 
of stopping the material as it slides over the inclined plane formed by 
the fixed wall 40. Finally, the device comprises a removal member such as 
a vibrating belt 5 (FIG. 1) or an inclined vibrating ramp 5' (FIG. 2). 
Each hopper 1 or 4 is opened by tilting sufficiently its movable wall 11 or 
41. In FIG. 2 the full lines represent the intermediate hopper 4 open, and 
the weighing hopper 1 closed, and in a phantom view the intermediate 
hopper 4 closed and the weighing hopper open. Also shown in this same FIG. 
2 are the unit 31 controlling the extractor 30, the unit 51 controlling 
the removal member 5, and a structure 6 for supporting the various members 
of the metering device. 
FIG. 3 illustrates the design of the rapid stabilization scales 2. A rod 20 
is connected on the one hand to the fixed wall 10 of the weighing hopper 1 
and on the other hand to the balance arm 21 of the scales. With the scales 
at zero, the rod 20 is oriented vertically in order to apply the weight on 
the balance arm without any component oriented in the direction of the 
balance arm 21. 
The balance arm 21 supports a winding 29 mounted on a plate 28. The balance 
arm can turn about an axle 22, and its dead weight together with the tare 
of the weighing hopper 1 is compensated by a tare spring 23. The winding 
29 is situated in the air gap of a permanent magnet 27. A zero detector is 
formed by a plate 24 integral with the balance arm 21, a lamp 26 and two 
photodiodes 25 mounted in bridge configuration. At zero there is no light 
on the two photodiodes 25 and the bridge is at equilibrium. When a load in 
the weighing hopper 1 causes the balance arm to drop, the small resulting 
movement is detected by an increase in the light which it produces on one 
of the two photodiodes 25, and the difference in resistance then existing 
between the two photodiodes creates a voltage at the terminals of the 
bridge. This voltage acts on an operational amplifier, which transmits a 
current to the winding 29 in order to bring the balance arm 21 back to 
zero, and the bridge to equilibrium. The electromagnetic force therefore 
exactly compensates for the load in the weighing hopper 1. After 
calibration, the measurement of the current transmitted to the winding 29 
is a measure of the weight of material contained in the weighing hopper 1. 
Different variations of the metering method described hereinabove are 
conceivable. The introduction of material into the weighing hopper can be 
carried out by withdrawing successive roughly measured batches. The 
quantity of material in each batch is preferably regulated as a function 
of the mean weight of the preceding weighings, or as a function of the 
time interval elapsing between two successive removals of a weighing, and 
as a function of the desired theoretical output. 
Perhaps preferable to this discontinuous method is a method in which 
material is withdrawn continuously from a storage reservoir, this flow is 
transferred, during a given time, to the weighing hopper, then this 
continuous flow of material is branched off in order to reintroduce it to 
the reservoir, during the time necessary to record the weight of material 
already introduced into the weighing hopper, then the quantity of material 
present in the weighing hopper is removed, and, as soon as the weighing 
hopper has been emptied of its contents, the flow of material is again 
transferred briefly in order to allow material to enter once more into the 
weighing hopper, and so on. In this case the continuously withdrawn flow 
is preferably regulated as a function of the mean weight of the preceding 
weighings or of the time interval elapsing between two successive removals 
of a weighing, and as a function of the desired theoretical output. 
Described in greater detail hereinbelow is a metering device in which 
successive batches are removed from the reservoir 3, which batches are 
introduced one after another into the intermediate hopper 4. At each cycle 
the volumetric extractor 30 transfers a roughly gauged volume into the 
intermediate hopper 4, assumed to be closed at the start. Upon an order 
from the central unit (designated CPU in FIG. 2) controlling the metering 
system, the intermediate hopper 4 opens, and this releases the material 
held for the time being by the said intermediate hopper 4. 
At this stage no accurate metering has been carried out. The system 
described has the sole purpose of dividing a flow, which should 
theoretically be continuous, into successive unit quantities. Each of 
these quantities can comprise a mass fluctuating within very wide 
proportions, for example + or -20%. 
As soon as the intermediate hopper 4 is closed, after a delay just 
sufficient to allow the falling stream of material present between the 
intermediate hopper 4 and the weighing hopper 1 to have disappeared, and 
which allows the scales 2 to stabilize, the latter record the weight of 
the material accumulated in the weighing hopper 1. 
The central unit controlling the metering device then triggers said 
sequence of operations (opening the weighing hopper, closing it, opening 
the intermediate hopper, closing it). The time interval elapsing between 
two sequences is calculated on the basis of the weight of material 
recorded during the weighing preceding that one carried out just before 
triggering a sequence of operations, or it could be calculated on the 
basis of one of the preceding weighings. 
The quantity of material withdrawn by the volumetric extractor 30 is itself 
preferably regulated as a function of a chosen number of preceding 
weighings, or of a chosen number of time intervals, and by comparing these 
values to desired mean values. 
The nominal output of the metering installation expressed in mass per unit 
of time is an operating parameter of this installation. On the basis of a 
set nominal output, and a weight measurement, it is possible to calculate 
the time during which a known weight quantity must be released in order to 
obtain a constant theoretical output. The metering device in this way 
releases each quantity of known mass after a time which is a function, not 
of this mass, but of the mass of the previously released quantity. 
Referring to FIG. 4 it will be seen that P.sub.0 represents the weight of 
the first weighing and P.sub.1 represents the weight of the second 
weighing, the latter is released at the end of a time interval t.sub.0 
which is a function of the weight of the preceding weighing P.sub.0. In 
the same way, if P.sub.2 represents the weight of the third weighing, the 
latter is released at the end of a time interval t.sub.1 which is a 
function of the preceding weighing P.sub.1. The straight broken lines 
between the top of each weighing and the base of the following weighing 
are of constant gradient. In this way an output (weight/time) can be 
readily formed which is continuous and constant overall. However, this 
output is pulsed since it consists of unit quantities distributed over 
time. 
It is very easy to smooth the output discharged from the metering system 
formed in this way by using, for example downstream of the weighing 
hopper, a vibrating ramp 5' or a vibrating belt 5, which will allow each 
weighing to spread out along the ramp before joining up with the receding 
weighing to again form a substantially continuous flow of material. The 
time intervals can be adjusted by any suitable means, for example, by the 
central control unit CPU shown in FIG. 2. 
The invention can have a very advantageous application in the continuous 
preparation of the rubber mixtures which are used in the pneumatic tire 
industry. It can be used in particular for performing the continuous 
metering of most of the chemical products used as additives in the base 
elastomers, since most of these can be reduced very easily into powder or 
granules. It can also be used to perform the continuous metering of the 
elastomers themselves when they have being shredded beforehand. The output 
ranges used can vary from a hundred grams or so per hour to several 
hundred kilos or so per hour.