GB2121542A

GB2121542A - A procedure for measuring the throughput volume or mass or the grain size of divided products, and apparatus for putting this procedure into effect

Info

Publication number: GB2121542A
Application number: GB08309938A
Authority: GB
Inventors: Gerard Lavanchy
Original assignee: HASLER FRERES SA
Current assignee: HASLER FRERES SA
Priority date: 1982-04-16
Filing date: 1983-04-13
Publication date: 1983-12-21
Also published as: GB8309938D0; IT8320618A0; IT1234921B; CH650590A5; GB2121542B; DE3313464A1

Abstract

The present invention relates to a procedure and apparatus for measuring the throughput or the grain size of divided products. The apparatus comprises essentially a hopper (11), a detection electrode (13) held at a potential which is different from that of the hopper, an amplifier (18) and a measuring circuit comprising, for example, three branches (7, 8 and 9). At the time of their passage over the electrode (13) the particles discharged via the hopper (11) are charged. The current coming from the electrode (13) is amplified, and its continuous and pulsating components are measured by measuring instruments (20 and 24), or are used by a counter (26). This apparatus may be used for determining the grain size of a product when its throughput is known, or the throughput of a divided product when its grain size is known. <IMAGE>

Description

SPECIFICATION A procedure for measuring the throughput volume or mass or the grain size of divided products, and apparatus for putting this procedure into effect The present invention relates to a procedure for measuring the throughput volume or mass or the grain size of divided products flowing from storage or production equipment.

This invention also relates to apparatus for putting this procedure into effect.

Numerous installations use devices which deliver doses by volume or weight for the purpose of distributing divided products in the form of powder or granules. These products frequently display a tendency to block the ducts provided to conduct them to the desired location. Furthermore, anomalies in throughput requiring prompt intervention may occur in the dosing devices. As a result, it is often necessary to ascertain periodically that the product is flowing normally, particularly when the system is functioning without permanent human supervision.

Again, many manufacturing processes result in the production of divided products whereof it is desired to know the throughput and/or grain size characteristics.

The present invention proposes a solution which in particular enables the flow of such products to be controlled, certain parameters linked with this flow to be measured and, if necessary, allows intervention to bring this flow under control as a function of these measured parameters.

To this end, the procedure according to the invention is characterised in that at least one isolated electrode, polarised by a continuous voltage, is inserted in a zone where the product flows freely, that the intensity of the current supplied by the electrode and corresponding to the charge of the particles is measured directly or indirectly, this measurement being effected by an electronic measuring circuit comprising at least one measuring instrument, and that a signal is generated representing the throughput of a product with a known grain size, or the grain size of a product with a known volume or mass.

Every particle of the product will already be electrically charged with a charge dQ when it passes over the electrode, this charge being substantially proportional to the surface area of the particle and to the potential of the electrode. The total current measured gives a signal which is representative of the throughput, assuming that the grain size of the product is known, or of the grain size of the product, assuming that the throughput is known.

In addition, when the grain size is sufficiently coarse, the elementary charges taken at the electrode by the successive particles are translated by so many impulses which can be detected and counted in the form of numerical information. In the case of a fine powder, obviously only a pseudo-continuous current is measured.

According to a preferred embodiment of this procedure an analysis and/or a comparison of the continuous components and/or the pulsating components of the current flowing from the isolated electrode which is polarised by a continuous voltage source is carried out. In order to determine the grain size it is possible to measure the throughput volume or mass of the discharged product beforehand. The current measured is the product of a function f(D) of the throughput D of the product, and a function g(y) of the grain size w of this product.

In fact, the current intensity measured, which is directly proportional to the surface area of the particles, is a function of the square of the radius of the particles which are classed as spherical volumes.

The throughput directly linked to the volume of these particles is a function of the third power of their radius. The relationship between the throughput and the intensity of the current is proportional to the radius and therefore enables the grain size to be measured ifthethroughput has been determined beforehand. On the other hand, if the grain size is known, measurement of the intensity enables the throughput of the discharged product to be determined.

To effect mixtures of products with differing grain size a plurality of products are discharged through a corresponding number of hoppers, and at least one of the components of the measured current is used to control the throughput of the products flowing through these various hoppers. Due to the mixing of the different grain sizes, the relationships between the frequency of the impulses, their amplitude and the mean value of the current discharged make it possible, by comparison with experimental results, to determine the approximate grain size composition of the mixture. This information may be used either in itself, or as a parameter which enables the throughput of the components flowing through the various hoppers to be controlled. Naturally, it is also possible to control the throughput of a single product from a single discharge outlet.

Certain manufacturing processes, such as the grinding of cement, for example, produce substances with varying grain size. The grain size information obtained then makes it possible to intervene in the process to control the output from the grinding plant, for example, or to process some of the product afresh by recycling it, in order to increase the fineness.

The apparatus for putting this procedure into effect is characterised in that it comprises at least one hopper allowing the flow of at least one divided product, at least one isolated electrode positioned in the stream of product and polarised by a source of potential, and a measuring circuit comprising at least one measuring instrument for generating a signal representing the throughput of a product with a known grain size, or the grain size of a product with a known throughput volume or mass.

Thus, when it is a question of determining the mean grain size of the product being discharged, the apparatus is associated with a device for measuring the throughput volume or mass emerging from the discharge outlet.

According to a preferred embodiment, the measuring circuit comprises at least two measure ment instruments designed to suppiy signals linked respectively to the continuous and the pulsating components of the current flowing from the measuring electrode.

This measuring circuit is advantageously complemented by at least one circuit for counting the particles. When the product being processed is actually a mixture of components with differing grain size, the circuit advantageously comprises a plurality of circuits for counting the particles, each of which comprises an electronic amplifier with a different response threshold. This design enables particles of each of the components of the mixture to be counted in a relatively simple fashion.

Finally, when the product is to be made up of a mixture of components with differeing grain size, the apparatus comprises a plurality of outlets or discharge hoppers, and devices for regulating the throughput at each of these outlets as a function of the signals supplied by the measuring instruments.

In this case, a system for controlling the quantities discharged is obtained which allows relatively exact mixtures to be made up.

The present invention will be better understood with reference to the description of an embodiment example and the attached drawing, in which; Figure lisa schematic view of the apparatus according to the invention, Figure 2 is a schematic view of a different version, comprising a plurality of hoppers with controlled discharge, Figure 3 is a schematic view of a version which is intended for controlling a grinder, the milling fineness of which it is desired to monitor, and Figure 4 shows a different arrangement for the electrodes.

With reference to Figure 1, the apparatus described comprises a hopper 11 which symbolises a dosing appliance (not shown) and which is electrically connected to earth. From this hopper there flows a stream of product 12 which strikes an isolated electrode 13 constituting a detection electrode. For the sake of keeping the drawing simple, this electrode is represented by a simple plate, but it may be formed by a tubular passage, an open duct, a funnel, etc. From this electrode the product flows in the direction of a container 14which may naturally be replaced by a conveyor belt or any other similar equipment. A source 15 of continuous floating voltage polarises the electrode 13.The electrical current supplied by this voltage source is amplified by an amplifier 18 to the terminals of which a feedback resistance 10 is connected and which is protected by a resistance 16 connected in series and by limiting diodes 17 connected between the input of the amplifier 18 and earth. This current amplified by the amplifier 18 is conducted to a measuring circuit comprising, for example, three main branches 7,8 and 9.

The branch 7 comprises essentially an amplifierintegrator 19 associated with a low-pass filter 6 which is known per se and which is composed of a capacitor 5 and a resistance 4 connected in feed-back from the amplifier-integrator 19, a resistance 3 for adjusting the gain, series-connected to the input of the amplifier-integrator 19, and a continuous current measuring instrument 20, series-connected to the output of the amplifier-integrator 19. For a product with a constant grain size, the instrument 20 gives an indication which is directly proportional to the amount discharged by the dosing device.

The branch 8 comprises an amplifier 22 for alternating signals, the input of which is connected via a coupling capacitor 21 and the output of which is connected to a rectifier bridge 23 in which a continuous current measuring instrument 24 can be connected. A feed-back resistance 2 is connected between the input of the amplifier 22 and the output of the rectifier bridge 23.

At the measuring instrument 24 a rectified signal, proportional to the effective value, for example, corresponds to the peak value or to the mean value of the pulsating component of the signal issuing from the amplifier 18. This latter signal is directly linked to the dimension of the particles being conveyed, that is to say, it is proportional to the charge which they receive from the electrode, this charge itself being directly linked to their surface area. The frequency of this signal is consequently directly proportional to the rate of fall of these particles.

The branch 9 is composed of a threshold amplifiercomparator 25 of the Schmidt trigger type, and an impulse counter 26 which may be used either as a frequency meter or as a summing device. The branch 9 may be replaced by a group of similar branches, each comprising an electronic trigger with a different response threshold, together with a summing device.

The threshold amplifier-comparator 25 is used to shape the impulses destined for the counter or frequency meter 26. A particle, the minimum charge of which, that is to say, in effect, the minimum dimension of which, is defined by the response threshold of the comparator, corresponds to each impulse. If the impulse counter 26 is used as a frequency meter, its information will correspond to the throughput from the dosing device. In the case where it is used as a counter proper, it will integrate the number of particles discharged by the hopper during the operation. As mentioned earlier, the circuit branch may be replaced by a plurality of branches each comprising an amplifier-comparator with a different response threshold, each of which corresponds to a given minimum dimension of the particles in a mixture.

Since the currents measured are very weak, the measuring circuit constituted by the source 15 and the electrode 13 is protected by a screening circuit 27 which is known per se, connected to the pole of the source 15 which is opposite to the one to which the electrode 13 is connected. All the electronic circuits are known and do not require a more detailed description. However, it should be noted that the amplifier 18 should be an instrumentation amplifier with a very low input current, in view of the fact that, when the througput of the stream of product 12 is very small, the intensity of the measured current may be in the order of a few pico amperes.

One possibility for linearisation of the apparatus shown in Figure 1 consists, for example, in correcting the gain of the amplifier 19 automatically by modifying one or other of the feed-back resistances.

This effect may be obtained by replacing the resistance 3, which connects the input of this amplifier 19 to the output of the amplifier 18, by a resistance of the known LDR type on which there acts the light produced by a lamp connected via an appropriate amplifier (not shown) to the output of the amplifier 22 being used in the peak value rectifier circuit.

Thus, large grains producing current impulses with a greater amplitude will increase the gain of the amplifier 19, making the output signal from the latter no longer proportional to the surface area of the grains, but to their mass, which is linked to the surface area by the relationship M = g S3/2 where M is the mass of a grain, g is its specific mass, S the surface area and k a proportionality factor which is a function of the shape and which with adequate approximation is taken as being constant for a given product.

More generally, the continuous and the pulsating components of the signal issuing from the amplifier 19 may be processed, if desired, by spectrum analysing circuits. According to the nature of the product, information on the quantity discharged and the size of the grains is deduced from this, as described above.

With regard to apparatus which has the task of monitoring a process automatically, obviously the outputs of the amplifiers are not necessarily bound to be used to feed simple indicators such as those represented by the instruments 20 and 24 for the sake of ease of understanding. In actual fact, the signals issued by these amplifiers will serve as information destined for the control and/or monitoring unit for the process, which is not shown here and of which the dosing device 1 forms a part. This unit may be a micro-processor equipped with an analogue-digital converter interface.

Figure 2 shows a different version of the apparatus described with reference to Figure 1. This apparatus includes a dosing appliance, shown schematically as three hoppers 30,31 and 32 containing the products 33, 34 and 35 respectively, which are of differing grain size. The lower opening of these hoppers is controlled by valves 36, 37 and 38 respectively, which are actuated by mechanisms 39,40 and 41.

Depending on the opening of the valves 36,37 and 38, predetermined doses of the products 33,34 and 35, which may be of differing grain size, are discharged onto an endless conveyor 42 which pours all the products thus collected into the hopper 11, which is similar to that shown in Figure 1,from whence these products fall over the electrode 13, which is also identical to that in Figure 1.These products are then removed by a conveyor belt 43.

The electrode 13 is polarised as before by the floating source 15, and the intensity of the current is amplified and then measured in the measuring circuit 44. One output 45 of the measuring circuit 44 is connected to a display and recording device 46 which supplies information relative to the throughput, to the grain size or the number of particles of each of the products 33,34 and 35 with differing grain size. Three outputs 47,48 and 49 are connected to the control mechanisms 39, 40 and 41 respectively for controlling the throughput.

The throughput control system may be of the known proportional-differential-integral type, the actuator being the motor of the dosing appliance or a control throttle or a vibrator or the like. An integrating balance 50 associated with a tachometer which is integral with the drive motor of the belt supplies the information on the throughput of material transported by the conveyor 42.

Figure 3 shows, by way of example a modification of the application of the apparatus illustrated in Figure 1.

In addition to the elements described with reference to Figure 1, this apparatus comprises a crushing mill 61 of a conventional type, a device 62 for removing a sample of the product, and a device 63 for regulating the main throughput.

The extracted product is conducted to a container 64, symbolising its evacuation to a destination for a later phase in its preparation. The device 62 for removing a sample may be a doserfollowed by an integrating balance, both of known type, enabling a known quantity of the product to be discharged over the measuring electrode. The information relating to this amount of product is introduced at the input 65 of the measuring appliance 66, which is similar to that described above, the input 67 being connected to the electrode 13 via the voltage source 15. The input 65 is thus designed to correct the gain of the amplifier automatically when the device for removing a sample does not have a constant throughput.

As described above, at its output 68 the measuring appliance 66 supplies a signal which is a function of the fineness of the product samples. This signal may then be used for controlling the regulator of the main throughput 63, since, in the case of grinding mills of known type, the fineness of the product, that is to say, the degree to which it is ground, is inversely proportional to the throughput of the grinding mill.

In one version of equivalent apparatus (not shown) sampling is carried out on the product extracted from the grinding mill, a variable portion then being recycled, this portion being controlled automatically by the signal generated by the measuring appliance and characterising the fineness of the product.

Figure 4 shows a modified version of the arrangement described with reference to Figure 1.

The voltage source 75 is connected to the hopper 71 symbolising the dosing appliance, both the throughput and the mass of which it is desired to measure.

The stream 72 of the particles charged by their passage through the hopper then strikes the isolated measuring electrode 73 which is connected in this case directly to the input of the current amplifier 18 equipped with its feed-back resistance 10. This amplifier then measures the discharge current of the particles, this current being equal to the charging current with which the hopper should have supplied them.

The apparatus is exactly the same as that described with reference to Figure 1, except that it comprises two isolated electrodes, one of which may be the hopper as described above, but it eliminates the use of a source of floating potential and a screening circuit 27.

The present invention is not limited to the embodiments described, but various embodiments of it are possible, and it may be subjected to numerous modifications which will be obvious to a worker skilled in the art.

Claims

1. A procedure for measuring the throughput volume or mass or the grain size of divided products flowing from a storage or production arrangement, characterised in that a stream of the product is discharged through at least one orifice over an isolated detection electrode held at a potential which is different from that of the orifice, that the intensity of the current flowing from the said electrode connected in a measuring circuit comprising at least one measuring instrument is measured, and that a signal is generated which represents the throughput of a product with a known graain size, or the grain size of a product with a known throughput volume or mass.

2. A procedure according to Claim 1, characterised in that analysis and/or comparison of the continuous and/or pulsating components of the current flowing from the detection electrode is carried out.

3. A procedure according to Claim 2, characterised in that the throughput mass or volume of the product discharged via the said orifice is measured independently.

4. A procedure according to Claim 2, characterised in that at least one product with an established grain size is discharged through at least one orifice, and that at least one of the components of the measured current is used to control the throughput of the product flowing through this orifice.

5. A procedure according to Claim 2, charcterised in that at least one of the components of the measured current is used to monitor and/or control the grain size of a product emerging from a production plant.

6. Apparatus for putting the procedure according to any of Claims 1 to 4 into effect, characterised in that it comprises at least one feed device providing a stream of at least one divided product, an isolated electrode positioned in the stream of the product and polarised to a potential which is different from that of the feed device, and a measuring circuit comprising measuring means for generating a signal representing the throughput of a product with a known grain size, or the grain size of a product with a known throughput volume or mass.

7. Apparatus according to Claim 6, characterized in that the measuring means comprises at least two devices designed to suppy signals linked respectively to the continuous and the pulsating component of the current flowing from the electrode.

8. Apparatus according to Claim 6, characterized in that it comprises at least one circuit for counting the particles forming the said product.

9. Apparatus according to Claim 8, characterised in that it comprises a plurality of circuits for counting the particles, each comprising an electronic amplifier with a predetermined response threshold.

10. Apparatus according to Claim 6, characterised in that it comprises at least one independent measuring device for the throughput mass or volume of the product discharged via the hopper.

11. Apparatus according to Claim 6, characterised in that it comprises a plurality of hoppers designed to discharge a plurality of products with differing grain size, and devices for regulating the throughput of each of these hoppers as a function of the signals supplied by the measuring instruments.

12. Apparatus according to Claim 6, characterised in that it comprises means for controlling the grain size of a product coming from a production plant, as a function of at least one of the components of the measured current.

13. Process or apparatus for measuring the volumetric or mass throughput, or grain size of a divided product substantiaily as hereinbefore described with reference to, and as shown in, the accompanying drawings.