FIELD OF THE INVENTION
The invention concerns a metering method for substances, for example
raw materials, semi-finished pieces, ingredients in a mixture or suchlike, and
also the metering device suitable to achieve said method.
The invention can be applied substantially in every field where it
is necessary to make a load, metered according to unit of time, of precise quantities
of substances or ingredients to be fed to a particular treatment, for example
a mixing treatment with other substances in a kneading machine or extrusion machine.
However, it can be applied to particular advantage in the field of
pasta production or other similar food products, such as for example snacks, children's
food, potato chips or similar.
BACKGROUND OF THE INVENTION
In various industrial fields, there is a well-known need to feed
a determined and precise quantity of one or more substances or products to a processing
machine or plant, over a period of time or in defined quantities, in order to
mix, knead, extrude, or draw the substances, or other required processes.
To feed particular materials in appropriately metered quantities,
the use of metering devices is known which, according to the metering system adopted,
are divided into two main categories: gravimetric and volumetric.
In the first category we find that type of device wherein the feed
of the material to be metered is regulated by means of control signals supplied
by weighing elements, which are able to detect, at pre-set intervals, the loss
of weight of the container from which the product is unloaded. The signals are
used to regulate the speed of extraction of the product from the container in order
to establish as regular and constant a flow as possible.
There are various types of gravimetric metering devices on the market,
which substantially comprise a mechanical part and an electric command panel. The
mechanical part normally comprises a hopper, possibly associated with a stirrer
member, which cooperates in the lower part with an extraction screw. The entire
structure is supported by means of a system comprising weighing means, normally
of the load cell type, connected to the electric panel to send signals relating
to the loss of weight of the hopper which feeds the product.
The electric panel comprises a power part, to command the motors
with variable rpm, and a control part, which can be programmed and personalized,
to manage the weight and hence the metering of the product.
A first problem with conventional gravimetric metering devices is
that the electric panel is often very complex and expensive, it can be installed
only by specialists, the relative management software can be modified only by the
manufacturers and not by the users, assistance and spares must be managed only
by the manufacturers.
Therefore, every little malfunction or other type of problem entails
long downtimes of the machine, with considerable loss of productivity.
Another shortcoming of conventional gravimetric metering devices
is the effect of the disturbances to the weight signals caused by the vibrations
introduced by the various devices of the plant; such disturbances affect the correctness
of the metering and feed of the substances.
The present Applicant has devised and embodied this invention to
overcome these shortcomings and to obtain other advantages as shown hereafter.
SUMMARY OF THE INVENTION
The invention is set forth and characterized in the respective main
claims, while the dependent claims describe other additional characteristics of
The main purpose of the invention is to achieve a gravimetric-type
metering device wherein the electric power and control part can be made entirely
with components freely available on the market, in order to allow any necessary
assistance, replacements or maintenance to be made even by the final users.
Another purpose is to adopt a method to control and manage the metering
wherein the weight signals detected by the weighing means associated with the product
container can be processed in such a manner as to eliminate any possible disturbance
or spurious signal, thus optimizing the regularity and precision of feed.
In accordance with these purposes, a gravimetric metering device
according to the invention comprises a substantially conventional mechanical part,
consisting of at least a hopper to feed the material, possibly associated with
at least a stirrer member, extraction means, normally a screw or belt, associated
in the lower part with the outlet from said hopper and weight detection means associated
at least with said hopper and able to detect the weight losses caused by the discharge
of the material.
According to one characteristic of the invention, said weight detection
means are connected to an inlet of a PLC-type processing and control unit by means
of at least a signal conversion unit and at least a communication interface unit,
and an outlet of said PLC-type processing and control unit is connected to a motor
of said extraction means by means of at least a communication interface unit and
a device to regulate the speed of said motor.
According to a variant, one outlet of said processing and control
unit is connected to the motor of said stirrer member by means of at least a communication
interface unit and a device to regulate the speed.
According to another variant, the motor of the stirrer member works
at a fixed speed.
Said PLC-type processing unit is suitable to receive the weight values
as obtained from the electric signals transmitted by the load cells, to process
them by means of an appropriate calculation algorithm, and to supply at output
a command signal to regulate the speed of the motor of the extraction means and/or
the stirrer member in order to maintain the quantity of material fed by the metering
device at a desired value.
According to the invention, the processing performed by said calculation
- at least a step of filtering the signals arriving from the weight detection
- the quantity of material unloaded from the metering device in terms of material
flow (unit of weight per unit of time, for example kg/h) is obtained from said
weight signals and the flow signal thus obtained is filtered;
- said filtered flow signal is sent, together with a filtered signal relating
to the set point metering value set by the operator, to a unit to regulate the
speed of the motor of the extraction means;
- the value relating to the flow signal and the set point value are compared
by said regulation unit, in order to output in feedback a signal to possibly correct
the speed of the motor of the extraction means in the event that said values be
In one solution of the invention, said processing and control unit
is connected on line, by means of a communication interface such as a modem or
similar, to a centralized management and control system, by means of which it
is possible to perform diagnostic and control procedures, and possibly to intervene
to make modifications.
In one embodiment of the invention, the PLC-type processing and control
unit is suitable to activate periodic cycles to fill the hopper, de-activating
the gravimetric control, when the residual material inside the hopper has reached
a pre-determined minimum level and until the material reaches a pre-determined
maximum level again.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other characteristics of the invention will become clear
from the following description of the preferential form of embodiment, given with
reference to the attached drawings wherein:
DETAILED DESCRIPTION OF PREFERENTIAL EMBODIMENT
- Fig. 1 shows a diagram of the principle of the gravimetric metering device
according to the invention;
- Fig. 2 shows a flow chart of the metering control method according to the invention;
- Fig. 3 shows more specifically the flow chart of the control algorithm of the
method according to the invention.
With reference to Fig. 1, a gravimetric metering device 10 for substances
and products comprises a hopper-type container 11 with a substantially vertical
axis inside which the material which has to be fed in precisely metered quantities
Inside the hopper 11 there is a stirrer member 12 with blades, associated
with its own motor 13, the function of which is to regulate the descent of the
material towards the lower outlet mouth 14 of the hopper 11, preventing the material
from forming connections and accumulations.
Said outlet mouth 14 is associated with a screw 15, with a substantially
horizontal axis, the function of which is to extract a desired quantity of material
from the hopper 11 and feed it towards another processing device or plant located
downstream, and not shown here.
The screw 15 is associated with its own motor 16 whose speed of rotation,
with a desired reduction ratio, determines the speed of rotation of the screw 15
and hence the quantity of material unloaded from the hopper 11 in the unit of
In cooperation with the structure which supports the hopper 11 and
the screw 15, there is a weighing system 18 comprising a plurality of load cells
19, normally from two to four (three in the case shown here) associated with a
conversion unit 20 suitable to calculate a value of weight from the electric signals
sent by the load cells 19.
The device 10 also comprises a PLC-type processing and control unit
21 able to control and manage the gravimetric metering procedure.
To be more exact, the unit 21 is suitable to receive as input the
setting signals in order to set and control the system through an operator interface
22, and to intervene in feedback on the extraction system in the event that the
quantity of material unloaded, as detected by the weighing system 18, does not
correspond to the setting value set by the operator.
An inlet of the unit 21 is connected, by means of a communication
interface 23, to the unit 20 which calculates the value of weight, or more correctly,
the weight losses, of the hopper 11/screw 15 system starting from the electric
signals emitted by the load cells 19. Said value relating to weight is sent to
the unit 21 periodically according to a pre-defined sampling interval.
In the event that the weight loss values, and hence the quantity
of material unloaded from the hopper 11 in the unit of time, are not coherent with
the pre-set value, the unit 21 intervenes in feedback and acts, by means of a
communication interface 24, on a unit 25 to regulate the speed of the motor 16
of the screw 15.
Therefore, based on this comparison and regulation, which are performed
with a frequency equal to the frequency of sampling with which the weight is detected,
the unit 21 is able to constantly maintain a desired pattern of the unloading
of the material from the hopper 11, keeping a constant outlet flow corresponding
to the correct quantity which must be fed to the device or plant located downstream.
A communication interface 24 and a regulation unit 25 are also connected,
in this case, to the unit 21 to control the speed of the motor 13 of the stirrer
The components associated with the motor 13 could be omitted, however,
and the stirrer 12 in this case would function at a fixed speed.
In the preferential embodiment, the regulation units 25 are inverters.
According to a variant, the motors 13 and 16 are direct current motors
and the regulation units 25 are able to regulate the current fed to the relative
motors in order to condition the speed thereof.
The principle with which the PLC-type unit 21 carries out the comparison
and regulation is shown schematically in the flow charts in Figs. 2 and 3.
As can be seen in Fig. 2, the control method according to the invention
provides a main procedure, identified in general by the block 29a and carried out
cyclically, which comprises a plurality of sub-procedures to manage the various
components of the device 10 and to interface with the outside.
To be more exact, said sub-procedures comprise: a block 26a to manage
the data exchanged with the outside devices, a block 26b wherein all the types
of alarm are managed which can occur during functioning, a block 26c relating to
the management of the interfaces, a block 26d suitable to calculate the parameters
and control the speed of the motor 16 of the screw 15, a block 26e which manages
the functioning of the motors 16 and 13, and a block 26f which manages the filling
steps of the hopper 11 during which the gravimetric control is excluded.
The control procedure also provides a calculation procedure, identified
in general by the block 29b, which is activated at the same frequency at which
the weight is detected as defined by the sampling interval, for example in the
range of 50 ms, as established by the synchronization block 27a.
Said calculation procedure provides two operations performed simultaneously
with every sampling period: an operation indicated by the block 28a, to acquire
the value of weight as found from the electric signals transmitted by the load
cells 19 and converted by the block 20, and an operation, indicated by the block
28b, to control the flow of material, to compare it with the setting value, and
possibly to supply a command in feedback to intervene on the motor 16 of the screw
Such operation 28b is made clear in the flow chart in Fig. 3.
It comprises a step 30a wherein the weight is acquired by means of
the signals of the load cells 19, a step 30b wherein said weight signal is filtered,
and a step 30c wherein the signal relating to the flow of material found from
said weight signal is filtered.
In parallel, the setting value, set by the operator in a step 30a,
is subjected to filtering in a step 30e.
Then, the filtered signal relating to the flow of material detected
with the sampling frequency and the filtered signal relating to the setting value
are sent together to a block 31 able to compare said values; said block 31 is able
to calculate by how much the speed of the motor 16 of the screw 15 has to be modified
so that the signal relating to the flow of material coincides with the setting
The result of this calculation is then sent to the inverter 25 suitable
to intervene on the motor 16 of the screw 15, and hence on the entire feed and
metering system, to restore the correct functioning.
Said control and regulation procedure, as we have said, is repeated
at the frequency of sampling with every subsequent detection of the quantity of
material, in weight, unloaded from the hopper 11.
It is obvious that modifications and variants may be made to the
metering method and relative device as described heretofore, all of which shall
come within the field of protection of the present invention.