GB2270771A

GB2270771A - Controlling bag filling by check weighing

Info

Publication number: GB2270771A
Application number: GB9319096A
Authority: GB
Inventors: Kenneth Victor Shail
Original assignee: Sparc Systems Ltd
Current assignee: Sparc Systems Ltd
Priority date: 1992-09-18
Filing date: 1993-09-15
Publication date: 1994-03-23
Anticipated expiration: 2013-09-15
Also published as: DE4331879A1; GB9319096D0; GB2270771B; GB9219773D0

Abstract

Apparatus for and method of filling a bag at a filing station in which a feed means 15, which may be an auger screw 14, is controlled by a control system 25, each filled bag 23 being directly received on a checkweigher (17, Figs 1 - 3 not shown) comprising weight transducer 24 and weighed, the weight being compared in the control system with a target weight and any discrepancy being translated into adjustment of the feed means before completion of the filling of the bag immediately subsequent to the one weighed. Interruption of the feed means may occur when the bag being filled 22 is substantially 75% of the filled weight of the preceding bag 23, feed recommencing only after adjustment of the feed means. Control system 25 may control a position drive, eg a servo motor or electric motor and clutch/brake unit, and the check-weigher may be disposed directly beneath the filled bag discharge position (see Figs 1 and 2). <IMAGE>

Description

1 2270771 WEIGH-FILLING METHOD AND APPARATUS This invention relates to a

method of and apparatus for weigh-filling a pouch, bag or the like, particularly with a fluent material, such as a powder.

Currently powders are fed to fill polyethylene, polypropylene or laminate pouches or bags. These are formed from flat film, around a shoulder, sealed into a tube form, filled, and then end-sealed. The machine carrying out these operations is known as a form-fillandseal machine. With such a machine, there is a problem in regard to dust which can linger in the air after filling. The dust is often trapped between respective layers of film when the bag is closed, contaminating the surface:and thus causing faulty seals. Machine operation cycle times have to be extended to allow the dust to settle, thus slowing down production.

The amount of dust in the air can be reduced by filling directly into the bag and not dropping the complete fill from above. Siich an arrangement is known and currently employs an auger screw arranged with its discharge end at the mouth of the bag. Accordingly, the powder has to drop only from the mouth to the bottom of the bag. Furthermore it is also possible to have the discharge end of the auger inside the bag and progressively to withdraw the bag as it is filled. This is known as bottom-up, filling.

A basic auger filler is a volume filler. The weight of the powder dispensed is a function of: a) the number of revolutions (or part thereof) of the auger, b) the bulk density of the powder, c) the filling efficiency of the auger, i.e. how well the powder occupies the volume of the auger.

2 Whilst a) can be controlled accurately, b) and c) are subject to relatively short term variations, thus causing pro-rata variations in the weight of product in the bag.

one solution is to follow the form-filland-seal machine with a belt type in-line automatic check weighing machine, which weighs each filled bag by passing it over a weigh platform'. The average weigh of a batch of filled bags is then calculated and a signal sent to the auger filler in an attempt to compensate for any variation from the desired weight of a filled bag.

However it can be difficult to mount such a check weighing machine close to the form-fill-and-seal machine and accordingly several bags are normally en route between filling and check weighing. The short term variations in b) and c) can easily have occurred before the bag is checkweighed. The delay also necessitates a gradual adjustment to prevent instability in the control loop. The whole operation is generally known as 'trend feedback', because it can only correct for long-term trends in the pack weight.

An object of the invention is to provide a method and apparatus for overcoming the aforementioned problems in an effective manner.

According to a first aspect of the invention there is provided a method of filling a bag at a filling station with material by controllable material feed means to provide a target weight for said bag, which filled bag then leaves the filling station, comprising weighing a filled bag received from said filling station, comparing the weight of the weighed bag with said target weight and, if there is more than a predetermined discrepancy between the two weights, adjusting the material feed 3 means accordingly before completion of the filling at the filling station of the bag immediately subsequent to the weighed bag, and so on for each weighed bag and the bag immediately subsequent to it.

According to a second aspect of the invention there is provided apparatus for filling a bag with material to achieve a target weight for said bag, comprising a filling station, controllable material feed means at the filling station for filling a bag at the filling station, in use, with material, weighing means for receiving, in use, a filled bag from the filling station, and control means for comparing, in use, the weight of a weighed bag with said target weight and, if there is more than a predetermined discrepancy between the two weights, adjusting said material feed means accordingly before completion of the filling at the filling station of the bag immediately subsequent to the weighed bag.

As used herein, the meaning of the words fill',,filled' and filling, in relation to the passage of material into bags is not restricted to that where the whole internal volume of a bag is occupied by material, but includes that where only part of said internal volume is occupied by material (to provide the target weight). 'Bag' is used herein to include all suitable containers and is not limited in respect of material or form.

Preferably said weighing and any required adjustment of the feed means take place after filling of said immediately subsequent bag has begun. Conveniently operation of the feed means is interrupted whilst adjustment is made thereto. Desirably said feed means is an auger screw, rotation thereof being interrupted, for example, when approximately 75! of the dose, based on the number of revolutions of the auger required to fill the 4 previous bag to its target weight, has been delivered. Advantageously the control means proportions the volume of each material dose to a bag directly on the number of revolutions and achieved weight of the previous dose, so that 100% correction from one bag to the next is obtained.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a schematic side view of apparatus of the invention for filling a bag by the method of the invention, Figure 2 is a front view of the apparatus of Figure 1, Figure 3 is an enlarged scale side view of an automatic checkweigher of the apparatus of Figures 1 and 2, and Figure 4 is a block diagram of a control system for the apparatus of Figures 1 and 2.

Apparatus according to the present invention is shown in Figures 1 and 2 and this is used for carrying out the method of the invention. In essence the apparatus comprises a known form-fill-and-seal machine which has an auger screw for feeding fluent material. The known machine is however, in the present invention, adapted both by-the addition of an automatic checkweigher immediately below an output of the machine and also by means of the method for controlling the auger screw in relation to information from the checkweigher.

The apparatus shown in Figures 1 and 2 comprises a reel of film 10 intended to form, at the machine, bags or the like for receiving the fluent material, such as a powder. The film is, for example, of polyethylene, polypropylene or a suitable plastics laminate. The film is fed by known conveying means to a forming shoulder 11 at the front of the machine where the film is formed into a bag or pouch. Each bag is sealed at vertical sealing means 12 also at the front of the machine, below the forming shoulder. The top of the machine mounts a motor 13 of known type and having associated components to be described hereinafter. The motor drives an auger screw 14 which extends vertically downwards centrally through a hopper 15, the discharge end of the auger screw being either at the mouth of the bag formed by way of the shoulder 11 and sealing means 12, or alternatively inside the bag for progressive withdrawal as the bag is filled.

As part of the sealing of the bag, there are provided horizontal sealing jaws 16 and these end-seal a filled bag as it drops down from the defined filling station at the front of the machine.

Disposed immediately below the position at which a filled bag is released from the filling station is a checkweigher 17, this enabling the bag to be checkweighed as soon as it is closed by the jaws 16.

Although there are obviously many way-of weighing bags-leaving the filling station, a convenient arrangement is provided by the checkweigher 17 shown in detail in Figure 3. Shown in this figure is an inclinedplate automatic.checkweigher designed specifically to work directly under formfill-and-seal machines. A base of the checkweigher carries a loadcell 18 on which is mounted an inclined weighing trough 19. At the lower end of this trough are arranged holding fingers 20 supported by the loadcell for rotational movement into and out of the lower end of the trough 19.

Also mounted on the base, separately from the loadcell 18, is a pusher 21 pivotally movable by suitable 6 means, such as hydraulic pistons, between a retracted position, shown in Figure 3 where it is swung downwardly from the lower end of the trough 19, and an extended position where it is in line with the trough so that its upper end is in juxtaposition with the lower end of the trough 19.

With the trough empty, the pusher is in its extended position and the fingers are in a retracted position, in which they are received in slots in the pusher. When a filled bag arrives onto the trough 19, it slides down the inside thereof until it is stopped by the upper surface of the pusher. Since the pusher is not mounted on theloadcell, the kinetic energy of the bag and its contents are notimparted thereto, instead being absorbed through the base of the checkweigher. A photocell detects the arrival of the bag at the pusher and immediately starts the holding fingers rotating from their retracted to their extended positions at the bottom of the trough 19 where they take the weight of the bag. Very shortly afterwards, the pusher retracts to its position shown in Figure 3, thus leaving the bag totally supported on the loadcell by the fingers. As will be described, the loadcell then provides a weight signal which is transmitted to a control system of the apparatus of the invention, this signal being stored in the control system memory, whereupon it causes the fingers to rotate back to their retracted position. The bag therefore now slides onto the sloping front surface of the pusher, and after a short delay the pusher returns to its extended, upper position to help eject the bag forward and be ready to receive the next one, the cycle then repeating. For simplicity the drive cylinder for the holding fingers is not shown in Figure 3, although this would be mounted on the loadcell.

As well as the provision of the checkweigher 7 immediately below the discharge of the form-fill-and-seal machine, enabling a bag to be check weighed as soon as it is closed, the present invention also relates to the use of software in a control system of the filling machine which enables the volume of powder to be adjusted according to the weight of the previous bag. This reduces the effect of long and short-term variations in b) and c) referred to in the introduction, to only those variations which occur from one bag to the next.

The simplest way to carry out this adjustment would be to weigh the previous bag before starting to fill the immediately subsequent one. Although this is according to the invention, it is disadvantageous in having an adverse effect on production speed. Accordingly a preferred embodiment involves starting to fill a bag at the filling station before the immediately preceding bag is weighed, then pausing when 75Cj or approximately 75% of the dose (based on the previous number of revolutions of the auger screw) has been fed into the bag. By this time weight information on the previous bag should be available. If not, the auger screw can, in one embodiment, stop rotating until this weight information arrives at the control system which itself controls the auger drive shaft. A microprocessor in the control system then calculates the remaining number of revolutions (or part thereof) to complete the dose, where the total number of revolutions in that dose (number of revolutions of previous dose) x (target weight) (actual weight of previous dose) In other words the control system proportions the volume of each dose directly on the number of revolutions and achieved weight of the immediately preceding dose. The method therefore obtains 100% correction from one bag to the next. The target weight is the weight marked on the bag or some other ideal fill weight that the user 8 wishes-to achieve in each bag. Of course the difference between a bag weight and a dose weight will merely be the weight of the material of the bag itself and the compared weights may or may not include this weight each time. Although it will normally be desirable for the exact target weight to be met each time, i.e. the predetermined discrepancy allowed is zero, a small predetermined difference from the target weight could possibly be allowed without adjusting the number of auger screw revolutions.

The 75% figure mentioned above can be varied and is subject to secondary software control. It is desirable to keep it as high as possible to allow maximum weighing time consistent with allowing sufficient remaining auger revolutions on which to make correction for the wildest variation in weight measured so far. However as mentioned above, the weighing of the previous bag and/or the adjustment of auger revolution can be carried out before the start of the filling of the immediately subsequent bag. Clearly the adjustment must be made prior to completion of the filling of said immediately subsequent bag. As stated, said predetermined difference or discrepancy in the compared weights is, in a prefered embodiment, zero.

Figure 4 shows the hopper in the form of an auger filler 15 with an auger drive shaft 14B extending centrally therethrough, a bag 22 being filled being shown at the d ischarge end of the auger filler, the previously filled bag 23 being shown on a weight transducer 24 which is part of the checkweigher 17. The weight signal from the transducer 24 is transmitted to a control system, shown generally at 25 and including, as mentioned above, a microprocessor. This control system 25 controls a position drive 26 for the motor 13. The position drive which turns the auger screw can be a servo-motor with a 9 shaft encoder on its output shaft or an electric motor of any type followed by an electrical clutch/brake unit. The latter employs a toothed metal disc with an inductive proximity switch to sense each tooth. A control system then counts teeth to determine the number of auger revolutions or part revolutions.

Although described with an auger screw, it will be appreciated that the apparatus and method of the invention are equally relevant to any controllable material feed means, the invention also not being limited to the feed of fluent material, and therefore including the feed of alternative material such as piece-part products.

1 1

Claims

1. A method of filling a bag at a filling station with material by a controllable material feed means to provide a target weight for said bag, which filled bag then leaves the filling station, comprising weighing a filled bag received from said filling station, comparing the weight of the filled bag with said target weight and, if there is more than a predetermined discrepancy between the two weights, adjusting the material feed means accordingly before completion of the filling at the filling station of the bag immediately subsequent to the weighed bag, and so on for each weighed bag and the bag immediately subsequent to it.

2. A method as claimed in claim-1, in which any adjustment of the material feed means takes place after commencement of the filling at the filling station of the bag immediately subsequent to the weighed bag.

3. A method as claimed in claim 2, in which the filling is interrupted should any adjustment of the material feed means be necessary and does not recommence until such adjustment has been made.

4. A method as claimed in claim 3, in which the interruption occurs when the weight of the bag being filled is 75% or substantially 75% of the filled weight of the preceding bag.

A method as claimed in any one of the preceding claims, in which the material feed means includes an auger screw which feeds material to each bag at the filling station.

6. A method as claimed in claim 4, in which the material feed means includes an auger screw and said 75915 or substantially 75% is calculated on the basis of the previous number of revolutions of the auger screw to fill said weighed bag and the total number of auger screw revolutions for the bag being filled is calculated as: number of revolutions to f ill Previous bacr x tarciet wc-ht actual weight of previous bag

7. A method as claimed in any one of the preceding claims, in which each filled bag is weighed by means of a checkweigher disposed directly beneath the position of discharge of a filled bag from the filling station.

8. A method as claimed in claim 7, in which computer control means recieve the weight of a bag weighed by the checkweigher, compare this weight with said target weight and if the weights differ by more than said predetermined discrepancy effect adjustment of said material feed means accordingly.

9. Apparatus for filling a bag with material to achieve a target weight for said bag, comprising a filling station, controllable material feed means at the filling station for filling a bag at the filling station, in use, with material, weighing means for recieving, in use, a filled bag from the filling station, and control means for comparing, in use, the weight of a weighed bag with said target weight and, if there is more than a predetermined discrepancy between the two weights, adjusting said material feed means accordingly before completion of the filling at the filling station of the bag immediately subsequent to the weighed bag.

10. Apparatus as claimed in claim 9, in which the control means are arranged to effect any adjustment of the material feed means, in use, after commencement of 12 the filling at the filling station of the bag immediately subsequent to the weighed bag.

11. Apparatus as claimed in claim 10, in which the control means are arranged to interrupt filling should any adjustment of the material feed means be necessary and not to recommence until such adjustment has been made.

12. Apparatus as claimed in claim 11, in which the control means are arranged to effect said interruption when the weight of the bag being filled is 75% or substantially 75% of the filled weight of the preceding bag.

13. Apparatus as claimed in any one of claims 9 to 12, in which the material feed means includes an auger screw to feed material to each bag at the filling station.

14. Apparatus as claimed in claim 13, in which the control means are arranged to control,in use, the number of revolutions of the auger screw in each bag filling operation and thus the tot al weight of material supplied.

is. Apparatus as claimed in claim 14, in which the control system controls a position drive which turns the auger screw.

16. Apparatus as claimed in claim 15, in which the position drive is a servo-motor with a shaft encoder on its output shaft.

17. Apparatus as claimed in claim 15, in which the position drive is an electric motor followed by an electrical clutch/brake unit.

13

18. Apparatus as claimed in claim 17, in which the clutch/brake unit has a toothed metal disc with an inductive proximity switch to sense each tooth, the control sysytem being arranged to count said teeth,in use, to determine the auger screw revolutions.

19. Apparatus as claimed in any one of claims 9 to 18, in which a checkweigher is disposed directly beneath the position of discharge in use, of a filled bag from the filling station.

20. Apparatus as claimed in claim 19, in which the checkweigher includes a weight transducer from which, in use, is transmitted a weight signal for a weighed bag to said control means.

21. A method of filling a bag substantially as hereinbefore described, with reference to, and as shown in the accompanying drawings.

22. Apparatus for filling a bag substantially as hereinbefore described, with reference to, and as shown in the accompanying drawings.