GB2234076A

GB2234076A - Method and apparatus for detecting leaking gas-filled packs

Info

Publication number: GB2234076A
Application number: GB8913097A
Authority: GB
Inventors: Stephen John Culley; Andrew Knapp; Steven Denny
Original assignee: University of Bath
Current assignee: University of Bath
Priority date: 1989-06-07
Filing date: 1989-06-07
Publication date: 1991-01-23
Also published as: GB8913097D0

Abstract

Apparatus for testing for leaks in a gas-filled pack is placed in a packaging line and comprises two upper conveyors (10A, 10B), each connected to a load cell (12A, 12B) and positioned above a lower, synchronized conveyor (8). A gas-filled pack on the lower conveyor (8) passes under and is compressed by the first and second conveyors (10A, 10B), in succession, the pack being detected by sensors (16) connected to a control unit (6) which causes the force required to displace the pack lid to be measured by a respective load cell (12A, 12B) at regular intervals of travel. A microprocessor determines from a comparison of the two force/displacement relationships so generated whether the pack leaks. In an alternative arrangement using just one load cell, the lid of the pack is displaced by a vertically reciprocal load cell whilst the load cell is moved along the packaging line by a chain drive moved at the speed of a pack by a stepper moter. When the vertical return stroke of the load cell is completed the actuator and load cell are returned to their initial horizontal position. <IMAGE>

Description

Method and Apparatus for Detectina Leaking Gas-filled Packs The invention relates to a method and apparatus for detecting leaking gas-filled packs, particularly, but not exclusively, food packs.

During the packaging of certain food packs, a film is placed over a tray containing a food product and air is then evacuated from the pack and replaced by a selected gas or gas mixture before sealing down of the lid. If there is some fault in the pack, for example if the packaging material or the seal around the pack lid is defective, air may enter the pack which could result in deterioration of the food. In order to guarantee shelf life, manufacturers currently perform quality control tests, for example, by removing packs at random from the packaging line and analysing a sample of the gas inside the pack. Alternatively, employees may inspect the packs or squeeze them by hand.Pack tops are usually slightly domed owing to the gas pressure inside the packs slightly exceeding atmospheric pressure and if a pack with a leak is squeezed it will not return to its original domed shape after squeezing. The latter method, however, is inefficient and labour intensive.

The invention provides a method of testing a gasfilled pack for leaks, which comprises using a device to apply a force to displace a portion of the pack inwardly and determining from the force/displacement relationship whether the pack leaks.

Conveniently the device may displace the top of the pack.

The invention is particularly useful in the case where the pack contains a food product and the gas filling the pack is not simply air.

The pack may have a body defining a tray and a film of plastics material extending over the top of the tray and sealed, for example, by heat sealing, to the tray.

Such packs are for example commonly provided for fish and meat products, for example bacon, mince and ham.

In carrying out the method of the invention it is possible to measure the force that has to be applied to generate a given displacement, or to measure the displacement caused by a given force or to measure both force and displacement. In the case where the force is measured, the force applied by the device to effect the displacement may be measured directly (for example by means of a load cell) or indirectly (for example by a measurement of pressure or some other variation).

Similarly, in the case where the displacement is measured, the displacement caused by the force applied by the device may be measured directly (for example by measuring the changes in dimensions of the pack) or indirectly (for example by measuring the displacement of the force applying device). The displacement may be a uniform displacement of a particular portion of the pack or it may be a non-uniform displacement with some portions of the pack being displaced more than others.

There are a variety of ways in which the force/ displacement relationship may be used to indicate whether the pack leaks. In one preferred method, the force/ displacement relationship during increase of the force applied to the pack is compared with the force/ displacement relationship during decrease of the applied force and the result of the comparison used in the determination of whether the pack leaks. An alternative approach is to consider the area under a graph of force against time in a case where the displacement of the portion of the pack is steadily increased and then reduced. In the case of a leaking pack the area under the graph will be significantly less. Another possibility is to carry out the step of using a device to apply force to displace a portion of the pack inwardly a plurality of times and to compare the force/displacement relationship on each occasion.With a non-leaking pack the force/displacement relationship should not vary, whereas with a leaking pack there will be a variation.

Preferably a displacement of a pack takes place while it is moving. This enables packs to be tested while they are moving along a packaging line without slowing down the movement of the pack. The pack may be displaced by causing it to pass through a space whose dimension measured in at least one direction is less than the corresponding dimension of the pack. For example the pack may be required to pass through a space the vertical dimension of which is less than the height of the pack. In such a case the leading portion of the top of the pack is displaced downwardly as it passes through the space and the remaining part of the top of the pack is similarly displaced as the pack continues its passage through the space.The pack may be passed between a pair of conveyors spaced apart by a distance less than the corresponding dimension of the pack, whereby a portion of the pack is displaced as the pack passes between the conveyors. The conveyors may have conveyor belts but other forms of conveyor, for example roller conveyors, may be employed, if preferred. Where such conveyors are used, the force applied by one of the conveyors to the pack may be measured.

An alternative to employing a pair of conveyors and one which still enables the displacement of the pack to take place while it is moving is for the pack to be displaced by a displacing head mounted for movement towards and away from the pack on a carrier which itself moves in synchronization with the pack from a first position during application of a force to the pack and thereafter returns to the first position.

The present invention also provides an apparatus for detecting a leak in a gas-filled pack, which comprises a device for applying a force to the pack to displace a portion of the pack inwardly and means for determining from the force/displacement relationship whether the pack leaks.

The apparatus may be arranged to carry out any of the forms of method defined above.

The present invention further provides a packaging line including an apparatus for detecting a leak in a gas-filled pack, the apparatus being as defined above.

Certain embodiments of the invention will now be described in greater detail by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of one type of inert gas filled food pack; Figure 2 is a schematic side view of a two station testing machine; Figures 3A and 3B are respectively side and end views of an upper conveyor unit of the machine of Figure 2; Figures 4A and 4B are respectively side and end views of a prototype two station testing machine embodying the design principles of Figures 2, 3A and 3B; Figures 5A and 5B are graphs of applied force against distance travelled by a pack under respective testing stations; Figure 6 is a graph that illustrates how a "skew factor" may be calculated; Figure 7 is a perspective view from below of a food pack having an indented lid;; Figure 8 is a schematic side view of a reciprocating head machine for use in an alternative embodiment of the invention.

The gas-filled pack of Figure 1 comprises a tray 2 over the top of which a film 1 of plastics material is secured with the film 1 sealed to the edge 3 of the tray, for example by heat sealing.

Figure 2 shows a two station testing machine that is suitable for testing packs of the type shown in Figure 1.

The machine comprises two upper conveyor units 4A, 4B, a lower conveyor unit 5 and a control unit 6. The machine is placed in a packaging line and is supported on adjustable feet 7 (see Figures 4A and 4B) which allow the height of the machine to be altered as required.

The lower conveyor unit 5 is of a generally conventional design and comprises a lower conveyor 8 which forms part of the packaging line and an induction motor 9 which drives the conveyor 8.

The upper conveyor units are the same and one is shown in Figures 3A and 3B. It comprises a conveyor 10 for compressing a pack passing beneath it, a stepper motor 11 and a load cell 12 for measuring the force applied by the conveyor. The structure supporting each conveyor is very rigid, ensuring that when the packs are compressed, undesired deflection is kept to a minimum.

Referring now also to Figures 4A and 4B, which shows the safety covers removed from the prototype, each upper conveyor 10 is aligned with, and positioned above, the lower conveyor 8. The vertical distance separating the two conveyors is approximately equal to but less than the overall pack height; the length of each upper conveyor is in the described embodiment the same as the pack length. If desired the length of each upper conveyor may be longer than the pack length.

In the described embodiment each of the conveyors has a conveyor belt which has a coarse texture so that any leaks in the pack are not sealed as the pack moves under an upper conveyor.

In order to accommodate packs of various heights the heights of the upper conveyors can be adjusted. A coarse adjustment means 13 is provided consisting of two parallel series of holes 13A in each of two side frames 13B fixed to the lower conveyor unit. A mounting member 13C of the upper conveyor unit is fixed to the side frames by bolts 13D passing through a selected pair of holes 13A in each side frame. Selection of the holes 13A through which the bolts pass provides a coarse adjustment. A fine adjustment means 14 is also provided. The fine adjustment means is a screw adjustment provided by four screws 14A (two of which are visible in Figure 4B there being two more behind the two shown), which finely adjust the vertical position of the upper conveyor 10 relative to the mounting member 13C.Four locking bolts 14D are provided for locking the upper conveyor to the side frames after the screws 14A have been adjusted. As shown in Figure 4A, the bolts pass through vertical slots 14E in the side frames. Both upper conveyors are adjusted to the same height above the lower conveyor.

Pairs of alignment guides 15 provided on opposite sides of the lower conveyor 8 position the packs on the lower conveyor in alignment with the upper conveyors 10 before they pass under the upper conveyors.

The distance between the alignment guides 15 can be altered to allow packs of varying widths to be guided under the conveyors 10.

In order to detect a pack approaching or leaving an upper conveyor 10, a respective optical sensor 16 (Figure 2) is provided midway between each upper conveyor 10 and the lower conveyor 8, the sensor being positioned halfway along the length of the upper conveyor.

As described briefly earlier, the lower conveyor 8 is driven by an induction motor 9 and each upper conveyor 10 is driven by a stepper motor 11. The speeds of the lower conveyor 8 and the upper conveyors 10 are synchronized.

The machine is fully automated, being controlled by a control unit 6 comprising a microprocessor. Signals from each of the optical sensors and load cells are relayed to the microprocessor. The load cell is linked to an analogue to digital converter that is connected to the microprocessor.

During the packaging operation, food packs move along the production line at a rate of for example 60 to 80 packs per minute. After a food product has been sealed in a pack the lid of the pack may be slightly convex or concave in shape because the pressure of the gas in the pack may not be atmospheric pressure. A pack passes onto the lower conveyor 8 of the testing machine and is subsequently positioned by the alignment guides 15 before passing under the first upper conveyor 10 (the conveyor on the left in Figure 4A and on the right in Figure 2). As this occurs, the lid 1 of the pack is displaced inwardly by the force exerted on it by the upper conveyor 10. When half the pack has moved under the upper conveyor 10, the front end of the pack will interrupt the beam of the optical sensor; this causes the microprocessor to commence sampling the force recorded by the load cell.Once the trailing end of the pack has passed the optical sensor, the beam is reestablished, thus causing the microprocessor to cease sampling. The samples from the load cell are stored in a buffer memory of the microprocessor. The pack then passes under the second upper conveyor 10 (the conveyor on the right in Figure 4A) where the sampling process is repeated.

The microprocessor takes an appropriate number of samples (for example 20 to 100) at each testing station.

The samples are taken at regular intervals of travel of the pack between the conveyors in such a way that changes in the speeds of the conveyors do not affect the number of samples taken. The samples from the load cell associated with the first upper conveyor are stored in the microprocessor memory and are subsequently compared with those obtained from the load cell associated with the second upper conveyor. If, upon analysis, the microprocessor detects a leaking pack, it initiates an ejection mechanism that removes that pack from the packaging line further down the line.

Figures 5A and 5B show how the microprocessor may determine whether a pack is leaking or not. Each graph shows, for both a sound pack and a defective pack, how it may be predicted that the force applied by the upper conveyor changes with the distance D that the pack has moved under the upper conveyor. The distance D is approximately proportional to the total inward vertical displacement of the pack lid and hence to the volumetic displacement and, for a constant conveyor speed, is proportional to time. It will be seen that as a defective pack moves under the first upper conveyor (Figure 5A), the force builds up but to a lower value for a defective pack than for a sound pack because the displacement causes some of the gas to be expelled from the pack.When the defective pack moves under the second upper conveyor (Figure 5B), far less force is necessary to move the lid to the same displaced amount.

The microprocessor can calculate the areas under each graph for a particular pack and, by comparing the magnitudes of the respective areas, determine whether a pack is leaking (in which case the second area is substantially reduced).

It will be appreciated that the prototype machine described above is a simple form of apparatus that is suitable for carrying out the invention. It would be possible to incorporate a number of developments and refinements in a final version of the machine. For example, the manually adjusted coarse and fine adjustment mechanisms for the upper conveyors 10 described above may be replaced with automatically adjustable means.

Various methods may be adopted for synchronizing the speeds of the lower conveyor 8 and the two upper conveyors 10. In one example an encoder is fitted to the drive for the lower conveyor 8 and pulses from the encoder used to drive the stepper motors 11. In that case, the speeds of the upper conveyors 10 are adjusted according to the speed of the lower conveyor 8.

Alternative embodiments of the invention may be adopted, depending on the type of packaging line and the type of pack.

Instead of a two station testing machine, and especially, but not only, if space is of paramount importance, a single station testing machine may be used.

The single station may resemble one of the stations described in the machine above, but in this case a different test for a leaking pack must be made. Referring more particularly to Figure 6, which shows a graph similar to that of Figure SA or Figure 5B, it would be possible instead of calculating the area under the graph, for the microprocessor to calculate a "skew factor".

The "skew factor" is a measure of how quickly the force rises with displacement (or time) compared with how quickly the force falls. As can be seen from Figure 6, in the case of a leaking pack, not only does the force not reach as high a value as for the sound pack, it also falls off in a shorter time compared with the time in which it initially rose. Of course, the extent of the asymmetry will depend on a number of factors, such as the size of the leak and the speed at which the force is applied and removed. An alternative would be for the maximum forces required to displace different packs by an equal maximum vertical displacement to be compared, although this is liable to introduce errors due to variations in pack heights and internal pressures.It should also be appreciated that either or both of these calculations may be used in a double station testing machine as a further check for a leaking pack.

It will be appreciated that a sampling conveyor can only readily be used for packs having lids that are not indented. For packs of the kind in Figure 7, that have indented lids (that is lids that lie in a plane below the uppermost portions of the periphery of the tray) an alternative form of apparatus is desirable.

One such alternative apparatus having a reciprocating head is shown in Figure 8 in which parts corresponding to parts shown in the earlier drawings are designated by the same reference numeral. In this arrangement the lid 1 of a pack is displaced by a load cell, which is moved up and down by a linear actuator 24.

The position of the load cell is noted by a position sensor 22 and relayed to a control unit such as a microprocessor. The load cell is moved along the packaging line by means of a drive system comprising a chain 20 that passes around a pulley driven by a stepper motor 21.

When an optical sensor detects the pack, the drive system accelerates the load cell to match the speed and longitudinal position of the pack (forward stroke) and the linear actuator 24 is operated to compress the pack via the load cell. The linear actuator and load cell are mounted by linear bearings on horizontal support rails 23. After the linear actuator has completed its vertical return stroke, the horizontal motion of the load cell and actuator in the direction of movement of the pack is brought to a halt and then the actuator and load cell returned to their initial position.

Instead of using a chain and stepper motor to provide the horizontal drive of the load cell and linear actuator, a pneumatic actuator may be used. The linear actuator 24 may be arranged to move the load cell downwardly until a given force on the lid 1 of the pack is obtained; the microprocessor can obtain readings of the force as it increases and reduces and thereby calculate the skew factor.

For production lines where the throughput of packs is less than about 60 packs/min, it may be feasible to decelerate the pack on the lower conveyor and test the pack, while stationary, before accelerating the pack again to rejoin the packaging line. As in the case of the reciprocating head, the same maximum force may be applied to each pack and the skew factor ascertained.

Claims

1. A method of testing a gas-filled pack for leaks, which comprises using a device to apply a force to displace a portion of the pack inwardly and determining from the force/displacement relationship whether the pack leaks.

2. A method according to claim 1, in which the device displaces the top of the pack.

3. A method according to claim 1 or 2, in which the pack contains a food product and the gas filling the pack is substantially inert with respect to the food product.

4. A method according to any preceding claim, in which the pack has a body defining a tray and a film of plastics material extending over the top of the tray and sealed to the tray.

5. A method according to any preceding claim, in which the force applied by the device to effect the displacement is measured directly or indirectly.

6. A method according to any preceding claim, in which the displacement caused by the force applied by the device is measured directly or indirectly.

7. A method according to any preceding claim in which the force/displacement relationship during increase of the force applied to the pack is compared with the force/displacement relationship during decrease of the applied force and the result of the comparison used in the determination of whether the pack leaks.

8. A method according to any preceding claim in which the displacement of the pack takes place while it is moving.

9. A method according to claim 8 in which the pack is displaced by causing it to pass through a space where dimensions measured in at least one direction are less than the corresponding dimensions of the pack.

10. A method according to claim 8 or 9, in which the pack is passed between a pair of conveyors spaced apart by a distance less than the corresponding dimension of the pack whereby a portion of the pack is displaced as the pack passes between the conveyors.

11. A method according to claim 10 in which the force applied by one of the conveyors to the pack is measured.

12. A method according to claim 8 in which the pack is displaced by a displacing head mounted for movement towards and away from the pack on a carrier which itself moves in synchronization with the pack from a first position during application of a force to the pack and thereafter returns to the first position.

13. A method according to any preceding claim in which the step of using a device to apply a force to displace a portion of the pack inwardly is carried out a plurality of times.

14. A method according to claim 13 in which the force/displacement relationship during a first application of the force is compared with the force/displacement relationship during a second application of the force and the result of the comparison used in the determination of whether the pack leaks.

15. A method substantially as hereinbefore described with reference to, and as shown in the drawings.

16. An apparatus for detecting a leak in a gasfilled pack, which comprises a device for applying a force to the pack to displace a portion of the pack inwardly and means for determining from the force/ displacement relationship whether the pack leaks.

17. An apparatus according to claim 16, in which the device is arranged to displace the top of the pack.

18. An apparatus according to claim 16 or 17, in which the determining means includes a measuring device for measuring directly or indirectly the displacement of the pack.

19. An apparatus according to any one of claims 16 to 18, in which the determining means includes a measuring device for measuring directly or indirectly the force applied to the pack.

20. An apparatus according to any of claims 16 to 19, in which the force applying device is arranged to apply a force to the pack while it is moving.

21. An apparatus according to claim 20 in which the force applying device comprises a conveyor which is arranged to be pressed against the pack.

22. An apparatus according to claim 21, in which the determining means includes a device for measuring the force applied to the pack.

23. An apparatus according to claim 20 in which a pair of force applying devices are provided for applying forces in turn on a pack.

24. An apparatus according to claim 20, in which the force applying device includes a displacing head and a carrier, the displacing head being mounted on the carrier for movement towards and away from the pack, and the carrier being mounted for movement along a path parallel to the path of the pack.

25. An apparatus for detecting a leak in a gasfilled pack, the apparatus being substantially as hereinbefore described with reference to, and as shown in the drawings.

26. A packaging line including an apparatus according to any of claims 16 to 26.