GB2234357A - Weighing machine - Google Patents

Abstract

A weighing machine intended to weigh a continuous stream of substantially similar packages 2, 2A, 2B, 2C, 2D, 2E being fed along a conveyed path 1, has a first weigh head 4 a second weigh head 13 downstream of the first weigh head, and means 11, 12 intermediate the two weigh heads to transfer a selected package after it has been weighed from the stream of packages to the second weigh head 13 so that it may be reweighed. Thereby if the second weight signal varies by an unacceptable amount from the first weight signal, a change may be made automatically or manually to the calibration and/or effective zero setting of the first weigh head so that packages, e.g. cans, with acceptable weights are no longer rejected because the tare weight of the first weigh head has been altered by contamination from the cans or by high temperature of the cans. <IMAGE>

Description

IMPROVEMENTS RELATING TO WEIGHING MACHINES This invention relates to weighing machines.

More particularly, it applies to weighing machines which are intended to weigh a continuous stream of substantially similar packages which may be fed by conventional means across weight measuring apparatus referred to below as a weigh head.

Conventionally the packages may be cans and for simplicity the packages will be referred to below as cans. However, it will be appreciated that the invention is equally applicable to other packages and is not restricted to the weighing of cans.

It may be desired to use a weighing machine to weigh cans containing a fluid or semi fluid product before the can is sealed. It may happen that some of the product may be spilt from the cans due to perturbations of the can while in transit across the machine. Even if cans are originally open but sealed at an earlier stage, product may still be spilt at this earlier stage. In either case product may, for example, run down the side of the can and some of it may then adhere to the weigh head when clearly its weight will add to the tare weight of the weigh head, and the weight measured as being the weight of the can will be in error.

Again cans arriving at a weigh head to be weighed may be very hot if either empty or full depending on the process to which they have already been subjected, for example, initial manufacture of the cans or the filling of cans with hot product.

Such hot cans may cause the weigh head itself to become abnormally hot, and although the weight/temperature coefficient of the weigh head may be acceptable under normal ambient variation conditions it may happen that with large changes of the weigh head temperature an unacceptable error will be introduced into the measured weight.

If a real or apparent error in weighing cans occurs, the electronic means which is conventionally part of a weighing machine will generate signals to operate means to remove the relevant cans from the normal transfer means of the machine and direct such cans on to, for example, a reject conveyor. Because the weighing process is continuous and particularly if the process operates at high speed, it will be realised that the number of reject cans will be objectionably high and particularly objectionable if the bulk of the cans rejected were, in fact, acceptable. Also it may be that a can that is inadequate will not be rejected.

An object of the invention is to compensate for such sources of error as those above.

According to the invention a weighing machine having a weigh head for weighing a continuous stream of substantially similar packages being fed along a conveying path is provided with a second weigh head downstream of the first weigh head, and means intermediate the two weigh heads to transfer a selected package after it has been weighed from the stream of packages to the second weigh head so that it may be reweighed. If the second weight signal varies by an unacceptable amount from the first weight signal, a change may be made to the calibration and/or effective zero setting of the first weigh head, so that packages, e.g. cans, with acceptable weights are no longer rejected.

Preferably an intermediate conveyor is provided to assist the transfer means, and also means to return the selected package when reweighed to the intermediate conveyor or to the original conveying path.

It is also preferred that means are provided to compare the first and second weight readings of the selected package, and means to adjust the calibration and/or tare weight setting of the first weigh head in accordance with this comparison.

By way of example, a specific embodiment of weighing machine according to the invention will be described with reference to the accompanying diagrammatic drawing which shows a typical weighing system in plan view.

Referring to the drawing, a conveyor 1 carries a continuous stream of cans 2, 2A, 2B, etc to a scroll accelerating and spacing means 3 which directs the cans for example 2C, 2D, 2E over a weigh head 4 where each can in turn is weighed. The weigh head 4, as explained above, may become contaminated with some of the cans contents. If the weigh head 4 is not so contaminated and provided the weight of the can is within an acceptable band of weight limits, the weighed can will be allowed to continue on conveyor 1. In this embodiment, guides 5 will then urge the accepted can on to a second conveyor 6 which is provided by the customer to transfer cans to further process operations.

If the weight of the weighed can is not within acceptable limits, or again if the weigh head has become contaminated as above, a reject signal will be generated by conventional electronic means 7 and the can will be removed from the conveyor 1. This removal is effected by, for example, piston push rod means or an air jet 8 whose instant of operation is timed from a signal generated by can detecting means 9 placed on either side of the weigh head 4. The can will then be directed on to a further conveyor 10 whose primary use is to carry rejected cans to the customer's temporary store. Conveyor 10 is referred to below as the reject conveyor.

The invention concerns the provision of means to effectively reject a selected can in a regular sequence, for example, one can in fifty or one can in a hundred or as convenient to the customer, onto the reject conveyor, and means to reweigh this can. For this purpose, a stop 11 is temporarily placed across the reject conveyor 10 to halt the can and push rod means 12 are provided to push the can on to a second weigh head 13 placed adjacent to the reject conveyor and thereby downstream of the first weigh head.

After the second weighing operation, further push rod means 14 pushes the can back, in this embodiment, onto the reject conveyor 10. Alternatively, if desired, the can may be pushed by the means 14 on to conveyor 6. The can then continues its transfer to the customer's reject temporary store or his further packaging process, respectively.

The sequence of operations described is, in this embodiment, originated in an electronic control unit 15. The unit 15 takes signals generated by the can detecting means 9 at each fiftieth/hundredth signal, or as convenient to the customer, and simulates a reject signal to operate the removal means 8. Since the speed of the reject conveyor 10 may not be suitably related to the machine conveyor speed, in this embodiment a further can detecting means 16 is placed across the reject conveyor 10.

Providing that the control unit 15 has instructed the detecting means 16 to do so, the detecting means 16 generates a signal which causes the can stop means 11 and the push rod means 12 to operate. The can is then weighed and returned to conveyor 10 or conveyor 6. When a can is detected by detecting means 9, at substantially the same instant a signal is generated by the control unit 15 to eject a can for further weighing. The weight signal of the weigh head 4 is stored in the control unit 15. When the can has arrived at the second weigh head 13 after the operation of the respective means 16, 11 and 12, the second weight signal is directed to the control unit 15. The signals from the two weigh heads are then compared. If there is a difference which is deemed to be unacceptable then a signal may be directed to the first weigh head 4 to adjust its calibration and/or zero weight.Alternatively, it may be deemed not convenient to provide immediate adjustment in this way. In this case, electronic storage means may by provided in the control unit 15 to sense whether the above difference is sustained in a number of sampled cans. In a further embodiment warning means may be operated at a monitoring station so that adjustment of the tare weight is at an operator's discretion. In other words the correction means may be automatic or manual. Adjustment means are provided which may be electrical, electronic or mechanical, as desired. The adjustment means may also be combined with or independent of the control unit 15.

In this embodiment, it is convenient to use the conventional reject conveyor 10 as an intermediate part of the selected can transfer means. However, if desired, a further conveyor may be provided specifically to carry out the can transfer operation. In this case, the respective means 8, 11, 12 and 13 are repositioned adjacent to this further conveyor. Similarly, it may be convenient to use a rejected can as the selected can.

It will be appreciated that there is a time period between weighing cycles at the second weigh head 13. During these dormant periods the weigh head 13 can be self checking, automatically calibrated or manually calibrated maintaining high degrees of accuracy.

As explained above, the term weigh head is intended to include any type of weight measuring apparatus, or similar measuring device which may be influenced by debris, product or environmental conditions.

Claims (4)

1. A weighing machine having a weigh head for weighing a continuous stream of substantially similar packages being fed along a conveyed path, the weighing machine being provided with a second weigh head downstream of the first weigh head, and means intermediate the two weigh heads to transfer a selected package after it has been weighed from the stream of packages to the second weigh head so that it may be reweighed.

2. A weighing machine as claimed in Claim 1, including an intermediate conveyor to assist the transfer means, and also means to return the selected package when reweighed to the intermediate conveyor or to the original conveying path.

3. A weighing machine as claimed in Claim 1 or Claim 2, including means to compare the first and second weight readings of the selected package, and means to adjust the calibration and/or tare weight setting of the first weigh head in accordance with this comparison.

4. A weighing machine substantially as hereinbefore described with reference to and as shown in the accompanying drawing.