US3814195A - Weight quality control determination - Google Patents

Abstract

A weighing apparatus consists essentially of a three pan balance with the pans at different pre-set distances from a fulcrum. Preferably a second balance, an equal arm two pan balance is also provided. These balances enable a method of weight determination to be rapidly carried out to establish the maximum weight that can be marked on a given set of packs while satisfying statistically acceptable practice or legislation.

Description

United States Patent 1191 Thorsson 177/200 Gilbert, June 4, 11974 WEIGHT QUALITY CONTROL 2,802,658 8/l957 Hensgen et al. 177/1 DETERMINATION 1 I 3,708,025 l/l9 73 Soler et al, l77/l, 75] Inventor: Paul Gilbert, Kent, England 1 [73] Assignee: Lever Brothers Company, New Pr'mary Exammer*Ge.orge Mmer York s7 ABSTRACT [22] Filed: Aug. 31,1973

. 1 A weighing apparatus consists essentiallyof a three [21] Appl 393387 pan balance with the pans at different pre-set distances from'a fulcrum. Preferably a second balance, [30] Foreign Application Priority Data an equal arm two pan balance is also provided. These Sept. 4, 1972 Great'Britain 40842/72 balances enable a method of Weight determination to be rapidly carried out to establish the maximum 521 vus. (:1 177/1, 177/200, [77/246 weight that can be marked on a given set of packs 511 11111. (:1. .1 G0lg 19/00, GOlg 1/18 while satisfying Statistically acceptable Practice or g- [58] Field of Search 177/1, 200, 246 lslation- [56] References Cited 9 Claims, 3 Drawing Figures UNITED STATES PATENTS l0ll949 PATENTEDJUH 4 19m SHEET 2 [1F 2 The present invention relates to weight control for a series of articles of nominal declared weight.

The term nominal declared weight isused in this specification to define the weight declared to the con-. su'mer,,usually it is the weight marked on the pack. lt willbe recognised that when packs are filled by machinery to a'given weight, i.e., target weight, there will be a spread about this target weight, some packs being lighter and some heavier, and that this variation will usually be in accordance with the normal distribution curve. Good practice and in some cases legislation requires that a large proportion of the packs will contain at least the weight marked on the pack and generally,

to be safe, slightly more. Thus the filling machinery will fill to a target weight whichis greater than the nominal declared weight. In some case the minimum difference between target weight and nominal declared weight is effectively defined by legislation in terms of the standard deviation of the normal distribution curve.

In practice during operation of a machine at a given target weight, fluctuations in external parameters, faults, variations in product ingredients and so on will result in variation in the spread of th normal distribution curve. Toovercome this and ensure safety at all timesit is usual to set the target weight slightly higher than it need be. Thus more productis filled into the packs than is strictly necessary and the extra is given away to the consumer. Thus higher production costs result from the give away.

in the ideal situation the target weight would be adjusted so that there was always the optimum minimum difference between the target weight and the nominal declared weight on the packet. However this requires calculation of the standard deviation and in practice this is usually difficult to effect sufficiently quickly and easily to be of use.

The present invention relates to a method and apparatus for this purpose.

Accordingly the invention provides amethod of deerated between mean weight and declared weight, and

this will be explained further at a later point in this Specification. This will be selected by the operator dependent on weight legislation or the requirement of weight inspectors and on the degree of safety in weight making that is required.

By obtaining the difference between the two specified mathematical products the mean sample range is being derived and from this, standard deviation and hencemaximum safe declared weight is obtained. The conversion of mean sample range to standard deviation is standard theory in statistics (see p 155 of Facts from Figures by M .I Moroneypublished by Penguin Books Limited).

It should be explained that the term we use for the maximum safe value of the declared weight is often referred to as the risk point or a defined percentage of the normal distribution curve and may not be the same as the nominal declared weight marked on the pack. it is the maximum value which the nominal declared weight can safely be without infringing regulations or practice requirements.

For example for certain products in the UK, weight legislation is interpreted by practice to require that 97% percent of production must be above the declared weight (the legal requirements on this differ from countermining the maximum safe value of declared weight g for a series of articles of nominal declared weight comprising:

a. forming 'a sample group from a pre-determined numberof articles within said series,

b. sub-dividing the articles in the group into equalnumbered sub-groups,

a c. for each sub group establishing the heaviestarticle,

H, and the lightest article, L, in that sub-group,

d. placing all the established lighter articles, L, at a first station of a weighing instrument,

e. placing all the established heavier articles, H, at a second station of the weighing instrument,

f. making a weight measurement by means of the weighing instrument to establish the difference between the product of total weight of the lighter articles, L, and [z 0.5/n], and the product of total weight of the heavier articles, H, and [z 0.5/n], said difference being the maximum safe value of the declared weight, where n is the number of articles in said sample group within the series and z is a constant dependent on the tolerated difference between declared weight and means weight of the series of articles.

The constant 2 usually lies between 1.5 and 3 and is proportional to the number of standard deviations toltry to country). This means that mean or target weight has to be 1.96 standard deviations above the declared weight. The two constants would be selected to achieve this requirement, and then from performing the method to obtain maximum safe value of declared weight we would compare the answer obtained with the nominal weight declared on the pack.

If the nominal declared weight marked on the pack is greater than the value of declared weight obtained by the process of the invention, it means that these packs the pack, it means that the filling machine is putting more material in the pack than it needs to and should be adjusted accordingly. In practice it is permissable to alter the target weight by-almost the amount of this difference since the whole of the normal distribution curve will be shifted equally,.and the target weight will then be just above the minimum legally permitted difference from the nominal declared weight.

it should be noted that it is particularly convenient to compare a mathematical product of weight and a constant with another-such mathematical product on a beam balance since these products can be represented as moments about a fulcrum. Thus, the weight measurement can be made on a beam balance having a fulcrum and three weigh stations at different predetermined distances from said fulcrum, the first and second stations being arranged to act in opposition to one another and the third station providing a position for measurement of the maximum safe value of the declared weight.

Thus the invention also provides a weighing apparatus for determining the maximum safe value of declared weight for a series of articles of nominal declared weight, comprising a beam balance having three 3 weigh stations effectively locatedat different-distances from a pivot point, said distances being in the ratio 1: (z 0.5/n): (z 0.5/n) where n is the number of articles in a sample group within the series'and z is a con It will be recognised that the larger the sample the greater the accuracy or significance of the determined safe maximum value of the declared weight. Where the subgroups consist of two articles, it is desirable that there should be at least four sub-groups to obtain a significant result. Five sub-groups is reasonably significant while remaining easy to manipulate. Thus the value of n, the number of articles in a sample group should desirably lie in the range 8 to 12 and preferably be 10.

When the sub-groups are in twos, these distances should be such that the following relationship is satisfied when the beam is in equilibrium: maximum safe declared weight I (0.5 0.8862K) H (0.5 0.8862K) where K is a factor defining the permitted number of under weight articles tolerated by legislation or'practice in termsof the number of standard deviations permitted between the mean weight and declared weight, and L and Pi are the average weights of the lighter and heavier articles respectively. The constants (z 0.5/n) and (z 0.5/n) are based on this relationship The derrivation of the formula will be explained in greater detail in relation to the described example.

A convenient apparatus may comprise a beam balance as specified above as well as a second weigher, preferably a two pan equal arm beam balance, for determining the lighter and heavier articles.

An embodiment of the inventinon will now be de-' scribed by way of example with reference to the accompanying diagrammatic drawings in which:

FIG. 1 is a perspective view of a weighing machine;

FIG. 2 is a diagram illustrating the principal of this machine; and

FIG. 3 is a graph depicting normal distribution weight spread.

FIGS. 1 and 2 showa simple apparatus for carrying out the invention. However reference will first be made to FIG. 3 which shows graphically what the invention sets out to do.

The full line curve is a conventional normal distribution curve ofweight W against numbers of articles N at any given weight. The peak of this curve represents the meanweight, and in practice a filling machine will be filling to a target weight T W corresponding to this mean weight. Then, dependent on legal requirements or accepted practice, therewill be a point on thiscurve which indicates the maximum value of weight which could be marked on the pack (this might be that 97% percent of the articles are above the declared weight in some countries and in others the requirements might be more, or less stringent than this, but the principle remains much the same). This is marked on the curve MVDW.

In practice a particular weight we refer to as nominal declared weightNDW is normally marked on the pack and the aim is to sell product as closely above that weight as is technically and legally possible.

Thus in'the example shown MVDW is above NDW so that the legal, or practice, requirements are satisfied, however the articles are of a greater weight than is necessary and the operation is not economically at the optimum. To achieve this aim therefore the target weight of the filling machine is shifted downwards to a new target weight TWl by an amount which brings MVDW downwards to a point where it will virtually co-incide with NDW and the whole of the normal distribution curve is shifted to the position shown dotted.

The present invention uses a particular form of weigh scale which for a given group of articles automatically computes the position of MVDW for that group and hence enables the target weight or the nominal declared weight to be adjusted accordingly.

The weighing machine has five weigh pans P1, P2, L, H and DW. The linkage connecting these pans is shown in FIG. 2,'the points F in each case indicating a fixed fulcrum.

The pans P1 and P2 are connected by a simple equal arm balance acting about Fulcrum F1 and their function is to distinguish between the heavier and the lighter article in any pair.

The pans H, L and DW are connected together by a more complex linkage. The pan H is intended to take all the heavy articles in a sample and is located a distance A from the fulcrum F2. The pan L for the lighter articles is located a distance B from fulcrum F2 on its other side. At a further distance C an upward force due to the pan DW (for declared weight) is applied to a pivotal connection PC. This upward force is transmitted via an equal arm balance pivoted about fulcrum F3, but in an alternative arrangement the pan DW is located .the same distance C on the other side of the fulcrum F2, and directly connected without the additional equal arm balance.

The distances A, B and C are selected in dependence Take ten articles from the production line, place the first two on scale pans P1 and P2 to establish the heavier and the lighter, place the heavier on pan H and the lighter on pan L, repeat the sequence with the re maining articles in random pairs thus ending with five articles on each of pans H and L, and finally place weights on pan DW until the complex scale balances. The weight at pan DW then indicates the maximum safe declared weight.

in practice an empty packet together with weights amounting to the nominal declared weight (i.e., the weight marked on the packet) is placed on pan DW initially and weight correction to achieve balance is noted. This correction is the amount by which the target weight should be adjusted for optimum performance.

The weigh balance is establishing the relationship A E H B E L CDW where A, B and C are constants H is the sum of the heavy articles L is the sum of the light articles, and

DW is the maximum safe declared weight.

This relationship is appropriate because of the following:

Declared weight target (or mean) weight K standard deviations where K is the factor defining risk level.

But standard deviation can be derived from mean sample range by the standard formula where A is a constant dependent on the number of articles from which the range is derived, m is the number of ranges and Hr and Lr are the heavier and lighter articles in each range.- And l 7! target weight X r where n is the number of packs in the sample and X, is the weight of each article in the sample. When a number of ranges of samples are taken and each range is simply two articles, A0 is equal to 0.8862. Then by substitution and algebra the following relationship can .be directly derived.

Declared weight I (0.5 0.8862K) H (0.5 0.886210 this is the same as the relationship and [(2 0.5)/n] but it will be seen that these are the I same as B and A when 2 0.8862K and each range consists of two articles. Thus in the example described where sample groups of 10 are taken and if we are working on a permitted difference between declared weight and mean weight of 1.96 standard deviations the arm ratios are 1:0.224z0. l 24 with z being equal to 1.74. If we are working to a greater degree of security where K 3 standard deviations, z is equal to 2.66 and the arm ratios are then 120.3 16:0.216.

What is claimed is:

l. A method of determining the maximum safe value of declared weight for a series of articles of nominal declaredweight comprising:

a. forming a sample group from a pre-determined number of articles within said series,

b. sub-dividing the articles in the group into equal numbered sub-groups,

c. for each sub-group establishing the heaviest article, H, and the lightest article, L, in that sub-group,

d. placing all the established lighter articles, L, at a first station of a weighing instrument, the weighing instrument comprising a balance beam having three weigh stations effectively located at different distances from a pivot point, said distances being in the ratio of 1: (z 0.5)/n: (z 0.5)/n where n is the number of articles in the sample group and z is a constant dependent on the tolerated difference between declared weight and mean weight of the series of articles,

e. placing all the established heavier articles, H, at a second station of the weighing instrument f. adjusting the weight on a third station of the weighing instrument until the three weigh stations on the balance beam are balanced, the final weight on the third station being the maximum safe value of declared weight for the series of articles.

2. A method according to claim 11 in which each subgroup consists of two articles.

3. A method according to claim 1 in which each sample group, and the value of n, lies in the range 8 to l2.

4. A method according to claim 1 in which each sample group, and the value of n, is 10.

5. A weighing apparatus for determining the maximum safe value of declared weight for a series of n articles of nominal declared weight, comprising a beam balance having three weigh stations effectively located at different distances from a pivot point, said distances being in the ratio 1 (z 0.5)/n (z 0.5)/n where n is the number of articles in a sample group within the series and z is a constant dependent on the tolerated difference between declared weight and mean weight of the series of articles, some of the articles being rela-- tively heavier while the others are relatively lighter, the weigh stations having moment arms (2 0.5 )/n and (z 0.5)/n supporting equal numbers of relatively lighter and relatively heavier articles, respectively, the weigh station having moment arm 1 having balance weights placed thereon until the scale balances, the amount of balance weight indicating the maximum safe declared weight.

6. A weighing apparatus according to claim 5 which further comprises a second balance, said second balance being arranged to distinguish between lighter and heavier articles in subgroups from the sample group of articles.

7. A weighing apparatus according to claim 5 where the constant z lies between 1.5 and 3.

Claims (9)

1. A method of determining the maximum safe value of declared weight for a series of articles of nominal declared weight comprising: a. forming a sample group from a pre-determined number of articles within said series, b. sub-dividing the articles in the group into equal numbered sub-groups, c. for each sub-group establishing the heaviest article, H, and the lightest article, L, in that sub-group, d. placing all the established lighter articles, L, at a first station of a weighing instrument, the weighing instrument comprising a balance beam having three weigh stations effectively located at different distances from a pivot point, said distances being in the ratio of 1: (z + 0.5)/n: (z 0.5)/n where n is the number of articles in the sample group and z is a constant dependent on the tolerated difference between declared weight and mean weight of the series of articles, e. placing all the established heavier articles, H, at a second station of the weighing instrument f. adjusting the weight on a third station of the weighing instrument until the three weigh stations on the balance beam are balanced, the final weight on the third station being the maximum safe value of declared weight for the series of articles.

2. A method according to claim 1 in which each sub-group consists of two articles.

3. A method according to claim 1 in which each sample group, and the value of n, lies in the range 8 to 12.

4. A method according to claim 1 in which each sample group, and the value of n, is 10.

5. A weighing apparatus for determining the maximum safe value of declared weight for a series of n articles of nominal declared weight, comprising a beam balance having three weigh stations effectively located at different distances from a pivot point, said distances being in the ratio 1 : (z + 0.5)/n : (z - 0.5)/n where n is the number of articles in a sample group within the series and z is a constant dependent on the tolerated difference between declared weight and mean weight of the series of articles, some of the articles being relatively heavier while the others are relatively lighter, the weigh stations having moment arms (z + 0.5)/n and (z - 0.5)/n supporting equal numbers of relatively lighter and relatively heavier articles, respectively, the weigh station having moment arm 1 having balance weights placed thereon until the scale balances, the amount of balance weight indicating the maximum safe declared weight.

6. A weighing apparatus according to claim 5 which further comprises a second balance, said second balance being arranged to distinguish between lighter and heavier articles in subgroups from the sample group of articles.

7. A weighing apparatus according to claim 5 where the constant n is an even number lying in the range 8 to 12.

8. A weighing apparatus according to claim 7 where the constant n is 10.

9. A weighing apparatus according to claim 7 where the constant z lies between 1.5 and 3.

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