It is a well recognized serious problem in the packaging industry that package filling machines, even though set to dispense a change at a given weight, will produce packages of varying weights. Ideally, package filling machines would produce packages none of which are underweight, and all of which are either at the minimum package weight desired, or in excess thereof by such a small amount that "giveaway" is minimized. Variations in the weight of charges of product dispensed into a package, and the final package weight, will necessarily vary, regardless of the weight settings, due to the presence of many variables, such as product density, feed characteristics of the product, the physical condition of the filler, and the like.
There have been many attempts to solve this problem of assuring minimum package weight according to government regulations, and, at the same time, minimizing "giveaway" of the product, such as by the use of servo systems which automatically adjust certain predetermined weight settings of the weighing means. In some instances the servo is activated in response to every package which is under or over a preselected weight range, but this unnecessarily changes the weight setting upon the occasional occurrence of weight deviations outside of said range, and thus the setting remains in error until corrected by a servo signal from some subsequent package.
If the weights of a number of packages are plotted as the number of packages against the package weight, the resulting graph will resemble a probability curve, as shown, for example, in Muskat et al Patent No. 3,805,903. In certain instances, systems have operated on the basis of locating the servo reference weight at the center of the probability curve. Such systems operate on the principle that half of the package weights must be less (or more) than the servo reference weight. This is undesirable because, when the curve broadens due to certain conditions at the filler, an excess number of packages will be rejected as below (or above) the critical limit for the minimum acceptable package weight.
Furthermore, when there are in fact underweight packages, and this fact is determined downstream of a plurality of packages moving on a conveyor, by checkweighing at some downstream point, the package may be rejected and thereby lost. One of the features of the present invention is to obviate this disadvantage by utilizing a single weigh bucket and checkweighing the charge of product therein one or more times to be certain that the weight thereof is at least equal to the minimum package weight before it is discharged into a package.
Examples of United States patents which disclose attempts to solve the problems mentioned above, include Pat. Nos. 3,620,316; 3,643,752 and 3,805,903.