Combinational weighing method and apparatus

Combinational weighing is carried out firstly by calculating combinations of weight signals from a plurality of weighing devices each weighing a batch of objects to be packaged and selecting a combination having a total weight satisfying a specified criterion with respect to a target value, and secondly by re-checking the weight signals from the weighing devices of the selected combination before discharging the objects therefrom. The value of the difference between the total weights obtained at the time of the combinational calculations and at the re-checking time is stored cumulatively after each cycle of operation, and a new target value is calculated and set after each cycle from many stored difference values from previous cycles in order to improve the work efficiency and the yield at the same time. According to a preferred embodiment, the difference between the weight values obtained at these two times of measurement is cumulatively stored after each cycle for all weighing devices and a new target value is calculated by considering the distributions of past difference values corresponding to the individual weighing devices of the selected combination.

BACKGROUND OF THE INVENTION 
This invention relates to a method of and an apparatus for combinational 
weighing such that a large number of objects, such as food items having a 
desired total weight, can be packaged inside a bag. 
For collecting a large number of objects for automatic packaging, it has 
been known to weigh the objects separately in batches by using a plurality 
of weighing devices and to make combinations of the weight signals from 
these weighing devices. A lower limit value is set, and a target value is 
obtained by adding a certain correction value to this lower limit value. A 
particular combination with a total weight close to this target value 
within a certain allowable range is selected, and the batches of objects 
from the selected combination of weighing devices are discharged into an 
automatic packaging machine. 
In other words, in order to make it absolutely certain that no merchandise 
with insufficient weight will be produced, a target value C is set between 
a certain specified lower limit value A and an upper limit value B by 
adding a certain specified correction value .alpha. to the lower limit 
value A, as shown in FIG. 3, and a combination is selected such that its 
total weight will not be less than this target value C. Before the objects 
are discharged from the selected combination of weighing devices, however, 
the total weight is checked again by inputting and adding the weight 
signals from the selected weighing devices, as disclosed, for example, in 
Japanese Patent Publication Tokko 2-54885. The weighed objects are not 
discharged if their total weight is found to be less than the specified 
lower limit value such that no merchandise with insufficient weight will 
be produced. 
The correction value .alpha. is determined in view of the small variations 
in the weight signals from the individual weighing devices, which may be 
caused by the vibrations of the weighing devices when objects are dropped 
onto the associated weigh hoppers as well as the vibrations of the floor. 
If a weight signal is received at time D for combinational calculations 
and the total weight of the selected combination is checked at a later 
time E, there is likely to be a difference .DELTA. in signal level between 
the two points in time D and E, as shown in FIG. 4. The level difference 
.DELTA. may be positive or negative. If it is negative, the weight at the 
time of the re-check may be less than the lower limit value A, and the 
discharge of the objects from the selected combination of weighing devices 
is prevented in order to avoid the production of an underweight product. 
For this reason, the correction value .alpha. is selected such that the 
effects of the signal level difference .DELTA. can be eliminated and the 
reduction in yield because of the occurrence of underweight combination 
can be better controlled. 
If the distribution of the signal level difference .DELTA. (that is, the 
distribution of the differences in measured weight value) is assumed to be 
a normal distribution with standard deviation given by .sigma..sub.1, and 
if N weighing devices are selected by combinational calculations, the 
distribution of total weights is also a normal distribution, but its 
standard deviation .sigma..sub.N becomes .sigma..sub.1 .sqroot.N. Since 
99.8% of the level differences will fall within three times this standard 
deviation .sigma..sub.N, or within the width of .+-.3.sigma..sub.N, if the 
correction value .alpha. is set equal to 3.sigma..sub.N =3.sigma..sub.1 
.sqroot.N, the fraction of selected combinations considered underweight 
will be reduced to less than 0.2%. 
It is not desirable, however, to change the target value whenever there is 
a change in the value of N during combinational calculations, because this 
will adversely affect the time required for the weighing. In order to 
carry out combinational calculations efficiently, an average number 
N.sub.L of the selected weighing devices is used to determine the 
correction value .alpha. as 3.sigma..sub.1 .sqroot.N.sub.L. If the lower 
limit value is set to 100 g and ten weighing devices are used, the 
standard deviation .sigma..sub.1 is empirically set equal to 0.1 g and the 
average number N.sub.L of combined weighing devices is set equal to 4 such 
that the correction value .alpha. becomes 0.6 
(=3.times.0.1.times..sqroot.4)g. If the correction value .alpha. is 
increased, underweight combinations will occur less frequently and the 
work efficiency will accordingly improve, but since the average amount of 
the objects that are put in a bag will increase, the yield is adversely 
affected. 
In summary, it is desired to provide an improved combinational weighing 
method, as well as an apparatus therefor, capable of reducing the 
frequency of occurrence of underweight combination at the time of the 
re-check while improving both the work efficiency of combinational 
weighing and the yield. With a prior art combinational weighing apparatus, 
however, the correction value is a constant which is preliminarily 
determined from the average number N.sub.L and the empirically determined 
value of the standard deviation. This makes it difficult to improve both 
the yield and the work efficiency for all combinational weighing apparatus 
having individually different characteristics. 
SUMMARY OF THE INVENTION 
It is therefore a basic object of this invention to provide a combinational 
weighing method and an apparatus therefor, capable of improving both the 
yield and the work efficiency. 
A combinational weighing method according to this invention, with which the 
above basic objects can be accomplished, may be characterized by the steps 
of detecting the difference between measured combined weight values at the 
time of combinational calculations and the re-check time and setting the 
correction value on the basis of the past values of the difference such 
that desired work efficiency and yield can be achieved. With such a method 
according to the invention, the difference in measured combined weight 
value is detected between the time of combinational calculations and the 
re-check time, and a new correction value is set on the basis of not only 
this difference value but also the past difference values obtained in 
earlier cycles of the combinational calculations. This newly calculated 
correction value is added to the specified lower limit value to obtain a 
new target value, and the next cycle of combinational calculations is 
carried out by using this new target value. 
A combinational weighing apparatus for using this method may be 
characterized as comprising difference detecting means for detecting the 
difference in measured combined weight between the time of combinational 
calculations and the re-check time, correction value calculating means for 
using past difference values detected by the difference detecting means 
and thereby calculating a correction value appropriately so as to obtain 
desired yield and work efficiency, and setting means for setting the 
correction value thus calculated by the correction value calculating means 
in the combination calculating means. 
It is not always to be taken for granted, however, that the distribution of 
measured weight differences is about the same among the weighing devices 
of an apparatus for combinational weighing and that the number of weighing 
devices selected is approximately constant. Consider a situation, for 
example, where an air conditioner is installed near the combinational 
weighing apparatus such that some of its weighing devices which are more 
effectively cooled thereby tend to have larger differences in weight 
values while those not being effectively cooled have smaller differences. 
In such a situation, if only those weighing devices with large differences 
are selected and combined, there is an increased probability that the 
total weight at the re-check time becomes smaller than the lower limit 
value that has been set. This would affect the work efficiency adversely. 
If only those weighing devices with small differences are selected and 
combined, on the other hand, there is an increased probability that the 
total weight becomes significantly larger than the lower limit value. This 
would adversely affect the yield. 
It is therefore a further object of the invention to provide a 
combinational weighing method and an apparatus therefor, capable of 
achieving the aforementioned basic object in spite of effects from the 
environment as described above. 
A combinational weighing method, with which the aforementioned further 
object can also be achieved, may be characterized by the steps of 
detecting the difference in measured weight values for each individual 
weighing device and setting the correction values on the basis of the past 
difference values on those weighing devices in the selected combination. 
Similarly, a combinational weighing apparatus, with which the 
aforementioned further object can also be achieved, may be characterized 
as comprising difference detecting means for detecting the differences in 
measures weight values for individual weighing devices and correction 
value calculating means for using past difference values from the weighing 
devices of the selected combination to calculate a correction value.

DETAILED DESCRIPTION OF THE INVENTION 
A weighing apparatus, embodying the present invention to achieve the 
aforementioned basic object, may be of the structure schematically shown 
in FIG. 1, comprising a conically shaped dispersion feeder 2 placed below 
a conveyor 1 for receiving objects M such as food items to be weighed from 
a production line, transporting them to a position above the dispersion 
feeder 2 and dropping them at the center of the dispersion feeder 2. A 
plurality of radially oriented feeders 3 are disposed around the outer 
periphery of the dispersion feeder 2 and are adapted to transport the 
objects M radially outward, while vibrating, and to drop them through pool 
hoppers 4 disposed therebelow into a plurality of weighing devices 5. 
Each of the weighing devices 5 is composed of a load cell 52 and a weigh 
hopper 51 for receiving a batch of the objects M discharged from the 
corresponding one of the pool hoppers 4. The load cell 52 has one of its 
vertical edges affixed to a frame structure 50, the other edge supporting 
the weigh hopper 51 so as to detect the weight of the objects M supplied 
into this weigh hopper 51 and to output a weight signal indicative of the 
measured weight to a central processing unit (CPU), shown at 10 in FIG. 2 
and to be described below. A discharge chute 6 is provided below the weigh 
hoppers 51, and an automatic packaging machine 7 is disposed further below 
the chute 6. 
In order to input the weight signals from the load cells 52 into the CPU 
10, each load cell 52 is connected to an amplifier 11 and a filter 12, and 
there is a multiplexer 13 connected to the output sides of these filters 
12. The weight signals are inputted into the CPU 10 from this multiplexer 
13 through an analog-to-digital converter (A/D) 14. The CPU 10 is also 
connected, on its input side, to an input means 15 for retrieving various 
data from a memory device 17 and inputting them into the CPU 10 in 
response to the user's operation on selection keys 16. On the output side 
of the CPU 10, there is a hopper control circuit 18 for selectively 
opening and closing the individual weigh hoppers 51. 
The CPU 10 includes a combination calculating means 19, a re-check means 
20, a difference detecting means 21, correction value calculating means 
22, correction value setting means 23, a memory means for storing a lower 
limit value 24, a memory means for storing an upper limit value 25, and a 
memory means for storing difference values 27. 
The combination calculating means 19 is for carrying out combinational 
calculations of the weight signals from the load cells 52 and selecting a 
combination within the allowable range A-B and near the target value C 
calculated by adding a correction value .alpha. to a specified lower limit 
value A, as explained above with reference to FIG. 3. 
After a combination has been selected by the combination calculating means 
19 but before the weighed objects M are discharged from the weigh hoppers 
51 of the weighing devices 5 of the selected combination, the combination 
calculating means 19 outputs to the re-check means 20 a signal indicating 
which of the weighing devices 5 have been selected. Upon receiving this 
signal, the re-check means 20 receives weight signals from the load cells 
52 of the weighing devices of the selected combination and adds them 
together to re-check the result of the combinational calculations. If this 
re-check value obtained by the re-check means 20 falls between A and B, or 
within the allowable range, the signal from the combination calculating 
means 19, indicative of the selected combination, is directly transmitted 
to the hopper control circuit 18 and the weigh hoppers 51 of the selected 
combination are opened. If the re-check value is below the lower limit 
value A, no signal is transmitted from the re-check means 20 to the hopper 
control means 18, and the weigh hoppers 51 are not opened to discharge the 
objects M therein. 
The difference detecting means 21 is for detecting the difference .DELTA. 
between the total weight calculated and selected by the combination 
calculating means 19 at the time of the combination calculations and the 
re-check value obtained by the re-check means 20 at the re-check time. 
The difference value .DELTA. thus detected is stored in the 
difference-storing memory means 27 for storing not only the difference 
value .DELTA. which has just been detected by the difference detecting 
means 21 for the cycle of combinational calculations completed by the 
combination calculating means 19, but also all of the difference values 
similarly detected in the earlier cycles of combination calculations. 
The correction value calculating means 22 is for calculating a new 
correction value .alpha. on the basis of the difference values .DELTA. 
obtained earlier by the difference detecting means 21 and now stored in 
the difference-storing memory means 27 such that desired work efficiency 
and yield can be attained. The new correction value .alpha. may be set 
equal to the largest of the absolute values of the differences .DELTA. 
currently stored in the difference-storing memory means 27. The correction 
value .alpha., so set, may be too large because it will tend to reduce the 
yield somewhat, but the work efficiency can be made close to 100%. This 
method of setting the correction value .alpha. is particularly suited when 
the distribution of the aforementioned difference values is uniform. If 
the difference values .DELTA. are distributed as shown in FIG. 3 in the 
form of a normal distribution, on the other hand, it is generally 
advantageous to set the correction value .alpha. equal to three times the 
standard deviation .sigma..sub.1 of this distribution which is calculated 
each time. If the correction value .alpha. is so set, 99.8% of the 
difference values .DELTA. may be expected to become included, as explained 
above. Thus, not only is the work efficiency improved but also the 
correction value .alpha. can be made smaller and hence the yield can be 
improved, too. As a further alternative, the correction value .alpha. may 
be set equal to the average of the absolute values of the difference 
values .DELTA. obtained previously and now in the difference-storing 
memory means 27. This method is advantageous in that the correction value 
.alpha. can be made even smaller and hence that the yield can be improved 
still further, although the work efficiency may be reduced somewhat. 
The correction value setting means 23 includes an updating means 23a and a 
memory means 23b. The correction value .alpha., calculated by the 
correction value calculating means 22, is inputted into the updating means 
23a to have the older value updated, and the updated correction value 
.alpha. is stored in the memory means 23b and transmitted to the 
combination calculating means 19. 
Next, a combinational weighing method using the weighing apparatus 
explained above will be described. 
As shown in FIG. 1, the objects M to be weighed are supplied from the 
conveyor 1 to the dispersion feeder 2 and then dropped into the pool 
hoppers 4 through the vibrating radial feeders 3. They are further dropped 
from these pool hoppers 4 down into the weigh hoppers 51 of the weighing 
devices 5 disposed below the pool hoppers 4, and the batches of the 
objects M thus placed in the plurality of weigh hoppers 51 are 
respectively weighed by the associated load cells 52. The weight signals 
indicative of the measured weights outputted from the load cells 52 are 
received by the combination calculating means 19 inside the CPU 10. The 
user operates the selection keys 16 to select from the memory device 17 a 
correction value .alpha. for the objects M, a lower limit value A and an 
upper limit value B and causes the input means 15 to temporarily store 
them respectively in the memory means 23b, 24 and 25. These values are 
then transmitted therefrom into the combination calculating means 19, by 
which combinations of the weight signals from the load cells 52 are 
calculated and a particular combination is selected which is the closest, 
within the range A-B, to the target value C obtained by adding the lower 
limit value A and the correction value .alpha. together. 
After the combination calculations are done by the combination calculating 
means 19, the weight signals from the load cells 52 associated with the 
selected weighing devices 5 are added together by the re-check means 20 to 
re-check the combinational calculations by the combination calculating 
means 19. If the sum thus obtained by the re-check means 20 (or the 
re-check value) is within the allowable range A-B, the hopper control 
means 18 causes the weigh hoppers 51 of the selected weighing devices 5 to 
be opened, thereby dropping the objects M from these weigh hoppers 51 down 
to the discharge chute 6 shown in FIG. 1 to be packaged by the automatic 
packaging machine 7. If the re-check value is less than A, on the other 
hand, the weighed objects M are not discharged and the occurrence of 
underweight products can be thereby prevented. 
The difference .DELTA. between the combined weight calculated by the 
combination calculating means 19 and the re-check value obtained by the 
re-check means 20 is calculated by the difference detecting means 21 and 
is cumulatively stored in the difference-storing memory means 27. In other 
words, not only the difference value .DELTA. most recently obtained but 
also the differences .DELTA. obtained in earlier-performed cycles of 
combinational calculations are stored therein and used by the correction 
value calculating means 22 to determine an optimum correction value 
.alpha. that will improve both the work efficiency and the yield. The new 
correction value .alpha., thus determined, is transmitted to the updating 
means 23a to update the previously determined correction value, and the 
updated correction value .alpha. is stored in the memory means 23b. In the 
next cycle of combinational calculations, the new correction value 
.alpha., retrieved from the memory means 23b together with the lower and 
upper limit values A and B respectively stored in the memory means 24 and 
25, is inputted to the combination calculating means 19, and a new target 
value C is obtained by adding the lower limit value A and the newly 
updated correction value .alpha.. 
In summary, the difference between the total weight of the selected 
combination obtained by the combination calculating means and the re-check 
value obtained somewhat later is detected after each cycle of 
combinational calculations, and the target value for the combinational 
calculations is updated after each cycle by using all these difference 
values obtained in the earlier cycles. As a result, both the work 
efficiency of the weighing apparatus and its yield can be improved at the 
same time. 
Next, another combinational weighing method and apparatus according to a 
second embodiment of the invention, with which the aforementioned further 
object can be achieved, will be described with reference to FIG. 5, 
wherein components which are substantially equivalent to those in FIG. 2 
and explained above with reference thereto are indicated by the same 
numerals. FIG. 5 shows another CPU 10' for replacing the one shown in FIG. 
2. In other words, the combinational weighing apparatus to be described 
below may be viewed as being structures as shown in FIG. 1. 
The CPU 10' shown in FIG. 5 is different from the CPU 10 in FIG. 2 in that 
there are included therein a dispersion calculating means 32, a memory 
means for storing dispersions 33, a correction value calculating means 34 
and a correction value setting means 35. The apparatus also additionally 
includes a dispersion updating means 38. 
According to the second embodiment of the invention, the dispersions 
V.sub.i of the distributions of the past differences in the weight values 
for the individual weighing devices are also stored in the memory device 
17. After the combination calculating means 19 has selected a combination, 
these dispersion values V.sub.i are retrieved and temporarily stored in 
the dispersion-storing memory means 33. When a signal c.sub.1 indicative 
of a selected combination is received from the combination calculating 
means 19, the correction value calculating means 34 uses the dispersions 
V.sub.i of the distributions of the weight values by these selected 
weighing devices to calculate a new correction value .alpha., as will be 
explained more in detail below, and this correction value .alpha. is set 
in the combinational calculation means 19 by the correction value setting 
means 35. 
The re-check means 20 functions generally as explained above with reference 
to FIG. 2, and the difference detecting means 21 detects the difference 
.DELTA. between weight values from each of the selected weigh hoppers 51 
at the time of the combination calculations and the time of the re-check. 
The dispersion calculating means 32 is for calculating new dispersion 
values V.sub.i from these difference values .DELTA. for the selected weigh 
hoppers 51 obtained by the difference detecting means 21 as above and the 
previous dispersion values V.sub.i based on the past data from the 
dispersion-storing memory means 33. These newly obtained dispersion values 
V.sub.i are stored in the memory device 17 by the dispersion updating 
means 38, updating the older values stored in the memory device 17. 
The correction value calculating means 34 treats the difference values 
.DELTA. obtained by the difference detecting means 21 as time-sequenced 
data and estimates the dispersions V.sub.i corresponding to the weigh 
hoppers 51 of the selected combination by the means square method to 
calculate the new correction value .alpha. as follows. 
Let V.sub.i be the dispersion of the distribution associated with one of 
the weigh hoppers 51 (identified by dummy index i). Then, the dispersion V 
of the distribution of the differences in total measured weight by the N 
weigh hoppers of a selected combination will be: 
EQU V=.SIGMA..sub.i=1.sup.N V.sub.i. 
If the standard deviation is denoted by .sigma., it is given by: 
EQU .sigma.=.sqroot.V=.sqroot.(.SIGMA..sub.i=1.sup.N V.sub.i). 
Consider a case, for example, where the combined weight from the second, 
fourth, fifth and eighth weigh hoppers is going to be calculated. In this 
situation, the dispersion V=V.sub.2 +V.sub.4 +V.sub.5 +V.sub.8 of the 
distribution of the total of the differences .DELTA..sub.2, .DELTA..sub.4, 
.DELTA..sub.5 and .DELTA..sub.8 of these individual weigh hoppers is 
calculated to obtain the standard deviation .sigma.=.sqroot.V. If the 
differences for all of these weigh hoppers 51 are normally distributed as 
shown in FIG. 3, the correction value .alpha. is set equal to 3.sigma. 
(that is, three times the standard deviation). The correction value 
.alpha. thus obtained is added to the lower limit value A to obtain a new 
target value C for the next cycle of combinational calculations. 
A combinational weighing method according to the second embodiment of the 
invention is different from the method described above with reference to 
FIG. 2 in that the selection keys 16 are operated to cause not only a 
lower limit value A and an upper limit value B but also initial dispersion 
values V.sub.i of the distributions of the differences in weight values 
corresponding to the selected objects M to be retrieved by the input means 
15 from the memory device 17 to be temporarily stored respectively in the 
memory means 24, 25 and 33. These initial dispersion values V.sub.i are 
inputted to the correction value calculating means 34 whereby an initial 
correction value .alpha. is calculated and the correction value setting 
means 35 serves to input this initial correction value .alpha. to the 
combination calculating means 19. The lower and upper limit values A and B 
are also received by the combination calculating means 19. 
When the combination signal c.sub.1 indicating the selected combination is 
transmitted from the combination calculating means 19 to the correction 
value calculating means 34, the dispersions V.sub.i of the distributions 
of the differences of the weight values from the selected weigh hoppers 51 
are retrieved thereby from the dispersion-storing memory means 33, and a 
correction value .alpha. corresponding to this combination of weigh 
hoppers 51 is calculated on the basis of these dispersion values V.sub.i 
and is set in the combination calculating means 19. The combination 
calculating means 19 then calculates combinations of weight signals from 
the load cells 52 in known manners. In other words, as many correction 
values .alpha. as there are combinations are calculated. If four out of 
ten weigh hoppers 10 are to be combined, .sub.10 C.sub.4 different 
correction values .alpha. are calculated. 
As the objects M are weighed, the difference values .DELTA. are detected by 
the difference detecting means 21 for each of the selected weigh hoppers 
51 and inputted to the dispersion calculating means 32 which serves to 
obtain new dispersion values V.sub.i on the basis of all of the past 
difference values .DELTA. inclusive of those which have just been 
calculated. Such newly obtained dispersion values V.sub.i are used by the 
dispersion updating means 38 to update the old values stored in the memory 
device 17. After combinational calculations are completed and a 
combination of weigh hoppers 51 with a total weight within the range A-B 
is selected, a completion signal e is transmitted to the input means 15. 
The input means 15 thereupon retrieves the updated dispersion values 
V.sub.i from the memory device 17 and temporarily stores them in the 
dispersion-storing memory means 33. A new correction values .alpha. is 
calculated by the correction value calculating means 34 on the basis of 
these temporarily stored dispersion values V.sub.i and is inputted to the 
combination calculating means 19 to determine a new target value C for the 
next selection. 
With such a method according to the second embodiment of the invention, the 
probability of the weight of objects M becoming less than the lower limit 
value A becomes extremely small. As a result, the work efficiency can be 
improved and the yield is also improved because the correction value 
.alpha. can be made as small as possible. 
Moreover, since a correction value .alpha. is calculated for each selected 
combination on the basis of the differences .DELTA. of the weight values 
from the selected weight hoppers 51 and a new target value C is determined 
accordingly for each different combination of the weigh hoppers 51, 
effects of the environment, for example, can be minimized and both the 
work efficiency and the yield can be improved. 
By way of the same example considered above, the correction value .alpha. 
may be set equal to the largest of the absolute values of .DELTA..sub.2, 
.DELTA..sub.4, .DELTA..sub.5 and .DELTA..sub.8. In this case, the 
correction value .alpha. becomes larger and hence the yield is adversely 
affected, but the work efficiency can be increased to nearly 100%. This is 
particularly suited when the differences .DELTA. are uniformly 
distributed. 
If the correction value .alpha. is set equal to the average of the absolute 
values of the differences .DELTA., on the other hand, the correction value 
.alpha. can be made even smaller and hence the yield can be improved 
although the work efficiency is adversely affected somewhat. 
The present invention is not limited to the kind of combinational weighing 
disclosed above. The invention can be applied, for example, to the kind of 
combinational weighing apparatus having memory hoppers below the weigh 
hoppers 51 and opening those of the weigh hoppers not selected in a 
combinational calculation to transfer their contents down to the 
corresponding ones of the memory hoppers such that the objects in such 
memory hoppers can also participate in the next cycle of combinational 
calculations. 
In summary, combinational weighing apparatus and methods according to the 
present invention can improve both the work efficiency and the yield at 
the same time, without being influenced by environmental conditions.