Weighing machine with weight detecting conveyor

A weighing machine has a weighing conveyor with a conveyor belt supported by a load cell and uses a digital filter to eliminate high-frequency components of weight signals from the weighing conveyor. In order to improve accuracy of measurement, operating conditions of the digital filter are set with regard to the length of the object to be weighed. Alternatively, time required for the object to be completely on the conveyor belt may be calculated or the timing for the zero-point adjustment for the load cell may be adjusted accordingly.

This invention relates to a weighing machine of a kind having a conveyor 
belt supported by a load cell for measuring the weight of a target object 
while receiving it from a feed-in conveyor belt and transporting it onto a 
discharge conveyor belt. 
BACKGROUND OF THE INVENTION 
For measuring the weight of a target object such as a packaged product, it 
has been known to make use of a so-called weighing conveyor with a 
conveyor belt supported by a load cell serving as a weight detecting 
means. At one end of the weighing conveyor is a feed-in conveyor belt from 
which a packaged product is delivered. After its weight is measured, it is 
discharged onto a discharge conveyor belt and, if necessary, a selection 
mechanism is activated. 
One of the problems associated with prior art weighing machines of this 
type has been that the result of measurement was severely affected by the 
length of the target object in the direction of its transportation 
(hereinafter simply referred to as its length). This problem has come to 
exist in the following three stages. 
Firstly, a weight detecting means thus structured is directly subjected to 
the vibrations of the conveyor which it supports. As a result, the 
oscillatory load from the conveyor belt is added to the weight of the 
target object, and a correct weight value cannot be obtained by a direct 
measurement. For this reason, output signals from the weight detecting 
means are passed through a low pass filter for removing the components 
with relatively high frequencies caused by the aforementioned vibrations. 
Since a low pass filter has an extremely large time constant, however, a 
stable output therefrom cannot be obtained until a certain length of time 
elapses after a target object is brought onto the weighing conveyor. In 
the case of a target object which is elongated in the direction of its 
transportation (hereinafter referred to as a long object), the entirety of 
the object can remain on the conveyor only for a short time, and an error 
is likely to occur if there is only a brief period of time available for 
the measurement. 
In view of the problems of this type, Japanese Patent Publication Tokkai 
60-79227 disclosed a weighing machine having a speed detector for the 
weighing conveyor such that the frequency band of the low pass filter can 
be adjusted according to the speed signal outputted from the speed 
detector. Such a weighing machine is capable of selecting an optimum 
frequency band for a given speed of the weighing conveyor, but there still 
remains the problem of measurement errors when objects of different 
lengths are to be weighed. 
Secondly, since an error is more likely to occur in the measurement of a 
long object, a method has been considered whereby moment-by-moment weight 
data are prevented from being taken between the time when the target 
object reaches the weighing conveyor and the later time when the low pass 
filter begins to output stable weight data. In other words, a detector for 
the target object is provided on the object-receiving side of the weighing 
conveyor, and weight data are taken or not taken according to the 
detection signals from this detector. Such a detector usually comprises a 
light-emitting element and a light-receiving element placed near the 
weighing conveyor such that the reflected light from the target object or 
the screening of the light thereby may be detected. In the case of an 
odd-shaped target object, that is, if its top surface has protrusions and 
indentations, a single object may screen the light twice while passing by 
such a detector, and the outputted signals may indicate that two objects 
have passed. This will cause an error in establishing the reference time 
for data processing and result in incorrect measurements. 
In order to overcome this difficulty, it has been known to enter from a 
data input means, such as a keyboard, a so-called detection inhibiting 
time period during which, after a detection signal is outputted from the 
target object detector, signals from this detector are prevented from 
being accepted. 
If the length of the target object or the conveyor speed is varied, 
however, a new value of the detection inhibiting time period must be 
determined by measuring the length of the target object and dividing it by 
the belt speed, and the value thus determined must be entered through the 
keyboard. 
Thirdly, a weighing machine of this type is adapted to keep determining an 
initial load value such as the weight of the conveyor, even when it is not 
loaded with any target object to be weighed. When there is no target 
object, such an initial load value is stored as the zero-point value, and 
the true weight of a target object is obtained by subtracting this 
zero-point value from the measured weight value. For this reason, it is 
extremely important for a weighing machine of this type to detect a 
no-load condition, and many methods for this purpose have been considered 
such as the method of monitoring the waveform of the signals from the 
weight detecting means in order to detect a no-load condition or that of 
using a timer to preliminarily set a time interval during which a no-load 
condition may be expected to have been established. 
According to the former method, a zero-point adjustment circuit is 
activated after a no-load condition is detected. Thus, there is a time 
delay between the detection of a no-load condition and the actual start of 
a zero-point adjustment procedure, and this gives rise to a problem of 
reduced operating efficiency. By the latter method, on the other hand, the 
user cannot adjust to changes in the length of the target object or to the 
belt speed of the weighing conveyor. Thus, the timer would be set for a 
longer period than necessary in order to be on the safer side, and this 
also results in wasted time. In summary, there always remained a problem 
of reduced work efficiency for the weighing machine because of the waste 
of time between when a zero-point adjustment becomes possible and when an 
automatic zero-point adjustment procedure is actually started. 
The present invention is for the purpose of eliminating these problems and 
its object is to provide a weighing machine of which the operation is not 
affected by changes in the length of the target object. 
SUMMARY OF THE INVENTION 
In order to accomplish the above and other objects by automatically setting 
optimum filter characteristics independent of any particular choice in the 
speed of the weighing conveyor or the length of the target object, a 
weighing machine embodying the present invention comprises a weighing 
conveyor having a weight detecting means supporting a conveyor belt for 
transporting a target object, data inputting means for inputting the belt 
speed of this weighing conveyor and the length of a target object, belt 
speed adjusting means for adjusting this belt speed to a set value, data 
processing condition setting means and digital filter means. The data 
processing condition setting means is for calculating optimum filtering 
conditions on the basis of the belt speed and the length of the target 
object. The digital filter means is adapted to receive a control signal 
from the data processing condition setting means and to thereby digitally 
filter weight signals from the weight detecting means. 
If the belt speed of the weighing conveyor and the length of the target 
object are preliminarily inputted through the data inputting means, 
optimum conditions for the digital filter means are automatically set by 
the data processing condition setting means, and the digital filter means 
functions under these conditions. In short, highly reliable weight data 
can be obtained independent of the length of the target object. 
According to a preferred embodiment of the invention, the data processing 
condition setting means is provided with thinning factor calculating means 
for determining operating conditions for the digital filter means for 
eliminating high-frequency components in weight signals such that highly 
reliable weight data can be obtained independent of the measurement 
frequency of the weight detecting means. 
According to another preferred embodiment of the invention, the 
aforementioned thinning factor calculating means calculates a thinning 
factor on the basis of the difference between the detection inhibiting 
time during which a target object is completely transferred from the 
feed-in conveyor onto the weighing conveyor and the object transportation 
time during which the target object is transported by the weighing 
conveyor, and also of the number of necessary weight data outputted from 
the weight detecting means. In other words, the time during which the 
target object is completely on the weighing conveyor is obtained from the 
difference between the object transportation time and the detection 
inhibiting time, and the thinning factor is calculated on the basis of 
this time period and the number of necessary weight data. Thus, the 
digital filter means eliminates high-frequency components of weight 
signals by using the largest thinning factor that can be set. Accordingly, 
high-frequency components can be eliminated with improved efficiency. 
In order to attain the aforementioned objects by automatically setting a 
detection inhibiting time on the basis of an inputted length value of the 
target object, a weighing machine according to the present invention may 
comprise a weighing conveyor, an object detecting means disposed on the 
feed-in side of this weighing conveyor, a data inputting means for 
inputting a length value of the target object, a detection inhibiting time 
calculating means and a detection inhibiting means. The detection 
inhibiting time calculating means is for calculating, on the basis of the 
inputted length value of the target object and the set belt speed, a 
transportation time corresponding to the length of the target object as 
the detection inhibiting time period. The detection inhibiting means 
serves to cause the detection signals from the object detecting means to 
be ignored until the aforementioned detection inhibiting time elapses from 
the time when the front end of the target object is detected by the object 
detecting means. 
When the data inputting means of a weighing machine thus structured sets a 
length value of a target object, a corresponding transportation time is 
calculated from the belt speed which is preliminarily set and this length 
value, and this is accepted as the detection inhibiting time. Accordingly, 
the detection inhibiting time for ignoring the quasi-signals associated 
with the ON-OFF operations of the object detecting means is automatically 
set. Thus, there is no longer the need to calculate a new detection 
inhibiting time whenever the length of the target object is changed. As a 
result, the weighing operation can be simplified, the possibility of an 
error in setting a detection inhibiting time can be reduced to a minimum, 
and the reliability of measurement can be improved. 
In order to attain the aforementioned objects of the invention by 
automatically setting a detection inhibiting time by transporting a target 
object just once, a weighing machine according to another embodiment of 
the present invention may include a weighing conveyor, an object detecting 
means as described above, means for measuring the output time of detection 
signals from the detecting means when a target object is transported onto 
the weighing conveyor and storing this output time as the detection 
inhibiting time corresponding to the length of the target object, and a 
detection inhibiting means as described above. When a target object is 
sent onto the weighing conveyor in a registration mode of operation of 
such a weighing machine, it is detected by the object detecting means and 
a detection signal is outputted. The output time of this detection signal 
is measured and the transportation time corresponding to the length of the 
target object is thereby determined and automatically set as the detection 
inhibiting time. When detection signals are outputted thereafter from the 
object detecting means in a weighing mode of operation, these signals are 
ignored from this time on until the aforementioned detection inhibiting 
time has elapsed. In this manner, signals can be processed with improved 
reliability after the front end of a target object is detected with 
certainty. 
Since the length of the target object is automatically measured while it is 
being transported and the detection inhibiting time is thereby determined, 
a weighing machine according to this embodiment of the invention does not 
require the input of any data from a keyboard and hence can measure the 
weights of target objects efficiently even when their lengths vary 
frequently. 
In order to attain the aforementioned objects of the invention by 
automatically setting an automatic zero-point adjustment inhibiting time 
corresponding to the length of a target object and to thereby prevent the 
occurrence of wasted time, a weighing machine according to still another 
embodiment of the present invention includes a weighing conveyor, a data 
processing condition setting means for calculating optimum filtering 
conditions from the belt speed of this weighing conveyor and the length of 
the target object, a digital filter means for receiving a control signal 
from this data processing condition setting means to thereby digitally 
filter weight signals from weight detecting means, and an automatic 
zero-point adjustment inhibiting time calculating means for calculating a 
zero-point adjustment inhibiting time on the basis of the length and the 
belt speed of the weighing conveyor, the length of the target object in 
the direction of its transportation and the filtering conditions. 
With a weighing machine according to this embodiment, since optimum 
filtering conditions for its digital filter are set from the belt speed 
and the length of the target object, and the digital filter is operated 
under such optimum conditions, it is possible to obtain weight data which 
may be considered most dependable corresponding to the lengths of the 
target objects and the belt speed. 
Moreover, zero-point adjustment is inhibited for a certain period of time 
from the time when a target object is transported onto the weighing 
conveyor, this period of time corresponding to the sum of a loading time 
determined by the belt speed, the length of the weighing conveyor and the 
length of the target object and a filter response time determined by the 
filtering conditions. As a result, a zero-point adjustment process can be 
initiated as soon as the weighing conveyor stabilizes under a no-load 
condition. In other words, zero-point adjustment can be effected 
efficiently without wasting any free time after a stable no-load condition 
is established and, since there are increased opportunities for a 
zero-point adjustment, corrections can be effected corresponding to 
changes in the length of target objects and weighing operations can be 
effected with higher accuracies.

DETAILED DESCRIPTION OF THE INVENTION 
In what follows, the present invention will be described by way of examples 
with the help of the drawings. 
With reference to FIGS. 1-5 which describe a weighing machine according to 
a first embodiment of the invention, its weighing conveyor 1 includes a 
conveyor belt 4 which is stretched between a drive roller 2 connected to a 
motor (not shown) and an idler roller 3 and is supported by a load cell 5 
serving as a weight detecting means. The weight of a target object M, 
which is transported onto the weighing conveyor 1 from a feed-in conveyor 
belt 6, is measured according to a timing schedule to be described below 
and is judged either acceptable or unacceptable in view of a target weight 
value. If it is adjudged unacceptable, it is discarded by a sorting means 
(not shown) disposed at a downstream location. If it is adjudged 
acceptable, it passes through the sorting means and is transported by 
means of a discharge conveyor 11 to a receiving station or the like. 
On the receiving side of the weighing conveyor 1, there is an object 
detector 7 for optically detecting the presence of a target object M 
transported by the feed-in conveyor belt 6. The object detector 7 is 
composed of an optical sensor of the transmissive or reflective type, 
having a light-emitting element and a light-receiving element such that a 
detection signal will be transmitted if detection light emitted from the 
light-emitting element is blocked by the target object M and fails to be 
received by the light-receiving element. 
The load cell 5 is adapted to output an analog signal indicative of the 
measured weight value. The analog signal outputted from the load cell 5 
passes through a preamplifier 8 and is received by an analog-to-digital 
converter 9. The digital signals outputted from the analog-to-digital 
converter 9 are transmitted to a digital filter 10A, which is so 
structured as to extract a direct current component as a weight signal 
from the received digital signals. 
As shown in FIG. 2, the digital filter 10A has its control unit 19 and 
program memory 12 connected to each other through a program bus 13, and a 
register 14, a data memory 15, an arithmetic and logic operation unit 
(ALU) 16 and a multiplication unit 17 connected together through a data 
bus 18 such that the take-in time for filter constants and the weight 
signal can be freely set by means of a control device 20 to be described 
below. 
With reference again to FIG. 1, the control device 20 is composed of a 
microcomputer having a central processing unit (CPU) 21, a read-only 
memory (ROM) 22, a random-access memory (RAM) 23 and interface circuits 
24. The control device 20 is adapted to receive data from the object 
detector 7 and a keyboard 26 (serving as a data inputting means for 
inputting various constants), and also weight data from a weight data 
memory circuit 60 to be described below. The control device 20 is also 
programmed to output filter constants to the digital filter 10A or weight 
data which, after the weight of a target object has been measured, may be 
believed the most trustworthy. The weight data memory circuit 60 stores a 
plurality of weight data outputted from the digital filter 10A. 
FIG. 3 shows the functions of the microcomputer of which the control device 
20 is composed, and the control device 20 is programmed as follows. 
In FIG. 3, numeral 23a indicates a set value memory means which forms a 
part of the RAM 23 (shown in FIG. 1) and serves to store inputted values 
of the belt speed V and the length L.sub.m for a target object received 
from the keyboard 26. The speed of the weighing conveyor 1 is controlled 
by a belt speed control means 71 so as to be equal to the set value V 
stored in the set value memory means 23a. 
Numeral 72 indicates a detection inhibiting time calculating means for 
calculating detection inhibiting time T.sub.01 from the belt speed V and 
the length L.sub.m of the target object according to Equation (1) given as 
follows: 
EQU T.sub.01 =(1/2){(L.sub.m /V)+(L.sub.m /V.sub.fc)} Eq. (1) 
where V.sub.fc is the speed of the feed-in conveyer belt 6 and the 
detection inhibiting time T.sub.01 is defined, as illustrated in FIG. 4, 
as the time interval from when a detection signal is outputted from the 
object detector 7 until the target object is completely on the weighing 
conveyor 1. 
Numeral 73 indicates an object transportation time calculating means for 
calculating object transportation time T.sub.11 from the belt speed V and 
the length L.sub.m of the target object according to Equation (2) given as 
follows:ps 
EQU T.sub.11 ={(L.sub.c -L.sub.m)/V}+T.sub.01 Eq. (2) 
where L.sub.c is the length of the weighing conveyor 1, and the object 
transportation time T.sub.11 is defined, as also illustrated in FIG. 4, as 
the time interval from when the target object M reaches the object 
detector 7 until the front end of the target object M reaches the distal 
end of the weighing conveyor 1 away from the feed-in conveyor belt 6. 
Numeral 74 indicates a thinning factor calculating means for determining 
what is herein referred to as the thinning factor N from the detection 
inhibiting time T.sub.01, the object transportation time T.sub.11 and the 
number P (such as 4) of weight data which may be considered necessary for 
obtaining a reliable weight value. The thinning factor N is a number which 
controls the response at the digital filter 10A, that is, the output 
interval of sampled weight signals expressed as the multiple of the 
sampling period .DELTA.T. More precisely, the thinning factor N will be 
herein defined as the number representing the ratio between the sampling 
period and the period of inputted signal. 
Numeral 75 indicates a filtering time calculating means for calculating the 
response time T.sub.n1 of the digital filter from the thinning factor N. 
Numeral 76 indicates a sampling time calculating means for calculating the 
sampling time T.sub.31, during which sampling can be performed. It is 
calculated from the detection inhibiting time T.sub.01, the object 
transportation time T.sub.11 and the response time T.sub.n1 of the digital 
filter according to Equation (3) given as follows: 
EQU T.sub.31 =T.sub.11 -T.sub.01 -T.sub.n1 Eq. (3) 
This is illustrated in FIG. 4. The detection inhibiting time calculating 
means 72, the object transportation time calculating means 73 and the 
thinning factor calculating means 74 constitute what is herein referred to 
as a data processing condition setting means 70. 
With reference still to FIG. 3, numeral 77 indicates a weight data 
outputting means for outputting the value considered the most trustworthy 
of the plurality of weight data stored in the weight data memory circuit 
60. Numeral 78 indicates an automatic zero-point adjustment inhibiting 
time calculating means for calculating a zero-point adjustment inhibiting 
time T.sub.41 from the belt speed V, the length L.sub.m of the target 
object and the response time T.sub.n1 of the digital filter according to 
Equation (4) given as follows: 
EQU T.sub.41 =(L.sub.m /V)+T.sub.n1. Eq. (4) 
The digital filter 10A may be either of a finite impulse response (FIR) 
type or of an infinite impulse response (IIR) type. The output from a FIR 
type digital filter is a sum of n input signals, each multiplied by a 
weight factor, where n is an integer. The output from an IIR type digital 
filter is an weighed sum of n input signals and m output signals where n 
and m are same or different integers. Generally, a FIR type is used where 
a quick response is desired from a rapidly varying input signal, while an 
IIR type may be preferred where the input is relatively stable but there 
are occasional noise components and it is desired to reduce the effects of 
such noise. The FIR and IIR types may be used in combination, and 
multi-stage filters may be used, depending on the system structure. 
The operation of a FIR type digital filter with three stages will be 
explained next with reference to FIG. 5 wherein Line (I) represents weight 
signals outputted from the load cell 5 at regular intervals of .DELTA.T 
(such as 0.5 millisecond). Let us assume that the first stage filter is of 
a FIR type with tap number=32 and thinning factor 1. Thus, the input to 
the first stage filter is at the same intervals as .DELTA.T as shown in 
Line (II) and, as soon as the 32nd weight signal is received thereby, 
filtered signals begin to be outputted therefrom sequentially at the same 
intervals as shown in Line (III). Let us assume next that the second stage 
filter is also of a FIR type but with tap number=32 and thinning factor 2. 
Such a filter will sample only every other signal outputted from the first 
stage filter as shown in Line (IV) because its sampling interval is 
2.DELTA.T. As soon as the 32nd signal is sampled by the second stage 
filter (simultaneously as the 94th signal is being sampled by the first 
stage filter as shown in FIG. 5), it begins to output filtered signals 
sequentially at the same intervals (2.DELTA.T) as shown in Line (V). Let 
us assume further that the third stage filter is also of a FIR type with 
tap number=32 but that its thinning factor is 3. This means that the third 
stage filter, sampling signals outputted from the second stage filter at 
intervals of 3.times.2.DELTA.T=6.DELTA.T. In other words, the total 
thinning factor due to all of these three stages is N=1.times.2.times.3=6, 
and only one out of every three signals from the second stage filter is 
sampled by the third stage filter as shown in Line (VI). Since its tap 
number is 32, the first filtered output signal therefrom will be obtained 
only after the 32nd signal is sampled thereby (at the same time, for 
example, as the 3.times.(32-1)+1=94th signal is being outputted from the 
second stage filter as shown in FIG. 5. It is to be noted from FIG. 5 that 
a time interval of (94+31.times.N).DELTA.T (=280.DELTA.T if N=6) will be 
required in this example in order to obtain the first filtered output 
signal from the third stage filter. As the thinning factor is increased, 
the lower limit of the signal frequency that can pass through the digital 
filter 10A becomes lower. Accordingly, the oscillatory load from the 
weighing conveyor 1 and the target object M which generate high-frequency 
components can be effectively removed. 
Before a weighing operation is started, the belt speed V and the length 
L.sub.m of the target objects to be weighed are inputted from the keyboard 
26 shown in FIG. 3. The weighing conveyor 1 is driven thereupon by the 
control device 20 in accordance with the inputted data, and the detection 
inhibiting time calculating means 72 and the object transportation time 
calculating means 73 calculate the detection inhibiting time T.sub.01 and 
the object transportation time T.sub.11, respectively, outputting 
corresponding signals to the thinning factor calculating means 74. What 
the thinning factor calculating means 74 actually does, according to the 
present invention, is to determine how large the overall thinning factor N 
of the digital filter 10A can be set for given values of the fundamental 
sampling period .DELTA.T, the detection inhibiting time T.sub.01, the 
object transportation time T.sub.11 and the value of P defined above (such 
as 4), and to output a signal indicative of the value of N so determined. 
Since {94+(32-1).times.N+(P-1) N}.DELTA.T={94+(3+ P).times.N}.DELTA.T 
(94+34.times.N).DELTA.T (if P=4) should not exceed (T.sub.11 -T.sub.01), 
the largest allowable value of the thinning factor in the case of this 
example is obtained by the formula N={(T.sub.11 -T.sub.01 
/.DELTA.T-94}/34. In summary, the thinning factor calculating means 74 is 
adapted to output to the digital filter 10A a signal representative of the 
largest possible value for the thinning factor N such that the selected 
number P of weight data can be obtained within the sampling time T.sub.31 
during which filtering operation is allowed. 
It is to be noted regarding the example considered above that the tap 
number was 32 in all three stages of the digital filter 10A. The formula 
for calculating the largest possible value of the overall thinning factor 
N of the digital filter 10A will depend not only on the number of stages 
as well as the tap number and the thinning factor for each of the stages. 
In what follows, these parameters which together determine the overall 
thinning factor N of the digital filter 10A will be summarily referred to 
as the filtering conditions, or filtering parameters. It is further to be 
noted that one or more of the stages may be of an IIR type, requiring a 
still different formula for calculating the overall thinning factor N. 
When a target object M is placed on the feed-in conveyor belt 6 under the 
condition described above and reaches near the front end of the weighing 
conveyor 1 as shown in FIG. 4, a detection signal is outputted from the 
object detector 7, and the control device 20 prevents the detection of the 
target object M for a period of time given by the detection inhibiting 
time T.sub.01 from the moment when this detection signal is inputted. The 
purpose of this inhibiting operation is to prevent the object detector 7 
from misinterpreting a single target object M, depending on its shape, as 
a plurality of objects as this target object M is transferred from the 
feed-in conveyor belt 6 onto the weighing conveyor 1. After the detection 
inhibiting time T.sub.01 has elapsed and the target object M is completely 
on the weighing conveyor 1, the control device 20 of FIG. 3 outputs a 
command to the digital filter 10A to have the weight signals from the load 
cell 5 sampled at constant intervals .DELTA.T. 
These weight signals, thus sampled at the constant period .DELTA.T, are 
sequentially thinned out by the digital filter 10A by the thinning factor 
N determined by the control device 20, as explained above, such that 
high-frequency components are removed from the weight signals. 
After the response time T.sub.n1 of the digital filter of FIG. 4 has 
elapsed and data begin to be outputted from the digital filter 10A, the 
control device 20 causes these weight data to be transmitted to and stored 
in the weight data memory circuit 60. Still after the elapse of time 
period T.sub.31, which is preliminarily obtained by calculation, the 
control device 20 stops the sampling operations by the digital filter 10A 
so as to prevent weight data with low reliability from being taken in. 
The weight data outputting means 77 of the control device 20 reads out a 
plurality of weight data stored in the weight data memory circuit 60, 
selects and outputs a value considered the most trustworthy of the 
plurality of weight data obtained during the sampling time of T.sub.31. 
The value to be selected may be the highest one, the average or the value 
which occurs most frequently. 
After the elapse of zero-point adjustment inhibiting time T.sub.41 from the 
moment when one of the target objects M is discharged from the weighing 
conveyor 1, the automatic zero-point adjustment function for the weighing 
conveyor 1 is activated because the weighing conveyor 1 is now in a 
condition to allow such automatic zero-point adjustment. If another object 
to be weighed is transported onto the weighing conveyor 1 from the feed-in 
conveyor belt 6 when the automatic zero-point adjustment is about to be 
started, however, the zero-point adjustment is not carried out, and the 
control device 20 measures the weight of the newly arrived object by a 
similar process and outputs a weight value therefor considered to be the 
most trustworthy. When the length L.sub.m of the target object to be 
weighed or the belt speed V of the weighing conveyor 1 is to be changed, 
the new value is inputted from the keyboard 26. The control device 20 
calculates the detection inhibiting time T.sub.01, the object 
transportation time T.sub.11, the sampling time T.sub.31, the thinning 
factor N, the response time T.sub.n1 of the digital filter and the 
zero-point adjustment inhibiting time T.sub.41 from the newly inputted 
value of the length L.sub.m or the belt speed V. The data processing 
condition setting means 70 changes the filter constants of the digital 
filter 10A on the basis of the new values of the detection inhibiting time 
T.sub.01, the object transportation time T.sub.11 and the thinning factor 
N. Thereafter, the digital filter 10A performs filtering operations under 
optimum conditions for the new values of the belt speed V and the length 
L.sub.m of the target object. 
With the belt speed V of the weighing conveyor 1 known, data on the length 
L.sub.m of a target object M may be obtained automatically by measuring a 
traveling time of the object M. 
A second embodiment of the present invention is described next with 
reference to FIGS. 6-8 wherein like components are generally indicated by 
the same numerical symbols. In FIGS. 6 and 7, numeral 10 indicates a 
digital filter, outputs from which are adapted to be received by the 
control device 20. The digital filter 10 is comprised of a digital signal 
processor and programmed so as to function as a low pass filter of a 
finite impulse response (FIR) type. The control device 20 is programmed so 
as to receive a signal from the object detector 7, data from the keyboard 
26 (serving as data inputting means for inputting various data) and weight 
signals from the digital filter 10 so as not only to perform various 
functions to be described below but also to output control parameters to 
the digital filter 10 and display data to a display device (not shown). 
With reference next to FIG. 7 which shows the functions of the 
microcomputer constituting the control device 20 of FIG. 6, the RAM 23 
stores the length L.sub.c of the weighing conveyor, the length L.sub.m of 
the target object to be weighed, the sampling time period T.sub.3 during 
which sampling may be carried out and the belt speed V. The ROM 22 stores 
a program which enables the control device 20 to perform its intended 
functions to be described below. 
With reference still to FIG. 7, numeral 30 indicates a detection inhibiting 
time calculating means for calculating a transportation time (as detection 
inhibiting time T.sub.0 =L.sub.m /V) corresponding to the length L.sub.m 
of the target object and outputting this value as a signal to a detection 
inhibiting means 32. The detection inhibiting means 32 serves to start a 
measurement starting timer 31 when a detection signal is received from the 
object detector 7 and to cause the detection signal to be ignored, 
although it may be repeatedly switched on and off, until a time period 
equal to the detection inhibiting time T.sub.0 elapses from the start of 
the timer 31. 
Numeral 33 indicates an object transportation time calculating means for 
calculating the object transportation time T.sub.1 (=L.sub.c /V) for the 
target object M on the weighing conveyor 1 by using the stored values of 
the length L.sub.c and the belt speed V. Numeral 34 indicates a filter 
response time calculating means for calculating the response time T.sub.n 
of the digital filter 10 as shown in FIG. 8 by using the calculated value 
of the object transportation time T.sub.1, the set value of the sampling 
time T.sub.3 and the detection inhibiting time T.sub.0 as T.sub.n =T.sub.1 
-T.sub.0 -T.sub.3. Numeral 35 indicates a filter constant calculating 
means for calculating filter constants from this response time T.sub.n of 
the digital filter and outputting them as signals to the digital filter 10 
to thereby determine filter characteristics. The object transportation 
time calculating means 33, the filter response time calculating means 34 
and the filter constant calculating means 35 constitute a data processing 
condition setting means. Numeral 36 indicates a measurement timing 
calculating means for calculating the unstable time T.sub. 2 between when 
a target object M is brought onto the weighing conveyor 1 and when weight 
signals begin to be taken in. The unstable time T.sub.2 may be calculated 
either from the response time T.sub.n of the digital filter and the 
detection inhibiting time T.sub.0 according to the relationship T.sub.2 
=T.sub.n +T.sub.0 or from the object transportation time T.sub.1 and the 
sampling time T.sub.3 according to the relationship T.sub.2 =T.sub.1 
-T.sub.3. 
The measurement starting timer 31 of FIG. 7 starts its time counting 
operation when the object detector 7 detects the front end of the target 
object M and stops it when it counts up a time period equal to the 
unstable time T.sub.2 which has been set. Thereafter, the weight signals 
outputted at constant intervals from the digital filter 10 are 
sequentially stored in a weight data memory 37 formed on a specified area 
of the RAM 23. 
Numeral 39 indicates an automatic zero-point adjustment inhibiting time 
calculating means for calculating the zero-point adjustment inhibiting 
time T.sub.4 from the transportation time T.sub.1 for the object M on the 
weighing conveyor 1 and the unstable time T.sub.2 according to the 
relationship T.sub.4 =T.sub.1 +T.sub.2 as illustrated in FIG. 8 and 
setting its value in another timer 38 (shown in FIG. 7) herein referred to 
as the zero-point input inhibiting timer. With the zero-point adjustment 
inhibiting time T.sub.4 thus set, the zero-point input inhibiting timer 38 
starts its time counting operation when the front end of the target object 
M to be weighed is detected by the object detector 7 and stops it after an 
elapse of this zero-point adjustment inhibiting time T.sub.4. Thereafter, 
the weight signals outputted at constant intervals from the digital filter 
10 are stored in another memory device 40 herein referred to as the 
zero-point memory. 
Numeral 41 indicates a weight calculating means for calculating the net 
weight of the target object M by subtracting the zero-point value stored 
in the zero-point memory 40 from the data in the weight data memory 37. 
Numeral 42 indicates a belt speed control means for adjusting the 
transportation speeds of the weighing conveyor 1 and the feed-in conveyor 
belt 6 according to the inputted belt speed V. 
Next, the operation of the weighing machine described above by way of FIGS. 
7 and 8 will be explained. First, the weighing machine is set in a 
registration mode from the keyboard 26, and the length L.sub.c of the 
weighing conveyor 1, the length L.sub.m of the target object M to be 
weighed, the sampling time T.sub.3 during which sampling is to be allowed 
and, if necessary, also the belt speed V are inputted. The control device 
20 causes these data to be stored on a specified area of the RAM 23 and 
performs certain specified calculations by using them. For example, the 
detection inhibiting time calculating means 30 determines a detection 
inhibiting time T.sub.0 (=L.sub.m /V) from the belt speed V and the length 
L.sub.m of the target object, although the detection inhibiting time 
T.sub.0 may also be obtained in the same way as the value of T.sub.01 was 
obtained by Equation (1), as explained above with reference to a different 
embodiment of the present invention. Similarly, the object transportation 
time calculating means 33 determines an object transportation time T.sub.1 
(=L.sub.c /V) from the length L.sub.c of the weighing conveyor and the 
belt speed V, although the object transportation time T.sub.1 may also be 
obtained as the value of T.sub.11 was obtained by Equation (2). The filter 
response time calculating means 34 calculates T.sub.n =T.sub.1 -T.sub.0 
-T.sub.3 to determine a response time T.sub.n for the digital filter from 
the object transportation time T.sub.1, the sampling time T.sub.3 and the 
detection inhibiting time T.sub.0. If the filtering time is sufficiently 
long, however, the response time may be set shorter than the calculated 
value of T.sub.n. 
The filter constant calculating means 35 calculates filter constants from 
this response time T.sub.n of the digital filter and outputs them as 
signals to the digital filter 10 to thereby determine filter 
characteristics. In this manner, the response time T.sub.n can be made 
longer if the length L.sub.m of the target object is small such that the 
digital filter 10 can be operated more effectively. In the case of a 
longer target object with a large value of L.sub.m, on the other hand, the 
response time T.sub.n of the digital filter may be made shorter such that 
the weighing capability will not be adversely affected. A short response 
time T.sub.n of the digital filter may give rise to the danger of external 
disturbances, but sufficiently accurate weight data can be obtained as a 
practical matter, for example, by subjecting the weight values to a 
translational averaging process. 
The measurement timing calculating means 36 of FIG. 7 determines the 
unstable time T.sub.2 =T.sub.n +T.sub.0. The unstable time T.sub.2 may 
also be calculated from the object transportation time T.sub.1 and the 
sampling time T.sub.3. The value of the unstable time T.sub.2 thus 
calculated is set in the measurement starting timer 31. The belt speed 
control means 42 adjusts the transportation speeds of the conveyors 1, 6 
and 11 according to the inputted value of the belt speed V. In 
applications where there is no need to change the belt speed V, the belt 
speed control means 42 may be absent. 
When the registration is completed, the keyboard 26 is operated to change 
the mode of operation from the registration mode to the weighing mode. 
When a target object M to be weighed reaches the position of the object 
detector 7 from the feed-in conveyor 6, a detection signal is outputted 
from the object detector 7. When this detection signal is received by the 
control device 20, the detection inhibiting means 32 activates the 
measurement starting timer 31. Until the detection inhibiting time T.sub.0 
elapses thereafter, that is, until the whole of the target object M passes 
the position of the object detector 7, all changes in the signals from the 
object detector 7 are ignored. Thus, it is the signal which was outputted 
when the front end of the target object M reached the object detector 7 
that is used as the reference time for the subsequent data processing. 
The digital filter 10 operates in synchronism with the analog-to-digital 
converter 9 to sequentially receive numerical data therefrom, to carry out 
filtering operations according to a specified formula by using inputted 
new numerical data and a row of old numerical data, and to output a result 
to the control device 20. The measurement starting counter 31 starts 
counting when the target object M is detected, stops counting when the 
unstable time T.sub.2 shown in FIG. 8 has elapsed and stores on the weight 
data memory 37 the weight data outputted from the digital filter 10 of 
FIG. 7. The weight calculating means 41 subtracts the zero-point value 
stored in the zero-point memory 40 from the weight data stored in the 
weight data memory 37, thereby determining the net weight of the target 
object M and outputting its value to a display device (not shown). By the 
time the display is made, the target object M is already on its way to be 
discharged from the weighing conveyor 1. 
The weighing conveyor 1 becomes unloaded as soon as the target object M is 
completely discharged from it, but it is not in a perfect zero-load 
condition until the unstable time period T.sub.2 elapses because the 
vibrations of the conveyor 1 have not been sufficiently attenuated. In 
other words, the signal from the load cell 5 cannot be considered to 
represent a true zero-load condition. Thus, the automatic zero-point 
adjustment inhibiting time calculating means 39 calculates the zero-point 
adjustment inhibiting time T.sub.4 from the object transportation time 
T.sub.1 and the value of the unstable time T.sub.2 by the relationship 
T.sub.4 =T.sub.1 +T.sub.2 and sets this value in the zero-point input 
inhibiting timer 38. The zero-point input inhibiting timer 38 begins to 
count in synchronism with the measurement starting timer 31 when the front 
end of the target object M is detected, stops counting after the 
zero-point adjustment inhibiting time T.sub.4 has elapsed and thereafter 
stores the weight signals outputted from the digital filter 10 on the 
zero-point memory 40. Accurate weight values can thus be obtained because 
the zero-point memory 40 now stores a weight signal corresponding to what 
may be considered a stabilized condition after the vibrations of the 
weighing conveyor 1 have been sufficiently attenuated. 
If a next target object M is detected before the elapse of this zero-point 
adjustment inhibiting time T.sub.4, the zero-point input inhibiting timer 
38 is reset and starts its counting operation from the beginning. Thus, 
the zero-point is updated only when two successive target objects are 
transported with a longer time interval in between than the zero-point 
adjustment inhibiting time T.sub.4. If this interval is short, the 
previous zero-point value is used. In this manner, variations in the 
zero-point of the weighing machine caused by time changes can be corrected 
without adversely affecting the work efficiently. 
If the length L.sub.m of the target objects M to be weighed or the belt 
speed V is to be changed, the new value is entered from the keyboard 26. 
On the basis of the new value of the length L.sub.m or the belt speed V, 
the control device 20 calculates the detection inhibiting time T.sub.0, 
the object transportation time T.sub.1 and the response time T.sub.n for 
the digital filter and changes the filter constants of the digital filter 
10 on the basis of these data. Thus, the digital filter 10 not only 
carries out its filtering operation under conditions that are optimum to 
the changed values of the belt speed V and the length L.sub.m but also 
changes the value of the zero-point adjustment inhibiting time T.sub.4. 
As an alternative to the embodiment of the invention described above, 
different values of L.sub.m may be preliminarily stored in a memory device 
for each kind of objects to be weighed such that the user has only to 
input a new call number from the keyboard 26 when it is desired to change 
the value of the length L.sub.m. Regarding the belt speed V, it may be set 
at a fixed value or a speed detector may be provided to detect the speed 
and register its value. 
FIG. 9 shows a weighing machine according to still another embodiment of 
the invention, characterized wherein a target object M is transported in 
the registration mode of operation such that the detection inhibiting time 
T.sub.0 can be set automatically. It includes a transportation time 
measuring means 50 for measuring the time at which the detection signal is 
outputted from the object detector 7 to thereby store, as the detection 
inhibiting time T.sub.0, the transportation time corresponding to the 
length L.sub.m of the target object to be weighed. This transportation 
time measuring means 50 is designed such that, while it is measuring the 
output time of a signal from the object detector 7, occurrence of 
OFF-conditions (that is, no-detection conditions) shorter than a specified 
minimum length will be ignored and hence that an accurate value of the 
detection inhibiting time T.sub.0 can be obtained independent of the shape 
of the target object M. 
A weighing machine according to this embodiment may also be designed such 
that different values of the detection inhibiting time T.sub.0 are 
preliminarily calculated and registered for target objects of different 
lengths and a right value of T.sub.0 corresponding to a given target 
object can be called by its call number. If the detection inhibiting time 
T.sub.0 is to be automatically set by merely transporting a target object 
M to be weighed, the object M is sent from the feed-in conveyor belt 6 
onto the weighing conveyor 1 in the registration mode of operation. The 
object detector 7 outputs detection signals as long as it keeps detecting 
the object M. The length L.sub.m of the target object M can be determined 
by measuring the output time of these detection signals by the 
transportation time measuring means 50, and the length L.sub.m thus 
determined is set in the detection inhibiting means 32 as the detection 
inhibiting time T.sub.0. 
In all of the embodiments of the invention described above, data such as 
the belt speed V, the length L.sub.c of the weighing conveyor, the length 
L.sub.m of the target object to be weighed and the sampling time T.sub.3 
are inputted through a keyboard serving as data inputting means. Since the 
length L.sub.m of the target object will have to be inputted each time 
objects of a different size are to be weighed, it may be a practical idea 
to paste a scale on a structure such as a cover disposed along the travel 
path of the weighing conveyor such that the object M to be weighed can be 
placed against it to have its approximate length determined directly. 
It is also to be remembered that no particular selection of filtering 
parameters for the filtering operation is intended to limit the scope of 
the invention. In what follows, there will be shown results of an 
experiment performed for testing. For this experiment, weight signals were 
sampled at intervals of 0.5 millisecond by a digital filter with three FIR 
type stages. The first stage had tap number=32 and performed simple 
averaging on 32 inputted weight data. The attenuation was small but the 
cutoff frequency was as low as 14.4 Hz and the gain became less than -20 
dB for the first time at 58 Hz as shown in FIG. 10. Since this is 
effective in the lower frequency region, it is considered useful for the 
attenuation of driving noise. The second stage was a filter with tap 
number=32 and weight factors determined by a hanning window function. The 
thinning factor for the second stage was 3. This filter had only low 
filtering effects in the low frequency region but high attenuation was 
obtained in the high frequency region as shown in FIG. 11. The third 
filter was a Chebyshev filter with tap number selectively either 32 or 64, 
having three Chebyshev characteristics which were selectable corresponding 
to each of these tap numbers such that its filtering characteristics could 
be varied by properly selecting filtering parameters. Higher filtering 
effects can be obtained with tap number=64 but the response delay becomes 
larger. The thinning factor for the third stage was made variable between 
3 and 255 such that the cutoff frequency could be lowered by increasing 
the thinning factor. If the thinning factor is increased, the filtering 
effect becomes higher but the response delay becomes greater. Chebyshev 
filters have superior cutoff characteristics and steep attenuation 
property, and it was possible to obtain a high, stable attenuation rate 
from a relatively low frequency region. FIG. 12 shows the attenuation by 
the third stage filter with tap number=32. FIG. 13 shows the total 
attenuation from all three stages. According to a preferred embodiment of 
the present invention, filtering conditions, such as the tap numbers and 
the thinning factors of its individual stages are made selectable 
according to the processing capability of the system such that a maximum 
filtering effect can be automatically obtained. 
This invention is applicable to weighing machines for continuously weighing 
objects such as merchandises which are already weighed and packaged.