Weigh feeder

A digital weigh feeder includes a conveyor belt for transporting material thereon. The conveyor belt has an average tare weight. A digital weight transmitter provides at least first and second outputs having first and second frequencies respectively. The ratio of the first and second frequencies is indicative of the weight of material on the belt. A weight processor digitally computes a scaled ratio of the first and second frequencies. An auto tare circuit digitally computes the average tare weight of the conveyor belt. The computed average tare weight is substracted from the computed scaled ratio of the first and second frequencies to obtain a reading of the actual weight of material on the belt.

BACKGROUND OF THE INVENTION 
The present invention is directed to a weigh feeder. In particular, the 
invention is directed to a weigh feeder which comprises a digital weight 
transmitter operatively associated with a conveyor belt for providing 
plural frequency signals which are indicative of the weight of material on 
the belt. 
Digital weigh feeders are well known. For example, see U.S. Pat. No. 
3,724,720 entitled "Digital Mass Flow Control System" assigned to the 
assignee herein. These weigh feeders are termed "true rate" systems since 
they control the true or instantaneous rate of the mass flow of material. 
In such systems, belt speed is automatically adjusted as a function of the 
weight of material sensed on the belt to achieve a constant feed or mass 
flow rate. Specifically, belt speed and belt loading are sensed and 
multiplied together to obtain an actual mass flow feedback signal. This 
feedback signal is compared to a desired flow rate or set point signal. 
Any difference between the two signals causes a change in belt speed to 
achieve a steady flow of material. 
The heart of any weigh feeder is the weight sensing component. The 
performance demands on such a component may be awesome. The ideal weight 
sensing component must be highly precise, exactly repeatable, completely 
linear and capable of operating over a broad range of loading conditions 
while remaining rugged and stable through long use with little or no 
calibration. The ideal weight sensing component would require no 
stabilization time, would not deflect under loading, and would be immune 
to vibration and temperature extremes. Heretofore, the weight sensing 
components used in weigh feeders fell far short of the ideal in respect to 
many of the above requirements. As such, the weigh sensing components 
considerably limited the performance of weigh feeder systems. 
A vibrating string digital weight transmitter closely approximates the 
above criteria for an ideal weight sensing component. Such a digital 
weight transmitter is described in U.S. Pat. Nos. 3,411,347, 3,423,999, 
3,621,713, 3,763,971, 3,724,573 and 3,805,605. The transmitter offers 
unexcelled accuracy and stability. The transmitter senses a load digitally 
so that loading information is not subject to inaccuracies due to drift 
which inaccuracies are common in analog weight sensors. The vibrating 
string transmitter is extremely accurate and may provide repeatability of 
.+-.0.003%, linearity of .+-.0.03% and long term stability of .+-.0.03% 
over considerable periods of time. The vibrating string transmitter 
closely approximates a defectionless load sensing system and is virtually 
immune to temperature and vibration extremes which typify the weigh feeder 
environment. 
The vibrating string digital weight transmitter provides plural output 
signals at different frequencies. The ratio of the frequencies provides an 
indication of the weight being sensed by the transmitter. Weigh feeder 
systems known in the art are not compatible with such weight transmitters. 
Accordingly, such weigh feeders cannot secure the benefits in accuracy, 
stability and reliability of a vibrating string transmitter. 
An advantage of the present invention is that it is totally compatible with 
a vibrating string digital weight transmitter. 
Another disadvantage of the invention is that it is much more accurate, 
stable and reliable than weigh feeders known in the art. 
A further advantage of the invention is that it automatically computes the 
average tare weight of a conveyor belt and provides an accurate indication 
of the actual weight of material on the belt. 
Other advantages appear hereinafter. 
SUMMARY OF THE INVENTION 
In a digital weigh feeder including a conveyor belt for transporting 
material thereon, a digital weight transmitter operatively associated with 
the belt for providing at least first and second outputs having first and 
second frequencies respectively. The ratio of the first and second 
frequencies is indicative of the weight of material on the belt. A first 
means digitally computes a scaled ratio of the first and second 
frequencies. A second means operatively associated with the first means 
digitally computes the average tare weight of the belt. A third means 
digitally subtracts the computed average tare weight from the computed 
scale ratio of the first and second frequencies to obtain an indication of 
the actual weight of material on the belt. 
For the purpose of illustrating the invention, there is shown in the 
drawings a form which is presently preferred; it being understood, 
however, that this invention is not limited to the precise arrangements 
and instumentalities shown.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawings in detail, wherein like numerals indicate like 
elements, there is shown in FIG. 1 a weigh feeder 10 in accordance with 
the principles of the present invention. The present invention is directed 
in particular to the weight/tare section 12 of weigh feeder 10. The 
operation of the remaining components of the weigh feeder are well known 
and are described in exacting detail in U.S. Pat. No. 3,724,720 making 
further detailed description herein unnecessary. 
The weigh feeder is capable of operating in a speed or volumetric mode and 
in a mass or gravimetric mode. In the speed mode, it is presumed that 
material is being deposited at a constant rate on the belt 14. 
Accordingly, the speed of a belt motor 16 is maintained constant to 
preserve a steady volumetric flow of material. In the speed or volumetric 
mode, a mode switch 18 is thrown to the speed position. This causes a dual 
data selector such as a RCA 4539 dual 1 of 4 selector to pass the output 
of a belt speed encoder 22 to the feedback input of a controller 24. In 
addition, the data selector 20 passes the set point output of a set point 
circuit 26 to the command input of controller 24. The set point output of 
set point circuit 26 represents the desired speed of operation of the belt 
14 in the speed of volumetric mode. The controller 24 operates a servo 
amplifier 28 to control the belt speed via belt motor 16. Controller 24 
typically comprises an up/down counter and input synchronizing circuitry 
for computing the difference between the command and feedback signals, as 
disclosed in U.S. Pat. No. 3,724,720. 
In the mass or gravimetric mode, the mode switch 18 is thrown to the mass 
position. This causes the dual data selector 20 to transmit the output of 
a rate multiplier 30 to the feedback input of controller 24. In addition, 
the data selector 20 transmits the set point output of set point circuit 
26 to the command input of controller 24. The set point output of set 
point circuit 26 represents the desired mass flow rate in the mass or 
gravimetric mode. The controller 24 operates the servo amplifier 28 as a 
function of the difference between the command and feedback signals at the 
output of the dual data selector 20. In particular, the controller 24 
operates the servo amplifier 28 to vary the speed of belt motor 16 to 
maintain a steady mass flow rate of material. 
In the weigh feeder described in U.S. Pat. No. 3,724,720, the rate 
multiplier 30 multiplies the output of the belt speed encoder 22 with a 
mass or weight signal provided by a load cell-digital volt meter pair to 
provide an output signal which is indicative of the rate of mass flow of 
material on the belt 14. The set point, mass flow and belt speed can be 
selectively displayed by means of a display circuit 32 as described in the 
patent. Alternatively, total mass can be displayed by means of a divider 
34 and totalizer 36. The reader is referred to U.S. Pat. No. 3,724,720 for 
further details of the structure and operation of the components described 
above. The remainder of the description of the weigh feeder herein is 
directed to the weigh/tare section 12 of the present invention. 
Weigh/tare section 12 includes a digital vibrating string weight 
transmitter 38 preferably associated with a weigh deck 40 disposed beneath 
the belt 14. The weigh deck 40 transmits the belt load to the transmitter 
38. The weigh deck 40 per se does not comprise the present invention. 
The digital weight transmitter 38 generates two separate output signals 
having frequencies f1 and f2 respectively. Frequencies f1 and f2 vary as a 
function of the load applied to belt 14. The ratio of the frequencies f1 
and f2 provides an indication of the weight of material on the belt 14. 
That indication, however, will include the tare weight of the belt itself. 
Thus, to obtain a measurement of the actual weight of material on the 
belt, it is necessary to compensate the weight measurement by the tare 
weight of the belt. In mathematical terms, the actual weight of material 
on the belt is given by: 
EQU Actual weight=K1(f1/f2)-K2 (1) 
where K1 is a constant determined by the full scale range of the weight 
transmitter 38 and K2 is a constant equal to the average tare weight of 
the belt 14. 
The weight K1(f1/f2) is digitally computed by a weight processor 42. See 
FIG. 1. The average tare weight of the belt 14 is digitally computed by an 
auto tare circuit 44. The output of the weight processor 42 represents the 
actual weight of material on the belt compensated by the average tare 
weight of the belt. This output is transmitted to the rate multiplier 30 
for purposes of mass flow and belt speed computations as described above. 
The weigh feeder 10 operates in either of two modes: a normal mode and a 
tare mode. In the normal mode, the weight/tare section 12 transmits the 
set point output of set point circuit 26 to the command input of 
controller 24. In addition, in the normal mode, the weight/tare section 12 
either transmits the output of belt speed encoder 22 to the feedback input 
of controller 24 when mode switch 18 is thrown to the speed or volumetric 
position or the section 12 transmits the output of rate multiplier 30 to 
the feedback input of controller 24 when switch 18 is thrown to the mass 
or gravimetric position. In the tare mode, the weight/tare section 12 
transmits an ATRM signal to the command input of controller 24. The ATRM 
signal represents the belt speed necessary for the belt to traverse an 
integral number of loops to complete the average tare weight calculation 
as will be described more fully hereinafter. In addition, in the tare 
mode, the weight/tare section 12 transmits the output of belt speed 
encoder 22 to the feedback input of controller 24. The operation of the 
weigh feeder 10 in the normal and tare modes is described in greater 
detail below. 
Operation In The Normal Mode 
In the normal mode of operation, the weight processor 42 provides an output 
indicative of the actual tared weight of material on the belt 14 in 
response to the f1 and f2 outputs of the digital weight transmitter 38. 
The weight processor 42 is shown in detail in block diagram form in FIG. 
2A. The f1 output of the digital weight transmitter is passed through an 
opto-isolator 46 and a bandpass filter 48. The bandpass filter 48 is 
centered at a typical f1 frequency and has a band width of approximately 
1.25 Khz. The optoisolator 46 and bandpass filter 48 are well known signal 
processing components used to separate the input signal from background 
noise. The BPO output of bandpass filter 48 is fed to a zero cross 
detector 50. The zero cross detector 50 shapes the BPO signal to provide a 
well-defined digital pulse train ZCD at the input to a frequency 
multiplier 52. 
A preferred form for the zero cross detector 50 and frequency multiplier 52 
is shown in FIG. 2B. The zero cross bias or threshold is provided by a 
Zener diode 56. The excursions of the BPO output of bandpass filter 48 
through the zero cross bias are detected by amplifier 58. The ZCD output 
of amplifier 58 is a digital pulse train which drives a phase locked loop 
60 in frequency multiplier 52. The frequency of the ZCD signal is f1. The 
output of the phase locked loop 60 is fed to a selectable divider 62. The 
divider 62 may be a 7 stage binary counter such as a RCA 4024 type 
counter. Only the first 6 stages of the counter are used in the preferred 
embodiment described herein as indicated by the lines Q1-Q6. The output of 
the sixth stage of the divider is coupled to the input of the phase locked 
loop 60. The output of the phase locked loop 60 is a pulse train having a 
frequency 64 times frequency f1. The Q1 output of the first stage of the 
divider will be a pulse train having a frequency of 32 times f1. The Q2 
output of the second divider stage will have a frequency of 16 times f1. 
The Q3 output of the third divider stage will have a frequency of 8 times 
f1. The Q4 output of the fourth divider stage will have a frequency of 4 
times f1. And the Q5 output of the fifth divider stage will have a 
frequency of 2 times f1. 
The output of the frequency multiplier 52, designated Nf1, will be a pulse 
train having a frequency which is a multiple N of the frequency f1 
depending upon the selected position of switch 64. In the preferred 
embodiment herein, switch 64 is actually a jumper which may be selectively 
connected between the frequency multiplier output Nf1 and the Q0-Q5 lines. 
The particular value of N to be selected will be a function of the full 
scale range of the digital weight transmitter 38 as will be described in 
detail hereinafter. 
The f2 signal output of the digital weight transmitter 38 is fed to an 
opto-isolator 66, bandpass filter 68 and zero cross detector 70. See FIG. 
2A. Opto-isolators 46 and 66 are identical. Bandpass filters 48 and 68 are 
of approximately the same band width but of different center frequencies. 
Whereas bandpass filter 48 has a center frequency equal to frequency f1, 
bandpass filter 68 has a center frequency equal to frequency f2. Zero 
cross detectors 50 and 70 are identical. The output of zero cross detector 
70 is shaped by a Schmitt trigger inverter 72. The output of the Schmitt 
trigger inverter is a pulse train having a frequency equal to frequency 
f2. 
The output of the Schmitt trigger inverter is transmitted to the clock 
input of an edge detector flip-flop 74 via inverter 76. The output of the 
inverter is designated f2. The Nf1 and f2 signals are the digital signals 
which are manipulated by the remaining portion of the weight processor 42 
to compute the actual weight of material on the belt 14 according to 
equation (1) above. 
The equation for the computation of actual weight of material on the belt 
as given above can be rewritten in the following form: 
EQU Actual weight=(N.times.c1)(f1/f2)-K2 (2) 
where the constant K1 has been replaced by the term N.times.c1 and N is the 
multiplication factor of frequency multiplier 52. This equation can be 
rearranged further as follows: 
EQU Actual weight=(N.times.f1)(c1.times.P2)-K2 (3) 
where P2=1/f2. 
The quantity c1.times.P2 defines a sample time window for computing the 
weight of material on the belt. It should be noted that the sample window 
itself will vary since P2 is a function of frequency f2 which varies with 
the load on the belt. 
The constants N and c1 depend on the full scale range of the digital weight 
transmitter 38. The constants N and c1 are chosen to take on the values 
given in Table 1 below to ensure that a maximum of 1000 pulses appear at 
the WPRM output of the weight processor 42 during a single sample window 
when full scale weight is on the belt. 
Table 1 
______________________________________ 
Digital Weight Transmitter 
Full Scale Range (Kg)* 
N c1 
______________________________________ 
0.200 64 4000 
0.400 32 4000 
0.600 32 2667 
0.800 16 4000 
1.000 16 3200 
2.000 8 3200 
4.000 4 3200 
6.000 4 2133 
______________________________________ 
*For 6kg maximum full scale range weight transmitter c1 = (5.12 .times. 
10.sup.4)/N .times. full scale range); maximum WPRM = 1000 pulses at full 
scale range load 
For loads less than the full scale load, no more than 1000 WPRM pulses will 
be produced by the weight processor per sample window. This is guaranteed 
by the choice of N and c1 as indicated by Table 1 above. 
The length of a sample window is equal to c1.times.P2. The factor c1 
therefore determines the length of a sample window. The length of a sample 
window is determined by sample window counter 78 in weight processor 42. 
See FIG. 2A. The sample window counter 78 is a 16 bit presettable down 
counter whose preset inputs are connected to plural c1 switches 80. 
Switches 80 comprise a bank of parallel connected rocker switches, each 
rocker switch being coupled to a preset input of counter 78. The c1 
switches 80 are selectively placed in open and closed positions to preset 
counter 78. These switches are programmable in the factory or the field 
prior to operation of the weigh feeder. During operation of the weigh 
feeder, the switches would not be accessible to the operator. 
The weight processor 42 also includes a tare counter 82. Tare counter 82 is 
a 24 bit presettable down counter which is preset to the average tare 
weight computed by auto tare circuit 44 as will be described in detail 
hereinafter. The tare counter 82 accounts for the constant K2 in equation 
(3) above. 
In operation, the first f2 pulse at the output of Schmitt trigger inverter 
72 is transmitted by inverter 76 to toggle edge detector flip-flop 74. 
When toggled, the Q output of the flip-flop enables AND gate 84 to pass 
the f2 output of Schmitt trigger 72 to the clock input of the sample 
window counter 78. At the same time, the Q output of edge detector 
flip-flop 74 enables AND gate 86 to pass the Nf1 pulses at the output of 
frequency multiplier 52 to the clock input of tare counter 82. Tare 
counter 82 counts down the Nf1 pulses and sample window counter 78 counts 
down the f2 pulses. 
For typical belt loads, the frequency f1 is between approximately 14.5 Khz 
and 16 Khz and the ratio f1/f2 fluctuates between approximately 1.017 and 
1.250. Accordingly, the frequency of the Nf1 pulses is much greater than 
the frequency f2. As a result, the tare counter 82 will count down to zero 
before sample window counter 78. When the tare counter 82 reaches the zero 
count, its A output disables AND gate 86 from passing any further Nf1 
pulses to the counter. At the same time, the A output of the counter 
disables an AND gate 88 which in turn enables an OR gate 90 to pass the 
Nf1 pulses at the output of frequency multiplier 52 to a rate multiplier 
92. 
Ideally, the rate multiplier 92 is set at 0.5000. At this setting, under 
ideal conditions, the WPRM output of rate multiplier 92 will consist of 
1000 pulses for each sample window set by counter 78 when full scale load 
is on the belt 14. If the load on the belt is less than full scale, the 
number of pulses at the WPRM output of rate multiplier 92 per sample 
window will be less than 1000. In any case, the number of pulses appearing 
at the WPRM output of rate multiplier 92 per sample window will indicate 
the actual weight of material on the belt 14. It should be noted that the 
maximum limit of 1000 pulses is arbitrarily chosen to represent full scale 
loading. Other limits can also be chosen with slight modification to the 
logic circuitry described herein as will be obvious to the person of 
ordinary skill in the art. 
In practice, the setting of rate multiplier 92 required to obtain 1000 
pulses at the WPRM output under full scale load conditions may vary 
somewhat from 0.5000. Thus, in practice, inaccuracies in the weight 
measurement introduced by the mechanical components of the weigh feeder, 
such as inaccuracies in the weigh deck dimensions, will cause more or less 
than 1000 pulses to appear at the WPRM output of the rate multiplier 
during a sample window when the rate multiplier is set at 0.5000. These 
inaccuracies are easily compensated for by manipulating the setting of the 
rate multiplier to obtain the required number of pulses at the WPRM output 
under full scale load conditions. 
As indicated above, the rate multiplier 92 passes a certain number of Nf1 
pulses to its WPRM output after the tare counter 82 reaches a zero count 
and before the sample window ends. This is indicated by the time interval 
T1 in FIG. 4A. The end of the sample window is determined by counter 78 in 
conjunction with an OR gate 94. See FIG. 2A. The sample window ends when 
the counter 78 reaches the zero count. At this time, the B output of 
counter 78 disables AND gate 84 from passing any further f2 pulses to the 
counter. At the same time, the B output of the counter causes a 
counter/decoder 96 to be reset via OR gate 94 and inverter 98. The B 
output of the counter also causes edge detector flip-flop 74 to be reset 
via OR gate 94. 
When reset, the Q output of the edge detector flip-flop presets the tare 
counter 82 and sample window counter 78 in preparation for the computation 
of weight for the next sample window. In addition, when the edge detector 
flip-flop 74 is reset, the Q output of the flip-flop disables OR gate 90 
from passing any further Nf1 pulses to the rate multiplier 92. This marks 
the end of the T1 interval shown in FIG. 4A. 
During the T1 interval, the rate multiplier 92 transmits the WPRM pulses to 
a 4 decade counter 100. The counter 100 counts the WPRM pulses to provide 
a digital indication of the weight of material on the belt sensed during 
the sample window. The sequence of operation of the counter 100 is 
controlled by the counter/decoder 96. The counter/decoder 96 is preferably 
a RCA 4022 type counter/divider with 8 decoded outputs Q0-Q7. Only the Q2, 
Q3 and Q4 outputs of the counter/decoder are used. 
The counter/decoder is clocked by a 250 Khz clock oscillator 102. After the 
counter/decoder 96 is reset at the end of the T1 interval, it begins to 
count the pulses generated by clock oscillator 102. When the 
counter/decoder counts to two, its Q2 output enables a latch circuit 104 
to transmit the count of WPRM pulses maintained in counter 100 to the mass 
flow control rate multiplier 30. The rate multiplier will then perform a 
conventional computation of belt speed times weight to derive the mass 
flow signal used to control mass flow in the mass or gravimetric mode as 
described in U.S. Pat. No. 3,724,720. When the counter/decoder 96 reaches 
a count of three, the Q3 output of the counter/decoder resets the counter 
100 in preparation for the next computation of weight for the next sample 
window. When the counter/decoder counts to four, the Q4 output of the 
counter/decoder disables the counter/decoder from counting any further. 
Thereafter, the counter/decoder will be reset at the end of the next 
sample window by the B output of the sample window counter 78 as 
previously described. 
Operation in the Tare Mode 
In the foregoing description of operation in the normal mode, it was 
presumed that the tare counter 82 had been preset to the average tare 
weight of the belt 14. In the tare mode, the average tare weight of the 
belt is computed and supplied to the preset inputs of the tare counter 82. 
The auto tare circuit 44 digitally computes the average tare weight of the 
belt in the tare mode. The auto tare circuit 44 is shown in detailed block 
diagram form in FIG. 3. 
In the tare mode, the tare weight of the belt 14 is averaged in respect to 
the number of sample windows per each complete loop of travel of the belt. 
The number of sample windows per belt loop is predetermined by 
manipulation of divisor switches 106 and 108. See FIG. 3. Divisor switches 
106 and 108 control the operation of selectable dividers 110 and 112 
respectively. Selectable dividers 110 and 112 comprise identical 12 bits 
counters 114 and 116. The sixth, eighth and tenth bit outputs of counters 
114 and 116 are selectively tapped by switches 106 and 108 to average the 
belt tare weight over 64, 256 or 1024 sample windows. 
During a tare calculation, the belt speed must be controlled to provide an 
integral number of loops of travel of the belt for the selected number of 
sample windows. This is accomplished by driving the command input of the 
controller 24 with the output, designated ATRM, of an auto tare rate 
multiplier 118. The feedback input to the controller, in the tare mode, is 
the output of belt speed encoder 22. Selection of the command and feedback 
inputs to the controller 24 in the tare mode is governed by dual data 
selector 20 as previously described. The operation of the dual data 
selector is summarized in Table 2 below. 
Table 2 
______________________________________ 
X Y Command Line 
Feedback Line 
Mode 
______________________________________ 
0 0 Set point Speed Normal/Volumetric 
1 0 Set point Mass Normal/Gravimetric 
0 1 ATRM Speed Tare 
1 1 ATRM Speed Tare 
______________________________________ 
From Table 2, it should be evident that the speed of the belt is always 
governed by the ATRM output of rate multiplier 118 in the tare mode. 
The number of pulses appearing at the ATRM output of rate multiplier 118 is 
determined by the number of f2 pulses generated at the output of inverter 
76, see FIG. 2A, and the setting of percentage switches 120. In the 
preferred embodiment herein, rate multiplier 118 is a three decade 
multiplier which passes a fixed percentage of the f2 pulses as determined 
by the setting of percentage switches 120. Percentage switches 120 
comprise three decades of BCD switches for setting the decades of rate 
multiplier 118. 
The setting S of the percentage switches is determined by the following 
equation: 
EQU S=(B.times.E)/(R.times.c1.times.F) (4) 
where B is the belt length in feet, E is the number of belt speed encoder 
output pulses at full scale belt seed, R is the number of sample windows 
selected by operation of switches 106 and 108, and F is the full scale 
belt speed in feet per minute. 
In operation, the number of sample windows R is selected by setting 
switches 106 and 108 to the appropriate bit outputs of counters 114 and 
116. This fixes the time interval during which the average tare belt 
weight is to be computed. To initiate operation in the tare mode, a tare 
switch or button 122 is depressed to generate a reset pulse at the reset 
inputs of flip-flops 124 and 126 via a Schmitt trigger inverter 128 and 
differentiator circuit 130. When reset, the Q output of flip-flop 126 
disables a bank of tri-state gates 132. The tri-state gates 132 comprise 
24 conventional tri-state gates, each of which connects a bit output from 
24 bit counter 133 to the preset input of a corresponding bit in 24 bit 
tare counter 82. The state of each of the gates is controlled by the 
enable/disable input which is connected to the Q output of flip-flop 126. 
The output of each of the gates is connected to a pull-down resistor 
coupled to ground. When the gates are disabled, their outputs are open and 
the preset inputs to the 24 bit tare counter 82 sill therefore be pulled 
to a binary low state indicating a zero preset count. Accordingly, at the 
beginning of operation in the tare mode, the tare counter 82 will be 
preset to zero. 
When the tare counter 82 is preset to zero, the A output of the counter 
will enable OR gate 90 to pass the Nf1 output pulses from frequency 
multiplier 52 to the 12 bit counter 114. See FIGS. 2A and 3. The counter 
114 counts the Nf1 pulses and divides that count by the divisor R. The 
divided count output of counter 114 represents the average tare belt 
weight. The divided count is passed via Schmitt trigger inverter 134 to 
the 24 bit counter 133. 
At the beginning of the first sample window, the Q output of edge detector 
flip-flop 74 toggles flip-flop 124. When toggled, the Q output of 
flip-flop 124 releases the reset on 24 bit counter 133. Accordingly, the 
counter 133 is free to count the pulses at the output of Schmitt trigger 
inverter 134 throughout the tare mode. 
At the end of each sample window, the Q output of edge detector flip-flop 
74 clocks 12 bit counter 116. When the preselected number of sample 
windows has been counted by counter 116, the counter toggles flip-flop 
126. Accordingly, the Q output of flip-flop 126 enables the tri-state 
gates 132. When enabled, the tri-states gates pass the average tare count 
maintained in counter 133 to the preset inputs of tare counter 82. The 
tare counter 82 will therefore be preset to the average belt tare count 
during the normal mode of operation. 
To ensure that the belt 14 traverses an integral number of loops during the 
tare mode, the rate multiplier 118 commands the controller 24 as 
previously described. When the tare button 122 is depressed, flip-flop 124 
is reset so that the Q output of the flip-flop resets the rate multiplier 
via OR gate 136 while disabling OR gate 138 from passing any f2 pulses to 
the rate multiplier. Thereafter, at the beginning of the first sample 
window, the Q output of edge detector flip-flops 74 toggles flip-flop 124 
as previously described to cause the Q output of flip-flop 124 to enable 
OR gate 138 via OR gate 136. As a result, OR gate 138 passes the f2 pulses 
to the clock input of rate multiplier 118. 
The rate multiplier 118 passes a fixed percentage of the f2 pulses to its 
ATRM output as determined by the setting of percentage switches 122 as 
previously explained. The controller 24, then, controls the belt speed to 
ensure that an integral number of revolutions of the belt is obtained 
during the tare mode. At the end of the tare mode, counter 116 toggles 
flip-flop 126 as previously described to cause the Q output of flip-flop 
126 to reset the rate multiplier 118 while disabling OR gate 138 from 
passing any further f2 pulses to the rate multiplier. 
During the tare mode, OR gate 136 controls the operation of dual data 
selector 20 via a Schmitt trigger inverter 140. The dual data selector 20 
operates in response to the output of the Schmitt trigger inverter as 
indicated in Table 2 above. 
An advantage of the invention is that it automatically computes the tare 
weight of a conveyor belt and provides an accurate indication of the 
actual weight of material on the belt with extreme accuracy and 
reliability. The invention is totally compatible with a vibrating string 
digital weight transmitter. Further, the invention comprises commercially 
available modular digital circuits to facilitate construction and repair. 
Preferably, the modular circuits are CMOS circuits so that power 
consumption is relatively low. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof and, 
accordingly, reference should be made to the appended claims, rather than 
to the foregoing specification as indicating the scope of the invention.