A loss-in-weight gravimetric feeding system is disclosed which includes a prefeeder for receiving and discharging material and at least first and second feeders for receiving material from the prefeeder and discharging the received material to a common collector. The system includes a device for alternately diverting material discharged from the prefeeder to the first and second feeders, circuits for generating a first and second weight signal representative of the weight of material in the first and second feeders respectively, and structure for generating a reference signal representative of a desired mass flow rate of material from a feeder. The system further includes a circuit for alternately comparing the reference signal to the first and second weight signals in synchronism with the diverting device for controlling each of the feeders based on the comparison between the reference signal and the weight signal associated with the feeder.

BACKGROUND OF THE INVENTION 
Loss-in-weight feeders are in widespread use in the material handling 
industry and are used to deliver particulate and other materials at 
precise feed rates. Typically, a loss-in-weight feeder measures the 
decreases in weight of material contained in the feeder over a period of 
time. The weight loss by the feeder in that period of time is exactly 
equal to the weight of material delivered to the next step in the material 
handling process. This mode of operation, because it measures change in 
weight per unit time, is referred to as "gravimetric" operation. 
An advantage of loss-in-weight feeders is that there is no possibility of 
feed rate errors due to storage or material accumulation in the material 
handling system. Thus, for example, if it is determined that the weight of 
a loss-in-weight feeder has descreased 30 pounds in one hour, then it can 
be said with certainty that those 30 pounds have been delivered to the 
next step in the process and are not "lost" due to storage or spillage 
somewhere in the system upstream of the feeder. 
A disadvantage of present loss-in-weight feeding systems is that, in 
measuring weight loss from a feeder, accurate weight measurements cannot 
be made when material is being added to the feeder during refill. While 
the feeder is being refilled, loss-in-weight measurements are impossible 
because material is being added to the feeder at an uncontrolled rate. In 
addition, the weighing device, usually a scale, is subjected to impact 
forces generated by the added material, entrapped air, and other factors 
which result in weight readings that vary widely and are inaccurate. 
During refill, therefore, it is customary to operate loss-in-weight 
feeders in a volumetric mode (which delivers a given volume of material 
per unit time) rather than in a gravimetric mode which would produce 
inaccurate feed control. Because refill may constitute a substantial 
portion of the feeder operating cycle, the overall accuracy of the system 
may be significantly reduced. 
The necessity for switching to a volumetric mode of operation during refill 
has additional disadvantages as well. In order to achieve high feed rates, 
refill time must be minimized. This requires expensive, high-quality, 
critically-damped scales to make certain that perturbations introduced 
during refill have been damped out when the system re-enters the 
gravimetric mode. If the chosen scale is under-damped or over-damped, 
large hoppers are required to ensure that the system will operate in 
gravimetric control for a long enough time to provide stable weight data. 
However, the use of large hoppers to accomodate high feed rates can also 
result in poor accuracy because larger hoppers are more easily affected by 
outside forces than small hoppers. In order to minimize inaccuracy, 
therefore, feed rates must generally be kept low. 
Accordingly, the present invention has been developed to substantially 
reduce the foregoing problems and to produce an improved loss-in-weight 
feeding system which allows gravimetric operation for 100 percent of the 
operating time. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the loss-in-weight gravimetric 
feeding system includes a prefeeder for receiving and discharging material 
and at least first and second feeders for receiving material from the 
prefeeder and discharging the received material to a common collector. The 
invention includes means for alternately diverting material discharged 
from the prefeeder to the first and second feeders, means for generating a 
first and second weight signal representative of the weight of material in 
the first and second feeders respectively, and means for generating a 
reference signal representative of a desired mass flow rate of material 
from a feeder. The invention further includes means for alternately 
comparing the reference signal to the first and second weight signals in 
synchronism with the diverter means and means for controlling each of the 
feeders based on the comparison between the reference signal and the 
weight signal associated with the feeder. 
It is therefore a feature of the invention to provide an accurate 
loss-in-weight feeding system. 
It is another feature of the invention to provide a continuous 
loss-in-weight gravimetric feeding system for feeding at high feed rates. 
It is still another feature of the invention to provide a continuous 
loss-in-weight feeding system which is compact and easily implemented. 
A still further feature of the invention is to provide a continuous 
loss-in-weight feeding system which obviates the need for volumetric 
control during refill.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring now to the drawings, wherein like numerals indicate like 
elements, there is shown in FIG. 1 a loss-in-weight feeder 10 in 
accordance with the present invention. 
Material to be fed by the system is put into the inlet 42 of prefeeder 12. 
Material inflow is indicated by .phi..sub.T. Prefeeder 12 may be any 
conventional feeding apparatus. Prefeeder 12 has a hopper 13 provided with 
a feed screw, auger or other conventional material feed apparatus 15, 
which is controlled by feed motor 14. Located below prefeeder 12 are two 
feeders 16 and 18. Feeders 16 and 18 have inlets 46 and 48, respectively, 
and comprise material feeding apparatus such as a feed screw or other 
conventional feeding apparatus 17 and 19, respectively, controlled by feed 
motors 20 and 22, respectively. Although for convenience the term "feed 
screw" is used in this description, it should be understood that the 
invention includes the use of any suitable material feeding apparatus and 
is not limited to the use of a feed screw. 
Feeders 16 and 18 are suspended from scales 24 and 26, respectively, which 
weigh the feeders 16 and 18 and the material therein. Scales 24 and 26 are 
conventional scales and generate electrical signals representative of the 
weight of feeders 16 and 18. The weight signals generated by scales 24 and 
26 will be referred to herein as weight "A" and weight "B" signals, 
respectively. (Although the term "weight" is used throughout this 
description, mass rather than weight may be sensed. Accordingly, the term 
weight should be read as including either weight or mass, without 
departing from the scope of the invention.) Feeders 16 and 18 also have 
outlets 50 and 52, respectively, from which material being fed is 
discharged to a common collector 28, which may be any suitable collector. 
Located immediately below the outlet 44 of prefeeder 12 is a diverter valve 
30. In the embodiment illustrated, diverter valve 30 is a blade-shaped 
member which pivots at one end 36 between a first position (shown in solid 
lines) and a second position 30' (shown in broken lines). Diverter valve 
30 serves to divert the flow of material from prefeeder 12 alternately to 
feeders 16 and 18 so that changing the position of diverter valve 30 from 
the first position to the second position 30' will cause feeders 16 and 18 
to be filled alternately with material from prefeeder 12. 
In the illustrated embodiment of the invention, diverter valve 30 is biased 
by spring 34 which is connected to diverter valve 30 at a point 38 
somewhere above pivot 36. Spring 34 is anchored to a fixed member at 40. 
Spring 34 serves to urge diverter valve 30 to the first position. Diverter 
valve 30 is moved from the first position to the second position 30' by 
means of a push rod 31 which pushes diverter valve 30 against the force 
exerted by spring 34 to position 30'. Push rod 31 is shown in FIG. 1 as 
being actuated by an electric solenoid 32, but may be actuated by any 
suitable means, such as a hydraulic or pneumatic piston. When solenoid 32 
is energized, push rod 31 pushes against diverter valve 30 and causes the 
valve 30 to move to position 30'. When solenoid 32 is de-energized, push 
rod 31 retracts, and diverter valve 30 moves from the second position 30' 
back to the first position under the force of spring 34. 
It should be understood that the particular mechanism which causes diverter 
valve 30 to change position is not critical to the invention. Thus, for 
example, diverter valve 30 may be caused to move by use of two opposed 
solenoids or two opposed pistons, or any other mechanism for effecting 
movement of diverter valve 30, without departing from the instant 
invention. Likewise, it should be understood that any other type of 
diverter valve may be used without departing from the scope of the 
invention. 
In operation, prefeeder 12 discharges material continuously. When diverter 
valve 30 is in the first position, material added to the system flows 
through prefeeder 12 and is diverted by diverter valve 30 into feeder 16. 
When diverter valve 30 is in position 30', the material discharged by 
prefeeder 12 is diverted to feeder 18. Diverter valve 30 is in the first 
position for 50 percent of the time and is in the second position 30' for 
50 percent of the time, so that the material discharged by prefeeder 12 is 
alternately directed to feeders 16 and 18. That is, the period of time in 
which diverter valve 30 diverts material to feeder 16 is substantially 
equal to the period of time in which diverter valve 30 diverts material to 
feeder 18. The volumetric capacity of prefeeder 12 is preferably less than 
or equal to 80 percent of the volumetric capacity of feeders 16 and 18, so 
that there is not possibility of overfilling feeders 16 and 18 or of 
having material "back up" in the system. 
A block diagram of the control circuitry for the invention is shown in FIG. 
2. Controller 58 includes a control panel 54, which has a row of switches 
56 to enable an operator to select the desired mass flow rate of the 
system. The output of control panel 54 is a signal which is a 
negative-going ramp, the slope of which is representative of the desired 
rate of decrease of the weight of feeders 16 or 18 during discharge, i.e., 
the desired mass flow rate of feeders 16 or 18. The ramp wave-form is 
generated by ramp generator 76, shown in FIG. 3. The manner in which the 
ramp wave-form may be generated will be understood by persons familiar 
with loss-in-weight feeding systems. For convenience, the ramp wave-form 
generated by ramp generator 76 may be referred to as the "set point ramp". 
The set point ramp wave-form is shown in FIG. 4. 
Although the set point ramp may be generated in a conventional manner, it 
must be emphasized that the set point ramp of the present invention is not 
a conventional loss-in-weight set point ramp. The set point ramp of the 
present invention differs from a conventional set point ramp primarily in 
that no portion of the set point ramp of the present invention corresponds 
to the period of time a feeder is being refilled. Each cycle of the 
present set point ramp represents the loss in weight of one or the other 
of feeders 16 and 18 during discharge. Other differences between the set 
point ramp of the present invention and the prior art are shown in FIG. 5. 
Instead of utilizing the integrating type of controller described above 
(i.e., generating a set-point ramp and comparing slope), a differentiating 
type of controller may be used without departing from the scope of the 
invention. Differentiating controllers, which generate a fixed setpoint 
and measure actual mass flow by computing weight loss over a period of 
time, are well-known in the art and need not be described in detail. 
One output of controller 58 is a drive command frequency, which is a series 
of pulses representative of the speed at which feeder motors 20 and 22 
must be driven to obtain an actual mass flow rate equal to the desired 
mass flow rate selected at control panel 54. The weight output signal from 
selector 72 is compared to the set point ramp in comparator circuit 78. 
Comparator circuit 78 is a conventional comparator circuit which compares 
the instantaneous slope of the weight output signal with the instantaneous 
slope of the set point ramp and generates an error signal E representative 
of the difference between the weight output signals and the set point 
ramp. Error signal E is sent to pulse generator 80, which generates a 
series of pulses called the drive command signal, shown in FIG. 3 as the 
Drive CMD Freq. The frequency of the pulses varies with the error signal E 
so that, for example, when the error signal E indicates an actual mass 
flow rate greater than desired, the frequency of the pulses decreases, 
causing feed motors 20 and 22 to run more slowly and thus reduce actual 
mass flow rate. Conversely, if the error signal E indicates a mass flow 
rate less than desired, the frequency of the pulses increases, causing 
motors 20 and 22 to run faster and thus increase actual mass flow rate. 
Error signal E also controls the speed of feed motor 14 associated with 
prefeeder 12 so that the speeds of motors 20 and 22 and the speed of motor 
14 increase or decrease in the same proportion. The relationship between 
motors 20 and 22 and motor 14 is discussed more fully below. The relation 
between the error signal E and the frequency of the pulses is shown in 
FIG. 6. The way in which the error signal and the drive command signal may 
be generated will be understood by those versed in the art. 
It should also be understood that, although the drive command signal 
described herein is a series of pulses controlled by the error signal E, 
the drive command signal could be a voltage, current or any other signal 
generated in response to error signal E for controlling the motors 14, 20 
and 22 without departing from the scope of the invention. 
The other output of controller 58 is the A/B SELECT signal. The A/B SELECT 
signal is a square wave having a period twice that of the set point ramp. 
Each transition of the A/B SELECT signal causes diverter valve 30 to 
change position. Likewise, each transition causes selector circuit 72 to 
switch from one weight signal to the other, as will be explained more 
fully below. The transitions of the A/B SELECT signal occur at the 
respective minimum weights of feeders 16 and 18, as shown in FIG. 4. As 
presently preferred, the weight signal from selector circuit 72 is split 
in controller 58 and is sent simultaneously to comparator circuit 78 (as 
described above) and to level detector 82. Level detector 82 is a 
conventional level detecting circuit. When level detector 82 senses that 
the weight signal from selector circuit 72 is at a minimum, it signals a 
logic element 84, shown in FIG. 3 as a flip-flop but which can be any 
suitable logic element, to change state. The output of logic element 84 is 
the A/B SELECT signal. 
The drive command frequency output of controller 58 is sent to two 
identical frequency/analog converters 60 and 62, where the drive command 
frequency is converted to an analog voltage representative of desired feed 
motor speed. Frequency/analog converters are conventional elements and 
need not be described further. The outputs of frequency/analog converters 
60 and 62 are sent to motor drive circuits 68 and 70 respectively. Motor 
drive circuits 68 and 70 are conventional circuits. Any conventional motor 
and motor drive may be used. 
The output of motor drive circuit 68 drives feed motors 20 and 22 
associated with feeders 16 and 18 respectively. Feed motors 20 and 22 are 
driven in parallel, so that both motors 20 and 22 are driven at precisely 
the same speed. The output of motor drive circuit 70 drives feed motor 14 
associated with prefeeder 12. Were it not for variable resistors 64 and 
66, feed motor 14 would necessarily be driven at the same speed as feed 
motors 20 and 22, because the same drive command frequency from controller 
58 is used to control all three motors. Variable resistors 64 and 66 are 
provided to "trim" the speeds of the motors so that prefeeder motor 14 may 
be caused to run more slowly than feed motors 20 and 22. This is to ensure 
that the maximum volumetric flow of prefeeder 12 will be less than the 
maximum volumetric flow of feeders 16 and 18, so that there is no 
possibility of overfeeding feeders 16 and 18 or of accumulating material 
in the feeders. 
All three motors are run continuously when the system is in operation and, 
because all are driven by the same drive command frequency, the motors 
"track" each other. That is, the speeds of motors 20 and 22 and the speed 
of motor 14 increase or decrease in the same proportion. The precise 
speeds of feed motors 14, 20 and 22 are not critical so long as (1) the 
speed of motor 14 is less than the speed of motors 20 and 22 so that 
maximum volumetric flow of prefeeder 12 is preferably less than or equal 
to 80% of maximum volumetric flow of feeders 16 and 18, and (2) the speed 
of motor 14 "tracks" the speed of motors 20 and 22 so that the ratio of 
volumetric flow of prefeeder 12 to volumetric flow of feeders 16 and 18 
remains constant, preventing storage in feeders 16 and 18 and minimizing 
prefeeder-to-collector delay in feed rate adjustments. 
In the illustrated embodiment, and as presently preferred, the motor speed 
control is operated in an "open loop" configuration. That is, no feedback 
of motor speed information is used to control motor speed. Motor speed is 
controlled by comparing the loss-in-weight signal representative of 
material fed from feeders 16 and 18 to the set point ramp signal generated 
by control panel 54. However, it should be understood that a closed loop 
motor control with motor speed feedback may be used for even greater 
accuracy if desired. 
The weight output signals of scales 24 and 26 (weight "A" and weight "B" 
signals, respectively) are sent to selector circuit 72, which is a 
conventional time multiplexing circuit. The output of selector circuit 72 
is described more fully below. The output of selector circuit 72 is sent 
to controller 58, where, as shown in FIG. 3, it is compared to the set 
point ramp generated by control panel 54 to generate the drive command 
frequency signal. 
Selector circuit 72 is controlled by the A/B SELECT signal output of 
controller 58. The A/B SELECT signal causes selector circuit 72 to 
alternately transmit the discharge portions of the weight output signals 
of scales 24 and 26 to controller 58 for comparison to the set point ramp 
in proper timed relationship as discussed more fully below. The A/B SELECT 
signal is also sent to diverter valve control circuit 74, which actuates 
diverter solenoid 32 so that the selector circuit 72 and the diverter 
valve control circuit 74 are operated in synchronism. 
Referring to FIG. 4, the weight and control signals are shown in their time 
relationship. All of the signals shown in FIG. 4 have been idealized for 
the sake of clarity. In actual practice, the signals shown in FIG. 4 may 
differ from their idealized form without departing from the instant 
invention. As described in connection with FIG. 1, diverter valve 30 is 
operated to cause feeders 16 and 18 to be filled alternately. Thus, for 
example, when diverter valve 30 is in the first position, material is 
added to feeder 16 and the weight of feeder 16 will increase as indicated 
by the top curve in FIG. 4 (weight "A" signal). Meanwhile, the material in 
feeder 18 is being discharged, and the weight of feeder 18 will decrease, 
as shown in the third curve in FIG. 4 (weight "B" signal). When controller 
52 senses that the weight of feeder 18 has reached a preselected minimum, 
(point "a" on the weight "B" signal curve), the A/B SELECT signal changes 
state and causes diverter valve 30 to move to its second position 30', 
halting the flow of material to feeder 16. Because feed motor 20 is run 
continuously, the material which was added to feeder 16 will now begin to 
discharge, and the weight of material in feeder 16 decreases (from point 
"a" to point "b" on the weight "A" signal curve). The rate at which 
material is added to feeder 16 (i.e. the discharge rate of prefeeder 12) 
is chosen to equal the discharge rate of feeder 16. Thus, since input and 
output rates are equal, there is no storage of material in feeder 16. The 
feed rate of feeder 16 is chosen such that material begins to be 
discharged from feeder 16 at precisely the same moment that diverter valve 
30 changes position to 30' and feeder 18 begins filling. 
At point "b" on the weight "A" signal curve, feeder 16 has discharged all 
of the material contained therein. When controller 58 senses that the 
weight of feeder 16 has reached a preselected minimum, the A/B SELECT 
signal again changes state and causes diverter valve 30 to move from its 
second position 30' to the first position. Material now begins to refill 
feeder 16. The weight of material in feeder 16 therefore begins to 
increase from point "b" to point "c" on the curve, repeating the 
above-described cycle. 
As shown by the second curve in FIG. 4, while feeder 16 is discharging 
(between points "a" and "b"), the flow of material from feeder 16 is 
constant. While feeder 16 is being filled (between points "b" and "c"), 
the flow from feeder 16 is zero. Feeder 18 behaves in the identical manner 
as feeder 16, except that, because it is filled alternately, the curves 
representing the change in weight of material in feeder 18 and the 
discharge of feeder 18 are shifted in time relationship by 180.degree. 
from the curves for feeder 16. 
The fifth curve in FIG. 4 is the set point ramp signal generated by control 
panel 54. The set point ramp signal is a negative-going ramp with a period 
T equal to the period of time feeders 16 and 18 require to discharge the 
material contained therein to obtain the desired mass flow rate selected 
at control panel 54. The seventh curve in FIG. 3 is the output from 
selector circuit 72. Selector circuit 72 selects the weight "A" and weight 
"B" signals from scales 24 and 26 so that only the portion of the weight 
"A" and weight "B" signals during discharge of the associated feeder are 
utilized. Each transition of the A/B SELECT signal (the sixth curve in 
FIG. 4) causes selector circuit 72 to switch from one weight signal to the 
other. The set point ramp and the weight output from selector circuit 72 
are compared in controller 58 to generate the drive command frequency as 
described above. 
The flow of material through feeder 16, for example, may be readily 
understood by reference to FIG. 4. The flow of material through feeder 18 
is identical to that of feeder 16. The speed of feed screw 15 is 
substantially constant in order to achieve a constant feed rate. Since the 
speed of feed screw 15 is constant, the transit time of material through 
feeder 16 is constant. The speed of feed screw 15 is chosen so that 
material added to inlet 46 of feeder 16 at time t.sub.0 reaches outlet 50 
at time t.sub.0 +T. That is, the transit time of material from inlet 46 to 
outlet 50 is T. It will also be observed from FIG. 4 that diverter valve 
30 is moved after each period T of the set point ramp. Thus, since the 
transit time is T, material added to the feeder during one period is 
discharged during the next period (when no material is added to feeder 
16). Material added to feeder 16 at the beginning of a period T will 
transit feeder 16 in that period, reaching outlet 50 at the beginning of 
the next period. Material added at the end of a period T will transit the 
feeder during the next period and will reach outlet 50 at the end of that 
next period. Thus, there will be no storage of material in feeders 16 or 
18. 
When the system is initially started, controller 58 signals selector 
circuit 72 (by means of the A/B SELECT signal) to send to controller 58 
the weight signal from a preselected one of feeders 16 and 18, for 
example, feeder 18. Thus, the system always begins by "looking at" the 
weight signal of a preselected scale, in this example, always scale 26. If 
there is no material in feeder 18 (for example, because diverter valve 30 
is in the first position), the weight signal from scale 26 will be at its 
minimum, corresponding to point "a" on the weight "B" signal curve in FIG. 
4. Accordingly, controller 58 is programmed to cause diverter valve 30 to 
move to the second position 30' to begin filling feeder 18. Diverter valve 
30 remains in the second position 30' until controller 58 senses that the 
weight of feeder 18 is at a preselected maximum. At that point, diverter 
valve 30 is moved to the first position, feeder 16 begins filling and 
feeder 18 begins discharging. 
When controller 58 senses that the weight of feeder 18 is at the minimum, 
it signals (by means of the A/B SELECT signal) diverter valve control 74 
to energize solenoid 32. Diverter valve 30 is thereby moved to the second 
position 30' and material being discharged by prefeeder 12 is diverted to 
feeder 18 to begin filling it. Simultaneously, controller 58 signals 
selector circuit 72 to transmit to controller 58 the weight signal from 
scale 24. The material in feeder 16 is now discharged until feeder 16 is 
empty and the weight of feeder 16 is at a minimum. While feeder 16 is 
discharging, feeder 18 is being filled. When feeder 16 is empty, feeder 18 
will be at its maximum weight. When controller 58 senses that the weight 
of feeder 16 is at the minimum, it signals diverter valve 74 to 
de-energize solenoid 32. Diverter valve 30 is returned to the first 
position and the material being discharged by prefeeder 12 is now diverted 
to feeder 16, which begins to fill while feeder 18 begins to discharge. 
This cycle then repeats as long as the system remains in operation. The 
way in which controller 58 may be programmed to carry out the 
above-described functions will be readily apparent to persons familiar 
with loss-in-weight feeding systems and need not be described here in 
detail. 
As presently preferred, controller 58 senses minimum and maximum weights of 
feeders 16 and 18 by conventional level detecting techniques. However, it 
is understood that any other method of sensing minimum or maximum such as 
computing change of slope, for example, may be employed without departing 
from the instant invention. 
By utilizing the weight output of scales 24 and 26 only while their 
associated feeders 16 and 18, respectively, are discharging, controller 58 
is able to make pure loss-in-weight measurements on the discharging 
feeder. Controller 58 is always performing pure loss-in-weight 
measurements on one of the two feeders, so that there is no period of time 
in which the system must be operated in volumetric control. 
It should be understood that the invention is not limited to the use of two 
feeders operating at a fifty percent duty cycle. For example, feeder 16 
may have twice the capacity of feeder 18. In this case, the system would 
operate with feeder 16 on a duty cycle of 2/3 and feeder 18 on a duty 
cycle of 1/3 to provide a constant feed rate. Likewise, the example, four 
feeders of equal capacity may be used, each operating on a duty cycle of 
twenty-five percent. Thus, any arrangement of feeder capacity, number of 
feeders and duty cycles may be employed without departing from the scope 
of the present invention. 
It is also possible to operate the system using only a single scale. For 
example, scale 26 may be eliminated. In this embodiment, loss-in-weight 
measurements are made on feeder 16 during the period of time it is 
discharging. That is, weight signal "A" is compared to the set point ramp 
by controller 58 while feeder 16 is discharging. The result of the 
comparison (error signal E) is stored, in conventional fashion, in memory 
within controller 58. When feeder 16 is empty, diverter valve 30 is moved 
to the first position as already described, feeder 16 begins to refill and 
feeder 18 begins to discharge. However, in this embodiment, instead of 
comparing weight signal "B" to the set point ramp when feeder 18 is 
discharging, controller 58 recalls from memory error signal E stored when 
feeder 16 was discharging. The recalled error signal E from feeder 16 is 
used to control the speed of feed motor 22 in feeder 18. That is, because 
feeders 16 and 18 are identical, it is assumed that the weight signal "A" 
during discharge of feeder 16 is identical to the weight signal "B" when 
feeder 18 is discharging. Thus, both feeders 16 and 18 are controlled by 
the weight signal of one of them. The error signal E is updated for each 
discharge cycle of feeder 16. 
This embodiment is of course inherently less accurate than the 
first-described embodiment because during the time feeder 18 is 
discharging no actual loss-in-weight measurements are being made. Thus, 
during the time feeder 18 is discharging, the system actually is operating 
in what may be called "quasi-gravimetric" control, which is an 
approximation of gravimetric control, rather than true gravimetric 
control. However, this embodiment is also less complex than the first 
embodiment, and for certain applications may provide suitable accuracy. 
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.