Rod weight control for a cigarette making machine

A control system for a cigarette making machine to provide improved cigarette rod weight control by continual adjustment of a servo-positioned ecreteur knife at the tobacco supply vacuum belt of the cigarette maker. The servo is positioned by a dual mode control system wherein one mode is based on long term feedback of cigarette rod density, measured by a .beta. ray gauge and a second mode based on a high speed adaptive feed-forward control based on tobacco density on the supply vacuum belt measured by a pneumatic gauge.

BACKGROUND OF THE INVENTION 
In the manufacture of cigarettes on modern, high speed cigarette makers, 
extreme care is taken to assure that the cigarette rod is held to a 
specified firmness or density. If the cigarette rod is too dense, 
excessive tobacco will be used and the resulting cigarette may not have 
the desired smoking characteristics such as draft resistance, puff count, 
etc. If too little tobacco is used, the resulting cigarette will be 
undesirably soft, tobacco may fall out of the end of the rod, and as is 
the case with too dense a cigarette rod, the smoking characteristics will 
vary from the designed parameters. 
Modern cigarette machines produce cigarettes at speeds approaching 8000 
cigarettes per minute and an attempt is made to continuously monitor the 
density of the cigarette rod at that speed. Numerous pneumatic devices 
have been suggested such as those disclosed in U.S. Pat. Nos. 3,411,513; 
3,595,067, and 3,850,029 and in British Pat. No. 1,372,056. These devices 
are (1) of the type employing floating nozzles issuing pressurized air 
onto the cigarette wrapper and either measure deformation of the wrapper 
or displacement of the nozzle based on the change in back pressure or (2) 
of a type providing a pressurized on-line chamber arrangement and 
observing pressure changes caused by variation of firmness or dimensions 
of cigarettes passing therethrough. 
Alternate, non-pneumatic approaches to firmness measurement are set forth 
in U.S. Pat. No. 2,667,172 and South African Patent Application No. 
73/9394. These devices suggest the use of tobacco rod forming elements of 
existing machinery for the additional function of providing output 
indication of rod firmness. In U.S. Pat. No. 2,667,172, an elongated short 
tongue is provided with a strain gauge positioned proximate its 
compression foot and a second gauge at the short tongue support clamp or 
beam. These gauges are in spaced longitudinal alignment whereby 
longitudinal pressures exerted by tobacco against the tongue may be 
sensed. This sensing apparatus is separate in function and operation from 
the tobacco rod firmness sensing apparatus of the South African 
Application. The latter apparatus comprises a single tongue support beam 
and foot adapted to provide output indication of vertical strain placed on 
the short tongue, i.e. movement of its foot transverse to the direction of 
movement of tobacco engaged thereby. The South African patent application 
relates also to specially constructed short tongues, disclosing a first 
embodiment wherein the compression foot of a short tongue is split into 
two successive longitudinal sections, each having a separate support 
flange with one support flange having a strain gauge thereon, and a 
further embodiment wherein the front compression foot section is further 
split into three circumferential segments, each having an independent 
support flange with a strain gauge thereon. Such support flanges are stems 
having one end terminating at the compression foot and an opposite end 
terminating at the tongue cantilever support beam. 
A more recent advance to measure the firmness or density of the tobacco rod 
in a cigarette maker is to subject the formed tobacco rod to the radiation 
of beta or other suitable rays which are absorbed by the material in known 
proportion to its mass, and to determine the absorption by an ionization 
chamber. This system is disclosed in U.S. Pat. No. 2,704,079. 
Once a signal is produced, either by a pneumatic sensor, a strain gauge 
type sensor or a beta ray type gauge, it can be suitably manipulated for 
feedback control of the amount of tobacco in the rod and therefore the 
cigarette rod density. This is normally accomplished by using the signal 
to control an ecreteur knife motor to adjust the position of the ecreteur 
knife relative to the tobacco supply vacuum belt. This belt is normally 
provided with excess tobacco and the ecreteur knife is utilized to slice 
off the excess tobacco to provide an accurate quantity of tobacco to the 
cigarette rod forming portion of the cigarette machine and produce rods of 
the desired density. 
Besides sensing the firmness of the finished rod, a further refinement to 
more accurately control rod density employs a second, pneumatic sensor 
adjacent the tobacco vacuum feed belt to determine the density of the 
tobacco on the belt upstream from the ecreteur knife. The feed-forward 
signal form this second sensor is used in conjunction with the signal from 
the rod density control to control the position of the ecreteur knife. 
A cigarette maker presently available which uses two input signals for 
weight control is offered by Molins Limited. The "Modic" controller on 
this machine uses the first signal in a threshold type feedback system 
such that when the signal from a .beta. gauge indicates a rod density 
error of more than 1/2%, the ecreteur knife motor is energized in the 
proper direction to bring the error to within .+-.1/2% of the desired 
density. The ecreteur motor operates at a set speed and therefore responds 
at the same rate to correct minor or major error signals. The second 
signal (pneumatic sensor) drives a separate hydraulic positioner with a 
bellowscontrolled valve to provide more rapid, gross positioning of the 
ecreteur knife in a feed-forward arrangement. 
A second feedback system, currently in use, is the "Accuray" Controller 
manufactured by Accuray Corporation. This system utilizes a digital 
computer to provide integral and proportional processing of the signals 
from a .beta. gauge to provide a loop control output signal for 
controlling the ecreteur motor and thereby provide a system with no net 
error. The "Accuray", like the Molins Modic, may also be employed in 
conjunction with a second, pneumatic, sensor. 
However, these systems are not completely satisfactory in that the ecreteur 
knife positioning motor responds to the control signals at a fixed rate 
irrespective of the degree of error in rod density. Furthermore, there is 
no provision to automatically tune the feedforward systems to compensate 
for process changes. At the high output speeds of current cigarette 
machines, this can result in excessive production out of specifications 
before the error is corrected. 
SUMMARY OF THE INVENTION 
The subject invention relates to a cigarette maker rod density control 
wherein the quantity of tobacco in the rod and therefore the rod density 
is controlled by servo-controlling the ecreteur knife position in response 
to a signal produced by summing a proportional plus integral feedback 
signal from a .beta. rod density gauge and adding to this a feed-forward 
signal from a pneumatic tobacco bed density gauge to provide a high speed 
no-net error density control of a cigarette rod.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing, a portion of a cigarette making machine is 
illustrated including a vacuum tobacco feed belt 3 adapted to feed a bed 
of tobacco 5 to the rod forming area 7 of the cigarette making machine. A 
variable position ecreteur knife 9 is provided to remove or slice off 
excess tobacco from tobacco feed belt 3 such that the resulting cigarette 
rod 11 contains the amount of tobacco necessary to provide the desired rod 
density. 
The ecreteur knife 9 is positioned by a servo 13 which is controlled by 
computer 15. For purposes of the present invention, a high speed servo 
solenoid positioner of the type manufactured by Ledex, Inc., model number 
20, driven by a model 181695-001 controller, may be modified by installing 
lead-lag compensation and increasing the open-loop gain to extend the 
bandwidth and accuracy. This servo will respond to a continuously variable 
signal from the controlling computer rather than to threshold type 
signals. 
The controlling computer 15 may be selected from any number of readily 
available general purpose data acquisition and control computers which 
would be suitable for this purpose, such as the MAC-SYM 350 computer 
manufactured by Analog Devices, Inc. The control computer provides an 
output control signal C(t) to the servo 13 in response to several inputs. 
A first "set-point" input M.sub.c provides a base line signal 
corresponding to the desired cigarette rod density or weight. A second 
input is provided by a .beta. mass density gauge comprising a .beta. ray 
source 17 and a .beta. ray detector 19. Alternatively, the .beta.-ray 
detector, 19, may be arranged so that its output, M.sub.e (t), represents 
the difference between M.sub.c and the actual rod weight. In this case, 
M.sub.c would not appear as a separate input to control computer 15. The 
signal from detector 19 may be filtered through a low pass filter 21 such 
as a model 3750 adjustable filter manufactured by the Krohn-Hite 
Corporation set at a high pass frequency of 1 HZ to provide a low pass 
filtered signal M.sub.1 (t) to the computer. The signal from detector 19 
may also be filtered through a low-pass filter 23 such as the filter 
described above set at a high pass frequency of 25 HZ to provide a 
low-pass filtered signal M.sub.h (h) to the computer. A fourth signal is 
provided to the computer from a vacuum sensor transducer 25, for example, 
a Model DP-15 differential pressure transducer manufactured by the 
Validyne Engineering Corporation. 
This transducer is utilized to provide an indication of the thickness or 
density of the tobacco bed on the vacuum belt 3 upstream of the ecreteur 
knife. The signal from the transducer 25 is filtered by a low-pass filter 
26 of the type described above set in the low pass mode with a hi-pass 
frequency setting of 20 HZ to provide a signal V(t) as the fourth input to 
the computer. 
With the aforementioned inputs to the computer, a dual mode control system 
is provided wherein one mode is based on long-term feedback of cigarette 
rod density and the second mode effects high-speed adaptive feed-forward 
control. 
Operation of the control computer may be understood by reference to the 
drawing in which the dual mode control is apparent. The controller output, 
C(t) is formed by summing two components. Once of these C.sub.1 (t), is 
simply the result of forming the difference between signal M.sub.c and the 
actual rod weight filtered signal M.sub.1 (t) and then applying the 
differnece (or error) to a classical proportional-plus-integral controller 
27 to provide the desired output signal. The signal M.sub.e (t) from the 
.beta. gauge detector is initially filtered to prevent system 
instabilities resulting from a transport lag of about 1/4 second between 
the time tobacco passes the ecreteur knife and the time it is read by the 
.beta. gauge. 
The second control signal C.sub.2 (t), is derived by multiplying the vacuum 
signal, V(t), by a factor k. Since the vacuum signal is related to tobacco 
density measured just before the ecreteur knife, the C.sub.2 (t) signal 
component is capable of responding very rapidly. However, since the 
trimming occurs after the measurement is made, there is no way to 
incorporate feedback in the C.sub.2 computation. This component is thus 
determined in a feed-forward sense. 
In principle, there is a value of k which would cause C.sub.2 (t) to be the 
exact signal necessary to cause rapid rod density variations to be 
minimized for given machine and tobacco filler conditions. However, if 
those conditions change, a slightly different k is needed. Therefore, the 
value of k is continually adjusted in the disclosed control system to 
minimize the variance in the M.sub.h (t) signal. Whether to increase or 
decrease k is determined by digitally processing at block 31 the signal 
M.sub.h (t), the variance of M.sub.h (t) (from block 29), and a version of 
V(t) passed through a time delay block 33. The time delay is set equal to 
the transport lag encountered by the tobacco in moving from pneumatic 
sensor 25 to the .beta.-gauge 19. The processing at 31 involves 
calculating the cross-correlation of M.sub.h (t) and V(t) at the transport 
lag and using the result along with the M.sub.h (t) variance to make 
update decisions on k. Because k is continually updated, this portion of 
the system can be considered an "adaptive feed-forward control". 
With the system described, the control does not operate only when a 
threshold error level is exceeded but operates continuously to yield zero 
average weight error by driving a single high speed servo ecreteur 
positioner. Furthermore, by continually updating the feed-forward gain, k, 
short-term rod density variations are minimized. 
While we have described the preferred embodiment of our invention it is to 
be understood that the invention is not limited thereto but may be 
otherwise embodied within the scope of the following claims.