US 3783373 A
A device for monitoring density of a rod of tobacco in a cigarette-making machine comprises a source of microwave radiation and a waveguide through which said radiation is propagated in the evanescent mode to a detector. The rod is passed across the path of the radiation, through apertures in the walls of the waveguide. The radiation may be split into two beams, one beam being directed through said waveguide and the other beam directed through a second, similar waveguide to a second detector providing a reference signal.
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Description (OCR text may contain errors)
United States Patent 3,783,373
Jawor 1 Jan. 1, 1974 FOR DENSITY 3,115,131 12/1963 Holliday 324/58 A AND MOISTURE 3,265,873 8/1966 Sawyer 324/585 A X 75 l T d B I L d 3,265,967 8/1966 Heald 324/58 A X t 1 1 or 2 32: mm Jawo" FOREIGN PATENTS OR APPLICATIONS 656,110 1/1963 Canada 324/585 A  Assignee: Molins Limited, Deptford, England 22 i F b 8 1972 Primary Examiner-Stanley T. Krawczewicz AttorneyPaul M. Craig, Jr. et a].
 Appl. No.: 224,798
-  ABSTRACT  Foreign Application Priority Data A device for monitoring density of a rod of tobacco in I Feb. 11, 1971 Great Britain 4495 71 a Cigarette-making machine comprises a Source of crowave radiation and a waveguide through which 52 us. Cl 324/585 A said radiation is propagated in the evanescent mode to [51 Int. Cl G0lr 27 04 a deteeter- The red is Passed across the P of the  Field of Search 324/585 A, 58 A diatieh, through apertures in the Walls of the Waveguide. The radiation may be split into two beams, one 56 References Cited beam being directed through said waveguide and the UNITED STATES PATENTS other beam directed through a second, similar waveguide to a second detector providing a reference sig- 2,532,8l7 12/1950 Lafferty et a1. 324/585 A naL 3,034,046 5/1962 Sasaki 324/585 A 3,060,421 10/1962 Rideout 324/585 A UX 6 Claims, 2 Drawing Figures SOURCE FZ/ DUMMY LOADi DIRECTIONAL e 1:] H ER 1 COUPLER I --:/H/Z;DET1E 1;;1; TUNED ANALOG E [QZ //0 //3FIIIEI can /0/ T 1 7 D o 1 //2 I W /zs w w A5? /2/ W was rum /Z6 FILTER A33 ANALOG 5/ urnvonx APPARATUS FOR MEASURING DENSITY AND MOISTURE This invention relates to devices for monitoring the density of a rod of material to be tested, e.g. a rod of tobacco in a continuous-rod cigarette-making machine. As will later become apparent, a device embodying the invention may also be used to monitor density of fluids.
In a continuous-rod cigarette-making machine, which forms tobacco into a continuous rod, wraps the rod in paper and cuts the wrapped rod into lengths constituting individual cigarettes, it is necessary to obtain the least possible variation of the mass per unit length of the tobacco rod from its desired value. Hence various means have been devised for monitoring the mass per unit length of the tobacco rod (for convenience termed the density) and for controlling feed and/or trimming devices in accordance with the density detected. Notably, a device has been employed in which Beta-radiation is passed through the rod and the emergent radiation employed to generate a control signal for the tobacco feed and/or trimming devices, as the attenuation of the Beta-radiation passing through the tobacco varies with the tobacco density. While such Beta-ray devices have proved satisfactory in many ways, there are inconveniences due to the presence of radioactive material as a source of Beta-radiation and some expense is involved in handling such material with proper safety precautions. Moreover, the radiation emission level is not truly constant as Betaradiation is essentially a random emission of particles with a continuous and non-uniform energy spectrum.
It is an object of the present invention to provide an improved device for monitoringtobacco rod density, not requiring the use of radioactive material.
According to the invention, there is provided a de vice for monitoring the density of a rod of tobacco or the like in a continuous rod cigarette-making machine, comprisinga source of microwave radiation, a detector for said radiation, and a waveguide providing a path for said radiation from said source to travel to said detector, in which said waveguide includes a portion of such narrow cross-section that said radiation is propagated therethrough in the evanescent mode, said portion of waveguide having apertured walls to permit said rod to be fed across the path of said radiation.
By use of microwave radiation, no radioactive material is required and, of course, the source may be deenergised whenever the monitoring device is not required to operate.
The detector provides an output signal which will be affected by any change suffered by the microwave radi ation in its passage through the waveguide and, in particular, will be affected by any attenuation of the radiation caused by the presence of the rod of tobacco or the like. Such attenuation may arise from absorption of the radiation by the tobacco and/or detuning of the waveguide i.e. alteration of its Q factor by the presence of the tobacco.
It is desirable to provide automatic compensation for any fluctuation in the output of the source. Preferably the source includes a microwave generator (e.g. a Gunn device in a cavity resonator) delivering its output to a directional coupler arranged to split the microwave radiation into two beams of equal intensity. With this arrangement, the two beams are directed to separate detector devices via separate but similar waveguides,
said waveguides having apertured walls to permit the tobacco rod to traverse one of the waveguides so that at least part of the microwave beam travelling through that one waveguide must pass through the rod, and thus the detector device receiving the beam through said one waveguide produces a measuring signal which varies with changes of rod density. The detector device associated with the other waveguide produces a reference signal, and comparison of the measuring signal and the reference signal (e.g. by subtracting one from the other) yields a control signal which is unaffected by any change in the output of the source.
In that portion of the waveguide which is of narrow cross-section, so that the radiation is propagated therethrough in the evanescent mode, the characteristic impedance is for practical purposes purely inductive, so that by providing appropriate capacitance (e.g. one or more tuning screws) complete transmission of the microwave energy may be obtained over a narrow frequency band. The reduced cross-section of this portion makes it an especially convenient location for the apertures in the walls of the waveguide through which the tobacco rod may be fed.
The performance of a device embodying the invention is liable to be adversely affected by variations in moisture content of the tobacco, as a change of moisture content may affect the device in the same way as a change of tobacco density. In circumstances where the moisture content is liable to vary, the monitoring device may be duplicated, the two parts of the device generating at different frequencies (e.g. l0 GHs and 30 GI-Iz) and from the two signals produced secondary signals may be derived (e.g. by an analogue network) to indicate the tobacco density and moisture content separately. In such a duplicated device, the arrangement is such that the lower-frequency radiation (e.g. l0 Gl-Iz) is propagated in the evanescent mode in the narrow portion of the waveguide, but the higher-frequency radiation (e.g. 30 Gl-Iz) is propagated in the normal mode throughout. I
In order that the invention may be well understood, preferred embodiments thereof will now be described with reference to the accompanying diagrammatic drawings, in which:
FIG. 1 illustrates one form of monitoring device embodying the invention, and
FIG. 2 illustrates a more complex device.
Referring first to FIG. 1, a source 1 of microwave radiation has its output connected directly to a 3db directional coupler 2 also connected to a dummy load 3 and to two waveguides 4, 5. The microwave radiation delivered by the source 1 is accordingly divided equally between the two waveguides 4, 5 and a microwave beam passes through each of said waveguides to a pair of detector units 6, 7; the detector unit 6 is coupled to the waveguide 4 and contains a diode 8 delivering its outputs to an amplifier 10, while unit 7 is coupled to the waveguide 5 and contains a diode 9 similarly connected to an amplifier 11. Corresponding parts of the device are mutually similar, i.e. waveguides 4 and 5 are of similar dimension to one another, diode 8 is similar to diode 9, and amplifier 10 is similar to amplifier 11.
Each of the waveguides 4, 5 is of such dimensions as to operate in the evanescent mode, e.g. the source 1 conveniently operates at a frequency of 10 GHz, and for this frequency each of the waveguides 4, 5 may have an inner cross-section 8.5 mm. by 4.0 mm. and a length of 34 mm. Each of said waveguides is provided with four tuning screws 12. Halfway along each of the waveguides 4, the side walls of the waveguide have mutually similar apertures 13, 14 respectively of a size to permit passage of a tobacco rod (travelling normal to the plane of the drawing) of which the density is to be monitored. Adjacent to these apertures each waveguide has screens closing the waveguide against entry of foreign bodies, said screens being of low-loss glass to minimise attenuations of the microwave beams. If desired, wavetraps (not shown) of conventional form may be fitted to the apertures 13, 14 to minimize escape of microwave radiation. It may also be of advantage to provide a polarisation filter (not shown) between the source 1 and coupler 2 to reduce the possibility of reflected radiation returning to the source; such a filter may also be provided between each of the wave-guides 4, 5 and its associated detector unit 6, 7, as in part the presence of the tobacco rod may rotate the polarisation plane of the microwave radiation, hence a polarisation filter in the position mentioned serves to increase the effect of the rods presence on the radiation level at the detector unit.
It should be noted that the source 1 includes means for modulating the microwave output, as this allows the amplifiers 10, 11 to be ac amplifiers. With a microwave frequency of IOGI-Iz, the modulation may conveniently be a IOKI-Iz square wave, e.g. provided by a simple multivibrator. If desired, diodes or other detectors may be provided at the outputs of the amplifiers 10, 11 so that the output signals are do. voltages; this is especially convenient as a comparing device 16, e.g. an analogue network can be coupled directly to these outputs to derive therefrom a single signal representa! tive of the rod density.
Now turning to FIG. 2, this'illustrates a more complex monitoring device, also embodying the invention, which provides for the fact that material such as tobacco normally contains moisture which affects the results obtained with a device such as that of FIG. 1 so that the device of FIG. 1 cannot be used to determine the density of the material unless the proportion of moisture present is separately determined.
The device shown in FIG. 2 includes many parts corresponding to those found in FIG. 1, namely a source 101, coupler 102, dummy load 103, two waveguides 104', 105, two detector units 106, 107, two diodes 108, 109, and two amplifiers 110, 111 (corresponding respectively to the parts 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11 of FIG. 1). Additionally, a second microwave source 121 is provided, delivering its output to a hybrid (magic-T") junction 122 from which said output divides equally in the two possible directions of propagation along a waveguide 123 extending at right-angles to the waveguides 104, 105. The waveguides 104, 105 are apertured in the same manner as the waveguides 4, 5, of FIG. 1 so that a tobacco rod extending normal to the plane of the drawing may pass across the path of microwave radiation travelling along the wave-guide 104 and so that the two waveguides 104, 105 are mutually similar although no rod is passed through the apertures of waveguide 105. Also, however, the waveguides 104, 105 are apertured to permit passage of microwave radiation travelling along the waveguide 123 which intersects both of the waveguides 104, 105. The hybrid junction 122 is symmetrically placed between the waveguides 104, 105 and the waveguide 123 extends for equal distances on either side of the junction 122, the portions of the waveguide 123 are functionally separate waveguides providing mutually similar paths for microwave radiation from the junction 122 to each of two detector units containing diodes 124, symmetrically mounted adjacent to the ends of the waveguide 123 and serving as detectors, coupled respectively to amplifiers 126, 127.
Each of the sources 101, 121 is arranged to produce a modulated microwave output, the carrier frequencies and modulation frequencies being however different. Conveniently for example the source 101 may produce a IOGHz carrier modulated at IOKHz, while the source 121 produces a 3OGI-lz carrier modulated at lMHz. The diodes 108, 109, 124 and 125 all serve as demodulators so that the amplifiers 110, 111, 126, 127 are all a.c. amplifiers operating at the modulation frequencies, i.e., amplifiers 110, 111 operate at lOKI-Iz and amplifiers 126, 127 at lMI-Iz. The outputs of the four amplifiers 110, 111, 126, 127 are fed into tuned filters 112, 113, 128, 129 respectively each of which filters is sharply tuned to the appropriate modulation frequency to minimize any cross-talk and includes a rectifier at its output so that the four tuned filters deliver do. outputs. These do. outputs are fed to a pair of analogue comparing circuits 114, 130, the circuit 114 receiving the outputs from filters 112, 113 and the circuit 130 receiving the output from filters 128, 129. Each of the analogue circuits is arranged to produce a single output representing the ratio between the two do. signals it receives. The two signals supplied tothe circuit 114 comprise an information signal (i.e., one derived from microwave radiation which has passed through waveguide 104 and hence through the tobacco rod) and a reference signal (i.e. one derived from microwave radiation which has passed through waveguide 105 and thus has not passed through the rod, but otherwise has been treated in exactly the same manner as the radiation from which the information signal is derived). The circuit 114 produces an output representing the ratio;
Information signal/Reference signal R and it can readily be shown that R l (aV bW) where V and W are respectively the volumes of solid (tobacco) and of water in the path of the radiation from which the information signal is derived, and a, b are constants. A similar equation, with different constants, is applicable to the output from the circuit 130, hence these two equations enable the values of V and W to be found from the values of the outputs from circuits 114, 130. Conveniently computing means in the form of an analogue network 131 for solving said equations may be connected to receive the outputs of the circuits 1 14, 130, allowing the values of V and W to be continuously calculated and delivered (as varying voltages or otherwise) at separate outputs 132, 133 as a rod is moved across the path of the microwave radiation travelling along waveguide 104.
The waveguide dimensions are such that the IOGHz radiation from source 101 is propagated through the waveguides 104, 105 in the evanescent mode while the 30GHz radiation from source 121 is propagated through the waveguide 123 in the normal mode.
What I claim as my invention and desire to secure by letters Patent is:
l. A device for monitoring the density of a rod of tobacco or the like in a continuous rod cigarette-making machine, comprising a source of microwave radiation, a first detector for said radiation, a waveguide providing a path for said radiation from said source to travel to said detector, in which said waveguide includes a portion of such narrow cross-section that said radiation is propagated therethrough in the evanescent mode, said portion of waveguide having apertured walls to permit said rod to be fed across the path of said radiation, a second detector, means in the form of a directional coupler for splitting the radiation from said source into two beams and directing one of said beams into said waveguide, a second similar waveguide arranged to receive the other of said beams and provide a path for said other beam to travel to said second detector, means for comparing output signals delivered by said first detector with output signals delivered by said second detector, a second source of microwave radiation, a further detector, and a further waveguide arranged to receive radiation from said second source and providing a path for said radiation to travel to said further detector, said second source being arranged to operate at a frequency different from that of the firstmentioned source and said further waveguide being arranged to intersect the narrow portion of said waveguide so that said rod can be fed simultaneously across the paths of radiation in both said narrow portion and said further waveguide.
2. A device as claimed in claim 1, in which said secand source is arranged to operate at such a frequency that microwave radiation therefrom is propagated through said further waveguide in the normal mode.
3. A device as claimed in claim 1, including an additional detector, an additional waveguide similar to said further waveguide and providing a path for radiation from said second source to said additional detector, and further means for comparing output signals delivered by said further detector with output signals delivered by said additional detector.
4. A device as claimed in claim 3, including computing means connected to receive outputs from both the comparing means and from the further comparing means.
5. A device as claimed in claim 4 in which said computing means is an analogue network.
6. A device as claimed in claim 3 in which said further waveguide and said additional waveguide are integral with one another and extend in opposite directions from the second source.