Patent Abstract:
A method for separating out foreign bodies from a material flow includes generating a material flow and transferring the material flow to a measuring section. At least one foreign body is detected in the material flow at the measuring section and the foreign body is removing from the material flow. The method includes moving the airflow along with and at substantially the same speed as the material flow between the detecting and removing steps.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority of German Patent Application No. 10 2004 017 595.0, filed on Apr. 7, 2004, the subject matter of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to a method and an apparatus for separating out foreign objects from a material flow. A method and apparatus of this type are disclosed, for example in German patent document DE 199 18 774 A1 and corresponding U.S. Pat. No. 6,332,543 B1.  
       SUMMARY OF THE INVENTION  
       [0003]     It is an object of the present invention to provide an efficient method and apparatus for separating out foreign bodies from a material flow.  
         [0004]     The above and other objects are accomplished according to the invention by the provision of a method for separating out foreign bodies from a material flow, comprising: generating a material flow; transferring the material flow to a measuring section; detecting at least one foreign body in the material flow at the measuring section; removing the foreign body from the material flow; and moving an airflow along with and at substantially the same speed as the material flow between the detecting and removing steps.  
         [0005]     By generating or providing an airflow with substantially the same speed as the material flow, at least for the time period between detection and removal of the foreign bodies, it is ensured that lightweight material flow components such as tobacco strips, which have a higher air resistance than foreign bodies such as rocks, do not travel different distances between the steps of detecting and removing of the foreign bodies. A certain amount of time is generally required for the detection of foreign bodies, so that a corresponding spatial and time distance exists between the location for detecting the foreign body and/or the measuring location, and the location where the foreign body is separated out from the flow. The method according to the invention thus prevents different material-flow components from moving at different speeds and furthermore prevents the separating out of the wrong components. Within the framework of this invention, the material flow refers in particular to a tobacco flow.  
         [0006]     The method can be realized particularly easily if the material flow in the measuring section follows a free-fall trajectory. On the other hand, it is possible for the apparatus to have a relatively flat structural design if the material flow in the measuring section follows a parabolic flight trajectory.  
         [0007]     The material flow speed and the airflow speed in the measuring section increases, in particular if the gravitational force acting upon the material contributes to the acceleration.  
         [0008]     The flow of air in the measuring section is preferably generated from the outside, for example by blowing air into or suctioning air out of the measuring section. This airflow can help generate an airflow for which the speed matches that of the material flow.  
         [0009]     For an embodiment where the measuring section is oriented at least partially in the direction of the earth&#39;s center, it is advantageous if the material flow in the measuring section moves inside a channel having a decreasing cross section in the material-flow direction, at least in some sections. Owing to the fact that the material flow moves in a downward direction, at least in some sections, the gravitational pull of the earth will accelerate the downward movement of the flow. To prevent the various components of the material flow from moving at different speeds, as a result of different air resistances, this embodiment of the method, makes use of the fact that the flow speed inside a channel with decreasing cross-sectional area will automatically increase. The cross-sectional area of at least some sections is preferably a function that depends on the square root of the drop depth for the material flow.  
         [0010]     The object is furthermore achieved with an apparatus for separating out foreign bodies from a material flow, comprising: a feeding device including a channel having an inlet for receiving the material flow and having an end with an outlet, the channel cross section being reduced in a movement direction of the material flow, at least in some sections; a foreign body detection device having a scanning zone in the channel to detect foreign bodies in the material flow; and a removing device having a separating out location at the outlet to separate out the foreign bodies from the material flow.  
         [0011]     An extremely efficient separating out of foreign bodies is consequently possible. Within the framework of this invention, the tobacco-flow outlet side in particular refers to the channel outlet in a downstream direction of the flow.  
         [0012]     The apparatus according to the invention can be realized particularly easily if the channel has a rectangular cross section, wherein it is advantageous if the channel is positioned substantially vertically, at least in some sections. It is furthermore advantageous if an impact surface is provided on the channel inlet side for deflecting a tobacco flow, for example supplied by a standard conveyor as described in German patent document DE 199 18 774 A1, wherein the flow is subsequently conveyed in the direction of the channel.  
         [0013]     The apparatus can have a relatively flat structural design if at least one channel wall is designed to follow at least in some sections a parabolic flight trajectory.  
         [0014]     The cross-sectional area of a particularly preferred embodiment of the device according to the invention is a function that depends on the square root of the drop depth for the material flow, at least in some sections. A suction-air generator or a blast air generator is preferably provided to help generate a corresponding conveying airflow in the channel.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The invention is described in the following without restricting the general inventive idea with the aid of embodiments and by referring to the drawings, to which reference is expressly made for all details not further explained in the text.  
         [0016]      FIG. 1  is a schematic cross-sectional representation of first embodiment of the device according to the invention, and  
         [0017]      FIG. 2  is a schematic cross-sectional representation of a second embodiment of the device according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]      FIG. 1  shows a schematic cross-sectional representation of a first embodiment of the apparatus according to the invention for separating out foreign bodies. The material flow  10  is conveyed on a conveyor belt  13 , that moves in the direction of the arrows and is deflected by a deflection roller  14 , wherein the material is conveyed in the x direction until it hits an impact surface  30 . The material flow  10 , e.g. a tobacco flow containing tobacco  11  and foreign bodies  12 , subsequently drops vertically downward into a chute and/or channel  20 . The channel  20  comprises walls  27  and  28 , having a constant depth in z direction which is perpendicular to the drawing plane for this embodiment. The width x in this case is a function of the drop depth y, meaning it is a square root function which is explained in further detail below.  
         [0019]     The material flow  10  drops downward and moves through a measuring section  15  which comprises a scanning zone  25  and ends in a separating-out location  26 . A window  22  is provided in the region of scanning zone  25  for allowing the light  23 , emitted by an optical detection device  21 , to pass through. The optical detection device  21  can comprise a laser for emitting a light beam  23  as well as a CCD line for measuring the light beam  23  reflection on the material flow  10  passing by. The CCD line preferably ensures the detection of foreign bodies  12  in the material flow  10  by means of a local resolution in z-direction. The optical detection device  21  can be embodied as described in German Patent document DE 199 18 774 A1, U.S. Pat. No. 6,332,543 B1, and German Patent Application 10 2004 015 463.5. The aforementioned patents and applications are herewith incorporated by reference into the present application.  
         [0020]     Thus, it is easy to compute the time interval required for a detected foreign body  12  traveling at a predetermined material flow speed v in the movement direction  17  to reach the separating-out location  26 . To eliminate the role of the air resistance, the speed of airflow  16  must match the speed v of the material flow  10 , wherein this speed v increases with increasing drop depth y as a result of the gravitational pull of the earth.  
         [0021]     At the separating-out location  26 , the foreign body  12  is separated out or blown out into the separating-out chute  32  by a schematically indicated blow-out nozzle  24 . The material flow  10  which is freed of foreign bodies  12  then enters the conveying-away chute  31  in the form of a tobacco flow  33  composed of tobacco  11  and cleaned of foreign bodies  12 .  
         [0022]     A different type of foreign-body detection device can also be used in place of an optical detection device, e.g. a device using heat, sound waves, or microwaves.  
         [0023]     The respective airflow  16  for the embodiment shown in  FIG. 1  is automatically increased or is generated by the airflow  16  carried along by the material flow  10 . As a result, it is ensured that tobacco strips which have a higher air resistance due to a lower specific weight do not move at a slower speed than, for example, foreign bodies  12 . It is therefore possible to precisely define the time difference between the instant when the foreign bodies are detected and the instant when they are separated out and/or removed at the separating-out location  26 . Without this correspondingly adapted airflow  16 , an undesirable amount of material (tobacco) would be separated out.  
         [0024]     The apparatus described in German Patent document DE 199 18 774 A1 is used for measuring the material flow during a free fall, wherein a certain amount of time is required for evaluating the measuring signals. The material-flow components are subsequently blown out at a downstream location on the flight trajectory. In the process, the free-falling material is subjected to air resistance and the components in the flow of material are consequently delayed. Since the air resistance is primarily determined by the surface area, material components with large surfaces, such as tobacco strips, experience a higher delay than equally heavy components with a smaller surface, e.g. stones. Owing to this phenomenon, different types of material components move at different speeds between the detection location and the blow-out location. Thus, the material flow components in the apparatus disclosed in the foregoing German Patent document are subject to a type of speed dispersion, wherein this speed dispersion makes it impossible to precisely define the flight trajectory time for the various material flow components. As a result, material not representing a foreign body is erroneously blown out while at the same time foreign bodies are not blown out at all.  
         [0025]     By contrast, the solution according to our invention avoids the speed dispersion, thus preventing the blow-out of material not representing a foreign body while ensuring that foreign bodies are definitely blown out. The invention involves intentionally generating an airflow having a trajectory that matches the trajectory for the material flow and/or the material component to be measured, meaning it moves at the same rate and in the same direction. The material flow and/or the measured material consequently is not delayed by the surrounding air, thereby avoiding the speed dispersion.  
         [0026]     The material flow  10  for the exemplary embodiment shown in  FIG. 1  is initially supplied horizontally on the conveyor belt  13 , wherein the surface is positioned at height level y=0. At the end of the conveyor belt  13 , the material drops into a chute with a rectangular cross section A(y)=x(y)·z, wherein z is assumed to be constant. The chute cross section x(y) must then be configured as follows.  
         [0027]     The drop speed without air resistance is v y ={square root}{square root over (2·g·y)}, wherein g is the acceleration due to gravity. The air speed should equal the drop speed at the blow-out location  26  y A , thus making the drop speed at the blow-out location: v L (y A ).  
         [0028]     The following applies based on the equation of continuity in fluid mechanics:  
                 A   ⁡     (   y   )       ·       v   L     ⁡     (   y   )         =         A   ⁡     (     y   A     )       ·         v   L     ⁡     (     y   A     )       ⁢           ⁢     ⟶     z   =     const   .         ⁢     x   ⁡     (   y   )         ·       v   L     ⁡     (   y   )         =         x   A     ⁡     (     y   A     )       ·       v   L     ⁡     (     y   A     )                   (   1   )             
 
 The following is obtained from this:  
                 x   ⁡     (   y   )       =         x   A     ⁡     (     y   A     )       ·         2   ·   g   ·     y   A             2   ·   g   ·   y             ⁢     
     ⁢     and   ⁢     /     ⁢   or     ⁢     
     ⁢       x   ⁡     (   y   )       =         x   A     ⁡     (     y   A     )       ·           y   A     y       .                 (   2   )             
 
         [0029]     If the cross section and/or the width x in the embodiment according to  FIG. 1  is realized according to the last-mentioned formula, then the air speed equals the drop speed without air resistance, thus avoiding the speed dispersion. The channel  20  is left open in the upper region and the side wall  28  does not reach up to y=0. As a result of the impact surface  30  and the low speed in the range of y=0, the error that occurs is negligible.  
         [0030]      FIG. 2  shows an embodiment where the material flow  10  on the conveyor belt  13  moves with the speed v o  at the location where the material flow  10  leaves the conveyor belt  13 . The material flow  10  then follows a parabolic flight trajectory downward, toward the right side of  FIG. 2 , wherein the surface of conveyor belt  13  is again at the height level y=0. At point x=0 and y=0, the material in the material flow  10  leaves the surface of the conveyor belt  13  and moves along a parabolic flight trajectory. Without air resistance, the material in the material flow  10  moves along a parabolic trajectory which can be described by means of the following dependencies: 
        x=v 0 ·t     v x =v 0  (speed in x direction)  
         v   y     =         -   g     ·     x     v   0         ⁢           ⁢     (     speed   ⁢           ⁢   in   ⁢           ⁢   y   ⁢           ⁢   direction     )           
         
         [0033]     From this it follows:  
       v   =         v   0   2     +       (     g   ·     x     v   0         )     2             
 
         [0034]     The embodiment shown in  FIG. 2  shows a flow of air generated above the material and/or by the material in the material flow  10 , wherein the airflow speed and direction match the speed and direction of the material flow in channel  20 . This can be achieved by using the following formula to change the width d(x) of channel  20 , having a corresponding rectangular channel cross section and a constant depth z:  
               d   ⁡     (   x   )       =       d   0         1   +         g   2       v   0   4       ⁢     x   2                     (   3   )             
 
 wherein d(x) should behave accordingly, at least in the region between the scanning zone  25  and the separating-out location  26 . 
 
         [0035]     The invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art, that changes and modifications may be made without departing from the invention in its broader aspects, and the invention, therefore, as defined in the appended claims, is intended to cover all such changes and modifications that fall within the true spirit of the invention.

Technology Classification (CPC): 1