Abstract:
A method of applying labels to flat objects transported along a conveying path includes determining the acceleration time from the starting time of a pressure and applicator drum as far as the peripheral speed corresponding to the defined, average transport speed of the objects at a circumferential region accommodating the labels and measuring the times at which the front edge of the respective object passes two sensor devices located one after another in the transport path before the pressure and applicator drum and determining the starting time of the pressure and applicator drum in accordance with a mathematical relationship.

Description:
TECHNICAL FIELD 
   The invention relates to a method for the application of self-adhesive labels to flat objects transported along a conveying path, by means of a pressure and applicator drum driven under control. 
   BACKGROUND OF THE INVENTION 
   During the processing of flat items of mail, in particular letters, postcards and so on, by postal delivery companies, the task arises of secure and rapid application of labels to flat items of mail. One example of this is the automatic forwarding of items of mail. In this case, items of mail to be forwarded are sorted out and addressed in accordance with predefined data, which is stored in a database. These items of mail have a label stuck to them, which covers both the old address and a barcode possibly applied to the surface of the item of mail. The label is then provided with a new barcode and the appropriate new address. The application of the label is in this case carried out in apparatus which is integrated into automatic letter distribution systems. The arrival of items of mail in such distribution systems is different in terms of format, weight and thickness. The items of mail are conveyed in such systems at, for example, speeds of 3.6 m/sec, which places high requirements on the speed at which the application of the labels must be carried out, and also on the exact positioning of the labels. Furthermore, the handling and, in particular, the transport of the labels to the surface of the items of mail represents a general problem if the labels have a self-adhesive surface. 
   U.S. Pat. No. 5,200,007 describes an apparatus for the application of labels to flat items of mail, which are conveyed in separated form standing on edge along a conveying path by means of a conveying apparatus. It has a label conveying apparatus for conveying the labels, which are located on a substrate, said apparatus being controlled by a sensor device for determining the front edge of the item of mail and a pressure and applicator apparatus for applying/pressing the labels onto the surface of the items of mail. In this apparatus, the labels are printed with distribution information. Labeling at high transport speeds of the items of mail is not possible with this. 
   A labeling apparatus has also been disclosed in which the address labels are applied to items of mail by means of a round applicator drum which, in its surface, has openings with a controllable vacuum to hold the labels temporarily. The feeding and the cutting of the label webs is carried out in such a way that labels without addresses are not applied (U.S. Pat. No. 4,421,587). 
   DE 36 22 502 A1 describes a labeling head machine in which the labels on the labeling head, which presses the labels onto the object, are held with the aid of openings in the surface of the head, which are connected to a vacuum source. 
   If the label can be pushed onto the pressure and applicator drum only when the latter is at rest, during each application operation it is necessary for the drum to be accelerated from a standstill to a peripheral speed corresponding to the transport speed of the flat objects, at which speed the label is applied to the object transported past the pressure and applicator drum. The pressure roller and applicator drum are then braked in such a way that they come to a standstill in the initial position again after one revolution. 
   On account of various influences, for example belt expansion of a top belt system for transporting letters, the transport speed at which the letters are transported past the pressure and applicator drum varies continuously. 
   If the respective label is always to be placed accurately at a predefined point on the letter, in spite of the speed fluctuations, then a great deal of expenditure on open-loop and closed-loop control is needed for this. 
   SUMMARY OF THE INVENTION 
   The invention is based on the object of providing a method for the application of self-adhesive labels to flat objects transported along a conveying path at fluctuating speed, by means of a pressure and applicator drum driven under control, in which the drive of the pressure and applicator drum is regulated with little expenditure in such a way that each label is applied to the respective object at a defined distance from the front edge of the object. 
   On account of the surprising fact that the labels are applied to the objects with high quality even when the transport speed and the peripheral speed of the pressure and applicator drum differ (up to 50%), the pressure and applicator drum is operated at a fixed application peripheral speed corresponding to an average transport speed of the objects. In order that the desired distance of the label from the front edge of the object is maintained even given fluctuating transport speeds, the acceleration starting time merely needs to be varied appropriately as a function of the measured speed and position of the respective item of mail. 
   Since the pressure and applicator drum always needs to be accelerated to a standard peripheral speed corresponding to the average transport speed, the acceleration time for an acceleration profile is defined in the run-up thereto, taking into account the available drive torque and the moment of inertia resulting from the design of the pressure and applicator drum and is taken into account as a constant in determining the start time. 
   It is advantageous to match the acceleration profile to the periphery of the pressure and applicator drum and the average transport speed to the available acceleration time and the acceleration travel. For this purpose, the pressure and applicator drum is accelerated beyond the peripheral speed corresponding to the average transport speed and is then braked to the peripheral speed corresponding to the average transport speed. 
   It is also advantageous to match the braking phase to time conditions, by the peripheral speed being increased briefly above the application speed. 
   In a further advantageous refinement, a different acceleration profile is defined for each transport speed range. 
   The invention will be explained in more detail below in an exemplary embodiment, using the drawing, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic side view of a device for the application of self-adhesive, substrate-free labels to flat objects, 
       FIG. 2  shows a schematic side view of the device pivoted through 90° with respect to the illustration in  FIG. 1 , with specific times during the transport of an object, 
       FIG. 3  shows the speed-time sequence for the drives of the labeling device. 
   

   DETAILED DESCRIPTION 
   A self-adhesive label strip  1  is drawn off a supply roll  2  in strip form by a label strip unrolling device  3 , known per se, and is supplied to a label conveying and cutting device  4 . 
   In the label conveying and cutting device  4 , the label strip  1  is pushed forward onto a pressure and applicator drum  17  at right angles to the transport plane of the flat objects  20  in accordance with the requisite length and cut off with the pressure and applicator drum  17  at a standstill. To this end, the label strip  1  is guided over a shaped guide  7  which, in the label strip advance direction  5 , changes from a slightly curved or uncurved guide surface at the entry into a guide surface at the exit having curvature matched to the surface of the pressure and applicator drum  17  which accommodates the labels. 
   In the surface of the guide  7  there are openings which are connected to a vacuum source  6 . Integrated into the guide  7  is a motor-controlled friction wheel drive, comprising a friction wheel  8  with drive  9  and pressure roller  10 . The label strip  1  is therefore pushed in the direction of the labeling apparatus  11 , comprising the pressure and applicator drum  17  which, on the accommodating part, likewise has openings connected to a vacuum source  19 , a drum drive  18  and a spring-mounted opposing roll  26 , and is itself changed to a cylindrical shape. 
   After the label strip  1  has been pushed forward with the aid of the friction wheel drive as far as a height  12 , which can be determined freely within certain limits, above the knife device, the knife drive  13  drives the knife  14  over the label strip  1  onto the opposing knife  15  and cuts off a label  16 . The label  16  is then carried only by the pressure and applicator drum  17 . 
   On a suitable transport means, not illustrated, for example a top belt system in which flat objects are transported clamped in, the objects  20  to be labeled are led past the labeling apparatus  11  along a path on the side facing away from the label conveying and cutting device  4 . 
   During the cutting of the label strip  1 , the pressure and applicator drum  17  remains at a standstill. It is then accelerated with the label  16 . The regulation of the movement sequence of the pressure and applicator drum is carried out by means of a controller, as will be described in more detail, in such a way that when the part of the pressure and applicator drum  17  with the label  16  meets the path of the object, specifically accurate acceptance of the self-adhesive label  16  by the object  20  is possible. The circumference of the pressure and applicator drum  17  is shaped outward in a first region and in a second region. The first circumferential region has a circular curvature, the center being located in the axis of rotation. In the second circumferential region, the circumferential surface is displaced inward until disruptive contact between the objects  20  which, for example, are provided with labels in a second, following device, is avoided when in the rest position. This circumferential surface can likewise have the same circular curvature as in the case of the first circumferential region, but the centre is displaced beyond the axis of rotation. The two circumferential regions are connected by appropriate transition regions. The effect of this is that, firstly, in the rest phases, that is to say during label cutting or during application pauses, the pressure and applicator drum  17  cannot project into the path of the objects, and therefore the objects  20  or parts thereof cannot be affected. Secondly, during application, a certain uniform pressure can be exerted against the flat objects in order to transfer the self-adhesive label  16 . In order to assist the transfer of the label to the objects  20 , an opposing pressure can be produced by the corotating pressure roller  26 . 
   This effect, achieved by the different circumferential regions of the pressure and applicator drum  17  with the axis of rotation physically fixed, can also be achieved if the pressure and applicator drum  17  is constructed as a circular cylinder and if it is mounted such that it can move in the direction of the objects  20 . For the purpose of application, it is then moved in the direction of the object such that the label can be pressed on and, in the event of non-application, it is moved back in such a way that contact with the objects  20  is avoided. This embodiment is not illustrated in the drawing, but can readily be implemented by those skilled in the art on the basis of their specialist ability. 
   A labeling device for letters is illustrated schematically in  FIG. 2 . 
   The labeling procedure always follows the following steps:
     1. label advance;   2. cut label;   3. measure current speed, determine the starting time for the pressure and applicator drum  17  and start the drum at the calculated time; in this case the movement sequence is a complex procedure which is composed of repeated acceleration, braking and constant speed phases;   4. apply the label;   5. brake the drum and stop it in the initial position.   

   The following illustration describes the basic labeling procedure. 
   The sequences of the labeling procedure are controlled by the course of the letter. The times t i  specified in the following text vary with the dynamics of successive labeling procedures.
     1. At the time t 0  (determined by the letter tracking of the machine control system), the higher-order machine control system sends a message related to the item of mail to the labeling controller, with the following information:
       letter ID,   label/no label,   label height.   
       2. The passing of the front edge of the item of mail t 1  at light barrier LB 1  starts the label advance. If the label advance is carried out at the end of the preceding application procedure, then it must be ensured that the pressure and applicator drum has reached the initial position before the start of cutting.   3. Start the knife drive at the time t 2 . The movement of the knife is synchronized such that the cutting begins only after the label advance has been completed. The knife drive completes a 360° revolution during each cut; the end position is monitored by a position transmitter.   4. Measurement of the current speed of the item of mail, using the light barriers LB 2  and LB 3 . From the measured running time t 4 −t 3 , which is a measure of the current speed, the starting time t 5  for the pressure and applicator drum  17  is calculated. This measurement ensures higher accuracy as compared with the usual indirect measurement of the speed via the belt cycle rate, since the belt is subject to oscillations.   

   Surprisingly, it has been established that a differential speed between the course of the letter and pressure and applicator drum  17  is uncritical in wide limits (&lt;50%) for the labeling application procedure. On this basis, for specific belt speed ranges, in each case a constant pressure and applicator drum speed is used as a basis in the concept. The belt speed-dependent correction of the horizontal label position is carried out by varying the starting time of the drum in relation to the front edge of the item of mail.
     5. Start the applicator drum t 5 . The acceleration procedure takes place in accordance with a trajectory determined in advance (off-line) and ensures the appropriate horizontal positioning accuracy. In this case, the front edge of the label meets the letter (merge point) at the time t 6 .   6. After the end of the application t 7 , the start of the braking phase takes place at the time t 8 , until the pressure and applicator drum  17  is located in the initial position again.   7. After the label has been applied to the item of mail, the pressure and applicator drum is moved into the initial position. This position is monitored by a position transmitter.   8. A label checker (FMS) comprises a colored mark sensor and reference light barrier (LB 4 ) for the front edge of the letter. The front edge of the letter is determined at the time t 7  and the front edge of the label is determined at the time t 8.  The difference represents the label position on the item of mail at the current measured speed v i  . Items of mail with missing labels or excessively large positioning deviations of the label are rejected. Systematic positioning deviations are used for the long-term readjustment of the applicator controller.   9. After t 8  a result message is sent to the higher-order machine control system, with the following content:
       letter ID (0 represents ‘phantom item of mail’),   label status (OK, failed),   label position (based on the front edge of the item of mail).   
       

   The speed-time sequence for the drives directly responsible for the labeling procedure is illustrated in  FIG. 3 .
         The cycle is triggered by a start message and begins with the label advance (t 11 ). During the advance, the speed of the item of mail is measured and the knife drive is started (t 12 ),   cut after the completion of the advance (t 12a , t 12b ),   drum start (t 15 ) after the end of the cut (t 12b ),   the label meets the item of mail at the time t 16  and is then rolled onto the item of mail,   the following label cycle begins even before the initial position of the label drum has been reached (t 20 ),   the drum initial position is reached (t 19 ),   in order to shorten the braking time of the pressure and applicator drum, this is again briefly accelerated above the application speed and then braked.       

   Here, the meanings are as follows:
     t 10  is the mail ID message from the higher-order control system   t 11  start the label advance   t 12  start the knife drive   T 12a , t 12b  are the start, end of the label cut   T 15  start the pressure and applicator drum   T 16  start the label application (merge point)   T 17  end of the application   T 18  start of the braking phase   T 20  start label advance of the next labeling cycle   T 19  reaching the drum initial position   

   In normal operation, the drive belts run at a constant speed. The current speed of the item of mail is determined by a run-time measurement between the measuring light barriers LB 2  and LB 3 . 
   The current speed of the item of mail is determined by the measurement of the run time t 9  of the item of mail between the measuring light barriers LB 2  at the position x 3  and LB 3  at the position x 4 . In the process, for a short time a constant speed of the item of mail between the light barrier LB 2  and the pressure and applicator drum  17  is assumed.
 
 t   9   =t   4   −t   3   (I)
 
   After the item of mail has passed the second measuring light barrier at x 4 , the run time (t 64 ) to the merge point (x 6 ) can be projected. 
               t   64     =         t   6     -     t   4       =       t   9     *         x   6     -     x   4           x   4     -     x   3                     (   II   )             
 
   The control concept assumes that, for a fixed range of the belt speed, the pressure and applicator drum  17  is always accelerated with the same speed trajectory and therefore always needs the same time from the start as far as the merge time. The drum acceleration time T Tr  is therefore a constant parameter from the belt speed range. The starting time t 5  of the drum, based on t 4 , is then given by (III).
 
 t   5   =t   4   +t   64   −T   Tr   (III)