Patent Publication Number: US-6220162-B1

Title: Method and apparatus for detecting collisions in printing machines

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method and an apparatus for detecting collisions in printing machines. 
     Printing machines typically have a number of rotationally driveable rotary bodies, each of which can be adjusted in a direction normal to the axis of rotation by means of an associated actuator. For example, in a typical flexographic printing machine, a plurality of inking units are arranged at a common reaction cylinder, and each inking unit has two such rotary bodies, i.e. a printing cylinder and an inking roller. During the printing operation the inking roller is in rolling engagement with the printing cylinder, and the printing cylinder itself is in rolling engagement with the printing medium which is guided around the reaction cylinder, so that the ink is transferred from the inking roller to the printing parts of the printing blocks of the printing cylinder and, then, a corresponding printed image is formed. In case of maintenance and retooling, for example, when the cylinders are exchanged, the inking roller is separated from the printing cylinder, and the printing cylinder is separated from the reaction cylinder. To this end, the inking roller and the printing cylinder are moved (adjusted) in a substantially radial direction relative to the reaction cylinder by means of their respective actuators. In this case there is a risk that the inking roller and the printing cylinder collide with one another or with other machine components, so that damages are caused. 
     It is therefore common practice to provide a monitoring system for detecting such collisions and for stopping the corresponding actuators immediately, in order to avoid damages or injuries of the operating personnel. 
     In conventional printing machines the detection of collisions is achieved by monitoring the driving torques of the actuator motors. When the rotary body hits an obstacle during the adjustment process, the driving torque transmitted from the actuator motor is increased, and when this driving torque exceeds a certain threshold value, this indicates that a collision has occurred, and the actuator motor is stopped. 
     The actuator, e.g. a spindle drive, generally has a large transmission ratio, so that even a comparatively small torque of the drive motor generates a high actuating force. Conversely, this means that the increase of the resistance opposing the adjusting movement in case of a collision leads to only a comparatively small increase in the transmitted torque. The collision detection system is therefore relatively slow and inaccurate. Although, in principle, the sensitivity can be increased by lowering the threshold value at which the actuator motor is stopped, this threshold value must always be selected so high that the sometimes considerably high frictional forces which occur during the adjusting movement can be overcome. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to increase the sensitivity in the detection of collisions. 
     In a method according to the present invention, this object is achieved by the feature that the rotary body is caused to rotate during the adjusting movement, and the rotary speed and/or the driving torque for the rotary movement is monitored. 
     When the rotary body collides with an obstacle during an adjustment operation, this blocks not only the further adjusting movement, but it also brakes the rotary movement. By monitoring the rotary speed and/or the torque of the rotary drive for the rotary body, this braking of the rotation can be detected with high sensitivity, so that, in case of a collision, a quicker and more sensitive response of the collision detection system is achieved. Another advantage of this solution is that the response sensitivity does not depend on the location at the circumference of the roller where the collision with the obstacle takes place. When, for example, the rotating body hits the obstacle in a glancing manner during the displacement, the displacement itself is scarcely blocked, but nevertheless the rotation is braked significantly, so that, even in this case, a more sensitive response of the collision detection system is assured. In particular, it is possible in this way to detect situations in which the rotary body is directly touched by an operator. Thus, injuries can be avoided reliably by immediately stopping the actuator and possibly also the rotary drive. 
     The solution according to the invention is particularly advantageous in printing machines of the single-drive type in which a separate drive motor is provided for the rotary drive of each rotary body. In this case, each rotary drive is provided with an angle increment sensor or torque sensor, anyway, for synchronising the rotary bodies, and this sensor and then be utilised as well for the collision detection, so that the construction of the collision detection system can be simplified. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be explained in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic side elevational view of a part of a printing machine to which the invention is applied; and 
     FIG. 2 is a schematic top plan view of the parts of the printing machine shown in FIG.  1  and of the collision detection system. 
    
    
     DETAILED DESCRIPTION 
     The printing machine shown in FIG. 1, for example, a flexographic printing machine, has a frame with two side members  10  and a reaction cylinder  12  rotatably supported therebetween. Attached to each side member  10  is a console  14  on which an inking unit  16  is mounted. In practice, a plurality of inking units can be arranged at one and the same reaction cylinder  12 . 
     The inking unit  16  comprises a printing cylinder  18  and an inking roller  20  with an associated chamber-type ink fountain  22 . The printing cylinder  18  and the inking roller  20  are rotatably supported in bearing blocks  24  and  26  which are arranged to be adjustable in the direction of double-arrows A and B on the top side of the console  14 . FIG. 1 shows the printing machine in a condition in which the printing cylinder  18  is separated from the reaction cylinder  12  and the inking roller  20  is separated from the printing cylinder  18 . During printing operation the printing cylinder  18  is adjusted against the reaction cylinder  12 , and the inking roller  20  is adjusted against the printing cylinder  18 . To enable these adjusting and separating movements, a spindle drive having an actuator motor  28  and  30 , respectively, and a drive spindle  32  and  34 , respectively, is associated with each of the bearing blocks  24 ,  26 . Each of the spindle drives is mounted on the console  14 . 
     As is shown in FIG. 2, the inking unit  16  further comprises two separate drive motors  36  for the printing cylinder  18  and the inking roller  20 . Each of these drive motors  36  is directly arranged on the shaft of the corresponding rotary body  18  or  20  on the drive side of the printing machine (top side in FIG.  2 ), so that each rotary body is driven for rotation by the associated drive motor  36  (single-drive). Synchronisation of the rotary bodies is controlled electronically in a known manner. 
     Each of the drive motors  36  has an integrated torque sensor T which supplies a torque signal to a control unit  38 , as is symbolised by arrows in FIG.  2 . The control unit  38  itself delivers control signals, in particular on- and off-signals to the actuator motors  28  and  30 . In the drawing, only the control signals for the actuator motors on the drive side are symbolised by arrows. It will be understood however that corresponding control signals are also supplied to the actuator motors on the opposite side of the machine frame. 
     When, for example, the printing cylinder  18  is displaced transversely to its rotational axis by means of the actuator motors  28 , collisions are monitored during this adjustment operation as follows. Before the adjustment operation begins, the printing cylinder  18  is driven for slow rotation by the drive motor  36 . The torque generated by the drive motor  36  under these conditions is monitored by means of the integrated torque sensor T and is continuously reported to the control unit  38 . When the outer circumference of the printing cylinder  18  collides for example with the reaction cylinder  12  or the inking roller  20 , the rotary movement is braked, and the detected driving torque is increased correspondingly. As soon as this driving torque exceeds an adjustable threshold value, the control unit  38  supplies an off-signal to the actuator motors  28  and  30 , and the displacement is stopped before the collision can lead to damages. 
     During the adjusting movements of the inking roller  20  the collision detection is performed in the same way. If the printing cylinder  18  and the inking roller  20  are displaced simultaneously, the collision detection for both rotary bodies is also performed simultaneously by the control unit  38 . In this case, the slow rotation of the printing cylinder  18  and the inking roller  20  occurs in the same direction, so that the rotation is braked and, correspondingly, a higher driving torque is generated, when the circumferential surfaces of the printing cylinder  18  and the inking roller come into engagement with one another. 
     The ink fountain  22  can be separated from the inking roller  20  in a known manner by means of an actuator which is not shown. During the adjustment operation the ink fountain is conveniently separated from the inking roller, so that the rotation of the inking roller is not braked by contact with the ink fountain. 
     In a modified embodiment the drive motors  36  have an integrated angle increment sensor Ω in place of the torque sensor T. In this case, the collision is detected on the basis of the decrease of the angular velocity when the printing cylinder and the inking roller, respectively, is braked due to a collision with the obstacle. In this embodiment it is not necessary that the rotary body is permanently driven during the adjustment operation. Since the printing cylinder and the inking roller are supported with low friction in roller bearings, it is sufficient to impart a rotation to the rotary body before the adjustment operation begins, so that the rotary body is coasting during the adjustment operation and the collision can be detected on the basis of an irregular decrease of the angular velocity.