Abstract:
The present invention relates to a device and a method for synchronizing processes which are executed on a plurality of units ( 2   a,    2   b,    2   c ), in particular, in paper-processing machines, the units ( 2   a,    2   b,    2   c ) being connected to a central control unit ( 1 ) via a data bus ( 6 ). The present invention is characterized in that the central control unit ( 1 ) is designed for distributing movement commands via a data bus ( 6 ) to a plurality of units ( 2   a,    2   b,    2   c ), the movement commands containing the instant at which these commands are executed, and that provision is made for the units ( 2   a,    2   b,    2   c ) which keep available the absolute time to synchronously execute these commands at the instant communicated via the commands.

Description:
[0001]    Priority to German Patent Application No. 101 56 209.8, filed Nov. 15, 2001 and hereby incorporated by reference herein, is claimed.  
         BACKGROUND INFORMATION  
         [0002]    The present invention relates to a device for synchronizing processes which are executed on a plurality of units, in particular, in paper-processing machines and a method for synchronizing a plurality of such units.  
           [0003]    Printing presses have a plurality of rotationally movable parts which have to be set into rotary motion in an angularly synchronous manner. To this end, in the past, these parts to be rotationally driven were coupled via a mechanical connection as, for example, a line shaft or a continuous gear train, which, however, had the disadvantage, among other things, that unpleasant vibrations occurred in such a machine. Therefore, recently the trend for the printing press parts to be rotationally driven is no longer to be mechanically synchronized with perfect angular accuracy but to be provided with separate electric drives which are then synchronized with each other via an electronic control. To this end, however, electronic circuits are required which correct the separate drive motors in such a manner that they work in an angularly synchronous manner. Moreover, the electronic controllers have to work rapidly and accurately. As a rule, greater difficulties occur especially when changing the operating speed of a printing press or of parts of a printing press. In this case, it must be ensured that the individual electric drives continue to work in an angularly synchronous manner after the speed change.  
           [0004]    A method and corresponding device for controlling the exact angular synchronism of a plurality of electric drives is known from German Patent DE 197 27 824 C1. This system is composed of a control computer, which represents a central master system, and a plurality of distributed drive controllers, which are arranged near the electric drive to be controlled. In this context, the distributed drive controllers and the master computer are interconnected via a synchronization bus. In this connection, the master system supplies machine speed setpoints to the distributed drive controllers via the synchronization bus. The rotational speed setpoints are locally generated from these machine speed setpoints in the respective distributed drive controllers. To allow angular synchronization between the individual drive units, the distributed drive controllers contain angle setpoint generators which are composed of a master pointer in the respective drive controller. When the machine speed setpoints have now been distributed to the drive controllers via the synchronization bus, then the master system sends a so-called “interrupt” to all drive controllers. This interrupt is used for synchronization and ensures that all distributed drive controllers run their computing cycles with the same current machine speed setpoint. The appropriate rotational speed and angle setpoints for the respective drive are then calculated in the distributed drive controllers. Such a drive control system for synchronizing a plurality of drive units offers the advantage that the synchronization bus is relieved since it only has to distribute the common machine speed setpoints and interrupts. Moreover, it is proposed to send so-called “asynchronous setpoint values”, such as an offset angle between several drive units, over a separate bus which is exclusively responsible for control and parameterization data.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    Starting from the prior art, an object of the present invention is to create an electrically synchronized drive combination of a plurality of electric drives without having to transmit time-critical setpoint values over a bus that assumes the communication between the drive units and a central control.  
           [0006]    The present invention provides a device for synchronizing processes which are executed on a plurality of units ( 2   a ,  2   b ,  2   c ), in particular, in paper-processing machines, the units ( 2   a ,  2   b ,  2   c ) being connected to a central control unit ( 1 ) via a data bus ( 6 ), wherein the central control unit ( 1 ) is designed for distributing movement commands via a data bus ( 6 ) to a plurality of units ( 2   a ,  2   b ,  2   c ), the movement commands containing the instant at which these commands are executed; and provision is made for the units ( 2   a ,  2   b ,  2   c ) which keep available the absolute time to synchronously execute these commands at the instant communicated via the commands.  
           [0007]    The present invention also provides a method for synchronizing processes which are executed on a plurality of units ( 2   a ,  2   b ,  2   c ), in particular, in paper-processing machines, the units ( 2   a ,  2   b ,  2   c ) being connected to a central control unit ( 1 ) via a data bus ( 6 ). The central control unit ( 1 ) distributes movement commands via a data bus ( 6 ) to a plurality of units ( 2   a ,  2   b ,  2   c ), the movement commands containing the instant at which these commands are executed; and the units ( 2   a ,  2   b ,  2   c ) which keep available an absolute time synchronously execute these commands at the instant communicated via the commands.  
           [0008]    The device and method according to the present invention offer the great advantage that no time-critical setpoint values have to be sent over the data bus. To this end, the device includes a central control computer which is connected with distributed units via a data bus; an absolute system time which can be synchronized via the data bus being kept available both on the central control computer and on the distributed drive units. Since all units of the device according to the present invention keep available this absolute system time, it is possible to send movement commands from the central control unit to the distributed units, the movement commands not only containing the position, speed and acceleration of particular forms of movement but, in particular, also the instant at which they are to be carried out in the distributed units. Therefore, the instant at which the movement commands arrive at the distributed units is unimportant as long as they arrive in time before the instant at which the movement command is to be executed. This can be ensured by sending the movement commands to the distributed units timely in advance. By freeing the data bus from time-critical setpoint values, it is made possible to use a fairly simple data field bus as, for example, a CAN (controller area network) bus to transmit the movement commands from the central computer to the distributed units. This provides a great price advantage over systems which are dependent on the transmission of time-critical setpoint values because these have to resort to fast and expensive drive bus technologies as, for example, SERCOS.  
           [0009]    If the distributed units have separate computers, then this allows the setpoint values to be locally generated from the movement commands which are sent by the central control unit. Then, a setpoint generator computes the position, speed and acceleration setpoints in the respective distributed unit so that all drives that are controlled by the distributed units work in an angularly synchronous manner.  
           [0010]    The use of a CAN bus as the data bus allows the drive concept according to the present invention to be implemented in a particularly cost-effective manner. CAN buses indeed have a relatively low data transfer rate but have proven efficient in industrial control technology and, in this case, are the means of choice since, due to the time-uncritical values that are transmitted over the field bus, no high demands need to be placed on speed.  
           [0011]    If, in addition to the arithmetic units for setpoint generation, further arithmetic units exist in the distributed units, then this offers the advantage that additional functions can be implemented on these further arithmetic units. In this manner, the setpoint values required for synchronization can be processed for an individual drive unit in such a way that special functions as, for example, an electronic cam disc are possible. Nevertheless, these distributed units provided with the special functions work with perfect angular accuracy and, consequently, synchronously to each other.  
           [0012]    Drive controllers present in the distributed units are used to compare the setpoint values, which are present in the distributed units and which have possibly been processed with special functions, to actual values present at the electric drive and, if necessary, to correct the actual values to the setpoint values. In this manner, the electric drives are corrected in such a manner that they are synchronous to each other.  
           [0013]    It is useful to define at least one of the distributed drive units as a so-called “master” while the other drive units are defined as so-called “slaves”. In this manner, it is possible to send so-called “positioning commands” to the drive units that are defined as slaves, the position commands making it possible, for example, to change the rotational angle of the slave units by a desired angle with respect to the master. To this end, the slave units receive a command which refers to the setpoint angle at a specific point in time and which assures that this differential angle is adjusted by increasing or reducing this setpoint rotational speed of the slave drive for a short time.  
           [0014]    If, in addition to the drive commands, data for parameterization and diagnosis are also sent via the existing data bus, then a further data bus, as recommended for these tasks in German Patent DE 197 27 824 C1, can be dispensed with. Since it is also possible to synchronize the global system time in all distributed units and the central control unit via this one data bus, no separate data bus is required for this either. Therefore, the communication required between the distributed units and the control unit for a method or device according to the present invention can be accomplished with a single data bus.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Further advantageous embodiments can be gathered from the Figures. An exemplary embodiment is described and explained in greater detail with reference to the Figures.  
         [0016]    [0016]FIG. 1 shows the design of a drive device according to the present invention, and  
         [0017]    [0017]FIG. 2 shows position, speed and acceleration diagrams which represent a so-called “positioning command” of a slave drive relative to a master drive. 
     
    
     DETAILED DESCRIPTION  
       [0018]    A possible design of a device according to the present invention is shown in FIG. 1. The device is composed of a central control unit  1  and three distributed units  2   a ,  2   b ,  2   c , the distributed units  2   a ,  2   b ,  2   c  representing the drive controls of electric drives which are not shown here. For communication between distributed units  2   a ,  2   b ,  2   c  and central control unit  1 , provision is made for a bus system  6 , which is a simple field bus as, for example, a CAN bus. Via field bus  6 , movement commands are sent from central control unit  1  to distributed units  2   a ,  2   b ,  2   c . Distributed units  2   a ,  2   b ,  2   c  are composed of an arithmetic unit  3   a ,  3   b ,  3   c , respectively, the arithmetic units generating setpoint values from the movement commands sent by the central control unit. These setpoint values can be further processed by further arithmetic units  4   a ,  4   b ,  4   c  in which special functions are stored so that each distributed unit  2   a ,  2   b ,  2   c  is able to perform different sequences of movements. This is useful, for example, if one of the electric drives controlled by distributed units  2   a ,  2   b ,  2   c  has to rotate slower or faster in a certain ratio to the other electric drives. In this context, the electric drives are controlled by drive controllers  5   a ,  5   b ,  5   c , which compare the actual values of the electric drives to the setpoint values from further arithmetic units  4   a ,  4   b ,  4   c  and correct them to the setpoints.  
         [0019]    As indicated in FIG. 1, it is possible to connect further distributed units to bus  6  in addition to the three distributed units  2   a ,  2   b ,  2   c  shown here until the maximum capacity of the bus system is reached. In the embodiment shown in FIG. 1, distributed drive boards  2   a ,  2   b ,  2   c  are designed to control the electric drives of a printing press. Printing presses have many rotationally movable driven parts which, as mentioned at the outset, are increasingly driven by separate electric motors but have to synchronized with each other as in the case of a mechanical line shaft or a gear train. To this end, one of the electric drives is selected to be a so-called “virtual master drive” while the other electric drives work as so-called “virtual slave drives”. Usable as master drive is, in particular, the drive board of the main drive of a printing press. The difference between the master drive and the slave drives lies only in that the slave drives receive further movement commands in addition to those sent to all drive boards  2   a ,  2   b ,  2   c , the further movement commands each affecting only one specific slave drive.  
         [0020]    The difference between main drive and slave drive also follows from FIG. 2. Here, three diagrams can be seen in which the acceleration, speed and angular position of a master drive and of a slave drive are plotted over time, respectively. As long as no slave-specific movement commands are given via bus system  6 , both the main drive and the auxiliary drives run completely synchronously with each other. However, if the intention is for one of the auxiliary drives to be rotated by a certain angle Δφ with respect to the main drive, then a process is initiated as shown in FIG. 2. In this process, central control unit  1  sends an additional movement command over field bus  6  to the respective distributed drive board. For instance, the intention is for the electric drive which is controlled by distributed drive board  2   c  to be rotated by a certain angle Δφ with respect to the main drive or to the other auxiliary drives. When verbally described, the movement command shown in FIG. 2 reads as follows: At instant t=10 sec, the angle of the slave drive is to be moved by angle Δφ relative to the actual angle at instant t=10 sec. This movement command is sent by central control unit  1  in coded form via field bus  6  to the respective distributed drive board, here  2   c . Then, the setpoint values for speed, acceleration and position are calculated on distributed drive board  2   c  using setpoint generator  2   c . The movement command gets executed in that at instant t=10 sec, the electric drive of the slave drive accelerates with constant acceleration to a higher rotational speed until instant t=15 sec, then maintains the speed constant for a further period of 5 sec and, subsequently, is decelerated during another 5 sec with constant negative acceleration to its original speed. Thus, it is guaranteed that at instant t=25 sec, the slave drive runs completely synchronously with the main drive or the other auxiliary drives again. Since the synchronization takes place on the basis of a uniform time base, this time is stored both in distributed drive boards  2   a ,  2   b ,  2   c  and in central control unit  1  as the so-called “absolute system time”. The system time can be generated by clock generators on the individual drive boards  2   a ,  2   b ,  2   c  and central control unit  1 , it being possible to synchronize the clock generators via field bus  6 .  
         [0021]    With regard to the exact procedure for establishing a uniform time base, reference is made to the two patent applications with the German reference numbers DE 101 11 647.0 and DE 100 59 270.8, both of which are hereby incorporated by reference herein, which deal with the temporal synchronization of processes, in particular, in the control of a printing press.  
         [0022]    In addition, data for parameterization and diagnosis of drive boards  2   a ,  2   b ,  2   c  can also be exchanged via field bus  6 . To enable reliable functioning of such a drive system, the movement commands always have to be sent by central control unit  1  to drive boards  2   a ,  2   b ,  2   c  in time before the instant at which they have to be executed. This lead time has to take into account the delay time on field bus  6  as well as system dead times. The arithmetic units on boards  2   a ,  2   b ,  2   c  work at the same computing speed so that no delays can occur between the individual boards  2   a ,  2   b ,  2   c  in the execution of the movement commands.  
         [0023]    List of Reference Numerals  
         [0024]    [0024] 1  central control unit  
         [0025]    [0025] 2   a ,  2   b ,  2   c  distributed control units  
         [0026]    [0026] 3   a ,  3   b ,  3   c  arithmetic units for setpoint generation  
         [0027]    [0027] 4   a ,  4   b ,  4   c  arithmetic units for carrying out special functions  
         [0028]    [0028] 5   a ,  5   b ,  5   c  drive controller  
         [0029]    [0029] 6  field bus