Abstract:
The present invention relates to an apparatus for controlling vibration of steel sheet being processed in a processing line. The apparatus includes: electromagnet devices for generating magnetic forces acting at right angles on the steel sheet; sensor devices for detecting separation distances between the steel sheet and the electromagnet devices; control devices for controlling a flow of excitation current through the electromagnet devices according to separation distances detected by the sensor devices; and actuator devices for adjusting the separation distance between the steel sheet and the electromagnet devices; wherein the actuator devices adjust the separation distance when a specific condition is attained in a positional relationship between the steel sheet and the electromagnet devices.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for controlling the vibration of a steel sheet being driven along the running surface of a processing facility in a steel rolling line or surface treating line in a steel mill. 
     2. Description of the Related Art 
     FIG. 27 shows a schematic diagram of a conventional apparatus for controlling the vibration of a steel sheet  101  being processed, by placing opposing electromagnets  102 A,  102 B on the front and back sides across the steel sheet  101 . 
     In such an apparatus, sensors  107 A,  107 B are placed inside the electromagnets  102 A,  102 B, respectively, for detecting the distances from the steel sheet  101  to respective electromagnets  102 A,  102 B, and the excitation currents passing through the coils in the electromagnets  102 A,  102 B are controlled according to the distances detected by the sensors  107 A,  107 B, so that the magnetic attraction forces can be adjusted in such a way to reduce the vibrations. 
     This vibration control apparatus comprises a plurality of pairs of electromagnets  102 ˜ 105  arranged transversely to the running direction of the steel sheet, as seen in a plan view of the steel sheet  101  shown in FIG.  28 . Pairs of sensors  107 ˜ 110  are placed in paired electromagnets  102 ˜ 105  so that the magnitudes of the excitation current can be adjusted according to respective separation distances detected by the paired sensors. 
     In such a vibration control apparatus, because of bowing in the steel being rolled, the path of the steel sheet can sometimes show a tendency to be closer to one or the other electromagnet depending on the type of steel being processed and the running speed. If the control of electromagnets is started under such a condition, the control apparatus, in its effort to correct bowing of the steel sheet, tries to deliver more current to the electromagnet that is farther away from the sheet. However, a considerable force is required when the steel sheet is thick so that it is necessary to supply a high current to develop the necessary magnitude of force. Under such a circumstance, excitation current may become saturated due to factors such as inadequate capacity of the amplifier for the electromagnet, which may result in virtual loss of vibration control. 
     Also, when starting or stopping the vibration control action of the apparatus, if the apparatus is simply turned on or off, the excitation current changes suddenly to cause the steel sheet to hunt for a balancing position thus resulting in wild oscillation, and in extreme cases, the surface of the steel may collide with the surface of the magnetic poles to cause scratches on the steel sheet. 
     Also, when starting the control action, if the steel sheet is vibrating with such a large amplitude that the electromagnets cannot be brought into a proper range for control action, it may be considered that the electromagnets may be brought into proper positions after starting the process line. However, if the gap is large and the steel sheet is outside the range of detection of the sensors and the sensors are not able to detect the sheet position properly, there is a possibility that the steel sheet can be induced into oscillation. 
     Also, in the control apparatus described above, the relationship between the electromagnet pairs and the running sheet is subject to continual change because of such factors as the variations in the sheet thickness and width of the steel roll to be processed. For this reason, if the gain of the control apparatus is fixed at a constant value, changes in thickness, for example, may make the steel sheet susceptible to vibration to such an extent that the sheet surface may touch the pole surfaces of the electromagnets, in some cases. 
     Also, widthwise snaking of the steel sheet may occur in such a way that the edge of the steel sheet  101  swings to the position shown by the dotted line in FIG.  28 . In such a case, the steel sheet  101  positions itself in an ambiguous-location between the pair of electromagnets  102  so that, in spite of the fact that the sensor pair  107  inside the electromagnet pair  102  cannot detect the distances to the steel sheet, the control action in this case would be based on the detected distance of the sensor pair  107  to the steel sheet, therefore, control action on the electromagnet pair  102  becomes impossible. Under such a circumstance, the steel sheet may undergo vibration or the surface of the sheet  101  may touch the pole surfaces of the electromagnet pair  102  to cause scratches on the sheet  101 . 
     Also, if the steel sheet moves completely out of the detection range of the pair of electromagnet placed near the edge of the steel sheet, power will be wasted by the pair of electromagnets that are out of the range of detecting the steel sheet. 
     All of the foregoing problems may also be caused by changes in the width of the steel sheet being processed, for example. 
     Also, this type of control apparatus is normally operated so that the steel sheet would pass through the center line between the pair of opposing electromagnets. But, when the type of steel being processed changes in a given roll, that is, when a welded joint is passing through, the electromagnets are sometimes moved away from their normal detection position to a standby position to avoid collision of the welded section with the electromagnets. If the move is made while the electromagnets are turned on, even though the position of the steel line has not changed, the relative distances between the steel sheet and the electromagnets would increase, so that the control apparatus judges that the steel sheet has moved in a direction away from the sensors, and increases the excitation current to the electromagnets. 
     In this case, because the electromagnets are moving away from the steel sheet, the current increases as the electromagnets are moved away, and ultimately the control apparatus capability reaches its saturation limit, and the apparatus becomes inoperable. In the worst case scenario, the magnets may be overheated and destroyed. 
     To avoid such phenomena from happening, power to the conventional apparatus is turned off when the electromagnets are to be moved to the standby position. In the absence of vibration control action, vibration can be introduced in the processing line, and particularly during the initial stage of preparing for the standby operation, in other words, while the distance of separation between the electromagnets and the steel sheet is small, there is a danger that the steel sheet may contact the electromagnets. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an apparatus for controlling vibration of a steel sheet being processed in a steel processing line, so that the processing line can be operated in a stable manner without having operational problems such as sheet vibration or loss of vibration control caused by such factors as snaking of the steel sheet or changes in the conditions of the sheet such as varying sheet thickness and width in the running sheet. 
     Also, it is another object of the present invention to provide a vibration control apparatus that permits the electromagnets to be retreated to a standby position without causing a line instability or excessive heating and damage to the electromagnets. 
     The object has been achieved in an apparatus for controlling vibration comprised by: electromagnet means for generating magnetic forces acting at right angles on the steel sheet; sensor means for detecting separation distances between the steel sheet and the electromagnet means; control means for controlling a flow of excitation current through the electromagnet means according to separation distances detected by the sensor means; and actuator means for adjusting the separation distance between the steel sheet and the electromagnet means; wherein the separation distance is adjusted by the actuator means when a specific condition is attained in a positional relationship between the steel sheet and the electromagnet means. 
     The present apparatus for controlling vibration may also be comprised by: electromagnet means for generating magnetic forces acting at right angles on the steel sheet; sensor means for detecting separation distances between the steel sheet and the electromagnet means; control means for controlling a flow of driving current through the electromagnet means according to separation distances detected by the sensor means; wherein a circuit gain for controlling the driving current is determined in accordance with information on the steel sheet, including thickness data, running speeds, joint locations, sheet widths and line tension data. 
     The present apparatus for controlling vibration may also be comprised by: electromagnet means for generating magnetic forces acting at right angles on the steel sheet; sensor means for detecting separation distances between the steel sheet and the electromagnet means; control means for controlling a flow of driving current through the electromagnet means according to a specific command value and separation distances detected by the sensor means; and moving means for moving the electromagnet means transversely to move away from the steel sheet so as to retreat to a standby position or to return to a detection position; wherein the moving means moves the electromagnet means to move away from the steel sheet to the standby position, according to sheet information including welded joint data, and to further perform a return operation to return to the detection position, and the control means alters the position command value when moving the moving means according to a distance to be moved, and further provides a return operation command. 
     The present apparatus for controlling vibration may also be comprised by: electromagnet means comprised by opposing pairs of electromagnets disposed in proximity of front and back surfaces of the steel sheet for generating magnetic forces acting at right angles to sheet surfaces; sensor means disposed so as to form opposing pairs of sensors for detecting respective separation distances between the steel sheet and the opposing pairs of electromagnets; control means for controlling a flow of driving current through the pairs of electromagnets according to differences in separation distances generated by the opposing pairs of sensors and specific position command values derived from the differences in separation distances; and moving means for moving-the electromagnet means transversely to the steel sheet so as to retreat to a standby position or to return to a detection position; wherein the moving means move the pairs of electromagnets to move away from the steel sheet to the standby position, according to sheet information including joint location data. 
     Any of the apparatuses described above is able to operate a processing line in a stable manner because an electromagnet requiring a higher flow of steady-state current than others in the sensor array is pushed closer to the sheet, in so doing, the supply of current to the electromagnet, which is most remote from the steel sheet, is reduced thereby reducing the load on the electromagnet and restoring the steady-state operation of the processing line. 
     The apparatus may be operated according to a condition that when the separation distance between an electromagnet and the sheet exceeds a specific value, an actuator device brings the electromagnet closer to a sheet steel to reduce the steady-state current flowing in the electromagnet to reduce its load to provide a stable vibration control. 
     The apparatus may be operated so that an electromagnet is moved by actuator means in a direction to nullify the low frequency components or direct current components, thereby reducing the load on the electromagnet and providing a stable operation of the processing line. 
     The apparatus may be operated so that a separation distance between a steel sheet and electromagnets is adjusted by paired electromagnets opposing each other across a steel sheet without altering the relative positions of the paired electromagnets, thereby reducing the load on the electromagnets and operating the line in a stable manner. 
     The apparatus may be operated so that, when starting or ending to control the excitation current, the apparatus adjusts the controlling gain and steady-state current in electromagnet means according to a ramp function, thereby preventing the generation of a phenomenon of “hunting”, i.e., oscillation of the strip of steel being processed. 
     The apparatus may be operated so that, when starting or ending to control a flow of excitation current to an electromagnet, the deviation in the steady-state location of an electromagnet in the integration means are reset to a zero, thereby reducing rapid changes in the excitation current and preventing “hunting”. 
     The entire operation of the vibration control apparatus is made smoother by using the present apparatus, because it is possible to bring the electromagnet closer to the steel sheet while soft-starting the vibration control system, or retreating the electromagnet away from the steel sheet by soft-stopping the vibration control means. 
     The present apparatus is controlled so that the controlling gain is determined according to detected distances of individual sensors, so that it is possible to prevent collision between the steel sheet and the pole surface of the electromagnet due to vibration caused by changes in the sheet condition such as sheet thickness and other parameters of the steel sheet being processed. 
     Also, internal judging means are provided in the apparatus so that when it is decided that a steel sheet is not present within a given range of a sensor, the controlling gain for this sensor is reduced to zero. For example, when the steel sheet is out of the range of detection of the sensor due to snaking or changes in the sheet width, the apparatus turns off the electromagnet corresponding to this sensor, thereby preventing waste of electrical energy. 
     Also, when snaking in the widthwise direction of the running sheet causes an uncertainty in detecting the edge of the steel sheet between a pair of electromagnets, the apparatus does not cause the paired electromagnets to become inoperative, thereby preventing loss of control of vibration or damage to the surface by collision of the sheet against the electromagnet. 
     The present apparatus is provided with a gain table based on information on a variety of steel sheets, including thickness data, running speeds, joint locations, sheet widths and line tension data, so that a controlling gain for each type of steel sheet is determined according to the gain table, thereby preventing vibration and resulting collision between the sheet and the pole surface of the electromagnet. 
     Also, even if the type of steel sheet varies within a given roll, stable operation can be continued by switching the electromagnets to be operated and suitably adjusting the controlling gain. 
     Also, if a weld joint is detected indicating a change in the type of steel to be processed, the controlling gain can be altered automatically so that manual alteration by a line operator is not required. 
     Also, in the present apparatus, the electromagnet means are disposed in such a way that electromagnets disposed on a front-side do not oppose electromagnets disposed on a back-side of a steel sheet, thereby preventing erroneous detection caused by mutual interference between the opposing electromagnets. 
     Also, the present apparatus is able to retreat the electromagnets to a standby position, or return the electromagnets to the detection position while performing vibration control by varying the position command value in accordance with a separation distance detected by a relevant pair of electromagnets, so that a flow of excessively high excitation current or overheating and damage to the electromagnets can be prevented. 
     Also, by detecting the separation distance using a pair of electromagnets across the steel sheet, obtaining a difference in the separation distance, and controlling the excitation current in accordance with the difference, the opposing pair of electromagnets can be retreated at the same time without altering the position command value, to prevent a flow of excessively high excitation current or overheating and damage to the electromagnets. 
     Also, the apparatus includes integration means which can be inactivated when the electromagnets are to be retreated so that even when the separation distance exceeds the sensor detection range, a flow of excessively high excitation current or overheating and damage to the electromagnets can be prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is aschematic block diagram of a vibration control apparatus in Embodiment 1. 
     FIG. 2A,  2 B are diagrams of an example of a plurality of pairs of electromagnets provided in the vibration control apparatus. 
     FIG.  3 A˜ 3 C are diagrams to illustrate the operation of the vibration control apparatus in Embodiment 1. 
     FIG. 4; is a schematic block diagram of the electrical control loop of the vibration control apparatus. 
     FIG. 5 is a block diagram of the internal structure of the vibration controller. 
     FIG. 6 is a graph to show changes in the steady-state current. 
     FIG. 7 is a block diagram of the internal structure of PID control means. 
     FIG. 8 is a graph to show changes in circuit gain caused by the control action. 
     FIG. 9 is a schematic circuit diagram of analogue integration circuit in the integration control means. 
     FIG. 10A,  10 B are schematic illustration of the hunting phenomenon. 
     FIG. 11 is a schematic diagram of a configuration used for mechanical and electrical control methods. 
     FIG. 12A,  12 B are diagrams illustrating the locations of the electromagnets for soft start. 
     FIG. 13A,  13 B are graphs to show the changes in gain and steady-state current during soft start. 
     FIG. 14 is a block diagram of the vibration control apparatus in Embodiment 2. 
     FIG. 15 is a side view of a pair of electromagnets. 
     FIG. 16 is a table for PID gain. 
     FIG. 17 is a block diagram of the vibration control apparatus in Embodiment 3. 
     FIG. 18 is a graph showing a relationship between the sensor output and threshold values. 
     FIG. 19 is a block diagram to shown the details of the internal structure of the vibration controller. 
     FIG. 20 is a side view of another pair of electromagnets. 
     FIG. 21 is a block diagram of the vibration control apparatus in Embodiment 5. 
     FIG. 22 is a block diagram of the control system in Embodiment 5. 
     FIG. 23 is an illustration of the electromagnets moving to the standby position. 
     FIG. 24 is a side view of the vibration control apparatus in Embodiment 6. 
     FIG. 25 is a block diagram of the control system in Embodiment 6. 
     FIG. 26 is a block diagram to show the internal structure of the vibration controller. 
     FIGS. 27,  28  are schematic diagrams of conventional vibration control apparatuses. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments shown in the following are provided for illustrative purposes only and are not meant to restrict the present invention in any way. Also, to achieve the object of the present invention, it is not always necessary to provide combinations of all the features presented in the examples. 
     Embodiment 1 
     Preferred embodiments will be explained with reference to the drawings. FIG. 1 shows a block diagram of the vibration control apparatus in Embodiment 1. The steel sheet  1  shown in its side view is moving from the bottom to the top of the diagram. An electromagnet  2 A faces the front surface of the steel sheet  1  and an electromagnet  2 B faces the back surface of the steel sheet  1 , and are placed opposite to each other with the steel sheet  1  intervening therebetween. A sensor  3 A is provided inside the electromagnet  2 A to detect the distance to the steel sheet  1  and a similar sensor  3 B is provided inside the electromagnet  2 B. The detection plane of sensor  3 A is coplanar with the pole surface of the electromagnet  2 A, and similarly the detection plane of sensor  3 B is coplanar with the pole surface of the electromagnet  2 B. Sensors  3 A,  3 B are also opposite to each other with the steel sheet  1  intervening therebetween. Electromagnet  2 A is installed on an electromagnetic (e/m) actuator  4 A and electromagnet  2 A is installed on an e/m actuator  4 B so that the distances between the respective electromagnet and the steel sheet  1  can be adjusted individually. 
     Output signals from sensors  3 A,  3 B are input into a (vibration) controller  5 , which also receives output signals from a sequencer  10 . Output signals from the controller  5  are input into amplifiers  6 A,  6 B, and the output signals from amplifier  6 A are input in the electromagnet  2 A and the output signals from amplifier  6 B are input in the electromagnet  2 B. 
     Further, the output from the controller  5  is input into lowpass circuits  7 A,  7 B, whose output signals are input into a comparator  8 . Output signals from the comparator  8  are input into an upper controller  9 , whose output is input into electromagnetic (e/m) actuators  4 A,  4 B. 
     Next, the operation of the control apparatus will be explained. Sensor  3 A detects the distance d A  from its detection plane to the surface of the steel sheet  1  and transmits the result to the controller  5 . Similarly, sensor  3 B detects the distance  B , from its detection plane to the surface of the steel sheet  1  and transmits the result to the controller  5 . The controller  5  outputs vibration control signals to amplifiers  6 A,  6 B according to the respective distance information received. 
     Amplifier  6 A supplies excitation current I A  to electromagnet  2 A, and amplifier  6 B supplies excitation current I B  to electromagnet  2 B, and the controller  5  controls amplifiers  6 A,  6 B in such a way that, if d A &lt;d B , and if d A &gt;d B , I A &gt;I B . By so doing, the steel sheet  1  is always pulled back to the central location between the electromagnets  2 A,  2 B. 
     The controller  5  outputs the same control signal, as the control signal sent to the amplifiers  6 A,  6 B, to the lowpass circuits  7 A,  7 B, respectively. Lowpass circuits  7 A,  7 B allow only the low frequency components in the respective control signals to be transmitted. The low-frequency components are compared in the comparator  8 , and the comparison results are sent to the upper controller  9 . The upper controller  9  operates the e/m actuators  4 A,  4 B on the basis of the respective results received so as to move the electromagnets  2 A,  2 B accordingly. 
     These control actions ensure that, when the steel sheet  1  comes closer to one or the other of the electromagnets  2 A,  2 B, the location of steel sheet  1  is adjusted by either the e/m actuator  4 A or  4 B so that the sheet  1  is always retained in the central location relative to the electromagnets  2 A and  2 B. 
     Two methods of moving the A-and B-side electromagnets may be considered: one method is to move the electromagnets independent of the other, and the other method is to move the electromagnet on the A- and B-sides at the same time along a parallel line. 
     Or, when the electromagnets are arranged in the width direction of the steel sheet  1 , as shown in FIG. 2A,  2 B, they may be moved together. 
     Accordingly, starting with the apparatus off and the sheet  1  is closer to the B-side, as illustrated in FIG. 3A, when the control apparatus is turned on to begin the vibration control process the following scenario may be experienced. Electromagnets  2 A,  2 B produce a centralizing force to bring the sheet  1  to the central location as illustrated in FIG.  3 B. If the force of attraction being applied by the electromagnet  2 A is too small for reasons such as the sheet  1  being too thick, a high excitation current flows in the electromagnet  2 A while little current flows in the electromagnet  2 B, and the control action becomes inoperative. 
     In such a situation, the e/m actuator  4 A is operated to bring the electromagnet  2 A closer to the sheet  1 , as illustrated in FIG. 3C, the attraction force exerted by the electromagnet  2 A increases to effect stable vibration control action. 
     In the above situation, the centralizing action can also be generated by moving the electromagnets  2 A,  2 B together to the left, without changing the interspacing of the electromagnets  2 A,  2 B. The construction of the apparatus may be simplified by providing one actuator to move both electromagnets  2 A,  2 B. 
     Next, the operation of the electrical control system will be explained. The electrical control loop section has been extracted from the overall control circuit, and is shown in FIG.  4 . 
     FIG. 5 shows the details of the internal structure of the vibration controller  5 . Output signals from sensors  3 A,  3 B showing the location of the steel sheet  1  and output signals from the position command means  11  are input into the difference detection means  12 , whose output signals are input into the proportional-integral-differential (PID) control means  13 . The PID control means  13  also receives gain command signals and integration reset signals output from the sequencer  10 . 
     Output signals from the PID control means  13  are input into the adder  14 A,  14 B, which also receive steady-state current command signals output from the sequencer  10 . Output signals from the adder  14 A are input into the current control means  15 A, and output signals from the adder  14 B are input into the current control means  15 B. Output signals from the current control means  15 A are input into the amplifier  6 A, and output signals from the current control means  15 B are input into the amplifier  6 B. 
     Next, the sequence of operation taking place inside the controller  5  will be explained. A difference between the sensor signal showing the location of the steel sheet  1  and the position command signal output from the position command means  11  is computed by the difference detection means  12 , and the computed difference is sent to the PID control means  13 . The PID control means  13  outputs control signals according to the input difference value. The control signal and the steady-state current command signal output from the sequencer  10  are added in the adders  14 A,  14 B. The summed values are respectively input into the current control means  15 A,  15 B, which output respective power command signals to the amplifiers  6 A,  6 B. 
     At the startup of the vibration control apparatus, the sequencer  10  outputs a steady-state current command signal so that the steady-state current input into electromagnets  2 A,  2 B will rise according to a ramp function as shown in FIG.  6 . At this time, electromagnets  2 A,  2 B rises simultaneously to the level of steady-state current. Similarly, when stopping the apparatus, the electromagnets on both A- and B-sides are deactivated by following the same ramp function. 
     Next, detailed configuration of the PID control means  13  will be explained with reference to FIG.  7 . The difference value output from the difference detection means  12  and the gain signal output from the sequencer  10  are input into the gain determination means  16 , whose output is input into the ratio control means  17 , integration control means  18  and the differentiation control means  19 . The integration control means  18  receives an integration reset signal output from the sequencer  10 . Output signals from the ratio control means  17 , integration control means  18  and differentiation control means  19  are input into the adder  20 , whose output is input into the adders  14 A,  14 B. 
     Next, the operation of the PID control means  13  will be explained. Similar to the case of controlling the steady-state current to the electromagnets  2 A,  2 B, at the time of starting and stopping the vibration control apparatus, the sequencer  10  outputs a gain command signal to vary the gain K in the PID control means  13  according to a ramp function, shown in FIG. 8, to the gain determination means  16 . The ratio control means  17 , integration control means  18  and differentiation control means  19  control the excitation current in the electromagnets, according to a gain K determined by the gain determination means  16 . 
     Next, the detailed configuration inside the integration control means  18  will be explained with reference to FIG.  9 . The integration control means  18  has an analogue integration circuit shown in FIG. 9, which is comprised by an amplifier  21 , resistors  22 , a condenser  23 , and a switch  24  connected to both ends of the condenser  23 . 
     Next, the operation of the integration control means  18  will be explained. The switch  24  is activated by the integration reset signal sent from the sequencer  10 . The switch  24  is normally in the off-position, but when the integration reset signal is received, it shifts to the on-position to short the ends of the condenser  23 , and resets the integration circuit. 
     At the time of starting the vibration control apparatus, an integration reset signal is sent from the sequencer  10  so that the switch  24  is turned on and the integration circuit is reset. Also, when the gain and steady-state current reach appropriate values, an integration reset signal is again sent to reset the integration circuit. 
     As described above, sudden increase in the excitation current is prevented, at the time of starting or stopping the apparatus, by varying the grain and steady-state current according to a ramp function, or by resetting the integrated value of the integration circuit, so as to eliminate hunting phenomenon, such as the one illustrated in FIG. 10A, and to enable to soft-start the apparatus in a stable manner as illustrated in FIG. 10B, for example. 
     Next, the operation of starting the electrical control while bringing the electromagnets closer to the steel sheet will be described. At the time of starting the vibration control apparatus, the electromagnets are moved from their initial positions to positions to create suitable gaps to the steel sheet, and based on the time internals required to move to these positions, the parameters for the soft-start operation, such as the steady-state current, gain and the rate of increase (slope) for the ramp function, are selected. 
     FIG. 11 shows a block diagram for only that part of the configuration to carry out the above-mentioned steps. The (vibration) controller  5  generates a system-start signal to operate the e/m actuator  4 A,  4 B to move the electromagnets  2 A,  2 B closer to the steel sheet  1 . At the same time, the controller  5  gradually increases the steady-state current portion of the excitation current to be supplied to the electromagnets  2 A,  2 B and the controlling gain for the excitation current to be supplied to the electromagnets  2 A,  2 B through the amplifiers  6 A,  6 B. 
     When the vibration control apparatus is started, the opposing electromagnets  2 A,  2 B are moved, at the same time, by the e/m actuators  4 A,  4 B in the direction to approach the steel sheet  1 , and when the inter-magnet distance between the electromagnets reach a certain value X as shown in FIG. 12A, the soft-start operation is commenced to gradually increase the gain and the steady-state current, and when an appropriate distance is reached as shown in FIG. 12B, the soft-start operation is ceased, and the vibration control apparatus transfers to a steady-state operation. 
     In this case, as shown in FIGS. 13A,  13 B, the time constant of the soft-start operation (i.e., the slope of the ramp function) is determined so that the gain and steady-state current will be at the appropriate values when the inter-magnet distance reaches an appropriate value. 
     Similarly, when the apparatus is to be stopped, soft-stop operation is used to separate the electromagnets gradually. 
     In the embodiment described above, the integration is performed using analogue circuits but is possible to carry out these operations using digital circuits or application softwares. 
     Embodiment 2 
     FIG. 14 shows Embodiment 2. In the diagram, the steel sheet  51  runs vertically from the bottom to top of the diagram at a running speed V m/min, and the electromagnet pairs  52 ˜ 56  are arranged transversely to the steel sheet  51 . Each of the electromagnet pairs  52 ˜ 56  is provided with respective internal sensor pairs  57 ˜ 61 . 
     FIG. 15 shows a side view of the electromagnet pair  52  and the steel sheet  51 . The electromagnet pair  52  is comprised by an electromagnet  52 A on the front-side and an electromagnet  52 B on the back-side of the steel sheet  51  disposed in such a way to oppose each other. The electromagnet pairs  53 ˜ 56  have the same structure. 
     The sensor pair  57  housed in the electromagnet pairs  52  is comprised by a sensor  57 A housed in the electromagnet  52 A disposed on the front-side of the steel sheet and a sensor  57 B housed in the electromagnet  52 B disposed on the back-side of the steel sheet and are disposed in such a way to oppose each other. Sensor pairs  58 ˜ 61  have the same structure. 
     Returning to explanation of FIG. 14, a weld joint detection sensor  62  is located A cm away from the transverse line of the electromagnet pairs  52 ˜ 56 , in the opposite direction to the running direction of the steel sheet  51 , for detecting the presence of welded joint  51   a.    
     Output signals from the weld joint detection sensor  62  are input into the upper controller  63 , whose output is input into the vibration controller  64 . Output signals from the controller  64  are input into the electromagnet pairs  52 ˜ 56 , and output signals from the sensor pairs  57 ˜ 61  housed in the electromagnet pairs  52 ˜ 56  are input into the vibration controller  64 . 
     In the vibration controller  64 , various information regarding the steel sheet to be processed, such as presence or absence of welded joints, the width of the steel sheet ahead of the welded joint, the width of the steel sheet following the welded joint, is stored in a table form. Driving parameters for the electromagnets are altered according to the contents in the table and the timing of welded joint detection. 
     Next, the operation of the vibration control apparatus will be explained. Sensor pairs  57 ˜ 61  detect the separation distance between the electromagnet pairs  52 ˜ 56  and the steel sheet  51 . In more detail, the sensor disposed on the front-side of the sheet  51 , for example the sensor  57 A in FIG. 15, detects the separation distance k A  to the front surface of the steel sheet  51 , and the sensor disposed on the back-side of the sheet  51 , for example the sensor  57 B in FIG. 15, detects the separation distance k B  to the back surface of the steel sheet  51 . Here, the detection surfaces of the sensors  57 A,  57 B are coplanar with the pole surface of the electromagnets  52 A,  52 B. The vibration controller  64  controls the electromagnet pairs  52 ˜ 56  according to the distances detected by the sensor pairs  57 ˜ 61  so as to control vibration of the steel sheet  51 . 
     If a welded joint  51   a  joining two different kinds of steels is detected in the running steel sheet  51  by the welded joint detection sensor  62 , the detected signals output from the welded joint detection sensor  62  are sent to the upper controller  63 , which outputs a control signal to the vibration controller  64 . Then, the controller  64  soft-stops the electromagnet pairs  52  and  56  when the welded joint  51   a  of the sheet  51  is at a point X m back of the transverse line of electromagnet pairs  52 ˜ 56 , thereby ceasing the operation of the electromagnet pairs  52  and  56 . 
     The sheet-stopping electromagnet pairs are pre-determined and stored in the vibration controller  64  according to the information input into therein. That is, in this case, the width of the sheet  51   b  preceding the weldedjoint  51   a  and the width of the sheet  51   c  succeeding the welded joint  51   a  have been input into the controller  64 , so that the sheet-stopping pair of electromagnets and those electromagnet pairs to be operated are determined on the basis of the installed positions of the electromagnet pairs  52 ˜ 56  in conjunction with the pre-input information. 
     After the steel sheet  51  has passed the transverse line of the electromagnet pairs  52 ˜ 56 , the vibration controller  64  renews the PID gain for controlling the electromagnet pairs  53 ˜ 55  according to the information such as the width and thickness of the steel sheet  51   c  that follows the welded joint  51   a.    
     More specifically, when an interval (A-X)/V min has elapsed after the welded joint  51   a  has passed the welded joint detection sensor  62 , the electromagnet pairs  52  and  56  are subjected to soft-stopping, i.e., a gradual lowering of the steady-state current and the PID gain. 
     At this point, based on the information such as sheet thickness and width of the steel sheet  51   c  that follow the previous steel sheet, the values of the PID gain for the electromagnet pairs  53 ˜ 55  are selected and after an elapsed interval of X/V min, the control mode is switched to the soft-mode. 
     The PTD gain is determined according to the sheet thickness in conjunction with a table, such as the one shown in FIG. 16, stored in the vibration controller  64 . If the values stored in the table do not match the input value, a PID gain can be computed by interpolation of the neighboring values. 
     Embodiment 3 
     Next, a vibration control apparatus in Embodiment 3 will be explained with reference to FIG.  17 . The steel sheet  51  travels from the bottom of the diagram towards the top of the diagram. A line of electromagnet pairs  52 ˜ 55  housing sensor pairs  57 ˜ 60  are arranged transversely to the steel sheet  51 . The structures of the electromagnets pairs  52 ˜ 55  and the sensor pairs  57 ˜ 60  are the same as those in Embodiment 2. 
     In this apparatus, an optical or magnetic displacement sensor  65 , disposed above the sheet  51 , detects snaking of the steel sheet  51  as a lateral left/right shift in the position of the steel sheet  51 , which is transverse to the travel direction of the steel sheet  51 . Output signals from the displacement sensor  65  are input into the upper controller  63 , whose output is input in the vibration controller  64 . Output signals from the controller  64  are input into the electromagnet pairs  52 ˜ 55 . Output signals from the sensor pairs  57 ˜ 60  housed in the respective electromagnets pairs  52 ˜ 55  are input into the controller  64 . The sensor pairs  57 ˜ 60  are placed in the center of the respective electromagnet pairs  52 ˜ 55 . 
     Next, the operation of the vibration control apparatus will be explained. The displacement sensor  65  successively detects the amount of lateral displacement of the running steel sheet  51 , and the detected results are successively input into the upper controller  63 . The upper controller  63  transmits the detected displacements and the pre-input information on sheet widths to the vibration controller  64 . 
     The vibration controller  64  computes the location of the edge of the sheet  51  from the lateral displacement information and the sheet width information, and determines the electromagnet pairs to be operated based on the computed edge location information and the positions of the electromagnet pairs  52 ˜ 55 . 
     Designating the center-to-center distance of the sensors  57 ,  60  by L, sheet width by B, outer diameter of the sensor head by D, and lateral shift by “a” (positive for a shift to the right), when a&gt;0 and B−a&lt;L+2D, the left-side electromagnet pair  52  is soft-stopped, and when a&lt;0 and B+a&lt;L+2D, the right-side electromagnet pair  50  is soft-stopped. The value of “a” should be less than the distance between the pair of electromagnets. 
     Embodiment 4 
     Next, a vibration control apparatus in Embodiment 4 will be explained. This apparatus is the same as the one shown in FIG. 17 in Embodiment 3. In this apparatus, shown in FIG. 19, an adder circuit  71  is provided to sum the output values from the front-side and back-side sensors. When the summed value computed by the adder circuit  71  exceeds a threshold value, the electromagnet pairs corresponding to the sensor pairs are soft-stopped. 
     Specifically, as shown in FIG. 15, when the steel sheet  51  is present between the sensor  57 A and sensor  58 B, respective distances to the steel sheet  51  can be determined. In this case, the output signal d 1  from the sensor  57 A is below a certain threshold value, as seen in FIG.  18 . However, when the sheet  51  moves out of the space defined by the sensor pairs, output signals d 2  from the sensor  57 A produce a constant value exceeding the threshold value, as seen in FIG.  18 . 
     The detailed configuration of the internal structure of the vibration controller  64  is shown in FIG.  19 . The controller  64  receives signals from the sensors  57 A and  57 B. These signals are input into a subtraction circuit  67   a  inside the controller  64  to compute a difference value between the two signals. A subtraction circuit  67   b  is provided to obtain a difference between the computed difference and the value provided by the position command circuit  66 . Output signals from the subtraction circuit  67   b  are input into the vibration controller  68 . Output signals from the vibration controller  68  are input into a current control means (A)  69  and a current control means (B)  70 . Output signals from the current control means (A)  69  and the current control means (B)  70  are input into electromagnet  52 A,  52 B, respectively, to operate each electromagnet. 
     Also, the signals from the sensor  57 A,  57 B to be input into the vibration controller  64  are also input into the adder circuit  71 . Output signals from the adder circuit  71  are input into the comparator  72 , where it is compared against the threshold value output from the threshold output means  73 . Output signals from the comparator  72  are input into the sequencer  74 , which outputs on/off control signal. 
     It should be noted that the descriptions given above relate to the electromagnet pairs  52  and sensor pairs  57 , but similar circuits are provided for the electromagnet pairs  53 ˜ 55  and sensor pairs  58 ˜ 60 . 
     Next, the operation of the vibration controller  64  will be explained. Here, the operation of the circuits related to only the electromagnet pairs  52  and sensor pairs  57  will be explained using FIG. 19, and explanations regarding similar operations of the electromagnet pairs  53 ˜ 55  and sensor pairs  58 ˜ 60  will be omitted. 
     The difference between the distance signals from the sensors  57 A and  57 B is computed by the subtraction circuit  67   a . This value represents a displacement value of the steel sheet  51  from the central position between the sensors  57 A,  57 B. A difference between this value and the position value given by the position command means  60  is computed by the subtraction circuit  67   b . The difference between the actual displacement and the command position is sent to the vibration controller  68 , which controls the current control means (A)  69  and the current control means (B)  70  according to the difference between the command value and the actual displacement value. The current control means (A)  69  and the current control means (B)  70  operate the respective electromagnets  52 A and  52 B. Accordingly, the steel sheet  51  is controlled so that its location coincides with the command value. 
     The distance values from the sensors  57 A,  57 B are input into the adder circuit  71  also to compute the sum of the distance values. The summed value is compared against the threshold value output from the threshold value output means  73 , and the result of comparison is forwarded to the sequencer  74 . When the summed value is greater than the threshold value, the sequencer  74  judges that the steel sheet  511  is not present between the sensor pairs  57 , and turns off the electromagnets pairs  52  housing the sensor pair  57 . When the power is turned off, control actions by the current control means (A)  69  and the current control means (B)  70  are nullified. When the summed value is less than the threshold value, it is judged that the steel sheet  51  is present between the sensor pairs  57 , and the electromagnet pairs  52  are turned on. When the power is turned on, control actions by the current control means (A)  69  and the current control means (B)  70  are activated. 
     It should be noted that other arrangements of the sensor pair are permissible as exemplified in FIG.  20 . In this case, sensors A, B are shifted relative to the other so that they are not opposite to each other. This arrangement enable to avoid a situation caused by mutual interference of the opposing sensors that the sum of the sensor output values when the sheet  51  is not present is less than the sum of the sensor output values when the sheet  51  is present. 
     Embodiment 5 
     Next, a vibration control apparatus in Embodiment 5 will be explained with reference to FIG.  21 . As shown in FIG. 21, vibration control electromagnets  52 A,  52 B are provided opposite to each other on both sides of the steel sheet  51 . A sensor  57 A is provided in one of the electromagnet  52 A. A plurality of pairs of electromagnets may be provided in some cases in either the longitudinal or transverse direction to the steel sheet  51 . 
     FIG. 22 shows a structure of the vibration control apparatus in Embodiment 5. The parts in FIG. 22 that are the same as those in FIG. 19 are give the same reference numerals, and their explanations are omitted. In this apparatus, because an inversion means  75  is provided between the vibration controller  68  and the current controlling means (B)  70 , electromagnets  52 A and  52 B are controlled in opposite manners. For example, when the driving current to the electromagnet  52 A is being increased, the driving current to electromagnet  52 B is being decreased. 
     Next, the operation of the apparatus will be explained. A welded joint represents a region of change in the running sheet from one type of steel to another type of steel, so that the weld section may be deformed or the sheet width may be quite different in the steels that is ahead of and following the welded joint. Therefore, there is a possibility that the deformed section can collide with the vibration control devices. To avoid such a situation, the electromagnets  52 A and  52 B are retreated from the sheet  51  to a standby position, that is, in a direction away from the back and front surfaces of the steel sheet  51 , as shown in FIG.  23 . 
     In such a case, the position command signal  66   a  in the control system, shown in FIG. 22, is altered according to the distance of movement of the sensor  57 A in the electromagnet  52 A. That is, when the electromagnet  52 A is pulled away from the steel sheet  51 , the sensor  57 A is also pulled away from the sheet  51 , and therefore, even though the location of the steel sheet  51  itself has not changed, the apparent location of the sheet  51  seen by the sensor  57 A is changed. The position command signal  66   a  is altered in accordance with the apparent change. 
     Accordingly, there would be no generation of magnetic forces to counter the movement of the steel sheet away from the electromagnet, and therefore, vibration control action can be continued during the standby operation without causing over-heating or damage to the electromagnets. 
     Embodiment 6 
     Next, a structure of the vibration control apparatus in Embodiment 6 will be explained with reference to FIG.  24 . In this apparatus, sensors  57 A,  57 B are provided in the interior of the electromagnets  52 A and  52 B positioned on both sides of the steel sheet  51 . The control system for the apparatus is shown in FIG.  25 . 
     According to this arrangement, a trigger value for the position command signal can be based on the difference in the distances from the steel sheet  51  to the sensor  57 A and  57 B. Therefore, the trigger value is zero when the steel sheet  51  is located exactly midway between the sensors  57 A,  57 B. 
     By adopting such a control structure, even during the interval of pulling the electromagnets  52 A and  52 B to the standby position, the trigger value may be left at zero to maintain the steel sheet  51  in the mid-position so that unnecessary magnetic forces are not generated and the vibration control action can be continued while carrying out the standby operation. 
     In each of the embodiments presented in the foregoing embodiments, the vibration control means  68  is operated according to the proportional-integral-differential (PID) control shown in FIG.  26 . The I-control (integral-control) mode operates in such a way to decrease the deviation between the command value and the actual sheet position value. However, in carrying out the standby process, as the sensors are pulled away from the sheet, the sensors move away from the sheet, and when the separation distances exceed the detection distance of the sensors, the I-control action can start to operate to increase the excitation current to the magnetic coils. 
     Therefore, during the standby operation including retreat- and return-periods, the I-control is turned off to prevent excess current to flow in the apparatus. During the retreating and returning operations, I-control naturally cannot be carried out, but the lack of I-control is not critical during such times, because precise control of the sheet position is often not required although the overall vibration control can still be exercised.