1. A control apparatus installed in a metal rolling facility which comprises a payoff reel (10) for feeding the metal sheet (1) to be rolledon the entrance side; at least a rolling mill (14, 15) whose rolls can be moved in the vertical direction; a takeup reel (19) for winding up the rolled metal on the exit side of the rolling mill (15); and plural electric motors (40, 41, 42, 42) for driving the payoff reel (10), the rolling mill (14, 15) and the takeup reel (19), wherein the control apparatus comprises inverters (50, 51, 52, 53) connected respectively with the plural electric motors (40, 41, 42, 42) to control the rotational speeds and/or torques of the motors (40, 41, 42, 42) by controlling the voltages and/or the frequencies of the voltages, applied to the electric motors (40, 41,42,42); a primary frequency detector (71) for detecting the primary frequencies of the inverters (50, 51, 52, 53); and a controller (61) for controlling the rotational speeds and/or torques of the motors (40, 41, 42, 42) on the basis of the detected primary frequencies of the inverters (50, 51, 52, 53) in order that the detected frequencies may not be reduced to zero.

2. A control apparatus installed in a metal rolling facility as claimed in claim 1, wherein a strip store mechanism (13) is provided on the entrance side of the rolling mill (14); and the operation of the metal rolling facility is accelerated or decelerated according to the length of metal sheet to be rolled remaining in store within the strip store mechanism (13).

3. A control apparatus installed in a metal rolling facility as claimed in claim 1 or 2, wherein the torques of the electric motors (40, 41, 42, 42) are changed by changing the tension exerted on the metal sheet (1) to be rolled so that the the primary frequencies of the inverters (50, 51, 52, 53) can be prevented from becoming zero.

4. A control apparatus installed in a metal rolling facility as claimed in claims 1 through 3, wherein the torques of the electric motors (40, 41, 42, 42) are changed by changing the target value for the thickness of the metal sheet (1) to be rolled or the target value for the load on the rolls of the rolling mill (14, 15).

5. A control apparatus installed in a metal rolling facility as claimed in claims 1 through 4, wherein the speed of feeding the metal sheet (1) to be rolled, the tension exerted on the metal sheet (1) to be rolled and/or the load on the rolls of the rolling mill (14, 15) are selectively changed in accordance with the operating condition of the metal rolling facility.

6. A control method for use in a metal rolling facility which comprises a payoff reel (10) for feeding the metal sheet (1) to be rolled on the entrance side; at least a rolling mill (14, 15) whose rolls can be moved in the vertical direction; a takeup reel (19) for winding up the rolled metal sheet on the exit side of the rolling mill (15); plural electric motors (40, 41, 42, 43) for driving the payoff reel (10), the rolling mill (14, 15) and the takeup reel (19); and inverters (50, 51, 52, 53) connected respectively with the plural electric motors (40, 41, 42, 43) to control the rotational speeds and/or torques of the motors (40, 41, 42, 43) by controlling the voltages and/or frequencies of the voltages, applied to the motors (40, 41, 42, 43), wherein the control method comprises a step of detecting the primary frequencies of the inverters (50, 51, 52, 53), and a step of controlling the rotational speeds and/or torques of the motors (40, 41, 42, 43) on the basis of the detected primary frequencies of the inverters (50, 51, 52, 53) in order that the detected frequencies may not be reduced to zero.

7. A control method for use in a metal rolling facility as claimed in claim 6, wherein a strip store mechanism (13) is provided on the entrance side of the rolling mill (14); and the metal rolling facility is accelerated or decelerated according to the length of metal sheet to be rolled remaining in store within the strip store mechanism (13).

8. A control method for use in a metal rolling facility as claimed in claim 6 or 7, wherein the torques of the electric motors (40, 41, 42, 42) are changed by changing the tension exerted on the metal sheet (1) to be rolled so that the the primary frequencies of the inverters (50, 51, 52, 53) can be prevented from becoming zero.

9. A control method for use in a metal rolling facility as claimed in claims 6 through 8, wherein the torques of the electric motors (40, 41, 42, 42) are changed by changing the target value for the thickness of the metal sheet (1) to be rolled or the target value for the load on the rolls of the rolling mill (14, 15).

10. A control method for use in a metal rolling facility as claimed in claims 6 through 9, wherein the speed of feeding the metal sheet (1) to be rolled, the tension exerted on the metal sheet (1) to be rolled and/or the load on the rolls of the rolling mill (14, 15) are selectively changed in accordance with the operating condition of the metal rolling facility.

11. A control apparatus installed in a metal rolling facility, substantially as herein described with reference to accompanying drawings and example.

BACKGROUND OF THE INVENTION
This invention relates to an apparatus and a method for controlling the operation of a cold-rolling facility.
In a cold-rolling facility, electric current concentrates on a part of the circuitry associated with the electric motors or on a particular one of the switching elements constituting the invertors if the the torque currents of the electric motors are increased when the rolling line is at a halt or operated at an extremely low speed. As a result, that part of the circuitry and/or the particular one of the switching elements are sometimes broken down. In order to prevent such an accident, therefore, the counter measure is taken that the torque currents are reduced or restricted when the rolling line is at a halt or that the rolling line is prevented from being operated at an extremely low speed.
In the metal rolling facility disclosed in the international publication pamphlet WO. 2008/062506, the operation of the metal rolling facility is continued at a low speed in a range of 0 mpm (meter/minute) through 50 mpm until the welding process for the metal to be rolled has been finished on the entrance side of the rolling line. Accordingly, electric motors and inverters having sufficiently large current capacities must be used in such a facility. In reality, in order that the rolling line may be operated not only at normal speed for normal rolling condition but also at an extremely low speed with the same rolling condition as in the normal rolling operation, an excessively large investment must be made into the existing rolling facility to meet the expected rolling capability.
It is customary in a cold-rolling facility that AC electric motors and inverters to energize the AC electric maters are used to drive the rolling mills, the recoiling reel and the payoff real. An AC motor rotates due to the torque generated by the alternating current flowing through its rotor or stator. The frequency of the alternating current is shifted from the rotational frequency of the electric motor by an amount corresponding to the slip frequency in the direction in which the torque is generated. When the cold-rolling facility is run at a low speed or at an extremely low speed, the rotational speed of each of the AC motors is slowed down so that the frequency on the primary side, i.e. motor side, of each of the inverters is also lowered. Hereafter, this frequency is referred to also as the "primary frequency "of the inverter. It should also be noted that although plural motors and plural inverters are used in the metal rolling
facility, a single motor and a single inverter are often mentioned and the operations thereof are described for the brevity of description in this specification where such mentioning and description will not cause an erroneous interpretation of the function and structure of the overall facility.
When the electric motor generates torque in the direction of rotation, the primary frequency of the inverter is higher than the rotational frequency of the electric motor by an amount equal to the slip frequency. On the other hand, when torque is generated in the direction opposite to the rotational direction of the electric motor, i.e. in the regenerative direction, the primary frequency becomes lower than the the rotational frequency of the electric motor by an amount equal to the slip frequency.
Accordingly, when the electric motor is operated at a low speed in the regenerative direction, the primary frequency of the inverter approaches zero, that is, the primary current of the inverter approaches direct current. An inverter is generally composed of switching elements and an electric motor is a sort of an inductive load. So, the electric resistance to the nearly direct current of the inductive load is very low. Namely, the voltage developed across the inductive load is low when current of low frequency follows through the inductive load. Consequently, the consumption of electric power usually increases at switching elements.
Further, when the primary frequency of the inverter is low, current of each phase tends to be held constant. As a result, the time for which current concentrates on a particular switching element is prolonged. It is therefore the general rule to provide a metal rolling facility with a protective circuit for limiting excessive current through switching elements and to prevent the electric motors and/or the inverters from being operated at such a slow speed, in order to prevent the switching elements from being damaged.
However, according to the metal rolling mill disclosed in the international publication pamphlet WO. 2008/062506, metal rolling operation must be continued at an extremely low speed while the metal sheets on the entrance side of the rolling mill are being welded together.
A first coil of metal sheet to be rolled is paid off from the payoff reel on the entrance side. When the payoff of the first coil of metal sheet from the payoff reel is completed, a second coil of metal sheet is to be paid off in like manner. The tail of the first coil and the head of the second coil are welded together by the welder and the continuous sheet of metal is subjected to rolling. On the exit side of the welder is provided a looper or strip storage mechanism for storing a certain length of metal sheet, which enables rolling operation to be
continued during welding by supplying the metal sheet to be rolled therefrom. If the maximum length of the metal sheet stored in the looper is made large enough, rolling operation can be continued at a relatively high speed.
In order to minimize the equipment investment to the facility, the looper is usually so designed that the length of metal sheet it can store is 100 meters at most. The shorter the length of metal sheet the looper can store, the smaller the equipment investment to the facility, and the simpler the structure of the looper. If the maximum length is in the order of several meters, a single stage of the vertical type looper can serve as such a strip storage mechanism.
In the case where the length of metal strip stored in the looper is limited short, rolling operation must not be halted but be continued only while welding is under way on the entrance side of the looper. Therefore, rolling speed must be extremely low during the time of welding.
With such a rolling mill as described above, even when it is operated at an extremely low rolling speed, the tension and the rolling torque sufficient enough for matel rolling must be generated. For this purpose, there is need for electric motors and inverters that perform satisfactorily functions even at an extremely low speed of rolling operation. Therefore, in order to realize a rolling facility having electric motors and inverters adapted for rolling operation at an extremely low speed, a costly equipment investment must be made to a rolling facility that has been designed compactly at low costs to adapt itself to the normal speed of metal rolling. This has been a problem with conventional metal rolling facilities.
SUMMARY OF THE INVENTION
The object of this invention is to provide a control apparatus and a control method, for use with a metal rolling facility which can perform rolling operations at extremely low speeds while the investment to the facility is restricted.
According to this invention, which has been made to attain the above object, there is provided a control apparatus installed in a metal rolling facility which comprises a payoff reel for feeding in the metal to be rolled; at least a rolling mill whose rolls can be moved in the vertical direction; a takeup reel for winding up the rolled metal on the exit side of the rolling mill; and plural electric motors for driving a payoff reel, the rolling mill and the takeup reel, wherein the control apparatus comprises inverters connected respectively with the plural electric motors to control the rotational speeds and/or torques of the motors by controlling the voltages and/or frequencies thereof applied to the motors, a primary frequency detector for detecting the primary frequencies of the inverters, and a control unit for controlling the rotational speeds
and/or torques of the motors on the basis of the detected primary frequencies of the inverters in order that the detected frequencies may not be reduced to zero.
Also, according to this invention, there is provided a control method for use in a metal rolling facility which comprises a payoff reel for feeding in the metal to be rolled; at least a rolling mill whose rolls can be moved in the vertical direction; a takeup reel for winding up the rolled metal on the exit side of the rolling mill; plural electric motors for driving a payoff reel, the rolling mill and the takeup reel; and inverters connected respectively with the plural electric motors to control the rotational speeds and/or torques of the motors by controlling the voltages and/or frequencies thereof applied to the motors, wherein the control method comprises a step of detecting the primary frequencies of the inverters, and a step of controlling the rotational speeds and/or torques of the motors on the basis of the detected primary frequencies of the inverters in order that the detected frequencies may not be reduced to zero.
As a result of preventing the primary frequencies of the inverters from nearing zero, there is provided a control apparatus and method for use in a metal rolling facility which can perform rolling operations at extremely low speeds without damaging the facility components, with a minimum investment to the facility, and which can maintain the high quality of the rolled metal as a product.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 schematically shows the entire structure of a control apparatus as an embodiment of this invention, along with the associated metal rolling facility;
Fig.2 graphically shows the relationship between the rotational speed of motor and the primary frequency of inverter;
Fig. 3 illustrates a procedure for preventing the primary frequency of the inverter from nearing zero by changing the rotational speed of motor;
Fig. 4 illustrates a procedure for preventing the primary frequency of the inverter from nearing zero by changing the torque of motor;
Fig. 5 schematically shows the structure of a primary frequency detector as an example;
Fig. 6 is a flow chart detailing the operation of the primary frequency detector shown in Fig. 5;
Fig. 7 schematically shows the structure of a primary frequency detector as another example;
Fig. 8 is a flow chart detailing the operation of the primary frequency detector
shown in Fig. 7;
Fig. 9 is a flow chart detailing the process of changing the operational setting in the rolling mill; and
Fig. 10 is a flow chart detailing the process of changing the operational setting in the payoff reel.
DETAILED DESCRIPTION OF THE EMBODIMENT
A control apparatus and method as embodiments of this invention will now be described as applied to a metal rolling facility having two stands of rolling mills. It is matter of course that the apparatus and method of this invention can be applied also to a metal rolling facility having one or more than two stands of rolling mills.
Fig. 1 schematically shows the structure of a control apparatus as an embodiment of this invention, for use in a two stand continuous rolling facility. The control apparatus of this embodiment includes a primarily frequency detector 71 for detecting the primary frequencies of the inverters and an operational setting modifier 72.
In this embodiment, as shown in Fig. 1, the coil of metal to be rolled is mounted on a payoff reel (supply reel) 10 and the metal to be rolled is fed into a pinch roll 11. Then, a strip storage, or looper, 13 stores a length of metal strip to be rolled long enough to continue rolling operation while welding is taking place. Under this condition, the metal is continuously fed into Nos. 1 and 2 stand mills 14 and 15 so as to be rolled to a desired thickness. The rolled metal sheet is then wound up on a tension reel (wind-up reel) 19 with a desired tension.
As the supply of the first coil of metal to be rolled is finished, a second coil of metal to be rolled is mounted on the payoff reel 10. Then, the tail of the metal strip of the first coil and the head of the metal strip of the second coil are welded together by a welder 12. While welding is taking place, it is necessary to stop the feed of the metal strip in the welder 12. Therefore, during the welding operation, the metal strip to be rolled should be supplied from the strip storage 13 at an extremely low speed so that rolling operation can be continued.
A master controller 61 outputs motor rotational speed commands 611 and 612 to inverters 51 and 52 which respectively energize electric motors 41 and 42 that drive the No. 1 and No. 2 stand mills 14 and 15, on the basis of the rolling mill speed command and the preset tension. The inverters 51 and 52 control the rotations of the electric motors 41 and 42 by controlling the currents flowing through the electric motors 41 and 42 and the frequencies of the currents, on the basis of the motor rotational speed commands 611 and 612.
The master controller 61 calculates the torque which an electric motor 40 to drive
the payoff reel 10 should generate on the basis of the preset tension on the entrance side of the rolling mill 14, and outputs a payoff reel current command 610 to an inverter 50. The inverter 50 for energizing the motor 40 to drive the payoff reel 10 controls its output voltage and frequency on the basis of the payoff reel current command 610 so that the current flowing through the electric motor 40 may become equal to the command value.
Also, on the exit side of the rolling mill 15, the master controller 61 calculates the value of the current which is to flow through the motor 43 to drive the tension reel 16 and the value of the torque which the motor 43 is to generate, on the basis of the preset tension on the exit side of the rolling mill 15, and outputs a current command 613 to an inverter 53 for energizing the motor 43 to drive the tension reel 16. The inverter 53 for energizing the motor 43 to drive the tension reel 19 controls its output voltage and frequency in such a manner that the current flowing through the tension reel motor 43 becomes equal to the command value.
The thickness of the metal sheet on the entrance side of the rolling mill 14 is measured by a metal sheet thickness detector 20, while the feed speed of the metal sheet to be rolled is detected by a metal sheet speed detector 30 disposed at the entrance to the rolling mill 14. Accordingly, through data tracking of work speed, an automatic gauge controller 62 obtains the thickness of the metal sheet at the No. 1 stand mill 14.
Further, the speed of the metal sheet at the exit of the No. 1 stand mill 14 is measured by a metal sheet speed detector 31 disposed at the exit of the No. 1 stand mill 14. Then, the automatic gauge controller 62 calculates the ratio of the metal sheet speed at the entrance to the the No. 1 stand mill 14 to the metal sheet speed at the exit of the No. 1 stand mill 14, and obtains the thickness of the metal sheet at the exit of the No. 1 stand mill 14 by multiplying the calculated ratio with the obtained metal sheet thickness at the entrance to the No. 1 stand mill 14. The thus obtained thickness of the metal sheet is referred to as the "Mass-flow gauge". The metal sheet thickness is controlled by adjusting the load onto the rolls of the No. 1 stand mill 14 and the positions of the rolls in the vertical direction so that the Mass-flow gauge may coincide with a preset value.
The same is true of the No. 2 stand mill 15: the metal sheet thickness at the entrance to the No. 2 stand mill 15 is measured by a metal sheet thickness detector 21 disposed at the exit of the No. 1 stand mill 14; the metal sheet thickness at the No. 2 stand mill 15 is obtained on the basis of the metal sheet speed detected by the metal sheet speed detector 31 disposed at the exit of the No. 1 stand mill 14; the metal sheet speed at the exit of the No. 2 stand mill 15 is detected by a metal sheet speed etector 32 disposed at the exit of the No. 2 stand mill 15; the ratio of the metal sheet speed at the exit from the No. 1 stand mill 14 to the metal sheet speed at
the exit of the No. 2 stand mill 15 is obtained; and the Mass-flow gauge is obtained by multiplying the thus obtained speed ratio with the metal sheet thickness at the No. 2 stand mill 15.
This Mass-flow gauge is further improved in precision as follows. Through data tracking up to the metal plate thickness detector 22 disposed at the exit of the No. 2 stand mill 15 on the basis of the metal sheet speed detected by a metal sheet speed detector 32 disposed at the exit of the No. 2 stand mill 15, the Mass-flow gauge associated with the No. 2 stand mill 15 is obtained at the position of the metal sheet thickness detector 22 disposed at the exit of the No. 2 stand mill 15. An error in the Mass-flow gauge associated with the the No. 2 stand mill 15 is then determined by comparing itself with the metal sheet thickness measured by the metal sheet thickness detector 22 disposed at the exit of the No. 2 stand mill 15. Finally, a precise Mass-flow gauge is thus obtained through the correction of the Mass-flow gauge associated with the No. 2 stand mill 15.
In order to render the precise Mass-flow gauge to a desired metal sheet thickness, the metal sheet thickness at the exit of the No. 2 stand mill 15 is controlled by inputting to the master controller 61 a modified metal sheet speed renewed at the No. 1 stand mill 14 and also by adjusting the positions of the rolls of the No. 2 stand mill 15 in the vertical direction. The procedure described just above is in general called the Mass-flow gauge control.
An embodiment of this invention provides a control apparatus and a control method for controlling a metal rolling facility which uses electric motors and inverters having capacities necesary and satisfactory for normal operations and which can perform rolling operation at an extremely low speed of 50 mpm or less, or especially of 30 mpm or less.
According to the embodiment of this invention, a primarily frequency detector 71 is provided to detect the primary frequencies of the inverters 50 ~ 53 for energizing the electric motors 40-43. Therefore, the primary frequency detector 71 also detects the primary frequencies of the electric motors, and an operational setting modifier 72 modifies the values of preset rolling speed, tension, rolling load and metal sheet thickness, so that control is made so as not to cause the primary frequencies to approach zero. In the following are described a control apparatus and a control method, which prevent the primary frequencies from approaching zero according to this invention.
Fig. 2 graphically shows the relationship between the rotational speed (or angular frequency in other words) of electric motor and the primary frequency of inverter. Fig. 3 illustrates a procedure for preventing the primary frequency of the inverter from being reduced to zero by changing the rotational speed of motor. Fig. 4 illustrates a procedure for preventing the
primary frequency of the inverter from being reduced to zero by changing the torque of motor while keeping the rotational speed of motor constant.
When such a metal rolling facility as described above is operated at an extremely low speed, the electric motor to drive the payoff reel on the entrance side of the rolling mill or the electric motor to drive the rolling mill is subjected to a regenerative operation. In the regenerative operation of the electric motor, torque is generated in the direction opposite to the rotational direction of the motor. As a result, slip frequency is generated in opposition to the rotational direction of the motor. Accordingly, the primary frequency of the inverter becomes lower than the frequency (or rotational speed in other words) of the electric motor so that the metal rolling facility is operated with the primary frequency of the inverter maintained in the close vicinity of zero. Consequently, electric current concentrates on some switching element and therefore the inverter may sometimes be broken down.
In order to control the primary frequency of the inverter, it is necessary to obtain the primary frequency itself first. In order to prevent the primary frequency of the inverter from becoming zero through the control of the primary frequency, it is necessary to obtain the primary frequency of the inverter when the rotational speed of the electric motor coincides roughly with the slip frequency. The primary frequency of the inverter may be obtained through calculation on the basis of the motor torque and rotational speed, or directly through measurement on the basis of of the current command to the inverter.
The methods for preventing the primary frequency of the inverter from becoming zero through the control of the primary frequency, are divided into two categories. One method is to modify the rotational speed of the electric motor as shown in Fig. 3. To modify the rotational speed of the electric motor, the run speed of the rolling mill line must be changed.
The other method is to change the slip frequency by changing the torque of the electric motor as shown in Fig. 4. In order to change the slip frequency by changing the torque of the electric motor, the target value of gauge, i.e metal sheet thickness, must be modified by changing the tension on the rolled metal sheet or the load on the rolls of the rolling mill.
If it is necessary to change the run speed of the rolling mill line, the speed command to the rolling mill line as a whole has only to be modified. However, there may be cases where the run speed cannot be increased due to the limitation on the time required for welding, the remaining length of metal sheet in store within the looper, etc. If the remaining length is less than a certain amount, the run speed of the rolling mill line is decreased.
The torque of the electric motor to drive the paayoff reel can be modified by modifying the tension on the metal sheet supplied from the payoff reel on the entrance side of the
rolling mill. The torque of the electric motor that drives the rolling mill, i.e. rolling torque, can be modified by changing the tension on the metal sheet on the entrance or exit side of that rolling mill or changing the rolling load through the modification of the target value of gauge.
In this way, according to this invention, control is made so as not to cause the primary frequency of the inverter to approach zero while monitoring the primary frequency. Namely, continuous operation can be made possible in the range of extremely low speeds by instantaneously and appropriately modifying rolling speed, tension, gauge setting, and rolling load in accordance with the intended operation at the time.
Further description will be made below of a control apparatus and method as another embodiment of this invention. Fig. 5 schematically shows the structure of an example of a primary frequency detector for detecting the frequency of the primary current (i.e. primary frequency) of the electric motor.
As shown in Fig. 5, the actually obtained rotational speed, torque current, and primary frequency of the electric motor are sent from motor speed controllers 51 and 52, and motor current controllers 50 and 53 to a primary frequency detector 71. As a result, the primary frequency of the electric motor is directly detected. The preventive frequency band Yrpm for which the control apparatus is prevented from being damaged as the primary frequency becomes zero, and the torque current X% for which the detection of the primary frequency becomong zero is started, are preset as control parameters.
Fig. 6 is a flow chart detailing the operation of the primary frequency detector shown in Fig. 5. In this operation, decision is made on whether or not the ratio of each of the torque currents of the respective electric motors to the rated current is larger than the preset ratio (X%) (S 61). When the ratio of each of the torque currents of the respective electric motors to the rated current is larger than the preset ratio (X%), the detection of the primary frequency is started.
The primary frequency is detected by the motor speed controller or the motor current controller (S 62). Then, decision is made on whether or not the absolute value of the detected primary frequency is smaller than Yrpm (S 63). And when the absolute value of the detected primary frequency is smaller than Yrpm, an operational setting modification demand is issued (S 64).
Fig. 7 schematically shows the structure of another example of a primary frequency detector for detecting the frequency of the primary current of the electric motor (i.e. primary frequency). The actually obtained rotational speed and torque current are sent from motor speed controllers 51 and 52, and motor current controllers 50 and 53 to a primary
frequency detector 71. The primary frequency is detected by estimating the primary frequency of the electric motor in the primary frequency detector 71. In this case, the slip frequency of each electric motor corresponding to the associated torque current is needed as a control parameter. As the slip frequency of each electric motor is a known quantity, it becomes possible to estimate the primary frequency of electric motor.
Fig. 8 is a flow chart detailing the operation of the primary frequency detector shown in Fig. 7. In this operation, decision is made on whether or not the ratio of each of the torque currents of the respective electric motors to the rated current is larger than the preset ratio (X%) (S 81). When the ratio of each of the torque currents of the respective electric motors to the rated current is larger than the preset ratio (X%), the detection of the primary frequency is started.
The estimated value of the primary frequency is calculated from the actually obtained torque current and rotational speed, and the slip frequency setting value (S 82). Then, decision is made on whether or not the absolute value of the calculated primary frequency is smaller than Yrpm (S 83). And when the absolute value of the calculated primary frequency is smaller than Yrpm, an operational setting modification demand is issued (S 84).
In the embodiments of this invention, the electric motor to drive the No. 1 stand mill and the electric motor to drive the payoff reel are supposed to be those for which the primary frequency becomes zero. Therefore, the setting of the speeds and the tensions associated with the No. 1 stand mill and the payoff reel is modified.
Fig. 9 is a flow chart detailing the process of changing the operational setting in the rolling mill. When an operational setting modification demand is issued from the primary frequency detector with respect to the electric motor to drive a rolling mill, the operational setting is modified (S 91). First, the length of metal sheet remaining in store within the looper is calculated (S 92a). The status of welding process in the welder 12 located on the entrance side of the looper 12 is checked and a necessary time for halt is calculated (S 92b). The maximum speed at which the rolling mill can be operated is calculated from the halt time on its entrance side and the remaining length of metal sheet in the looper (S 92c). If it is presumed that when the maximum speed is reached, the absolute value of the primary frequency becomes equal to or greater than Yrpm (S 93), then decision is made that acceleration is still possible so that the rolling mill is accelerated (S 94).
If the absolute value of the primary frequency does not become equal to or greater than Yrpm (S 93) even when the maximum speed is reached, then decision is made that acceleration is not possible so that the rolling mills are decelerated (S 95). Since the
deceleration increases friction between the rolls of the mill and the metal sheet to be rolled, the rolling load increases. So, decision is made on whether or not the rolling load is equal to or less than a preset value (S 96).
In the case where the rolling load is greater than the preset value, the torque of the electric motor and the upper and lower limits of the operational and mechanical tensions are previously set and if the tensions can be increased, the target values for the tensions on the entrance and exit sides of the rolling mill are increased (S 98).
In the case where the target values for the tensions cannot be increased as the tensions reached the upper limit, the tention values are maintained while the rolling mill is kept decelerated and the rolling load is restricted (S 99). In this case, an error occurs in the thickness of the rolled metal sheet at the exit of the No. 1 stand mill 14 but the No. 2 stand mill 15 adjusts the metal sheet thickness to a desired value.
Fig. 10 is a flow chart detailing the process of modifying the operational setting in the payoff reel. First, the status that the primary frequency of the electric motor to drive the payoff reel becomes zero, is detected. Then, when there is an operational setting modification demand (S 101), the storable length of metal sheet in the looper is calculated (S 102a), the time for operating the rolling mill at low speeds is calculated (S 102b), and the maximum speed attainable without halting the stand mill is calculated (S 102c).
If it is presumed that the absolute value of the primary frequency becomes equal to or greater than Yrpm when the calculated maximum speed of the payoff reel is reached, decision is made that acceleration is still possible (S 103) so that the payoff reel is accelerated (S 104). If the absolute value of the primary frequency does not become equal to or greater than Yrpm when the calculated maximum speed is reached, decision is made that acceleration is impossible (S 103) so that the payoff reel is decelerated (S 105).
In the case where the absolute value of the primary frequency becomes equal to or greater than Yrpm when the primary frequency is shifted by increasing the tension on the metal sheet under being rolled (S 106), the tension is increased (S 107). If the absolute value of the primary frequency does not reach Yrpm even when the tension is increased (S 106), the torque current is decreased by decreasing the tention and simultaneously the primary frequency is shifted in the direction opposite to the motor rotation. Also, at this time, the rolling load is checked and when it is equal to or greater than Z tons (S 109), limitation is set on the rolling load (S 110).
This invention is well applied for the control of the cold-rolling facility.

CLAIMS:
1. A control apparatus installed in a metal rolling facility which comprises
a payoff reel (10) for feeding the metal sheet (1) to be rolledon the entrance side;
at least a rolling mill (14, 15) whose rolls can be moved in the vertical direction;
a takeup reel (19) for winding up the rolled metal on the exit side of the rolling mill (15); and plural electric motors (40, 41, 42, 42) for driving the payoff reel (10), the rolling mill (14, 15) and the takeup reel (19),
wherein the control apparatus comprises
inverters (50, 51, 52, 53) connected respectively with the plural electric motors (40, 41, 42, 42) to control the rotational speeds and/or torques of the motors (40, 41, 42, 42) by controlling the voltages and/or the frequencies of the voltages, applied to the electric motors (40, 41,42,42);
a primary frequency detector (71) for detecting the primary frequencies of the inverters (50, 51, 52, 53); and
a controller (61) for controlling the rotational speeds and/or torques of the motors (40, 41, 42, 42) on the basis of the detected primary frequencies of the inverters (50, 51, 52, 53) in order that the detected frequencies may not be reduced to zero.
2. A control apparatus installed in a metal rolling facility as claimed in claim 1, wherein a strip store mechanism (13) is provided on the entrance side of the rolling mill (14); and the operation of the metal rolling facility is accelerated or decelerated according to the length of metal sheet to be rolled remaining in store within the strip store mechanism (13).
3. A control apparatus installed in a metal rolling facility as claimed in claim 1 or 2, wherein the torques of the electric motors (40, 41, 42, 42) are changed by changing the tension exerted on the metal sheet (1) to be rolled so that the the primary frequencies of the inverters (50, 51, 52, 53) can be prevented from becoming zero.
4. A control apparatus installed in a metal rolling facility as claimed in claims 1 through 3, wherein the torques of the electric motors (40, 41, 42, 42) are changed by changing the target value for the thickness of the metal sheet (1) to be rolled or the target value for the load on the rolls of the rolling mill (14, 15).
5. A control apparatus installed in a metal rolling facility as claimed in claims 1 through 4, wherein the speed of feeding the metal sheet (1) to be rolled, the tension exerted on the metal sheet (1) to be rolled and/or the load on the rolls of the rolling mill (14, 15) are selectively changed in accordance with the operating condition of the metal rolling facility.
6. A control method for use in a metal rolling facility which comprises
a payoff reel (10) for feeding the metal sheet (1) to be rolled on the entrance side;
at least a rolling mill (14, 15) whose rolls can be moved in the vertical direction;
a takeup reel (19) for winding up the rolled metal sheet on the exit side of the rolling mill (15);
plural electric motors (40, 41, 42, 43) for driving the payoff reel (10), the rolling mill (14, 15) and the takeup reel (19); and
inverters (50, 51, 52, 53) connected respectively with the plural electric motors (40, 41, 42, 43) to control the rotational speeds and/or torques of the motors (40, 41, 42, 43) by controlling the voltages and/or frequencies of the voltages, applied to the motors (40, 41, 42, 43), wherein the control method comprises
a step of detecting the primary frequencies of the inverters (50, 51, 52, 53), and
a step of controlling the rotational speeds and/or torques of the motors (40, 41, 42, 43) on the basis of the detected primary frequencies of the inverters (50, 51, 52, 53) in order that the detected frequencies may not be reduced to zero.
7. A control method for use in a metal rolling facility as claimed in claim 6, wherein a strip store mechanism (13) is provided on the entrance side of the rolling mill (14); and the metal rolling facility is accelerated or decelerated according to the length of metal sheet to be rolled remaining in store within the strip store mechanism (13).
8. A control method for use in a metal rolling facility as claimed in claim 6 or 7, wherein
the torques of the electric motors (40, 41, 42, 42) are changed by changing the tension exerted on the metal sheet (1) to be rolled so that the the primary frequencies of the inverters (50, 51, 52, 53) can be prevented from becoming zero.
9. A control method for use in a metal rolling facility as claimed in claims 6 through
8, wherein the torques of the electric motors (40, 41, 42, 42) are changed by changing the target
value for the thickness of the metal sheet (1) to be rolled or the target value for the load on the
rolls of the rolling mill (14, 15).
10. A control method for use in a metal rolling facility as claimed in claims 6 through
9, wherein the speed of feeding the metal sheet (1) to be rolled, the tension exerted on the metal
sheet (1) to be rolled and/or the load on the rolls of the rolling mill (14, 15) are selectively
changed in accordance with the operating condition of the metal rolling facility.
11. A control apparatus installed in a metal rolling facility, substantially as herein described with reference to accompanying drawings and example.