Thickness control system for a rolling mill

Abstract: A rolling mill (32) has a hydraulic roll-gap control system (66) for setting the roll gap between two work rolls (3, 4) of the rolling mill and a mill modulus control unit (54) for supplying a correction signal (C p ) to the hydraulic roll-gap control system based on the difference between a reference rolling pressure and the actual rolling pressure during rolling detected by a load cell (1). The rolling mill includes a thickness control system on at least the entry side of the rolling mill including a tension controller (33) which comprises means (35) for applying a force to the workpiece (30) in the direction of its thickness, that is to say perpendicular to the plane of the workpiece, means (37) for producing a signal (T) indicative of the tension in the workpiece, means (45) for comparing the said signal (T) with a reference signal (T ref ) and producing a different signal (ΔT) and means (40, 42) responsive to the different signal and arranged to control the force-applying means (35) to vary the tension in the workpiece so as to reduce the value of the different signal, i.e. maintain the tension substantially constant.

Description: The present invention relates to a thickness control system for a hydraulically loaded rolling mill to ensure highly responsive thickness control for a workpiece. Figure 1 is a schematic side view of an example of a known hydraulically loaded rolling mill, namely a single stand reversible cold rolling mill 32 having uncoiling and coiling reels 20 and 27 on the entry and exit sides. More specifically, a workpiece 30 to be rolled is fed from the reel 20 driven by a motor 19 and passes over a deflector roll 21 and is rolled between upper and lower work rolls 3 and 4. The rolled workpiece 30 passes over a further deflector roll 26 and is coiled by the reel 27 driven by a motor 28. The reel driving motors 19 and 28 are associated with respective reel-motor tension controllers 18 and 29 so as to maintain the tension of the workpiece on the entry and exit sides, respectively, constant. Generally, the tension controllers 18 and 29 serve to control the tensions in proportion to the motor currents. The rolling velocity or speed in the rolling line is controlled to a predetermined value by controlling a work-roll driving motor 23 by means of a speed controller 24. In Figure 1, reference numeral 1 denotes a load cell for detecting the rolling pressure; 2 and 5, upper and lower back-up rolls; 6, a hydraulic cylinder for setting the roll gap between the work rolls 3 and 4; 8, a servo valve connected through a piping 7 to the cylinder 6; 9, a displacement gauge for sensing displacement of a draft ram 6' in the cylinder 6; 10, a servo amplifier for transmitting a command in the form of a current signal to the servo valve 8; and 11, a coefficient multiplier for providing a control gain K G to amplify an output signal from a comparator 12 to control the draft position S' of the ram 6'. In a basic position control loop, an instruction signal R is compared with an output signal S from the displacement gauge 9 and a signal e representative of any deviation derived is multiplied by the gain K G in the coefficient multiplier 11. The opening of the servo valve 8 is controlled with the multiplied signal through the servo amplifier 10 to quantitatively adjust the supply of pressurised oil through the piping 7 to the cylinder 6, thereby controlling the position S' of the ram 6'. As a result, the lower back-up and work rolls 5 and 4 are displaced to adjust the roll gap between the work rolls 3 and 4 to a predetermined value by the components described above which together constitute a hydraulic roll-gap control system 66. Control of only the position S' of the ram 6' would cause errors in the roll gap between the work rolls 3 and 4 due to elongation of those components of the mill which are subjected to the rolling pressure. This problem is usually compensated for as follows: A reference rolling pressure P ref is stored at an appropriate time after start of rolling. Difference ΔP between the reference rolling pressure P ref and the actual rolling pressure during rolling, which is detected by the load cell 1, in the form of a signal P is calculated by a comparator or adder-subtractor 17 and then is divided by the mill modulus K m , which is specific to a mill, in the manner of a spring constant and has been measured in advance, in a coefficient multiplier 16 of a mill modulus control unit 54 to calculate the elongation of the mill. The calculated elongation is multiplied by a correcting gain c which will set a correction percentage, thereby obtaining a modifying signal C p which is used to modify the position S' of the ram 6'. This signal C p is supplied to the adder 13 as an instruction for the above basic position control loop to correct the position S' of the ram 6'. This procedure is generally called mill modulus control. In order to ensure that the thickness of the workpiece 30 on the exit side of the mill has a desired or reference value h ref , a signal h representative of the actual thickness of the workpiece sensed by a thickness gauge 25 (or a thickness gauge 22, if rolling in the reverse direction) on the exit side of the rolling mill 32 is compared with the reference value h ref by a comparator or adder-subtractor 31 to obtain a thickness deviation Δh. This deviation is passed through an integral controller 15 and is multiplied by a correction gain 1+(M/K e ) for correction into an actual draft position in a coefficient multiplier 14 to obtain a modifying signal C h for correction of the position S' of the ram 6'. The modifying signal C h is also supplied to the adder 13 as an instruction for the above basic position control loop to correct the position S' of the ram 6'. This procedure is called monitor AGC. M is a constant representative of the hardness of the workpiece 30 and has been measured in advance. K e is a controlled mill modulus and satisfies the equation: K e = K m /(1 - c) . When the position S' of the draft ram 6' is changed to control the thickness of the workpiece 30 in the rolling mill of Figure 1, the tensions applied to the workpiece 30 on the entry and exit sides fluctuate. For example, when the roll gap between the work rolls 3 and 4 is narrowed so as to decrease the thickness of the workpiece 30, the workpiece 30 will elongate and the tensions on the entry and exit sides will decrease. Such fluctuation of the tensions may be absorbed by change of the peripheral velocities of the reels 20 and 27 which have a large inertia; but, this absorptive response is generally slower by one or more orders of magnitude than the hydraulic roll-gap control. This means that, once the roll gap is changed and the tensions of the workpiece 30 on the entry and exit sides fluctuate, the tensions cannot be returned to their preset values as quickly as the hydraulic roll-gap control. As a result, the decrease of the tensions on the entry and exit sides will cause the deformation resistance of the workpiece 30 to apparently increase and thereby nullify the narrowing of the roll gap, with the disadvantageous result that the workpiece thickness is not decreased. Thus, when attempt is made to decrease the thickness of a workpiece being rolled by a rolling mill with a high-response hydraulic roll-gap control, the workpiece thickness cannot be decreased at a rate greater than the rate of responsive change of the peripheral velocities of the reels 20 and 27. Therefore, a disturbance in the thickness on the entry side of, say, 2-3 Hz or more cannot be eliminated by stiffening the mill by the above-mentioned mill modulus control since the thickness control is not responsive, for the reason mentioned above. It is often heard at rolling plants that the thickness control accuracy cannot be improved as expected even when the position S' of the ram 6' is controlled quickly by the hydraulic roll-gap control system 66. This is to be attributed to the reason discussed above. Figure 2 shows a computer simulation example done by the inventor, which supports the above-mentioned fact. The installation simulated is the single stand reversible cold rolling mill shown in Figure 1 where a workpiece of width of 1800 mm, entry side thickness of 0.52 mm, entry side setting tension of 1.36 tons and exit side setting tension of 2.35 tons is rolled at rolling speed of 1800 m/min to a thickness of 0.3 mm, the roll gap being decreased midway and stepwise by 10 µm. The assumption is that the response of the hydraulic roll-gap control is 20 Hz with 90 degrees phase lag in frequency response and the desired value is reached within 0.04 second or less in a step response. According to the simulated results, the thickness change Δh on the exit side reaches a steady value within about 1 second when the roll gap is changed by 10 µm. In the actual hydraulic roll-gap control system, the desired value of the roll gap is reached within 0.04 second while the thickness change occurs 25 times more slowly than this, which is attributed to the fact that the response in terms of change of peripheral velocities of the reels 20 and 27 on the entry and exit sides is very slow, as described above. Thus, the reels 20 and 27, where tensions are controlled by maintaining the motor currents constant, have substantial inertia including the motors 19 and 28 so that changes of the peripheral velocities of the reels to some steady value to suppress tension fluctuations occur over about 1 second. The object of the present invention is to overcome the above problems encountered in the known thickness control systems and, in particular, to provide a thickness control system for a rolling mill which can enhance the response of the thickness control whereby a rolled product of increased accuracy of thickness is produced. This object is achieved by a thickness control system according to Claim 1. It is preferred that the thickness control system includes a thickness gauge for detecting the thickness of the workpiece to be rolled, a speed detector for detecting the speed of the workpiece to be rolled, a roll gap change computing element arranged to produce a roll gap change signal from the signal from the thickness gauge, to calculate the timing of a change in the roll gap to accommodate a change in thickness detected by the thickness gauge and to supply the roll gap change signal to the hydraulic roll gap control system at the calculating timing. The tension controller in accordance with the invention may be provided on the entry side only or on both the entry and exit sides of the rolling mill and will contribute to rapid suppression of any tension fluctuations in the workpiece caused by a change in the roll gap. It is preferred that the thickness control system includes a thickness gauge for detecting the thickness of the rolled workpiece, a mill modulus computing element arranged to produce a signal representative of an optimum mill modulus from at least one of the signals produced by the load cell and the signal produced by the thickness gauge and a correction gain setter arranged to produce a correction gain signal based on the mill modulus signal and to supply it to the mill modulus control unit. The present invention also embraces a rolling mill including a thickness control system of the type referred to above. The tension controller in accordance with the invention is provided in conjunction with a tension controller of the known type which acts on the supply reel on the entry side and optionally also on the take-up reel on the exit side of the mill, e.g. by maintaining the current in the motor of the or each reel constant. Further features and details of the invention wil be apparent from the following description of certain specific embodiments which is given by way of example with reference to Figures 3 to 23 of the accompanying drawings, in which:- Figure 3 is a block diagram showing a first embodiment of the present invention; Figure 4 shows a specific example of the tension controllers 33 and 34 in Figure 3; Figures 5 to 7 show the results of a computer simulation of the response when the response of the reel-motor tension controllers 18 and 29 is assumed to be three times higher than in the conventional system of Figure 1 wherein Figure 5 shows the case where the response is assumed to be higher in the tension controllers 18 and 29 on the entry and exit sides; Figure 6 shows the case where the response is assumed to be higher only in the tension controller 29 on the exit side, and Figure 7 shows the case where the response is assumed to be higher only in the tension controller 18 on the entry side; Figure 8 is a block diagram showing a second embodiment of the present invention; Figure 9 illustrates a specific example of the tension controllers 48 and 49 in Figure 8; Figure 10 illustrates a specific example of the tension controller of a third embodiment of the present invention; Figure 11 is a block diagram of a specific example using an electromagnet or linear motor as the tension controller of a fourth embodiment of the present invention; Figure 12 is a diagram used to explain the tension control principle of the reels 20 and 27; Figure 13 is a block diagram showing the influence on the exit side thickness Δh when the roll gap ΔS is changed; Figure 14 is a block diagram used to explain the performance of the tension controller of the present invention; Figure 15 is a block diagram of a fifth embodiment of the present invention in which the control gain is corrected in acordance with the coil radius; Figure 16 is a block diagram showing a sixth embodiment of the present invention with the control gain being corrected in accordance with mill speed; Figure 17 is a diagram showing the results of a computer simuation of exit side thickness change and entry side tension fluctuation with entry side thickness change; Figure 18 is a diagram showing the results of a computer simulation of exit side thickness change and entry side tension fluctuation with entry side thickness change in the system of Figure 3; Figure 19 is a diagram showing the results of computer simulation of exit side thickness change and the entry side tension fluctuation with roll eccentricity in the conventional system of Figure 1; Figure 20 is a diagram showing the results of a computer simulation of exit side thickness change and entry side tension fluctuation with roll eccentricity in the system of Figure 3; Figure 21 is a block diagram of a seventh embodiment of the present invention; Figure 22 is a diagram showing the results of computer simulation in the case where the mill modulus is increased by a factor of three in the system of Figure 21; and Figure 23 is a diagram showing the results of a computer simulation in the case where the natural mill modulus is used in the system of Figure 21. Figure 3 shows a first embodiment of the present invention applied to a single stand reversible cold rolling mill in which tension controllers 33 and 34 are disposed on both the entry and exit sides of a rolling mill 32 which is otherwise the same as that shown in Figure 1. The component parts shown in Figure 1 are referred to by the same numerals and will not be described again. Figure 4 shows one construction of the tension controllers 33 and 34 in which a pressure roll 35 is rotatably supported on an arm 36 and engages the workpiece 30. A load detector or load cell 37 is mounted on a bearing of the pressure roll 35 to detect the reaction force exerted by the workpiece 30. The arm 36 is connected to a lever 38 and is pivotable about a shaft 39 to effect vertical movement of the roll 35. The lever 38 is further connected to a piston rod 41 extending through a hydraulic cylinder 40 and is rotatable about the shaft 39 by adjusting the supply of a liquid to the cylinder 40 by means of a servo valve 42. Rotational movement of the lever 38 causes the arm 36 connected thereto to swing, thereby moving the pressure roll 35 vertically. The servo valve 42 is adjusted as follows Based on the reaction force of the workpiece 30 detected by the load cell 37, the tension T of the workpiece 30 is obtained by a tension computing element 46 and is compared with a preset tension value T ref by a comparator or adder-subtractor 45 to obtain a deviation ΔT therefrom. The deviation ΔT is multiplied by a coefficient K T in a coefficient multiplier 44 and is used to control the servo valve 42 through a servo amplifier 43 to make the deviation ΔT zero. Due to the tension controllers 33 and 34, illustrated in Figure 4, any change of the roll gap causes a resultant tension fluctuation which is detected by the load cell 37 on the bearing of the pressure roll 35. In order to make this equal to the desired value T ref , the inflow and outflow of fluid into and from the hydraulic cylinder 40 is adjusted by the highly responsive servo valve 42 so that the pressure roll 35 is moved vertically and the tension of the workpiece 30 is promptly varied. Accordingly, any roll gap change by the hydraulic roll-gap control instantly infuences the exit side thickness of the workpiece 30 so that highly responsive thickness control can be effected in comparison with the conventional tension control utilising motor current. In the system of Figure 3, the reel-motor tension controllers 18 and 29 suppress relatively slow tension fluctuations and the tension controllers 33 and 34 absorb faster tension fluctuations. Figure 5 shows a simulation in which the response of the reel-motor tension controllers 18 and 29 on the entry and exit sides of the rolling mill 32 in Figure 1 is assumed to be three times higher than in Figure 2. When the roll gap is decreased stepwise by 10 µm, the exit side thickness Δh reaches a steady value after about 0.3 second, i.e. three times as quickly as in the simulation of Figure 2. The tension controllers 33 and 34 in Figure 4, which are as rapidly responsive as the hydraulic roll-gap control, can suppress tension fluctuations at a higher speed than the simulation example in Figure 5 to thereby control the thickness of the workpiece. Figure 6 is a simulation of the case where, in the rolling mill of Figure 1, the response of only the exit side reel-motor tension controller 29 is assumed to be three times higher whereas the response of the entry side reel-motor tension controller 18 is the same as in Figure 2. On the other hand, Figure 7 is a simulation of the case where the response of only the entry side tension controller 18 is assumed to be three times while the response of the exit side tension controller 29 is the same as in Figure 2. As is evident from Figures 6 and 7, providing a quick response time of only the entry side tension controller, which exerts a greater influence on the workpiece than the exit side tension controller, will attain substantially the same effects as providing a quick response of both the entry and exit side tension controllers in Figure 5. This means that, as regards the entry and exit side controllers 33 and 34 in the embodiment of Figure 3, control of only the entry side one 33 will suffice for attaining the required effect in the case of the rolling direction shown. Therefore, though a reversible rolling mill will require tension controllers on both sides of the mill, only an entry side tension controller will suffice for a non-reversible rolling mill. Figure 8 shows a second embodiment of the present invention in which load cells 50 on the bearings of the deflector rolls 21 and 26 detect the tensions in the workpiece 30. Based on the detected tensions, tension controllers 48 and 49 adjust the vertical position of the pressure rolls 35 (see Figure 9) to control the tension of the workpiece 30. The same components as in the first embodiment shown in Figures 3 and 4 are referred to by the same numerals. Figure 9 shows an example of the tension controllers 48 and 49 in Figure 8 which are substantially similar to the controllers 33 and 34 of the first embodiment shown in Figures 3 and 4 except that, instead of the load cell 37 for the pressure roll 35, a load cell 50 is mounted on each of the bearings for the deflector rolls 21 and 26 to detect the reaction force from the workpiece 30. Thus, when the roll gap is changed, the resultant tension fluctuation is detected by the load cell 50 on the bearing of the deflector roll 21 (26). To make this equal to the desired value T ref , the inflow and outflow of fluid into and out of the hydraulic cylinder 40 is adjusted by the highly responsive servo valve 42 so that the pressure roll 35 is vertically displaced to instantly change the tension on the workpiece 30. Accordingly, any roll gap change by the hydraulic roll-gap control promptly infuences the exit side thickness of the workpiece 30. As in the case of the first embodiment, the tension controllers 48 and 49 are combined with the conventional reel-motor tension controllers using motor current to achieve highly responsive thickness control. Figure 10 shows a third embodiment of the present invention in which the tension controller 61 utilizes a fluid film instead of a pressure roll and comprises a fluid pad 57, a control valve 58, a fluid source 59 and piping 60 for connecting these components. The components which are the same as in the first and second embodiments are referred to by the same numerals. The fluid pad 57 injects fluid from the source 59 through the valve 58 against the lower surface of the workpiece 30 to form a liquid film. This film supports the workpiece 30 by its pressure and imparts tension to it. The load cell 50 on the bearing for the deflector roll 21 (26) detects the reaction force from the workpiece 30. The output of the load cell 50 is inputted into a tension computing element 62 to obtain the tension T in the workpiece 30. The tension T thus obtained is compared with the tension reference value T ref by a comparator or adder-subtractor 63 to obtain a deviation ΔT therefrom. The coefficient multiplier 64 multiplies this deviation ΔT by a coefficient K TV and inputs it into a control valve regulator 65 which regulates the opening of the control valve 58 in response to the input signal and quantitatively controls the fluid emitted from the fluid pad 57. More specifically, in the case where the detected tension T is smaller than the tension reference value T ref , the control valve 58 is opened to increase the fluid flow rate to increase the tension. On the other hand, if the detected tension T is greater than the tension reference value T ref , the control valve 58 is throttled to decrease the fluid flow rate to decrease the tension. In this way, the tension of the workpiece 30 is controlled by the pressure of the fluid film to make the deviation ΔT zero. Figure 11 illustrates the fourth embodiment in which the tension controller 100 uses the attractive force of an electromagnet 101 in Figure 11 and the workpiece is ferromagnetic material such as iron. The components which are the same as in Figure 10 are referred to by the same numerals. Reference numeral 103 designates a regulator controlling the electromagnetic flux density. The electromagnet 101 is driven in response to the deviation ΔT of the detected tension T from the tension reference value T ref to generate a vertical attractive force on the workpiece 30 to control its tension. Instead of the electromagnet 101, linear motors may be disposed above and below the workpiece to impart tension in the workpiece by applying an attractive or repulsive force. In this case, the workpiece is limited to electrically conductive material. It has been explained in relation to Figures 5, 6, 7 and 2, that the speed of response of the thickness control can be improved by speeding up the response of the tension control. This will now be explained in more detail. Figure 12 illustrates the principle of the tension controllers for the reels 20 and 27. The torque τ of the motor 19 (28) required to generate a tension T in a coil 67 when the radius of the coil 67 is D is proportional to the product of D and T and is given by: τ ∞ T·D The output torque of the motor 19 (28) is expressed by: τ ∞ i·ø From (1) and (2) τ ∞ i·(ø/D) wherein i represents the motor current and ø the motor field magnetic flux. If the motor is controlled such that the coil radius D is proportional to the motor field magnetic flux ø, (ø/D) has a constant value and the tension T is proportional to motor current i. Thus, in the tension control for the reels 20 and 27, the coil radius D is made proportional to motor field magnetic flux ø and the required tension T is obtained by setting the motor current. This is the conventional tension control for the reels 20 and 27 during steady state rolling. As shown in Figure 2, with the conventional tension control, any roll gap change will result in a change of the exit side thickness of the rolled strip only after the response time of the tension control since the reels 20 and 27 have a substantial inertia and the response of the tension control is therefore relatively slow. Accordingly, the thickness accuracy cannot be improved with a high-response hydraulic roll-gap control. Figure 13 is a Bode diagram of the influence on the exit side thickness Δh when the roll gap ΔS is changed. The dotted line illustrates the response of a conventional tension controller while the solid line illustrates the response when the tension controller of the present invention (e.g. 48 in Figure 9) is disposed on the entry side of the rolling mill. In the conventional example shown by the dotted line, the influence of the roll gap ΔS is attenuated to as low as 1/1000 at 3.75 Hz. As described below, this sharp downward peak occurs due to the inertia of the reel 20 (27) and to resonance determined by the spring constant of the workpiece 30. By contrast, with the present invention as shown by solid line, the downward peak is displaced towards lower frequences and the peak attenuation is decreased to about 1/10. At 2-10 Hz, the characteristic becomes substantially flat at Δh/ΔS≒1 and the roll gap ΔS substantially determines the thickness Δh. Figure 14 is a block diagram explaining the performance and function of the tension controller of the present invention. The controller is omitted because of its quick response. The zone within the dotted line expresses the characteristics of the tension controller used in the present invention and the remainder expresses physical phenomena during the rolling operation. The symbols used are E : Young's modulus of workpiece, b : workpiece width, H : workpiece thickness, L 1 : distance between the rolling mill and reel, J : inertia moment of reel including coil, R : coil radius (= D/2), K t : gain of tension controller, S : Laplace operator, ΔV : rolling speed variation and ΔT b : backward tension fluctuation. The generation of actual tension fuctuation during a rolling operation and the functions or performance of the tension controller of the present invention are explained in this diagram. Firstly, the reel 20 (27) including the coil 67 (see Figure 12) is accelerated by the tension value T b which is proportional to the motor current value from a current controller (not shown) to generate a peripheral speed v of the reel at block 69. The reel peripheral speed v is disturbed by a speed change ΔV of the workpiece 30 due to tension fluctuations on the entry and exit sides of the mill 32 and/or due to the thickness variation of the workpiece 30, which causes speed unbalance through an adder 72. This is integrated (by the integrator 73) into an elongation difference Δℓ in the longitudinal direction of the workpiece 30 from which the tension stress change Δσ is calculated at block 76. The calculated tension stress change Δσ is multiplied by bH at block 78 so that the backward tension fluctuation ΔT b is obtained which is compared with the tension value T b in the adder 80 to obtain the deviation T b - ΔT b . Thus, the reel 20 (27) is driven by the deviation T b - ΔT b to compensate for the influence of ΔV. The compensatory response is slow, as already mentioned, because of the great inertia of the reel 20 (27) as shown in block 69. The above description relates to actual tension fluctuation during a rolling operation and the conventional tension fluctuation compensation by the reel 20 (27). By contrast, with the tension control system of the present invention, the tension fluctuation ΔT b is detected and is multiplied with conversion coefficient given by block 82 into an elongation change Δℓ r . The elongation change Δℓ r is multiplied by control gain K t in block 84 to obtain a control quantity Δℓ c which is used for tension control. As is evident from Figure 14, the response is much quicker as the inertia of the reels (block 69) is not involved. When the characteristics of the components within the dotted line of Figure 14 are not taken into account, the transfer function from ΔV to ΔT b is obtained from the following equation: The resonance frequency ω n is obtained from equation (4) as: ω n = EbH L 1 R 2 J and this value was 3.75 Hz in the conventional system shown by the dotted line in Figure 13. The transfer function from ΔV to ΔT b when utilising the characteristics of the tension control system of the present invention within the dotted line of Figure 14 is given by: G represents the dynamic characteristic of the tension controller (block 86 in Figure 14) and G = ω n 2 S 2 + 2ζω n S + ω n 2 From the equation (5), the resonance frequency ω n is given by: ω n = EbH L 1 · 1 1 + K t ·G R 2 J Thus, the tension controller of the present invention serves to change the Young's modulus of the workpiece 30 so that it alters the resonance frequency ω n caused by inertia of the reel 20 (27) and the spring constant (Young's modulus) of the workpiece 30 to a region where no influence is exerted on the thickness control. If a positive value is taken for K t , the resonant frequency is moved towards a lower frequency than the actual resonance frequency. If a negative value is taken, the resonant frequency is moved towards a higher frequency. The phenomena that the tension varies widely due to resonance of the reel 20 (27) and that the thickness is not changed even when the roll gap is changed, as seen in the conventional system, are thus prevented. Since the control of the roll gap directly influences the thickness, conventional thickness control modes such as feed forward AGC or BISRA (British Iron and Steel Research Association) AGC can be utilised effectively. Figure 15 shows a development of the invention based on the above concept. As is evident from equation (6), the inertia of a reel will alter as the coil radius R alters. In Figure 15, the radius R is detected by, e.g., an optical sensor 90. Based on the sensed value, a computing element 91 obtains a correction value ΔK t of the control gain K t and the control gain K t is changed accordingly. Figure 16 shows a further development of the invention in which the speed V of the workpiece 30 is detected by a detector 93. Based on the detected speed, the frequency of entry side thickness disturbance is calculated to obtain a required value ω n from which a correction quantity ΔK t of the control gain required is calculated, using equation (6), by the computing element 94 to thereby change the control gain K t . When a rolling mill is hardened so as to eliminate any entry side thickness disturbance by the mill modulus control, disturbances such as roll eccentricity generated by the mill itself naturally tend to influence the thickness, thereby disadvantageously resulting in a reduction of the thickness accuracy. This problem is conventionally solved with a so-called roll eccentricity elimination controller in which the roll eccentricity is obtained from e.g. a rolling pressure signal and on the basis of the obtained roll eccentricity the roll gap is corrected by moving it cyclically to counteract the eccentricity. However, this method cannot effectively eliminate the influence of eccentricity upon higher-speed rolling since the variation period of roll eccentricity is too quick to be effectively responded to and counteracted by the hydraulic roll-gap control. Figures 17 to 20 show the results of a computer simulation which the inventor has performed to review the above problem. The simulation was performed on a single stand cold rolling mill as shown in Figures 1 and 3. The workpiece, having an entry side thickness of 0.28 mm, a width of 1800 mm, an entry side setting tension of 1.42 tons and an exit side setting tension of 3.04 tons, was rolled to the desired thickness of 0.2 mm at rolling speed of 1800 m/min. The calculation was made on the assumption that the entry side thickness disturbance has an amplitude of ±4 µm and a fluctuating frequency of 5 Hz and that the roll eccentricity has an amplitude of ±3 µm and a fluctuating frequency of 6.53 Hz. Figures 17 and 18 represent cases where only the influence of entry side thickness fluctuation was studied. Figure 17 shows a case where the mill modulus is made ten times harder by the mill modulus control than in the conventional rolling mill 32 of Figure 1 and the exit side thickness fluctuation is 5.4 µm P-P to the entry side thickness fluctuation of 8 µm P-P . In the system of the present invention having the tension controller 33 on the entry side of the rolling mill as shown in Figure 3, the exit side thickness fluctuation can be decreased to 3.4 µm P-P , as is evident from Figure 18. This is because the entry side thickness fluctuation can be decreased by hardening the mill by the mill modulus control as the entry side tension fluctuation can be suppressed by the tension controller 33. By contrast, Figures 19 and 20 represent cases where only the influence of roll eccentricity was studied. Figure 19 shows a case where the mill modulus is made ten times harder by the mill modulus control than in the conventional rolling mill 32 of Figure 1 and where roll eccentricity of 6 µm P-P induced virtually no exit side thickness fluctuation at all. As regards the entry side tension fluctuation, the tension fluctuates to as high as 0.88 ton P-P so that roll eccentricity exerts almost no influence on thickness. On the other hand, when the tension controller 33 is disposed on the entry side of the rolling mill 32, as shown in Figure 20, the entry side tension fluctuation is substantially decreased to 0.2 ton P-P so that the exit side thickness fluctuation is increased up to 3.2 tons µm P-P . In other words, suppression of the entry side tension fluctuation will cause the change of roll gap due to roll eccentricity to exert an influence on the thickness of the workpiece. The above result reveals that, when the tension controllers 33 and/or 34 are disposed on the entry side or on both the entry and exit sides to adjust tension or tensions in the workpiece 30, both the factors attributable to the workpiece itself such as entry thickness disturbance, and factors attributable to the rolling mill, such as roll eccentricity, are to be taken into consideration. Figure 21 is a block diagram showing a seventh embodiment of the present invention. Those components which are the same as in Figure 3 are referred to by the same numerals. As shown in Figure 21, the tension controllers 33 and 34 to adjust the tensions applied to the workpiece 30 are disposed on the entry side or on both the entry and exit sides of the rolling mill 32. The thickness gauge 22 to detect thickness of the workpiece 30 and the speed detector 55 to detect the feed speed V of the workpiece 30 are disposed on the entry side of the rolling mill 32. Also, the thickness gauge 25 to detect thickness of the rolled workpiece 30 is disposed on the exit side of the rolling mill 32. Based on a signal t from the thickness gauge 22 on the entry side, a roll gap change computing element 51 calculates a roll gap change quantity necessary to counterbalance the entry side thickness disturbance. Based on a signal V S from the speed detector 55, the computing element 51 calculates the timing of the change of the roll gap, i.e. the timing where the entry side thickness disturbance will pass between the work rolls 3 and 4 of the rolling mill 32. The computing element 51 transmits, as an instruction to the basic position control, a roll gap change signal C F representative of the calculated quantity to the adder 13 at the calculated timing. Further, a mill modulus computing element 52 is provided by which an output signal P representative of the rolling pressure from the load cell 1 and/or a signal h representative of the exit side thickness from the thickness gauge 25 on the exit side is analyzed to obtain the frequency of the exit side thickness fluctuation and based thereon calculated an optimal mill modulus. A mill modulus signal K B representative of the optimal mill modulus is transmitted from the computing element 52 to a correction gain setter 53 which produces a correction gain on the basis of the signal K B and outputs a correction gain signal c to the mill modulus control unit 54. The above embodiment operates as follows: The tension controllers 33 and 34 measure tension fluctuations in the workpiece 30 and move the pressure roll or rolls 35, as shown in Figure 4, to reduce the fluctuations. Accordingly, the tension fluctuations due to roll gap change are quickly suppressed and the roll gap change influences the exit side thickness. The entry side thickness fluctuation is measured by the thickness gauge 22 on the entry side of the rolling mill 32 and the feed speed V of the workpiece 30 is measured by the speed detector 55. Based on the signals t and V S respectively from the thickness gauge 22 and the speed detector 55, the roll gap change quantity and the timing of the entry side thickness fluctuation passing between the upper and lower work rolls 3 and 4 of the rolling mill 32 are calculated by the roll gap change quantity computing element 51. The roll gap change quantity signal C F is outputted to the adder 13 of the basic position control loop. Thus, the roll gap between the work rolls 3 and 4 is adjusted and the entry side thickness fluctuation is eliminated. Also, based on the signal P from the load cell 1 and/or the signal h from the thickness gauge 25 on the exit side, the frequency component of the exit side thickness fluctuation is obtained and the optimal mill modulus for eliminating the influence of disturbances caused by the rolling mill 32 itself, such as roll eccentricity, is obtained by the mill modulus computing element 52. Based on the mill modulus signal K B outputted from the mill modulus computing element 52, a correction gain is produced by the correction gain setter 53 which outputs a correction gain signal c on the basis of which in turn the correction gain of the coefficient multiplier 16 in the mill modulus control unit 54 is changed. There is no need to supply both the signal P from the load cell 1 and the signal h from the exit side thickness gauge 25 to the mill modulus computing unit 52 and one of them will suffice. As shown in Figures 19 and 20, if roll eccentricity is the main cause of exit side thickness fluctuation, it is not desirable to harden or stiffen the mill by mill modulus control since this aggravates the exit side thickness fluctuation. However, in the Figure 21 embodiment, the mill modulus is set by the mill modulus control to make the mill softer in the event that the influence of roll eccentricity is considerable. Thus, exit side thickness fluctuation due to roll eccentricity is suppressed. On the other hand, the setting of the mill modulus by the mill modulus control to make the mill softer more or less means a stronger influence of entry side thickness disturbance on the exit side thickness fluctuation. However, in the Figure 21 embodiment, the entry side thickness fluctuation is measured by the thickness gauge 22 and the speed of the workpiece 30 is measured by the speed detector 55. The timing of the entry side thickness fluctuation passing between the work rolls 3 and 4 of the rolling mill 32 is obtained by the roll gap change quantity computing element 51 and the roll gap is changed from time to time in accordance therewith. Thus, the entry side thickness disturbance is suppressed and the influence of entry side thickness disturbance on the exit side thickness fluctuation can be reduced. Figures 22 and 23 show the results of computer simulations which were performed to show the effects of the embodiment of the present invention in the case where entry side thickness fluctuation and roll eccentricity are simultaneously present as disturbances. The conditions are the same as in Figures 17 to 20. Figure 22 shows the case where the mill modulus is increased by three times by the mill modulus control as compared to the Figure 21 embodiment (c = 0.67 when Ke = Km/(1 - c) ). Figure 23 shows the case where the natural mill modulus is used (c = 0). In Figure 2, the exit thickness fluctuation is about 3.4 µm due to the influence of roll eccentricity whereas in Figure 23 where the mill modulus is set to the optimal value, the fluctuation is decreased to about 2.6 µm and this demonstrates the excellent effects of the present invention. There is no need to calculate the optimal mill modulus at all times and it may be enough to calculate the optimal mill modulus only once according to the rolling pressure or exit side thickness and to preset the same. The above description relates to a single stand reversible cold rolling mill; however, it is to be understood that the present invention may also be applied to a non-reversible rolling mill for rolling in one direction. The tension in the workpiece may be detected from the reaction force of the workpiece on a roll or other components on the working path of the workpiece in place of the pressure roll and deflector roll.

Claims: A thickness control system for a single stand rolling mill (32) having a hydraulic roll gap control system (66) for setting the roll gap between two work rolls (3,4) of the rolling mill, a supply reel (20) on the entry side for supplying the workpiece (30) to the work rolls, a take-up reel (27) on the exit side, a tension controller (28) acting on at least the supply reel for controlling the tension in the workpiece (30) and a mill modulus control unit (54) for supplying a correction signal (C p ) to the hydraulic roll gap control system based on the difference between a reference rolling pressure and the actual rolling pressure during rolling detected by a load cell (1), characterised in that a further tension controller (33) is provided which includes means (35) for applying a force to the workpiece (30) in the direction of its thickness, means (37) for producing a signal (T) indicative of the tension in the workpiece, means (45) for comparing the said signal (T) with a reference signal (Tref) and producing a difference signal (ΔT) and means (40,42) responsive to the difference signal and arranged to control the force-applying means (35) to vary the tension in the workpiece so as to reduce the value of the difference signal. A system as claimed in claim 1 characterised by a thickness gauge (22) for detecting the thickness of the workpiece (30) to be rolled, a speed detector (55) for detecting the speed of the workpiece to be rolled, a roll gap change computing element (51) arranged to produce a roll gap change signal (C F ) from the signal from the thickness gauge (22), to calculate the timing of a change in the roll gap to accommodate a change in thickness detected by the thickness gauge (22) and to supply the roll gap change signal (C F ) to the hydraulic roll gap control system (66) at the calculating timing. A system as claimed in claim 2 characterised by a thickness gauge (25) for detecting the thickness of the rolled workpiece (30), a mill modulus computing element (52) arranged to produce a signal (K B ) representative of an optimum mill modulus from at least one of the signals (P) produced by the load cell (1) and the signal (h) produced by the thickness gauge (25) and a correction gain setter (53) arranged to produce a correction gain signal (c) based on the mill modulus signal (K B ) and to supply it to the mill modulus control unit (54). A system as claimed in any one of the preceding claims characterised in that the means for applying a force to the workpiece (30) comprises a pressure roll (35) and that the means arranged to control the pressure roll (35) include a hydraulic cylinder (40) arranged to move the pressure roll and a servo valve (42) which controls the supply of hydraulic fluid to the cylinder (40) in response to the difference signal (ΔT). A system as claimed in any one of claims 1 to 3 characterised in that the means for applying a force to the workpiece (30) includes a fluid support mechanism (57) arranged to produce a fluid film against the workpiece and a control valve (58) arranged to control the rate of fluid supply to the fluid film in response to the difference signal (ΔT). A system as claimed in any one of claims 1 to 3 characterised in that the means for applying a force to the workpiece (30) includes an electromagnet (101) or linear motor means arranged to exert an attractive or repulsive force on the workpiece and a regulator (103) arranged to vary the attractive or repulsive force in response to the difference signal (ΔT). A system as claimed in any one of the preceding claims characterised in that the means for producing a signal indicative of the tension in the workpiece (30) comprises a load cell (37;50) arranged to detect the force applied to a roll or other member, such as the pressure roll (35) or a deflection roll (21) arranged in the path of the workpiece. A system as claimed in any one of the preceding claims characterised by a sensor (90) arranged to produce a signal indicative of the radius of a coil (67) of the workpiece (30) on the entry side of the rolling mill and by a coefficient multiplier (44) through which the difference signal (ΔT) is supplied to the means (40,42) arranged to control the force applying means (35), the coefficient multiplier (44) being arranged to select either a negative or a positive control gain and the magnitude thereof in dependence on the value of the signal from the sensor (90). A system as claimed in any one of claims 1 to 7 characterised by a sensor (93) arranged to produce a signal indicative of the speed of the workpiece (30) to be rolled and by a coefficient multiplier (44) through which the difference signal (ΔT) is supplied to the means (40, 42) arranged to control the force applying means (35), the coefficient multiplier (44) being arranged to select either a negative or a positive control gain and the magnitude thereof in dependence on the value of the signal from the sensor (90).

Inventors: KUWANO HIROAKI, KUWANO, HIROAKI
Applicants: ISHIKAWAJIMA HARIMA HEAVY IND, ISHIKAWAJIMA-HARIMA JUKOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: B21B37
IPC codes: B21B37, B21B38