Patent GB-637785-A (GB)
Title: Improvements in and relating to screwdown control systems for rolling mills
IPC: B21B37/60
Filed: 1946-04-24
Abstract: 637,785. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. April 24, 1946, No. 12393. Convention date, April 4, 1945. [Class 38 (iii)] [Also in Group XXXVIII] In a rolling mill provided with a pair of rolls 1, 4 and movable supporting means for one of them whereby the pass opening can be adjusted automatically for the successive traverses of the material, a screw-down adjustment is effected by a motor fed by an adjustable voltage generator, and a selecting device for any particular pass opening is set to control Selsyn devices responsive to positional disagreement between the selector and the rollsupporting means, thereby controlling theenergization of the generator field winding to cause the motor to drive the roll-supporting means to the selected position, and means is provided for limiting the generator field current to predetermined values which vary in accordance with the positional disagreement. The control system is so arranged that the final movement of approach of the movable roll to a selected position is always in the same direction, regardless of whether the opening is being increased or decreased, and means is provided for preventing overshooting during movement in that direction only. An adjustable electrical differential is also provided for varying the position of correspondence between the roll-supporting means and the selector device to compensate for variations in the diameter of the rolls. In the example, the upper roll 4 is adjustable by screws 7 and 8 at its end journals operated by reduction gears 10, 11 driven by a D.C. motor 9. The motor also drives a gear train 54 which drives tachometer generators 12, 96 and also the rotors of a pair of coarse and fine transmitter Selsyns 45, 46 which are geared together in a 35: 1 ratio and energized from a single-phase source. The stators of Selsyns 45, 46 are connected over 3-phase connections to the stators of Selsyns 49, 50 provided with manually adjustable rotors which are geared together also in a 35: 1 ratio and function as induction regulators whereby the signal passed on to the receiver Selsyns can be adjusted to compensate for wear of the rolls. The rotors of Selsyns 49, 50 are connected over 3-phase leads 47, 48 to similar branch connections to the stators of a number of pairs of coarse and fine receiver Selsyns, of which only the first pair 41, 42 and the last pair 43, 44 are shown. The coarse and fine receiver Selsyns comprising each pair are geared together in a 35: 1 ratio and can be preset manually according to the roll spacing required for the various passes, the setting being indicated by a rotary pointer, and the pairs of rotors are connected selectively to an amplifier circuit 15 by operation of a rotary pointer 13b which drives a rotary switch arm 13a over contact studs each connected to energize the coil of a relay, the first stud being shown connected to relay coil 58c which operates to close contacts 58a, 58b to connect the coarse and fine Selsyns 41, 42 of the first pass to the amplifying circuit 15. Signals from the " fine " Selsyns are fed over conductors 61 to the primary winding of a transformer 36 and applied to the grids of a twin triode valve 29 in a " push pull " arrangement with output connected to the grids of a pair of triodes 19, 20 which have their anodes connected respectively to the oppositely wound reference field windings 16c, 16d of a crossarmature excited machine 16. Signals from the " coarse " Selsyns are fed over conductors 60 to the primary winding of a transformer 55a with secondary winding connected across neon type valves 56, 57 to resistances 38, 40 which are also in the " fine " signal circuit to the grids of valve 29. The valves 56, 57 ensure that only signals above' a certain critical voltage can be fed to the grids from transformer 55, the latter being so designed that the voltage is below this figure when the positional disagreement of the '' coarse transmitter Selsyn 45 and operative " coarse '' receiver Selsyn is five degrees or less. This also provides that, with the 35: 1 gear ratio between the rotors of associated " coarse " and " fine " Selsyns the control is taken over by the " coarse " receiver Selsyn before the rotor of the " fine " receiver is displaced 180 degrees relative to the rotor of " fine " transmitter 46. The cross-armature excited machine 16 is driven by any suitable means at a preferably constant speed and controls the energization of the field 17a of a generator 17 driven at constant speed by an induction motor 18, to supply the operating voltage of motor 9. When the position of the roll 4 does not correspond with its selected position, a signal, dependent on the direction and extent of divergence between the transmitter Selsyns and whichever pair of receiver Selsyns is determined by the selector 13, is applied to one or the other of the field coils 16c, 16d of the exciter 16, and the roll 4 is operated accordingly by motor 9. A feed.- back voltage proportional to the speed of motor 9 is derived from tachometer generator 96 by conductors 97 connected to tappings on a resistance 95 to impose on the control grids of the valve 29 a voltage opposing that supplied from the Selsyns. To this is also added a control voltage proportional to the rate of change of that supplied to motor 9 and derived from a transformer 98 with its primary winding across the motor terminals. The means for limiting the current in the field winding 17a of generator 17 includes a control field winding 16f of the exciter 16, which has one end connected to the mid-point of a resistance 65 connected across a source of substantially constant D.C. and the other end connected over a lead 70 to one terminal of the generator. Oppositely arranged rectifiers 67, 68 are connected to the ends of resistance 65 whereby the voltage across a resistance 69 in the supply circuit to motor 9 is compared with the reference voltage. When the voltage drop across the resistance 69 exceeds the reference voltage across one half of resistance 65 current flows in the field winding 16f in opposition to which of the directional control field windings 16c or 16d is predominantly energized, thereby acting to limit the current supply to motor 9. Means are provided to maintain a downward force on the roll 4 when the motor 9 is at rest in a selected position, by maintaining a small current through the motor armature. This current is limited to prevent overheating by means including a relay 71 energized through a rectifier fed by a tachometer generator 72 driven by the motor 9, so that when the motor is rotating the voltage developed by the tachometer maintains the relay contacts 71a open. When the motor 9 ceases to rotate the contacts 71a close and short-circuit the reference voltage potentiometer 65, with the result that the voltage across resistance 69 is applied practically unopposed to the exciter field 16.f and the armature current of motor 9 is thus limited to a low value. Overshooting of the roll 4 in its positional movement is prevented by providing a stabilizing circuit wherein the current limiting exciter field winding 16f is energized in response to the difference between a reference voltage derived from a centre-tapped resistance 74 connected across a rectifier 75, and the voltage across a resistance 78 in the circuit of the generator field 17a. The reference voltage across the resistance 74 and rectifier 75 is supplied by a transformer 77 and may be varied by inserting the reference coil 81a of saturable core reactor in the primary winding circuit, the saturating winding 81b of the reactor being fed with a direct current proportional to the positional disagreement between the active rollopening selector unit and the mill roll 4. This direct current is derived from an amplifier circuit 82 connected over conductors 83 to receive signals from the rotor of the active coarse control Selsyn, these signals being rectified and used to bias a valve 84 controlling an output valve 85 connected to the saturable reactor. The system is further stabilized by applying to the grid of the valve 84 a reverse polarity voltage obtained from the tachometer generator 72 through a rectifier 73. In addition, a further stabilization is effected through amplifier 15 by feeding the voltage derived from the " fine '' Selsyn to the gridcontrolling transformer 36 by way of selenium cell rectifiers 100, 101, so that no signal voltage is applied to the input circuit until there are a few degrees of positional disagreement between transmitting and receiving " fine " Selsyns. The movable contact arm 13a of the selector may be moved by means of an electric motor provided with a push-button control comprising an " up '' and a " down " push button, Fig. 2 (not shown), one of which is operated as required after setting a pointer on a scale indicating the total number of passes required, this operation also serving to set a limit switch in a corresponding position. The depression of the push button closes a holding circuit which maintains the system energized until the pilot motor actuating the switch contact arm has moved the latter to the next pass position, the mill roll in the meantime having also been moved correspondingly. At this stage a detent switch opens the push-button control circuit, necessitating a further depression of the button to operate the system to the next pass position, and so on until the selected position is reached.