Patent GB-913600-A (GB)
Title: Improvements in or relating to automatic control systems
IPC: H05B7/156
Filed: 1958-12-29
Abstract: 913,600. Automatic arc-furnace control. REAVELL & CO. Ltd. (G.P.E. Controls Inc.). Dec. 29, 1958, No. 41955/58. Class 38 (4). [Also in Group XXIX] An hydraulic position control servo system of the type having a movable jet pipe control element, is modified by the provision of means whereby the deviation is allowed to reach a predetermined value before the control action begins, the control action continuing until the deviation is reduced to zero. In the first embodiment, Fig. 1, as applied to the position control of an arc furnace electrode the jet pipe 18 is positioned by a signal from an impedance-current transducer 12-15, 30-33 against the bias of a spring 34 adjustment of which varies the set point. The piston 19 is initially restrained from movement by a spring 41 and fluid leaves the cylinder 20 via pipes 35, 36 at a flow ratio determined by the jet pipe position. The pipes 35, 36 are connected respectively to cylinder chambers 84 of a control valve unit 39 which includes two normally open valves allowing flow from pipes 35, 36 to opposite ends of a cylinder 40 containing a piston 42 biased by a spring 59. The control valve unit 39 also contains two normally closed valves in a by-pass connection between the ends of the cylinder 40. The piston 42 is connected to the jet 18 by a feed-back linkage 43-46. The arrangement is such that when the jet pipe 18 is initially moved fluid passes via pipes 35, 36 at different rates to the ends of the cylinder 40 causing movement of the piston 42. The feed-back linkage 43-46 stabilizes the jet pipe. Movement of the piston 42 continues until one or other of two abutments 62 on the piston-rod meets one or other of two fixed stops 60 and moves it to abut the other stop, The pressure in one or other of the two pipe lines 35, 36 builds up to actuate the control valves in the lines 35, 36 and to open one or other of the two by-pass valves interconnecting the ends of the cylinder 40. The jet pipe 18 immediately moves to the position corresponding to the input signal and effects corresponding movement of the piston 19 which actuates the pilot valve 22 controlling the supply of fluid to the servo-piston 26 mechanically connected to the arc-electrode of the furnace. In a modification, Fig. 3 (not shown), the dead-time is introduced by a mechanical lost-motion device located between the jet pipe actuated piston and the pilot valve. In another example, Fig. 4, the jet pipe 126 cooperates with a piston 127 in a cylinder 130 having a by-pass pipe 129 containing a normally open valve 131, so that initial deflection of the jet pipe does not result in movement of the piston 127. The moving coil 142 of an ammeter 141 is connected to a coil 144 which controls the position of the jet pipe 126. When the ammeter pointer 147 closes one or other of the switches 148, 149 the coil 139 is energized to energize and establish a holding circuit for the coil 132 which closes the by-pass valve 131. The piston 127 then moves an amount proportional to the input signal to actuate the pilot valve 133. The holding circuit is broken when the pointer 147 on reaching the zero position actuates the toggle switch 152.