Patent GB-1009413-A (GB)
Title: Improvements in and relating to the control of a plant
IPC: F22B35/10
Filed: 1964-08-06
Abstract: 1,009,413. Fluid pressure servomotor systems. SULZER BROTHERS Ltd. Aug. 6, 1964 [Aug. 16, 1963], No. 32077/64. Heading G3P. A method of controlling a plant having at least one control loop in which, as well as the controlled condition and the correcting condition, an extra variable is operative, the signal of such extra variable being modified before acting on the loop, the modification of the signal of the extra variable being different according to whether the extra variable is varying positively or negatively. One form of apparatus for modifying the extra variable signal is shown in Fig. 3 in which the extra signal is applied to a lever 43 to rotate a shaft carrying cams 44, 44<SP>1</SP>, 44<SP>11</SP>. Rotation of cam 44 acts through a device 53 to actuate a value 50 thereby providing an output signal in conduit 33. The device 53 comprises a fixed cylinder 58 which houses a piston 57. An adjustable restriction 62 is provided through the piston which restricts upward movement of the piston while a non-return value 60 allows flow through the piston when it moves downwardly. The piston 57 is connected to a movable cylinder 55 which houses a piston 54, loaded by a spring 63, the piston has an adjustable restriction 61 and non-return valve 59, the latter allowing flow through the piston when it moves in an upward direction. A piston rod 46 of the piston 54 carries a cam follower 45 while a piston rod 56 of the piston 57 carries an abutment for a spring 49 acting on a control valve 50. In operation if the cam 44 rotates to raise the rod 46, the piston 54 moves upwards in the cylinder 55, due to flow through the valve 59, and compresses the spring 63. The cylinder 55 is moved upwards at a slow rate determined by leakage through the restriction 62 of piston 57 and thus an abrupt upward movement of rod 46 results in a gradual upward movement of valve 50 to provide a gradual change in the output signal. When the cam 44 rotates in the opposite direction for downward movement of the rod 46, the valve 60 allows rapid movement of the piston 57 in the fired cylinder 58 and the piston 54 and cylinder 55 move together, thus an abrupt downward movement of rod 46 results in an abrupt downward movement of valve 50 and an abrupt change in the output signal. The device 53<SP>11</SP> is similar to the device 53 except that the valves 59<SP>11</SP>, 60<SP>11</SP> in the pistons 54<SP>11</SP>, 57<SP>11</SP> act in the opposite direction. The device 53<SP>11</SP> thus operates in an opposite manner to provide an abrupt change in output for upward movement and a gradual change in output for downward movement of the rod 4611. In a modification two devices 53 or 53<SP>11</SP> are arranged in series. In a further modification the input is applied to a rod 65, Fig. 5, of a piston 66 which acts through a spring 68 on the control valve 69. The piston 66 is of larger diameter than the valve 69 and the space above the piston is connected to an oil line 73 between non-return valves 75, 76 while the space below the valve 69 is connected to the line 73 between the valve75 and an adjustable bleed 77. When the rod is moved upwards, the spring 68 is unpressed and oil is forced by the piston 66 through valve 75 to the space below the valve thus providing an accelerated advance of the valve which subsequently returns to the position determined by the spring 68 due to leakage past the bleed 77. When the rod 65 waves downwards, the valve 69 moves downwards gradually under control of leakage through the bleed 77 and the piston 66 sucks in further oil past the valve 76. In an electromechanical modifying device, an abrupt rise in the voltage across input terminals 80, 81, Fig. 6, raises the potential in a line 93 and current flows through a diode 88 to rotate an electric motor 86 clockwise. The motor waves a slider 92 of a potentiometer 87 until the voltage tapped off equals the input voltage and this action provides a rapid change in the output signals at terminals 82, 83 and 84, 85. If the input voltage drops abruptly, current now flows through diode 90, resistor 89 and diode 91. An intermediate voltage is tapped off the resistor 89 via the tap 94 and applied to the right hand terminal of motor 86 to cause rotation in an anticlockwise direction at a reduced speed and thus the output signal is varied gradually. In an electronic modifying device, an increase in the potential of the input terminal 101, Fig. 7, causes flow of current through a diode 106 and resistors 107, 108, 109. This decays exponentially and produces a voltage drop across the output terminals 102,103. When the input potential is reduced abruptly, the charge of capacitor 105 is adjusted by current flowing through the resistors 109, 110 and diode 111. In a modification the capacitor is positioned in place of the resistor 109 and the resistor 108 is omitted. The Specification also describes a circuit of two resistors and one diode which gives a greater output for positive change and a smaller output for an equal negative change in the input. The control loop may comprise a valve controlling feed water to a boiler in response to steam output temperature in which the extra variable in steam flow. Two loops may also be used one controlling fuel flow and the other controlling air flow to a boiler burner. Each loop is controlled by the respective flow of fuel or air and the extra signal is obtained in response to steam pressure and flow. The signals are modified so that in the event of a load increase, the air supply is increased before the fuel and in the event of a load decrease the fuel supply is reduced before the air supply.