Patent GB-789191-A (GB)
Title: Electronic level-sensitive apparatus
IPC: G01F23/24
Filed: 1955-11-18
Abstract: 789,191. Electric correspondence control. GILBERT & BARKER MANUFACTURING CO. Nov. 18,1955 [Jan. 13, 1955], No. 33032/55. Class 40 (1). In an electronic level-sensitive control adapted for use with containers of liquid and of the type wherein the level sensing element 1 is movable by a reversible two-phase motor 4 having two windings 86, 88 and separate first and second circuits supply alternating current in proper phase relation to said windings, the first circuit supplying a fixed voltage of one phase, the second circuit being supplied with a first and fixed voltage of the other phase tending to actuate the motor to raise the element and a second and opposing voltage controlled by and variable with variations in the distance between the element and the level of liquid in the container 2 and tending to lower the element, the first and second voltages balancing when the element and liquid level are in a predetermined vertical relationship causing the motor to stop, variations in the liquid level causing a change in said second voltage to actuate the motor in one direction or the other to effect a corrective movement of the element to restore the predetermined relationship, means are provided for inhibiting the second voltage from said second circuit to cause the motor to lift the element independently of variations in the level of the liquid. The circuit is a modification of that described in Specification 750,858 and comprises a R.F. oscillator 46 from which a voltage is fed to the element 1. The element 1 is a wire which is mounted within a tubular metallic shield 5 which is suspended through an insulating link 7 from one end of a perforated tape 6. The tape extends out of the container 2 and passes over a series of idler rolls 10 to a wheel 11 which has pins or teeth to engage perforations of the tape. The wheel 11 is actuated from the motor 4 through reduction gearing to cause the tape to move at a low speed. The free end of the tape is wound on to a spring- actuated winding drum 12. Movement of the tape is limited by a stop pin 16 which is fixed to the tape 6 and which when the element 1 reaches the desired height jams between an idler roll 10 and a supplementary roll 17 and stalls the motor 4. The wheel 11 can be made to operate suitable potentiometers which form part of a telemetric system for transmitting the level indication to a remote location. A level indicator 18 is also located adjacent the motor 4 and is driven by the wheel 11. A radio frequency cable extends from the element 1 around idler rolls 20, 21 and thence round a weight actuated roll 22 to a clamp 24. From the clamp the cable 19 passes to the oscillator 46. The oscillator 46 is adjusted so that when the element 1 is not in the liquid it oscillates at the resonant frequency of the element. When the free end of the element is immersed in the liquid in the tank, the liquid causes a substantial decrease in the signal voltage by de-tuning the element and also by its presence in the field of the element, the voltage decreasing in value approximately in direct proportion to the extent of immersion of the element in the liquid until a certain limit is reached when the signal voltage completely disappears. The R.F. voltage from the oscillator is modualted at mains frequency from a winding 66 of a transformer 67 and thus the modulating voltage on the element 1 will depend upon the extent of immersion in the liquid. Part of the voltage on the element 1 is applied to a diode detector 36 and the modulating voltage is applied to a two-stage amplifier 74 and thence to a transformer 80; this voltage is the second voltage referred to above. The first fixed voltage referred to above is applied in series with the secondary of the transformer 80 from a winding 84 of the transformer 67 and the resultant between the two voltages is applied to a power amplifier 81 and thence to the winding 86 of the motor 4. The winding 88 of the motor 4 is supplied from the mains through a condenser 89. Thus when the modulating voltage from the element 1 is equal to the voltage from the winding 84 the resultant will be zero and the motor 4 will be inoperative; any difference between the voltages will cause the motor 4 to rotate in one direction or the other to maintain the element 1 in its predetermined relationship to the liquid. The means for inhibiting the second voltage can be a switch 90 which removes the modulating voltage from the oscillator or it can be a switch 91 which shortcircuits the secondary 83 of the transformer 80. In either case only the voltage from the secondary 84 is applied to the power amplifier 81 and this causes the motor 4 to operate to raise the element 1 from the liquid at its greatest speed. The switches 90 and 91 are provided adjacent the motor 4 but if it is desired to inhibit the second voltage from a distance this can be done by means of a relay 93 the contacts 92 of which short-circuit the secondary 83 of the transformer 80. The relay 93 is operated by a distant switch 94 when it is desired to raise the element 1 for testing purposes to see whether it is operative, but it can also be operated automatically when a motor 99 is switched on through a relay 106. The motor 99 drives a pump for circulating the liquid in the container. The relay 93 can also be operated when a motor 114 is switched on through a relay 116. The motor 114 drives a pump which fills the container with liquid. The relay 93 can also be operated at times when it is desired to inject compressed-air into the container ; this is done by providing a pressure actuated switch in the compressed-air line which when the compressed-air is applied to the container operetes the relay 93.