Patent GB-548171-A (GB)
Title: Improvements in or relating to electrical proportional control systems
IPC: G01R17/02
Filed: 1939-11-29
Abstract: 548,171. Automatic control systems ; inductances. WALLACE & TIERNAN PRODUCTS, Inc. Nov. 29, 1939, No. 31074. Convention date, Dec. 5, 1938. [Classes 38 (ii) and 38 (iv)] [Also in Group XXXVIII] A proportional control system comprises inductively established e.m.f.'s at a transmitter and receiver which are normally balanced in opposition, means for varying the e.m.f. at the transmitter in response to a controlling variable and means for causing an unbalance to restore the balanced condition at the receiver without causing flow of current between the transmitter and receiver and without causing appreciable phase changes. Current flow is prevented by using a thermionic valve to detect unbalance and phase change by the condition of the apparatus for producing the variable e.m.f.'s The system may be extended to provide control between a number of transmitters and receivers, conjointly or otherwise. The basic circuit is shown in Fig. 1, in which the transmitter 1 and receiver 10 comprise variably coupled coils 6, 8 and 12, 14 of which 8, 14 are connected in series with an A.C. source 22, the coupling at the transmitter being varied in accordance, for example with the head of liquid in chamber 2 dependent on the rate of flow over a weir and that at the receiver being varied in response to out-of-balance conditions by a motor 9 which also drives an indicator 11 or performs some control function as will appear later. The voltage outputs of coils 6, 12 are arranged to be in opposition in the input circuit of valve 16 the output of which, when!unbalance occurs, energizes shading coils 17 of the motor so that it drives in one direction or the other depending on the direction of displacement of the transmitter coil 6 as explained in Specification 493,457, [Group XXXVIII]. The range of movement of the receiver is adjusted by a potentiometer 12a and a similar potentiometer (referred to later as a " dosage " control) may be provided at the transmitter to adjust the response of the receiver in relation to movements of the transmitter makes correction for non-linearity in the relationship for example of the controlling variable (fluid flow) to movement of floats 3 (square law) or of the output of the movable coils in response to angular movement thereof (sine law). Constructions of variable couplings are described some of which obey a law other than a simple sine law. The receiver motor may control a variable gear-box by which one chemical reagent is added to another in proportion to the flow of the latter. Extensions of the system. In Fig. 8 (not shown) one transmitter may be selectively connected to a number of receivers by manual switches, all the fixed coils of the couplings being connected in series, this system being further extended in Fig. 9 (not shown) to the selective connection of a number of transmitters to a number of receivers. In Fig. 10 (not shown), the outputs of a number of transmitters are integrated in a single receiver. In Fig. 12 (not shown) comprising the application of the invention to a chlorination plant, the transmitter is provided with a number of the previously mentioned " dosage " potentiometers which may be pre-set and are selectively connected in circuit at different times of the day by a stepping switch controlled by a clock. Provision is also made for independent manual dosage control. Chlorination plant. Fig. 13, with additional control in response to chlorine content of treated fluid. The transmitter 312 is controlled by fluid head in a meter 300 and the receiver motor 315 controls the amount of chlorine fed from supply 301 over pipe 302 by means of rate controller 306 having a variable liquid head depending on the position of pipe 307 adjusted by the receiver motor 314. Dosage and range adjusting potentiometers 322, 323 explained above are provided. The additional control voltage is introduced over composite potentiometer 319 having one section connected in shunt of the flow transmitter rotor 313 and the other section 319 connected to the rotor of a similar transmitter 310a controlled by meter 310 to which is applied a voltage from cell 309 dependent on the chlorine content of the treated fluid. The chlorinator is described in U.S.A. Specifications 1,777,986, 1,777,987 and the cell 309 in U.S.A. Specifications 1,944,803 and 2,076,964. In a modification, Fig. 14 (not shown) the additional control voltage is introduced by a potentiometer directly controlled by the meter 310. Other modifications. Fig. 15 (not shown) comprises a chlorination plant in which an additional receiver operates recording mechanism. Cam controlled contacts connect the control receiver and recording receiver alternately to the transmitter and further contacts are manually adjusted to control the duration of the periods during which the control receiver is in operation, the recording receiver being in operation for a fixed period in each cycle or for the periods when the control receiver is not in operation. The control receiver thus receives impulses of constant frequency but the duration of the impulses may be varied. This serves to control the rate of response of the chlorinator, the response being slower for shorter impulses, whereby hunting may be minimized. Fig. 16 (not shown) comprises a system in which fluid and dry reagents are fed to a primary fluid in proportion to its flow. Separate receivers controlling the reagents are connected to a common transmitter responding to fluid flow. The receivers in this case are positioned in accordance with the actual rate of supply of the reagents so that the proportioning is directly checking. The fluid reagent is fed through a valve controlled by the respective receiver motor and the receiver inductive coupling is controlled directly by a raegent flow meter. The dry reagent is fed by a conveyer carried by weighing apparatus, the amount of reagent deposited on the conveyer being controlled by the respective receiver motor, and the inductive coupling of the receiver being controlled by the weighing apparatus. The amount of fluid reagent is recorded by a further receiver operated in conjunction with the reagent flow meter and inductor operating in this instance as a transmitter. Other modifications. Instead of using separate two-wire circuits between the stators and rotors of the transmitters and receivers, Fig. 11 (not shown) employs an arrangement in which the return circuit for the rotors comprises a simpler circuit between the stators. Instead of using variable inductive couplings at the transmitters and receivers, fixed transformers with potentiometers across their secondary windings may be employed. Construction of variable inductors. The stator 23, Fig. 3, comprises a former in two halves to facilitate assembly, each half carrying a winding. The rotor comprises a solid former of insulating material, Fig. 4B, or a ring, Fig. 4A, with a spherical soft iron core or an air core, with the winding in a circumferential groove or slot. This construction minimizes phase variations. An outer insulating ring pressed over the rotor carries an operating shaft 30 and pivot 31 which form part of the electrical connections to the rotor. The pivot is supported in a bearing 37 urged upwards by a spring and the shaft 30 and, carried in a bearing 33 supported on pillars 35, is driven by a gearwheel 43 frictionally mounted on the shaft. The inductor is enclosed in a screen 46. In a modification, the stator comprises a winding on a laminated core. An additional stationary aiding or opposing winding may be used in series with the rotor, Fig. 17 and 18 (not shown), to provide a resultant voltage varying according to a law differing from the simple sine law. A linear law may be obtained over a restricted range of movement. Specification 367,821, [Group I], also is referred to.