Patent GB-568204-A (GB)
Title: Improvement in measuring apparatus
IPC: G05D23/22
Filed: 1943-01-21
Abstract: 568,204. Transformers ; automatic control. BROWN INSTRUMENT CO. Jan. 21, 1943. No. 1097. Convention date, Dec. 1, 1941. [Classes 38 (ii) and 38 (iv)] [Also in Groups XIX, XXV, XXXVI, XXXVII and XXXVIII] Temperature.-The motor, e.g. of a selfbalancing measuring system applicable to automatic control, is of the rotating field type and comprises a winding connected to the power supply and another winding at right angles carrying control currents, which lead or lag the supply current by 90 degrees according to the required direction of rotation, and also a steady current and a current of harmonic frequency which have a maximum value when the system is balanced and exert a braking effect on the motor. Motor-control arrangements. In the system shown in Fig. 1, a thermocouple 13 generates a voltage in opposition to that of a motor-driven potentiometer in a circuit including an interrupter 18, driven from the power mains, and the primary of a transformer 19 having a primary impedance 5 to 10 times that of the rest of the potentiometer circuit. The output of the latter is amplified in a double triode tube 20 acting as two stages of a three-stage amplifier, the final stage of which is furnished by one of the sections of a second double-triode tube, the other section of which acts as the supply rectifier for all three stages. The output of the final stage is taken to all the grids in parallel of two double-triode output tubes. One anode in each tube is supplied in one phase and the other in opposite phase and the combined anode currents flow in the control windings 204, 205 of a squirrel-cage motor which drives the potentiometer and also indicating, recording and automatic control apparatus. The motor windings have a high ratio of inductance to resistance and the power windings 202, 203 are series tuned by a condenser 206 to the supply frequency. In the condition of balance, equal impulses are supplied by both sections of the double power tubes and this results in a current in the control windings having a double frequency (120 cycle) component and a D.C. component. In these conditions in which the cores concerned are saturated, the control windings are parallel tuned by a condenser 211 to 120 cycles. At full-speed, owing to the increased inductance of the control windings, the condenser tunes them to 60 cycles. The D.C. and 120 cycle components bring the motors quickly to rest when the point of balance is reached. When balance is destroyed, the impulses from the respective sections of the power tubes are unequal and this results in a reduction of the D.C. and 120 cycle components and the appearance of a 60-cycle component lagging or leading the mains current by 90 degrees and so causing the motor to rotate. At high speeds, owing to the distortion of the field, current in the control winding to assist rotation is derived by transformer action from the power windings. The design is such that the motor does not heat up when stalled and limit-switches are therefore unnecessary. In modified systems, Figs. 37 and 38 (not shown), the motor is controlled in accordance with the unbalance current of D.C. and A.C. bridges. In the latter case, the interrupter 18 is not used. Thermocouple and potentiometer circuit arrangements and calibration. As shown in Fig. 1, the thermocouple is associated with a potentiometer system comprising a slide-wire resistance driven from the motor by means of a cable passing round and secured at both ends to a drum 43 and connected across one of a series of three manganin resistances 30 ... 32 fed by a. battery 33 through a vernier rheostat with two wipers, one of which has a lostmotion connection and the other a direct connection to the control knob 38. The rheostat wires may be of copper or other material having a positive temperature coefficient of resistance to compensate for temperature changes in the battery. A further shunt circuit comprises a resistance 39 of copper which compensates for variations of temperatures at the cold junction and a resistance 40. Inaccuracies in the resistances 30 ... 32 are compensated by adjusting the wiping contact of the slide-wire resistance in relation to its driving drum 43. Inaccuracy in the resistance 31. across which the slide-wire resistance is connected is compensated by adjusting the diameter of the drum 43. To standardize the system, the switch 57 is moved down. This so alters the circuits that the direct-current input to interrupter 18 and transformer 19 represents the difference between the voltage of a standard cell 246 and the potential drop in resistance 40 due to current from the potentiometer battery 33 and this causes the motor to run. Rheostat 38 is then adjusted until the motor remains stationary. In a modified circuit arrangement, a further potentiometer is included in circuit with the thermocouple &c. to adjust the range of the system. Stray currents picked up by the thermocouple leads are limited by a resistance-condenser circuit 108. 111. Stray currents can also be reduced by special constructions of thermocouple, Figs. 34 to 36 (not shown). Checking for faults ; faulty operation ; alarm arrangements. If the signal input circuit fails in an automatic control system, there is a danger that the furnace will be dangerously overheated. To guard against this, a small permanent A.C. bias is provided by transformer 128 to produce the same effect as a rise in temperature and so shut down the furnace if Fig. 34 (not shown), failure of the power tubes, either by failing to pass current or by passing excessive current, causes the operation of an alarm. The alarm circuit includes the motor control winding and so causes the motor to drive to an extreme position. The motor may be driven in this way in the arrangement shown if the rectifier section of tube 21 fails by a short-circuit. The amplifier is designed so that if one of the amplifier tubes fails it may be replaced temporarily by one of the power tubes. To test for faulty operation, the key 57 is moved upwardly. In this position, the input circuit to the vibrator and transformer is shunted so that if the motor runs it can onlv be because the amplifier is faulty or because of inductive pick-up in the input circuit. Automatic control. When the temperature exceeds or falls below a predetermined value, an arm 226, Figs. 1 and 25, rocks a. member 230 of a follow-up valve to cause a pneumatic motor to give the fuel valve a new setting. The point at which the control operates is adjustable by means of a knob which controls the position of a pivoted lever 457. The arm 226 is coupled to a floating lever 436, one end of which is pivoted to the lever 457 and the other end of which is coupled to the shaft 408 that drives a recording pen. The setting of lever 457 is indicated by a pointer 290 coupled to it by means of an eccentric which is adjustable from the front of the apparatus by means of a screw-driver. Construction of transformer. Fig. 22. Each of the two primary windings 70, 70<SP>1</SP> and 71, 71<SP>1</SP> is distributed over two limbs of the core and the primary windings 76, 76<SP>1</SP> are enclosed in a shield on which is wound the secondary. The windings are such that external disturwindings 70, 71, 76, produce an equal and opposite current flow in windings 70<SP>1</SP>, 71<SP>1</SP>, 76<SP>1</SP>.