GB962650A - Electrical control systems - Google Patents

Abstract

962,650. Control systems. BAILEY METER CO. Aug. 5,1960 [Sept. 21, 1959], No.27318/60. Heading G1N. A control system comprises two alternative electrically operated means for controlling the position of a final element, balancing means generating a signal whose magnitude represents the difference between the outputs of the two means, and transfer means to apply this signal to the output of one or other of the controlling means during transitions from one to the other, to equalize the two outputs and thus ensure a smooth transfer of control. In Fig. 1 a flowmeter 16 positions the armature of a differential transformer 20 to give a signal which is converted to D.C. and compared with an adjustable reference value in a unit 32. Any difference is passed to a proportional D. C. feedback amplifier 40, the output of which operates a pneumatic unit 44 for controlling a valve 10 in the conduit 14 to be controlled. For manual operation, the unit 44 is disconnected from the amplifier 40 and joined to a potentiometer slider 54 which is driven by a motor 60 energized from an A.C. power supply via push-buttons 70, 72, so that operation of one button or the other causes the motor to run in one direction or the other. During automatic control, the signal from amplifier 40 is passed to a further servo amplifier 76 where it is compared with the signal from a potentiometer slider 78 gauged to the slider 54. Any difference causes energization of one or other of relays A and B, closure of the corresponding contact A1 or B1 (in parallel with push-buttons 70, 72) and operation of the motor to drive slider 78 to a balance point and move slider 54 correspondingly. On starting up for automatic control, button 90 is pressed to energize relay D. Contacts D1 and D2 close to prevent capacitor C (see below) from charging, contact D3 holds the relay when the button is released, contacts D4 and D5 allow contacts A1 and B1 to operate the motor 60, and contact D6 joins the output of amplifier 40 to the unit 44. On changing to manual control, button 88 is pressed to de-energize relay D and open contacts D1-D6. In addition, D7 joins the unit 44 to the output of the slider 54, which is now under the control of the push-buttons 70 and 72 due to closure of D8 and D9. Capacitor C is now connected by contacts D10 and D11 between the slider 54 and the output of amplifier 40, and therefore becomes charged if there is any potential difference between these points. There is no sudden change in the signal applied to unit 44 since the slider 54 has been continually adjusted during automatic operation. When the system is changed back to automatic control, the voltage across capacitor C is applied to the input of amplifier 40 so that the initial output signal is the same as that produced under manual control. The charge on the capacitor then leaks away gradually and the automatic control takes over without any sudden change in signal level. In a modification (Fig. 2, not shown) the motor also drives a transducer which thus senses the position of the valve 10 and passes this signal to the servo amplifier (which in this case is a magnetic amplifier) in conjunction with the signal from amplifier 40. The push-buttons 70 and 72 act on the relays of the servo amplifier instead of directly on the motor. In a further modification (Fig. 3, not shown) the capacitor is one which is normally associated with the amplifier 40 to introduce an integrated term into the feedback, but, on manual operation, it is disconnected from the feedback path and used as in Fig. 1. These two modifications may be combined (Fig.4 not shown). In Fig.5 the control is effected in accordance with two variables. The amplifier A in unit 134 receives a signal from a steam-pressure transducer 130 in the same way as in Fig. 1 and its output is passed to a further similar amplifier A in unit 138 in conjunction with a signal from a steam-flow meter 136. The output of the second amplifier is passed to the control point via D6 (automatic operation) or returned via D20 to capacitor C2, which acts as in Fig. 3 (manual operation). The system of the invention is applicable to power stations and process industries.