Patent GB-724473-A (GB)
Title: Automatic frequency control of electromechanical transducers
IPC: B06B1/02
Filed: 1953-04-24
Abstract: 724,473. Automatic frequency-control systems. CLEVITE CORPORATION. April 24, 1953 [May 10, 1952], No. 11347/53. Class 38 (4). The frequency of an oscillatory source supplying an electro-mechanical transducer is automatically regulated by a control voltage in such manner that the oscillator is maintained in frequency agreement with the mechanical resonance frequency of the transducer despite changes in that resonance frequency due to changes of temperature &c. The control voltage is derived by comparing the phases of the current supplied to the transducer and the voltage across it, and it is found that if this phase angle (or power factor) is kept constant, the apparatus operates with maximum efficiency at all temperatures within a predetermined range. The dielectric of the transducer is preferably of polycrystalline ceramic material such as barium titanate. The transducer may be of the kind described in Specification 648,403, [Group XXXIX], and the dielectric may be polarized in the manner described in U.S.A. Specification 2,486,750. The transducer may be used for various industrial operations, such as depolymerizing a polymer, distributing particles in suspension in a liquid, coagulating material suspended in a liquid and soldering. Fig. 3 is a block diagram of the system. An oscillator 18 supplies the terminals 20, 21 of the transducer through an amplifier 19. An oscillatory voltage in phase with the transducer current is taken off across the series resistor 28 and is passed through a pre-set phase shifter 25 and amplifier 26 to a phase comparer 27. The oscillatory voltage across the transducer is taken from a shunt potential-divider 22, 23 through an amplifier 24 to form a second input to the comparer 27. D.C. voltage varying with the phase difference is fed from the comparer 27 to a reactance valve 29 associated with the oscillator, and varies its frequency in such manner that the phase angle of the loaded transducer is kept constant at a value determined by the setting of the phase shifter 25. In a specific circuit arrangement, Fig. 4 (not shown), the phase shifter comprises a variablytuned circuit on the output side of the amplifier 24, and the phase detector comprises a pair of diode valves with their anodes connected in push-pull to the output of amplifier 26, and in parallel to the output of amplifier 24, their load resistors being connected in series in the grid circuit of the reactance valve 29. In a further circuit, Fig. 5, the back-coupled oscillator 90 feeds the transducer terminals 95, 96 through two intermediate tuned amplifiers 91, 92 and an output stage comprising two parallel-connected tubes 93, 94. Amplification at the valve 91 is controlled by a triode tube 138 connected in the cathode load. Its electrodes are earthed for oscillatory voltages, and its resistance is varied by adjusting its grid voltage by a tap on a resistor 139 connected between earth and a negative voltage source N. The phase-comparer comprises diodes 101, 102 with their anodes excited in push-pull by the output B derived from the transducer current, and with their anodes excited in parallel from the terminals A, C by the transducer voltage. The load resistors 106, 107 are connected in series with a resistor 130, giving a fixed negative voltage, in the grid circuit of the reactance tube 112. When a switch 111 is thrown, the automatic frequency control is disconnected, and a manual control is provided by a slider on the resistor 130 which is connected to the negative voltage point N through series resistors 131, 132 and a gas-filled tube 133 in shunt. The frequency of the oscillator may be " wobbled " by connecting a point in its cathode resistor 118 to the secondary of a mains transformer 100 by which the filaments of the later tube stages are supplied.