Patent GB-587172-A (GB)
Title: Improvements in or relating to high-frequency dielectric heating apparatus
IPC: H05B6/50
Filed: 1944-07-15
Abstract: 587,172. Automatic control systems. SANGAMO WESTON, Ltd. July 15, 1944, No. 13553. Convention date, Nov. 24, 1943. [Class 38 (iv)] In a high - frequency dielectric heating apparatus comprising a radio-frequency driving oscillator coupled to a heating head and including a resonant tank circuit the heating circuit is automatically maintained in resonance with the oscillator frequency. Control of the heating circuit frequency is effected by a.reversible tuning motor operated in the corrective sense by a direct voltage derived from a phase detector having inputs coupled respectively to the oscillator and heating tank circuits. As shown in Fig. 1, plastic material P to be heated forms the dielectric of a condenser C1 in a resonant circuit L1, C1, coupled aperiodically through a transmission line T1 to the output tank circuit L2, C2 of a driver oscillator O. The resonant circuits L1, C1, L2, C2 are loosely coupled, respectively, to resonant circuits L3, C3, L4, C4 connected to a phase detector bridge network B. When all circuits are in resonance, current in the L4, C4 circuit is in phase with that in the heating circuit L1, C1 and that in the L3, C3 circuit is in quadrature therewith. The two inputs to the bridge B which may be a known ring modulator containing non-linear elements in a balanced circuit arrangement, are thus in quadrature resulting in zero output from the bridge. Detuning of the heater circuit L 1, C 1 causes a phase shift in the L3 C3 input to the bridge which delivers a D.C. output the magnitude and direction of which depend on the direction of the phase deviation. This D.C. output operates' a polarized relay R to control a reversible motor M which mechanically adjusts a variable element.of the condenser C1, the inductance L1, or both, in the corrective sense. The reversible motor M may, for example, be of the rotatable, heated bimetallic strip or solenoid operated plunger type. Fig. 3 (not shown) illustrates a ring-modulator phase detector consisting of four diodes in series, the L3, C3 and L4, C4 circuits being connected across the respective diagonals of the rectifier bridge so formed and the output to the relay R being taken from the mid-points of the inductances L3, L4. In a modification, Fig. 4, thermocouples HT having a square law inputoutput characteristic are used as the non- linear elements in the phase detecting bridge B. The heaters H are arranged in a closed circuit across which the input circuits L3, C3, L4, C4 are connected and the thermocouples T are connected in a series circuit with relay R. In a preferred arrangement, the ends of inductance L3 are connected to the anodes of a pair of triodes V, Fig. 5. The centre tap of L3 is connected through a radio frequency choke rf to the junction of independently adjustable condensers C3 and to the tube cathodes through resistors r. The L4, C4 circuit is connected between the cathodes and grids and relay R is connected across the two cathodes. This arrangement avoids cross-coupling of the resonant input circuits. The transmission lines T1, T2 may be dispensed with by direct coupling L2, C2 to both L1, C1 and L4, C4.