Patent GB-664227-A (GB)
Title: Improvements in and relating to automatic tuning systems
IPC: H05B6/50
Filed: 1949-11-15
Abstract: 664,227. Automatic control systems. BRITISH THOMSON-HOUSTON CO., Ltd. Nov. 15, 1949 [Nov. 26, 1948], No. 29221/49. Class 38 (iv). An automatic tuning system for a load supplied with high frequency energy by a transmission line, comprises a variable reactance in series with the load, means for determining the phase angle between the current and voltage in the transmission line, means for varying automatically the reactance to maintain the phase angle at a minimum, a variable reactance in parallel with the load, means for determining the ratio of voltage to current in the transmission line, and means for varying the parallel reactance in dependence upon the ratio to maintain the impedance matching between the load and transmission line. As drawn in Fig. 1, in a high frequency heater the material 1 to be heated is placed between electrodes 2, 3 a variable capacitor 4 operated by a reversible motor 5 being connected in series with the electrodes and a variable inductor 6 operated by reversible motor 7 connected in parallel, the network being supplied with H.F. energy by generator 43 through transmission line 8. The phase sensitive control unit 13 and amplitude comparison control unit 14 both receive a voltage from probe 11 through line 12 which is proportional to and in phase with the voltage in line 8, and a voltage from loop 15 through line 16 which is proportional to but 90 degrees out of phase with the current in line 8, the control unit 13 adjusting capacitator 4 to maintain the phase angle between current and voltage in the transmission line 8 at zero, and the control unit 14 adjusting inductor 6 to maintain the ratio of voltage to current in the line 8 at a value equal to the characteristic impedance of the line. In this way compensation is obtained for changes in impedance which occur at the heater and the load is kept matched to the transmission line impedance. Figs. 2a and 2b show the circuit in detail the material 1 being supported by a conveyer belt between electrodes 2 and 3 which are supplied through capacitator 4 and transmission line 8 from a conventional generator 43 at a frequency of 13,560 Kc/s. A reversible motor 25 is controlled by an automatic frequency regulator (not shown) responsive to the frequency of the generator, and operates variable capacitator 10 to maintain the frequency within desired limits. When the anode circuit of the generator 43 is closed a motor 26 is energized and moves the inductor 9a to its position of maximum coupling with inductor 9b limit switch 42 then operating to open the circuit of solenoid 38 and deenergize the motor. The phase sensitive circuit receives a voltage from probe 11 which is applied to control electrode 44a of mixer valve 44, and the output of a piezo-electric oscillator 46, providing a frequency of 12,060 Kc/s, is applied to the control electrode 44b of valve 44, and control electrode 52b of mixer valve 52. A frequency of 1,500 Kc/s. appears in the output of valve 44, which functions as a limiter on large input signals, and is applied to the control electrode of limiter valve 56, so that the output voltage of tuned circuit 61, 62 remains constant for a wide variation of voltage from probe 11, this output being applied to control electrode 64a of valve 64 which functions as a phase detector as described in Specification 665,584. The voltage induced in loop 15 is connected via conductor 16, which has a length equal to that of conductor 12 so that equal phase delay occurs. to the control electrode 52a of valve 52, which acts as a mixer and limiter, the 1,500 Kc/s. output being applied to the control electrode of limiter valve 66 to provide a constant output for variations of voltage from loop 15, this output being impressed on control electrode 64b of valve 64. In the output of valve 64 a voltage variation corresponding to the variation in phase angle between the current and voltage in line 8 appears, and is applied through resistor 69 to the control grid of valve 68 whose cathode is maintained at a constant positive potential with respect to earth by a gas filled discharge tube 70. Normally closed relay 73 and normally open relay 74 are connected in the anode circuit of valve 68, both relays being open when the automatic tuning system is in balance. Variable resistors 71, 72, adjust the relative currents in the two relay solenoids. When relay 73 is closed, motor 5 driving capacitator 4 is operated in the dorward direction by three-phase contactor 78 through relays 75 and 77 and with relay 73 open and relay 74 closed the motor 5 is operated in the reverse direction through relays 79 and 81, capacitator 4 being operated at a rapid rate compared with inductor 6. For the amplitude comparison circuit a voltage from probe 11 is applied via conductor 12 to one anode of a double diode valve 84, and a voltage from loop 15 is applied via conductor 16 to the cathode of the second diode of valve 84, the anode of the second diode being connected to the cathode of the first diode, and an output is produced across resistor 87, as described in Specification 670,823, proportional to the difference in the amplitude of the voltages from probe 11 and loop 15. This ouput is applied to an amplifier valve 88 whose ouput is applied through resistor 82 to the control electrode of valve 89 whose cathode is maintained at a fixed positive potential above earth by gas filled discharge tube 90. Normally closed relay 94 and normally open relay 95 are connected in the anode circuit of valve 89, both relays being open when the system is balanced, closing of relay 94, closing relays 96 and 98 to operate the motor 7 in a forward direction through three phase contactor 99 to increase the inductance of inductor 6, and closing of relay 95. Closing relays 100 and 102 to operate motor 7 in the reverse direction. Motor 7 continues operation until the load impedance equals the impedance of transmission line 8. When the anode circuit of the high frequency generator is opened to stop operation, switch 22 returns to the lower position and energizes solenoid 102 to return inductor 6 to its minimum inductance position, when limit switch 105 is operated to stop the motor 7. Inductor 6 is described in Specification 659,726 [Group XL(b)]. When the operation is stopped switch 22 also energizes solenoid 107 to energize the motor 26 through switches 108, 109, 110, to return the inductor 9 in the generator tank circuit to its position of minimum coupling, when limit switch 111 opens and stops the motor. When motor 25 is operating one of the switches 112 or 113 is opened by solenoid 29 or 34 to prevent operation of motors 5, 7, or 26. In a modification, Fig. 3 (not shown) a variable inductance is substituted for the variable capacitor 4, and in a further modification, Fig. 4 (not shown) control voltages proportional to the standing wave ratio are provided from two probes spaced from <SP>1</SP>/ 10  to “ wavelength apart in the line 8, which control the amplitude comparison circuit so that the standing wave ratio is reduced to a minimum.