Patent GB-1330234-A (GB)
Title: Temperature control device
IPC: H05B6/14
Filed: 1971-07-23
Abstract: 1330234 Automatic control of temperature MASCHINENFABRIK RIETER A G 23 July 1971 [23 July 1970] 34688/71 Heading G3R [Also in Division G1] Relates to the temperature control of an inductively heated cylindrical transporting roll for continuous filaments, e. g. synthetic threads, or stationary iron core being coaxially mounted inside the roll and carrying a coil connectable to an A.C. source to produce atlernating magnetic flux in both the core and the roll walls which support the thread, wherein the effective resistance of the coil and its magnetic circuit including the roll is determined and used to control an electronic valve e.g. a thyristor whereby to control the supply to the induction coil. On/off or P.I.D. control may be effected. In Fig. 1, a roll 1 is mounted on a shaft 3 rotatable in a housing 2 and surrounds co-axially a tube 4 which is fixed to the housing. The tube carries a magnetic core 5 on which is wound the coil 6 the magnetic circuit being completed by the wall 7 of the roll. The wall also includes a ring 8 of copper, brass or aluminium. Current in the coil induces current in the ring 8 and wall 7 consequently heating the wall and filaments. In a control circuit arrangement Fig. 4, the ratio of the coil voltage Uu to the coil current UI is obtained in a divisor 22 as a measure of the effective resistance of the coil and roll assembly (the equivalent circuit of which is illustrated in Fig. 3 not shown and which is represented by an impedance 17 in Fig. 4). Since the resistance of the ring 8 and wall 7 varies with temperature of the roll wall, the said ratio is proportional to the temperature and is fed to a differential - amplifier 23 together with a desired temperature value in a line 24. If the temp. exceeds the pre-set value, a positive output from amplifier 23 is applied via a trigger circuit 25 to reset a flip-flop 27. This causes a control unit 28 to effect the turn-off of the electronic valve 20 thereby interrupting the heating current. Simultaneously an impulse is given to a delay element 26 and after a predetermined time (e.g. 2 secs) the delay element resets the flipflop 27 and consequently the valve 20 is again turned ON and heating is continued. However if the roll has not cooled sufficiently during the delay period the heating current is again interrupted. In another embodiment Fig. 5, by means of a two-way switch 32 operated by a device 33, a heating cycle and a measuring cycle are effected alternately. Thus during a heating cycle with switch 32 in position as shown, the induction heater 31 receives current via valve 30 from source 29. During temperature measuring with switch in the dashed line position, the heater impedance 31 is connected in one arm of an AC bridge 39 connected to an AC source 34 preferably of different frequency to source 29. The desired temperature is pre-set on R40. The bridge unbalance voltage proportional to temp. deviation is applied to a differential amplifier the output of which is fed to a phase discriminator 37 where it is compared with the phase of the A.C. source 34. The output of the discriminator 37 signifies the sense of the deviation i. e. whether temp. is too high or too low. In addition the amplifier output is applied to an amplitude discriminator 43. The discriminator outputs control a control signal generator 44 which effects phase control of the triggering of the valve 30 accordingly, for the next heating period. In a further embodiment, Fig. 7 (not shown) the control circuit is arranged so that at the desired roll temperature the coil voltage and coil current signals are equal, subtraction circuits being provided to determine the temperature deviation.