Patent GB-1328809-A (GB)
Title: Control apparatus
IPC: G05D23/19
Filed: 1971-04-19
Abstract: 1328809 Automatic control of temperature ROSEMOUNT ENG CO Ltd 19 April 1971 [12 Feb 1970] 6867/70 Heading G3R In a retroactive control system, e. g. for the control of a plurality of heaters in a nylon draw twist machine so as to stabilize the temperature of all the heaters at the same temperature, a plurality of controllers are connected to a common desired value set point. Proportional control is effected by modulating the respective sensed temperature values (which in effect changes the set-point value) by means of a common periodically varying signal (perturbation signal) so that the mark/space ratio of the ON/OFF output of each controller is dependent upon the magnitude of the deviation of the respective sensed values from the set point value.  Each basic controller circuit is formed by printed circuit techniques on a respective card, and likewise the set point source and periodically varying signal generator. The cards (e.g. in excess of one hundred) are inter-connected by lines 30 as in Fig.2 which depicts a "P" control arrangement. The controller card 21 for instance comprises a differential amplifier, the two inputs of which are connected across the output of a bridge, one branch of which comprises the set point voltage divider and the other branch comprises a resistor network including a resistance temperature sensor 34. Alternatively the sensor may be a thermocouple, Fig.6 (not shown). The bridge is connected between +12<SP>v</SP> and a common line 30. The amplifier output is connected to an AND gate 11, the other input of which may be permanently connected to a positive voltage so that a trigger unit 12 will turn ON a thyristor 13 as soon as the temperature sensed falls below the set point (or above if the controller is controlling a refrigerator). To achieve proportional control the output of a triangular waveform generator 24 is connected so as to perturb or modulate the sensed value signal. The peak to peak amplitude defines the allowable range of variation of the sensed temperature, the upper limit being the set point value. When the sensed temperature signal lies within the allowable range the controller output will comprise alternate periods of conduction and non-conduction of thyristor 13 at a fixed frequency equal to that of the triangular waveform but with variable mark/space ratio depending upon temperature deviation. In an alternative arrangement, Fig. 7 (not shown) the triangular wave is combined with the set point signal. To effect zero voltage switching of the thyristor and prevent radio interference, instead of connecting the other input of the AND gate to +12 V. D. C., it may be connected to a source producing a signal which remains at zero potential except for the first m. sec of each half-cycle of the mains supply. This if an error signal occurs half way through a mains half-cycle, firing of the thyristor is delayed until the next zero voltage cross-over. Alternatively phase angle control of the thyristor may be effected by employing a triangular wave at 100 Hz i.e. having a 10 m. sec period instead of a 1 sec period, and synchronized to the mains supply (50Hz). In this case the AND gate is held fully open by a positive D.C. voltage. The switching means may be a relay, instead of a thyristor, Fig.3 (not shown). Compensation for power supply variations may be effected by passing a common correction signal to the first input of each differential amplifier. A further correction signal may be applied to the first input of each amplifier, and which is derived from the average of the deviations appearing at the inputs of all the amplifiers, to compensate for errors in the functioning of individual controllers. Additionally an alarm device may be actuated if a sensed temperature deviates excessively and too rapidly from the set point, Fig. 5 (not shown).