Patent GB-1278033-A (GB)
Title: Proportioning temperature control apparatus
IPC: G05D23/19
Filed: 1970-05-14
Abstract: 1278033 Automatic temperature control systems GULTON INDUSTRIES Inc 14 May 1970 [26 May 1969] 23441/70 Heading G3R A temperature control system for heating or cooling has a variable proportional band equidistant on each side of a set point where the degree of intermittance of a cyclically variable control signal is varied proportionally from one band limit to the other. The periodicy of the cyclic signal is varied so that the frequency increases with bandwidth. An offset reducing circuit may be incorporated. The system is arranged so that in a heating system, heat is fully on below the lower limit that a substantially shut off above the upper limit. In the embodiments described a relaxation oscillator is employed embodying a capacitor which is charged and discharged by connecting it cyclically and alternately to two adjustable voltages which may be of the same or differing polarity, the capacitor voltage being added to an amplified thermocouple output voltage. Variation of one or other voltages varies the proportional bandwidth and rate of charge on which the periodicy is dependent. In one embodiment, Fig.6, temperature is sensed by a thermocouple 11 connected to a bridge circuit 23 which operates a galvanometer movement 22 having a pointer moving over a calibrated scale and a mask 24 which obscures light from a photo-cell 18, the output from which is added to capacitor 32 voltage which is charged and discharged respectively by variable sources 36, 38 of opposite polarity, the combined voltage being applied to a level responsive circuit 26 energizing a relay coil 28 controlling a contact 28-2 which connects the capacitor 32 to the alternative source, and through other contacts, a heater 12 and indicator lamps 15', 17'. When the sensed temperature is at the mid point, the heater is energized for periods equal to the de-energized times. In another embodiment the capacitor is charged and discharged by connecting it to two voltages of the same polarity derived from potentiometers, see block (34), Fig.10 (not shown). In a further embodiment, Fig.11 (not shown), the thermocouple is connected to a potential divider (231, 235, 237) and a temperature compensating resistor (239) also a capacitor (234). A high value resistor 244 prevents loss of control if the thermocouple becomes open circuited. Capacitor (234) is connected to a noise filter (248, 250). Another potential divider 231', 235', 237' &c.) charges another capacitor (250'). Terminals (F, B) are connected to a circuit in which a capacitor (32') is charged and discharged through a relay contact 28'2). A gating circuit fed by mains voltage alternately connects a capacitor (257) through field effect transistors (263, 263') to capacitors (250, 250') the capacitor (257) operating a signal level responsive circuit which actuates relay (28). An automatic offset compensation circuit Fig.14 (not shown) may be added to Fig.11 (not shown) by the addition of components (226, 226', 232, 32") having a large time constant, the action being that over a period capacitor (32") gradually charges up to the average value of capacitor (32').