Patent GB-687154-A (GB)
Title: Improvements in or relating to a regulating apparatus
IPC: G05D23/24
Filed: 1950-03-02
Abstract: 687,154. Automatic temperature control systems. GARRETT CORPORATION. March 2, 1950 [March 26, 1949], No. 5299/50. Class 38 (iv). The temperature of an aircraft cabin 10 is controlled by three temperature-sensitive resistances 18, 19, 20, Fig. 1, connected in an A.C. bridge circuit 30, Fig. 2, the output of which is applied to a valve 35 controlling a reversible servomotor 37 positioning a control valve 14 which controls the flow into the cabin of hot air and cold air from ducts 12, 13. A setting resistance 31 is included in the bridge circuit the output of which is of a phase and magnitude depending upon direction and magnitude of unbalance caused by deviation of the cabin temperature from a predetermined normal. This output is supplied through an amplifier 36 to the grids of the double triode valve 35 the anodes of which are supplied from a transformer 55 fed from a supply exactly in phase (or exactly 180 degrees out of phase) with the bridge supply 32. Thus, when the temperature deviates in one direction to unbalance the bridge positive half-cycles on the grids of the valve coincide with positive halfcycles on one of the anodes to cause that section of the valve to conduct, and if the temperature deviates the other way then positive halfcycles on the grid coincide with positive halfcycles on the other diode causing the other section of the valve to conduct. During negative half-cycles both sections of the valve are cut off. The current pulses in the anode circuits of the valve when either section conducts are sufficient to energize the appropriate relay coil 56 to make contacts 41, 60. Closure of a contact 41 causes energization in the appropriate direction of the control valve motor 37, manual control of which may be effected by a switch 46. In order to achieve a step-bystep control a variable negative bias is applied to the grids of the valve when the valve conducts by the contact 60 from a potential source 62. This potential is applied to the grid through a condenser resistance network 65 such that the potential or negative bias builds up gradually on the grid eventually cutting off the valve whereupon the negative bias gradually dies away, the valve becoming conducting again, such effect being rapid enough, however, to occur many times during the positive half-cycle on the grid from the bridge output. Thus for a large unbalance the frequency and length of the impulses delivered to the control valve motor will be greater than those for a small unbalance. A constant negative bias is applied, thus determining the sensitivity of the control, from a variable potential source 72. The sensitive resistances 18 and 19 are located in the air inlet duct of the cabin while the resistance 20 is located in the cabin. The resistance 18 responds immediately to changes of temperature in the duct, whereas the resistance 19 may be lagged so as to respond after a time lag as nearly as possible equal to the time taken for the new temperature to be manifest in the cabin itself by the resistance 20, thus avoiding hunting and allowing immediate adjustments for temperature variations both in the cabin itself and in the incoming air flow.