Patent GB-959072-A (GB)
Title: Improvements in or relating to computing systems
IPC: G06G7/70
Filed: 1962-06-19
Abstract: 959,072. Electric analogue calculating. LITTON INDUSTRIES Inc. June 19,1962 [June 20, 1961], No. 23476/62. Heading G4G. Relates to a computing system in which a desired output (M) is a discontinuous function of first and second input variables (Psi, Pt respectively),the first of which undergoes an abrupt change to produce the discontinuity, this discontinuity occurring at various absolute values of the first variable but at substantially the same predetermined value (X) of the ratio of the variables (Pt/Psi), and in which the first variable is corrected to a smooth and continuous function below and through the region of the discontinuity and is independently corrected to a smooth and continuous function above the discontinuity; switching from one correction to the other being effected when the ratio Pt/Psi abruptly shifts through the predetermined value. The system is of particular application in aircraft, where M represents Mach No. which is determined as a function of the total pressure Pt and the static pressure Ps in front of the aircraft. The instruments measuring the static pressure are subject to error as the speed of sound is approached and a typical variation of the ratio Psi/Ps (where Psi represents the indicated static pressure) with Mach No is indicated in Fig. 2. Three embodiments of the invention, for correcting for this error, are described. In one embodiment (Fig. 5) a pressure sensor 36 produces a signal proportional to Psi which is fed to the summing circuit 44, together with a correction term -#Ps. The output from the circuit 44, which is the correct value Ps, is fed as one input to the altitude computer 40. The correction term -#Ps is fed to the circuit 44 from a correction signal generator 50 through a switch 46, when this is set for values of Psi below the discontinuity, and from a circuit 56 (usually an earthing point) when the switch is set for values of Psi above the discontinuity. The operation of the switch is controlled by a relay 48, which is brought into action by a signal from the phase sensitive summing amplifier 58 fed from a ratio detection circuit 60, as indicated more clearly in Fig. 6. Inputs to the ratio detector are indicated static pressure Psi and total pressure Pt, applied as A. C. signals of predetermined phase to the transformer 62. The total pressure Pt is reduced in magnitude by the resistor combination 72, 74, which is adjusted so that at a given value of Pt/Psi the phase of signals on the secondary winding will be reversed. Since amplifier 58 amplifies signals of one phase only, set by the reference source 76, this change in phase will cause the position of the relay contacts 46 to be reversed at the appropriate time. Thus the subsonic correction term #Ps will be applied from the correction signal generator 50 until the pressure error goes through its discontinuity, when it is switched out by the switch 46. A plot of log (Pt/Psi) against Mach No. gives a straight line containing a jump at the discontinuity (Fig. 7), Fig. 8 indicates an embodiment wherein the logarithms of Psi and Pt are derived using logarithmic potentiometers connected to the output shafts of pressure servos 86 and 88, the shafts having positions representing Psi and Pt respectively. Signals representing +log Pt and -log Psi are fed from the logarithmic potentiometers 108, 106 ganged to the output shafts of the pressure servos to the summing network 114, to which also is applied a switching point set signal 116 which determines the level at which the signal applied to the amplifier 118 will be changed in phase. As before this change in phase is used to operate an amplifier and hence a relay which switches out the correction signal. Fig. 9 indicates a further embodiment operating similarly to that of Fig. 8 but using mechanical arrangements. The three dimensional cam 132 is used in place of the relay 120, and is actuated by a magnetic plunger 134 operating from the phase sensitive amplifier 118. The cam actuates a rack 140 coupled to one input of the mechanical differential 144, which, corrects the output on a shaft giving log Psi to an output corresponding to the desired log Ps. The tracks 150, 152 provide the static pressure correction terms for subsonic and supersonic conditions respectively, and the appropriate track is engaged by the actuation of the magnetic plunger 134. The additional cams 160, 162 and the differential 164 are used in the development of other desired output functions.