Patent GB-1471194-A (GB)
Title: Method and system for estimating the value of a variable of operation in a process
IPC: F02C9/00
Filed: 1974-07-05
Abstract: 1471194 Automatic control systems UNITED AIRCRAFT CORP 5 July 1974 [9 July 1973] 29928/74 Heading G3R [Also in Division G4] A process variable which cannot be measured directly is calculated by an arrangement in which signals corresponding to a plurality of variables, which are related to the required variable, are combined in a plurality of ways to produce a plurality of estimates of the required variable, these estimates are averaged and then each is compared with this average, any which differ by more than a predetermined amount are rejected and a new average formed of those remaining. The new average is compared with a desired value to effect control. As shown, four variables are measured, their signals S1 ... S4 being combined in pairs as shown in Fig. 3 to produce six estimates Y1 ... Y6 of the required variable. The measured signals are scaled by potentiometers P1 ... P12 which are set to correspond to the relationship between the measured variable and the required variable. This is determined by the use of given Formulae having partial differential coefficients which are determined experimentally on the system or a model. The values Y are then weighted by potentiometers P13 ... P18 so as to give more weight to the more reliable combinations, preferably by making the factors inversely proportional to the root-sum-square error in the value of Y that results from one standard deviation in error in each of the pair of signals S1 ... S4. These potentiometers are of double type to give two signals, the weighting factor W and the product YW, the weighted estimate. In box 17 Fig. 2, also Fig. 4 (not shown) the six values of YW are summed as also are the six values W and the quotient of these is obtained to give a single weighted average for the required variable. YA and YB refer to the weighted averages of two different required variables. In box 18, also Fig. 5 (not shown) each of the estimates Y1 ... Y6 is compared with the weighted average value YA or YB and the result compared with an error factor, E, divided by the respective weighting factor W. The error factor may be two standard deviations. Any which have a difference greater than the weighted error factor cause a logic circuit to operate a relay V to open a switch Q, thereby disconnecting the values YW and W from the control system. Those remaining are averaged at 20 also Fig. 6 (not shown) in the same way as they all were at 17 to produce a final weighted average estimate of the required variable YA or YB which is then compared with a command, or "required", value at 22 or 24 to form an error signal; this, in turn, is used to derive correcting signals Ui or U 2 . As an alternative feature the permitted error factor, E, may be increased in response to transients; in Fig. 7 (not shown) this is done by using the weighted average from 17 to drive a servomotor having a bigger lag than the system. The difference between the position of the motor and the input signal is proportional to the transient and a scaled proportion of this is added to the error factor (E). In a digital system the difference between successive samplings is used. In the particular embodiment Fig. 1, (not shown) the thrust and fuel/oxidizer ratio of a rocket engine is calculated from measurements of chamber pressure, fuel pump speed, oxidizer pump speed and turbine temperature, control being  effected by adjusting valves in the oxidizer flow both upstream and downstream of a pump. Mention is made of an alternative application in which, in a twin spool turbojet engine "low rotor speed", "high rotor speed", burner pressure and exhaust area are measured to estimate turbine inlet temperature and "low spool stall margin". These would be controlled by adjusting fuel flow and jet area.