Algorithms of automatic monitoring of the parameters of the electric rocket propulsion system are considered. The informative features of the subsystems of the electric rocket propulsion systems and the nature of their changes in the process of operation of the propulsion system are determined. The algorithms for detecting jump-like changes in informative features for different volumes of apriority information are presented.
Keywords: electric propulsion system, Hall thruster, measurements of the parameters, automatic control, algorithms of monitoring.
Electric rocket propulsion systems (ERPS) are increasingly used in the implementation of various space programs. The main advantage of the application of ERDU is a significant reduction in the mass of the propellant in comparison with chemical thrusters of low thrust. This is ensured by a significantly higher value of the specific impulse, by the possibility of multiple firings and by a long service life . The ERPS consists of blocks and subsystems of various physical nature: an electric rocket thruster (ERT), a system for storing and supplying the propellant (SSSP), a power supply system (PSS) and an automatic control and monitoring system (ACS) that unites these systems into a single whole.
The most promising electric rocket engine is currently considered to be the Hall engine. In such an engine, the process of ionization of the propellant takes place in crossed electric and magnetic fields, and the acceleration of the formed ions is performed by a longitudinal electric field .
Electric rocket propulsion systems require complex control and monitoring algorithms. The need for the implementation of complex algorithms for controlling and monitoring the parameters of the ERPS subsystems is conditioned by the specifics of providing the electric energy of the spacecraft, ensuring the reliability of the ERPS operation during a long service life and the desire to cover a wider range of tasks being solved by the same ERPS.
Specificity of spacecraft’s power supply is that, during long-term operation in space conditions, solar batteries, which until now are the main source of electric power for ERPS, are subject to considerable degradation, as a result of which, the electric power is significantly reduced. The automatic control and monitoring system of the ERPS should provide optimal modes of the ERPS in conditions of a significant change in the power of the primary power source.
As a result of the long-term autonomous functioning of the propulsion system, the parameters of its subsystems, in particular, the electric rocket engine, the system of storage and supply of the propellant and the power supply system, can be substantially changed, up to their failure. Therefore, the automatic control and monitoring system should ensure the maintenance of the optimum operating modes of the ERPS when the parameters are changed within certain limits, and in case of failures of individual elements, generate signals for the disconnection of the failed elements and connection of the backup ones.
The most informative parameter characterizing the normal operating mode of the Hall thruster is the magnitude of the discharge current (Id), which characterizes the ionization processes in the accelerating channel of the engine. At that, information on the state of thruster operation and closed electron drift is contained both in the average value of the discharge current, and in the level and frequency ranges of the discharge current oscillations [10, 11].
The storage and supply systems of the propellant included in the ERPS can be built on various physical principles, but the gas systems for inert gases – xenon (Xe), argon (Ar) – have become most widespread at present. In the gas SSSP, informative parameters are: values of the pressure (Pi) in the working tracts of the system; information about the condition of the valves; the heating current of the thermo-throttle (Ith); temperature parameters (Ti) at various points of the SSSP .
The power supply system of the ERPS contains a number of controlled and uncontrolled power supply sources that provide electrical operation modes for individual subsystems of the ERPS. The normal functioning of a given source can be judged by the values of the voltage (Vi) and the current (Ii) and their correspondence to the required values [12, 13].
Thus, in electric rocket engines, it is possible to measure a vector of variables that characterize the current state of the engine, and similar vectors of variables can be measured for the system of storage and supply of the propellant, the power supply system and the automatic control unit of the ERPS.
The automatic monitoring unit for the parameters of the ERPS, which is an integral part of the automatic control unit, must provide the solution of the following tasks: gathering information about the current state of the subsystems; processing of the current information in order to identify emergency and abnormal situations; processing of the current information about the parameters of subsystems in order to forecast the occurrence of emergency situations, etc.
Table 1 lists the main controlled parameters of the subsystems of the ERDU and the nature of their possible changes. An analysis of the nature of the possible change in the monitored parameters of the subsystems of the ERPS shows that, in the automatic monitoring unit of the ERDU along with the tolerance monitoring algorithms checking whether the monitored parameters are in a given tolerance ranges, algorithms for detecting a sudden change, as well as a slow drift of the monitored parameters should be implemented, for making decisions as for prevention of emergency and abnormal situations in individual subsystems and in ERPS as a whole.
Mathematical formulation of the problem of automatic monitoring of parameters of the ERPS. To automatically monitor the parameters of the subsystems of the ERPS, measurements of various informative characteristics that characterize the functioning and operability of the propulsion system are used. The model of such measurements can be a sequence of random variables – one-dimensional random signals X(1), X(2), X(3), …, X(k) or a sequence of random vectors [X(1)], [X(2)], …, [X (k)], a multidimensional discrete random signal, where [X(k)] = [X1(k), X2(k), …, Xm(k)]T; T is the sign of transposition. Properties of discrete signals and their use for solving forecasting and control problems are considered in . Obviously, the measured signals contain information on the state of the subsystems of the ERPS, and can be used for solving the tasks of monitoring.