Lecture
The continuous drive to improve the performance characteristics of aircraft engines makes it necessary to further develop their control systems. Modern aircraft engine control systems (ECS) operate under conditions characterized by a high intensification of control processes, a wide range of flight conditions and power-plant operating modes, and the need to simultaneously control several gas-dynamic parameters or a complex of parameters characterizing engine operating modes. All of this makes modern aircraft engines extremely complex control objects [1, 2]. Maximum efficiency of aircraft engine operation can be achieved only through the use of control systems designed with due regard for current requirements and methods of system optimization and synthesis, as well as the specific features of computer-aided design technology for aircraft engine control systems [31, 32].
The difficulty of designing an aircraft engine control system lies in the fact that:
• the system belongs to the class of multiply-connected, or multidimensional, systems, since it consists of a significant number of interconnected
and interacting, fairly complex subsystems;
• the aircraft engine control system is a multi-mode system, since the number of subsystems and the nature of their interaction during operation can change; moreover, individual subsystems can operate independently.
The tasks of automatic control of aircraft engines are [31, 32]:
• calculating the optimal thrust and efficiency characteristics of the power plant
at a given operating mode;
• the ability to change engine operating modes while meeting the requirements for the duration of transient processes, and maintaining a set operating mode;
• preserving the stability of working processes and the strength of the engine structure by preventing mechanical and thermal loads on its elements during steady-state and transient operating modes.
These requirements must be met under all flight conditions. Based on the tasks outlined above, control programs are formed that largely determine the engine characteristics, the structure of the aircraft engine control system, and consequently, the structure of the control system's
subsystems and the nature of their interaction.
Aircraft engine control programs are divided by their purpose depending on the operating mode used: control programs for maximum modes, engine start control mode, throttle modes, idle mode, acceleration and deceleration modes, and afterburner modes.
The synthesis of these control programs is generally carried out at the engine design stage as a result of solving problems using average statistical or calculated characteristics of engine components, based on the requirements imposed on the engine by the aircraft. When designing the aircraft engine control system, questions of the technical implementation of these programs are addressed with maximum accuracy, speed of response, and so on.
When designing an aircraft engine control system, both hydromechanical and electronic component bases are used in accordance with the requirements and operating conditions of the control system. The presence of both electronic
and hydromechanical bases is due to the following requirements:
• a minimal number of moving parts in the device (reduces the probability of failure and simplifies the device's design). This condition is met by electronic components;
• operability at high ambient and working medium temperatures is not a problem for either hydromechanical or pneumatic components;
• operability at negative temperatures is determined by the state of the working fluid and is therefore limited only for pneumatic components;
• high speed of response is fully satisfied by electronic components;
• operability under radiation conditions for hydraulic and pneumatic components is practically not reduced, but is limited for electronic components;
• insensitivity to shock loads is characteristic of electronic and pneumatic components;
• the possibility of miniaturization is realistic for electronic components in large-scale and very-large-scale integrated circuits;
• manufacturability of the design, which allows the use of modern mass-production methods and makes it possible to significantly reduce the labor intensity of the components, is an advantage of electronic components;
• sensitivity of hydromechanical and pneumatic components to contamination of the working medium requires a high degree of purity of the working fluid and can only be ensured with the help of special filtration equipment; no such requirements are imposed on electronic components.
Modern automatic aircraft engine control systems include three types of subsystems of varying structure and complexity [31, 32]:
• those having only hydromechanical equipment (for example, subsystems controlling acceleration and deceleration);
• those having only electronic equipment (subsystems for anti-icing system control, turbo-starter activation, engine operating-time estimation, etc.);
• those having electronic equipment as the primary controller, working together with a hydromechanical actuator, and a hydromechanical controller used as a backup in the event of failure of the electronic part (fuel flow control subsystem, inlet guide vane position control subsystem).
The fuel supply system, which occupies a central place in the aircraft engine control system, is structurally the third type of system, i.e., a combination of hydromechanical and electronic subsystems, in which the hydromechanical units (HMU) perform a dual role:
• as actuators of the main aircraft engine control system when the electronic subsystem (main automation) is operating;
• as a backup subsystem controlling the aircraft engine in the event of main automation failure.
In this regard, particularly high requirements are placed on the hydromechanical part when designing an aircraft engine control system. One of the main directions in the development of hydromechanical system design technology is the development and construction of mathematical models. Mathematical models of aircraft engine control systems are used both at the synthesis stage and at the stages of manufacturing, testing, and operation of automatic engine control systems, allowing one to:
• select the structure and parameters of the main and backup control-system subsystems well in advance, at the design and manufacturing stage of the hydromechanical units;
• reduce the volume of testing with actual control-system units at all stages of control-system development, from the unit test bench to the aircraft, aimed at refining the optimal structures and parameters of the main and backup automatic control system subsystems;
• reduce the volume of full-scale testing with the actual control system when eliminating defects identified during development or operation;
• automate lengthy testing of the electronic subsystem using a mathematical model of the control system's hydromechanical sub-
system, which reduces the time and costs at the stages of development and refinement of control-system units and subsystems.
Thus, the construction of reliable mathematical models of hydromechanical units of aircraft engine control systems is an important task in the design of such systems, the solution of which significantly affects the technical and economic performance of the engines being designed.
Based on a generalization of experience in building and using aircraft engine control system models, the following requirements can be formulated that must be taken into account when developing mathematical models:
• modularity, consisting in representing the model as a set of block-modules;
• high accuracy, consisting in matching the model's degree of complexity to the degree of reliability and the amount of initial information available;
• the ability to change the structural diagram of the object;
• the ability to update and expand the initial information about the object;
• universality of the model across all operating modes.
Implementation of these requirements makes it possible to develop effective methods for constructing mathematical models of aircraft engine control systems.
The modern approach to solving the problem of constructing mathematical models involves the use of identification methods.
Identification models are built from observations of the object in a closed loop according to the scheme shown in Fig. 3.1.
Fig. 3.1. Closed-loop identification scheme: R(t) – vector of input signals; Y(t) – vector of object output signals; Ym(t) – vector of model output signals; N(t) – vector of disturbances; ∆A(t) – matrix of model parameter changes
The identification approach to building HMU models expands the areas of application of mathematical deterministic models. This is especially evident in the following areas.
First, experience in the production of serial hydraulic units for aircraft engine control systems shows that the characteristics of a significant number of units at the acceptance-testing stage do not meet the standards specified in the technical specifications. The reasons leading to deviations in characteristics are insufficient design development of the units and low manufacturability. The use of identification methods in constructing mathematical models of units makes it possible to determine technological deviations, i.e., to solve the inverse design problem, which improves quality and reduces the time needed to develop units during experimental design work.
Second, as a result of the operation of hydromechanical units of aircraft engine control systems, changes occur in structural parameters
due to wear of mechanical components, which ultimately degrades the quality of system operation. Gradually accumulating, such damage can reach a critical state at which a partial failure of a component, unit, or the power plant as a whole occurs. Therefore, the task of timely detection of gradual failures is one of the pressing issues in the diagnostics of aircraft engine control systems, and identification models are used to solve it.
Third, automating the tuning process for newly created units requires the use of identification models in which adjustment elements are explicitly represented, and whose optimal position is determined by solving an optimization problem.
Thus, the application of identification methods in constructing mathematical models of hydromechanical units of aircraft engine control systems is a modern and promising approach to solving the problem of designing aircraft engine control systems, one that takes into account the specifics of design technology, all stages of the hydromechanical unit's life cycle, and diagnostic requirements.
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