A Study in Jet Engine Control Control Structure Selection and Multivariable Design
Doctoral thesis, 1999
Turbojet engines are becoming increasingly complex, with more control variables, to meet future demands on performance, fuel consumption and functionality. To utilise the potential of these engines, it is also necessary to use more advanced control concepts than conventionally used today. The trend is towards control concepts often referred to as "Smart Engines". Multivariable controllers are most often the cores of these systems.
There are many ways to design controllers for multivariable systems. A number of design methods for linear multivariable controllers have been developed during the last decades and applied to jet engines in various research projects. There are, however, few examples of jet engines in production today using these techniques. Linear design is only one part of the control design. To make the design work for real applications it is essential to handle all aspects of control design and there are some missing links here. One important part of this is how to select inputs and outputs and how to configure the control laws.
This thesis tries to grasp a very broad area of jet engine control. A case study is performed that covers a large portion of the control design. The objective of this is to demonstrate how a complete design can be done and also to point out the need for further research. Within this case study a control concept based on multivariable H-design technique is proposed. A simulation model of the RM12 engine for the JAS 39 Gripen fighter aircraft is used to demonstrate the concept. The study has been restricted to continuous time controllers, and 'normal mode' control. The linear H controllers have been designed in three design points to cover the selected engine operating range. These controllers are then tied together to a global nonlinear controller using a switching strategy. The multivariable control concept has been evaluated via extensive simulations to test various types of thrust transients, robustness to engine variations, and handling of disturbances. With the modular structure of the proposed concept, it might be a suitable basis for a future advanced jet engine control system.
A large part of the thesis is about methods for Control Structure Design (CSD). Some methods for selection of inputs, outputs and control configurations proposed in the literature are thoroughly evaluated. A toolbox for the selected methods has been developed in MATRIXx. The tools are applied to the jet engine model extended with a large number of candidate output signals. All combinations of inputs and outputs are evaluated to find the best possible control structures. Some of the selected structures are then further evaluated to see if a block decentralised control configuration can be used instead of a fully multivariable one. Control design is performed for the selected structures and the design results are compared with the predictions from the CSD methods. It is shown that the predictions mainly correspond to the design results but also that it is necessary to use a large portion of application insight when using the tools.
control structure design