To meet increasing and often conflicting demands on performance, stability, fuel consumption and
functionality, modem jet engines are becoming increasingly complex. Improved compressor
performance is a major factor in this development process. Optimum compressor efficiency is
achieved in operating regions close to flow instability. Surveying basic concepts and control methods
of compressor instabilities, an overview of the fundamentals of surge and rotating stall is presented.
To maximise the potential of an aero gas turbine compression system, it is proposed to use more
advanced control strategies, such as multi variable control.
Multivariable control may offer the prospect of lower safety margin requirements leading to
greater compressor efficiency. Alternatively, it may result in more agility in combat through
improved engine responses and prolonged engine life. A multivariable control technique is proposed
and tested on a Rolls-Royce three-spool high bypass ratio turbofan engine. Since elements of the 2x2
system can be represented by linear third order models, a muItivariable PID controller will be
sufficient provided the design requirements are not too rigorous. To have a simple and efficient
design, a systematic decentralised PI (PID) control design strategy is developed. Decoupling a given
2x2 process by a stable decoupler, the elements of the resulting diagonal matrix are approximated by
first (second) order plus dead time processes using the proposed model reduction techniques. Then,
SISO controllers are designed for each element using the developed tuning formulae.
Any practical design method should be simple, easy to apply, flexible, generic or extendable, and
applicable to complex control schemes to fulfil more demanding control requirements. It will be
advantageous if the design algorithm can also directly address the design requirements, be repeatable
for any control objective, constraint and category of processes, have a design parameter, and can
consider any number of objectives and constraints. Formulating the PI (PID) control design problem
as an optimisation problem, a non-dimensional tuning (NDT) method satisfying the above-mentioned
design properties is presented. For a given first (second) order plus dead time process, the NOT
method is used in conjunction with either a single-objective or a multi-objective optimisation
approach to design PI (PID) controllers satisfying conflicting design requirements. In addition,
considering load disturbance rejection as the primary design objective, a simple analytical PI tuning
method is presented. The design problem is constrained with a specified gain or phase margin.
Compared to the corresponding conventional SISO controller, it is demonstrated that the
resulting decentralised controller considerably improves the overall surge risk to the engine during
the transient manoeuvres while maintaining similar thrust levels. Due to non-linearity of jet engine
models, gain scheduling is necessary. Designing decentralised controllers at various operating points,
the gain-scheduled controller accommodates the non-linearity in engine dynamics over the full thrust
range.