2 research outputs found
H°°-design and the improvement of helicopter handling qualities
This thesis presents the results of a study into the use of Hꝏ-optimization for the design
of feedback control laws for improving the handling qualities of a Lynx helicopter. An
important improvement to the Hꝏ-optimization procedure is the reduction in the number
of iterative steps in the γ-iteration before convergence to the optimal γ. Some new
algorithms are derived which significantly reduce the computation time for the γ-iteration.
Both 2-block and 4-block cases are considered.
Control laws are designed for precise control of pitch and roll attitude, yaw rate and
heave velocity. Analysis of the raw helicopter showed the need for a stability augmentation
system as the dynamic characteristics of the unaugmented helicopter do not comply
with military helicopter handling qualities requirements. Results from current research
on helicopter handling qualities were used as guidelines in order to define the required
dynamic characteristics. A six-degree of freedom nonlinear simulation was used to analyse
the helicopter dynamic time histories.
A possible solution to the problem of incorporating helicopter handling qualities in the
design of robust controllers is to use a two-degree of freedom controller structure. This is
illustrated using both H2 and Hꝏ-optimization.
A piloted simulation study to assess the effectiveness of advanced control laws was initiated
at RAE, Bedford.The trials were carried out in the single seat cockpit flight simulator, at
the Flight Research Division and represent the first ever real-time piloted simulation using
a Hꝏ-controller.</p
Robustness of multivariable feedback systems
The robustness of the stability property of multivariable feedback
control systems with respect to model uncertainty is studied and
discussed. By introducing a topological notion of arcwise connectivity,
existing and new robust stability tests are combined and unified under a
common framework. The new switching-type robust stability test is easy
to apply, and does not require the nominal and perturbed plants to share
the same number of closed right half-plane poles, or zeros, or both. It
also highlights the importance of both the sensitivity matrix and the
complementary sensitivity matrix in determining the robust stability of a
feedback system. More specifically, it is shown that at those
frequencies where there is a possibility of an uncertain pole crossing
the jw-axis, robust stability is "maximized" by minimizing the maximum
singular value of the sensitivity matrix. At frequencies where there is
a likelihood of uncertain zeros crossing the imaginary axis, it is then
desirable to minimize the maximum singular value of the complementary
sensitivity matrix.
A robustness optimization problem is posed as a non-square
H∞-optimization problem. All solutions to the optimization problem are
derived, and parameterized by the solutions to an "equivalent" two-parameter
interpolation problem. Motivated by improvements in
disturbance rejection and robust stability, additional optimization
objectives are introduced to arrive at the 'best' solution.</p