British Rail has been designing active suspensions for some 16 years,
starting with tilt systems for the Advanced Passenger train. These
have been designed using classical control techniques requiring a
combination of experience, intuition and frequency response stability
techniques, such as Nichols' plots. In order to see if a more
systematic approach to control system design could produce
improvements in performance and implementation the current
investigation was instigated in which controllers designed using
classical techniques are compared with controllers designed using
linear optimal control theory.
The active suspension used for the investigation was an Electro
Magnetic active vertical suspension fitted to a service MkIII coach.
Design of the actuators is described in the thesis along with the
design of analogue and digital control systems.
Two classical control systems were designed. a simple "Sky Hook"
damper control system and a more sophisticated position control system
developed from British Rail’s experience with Maglev suspensions. A
regulator designed using linear optimal control theory was found to
give very good results in theory. However to implement the regulator
it was necessary to design a system observer. In order to achieve a
practically realisable observer considerable rationalisation of the
vehicle model was required, which drew heavily on experience gained
designing classical control systems.
The classical control systems proved to be much easier to commission
than the optimal controllers as they were designed with implementation
in mind. During track testing problems of interaction between
vehicles were encountered, as a result the biggest improvements in
ride were obtained with the simple Sky Hook damper, as it was less
specific to the vehicle than the other configurations.
With further development one of the optimal control systems considered
will probably turn out to be the most effective as it draws on the
attributes of both classical and optimal design techniques.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.