Adaptive motion control for a four wheel steered mobile robot.
For adaptive motion control of an autonomous vehicle, operating in a generally
structured environment, position and velocity feedback are required to ascertain the
vehicle location relative to a reference. Whilst the literature offers techniques for
guiding vehicles along external references, autonomous vehicles should be able to
navigate between despatch locations without the need to rely on external guidance
systems. Considerations of the vehicle stability and manoeuvrability favour a vehicle
design with four independently steered wheels.
A new motion control methodology has been proposed which utilises the geometric
relationship of the angular displacements and the rotations of the wheels to estimate the
longitudinal and lateral motions of the vehicle. The motion controller consists of three
building blocks: the motion control system comprising the position tracking and the
motion command generation; the electronic system comprising a data acquisition
system and proprietary power electronics; the mechanical system which includes an
undercarriage enabling permanent contact of the wheels with the floor. The
components have been designed not only to perform optimally in their specific
functions but also to ensure full compatibility within the integrated system.
For reliable deduction of the wheel rotations with a high degree of accuracy a dedicated
data acquisition interface has been developed, which enables data to be captured in
parallel from four optical encoders mounted directly on the wheel axles. Parallel
sampling of the angular wheel position and parallel actuation of all steering motors
improves the accuracy of the system state and gives a higher degree of certainty.
Considering only circular motion of the vehicle, a method for calculating the steering
angles and wheel speeds based on the complex notation is presented. By cumulating the
displacement vectors of the vehicle motion and the location of the centre of rotation
between consecutive samples of the controller, the path of the vehicle is estimated. The
difference between the nominal and the deduced centre of rotation is determined to
minimise deviations from the reference trajectory and to allow the controller to adapt to
changes in the road/tyre interface characteristics.
The individual mechanical and electronic components have been assembled and tested.
Additionally, the performance of the electronic interface has been evaluated on a
purpose built test-bed. For the experimental validation of the methodology, a simple
method of mapping the centre of curvature with a pen mounted at the nominal centre of
rotation has been proposed. Experiments have been conducted with both the steering
angles fixed to their theoretical values for the nominal centre of rotation and with the
proportional steering controller enabled. The results from the latter method have shown
a significantly reduced deviation from the nominal centre of rotation.
The data captured of the angular wheel positions and steering angle settings has been
analysed off-line. Good agreement is obtained between the deduced and the actual
centres of rotation for the measurements averaged over 1.5 seconds. A number of
different centres of rotation have been investigated and the required steering angles to
compensate for the deviation have been plotted to form a control surface for the motion
controller. The deviation between the estimated and the actual centre of curvature was
less than 1.6% and adequate results could be obtained with the proportional steering