  The development of mobile robot platforms for inspection has gained traction
in recent years with the rapid advancement in hardware and software. However,
conventional mobile robots are unable to address the challenge of operating in
extreme environments where the robot is required to traverse narrow gaps in
highly cluttered areas with restricted access. This paper presents MIRRAX, a
robot that has been designed to meet these challenges with the capability of
re-configuring itself to both access restricted environments through narrow
ports and navigate through tightly spaced obstacles. Controllers for the robot
are detailed, along with an analysis on the controllability of the robot given
the use of Mecanum wheels in a variable configuration. Characterisation on the
robot's performance identified suitable configurations for operating in narrow
environments. The minimum lateral footprint width achievable for stable
configuration ($<2^\text{o}$~roll) was 0.19~m. Experimental validation of the
robot's controllability shows good agreement with the theoretical analysis. A
further series of experiments shows the feasibility of the robot in addressing
the challenges above: the capability to reconfigure itself for restricted entry
through ports as small as 150mm diameter, and navigating through cluttered
environments. The paper also presents results from a deployment in a Magnox
facility at the Sellafield nuclear site in the UK - the first robot to ever do
so, for remote inspection and mapping.
