On the basis of the previous considerations, and taking into
account our experience in previous research projects [17], [18],
the navigation system of a powered wheelchair has been designed
as follows. It must interact with the users in order to
involve them in the guidance of the vehicle without limiting
the functionality and security of the system. The navigation
system allows two levels of autonomy [19]–[21] that will be described
in Section IV-C. The general architecture of the navigation
system is shown in Fig. 1, where two categories of navigation
sensors are used: dead-reckoning and external/environmental
sensors. The dead-reckoning system is based on odometric
sensors that allow the estimation of the mobile base position
with respect to a starting reference configuration. This has
been simply carried out by two incremental optical encoders
aligned with the axes of the driving wheels. The external/environmental
sensor system is based on a proper set of ultrasonic
range finders used for detecting obstacles in the environment.
In order to simplify the integration of the navigation module
in the existing wheelchairs, a control module has been introduced
which adapts the commands of the navigation module to
the commands of the mobile base actuators.
In the navigation system architecture shown in Fig. 1, the user
sends commands to the navigation module through the user interface.
The navigation module generates the control variables
and the control module translates these control variables in the
low level commands for the actuators of the driving wheels. The
main component of the whole guidance system is the navigation
module. The following section will give some details of the
sonar map of the environment and of the autonomy levels which
constitute the main elements of the navigation module.