\subsection{Sensors}\label{mysensors}

The wheelchair was originally equipped with ground truth and touch sensors. Additional sensors were mounted for providing more information about the environment, essential for implementing shared control. USARSim provides a host of configurable sensors that can be mounted on the wheelchair by including them in the system configuration file, through a line of code specifying the name, type, location and orientation of each sensor (Wang \& Balakirsky, 2007). Sonar sensors and infra-red (IR) sensors were the chosen sensors for this project, being the cheapest and most easily available. The IR sensor emits a line in the direction of orientation of the sensor and provides a range value equal to the distance of the first point of obstruction encountered by the beam from the sensor. The sonar sensor sends a number of lines within its beam cone and returns the closest point of obstruction. If no obstacle lies along the path of the beams emitted by a sensor, the maximum range is returned. The maximum and minimum range of the sensors can be configured. The beam width of the sonar sensor is also configurable. A point to note is that the IR sensor beam does not get reflected by transparent substances such as glass and hence such obstacles cannot be detected. \hyperref[fig:top]{Figure 3.2} shows a screen shot of the wheelchair in the simulator, illustrating the arrangement of the sonar and IR sensors.

\begin{figure}[t]
\centering
\includegraphics[width=0.8\textwidth]{top}
\caption[Top view of sensors]{Top view of the sensor arrangement showing all 9 sonar sensors (encircled in yellow) and 4 IR sensors (encircled in red)}
\label{fig:top}
\end{figure}

Sonar sensors were chosen to provide Obstacle Avoidance assistance since, compared to IR sensors, they have a wider range and can be used in the outdoor environment, being insensitive to bright light. The total number of sensors was decided empirically, while trying to minimise the number through effective algorithms. Five sensors were placed in front of the wheelchair, two at either corners directed at an angle of 28° in the front-back wheelchair axis, and two were placed on either side. The height of the sensors above the ground was decided by taking into consideration objects such as chairs, beds, sofas etc. commonly seen in a household, which range in height from about 30 to 60 cm (from personal observation and measurement). However, since testing was done in an environment having other kinds of obstacles, the height was adjusted to be suitable for most obstacles in the map and was set at 53cm above the ground (see \hyperref[fig:sideUp]{Figure 3.3} for relative height above ground). A suggestion for practically mounting the sensors at this height is to use the arrangement implemented in the MAid wheelchair (Prassler et al, 1999). The beam angle of the sensors was set at 30°. Higher beam angles caused incorrect behaviour of the Obstacle Avoidance assistance as it is based on fixed thresholds for determining risk of collision. The maximum range of the sonar sensor is 5m and minimum is 10cm. These were the default values provided in USARSim and since they were suitable for the current approach, they were not altered.

\begin{figure}[t]
\centering
\includegraphics[trim= 0mm 30mm 0mm 0mm, clip, width=0.8\textwidth]{sideUp}
\caption[Height of sonar sensors]{Side view of the wheelchair showing relative height of the sonar sensors, encircled in yellow. IR sensors are marked in red}
\label{fig:sideUp}
\end{figure}

For providing Wall Following assistance, IR sensors were used since wide range was not required and the smaller size and cheaper cost of these sensors could be taken advantage of. Four IR sensors were mounted - one each on the right and left sides and corners. They were placed at a distance of 5cm from the ground (see \hyperref[fig:sideIR]{Figure 3.4}). Placing the sensors close to the ground allows the use of Wall Following for following kerbs as well. The corner sensors were oriented at an angle of roughly 45° to the front-back axis of the wheelchair to facilitate the determination of wheelchair drift as explained in \hyperref[wfalgo]{Section 4.1.3}. The range of the IR sensors was set as 5cm to 1.5m in order to be able to detect a wall within 1m of it as well as to pull up close to the wall.

\begin{figure}[t]
\centering
\includegraphics[width=0.8\textwidth]{sideIR}
\caption[Side view of IR sensors]{Side view of the wheelchair showing IR sensors marked in red - left side and corner IR sensors are clearly visible; right side IR sensor is partially visible}
\label{fig:sideIR}
\end{figure}

It was beyond the scope of this project to modify the centre of mass of the wheelchair that would have been altered by the addition of the sensors. Hence, it is assumed that the sensors are light-weight and do not affect the centre of mass significantly.


