\section{Discussion of Wheelchair Behaviour}\label{chairBehaviour}

\begin{figure}[t]
\centering
\includegraphics[width=0.49\textwidth]{headon}
\vspace{5mm}
\includegraphics[width=0.49\textwidth]{TreeStr}
\caption[Case study - Large obstructions]{Examples of large obstacles appearing in front of the wheelchair, causing it to stop}
\label{fig:CSlargeOb}
\end{figure}

The behaviour of the wheelchair in the Wall Following mode was found to vary depending on the angle of inclination of the wheelchair to the wall, prior to starting the wall following behaviour. If the inclination was high (angle between side and corner sensors much lesser than  48.5° or much greater than 51.5°), the drift correction measures made the wheelchair follow a curved path or zigzag route, which took a few seconds to stabilise. Higher the velocity, more the curvature observed since the deflection of the wheelchair away or towards the wall is more, and greater distances are covered in the time between consecutive corrections (i.e., 1 second). It was found to follow an almost linear path when the angle of inclination was low. This behaviour occurs because the degree of deflection of the wheelchair, while moving right or left, was not handled separately for different inclinations of the wheelchair with respect to the optimal range of  48.5° and 51.5°. The curvature of the path followed by the wheelchair also depends on the distance from the wall, since the control algorithm strives to maintain the wheelchair between 40cm and 65cm from the wall.
 
The behaviour of the wheelchair due to the Obstacle Avoidance and Collision Avoidance assistive modes are explained with the help of a few examples. 

\textit{Case 1}: The response of the wheelchair to large obstructions directly ahead of the wheelchair, such as those shown in \hyperref[fig:CSlargeOb]{Figure 4.4}, depends on the angle of inclination of the wheelchair to the obstacle. If the wheelchair approaches the obstacle head-on, it will stop (Case ii, \hyperref[oaalgo]{Section 4.1.1}). If inclined to the right or left, it will move towards the other side (Case iii, \hyperref[oaalgo]{Section 4.1.1}).

\textit{Case 2}: Some objects lie beyond the field of view of the sonar sensors, as illustrated in \hyperref[fig:NarrowOb]{Figure 4.5}, and are not detected by them, or detected too late to make a corrective action. The narrow pole belongs to the former category. The traffic cone escaped the beam of the sensor due to its shape. This behaviour is attributed to the narrow beam of the sonar sensors (30°).

\begin{figure}[t]
\centering
\subfloat[Narrow pole]{\label{fig:NarrowPole}\includegraphics[width=0.49\textwidth]{NarrowObject}}
\vspace{5mm}
\subfloat[Traffice Cone]{\label{fig:TrafficCone}\includegraphics[width=0.49\textwidth]{TrafficCone}}
\caption[Narrow objects lying in blind spots]{The beam angle of the sonar sensors is not wide enough to detect all kinds of obstacles, such as the narrow pole or the tapering traffic cone, depending on the orientation of the wheelchair}
\label{fig:NarrowOb}
\end{figure}

\begin{figure}[t]
\centering
\includegraphics[width=0.49\textwidth]{sidesBlockedStop}
\caption[Case study - Narrow corridor]{Navigating through a narrow corridor causes the wheelchair to stop frequently due to obstructed sides and corners}
\label{fig:NarrowPassage}
\end{figure}

\textit{Case 3}: When both corners of the wheelchair are obstructed as shown in \hyperref[fig:NarrowPassage]{Figure 4.6}, the wheelchair tries to move away from the closer side, right in this case, if the other side is free of obstacles. In this example, the left side is also obstructed and this causes the wheelchair to stop. The user is given the opportunity to slowly manoeuvre the wheelchair through the narrow passage without colliding with either side. 

\textit{Case 4}: A corridor containing columns, forming many narrow passages, is shown in \hyperref[fig:ColumnsImage]{Figure 4.7}. This obstacle course forms the last part of the Training Course Virtual Environment map and was used for testing the behaviour of Obstacle Avoidance and particularly, Collision Avoidance. One of the major observations seen in this part of the circuit is collisions occurring between the side of the wheelchair and the columns or railings due to the lack of sensors on the sides.

Due to the maximum range restrictions of the IR and sonar sensors, the assistive behaviours were found to be more effective at lower velocities. As mentioned previously, the Wall Following mode can be executed only in the first two velocity levels, which are below 1m/s. Moreover, movement in narrow spaces would be almost impossible due to longer thresholds of higher velocity levels causing the Collision Avoidance to be activated more often. Hence the maximum velocity of the wheelchair in shared control mode has been temporarily restricted to less than 1m/s. After the algorithms are made more robust, lower thresholds may be used and higher speeds would be possible.

\begin{figure}[t]
\centering
\includegraphics[width=0.95\textwidth]{columnsImage}
\caption[Case study - Obstacle course of columns]{Obstacle course containing columns in the Training Course VE. The total width of the passage is about 1.8m. Some spaces, as shown here, are only as wide as the wheelchair, which spans about 0.65m}
\label{fig:ColumnsImage}
\end{figure}

\hyperref[fig:GoodOA]{Figure 4.8} shows how an obstacle in front the wheelchair was detected and avoided well in time. \hyperref[fig:CADeactivated]{Figure 4.9} shows an example of a case where collision avoidance prevents the user from going in the direction of obstruction. In this case, the wheelchair is obstructed on all three sides - front, left and right. The wheelchair user is required to reverse, re-align the position of the wheelchair and then proceed.

\begin{figure}[h]
\centering
\subfloat[Wheelchair oriented towards obstacle]{\label{fig:obsColStr}\includegraphics[width=0.49\textwidth]{RtCornerStrColumn}}
\vspace{5mm}
\subfloat[Successfully avoided obstacle]{\label{fig:obsColGood}\includegraphics[width=0.49\textwidth]{RightColumnCorner}}
\caption[Case study - Successful obstacle avoidance]{Example of successful obstacle avoidance assistance - The column ahead of the wheelchair (in (a)) is detected and the wheelchair is steered away from it (in (b))}
\label{fig:GoodOA}
\end{figure}

\begin{figure}[h]
\centering
\subfloat[Collision avoidance activated]{\label{fig:CAactivated}\includegraphics[width=0.49\textwidth]{CAFrontRightStop}}
\vspace{5mm}
\subfloat[Reversed and re-positioned wheelchair]{\label{fig:CAreverse}\includegraphics[width=0.49\textwidth]{backupStrAgainStopped}}
\caption[Case study - Collision avoidance]{Collision avoidance is activated due to obstructions on all three sides  (in (a)). The driver reverses the wheelchair and re-aligns it to proceed in the forward direction(in (b))}
\label{fig:CADeactivated}
\end{figure}


