
\section{Basic Principle}\label{principle}
The underlying principle of shared control of a wheelchair is to combine the navigational commands from both the user and the controller to produce the final motor commands to the wheelchair. The user input is received from any of the user interfaces as mentioned in Section 1.2. The controller's commands are based on environmental info received from sensors fitted to the wheelchair. Figure 1 ...

Various research groups have adopted different approaches to provide shared control. While the basic principles are more or less the same, the implementation differs depending on some characteristics of the wheelchair system such as:\\
\begin{itemize}
	\item Default behaviour - User or robot controlled
	\item Mode switching techniques - implicit by user intention plan prediction or explicit by user input
	\item Manner of assistance - Constant or adaptive
	\item Type of assistance - High level or low level
	\item Frequency of user command - If previous command is carried out until new command is issued
\end{itemize}
All agree that help should be provided only as and when required. Some (Yanco, Petry) provide assistance as a last resort to protecting the safety of the use. Others () try to estimate user intention, or (uridales) try to provide assistance based on performance or stress levels.

\section{Overview of some shared control approaches}\label{overview}
Many projects such as ... have implemented shared control.\\

popular approach is to add different modes or behaviour in a modular fashion

different algorithms have been used to achieve this, some more mathematically complex than others. 

olly Yanco in her PhD thesis devised an algorithm for indoor and outdoor navigation using common sense constraints. It is not limited to any particular environment and does not require prior knowledge of destination. A noteable advantage over other approaches which need a goal. However, this causes more chances of frustration, unless it is coupled with good user intention strategies.

Another approach to obstacle avoidance is Potential Field Methodolgy. Petry and .. have adopted this methodology with some modifications to overcome .. Uridales uses.

Another technique used is the vector f h. 


The Vector Field Histogram is used for obstacle avoidance assistance by the wheelchairs Sharioto and NavChair. Petry enlists the advantages and drawbacks of this method and uses Potential Field Theory instead for obstacle avoidance. Uridales also used this technique with ... modifications to compensate for the known drawback of...

In the MAIA project, the robot wheelchair, Sharioto, was integrated with the asynchronous IDIAP BCI (Philips, 2007). The shared control system of the wheelchair receives the user input as estimated probability distribution values for three commands, Forward, Left and Right, from the BCI system. The command with the highest probability is identified as the user's intention and is converted into translational and rotational velocity values, which is combined with the current motor signal values to produce new wheelchair motor signals for a smoother driving behavior (Millan, 2009). Simultaneously, the shared control system receives an 'appropriateness level' for three assisting behaviours, namely, collision avoidance, obstacle avoidance and orientation recovery. These levels are calculated based on information of the environment obtained with a laser scanner. A winner-takes-all algorithm is used to select the assisting behaviour with the  highest appropriateness level which then modifies the motor signal obtained using the user input to create the final motor signal.

\section{Prediction of user intention}\label{intention}
Carlson

\section{Mode Switching Techniques}\label{mode switching}


\section{Evaluation methods}\label{evaluation}

Petry - Within subject - Wilcoxin - for no. of collisions : qtytative, Likert scale- Qualitative


\section{Sensors used}\label{sensors}

Petry - 2 encoders, 8 sonar sensors, 8 IR