\subsection{Modes of Control}\label{controlmode}
An EPW may be controlled manually by the user, autonomously by an intelligent controller or in a shared manner by both the user and the wheelchair controller.
Standard EPWs are manually controlled by the user with the help of any of the access devices mentioned in \hyperref[accessdevice]{section 1.2.1}. In this case, the controller does no more than converting the user input into translational and rotational commands to be given to the motors. However the safety of the user may be compromised if precise control of the wheelchair is not possible due to limitations of the input device and/or the user suffers from lack of physical dexterity, cognitive abilities etc. (Velázquez, 2010). Hence robotic technology has been used to provide navigational assistance to the user.
Autonomous robotic wheelchairs (ARWs) are equipped with sensors to retrieve information about the environment. The controller in this case, takes inputs from the user as well as the sensors for decision making. The user needs only indicate the final destination and the control algorithms undertake global path planning and manoeuvre through obstacles. In the past two decades, there have been many projects across the world to develop autonomous wheelchairs. Simpson (2005) has provided a detailed review of \textquoteleft{smart}' wheelchairs. Some of the wheelchairs capable of autonomous navigation using landmark recognition or internal maps include Bremen autonomous wheelchair and those developed in projects such as VAHM (Véhicule Autonome pour Handicapé Moteur), SENARIO (Sensor Aided Intelligent Wheelchair Navigation System), TAO, TinMan etc. The OMNI (Office Wheelchair with High Manoeuvrability and Navigation Intelligence for People with Severe Handicap) project developed an omni-directional wheelchair for vocational rehabilitation (Hoyer \& Borgolte, 1996). The MAid (Mobility Aid for Elderly and Disabled People) project developed and tested control algorithms for manoeuvring a wheelchair in crowded public environments such as railway stations, malls etc. (Prassler et al, 1999).

The control algorithms used in ARWs serve the purpose of efficient manoeuvring of the wheelchair while ensuring safety of the user. However, the user of the wheelchair becomes a mere passenger and not the driver. As aforementioned, for people undergoing rehabilitation for SCI and SCD, a sense of independence is vital for their recovery and adaptation in society. Hence the assistance provided to the user must only \textit{aid} their existing skills and not suppress them. With this view in mind, several projects described above and other research groups have sought to provide semi-autonomous or shared control of the wheelchair. \hyperref[litreview]{Chapter 2} provides a detailed review of shared control approaches in assistive robotic wheelchairs.