\section{Acceptance of Shared Control Behaviours}\label{scAcceptance}

A common feedback given by all the volunteers was that they preferred manual control of the wheelchair over shared control.  It may be noted that all of them are able-bodied people who have sufficient skills and the confidence to perform the driving tasks without any assistance. The same may not be the case for people with disabilities as shown by the evaluation of the Hephaestus smart wheelchair system (Simpson et al, 1999), with the help of both able-bodied volunteers and those with disabilities, caused by conditions such as cerebral palsy and post-polio syndrome. While the able-bodied volunteers preferred not to be assisted by the smart wheelchair, the people with disabilities felt more secure driving with obstacle avoidance assistance. 

In re-directing the control of the wheelchair from the driver to the wheelchair controller, shared control is expected to lessen the sense of control felt by the user. However, the degree of the loss of control perceived by the user seems to be related to the magnitude of unpredictability of the shared control behaviours. It was observed that users accepted the shared control assistance if it was as per their expectations. However, they expressed frustration when they were unable to understand the behaviour of the wheelchair. This was clearly noticeable in narrow spaces where the wheelchair had stopped and prevented the user from moving in certain directions due to the Collision Avoidance assistive behaviour. Parikh et al (2005) have linked sense of control and frustration as inversely related feelings. Carlson \& Demiris (2010) have also provided a similar discussion relating to the lack of understanding of the system and the need for appropriate feedback.

An inference may be made at this point drawing from the discussion on \textquoteleft{feeling in control}'. The fact that users prefer to have as much control as possible over the wheelchair elicits the advantage of shared control wheelchairs over autonomous wheelchairs in ensuring the driver's safety while still maintaining his or her superiority.

It was noticed that the volunteers responded in different ways while the experiments were being conducted. A few volunteers were more cautious than others and slowed down the wheelchair when approaching obstacles, resulting in lesser collisions. Others focussed less on speed control. Some volunteers got more frustrated than others. Those who were able to understand the shared control behaviours more quickly, adapted to the system more easily. Hence the level of acceptance of the assistance provided by shared control was not similar among the volunteers. This is proven by the varied workload scores depicted in Figures \hyperref[fig:oaWork]{5.4} and \hyperref[fig:wfWork]{5.6}, in \hyperref[results]{Section 5.4}. 
