**4.4 Proximity sensors**

Ultrasonic proximity sensors are wired into the wheelchair drive system circuitry in order to assist the user in avoiding obstacles as well as in navigation. These sensors are crucial in assisting the user in preventing crashing into obstacles when they lose control of the wheelchair. The sensors convert electric signals into ultrasonic waves which are emitted towards the four sides of the wheelchair and are reflected back to the sensor upon encountering an obstacle. Measuring the time between sending and receiving the signal allows calculation of the distance between the wheelchair and the object and using a mathematical function model encoded in the microcontroller, they estimate the brake power required to stop the wheelchair. The brake power function is consists of an exponential decrease in the speed of the wheelchair as it is brought to a halt. This allows the wheelchair to make a smooth stop without agitating the user. For the sensors on the sides of the wheelchair, the microcontroller is coded to keep a safe constant distance from an obstacle thus helping the user from steering sideways into obstacles. These abilities make the wheelchair to be conscious of its surroundings when driving. The flow chart below summarises the algorithm followed on the decision making by the system (**Figure 7**).
