reduce or eliminate the user's task of driving a powerchair. Usually, a smart wheelchair is controlled via a computer, has a suite of sensors and applies techniques in mobile robotics, but this is not necessary. The type of sensors most frequently used by smart wheelchairs are the ultrasonic acoustic range finder (i.e. sonar) and infrared red (IR) range finder. The interface may consist of a conventional wheelchair joystick, a "sip-and-puff" device or a touch-sensitive display. This differs from a conventional powerchair, in which the user exerts manual control over speed and direction without intervention by the wheelchair's control system. Smart wheelchairs are designed for a variety of user types. Some are designed for users with cognitive impairments, such as dementia, these typically apply collision-avoidance techniques to ensure that users do not accidentally select a drive command that results in a collision. Others focus on users living with severe motor disabilities, such as cerebral palsy, or with quadriplegia, and the role of the smart wheelchair is to interpret small muscular activations as high-level commands and execute them. Such wheelchairs typically employ techniques from artificial intelligence, such as path-planning. Technological developments Recent technological advances are slowly improving wheelchair and powerchair technology. A variation on the manually-propelled wheelchair is the Leveraged Freedom Chair (LFC), designed by the MIT Mobility Lab. This wheelchair is designed to be low-cost, constructed with local materials, for users in developing countries. Engineering modifications have added hand-controlled levers to the LFC, to enable users to move the chair over uneven ground and minor obstacles, such as bumpy dirt roads, that are common in developing countries. It is under development, and has been tested in Kenya and India so far. The addition of geared, all-mechanical wheels for manual wheelchairs is a new development incorporating a hypocycloidal reduction gear into the wheel design. The 2-gear wheels can be added to a manual wheelchair. The geared wheels provide a user with additional assistance by providing leverage through gearing (like a bicycle, not a motor). The two-gear wheels offer two speed ratios- 1:1 (no help, no extra torque) and 2:1, providing 100% more hill climbing force. The low gear incorporates an automatic "hill hold" function which holds the wheelchair in place on a hill between pushes, but will allow the user to override the hill hold to roll the wheels backward if needed. The low gear also provides downhill control when descending. A