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Alternative wheelchair control
The normal electric powered wheelchair needs to be operated through joystick by hand. However, the hand function may be limited or even not available in some patients with severe disabilities. The idea of the alternative wheelchair control is to use other part of the body rather than hand to operate some sort of a proportional control joystick. The alternative wheelchair control includes sip-n-puff control, chin control, head control, speech control and tongue-operated solution - each of those can in some form be found on the market nowadays.
Sip-n-puff devices are widely used for controlling powered wheelchair. In sip-n-puff system, the user gives commands to the chair by “sipping” (inhaling) and “puffing” (exhaling) on a pneumatic tube. This method works, basically, on the amount of pressure applied to the pneumatic tube and whether the sign of the pressure is negative or positive (indicating sipping or puffing, respectively). Sharp sips and puffs can be used to change the speed and direction of the wheelchair. Steering is accomplished by lower-level sips and puffs.
Head control is the option for the individuals with good head movement ability. In head control devices, switches are mounted in the headrest and activated by head movement. Ideally six commands including mode and power on-off/emergency stop, as well as directional control is available. By being in proximity to the switch in the center pad, the patient moves the wheelchair forward. Activating the side pads moves the chair in the corresponding direction. A reset switch toggles between the forward and reverse functions.  Some new head controller can detect the position and movement of head using ultrasonic transducers or RF, and translate those movements into proportional control of the wheelchair.
Recently several research institutions have developed inertial sensing units for head control of powered wheelchair platforms [3-5]. The units use an accelerometer IC to measure the roll pitch and yaw orientation of the users head. Simply tilting the head on the forward/back axis gives proportional drive in that direction. The same happens when the head is tilted on the left/right axis. The head turning axis is left free so the user can still look around. The user must be able to return to a neutral position for safe operation.
Chin control is usually considered as different to head control. But a chip-mounted joystick requires head movement. The chin sits in a cup shaped joystick handle and is usually controlled by neck flexion, extension and rotation. This system is designed for a client with good head control. 
Speech systems are usually used with higher level spinal injury patients C1 – C4. The wheelchair is configured with a built in speech recognition system which recognizes a small vocabulary of words used to control directional movement e.g. forward, back , left, right etc. Some more advanced systems use speech recognition to recognize all spoken words so that the user may use it for navigation and communication through computer systems e.g. email, documents. 
Tongue operated solution
The only commercially available tongue controller is tongue touch keypad (TTK). It is introduced in early 90s. The system consists of 9 switches built into a dental mouthpiece. It fits in the roof of the mouth and is activated by the touch of the user’s tongue. It allows user to select latched or unlatched drive mode. User can adjust the wheelchair speed by touching the front pad to go faster or the rear pad to go slower.
 Torsten Felzer and Rainer Nordmann, Alternative Wheelchair Control, 2007
 Michelle L. Lange, Driving with Your Head: head controlled power wheelchair access methods, 2001
 SHOW-HONG, C., YU-LUEN, C., YING-HAN, C., JEN-CHENG, T. & TE-SON, K. Year. Head-controlled device with M3S-based for people with disabilities. In: Engineering in Medicine and Biology Society, 2003. Proceedings of the 25th Annual International Conference of the IEEE, 17-21 Sept. 2003 2003. 1587-1589 Vol.2.
 NGUYEN, H. T., KING, L. M. & KNIGHT, G. Year. Real-time head movement system and embedded Linux implementation for the control of power wheelchairs. In: Engineering in Medicine and Biology Society, 2004. IEMBS '04. 26th Annual International Conference of the IEEE, 1-5 Sept. 2004 2004. 4892-4895.
 MANDEL, C., ROFER, T. & FRESE, U. Year. Applying a 3DOF Orientation Tracker as a Human-Robot Interface for Autonomous Wheelchairs. In: Rehabilitation Robotics, 2007. ICORR 2007. IEEE 10th International Conference on, 13-15 June 2007 2007. 52-59.
 K.Arshak and D.Buckley. Review of Assistive Devices for Electric Powered Wheelchairs Navigation. ITB Journal, 2006