30-05-2012, 11:49 AM
Assistive technologies
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INTRODUCTION:
Assistive technologies are critical for people with severe disabilities to lead a self-supportive independent life. Persons severely disabled as a result of causes ranging from traumatic brain and spinal cord injuries to stroke generally find it extremely difficult to carry out everyday tasks without continuous help. Assistive technologies that would help them communicate their intentions and effectively control their environment, especially to operate a computer, would greatly improve the quality of life for this group of people and may even help them to be employed. A large group of assistive technology devices are available that are controlled by switches. integrated hand splint, blow-n-suck (sip-n-puff) device, chin control system, and electromyography (EMG) switch are all switch based systems and provide the user with limited degrees of freedom. A group of head-mounted assistive devices has been developed that emulatea computer mouse with head movements. Cursor movements in these devices are controlled bytracking an infrared beam emitted or reflected from a transmitter or reflector attached to theuser's glasses, cap, or headband. Tilt sensors and video based computer interfaces that can track afacial feature have also been implemented. One limitation of these devices is that only those people whose head movement is not inhibited may avail of the technology. Another limitation isthat the user's head should always be in positions within the range of the device sensors. For example the controller may not be accessible when the user is lying or not sitting in front of acomputer.Another category of computer access systems operate by tracking eye movements fromcorneal reflections and pupil position. Electro-oculographic (EOG) potential measurements havealso been used for detecting the eye movements.
Use of Tongue for Manipulation:
Since the tongue and the mouth occupy an amount of sensory and motor cortex that rivals that of the fingers and the hand, they are inherently capable of sophisticated motor control andmanipulation tasks. This is evident in their usefulness to the brain by the cranial nerve, whichgenerally escapes severe damage in spinal cord injuries. It is also the last to be affected in mostneuromuscular degenerative disorders. The tongue can move very fast and accurately within themouth cavity. It is thus suitable organ for manipulating assistive devices. The tongue muscle issimilar to the heart muscle in that it does not fatigue easily. Therefore, a tongue operated devicehas a very low rate of perceived exertion.An oral device involving the tongue is mostly hidden from sight, thus it is cosmeticallyinconspicuous and offers a degree of privacy for the user. The tongue muscle is not afflicted byrepetitive motion disorders that can arise when a few exoskeletal muscles and tendons areregularly used. The tongue is not influenced by the position of the rest of the body, which may beadjusted for maximum user comfort.
Working of Tongue Drive and its Block Diagram:
In Tongue Drive system, the motion of the tongue is traced by an array of Hall-effect magneticsensors, which measure the magnetic field generated by a small permanent magnet that iscontained within a nonmagnetic fixture and pierced on the tongue. The magnetic sensors aremounted on a dental retainer and attached on the outside of the teeth to measure the magneticfield from different angles and provide continuous real-time analog outputs.Fig. 1 shows the Tongue Drive System block diagram with two major units: one inside themouth, the mouthpiece, and the other outside, a portable body worn controller. Small batteriessuch as hearing aid button-sized cells are intended to power the mouthpiece for extendeddurations up to a mouth. The power management circuitry scans through the sensors and turnsthem on one at a time to save power. The time division multiplexes (TDM) analog outputs arethen digitized, modulated, and transmitted to the external controller unit across a wireless link.The magnetic field generated by the tracer inside and around the mouth varies as aresult of the tongue movements. These variations are detected by an array of sensitive magneticsensors mounted on a headset outside the mouth, similar to a head-worn microphone, or mountedon a dental retainer inside the mouth, similar to an orthodontic brace.
Sensor Signal Processing Algorithm:
The SSP algorithm running on the PC was developed in LabVIEW (National Instruments;Austin, Texas) and MATLAB (The Math Works; Natick, Massachusetts) environments. Itoperates in two phases: training and testing. The training phase uses principal componentsanalysis (PCA) to extract the most important features of the sensor output waveforms for eachspecific command [23].