According to data from the 1994 National Health Interview Survey on Disability approximately 527,000 persons in the U.S. use some type of vision device. The following tables provide a breakdown of technology use by age of person and type of device.
Table 2‑1: Number and Percent of persons using Vision Devices
|AT device||All Ages||Ages 44 and under||Ages 45-64||Ages 65 and over|
|Total number of people||Any vision device||527,000||123,000||135,000||268,000|
|Percentages of people using each device out of all those who use any vision device||Telescopic lenses||30%||32%||37%||26%|
The above table shows that the number of visually impaired users using Braille script to read is less as compared to that of the other devices. Amongst these, users under age of 44 use it the most while for other age groups, other devices are preferred.
Commercially available Braille displays use bimorph piezoelectric bending actuators to control individual Braille dots [3, 4]. Attempts at reducing cost of these devices have brought it down to current levels in the range of 2500-4000 USD (65-100 USD per Braille character), which are yet significantly high . Hence, recent attempts at developing affordable Braille displays have concentrated on developing new actuators, which are more cost-effective, as opposed to piezoelectric actuators. Notable actuation alternatives proposed and developed for this purpose include actuators based on electromagnetics, electrostatics, pneumatics, thermo-pneumatics and smart materials.
Smart materials used for electromechanical actuation include Shape Memory Alloys (SMAs), electro-active polymers (EAPs), electro-rheological (ER) fluids and magneto-rheological (MR) fluids. No commercial Braille display has been reported, which successfully exploits any of the above mentioned alternatives.
SMA based actuation, in general, offers certain distinct advantages compared to other actuation alternatives. When compared to other smart materials, SMAs offer higher actuation stress as well as strain, which is preferred for a Braille display due to definite requirements of stroke and resisting force . Compared to conventional actuation alternatives, SMAs offer a higher power density, allowing development of miniature actuators suited for the size and spacing requirements of Braille.
SMA wire in a ‘V’ shape configuration with perfectly chosen angle and length makes it possible to achieve perfect stiffness needed . There are various ways to attain variable stiffness in the SMA wire . Variable stiffness can be added to the system to enable users to press pins with the force they desire. From variety of SMA materials available, NiTi alloy is the best suited for actuation purposes due its actuation properties, and affordable cost .