Page 465 - Biomedical Engineering and Design Handbook Volume 2, Applications

TECHNOLOGY AND DISABILITIES 443

interface. One example of coded access used in electronic assistive devices is Morse code, in which
the selection set is the alphabet. An intermediate step [e.g., holding longer (dash) or shorter (dot)] is
necessary in order to make a selection. Two-switch Morse code is also used, in which one switch
sends dots and the other sends dashes. As long as either switch is held down, the dots or dashes are
sent. A major goal in the development of Morse code was its efficiency, and this can be very useful
when speed of entry is the goal. Codes are usually memorized, and this means that the selection set
need not be visually displayed. This allows use by individuals who have visual limitations. Coded
access also requires limited physical skill but significant cognitive skill, especially memory and
sequencing.

15.2.1 Methods of Activation Used for Control Interfaces
Control interfaces may be characterized by the way in which the consumer activates them (Cook and
Polgar, 2008). Three types of action by the user can result in activation of the control interface:
movement, respiration, and phonation. These are shown in Table 15.1. Movements can be detected
in three basic ways. First, a force may be generated by a body movement and detected by the
control interface. These mechanical control interfaces (e.g., switches, keyboard keys, joysticks,
mouses, and trackballs) represent the largest category. Many mechanically activated switches are avail-
able for control of assistive devices. Most often movement of the hand, head, arm, leg, or foot acti-
vates these switches. Hand, foot, or head movement (e.g., chin) is generally used to activate multiple
switch arrays, including some joysticks. These vary in the amount of force required, the sensory
feedback provided, and the degree to which they can be mounted to a wheelchair, table, or other
surface for easy access.
Electromagnetic control interfaces can also be used to detect movement at a distance through
either light (visible or IR) or radiofrequency (RF) energy. These interfaces include head-mounted light
sources or detectors and transmitters used with environmental control systems for remote control.
The third type of movement detection is electrical. These control interfaces sense bioelectric
signals. Switches of this type require no force, and a common example of this type of interface is

TABLE 15.1 Methods of Activation for Control Interfaces
Signal sent, user action
(what the body does) Signal detected Examples
1. Movement (eye, head, a. Mechanical control interface: activation a. Joystick, keyboard, tread
tongue, arms, legs) by the application of a force switch
b. Electromagnetic control interface: b. Light pointer, light detector,
activation by the receipt of electromagnetic remote radio transmitter
energy such as light or radio waves
c. Electrical control interface: activation c. EMG, EOG, capacitive or
by the detection of electrical signals from contact switch
the surface of the body
d. Proximity control interface: activation d. Heat-sensitive switches
by a movement close to the detector,
but without contact
2. Respiration a. Pneumatic control interface: activation a. Puff and sip
(inhalation/expiration) by the detection of respiratory airflow
or pressure
3. Phonation a. Sound or speech control interface: a. Sound switch, whistle
activation by the detection of articulated switch, automatic speech
sounds or speech recognition
Source: From Cook AM and Hussey SM: Assistive Technologies: Principles, and Practice, (2d ed.), St. Louis, 2002,
Mosby Yearbook Publishers, with permission.