Page 464 - Biomedical Engineering and Design Handbook Volume 2, Applications

442 REHABILITATION ENGINEERING AND PROSTHETICS DESIGN

Any item, piece of equipment or product system whether acquired commercially off the shelf, modi-
fied, or customized that is used to increase or improve functional capabilities of individuals with
disabilities.

This definition has also been incorporated into other legislation in the United States and is used as a
working definition in other countries as well (Cook and Polgar, 2008). Note that the definition
includes commercial, modified, and customized devices. This allows us to include products made for
the general population in our working definition of assistive technologies. This definition also
emphasizes functional capabilities of individuals who have disabilities. The inclusion of customized
devices emphasizes the role of the rehabilitation engineer in designing or modifying devices that
meet the unique needs of an individual person who has a disability.

15.2 CONTROL INTERFACES FOR ELECTRONIC
ASSISTIVE TECHNOLOGIES

There are three elements that make up the user interface for assistive technologies: the control inter-
face, the selection set, and the selection method (Cook and Polgar, 2008). The control interface is
the boundary between the user and an electronic or mechanical assistive technology device. This is
what allows the individual to operate, or control, the device. For electronic assistive technology sys-
tems, control interfaces include joysticks for powered wheelchairs, keyboards and mouse input
for computers, and communication devices and single switches used to control household devices
such as lights or radios. Alternative control interfaces to these are also used, and they are described
in later sections of this chapter.
The selection set is a presentation of the items from which the user makes choices. The elements
in the selection set correspond to the elements of a specific assistive technology device output.
Selection sets may have written letters, words and sentences, symbols used to represent ideas,
computer icons, or line drawings/pictures. They may be presented in visual (e.g., letters on keys),
tactile (e.g., Braille), or auditory (e.g., voice synthesis) form. We can define two selection methods
through which the user makes selections using the control interface: direct selection and indirect
selection (Cook and Polgar, 2008). For any particular application the three elements of the
human/technology interface will be chosen based on the best match to the consumer’s skills (motor,
sensory, linguistic, and cognitive) (Cook and Polgar, 2008).
The fastest and easiest selection method to understand and use is direct selection. In this method,
each possible choice in the selection set is available at all times and the user merely chooses the one
that she wants. Indirect selection methods were developed to provide access for individuals who
lacked the motor skills to use direct selection. Indirect selection methods are scanning, directed
scanning, and coded access. Each of the indirect selection methods involves one or more intermediate
steps between the user’s action and the entry of the choice into the device. One of the most com-
monly used methods is scanning. Although there are a variety of implementations of scanning, they
all rely on the basic principle of presenting the selection set choices to the user sequentially and
having the user indicate when his choice is presented. The indication may be by a single movement
of any body part. Since scanning is inherently slow, there have been a number of approaches that
increase the rate of selection (Cook and Polgar, 2008). These involve selecting groups of characters
first to narrow the choices, then selecting the desired item from the selected group.
In a hybrid approach, called directed scanning, the user first activates the control interface to select
the direction (vertically or horizontally) in which the selection set is scanned by the device. The user
then sends a signal (either by waiting or hitting an additional switch) to the processor to make the selec-
tion when the desired choice is reached. Joysticks or other arrays of switches (2–8 switches, including
diagonal directions) are the control interfaces that are typically used with directed scanning.
Coded access requires the individual to use a unique sequence of movements to select a code cor-
responding to each item in the selection set. These movements constitute a set of intermediate steps
that are required to make the selection using either a single switch or an array of switches as the control