Control Interfaces for Assistive Technologies

Key terms

Acceptance Time

Activation Characteristics

Sensory Characteristics

Spatial Characteristics

The user of an assistive device activates a control interface to provide an input to operate the device (e.g., turn it on, make a selection, or make it talk). Control interfaces come in many different sizes and shapes. The user’s needs and activities dictate the type of control interface that is chosen. The user’s needs are those elements of their motor, sensory, and cognitive abilities that influence the successful use of a control interface. User’s activities include a variety of tasks such as turning on a light using an enlarged light switch, or accessing a portable communication system using a single switch. In order for the person to know that they have successfully activated the device, it is necessary for there to be feedback from the device to the person. This usually takes place through a visual or auditory display or both. These displays are considered a component of the human/technology interface. Alternative displays for people with visual or auditory impairments are discussed in Chapters 8 and 9, respectively.

This chapter begins with a discussion of the characteristics that make control interfaces useful for particular individuals. Options for control interfaces for both direct and indirect selection are described with an emphasis on how to make their use successful.

Characteristics of control interfaces

Control interfaces differ according to their spatial, sensory, and activation characteristics.4,8 Consideration of these characteristics can be helpful when working to optimize the effectiveness of a particular device for an individual. The important factors are the placement and size of the control interface (which we call spatial characteristics), how the person uses the control interface to make a selection (activation characteristics), and what feedback is obtained as a result of the person using it (sensory characteristics). These characteristics and their use in control interface assessment and recommendation is discussed in detail by Cook and Polgar (2008).⁸

Spatial Characteristics

The spatial characteristics of a control interface are (1) its overall physical size (dimensions), shape, and weight; (2) the number of available targets contained within the control interface (e.g., a keyboard has over 100 targets while a joystick may have only 4); (3) the size of each target; and (4) the spacing between targets.

The target size and spacing should be matched to the individual’s fine and gross motor skills. Targets that are large and spaced far apart are useful for individuals with good range of motion but limited fine motor control (e.g., someone with a coordination disorder or a tremor). Small, closely spaced targets are useful for individuals with limited range of motion, muscle weakness, and accurate fine motor control (e.g., someone with arthritis). For example, a single control interface has one target, and the target size is the dimension (height and width) of the control interface (see Figure 6-10, A, for example). Typically, a single control interface can accommodate an individual who has limitations in range of motion and limited fine motor control. Control interface arrays (including joysticks) have two to five switches, each representing a different target and a different result when activated. The user’s range of motion required to access a switch array needs to be larger than for a single switch, but it can still be relatively small, depending on the spacing between the switches. The user’s fine motor control needs to be more refined than that required for a single control interface and less refined than that for a keyboard.

A contracted keyboard has keys (targets) of small size in close proximity to each other. Its overall size is also small. The keys on these keyboards range in size from 0.5 to 1.5 cm, and they require relatively fine resolution from the user. The requirement for the user’s range is moderate (less than 15 cm in both horizontal and vertical directions). Standard or commonly used keyboards require moderate range and relatively fine resolution of the user. Expanded keyboards have large overall size and enlarged target size, requiring relatively large range and fine resolution. Control interface arrays and keyboards can have from 2 to more than 100 targets.

Activation and Deactivation Characteristics

There are many characteristics related to the activation of the control interface.⁸ The effort expended and the amount of movement required to activate the control interface are important to consider when evaluating how effective a control interface is for an individual or when troubleshooting a situation that is not working well. Deactivation or the release of a control interface may be different than activation and also needs to be considered. While the choice of a control interface for a given individual will be part of the team assessment, the rehabilitation assistant (e.g., OTA, PTA) will often be the one interacting with the user on a regular basis. This frequency will allow the rehabilitation assistant to see if the person is able to use the device easily or if some adjustments are needed.

Control interfaces can be activated by a variety of methods that are shown in Table 6-1. The first column identifies the three ways the user can send a signal to the control interface (i.e., movement, respiration, and phonation); the middle column shows how each of these signals is detected by the control interface; and the column on the far right provides examples of each type of control interface.

Table 6-1

Method of Activation

Signal Sent, User Action (What the Body Does)	Signal Detected	Examples
1. Movement (eye, head, tongue, arms, legs)	1a. Mechanical switch: activation by the application of a force	1a. Joystick, keyboard, tread switch
	1b. Electromagnetic switch: activation by the receipt of electromagnetic energy such as light or radio waves	1b. Light pointer, light detector, remote radio transmitter
	1c. Electrical switch: activation by detection of electrical signals from the surface of the body	1c. EMG, EOG,* capacitive, or contact switch
	1d. Proximity switch: activation by a movement close to the detector but without contact	1d. Heat-sensitive switches
2. Respiration (inhalation-expiration)	2. Pneumatic switch: activation by detection of respiratory airflow or pressure	2. Puff and sip
3. Phonation	3. Sound or voice switch: activation by the detection of articulated sounds or speech	3. Sound switch, whistle switch, speech recognition

^*EMG: Electromyographic; EOG: electroculographic.

Movements by the user can be detected by the control interface in three basic ways. Mechanical control interfaces detect a bodily movement that generates a force. This mode of activation represents the largest category of control interfaces. Most control interfaces, keyboard keys, joysticks, and other controls (e.g., mouse or trackball) that require movement or force for activation fall into this category. Electromagnetic control interfaces do not require contact from the user’s body for activation. They detect movement at a distance through either light or radio frequency (RF) energy. Examples include head-mounted light sources and detectors and the transmitters used with EADLs (Electronic Aids for Daily Living) for remote control (similar to garage door openers). Electrical control interfaces are sensitive to electric currents generated by the body. One type, called a capacitive switch, detects static electricity on the surface of the body. This is similar to the game children play when they attempt to shock someone with static electricity. A common example of this type of interface is seen in some elevator buttons. The control interface requires no force, making them useful to individuals who have muscle weakness. Other electrical control interfaces use electrodes attached to the skin to detect the electromyographic (EMG) signal associated with muscle contraction. Electrodes placed near the eyes can measure eye movements and generate an electroculographic (EOG) signal based on them. Proximity control interfaces detect movement, without coming into contact with the body.

The second type of body-generated signal shown in Table 6-1 is respiration. The signal detected is either airflow or air pressure. The use of this type of control interface, generally called a sip-and-puff switch, requires that the user be able to place and maintain her lips around a tube and produce good control of airflow.

In order to activate a control interface the individual must exert some effort. The effort required varies from zero for touch switches upward to a relatively large amount for some mechanical switches. For a switch requiring physical movement, effort is the force required to activate the switch.

For an individual using a light pointer to choose from an array of different items, the effort required is sufficient head movement to aim the light beam at one item and move between items, and enough postural stability to hold the light beam on the desired item. Electrical interfaces require a range of effort from zero (for a capacitive switch) to relatively high for muscle force activation of an EMG.

The activation effort of pneumatic control interfaces is the amount of exhalation or inhalation required for activation, which can be either how hard (pressure) or how fast (flow) air is exhaled or inhaled. For example, some power-wheelchair processors use a system in which a hard puff (large effort and high pressure generated) is forward, a soft puff (small effort and low pressure generated) is a right turn, a hard sip is reverse, and a soft sip is a left turn. The difference in these control signals is based primarily on effort generated. Phonation signals also have a level of effort related (at the simplest control interface level) to volume or loudness.

There is also a force required to release, or deactivate, some control interfaces. Muscle contraction is necessary to remove, or release, the body part from the interface. Weiss (1990)¹⁷ measured both activation and deactivation forces for several mechanical switches and found that force was required to release the control interface in all cases, but the deactivation force was approximately one third to one half that required for activation.

Sensory Characteristics

The auditory, somatosensory, and visual feedback produced during the activation of the control interface comprises its sensory characteristics. Some control interfaces provide auditory feedback in the form of a click when activated. For example, keyboards that use mechanical switches for each key usually click when pressed, thus providing auditory feedback. Other keyboards have a smooth membrane surface that does not provide any natural auditory feedback. Often a tone is emitted to let the user know that a selection has been made.

When the interface is within the consumer’s visual field, visual data are obtained through observation of the placement and the movement of the control interface. For some individuals the type of visual data will mean the difference between successful and unsuccessful use of a control interface. For example, someone who has difficulty attending to objects in the environment may be more attentive to a control interface that is large and brightly colored.

The eye is sensitive to colors in the visual spectrum (from violet to red), but the eye is not equally sensitive to all colors in this range. If the eye is fixed and not allowed to rotate, the limits of color vision are 60 degrees to each side of the midline. Within this range, the response of the retina to colors is not equal for all wavelengths (colors). Figure 6-1 illustrates that blue objects are visible over the entire 60-degree range, whereas yellow, red, and green are recognizable only at points closer to the fixed (center) point of vision. This limitation on color sensitivity has a practical implication when working with individuals who rely on peripheral vision or who have difficulties in moving their eyes to track objects. If a target (e.g., a switch) is green or red, its position may limit the person’s ability to see the object. We can increase the visibility of the switch by using blue or yellow.

Figure 6-1 Color response of the eye differs with visual angle. (Modified from Woodson W. and Conover D. *Human engineering guide for equipment designers*, Berkeley, 1964, University of California Press.)

Somatosensory feedback is the tactile, kinesthetic, or proprioceptive input sensed on activation of the control interface. For example, the texture or “feel” of the activation surface provides tactile data. The position in space of the body part and its movement when the user activates the control interface provides proprioceptive feedback. Many mechanical control interfaces require movement and force for activation. The displacement of the control interface provides kinesthetic (movement) feedback, as well as tactile and proprioceptive feedback that are beneficial to the user. If there is not much movement, as with a membrane keyboard or touch screen, the sensory feedback is less and the individual may press harder than necessary, thinking that more force is needed to activate the keys. This extra, sustained force may result in errors, since many keyboards will repeat entries if a key is pressed for more than a second or two.*

Control interfaces that require more effort typically provide more sensory feedback. Likewise, switches that require very little effort provide very little sensory feedback. For example, a contact switch that is activated by an electric charge from the body (i.e., requiring only touch) does not provide the user with any somatosensory or auditory feedback. Many mechanical switches provide abundant feedback through the feel (tactile), an observable movement of the mechanism (visual), and an audible click (auditory).

Is it working? evaluating the effectiveness of a control interface

Selecting a control interface for an individual is a complex process.⁸ Once a control interface is selected and implemented for an individual, its effectiveness must be evaluated on an ongoing basis. The evaluation of effectiveness may be at the time of initial use or at any point where a change has occurred. The desired change in control method may be due to the individual (e.g., a degenerative condition that makes use a particular control interface difficult). Alternatively, the change may be required due to a change in technology. For example, a new wheelchair may be acquired and the control interfaces will have to be mounted to the new wheelchair and set up for the user, or the new wheelchair may use a different control interface to operate it. A new device to be controlled (e.g. a more advanced augmentative communication system) may be introduced, requiring changes in the control interface. Finally, the consumer may be experiencing difficulties (poor accuracy, excessive fatigue) using the control interface for the desired activities. In all of these cases, reevaluation of effectiveness is required.

Evaluating the effectiveness of a control interface that has been selected and installed can be challenging because of the large number of factors that can be involved. Figure 6-2 shows a systematic approach to evaluation of how well a control interface is working for an individual consumer. The process includes observation of the consumer carrying out the desired tasks. The range of possible tasks for which a control interface might be used includes controlling a power wheelchair (Chapter 12), making choices with an Electronic Aid to Daily Living (EADL; Chapter 14), using an augmentative communication system (Chapter 11) or cognitive assistive technologies (Chapter 10), or providing input to a computer as we discuss later in this chapter.

Figure 6-2 Evaluation of the effectiveness of a control interface in use.

When observing the consumer carry out the desired activity, it is important to note whether the speed and accuracy of selection are sufficient to accomplish the desired task and whether the effort expended results in fatigue during routine use. In general, accuracy is more important than speed. If indirect selection is being used, then accuracy is measured by whether the switch (or switches) can be pressed on command. If direct selection is being used with a keyboard, then accuracy involves not only hitting a key on a keyboard or a location on a touch screen, but also hitting the correct key or screen location. A consumer using an adapted mouse input could be asked to move the mouse pointer to a specific screen location and carry out other mouse functions such as click, double click, or dragging an icon to a new location. A user controlling a power wheelchair with a joystick may be asked to drive the wheelchair to a specific location or to turn in a specific direction.

Because they provide daily care and interact on a regular basis with individuals using assistive technologies, rehabilitation assistants (e.g., OTAs, PTAs) are often responsible for evaluating the effectiveness of a control interface. The first step is to ensure that the individual is properly positioned. The chosen anatomic site must be free to move as much as possible without restriction (e.g., from a head rest, wheelchair arm, or other constraint). The next step is to be sure that the control interface is placed in a position where the consumer can easily activate it without losing body position or exerting undue physical effort.

Once the individual and the control are properly positioned, it is possible to determine how accurately the consumer is using the control interface. In order to determine if the task was completed accurately, the observer must know what the consumer was trying to accomplish. The observer should direct the consumer to make specific selections using the control interface, keeping track of how long it takes to complete each task, how accurately it is done, and how much effort it requires.

Any limitations in accuracy or speed or high levels of fatigue require changes in the system. This may mean repositioning a control interface to make it easier to activate, choosing a control interface that requires less force or range of motion to activate it, or looking for an entire new control interface–anatomic site combination that is less fatiguing. It is also important to note the consumer’s evaluation of how successful the control interface–anatomic site combination is in meeting his or her needs.

Enhancing control: proper positioning and arm supports

Control enhancers are aids and strategies that enhance or extend the physical control (range and resolution) a person has available to use a control interface. In some cases a person’s control may be enhanced to the extent that he can select directly. In other cases control enhancers can minimize fatigue. Control enhancers include strategies such as varying the position or the characteristics of the control interface and devices such as mouthsticks, head and hand pointers, and arm supports.

The person and the control interface should both be positioned to maximize function. The importance of proper positioning to maximize an individual’s function is discussed in Chapter 4. A person’s position should be observed as he or she uses a device to be sure that activation of the control interface does not result in an undesirable change in body position. If inadequate positioning appears to be affecting the person’s ability to control an interface, it should be addressed. The position of the control interface can also affect the person’s ability to activate it, and changing the height or the angle of the control interface even slightly may dramatically improve the person’s control ability.

Features that enhance control are sometimes incorporated into the interface. For example, some joysticks have a feature, called tremor dampening, that allows adjustment of the joystick for people who have tremors. Tremor-dampening joysticks are able to distinguish between tremors, which are faster and smaller in amplitude, and intentional movements, which are slower and larger. The joystick is adjusted so that the tremors are disregarded and only intentional movements are detected. This adjustment enhances the ability of an individual who might otherwise be unable to operate a joystick to control a power wheelchair. A similar feature, called FilterKeys, is employed in Windows. When the FilterKeys feature is activated in Windows, brief keystrokes are ignored and the delay is lengthened before a key repeats itself when pressed for an extended period of time.

Head Pointers, Hand Pointers, and Mouthsticks

For individuals who lack functional movement in their arms and hands, a mouthstick or head pointer (Figure 6-3) can be used with head and neck movement to access a keyboard or perform other types of direct selection tasks (e.g., dialing a telephone number or turning pages in a book). A mechanical head pointer is a rod with a rubber tip that is attached to a head band. The individual can use the end of the head pointer to depress keys. Besides being able to move the head vertically and horizontally, the individual must have the ability to move the head forward to depress keys.

Figure 6-3 Control enhancers. A, Mouthstick. B, Head pointer.

There are also light pointers that can be worn on the head or held in the hand to control devices. One advantage of head-controlled light pointers is that it is not necessary for the user to move the head forward or backward. Light pointers are described in greater detail in the section on pointing interfaces.

Hand pointers (Figure 6-4, B) can be grasped with a gross hand grip. These devices include a projection with a rubber tip that can be used to press keys. These are sometimes referred to as typing aids. A pointing aid may help an individual who has the gross motor ability to move her arm and hand around a keyboard but has difficulty extending and isolating a finger to depress a key. There are commercially available aids that can be strapped onto the hand to assist in pointing, such as the typing aid shown in Figure 6-4, B. In some cases it is necessary to custom fabricate a pointing aid in order for it to fit the consumer’s hand appropriately. These custom-fabricated aids can range from complex hand splints to simple tools, such as a pencil with an enlarged eraser.

Figure 6-4 Control enhancers. A, Mobile arm support used to enhance the control in the upper extremity for accessing a control interface. B, Typing aid used to enhance a person’s ability to point and access a keyboard. (Courtesy Sammons Preston Co., Bolingbrook, Ill)

Mouthsticks are often used by individuals with quadriplegia as a result of a spinal cord injury. A mouthstick consists of a pointer attached to a mouthpiece. The user grips the mouthpiece between his teeth and moves his head to manipulate control interfaces or other objects. The shaft of the mouthstick can be made from a wooden dowel, a piece of plastic, or aluminum. In some cases, interchangeable tips for different functions (e.g., painting, writing, or typing) can be inserted into the distal end of the shaft. The mouthpiece can be a standard U shape that is gripped between the teeth or a custom-made insert. Mouthsticks are also available from several suppliers.* Use of a mouthstick requires good oral-motor control; later in this chapter we discuss training to develop these skills.

Mobile Arm Supports

Individuals who have weakness in the arm may not have enough strength to access the full range of a keyboard adequately. A mobile arm support (Figure 6-4, A), which props the arm and assists in arm movements by eliminating some of the effects of gravity, may then allow the individual to access a keyboard.

Positioning the Keyboard for Use

In all situations it is necessary to address the position and placement of the control interface so that it is optimally accessed by the user. Most keyboards are connected with a cable to the computer, which allows some latitude in positioning them so they are accessible. Keyboards can be placed on stands that raise them (e.g., for mouthstick use) or easels that tilt them (e.g., for easier hand access or foot access). Some keyboards (e.g., contracted keyboards) can be mounted to wheelchairs.

Control interfaces for direct selection

Direct selection is generally preferable to indirect selection because it is faster and more efficient. For that reason it is often worth the effort to work with direct selection approaches, if possible, before trying scanning or encoded access. Control interfaces for direct selection include various types of keyboards, pointing interfaces, speech recognition, and eye gaze. While the hands are preferred because of the inherent fine motor control, many individuals use foot or head control for direct selection, and automatic speech recognition allows a “hands free” approach to directly selecting items.

Several direct selection approaches utilize on-screen keyboards that employ a video image of the keyboard on the video screen, together with a cursor (see Figure 7-1, A). The user makes choices by moving the cursor to the desired key and selecting it (e.g., by clicking). A variety of mouse pointing methods or scanning can be used to position the cursor and make a selection (see Chapter 7).

The critical questions presented in Box 6-1 can assist in determining the effectiveness of a keyboard for a specific individual. Affirmative responses to all seven questions indicate that the control interface is meeting the consumer’s needs. A negative answer to any one or more questions will direct the rehabilitation assistant’s attention to the area or areas needing attention.

Box 6-1 Critical Questions for Evaluating Keyboard Use

1. Can the consumer reach all the keys on the keyboard?

2. Are the size, spacing, and sensory feedback of the keys appropriate?

3. Is the consumer’s speed of input adequate for the task?

4. Does the consumer target keys with approximately 75% accuracy?

5. Is the consumer able to simultaneously hold down a modifier key and select another key?

6. Is the consumer able to control the duration for which a key must be pressed before it repeats itself?

7. Does the consumer effectively use the standard keyboard layout?

In some situations, speed is of primary importance (e.g., in a work setting). In general, speed and accuracy are in opposition. That is, as speed increases, accuracy decreases. In some cases, the consumer may make selections so slowly and deliberately to be accurate that the use of the control interface under investigation becomes impractical. For example, if it takes several seconds to select a key, this rate may be equivalent to the use of scanning to make a selection. Because scanning takes much less physical effort, it should then be considered as an alternative to direct selection. The criterion for accuracy is somewhat subjective and subject to clinical judgment. We recommend that a goal of at least three out of four selections (75%) be established as a minimum for determining the effectiveness of a particular interface.

If the answer to any of the questions in Box 6-1 is determined to be “no,” then the use of a less limiting keyboard, a control enhancer, or modifications to the keyboard should be considered. For example, if a standard keyboard cannot be used because of a targeting problem, we may consider the following: (1) an enlarged keyboard with larger targets (less limiting), (2) a keyguard (modification), or (3) a typing aid (a control enhancer).

Keyboards

For written communication, a keyboard is typically considered the most efficient means of inputting information. The standard keyboard is the first choice for computer access. However, many individuals with disabilities are unable to use a standard keyboard. Fortunately, there are a number of alternatives. Table 6-2 provides examples of some commercially available alternatives to the standard keyboard.

Table 6-2

Alternative Keyboards for Direct Selection

Category	Description	Device Name/Manufacturer
Expanded keyboards	Generally membrane keyboards that have enlarged target areas, often programmable so that key size can be customized; useful for individuals with good range and poor resolution; also useful for individuals with limited cognitive/language skills or visual impairment.	IntelliKeys (IntelliTools); USB King Keyboard (TASH, Inc.); Expanded Keyboard (Able Net, Inc.); Big Keys Plus (Inclusive Technologies); Expanded Keyboard (Maltron)
Contracted keyboards	Miniature, full-function keyboards, typically with membrane overlay; useful for individuals with limited range of motion and good resolution.	USB Mini Keyboard (TASH, Inc.); Mini Keyboard (Able Net, Inc.); The Magic Wand Keyboard (In Touch Systems)
Touch screens/touch tablets	Activated by either breaking a very thin light beam or by a capacitive array that detects the electrical charge on the finger; the electrode array used to detect where the finger or pointer is touching is transparent; touch screen can be placed over the face of a monitor.	Touch Window (RiverDeep); MagicTouch (Laureate Learning Systems, Inc.)
TongueTouch Keypad	Battery-operated, radio frequency–transmitting device with nine pressure-sensitive keys activated by tongue; universal controller processes information sent from keypad to receiver.
Special-purpose keyboards	Keyboards on special-purpose devices, such as augmentative communication and environmental control devices; available keys may be much more limited in number or may be specific in function compared with standard keyboard.	See Chapter 11

Data from: RiverDeep, San Francisco, Calif. (http://rivapprod2.riverdeep.net/ ); Able Net, Inc., Minneapolis, MN, (http://www.ablenetinc.com/); Laureate Learning Systems, Inc., Winooski, Vt. (www.laureatelearning.com); IntelliTools, Frederick, CO (www.intellitools.com); Inclusive Technologies, (http://www.inclusive.co.uk/catalogue/index.html); In Touch Systems, Spring Valley, N.Y. (www.magicwandkeyboard.com); Maltron-USA (http://www.maltron-usa.com/expanded.htm); TASH, Inc., Ajax, Ontario, Canada, or Richmond, Va. (www.tashinc.com).

Standard Keyboards

Some individuals may have difficulty writing because of fatigue or minimally impaired motor control. A standard keyboard on a computer may be all that is needed to allow them to complete writing tasks effectively. Because it is readily available, the standard keyboard is the most desirable interface for direct selection for text entry. The standard keyboard typically has a full alphanumeric array consisting of letters; numbers; punctuation symbols; special characters such as “\,” “/,” “@,” “#,” “$,” and “%”; and special keys (e.g., END, DEL, SHIFT, CONTROL, and ALT). Key size and spacing and the amount of distance the keys travel vary depending on the type and manufacturer of the keyboard. Small keyboards are now readily available on the consumer market.

To keep the overall size down, laptop computers in particular have smaller keyboards. In many cases the laptop keyboard is flat, as opposed to the tiered key rows on a full-size keyboard. This can be more difficult for some individuals.

Ergonomic Keyboards

The term repetitive strain injury (RSI) encompasses several musculoskeletal disorders that develop as a result of sustained, repetitive movements.⁵ Carpal tunnel syndrome is the most common RSI. Standard keyboards place the hands in an unnatural position with the forearms pronated and the wrists extended and ulnarly deviated. This position causes strain on the tendons and nerves. Numerous alternatives to the standard keyboard have been developed in attempts to reduce this strain on the wrist and hands. These alternatives range from minor rearranging of the keys to major redesign of the keyboard shape and configuration.

Ergonomic keyboards attempt to reduce the strain placed on the hands and wrists during the repetitive motion of keying by putting the forearms, wrists, and hands in a neutral position, which is more natural and more comfortable for the typist. There are three basic ways in which the standard keyboard has been redesigned. The first and most common type of ergonomic keyboard is the fixed-split keyboard (e.g., Figure 6-5, A). The difference between these keyboards and standard keyboards is that the keys are spaced farther apart and the keyboard is curved, so that the hands are placed in a more neutral position. Many of these keyboards have a built-in wrist rest to support the wrists while typing.