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Pointing Devices

In the early days of computers, commands and data were input via a keyboard. On early computer monitor screens, the text entry position was denoted by a blinking underscore or vertical bar, called a cursor. At first, users had no control over the location of this cursor; later, directional arrow keys and key commands allowed users to select text entry points. In more recent operating systems that utilize a graphical user interface (GUI) , the cursor still indicates the point where text may be entered, but it is also a visible and moving on-screen pointer controlled with an input device, such as a mouse. The computer operator uses the pointing cursor to establish where the position indicator cursor should be placed, or to select a program to run or file to view. Typically, the pointing cursor appears on the screen as an arrow.

Since early computer use keyboard commands were difficult to learn and cryptic to non-computer specialists, computer manufacturers and software developers quickly embraced the point-and-click interfaces first popularized by the Apple Macintosh. However, keyboard arrows were no longer adequate as a way to move a cursor around the screen. The point-and-click concept required the user to move something that would cause a corresponding movement on the screen. This led to the development of input devices such as the mouse, the joystick, and other tools for controlling on-screen movement of the cursor. Although the mouse and its descendants are not replacements for the keyboard, they do supplement the keyboard in tasks for which it is ill suited.

Common Pointing Devices

Common pointing devices used to control on-screen movement include computer mice, touchpads, touch screens, joysticks, graphics tablets, and trackballs. Some of these devices, including the mouse and the joystick, can be added to a computer system according to the needs of a user. Other devices, such as touch screens, are integrated into specialized computer systems designed for particular purposes.

Computer Mouse.

A computer mouse is a small, hand-held, interactive input pointing device that, when rolled over a flat surface, controls placement of the cursor on a computer display. A computer mouse is analogous to a live mouse in that it is palm-size and mouse-shaped, with rounded corners. Originally, all mice were connected to computers with a wire suggestive of a tail; however, cordless mice are also available now.

A mouse can be a one-, two-, or three-button device. After a user positions the cursor on the computer display by moving the mouse, screen action can be controlled by single or multiple clicks of the mouse buttons. Screen icons can be activated with one click, or dragged across the computer display by a single click that is held as the mouse is moved from one location to another.

Traditional computer mice are electromechanical devices. A rubber-coated steel ball that protrudes from the bottom is detected by two orthogonal rollers, which also touch the ball, inside the plastic housing. These rollers act as transducers to convert the speed and direction of the rolling ball to electrical signals that are sent to a software driver to move the screen cursor. To provide better traction for the rolling ball, a mouse is generally operated on a flat, cushioned pad of foam.

There are also two other kinds of mice: optical mice and optomechanical mice. An optical mouse has no wheels or balls but instead depends on a light emitting diode (LED) and a photodetector. As the mouse moves across a grid of closely spaced lines on a specially designed mouse pad, the photodetector senses line crossings by changes in reflected light. An optomechanical mouse combines characteristics of the optical mouse and the standard electromechanical mouse: it is built with a moving ball and shafts with slits through which light can pass. As the mouse moves, the shafts rotate and light pulses strike the photodetector through the slits. The amount of cursor motion is proportional to the number of light pulses detected. No special mouse pad is required with optomechanical mice, and they are less vulnerable to dust and dirt-related failure than are mechanical mice.

Common to all types of mice are the serial communications sent back to the computer every time the mouse moves a certain distance (e.g., 0.25 millimeters or 0.01 inch or a mickey )one byte for x-movement, one byte for y-movement, and one or two bytes for button status. Low-level software in the computer converts the relative mouse movements to absolute cursor position on the display.

Touchpads.

Where using a mouse would be awkward, such as in a laptop computer configuration, or cursor movement is more important than characters, touchpads have become popular. These are generally built into a computer unit, and they often include clickable buttons that correspond to the buttons of a mouse. Beneath the top layer of the touchpad are two or more layers separated by a non-conducting dielectric ; each layer contains a grid of electrode rows and columns. The different layers create a capacitance (electric field) between them that may be drastically changed by the electric field of a human finger either touching or moving near the touchpad. Changes in capacitance are measured 100 times a second and translated into cursor movement.

Touch Screens.

A touch screen is a computer display screen that is sensitive to human touch, allowing the screen to function as an input pointing device. The user touches the screen itself to cause some action to take place.

There are three types of touch screen technology. A resistive touch screen panel is coated with a thin, metallic, electrically conductive and resistive layer that causes a change in the electrical current that is registered as a touch event and sent to the controller for processing. A surface wave touch screen uses ultrasonic waves that pass over the touch screen panel. When the panel is touched, a portion of the wave is absorbed and this change in the ultrasonic waves registers the position of the touch event and sends this information to the controller for processing. A capacitive touch screen panel is coated with a material that stores electrical charges. When the panel is touched, a small amount of charge is drawn to the point of contact. Circuits located at each corner of the panel measure the charge and send the information to the controller for processing.

Resistive touch screen panels are generally the most affordable but they offer only 75 percent clarity and the layer can be damaged by sharp objects. Resistive touch screen panels are not affected by outside elements such as dust or water. Surface wave touch screen panels are the most advanced of the three types, but they can be damaged by outside elements. Touch screens are especially popular in very small computers to eliminate the requirement of a keyboard space. They are also commonly used in retail situations such as gas stations, ATM bank machines, restaurants, and information kiosks.

Joysticks.

Joysticks are similar to mice in that they transmit X-Y hand coordinates to the computer to position a cursor on the screen. A joystick is connected to a yoke that pivots freely in all directions. Sensors detect movement and generate corresponding electric currents that are interpreted by a microcontroller as movement of the cursor. Later model joysticks also have a "tophat button" or contact switch that can be pressed to trigger screen activity. Joysticks are most commonly used in computer games and simulations to provide users with flexibility and quick response time when interacting with on-screen events.

Graphics Tablets.

Digitizing graphics tablets give artists the flexibility to translate precise pen movements into lines on a display. They can also "read" a user's handwriting through the use of specialized software. Graphics tablets use a variety of technologies, including pressure-sensitive sensors, optical sensors, magnetic sensors, and capacitive sensors, to determine the location of a pen on the pad. Some tablets require the use of a special pen that is attached to the tablet while others allow the use of any pointed object, including a wooden pencil. The resolution and accuracy of a graphics tablet depends on the technology employed. Graphics tablets can be used as mouse replacements but they are particularly suited for drawing.

Trackballs.

A trackball is a computer cursor control device usually built into the front of the keyboard, close to the user. Essentially, the trackball is an upside-down mouse that rotates in place within a socket. The user first rolls the ball to direct the cursor to the desired place on the screen and then clicks one of two buttons (identical to mouse buttons) near the trackball to select desktop objects or position the cursor for text entry. Like a touchpad, this pointing device is used in laptop computers and other applications where a mouse would be inconvenient or awkward to use.

Other Pointing Devices

Less commonly used pointing devices include eraser-like pointer sticks built into some laptop keyboards, as well as hand-held units such as light pens, which were among the earliest input units, and pen input devices. Pointing sticks, or eraserhead pointing devices, are so called because they look like a pencil eraser stuck somewhere between the G and K keys on the computer keyboard. When a user's finger puts pressure on the eraserhead, the pressure is passed to contacts underneath the keyboard which varies electric current; a microcontroller translates the variable electric current to cursor movement on the computer display.

Pen input devices and light pens allow users to bypass a keyboard and instead use the familiar hand movements of writing or drawing to control action on a computer screen or input graphics or text information. A photodetector in the tip of a light pen responds to points of light on the screen so when a point of light on the screen is lit, the light pen notifies the program that the current location is correct. By moving a pen around the screen, a cursor can be made to follow the pen. Pen input devices and light pens make it possible to activate a command, execute a task, or draw. Handwriting recognition software is also being developed to convert on-screen handwriting into text.

see also Game Controllers; Games; Input Devices; Mouse.

William Yurcik

Bibliography

Kolle, Iril C. Graphics Tablet Solutions. Cincinnati, OH: Muska & Lipman Publishing, 2001.

White, Ron. How Computers Work, 6th ed. Indianapolis, IN: Que, 2002.

Internet Resources

MouseSite. <http://sloan.stanford.edu/mousesite/MouseSitePg1.html>

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