A touchpad (also trackpad) is a pointing device consisting of specialized surface that can translate the motion and position of a user's fingers to a relative position on screen. They are a common feature of laptop computers and also used as a substitute for a computer mouse where desk space is scarce. Touchpads vary in size but are rarely made larger than 20 square centimeters (about 3 square inches). They can also be found on personal digital assistants (PDAs) and some portable media players.
Like mice, touchpads are relative motion devices. This means that the cursor on the screen will move in the same direction as the motion of a finger moving on the touchpad's surface. The buttons below or above the pad serve as standard mouse buttons. Depending on the model of touchpad and drivers behind it, you may also click by tapping your finger on the touchpad, and drag with a tap followed by a continuous pointing motion (a ‘click-and-a-half’). Touchpad drivers can also allow the use of multiple fingers to facilitate the other mouse buttons (commonly two-finger tapping for the center button).
Some touchpads also have “hotspots”: locations on the touchpad that indicate user intentions other than pointing. For example, on certain touchpads, moving the finger along an edge of the touch pad will act as a scroll wheel, controlling the scrollbar and scrolling the window that has the focus vertically or horizontally depending on which edge is stroked. However, these are driver dependent functions and can be disabled. Also, certain touchpad drivers allow for tap zones, regions whereby a tap will execute a function. For example, pausing the media player or launching an application.
Touchpads are primarily used in portable laptop computers, because a standard mouse requires a flat surface near the keyboard not always available outside of a standard computing environment. Because the touchpad's position is fixed relative to the keyboard, and very short finger movements are required to move the cursor across the display screen, many users find touchpads preferable, and desktop keyboards with built-in touchpads are available from specialist manufacturers. However, these features can also cause frustration when a user's thumb accidentally swipes over the touchpad while typing.
Touchpads are also the primary control interface for menu navigation on all of the currently produced iPod portable music players (except the iPod shuffle and iPod nano and classic), where they are referred to as “click wheels”. Creative Labs also uses a touchpad for their Zen line of MP3 players, beginning with the Zen Touch and most recently featured in the Zen Vision:M. The New Zune Line-up Zune 80, Zune 4/8 uses touch for the Zune pad
Apple's PowerBook 500 series was the first laptop to carry such a device, which Apple refers to as a “trackpad”. When introduced in May 1994, it replaced the trackball of previous PowerBook models. Apple's more recent laptops feature trackpads that can sense up to three fingers simultaneously, providing more options for input, such as the ability to bring up the context menu by tapping two fingers.
Psion PLC's Psion MC 200/400/600/WORD Series, introduced in 1989, came with a new mouse-replacing touchpad; however, the Psion's device more closely resembles a graphics tablet than a touchpad, as one would position the cursor by clicking on a specific point on the pad, instead of moving it in the direction of a stroke.
There are two principal means by which touchpads work. In the matrix approach, a series of conductors are arranged in an array of parallel lines in two layers, separated by an insulator and crossing each other at right angles to form a grid. A high frequency signal is applied sequentially between pairs in this two-dimensional grid array. The current that passes between the nodes is proportional to the capacitance. When a virtual ground, such as a finger, is placed over one of the intersections between the conductive layer some of the electrical field is shunted to this ground point, resulting in a change in the apparent capacitance at that location. This method received awarded to George Gerpheide in April 1994.
The capacitive shunt method, described in an application note by Analog Devices, senses the change in capacitance between a transmitter and receiver that are on opposite sides of the sensor. The transmitter creates an electric field which oscillates at 200-300 kHz. If a ground point, such as the finger, is placed between the transmitter and receiver, some of the field lines are shunted away, decreasing the apparent capacitance.