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Touchpad and a pointing stick on an IBM Laptop

Close up of a touchpad with a locking button. Horizontal and vertical scroll bars are clearly visible near the bottom and right sides of the touchpad.

A touchpad (also trackpad) is an input device commonly used in laptop computers. They are used to move the cursor, using motions of the user’s finger. They are a substitute for a computer mouse. 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.

Touchpads operate in one of a few different ways, all of which entail sensing the capacitance of a finger, or the capacitance between sensors. This is why they will not sense the tip of a pencil or other similar implement. Gloved fingers may be problematic (such as in a cleanroom environment) but can sometimes work. Moist, sweaty, or calloused fingers can be problematic for those touchpads that rely on measuring the capacitance between the sensors.

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 following by a continuous pointing motion (a ‘click-and-a-half’^{[citation needed]}). 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 your finger along the right edge of the touch pad will control the scrollbar and scroll the window that has the focus vertically. Moving the finger on the bottom of the touchpad often scrolls horizontally.

Some touchpads can perform special functions by either tapping in a special corner of the pad, or by tapping with two or more fingers.

Early Apollo desktop computers were equipped with a touchpad on the right side of the keyboard.^[1]

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^{[citation needed]}. 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 Shuffle and Touch), 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.

Apple's PowerBook 500 series was the first laptop to carry such a device, which Apple refers to as a “trackpad”. When introduced in 1994, it replaced the trackball of previous PowerBook models. Apple's more recent laptops feature trackpads that can sense two 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^[2], introduced in 1989, came with a new mouse-replacing touchpad^[3]; 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. The conductors in these layers are oriented orthogonally to one another. A high frequency signal is applied sequentially between pairs in the two-dimensional matrix created by the conductor 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 U.S. Patent 5,305,017  awarded to George Gerpheide in April 1994.

The capacitive shunt method, described in an application note by Analog Devices^[4], 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.

• Synaptics
• Alps Electric Corporation
• Cirque Corporation

Look up touchpad, trackpad in Wiktionary, the free dictionary.

• Pointing device
• Pointing stick
• Touchscreen