A touchpad (or trackpad) is a pointing device featuring a tactile sensor, a specialized surface that can translate the motion and position of a user's fingers to a relative position on screen. Touchpads are a common feature of laptop computers, and are also used as a substitute for a mouse where desk space is scarce. Because they vary in size, they can also be found on personal digital assistants (PDAs) and some portable media players. Wireless touchpads are also available as detached accessories.
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Touchpads operate in one of several ways, including capacitive sensing and conductance sensing. The most common technology used as of 2010[update] entails sensing the capacitive virtual ground effect of a finger, or the capacitance between sensors. Capacitance-based touchpads will not sense the tip of a pencil or other similar implement. Gloved fingers may also be problematic.
If the computer is powered by an external power supply unit (PSU), the PSU will influence the virtual ground effect, with the influence depending on the circuitry of the PSU. Laptop manufacturers account for this in the design of their PSUs, meaning that the use of generic PSUs can cause touchpads to malfunction and pointer movement to become erratic. This misbehaviour can be rectified by restoring proper grounding, either by touching a metallic part of the computer with the other hand, touching the (insulated) power supply with some part of the body, or using the computer on the lap instead of on a desk.
While touchpads, like touchscreens, are able to sense absolute position, resolution is limited by their size. For common use as a pointer device, the dragging motion of a finger is translated into a finer, relative motion of the cursor on the screen, analogous to the handling of a mouse that is lifted and put back on a surface. Hardware buttons equivalent to a standard mouse's left and right buttons are positioned below, above, or beside the touchpad. Netbooks sometimes employ the last as a way to save space.
Some touchpads and associated device driver software may interpret tapping the pad as a click, and a tap followed by a continuous pointing motion (a "click-and-a-half") can indicate dragging.[1] Tactile touchpads allow for clicking and dragging by incorporating button functionality into the surface of the touchpad itself.[2][3] To select, one presses down on the touchpad instead of a physical button. To drag, instead performing the "click-and-a-half" technique, one presses down while on the object, drags without releasing pressure and lets go when done. 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 have "hotspots", locations on the touchpad used for functionality beyond a mouse. 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. Apple uses two-finger dragging for scrolling on their trackpads. Also, some touchpad drivers support tap zones, regions where a tap will execute a function, for example, pausing a media player or launching an application. All of these functions are implemented in the touchpad device driver software, and can be disabled.
As touchpads began to be introduced in laptops in the late 1980s, there was often confusion as to what the product should be called. No consistent term was used, and references varied, such as: glidepoint, touch sensitive input device, touchpad, trackpad, and pointing device.[4][5][6]
Users were often presented the option to purchase a pointer stick, touchpad, or trackball. Combinations of the devices were common, though touchpads and trackballs were rarely included together.[7]
A touchpad was first developed for Psion's MC 200/400/600/WORD Series in 1989.[8] Cirque introduced the first widely available touchpad, branded as GlidePoint, in 1994.[9] The touchpads included in Apple Computers’ PowerBooks were based upon Cirque’s GlidePoint technology;[10] another early adopter of the GlidePoint pointing device was Sharp.[9] Later, Synaptics introduced their touchpad into the marketplace, branded the TouchPad. Epson was an early adopter of this product.[9]
Touchpads are primarily used in self-contained portable laptop computers and do not require a flat surface near the machine. The touchpad is close to the keyboard, and only very short finger movements are required to move the cursor across the display screen; while advantageous, this also makes it possible for a user's thumb to move the mouse cursor accidentally while typing. Touchpad functionality is available for desktop computers in keyboards with built-in touchpads.
Early Apollo desktop computers were equipped with a touchpad on the right side of the keyboard.[11]
One-dimensional touchpads are the primary control interface for menu navigation on second-generation and later iPod Classic portable music players, where they are referred to as "click wheels", since they only sense motion along one axis, which is wrapped around like a wheel. Creative Labs also uses a touchpad for their Zen line of MP3 players, beginning with the Zen Touch. The second-generation Microsoft Zune product line (the Zune 80/120 and Zune 4/8) uses touch for the Zune Pad.
Apple's PowerBook 500 series was its 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 five fingers simultaneously, providing more options for input, such as the ability to bring up the context menu by tapping two fingers. In late 2008 Apple's revisions of the MacBook and MacBook Pro incorporated a "Tactile Touchpad" design with button functionality incorporated into the tracking surface.[12][13][14]
Psion's MC 200/400/600/WORD Series,[8] introduced in 1989, came with a new mouse-replacing input device similar to a touchpad,[15] although more closely resembling a graphics tablet, as the cursor was positioned 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 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,[16] 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.
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