A touchpad is perhaps the most common kind of tactile sensor.
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Operation and function
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 capacitative 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 detailed construction of the PSU will influence the virtual ground effect; a touchpad may work properly with one PSU but be jerky or malfunction with another (this does not imply any electrical risk whatsoever, a delicate capacitative ground, not a contact ground, is at issue). This has been known to cause touchpad problems when a manufacturer's PSU, which will have been designed to work with the touchpad, is replaced by a different type. This effect can be checked by touching a metallic part of the computer with the other hand and seeing if operation is restored. In some cases touching the (insulated) power supply with some part of the body, or using the computer on the lap instead of on a desk, while working can restore correct operation.
While touchpads, like touchscreens, by their design 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 below, above or, to reduce the depth of the pad in compact devices such as netbooks, beside the pad.
On some touchpads and associated device driver software tapping the pad may be interpreted as a click, and a tap followed by a continuous pointing motion (a "click-and-a-half") can be used for dragging.[1] 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 the media player or launching an application. All of these functions are implemented in the touchpad device driver software, and can be disabled.
Edge motion
Edge motion is a property of a notebook computer's touchpad which lets the user scroll the computer screen without having to move the pointer to a scrollbar.Moving the finger along an edge of the touch pad acts as a scroll wheel, scrolling the window that has the focus (or that having mouse pointer over it, on Linux) vertically or horizontally depending on which edge is stroked.
When the scrolling operation is complete, the cursor retains its original position. This saves the labor of (1) moving the cursor to the scrollbar and (2) moving it back to where it was.
Touchpads in devices
Early Apollo desktop computers were equipped with a touchpad on the right side of the keyboard.[2]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.
One-dimensional touchpads are the primary control interface for menu navigation on the currently produced iPod Classic portable music player, 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 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 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 have a different touchpad design with the button incorporated into the tracking surface.[3]
Psion PLC's Psion MC 200/400/600/WORD Series,[4] introduced in 1989, came with a new mouse-replacing input device similar to a touchpad,[5] 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.
Close up of a touchpad with a locking button on a Compaq Presario Laptop. Horizontal and vertical scroll bars are clearly marked out and visible near the bottom and right sides of the touchpad.
Theory of operation
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,[6] 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.
Apple Inc. Multi-touch TrackPad
Trackpad on an Apple MacBook Pro
Scrolling TrackPad is the name for one of Apple Inc.'s patent-pending trackpads, used in their MacBook and MacBook Pro laptop computers. They were previously used in the PowerBook and iBook lines, prior to Apple's switch to Intel processors. It lets users scroll in an arbitrary direction by touching the pad with two fingers instead of one, and then moving their fingers across the pad in the direction they wish to scroll. A feature introduced in Mac OS X 10.5 (Leopard) allowed users to do various other combinations of gestures such as swiping four fingers up or down to activate exposé. There are also many applications available which are designed to extend the multi-touch options. For comparison, many laptop touchpads instead set aside an area along the right edge and bottom edge of the pad,| |
Trackpad of the Blackberry Curve 8520 in the red circle.
and moving a single finger in these areas performs a vertical or horizontal scroll operation, respectively. Current MacBooks, MacBook Pros and MacBook Airs have a Multi-Touch pad. Apple's newest multitouch trackpads also support pinching and rotating gestures, similar to those used on iPhone devices.
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