A touchpad is a pointing device that is made of a specialized surface that can translate the motion and position of a user’s fingers to a comparative position on screen. They are most commonly found in laptop computers, but are also used as substitutes for a computer mouse when desk space is limited. Touchpads may vary in size, but are rarely made larger than 6 square inches. They can also be found on personal digital assistants and some portable media players. A touchpad is the most common kind of tactile sensor.
Touchpads operate in one of many ways, including capacitive sensing and conductance sensing. The most common technology used today involves sensing the virtual ground effect of a finger, or the electrical capacity between sensors. Capacitance-based touchpads are not able to sense the tip of a pencil or other similar tool. If the computer is powered by an external power supply, its thorough construction will influence the virtual ground effect, and therefore the touchpad may or may not work. While touchpads are able to sense exact position, resolution is limited by their size. The dragging motion of a finger is converted into a higher quality, relative motion of the cursor on the screen, corresponding 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 near it. On some touchpads, tapping the pad may be translated as a click, and a tap followed by a continuous pointing motion can be translated as dragging. Touchpad drivers can also facilitate the other mouse buttons by using multiple fingers. Even some touchpads have hotspots, which are locations on the touchpad used for purposes beyond a mouse. For instance, on certain touchpads, moving the finger along an edge of the touch pad will act as a scroll wheel. Also, some touchpad drivers support tap zones, which are areas where a tap will execute a function. All of these functions are put into practice in the touchpad device driver software, and can be disabled as needed.
There are two main means by which touchpads work. In the matrix approach, a series of conductors are arranged in a collection of parallel lines in two layers that cross each other at right angles to form a grid and are separated by an insulator. A high frequency signal is applied consecutively between pairs in the grid. The current that passes between the nodes is relative to the electrical capacity. The capacitive shunt method detects the change in capacitance between a transmitter and receiver that are on opposing sides of the sensor. If a ground point is placed between the transmitter and receiver, some of the field lines are shunted away, thus decreasing the seeming capacitance.