A touchpad 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 in PDAs. Touchpads were originally known as trackpads. Touchpads operate by sensing the capacitance of a finger, or the capacitance between sensors. Capacitive sensors are laid out along the horizontal and vertical axes of the touchpad. The location of the finger is determined from the pattern of capacitance from these 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.
Touchpads are relative motion devices. That is, there is no isomorphism from the screen to the touchpad. Instead, relative motion of the user’s fingers causes relative motion of the cursor. 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’). 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 in horizontal direction.
