How a Smartphone Touch Screen Works
Touchscreens have largely replaced the mouse and keyboard as the most popular input terminal on all kinds of devices. They are usually the only input device on your phone or tablet and even several brands of laptops and point-of-sale terminals are equipped with touchscreens now. The reason for this quick rise in popularity is simple; touchscreens are compact and there’s essentially no limit to what you can do with them.
Types of Touchscreens
Touchscreens come in two basic types, capacitive and resistive. Most phones use the capacitive variety because it registers multiple touches and responds quickly. The drawback is that it cannot be activated by non-conducting objects like fingernails or gloves. Resistive touchscreens are often used in car GPS screens and ATMS. Most resistive touchscreens don’t have the ability for multi-touch, because of the multiple layers of displays and the larger amount of force it takes to register a response. They are more durable, cheaper and can be used with gloves or fingernails. Resistance touchscreens are rarely used these days on popular smart devices, so this article will be focused more on capacitive touchscreens.
The glass you find on modern smartphone screens is not ordinary glass, but a highly engineered material. Originally, lenses were made from plastic, but were replaced by glass as the technology improved. Today’s high-strength glass is chemically tempered and makes for a very thin and tough lens. You may have heard of Corning’s Gorilla Glass, the chemically tempered glass used on several popular brands of smartphone and tablets such as Samsung and Google. It is made to be especially hard and tough, but the possibility of breaking still exists. The reason is because the glass still needs to be very thin (less than 1 mm) to reduce size and weight. As material science advances, so will the material used for touch lens.
When you touch your smartphone screen, the touch is sensed through capacitive measurements underneath the glass. Capacitance describes the ability of anything to hold a “charge”. When you shuffle your feet across a carpet then touch a metal object and experience a static shock, that’s your body experiencing capacitance. Your body was acting as a capacitor, and storing the static electric energy picked up from the carpet. When you touch your phone, the screen senses your finger because of the increased capacitance your body introduces around an electrical grid embedded in the display.
The anatomy of a touchscreen begins with a conductive grid made up of horizontal conductors and vertical conductors (as shown in the figure). The grid lines are etched on opposite sides of a thin plastic insulating sheet that is glued to the back of the glass lens. The grid is energized by a voltage creating an electrostatic field. These conductive lines cross and form tiny capacitors. The circuitry in your device senses this and measures it. So, for example, when your phone senses that the capacitance on horizontal line #50 and the vertical line #100 changed, it knows you touched at that intersection on the screen. Actually, when you touch the screen, you register a lot of contact points and your phone has to figure out what you want.
Locating Your Touch
The circuitry inside is virtually transparent, that is why you can’t see the grid in your phone. The location of the touch point is determined by the position on the grid where the capacitance is changed by your finger. The “X” position is sensed by the horizontal set of lines, the “Y” position is sensed by the vertical lines. Since your finger contacts many points on the grid forming an area, the microprocessor locates the center of the area to calculate a touch point as shown in the figure below.
Touchscreens are not pressure sensitive, but they require a certain contact area before they register a touch. If you touch the screen very lightly it may not register until you push harder, or form a larger contact area. On the other hand, if you’re pushing too hard, and forming too big of a contact area, this is known as fat-finger syndrome. Try again with less force and with more precision. Often this contact area is offset and you learn to compensate by pressing what seems to the edge of a key instead of directly over it.
Another problem is “ghosting” or “falsing” the touchscreen. This occurs when there are water droplets on the screen that register capacitance and create a touch point. Sometimes if there is a dead spot in the conductive grid, the screen will misread contact in that area. Older devices used to require frequent recalibration to realign the touch points, but this is almost never an issue with today’s devices.
A gripe about capacitive touch is that it does not work with a gloved hand. This is because the glove acts like an insulator keeping your (conductive) finger from the glass. Many companies make special gloves that have conductive fibers woven into the finger areas, thereby transferring your finger capacitance to the screen. Some manufacturers have compensated for this issue by giving devices an “increased sensitivity” mode. This mode increases capacitance sensitivity and allows users to use their phone with thin gloves.
Replacing the lens/touchscreen is a bit more complicated and expensive today than it used to be, because current phones laminate the LCD display to the lens and touchscreen. If the lens is cracked but the conductors are still intact, all you would need is a glass repair. But if the conductors or any other important mechanism of the display breaks, the whole thing must be replaced.
The future of touchscreens is “gesturing” and some newer phones already have it. “Gesturing” is when your phone can respond to a hand waved over the screen without actually touching it. Infrared (IR) sensors located on the display perimeter make this possible but it is only able to collect limited information. While the possibilities with “gesturing” will probably grow, it is doubtful that it would ever replace a touchscreen completely.