Touchscreen technology drives growth in consumer electronics market

Smartphones and tablets are now ubiquitous. Businesses and consumers alike are using them in large numbers, and this year has seen a surge in tablet models. Touchscreen applications, first introduced on small form factor devices a few years ago, are quickly making their way to larger devices. For computing and electronic device manufacturers, this new market represents not only the latest consumer craze, but also a possible fundamental shift in how people interact with information and the computing hardware that provides it. The growth of this market has sparked a flurry of activity among device manufacturers, who are actively porting touchscreen technology to larger form factor hardware.

　　Still, the transition from small screens and simple touch applications to a new paradigm of interacting with full-size computers primarily with hands and fingers is not necessarily a straightforward one. Manufacturers will need to rethink the way consumers use touchscreens to meet new and more demanding requirements. Most importantly, the transition to larger screen sizes makes multi-touch capabilities essential. For example, smaller mobile phone displays can rely on single-finger touches to control and select phone actions. However, while a few finger strokes are sufficient on a 5-inch screen, how many strokes are needed on a 12-inch or 40-inch device, or when multiple users are interacting with both hands at the same time? What new popular applications will emerge for large-screen devices? How can manufacturers ensure that their devices support these applications?

　　The Basics of Touchscreen Technology

　　The basic principles of today's touchscreens are derived from early touch technologies that used (and still use) buttons, scroll wheels, and sliders. Over the years, the use of mechanical switches has continued to decline, with control technologies such as resistive membrane switches, piezoelectric switches, and capacitive sensing-based touch technologies leading the way.

　　Resistive touch technology

　　Resistive touch technology consists of a flexible top layer, an insulating spacer, and a bottom substrate. The top layer has a pattern printed on the top surface, and a conductive pattern using conductive ink, such as silver or carbon, is printed on the bottom surface. The substrate has a corresponding conductive pattern printed on it. The individual conductive layers are squeezed together through holes in the spacer to create a contact. To create tactile feedback, metal or plastic domes under the cover layer can be used to make a "click" when the switch is actuated, and embossing on the top layer can be used to guide the user's finger to the "sweet spot" of each switch. Despite this, membrane switches have many disadvantages. First, they are not true touch switches. Physical movement and physical pressure are required to make contact.

　　Similarly, resistive touch screens consist of multiple layers, the most important of which are two thin conductive layers with a small gap between them. Pressing a point on the outer surface of the screen will cause the two metal layers to connect at that point, which acts as a voltage divider, resulting in a change in current, which will be registered as a touch operation and sent to the controller for processing.

　　Figure 1 – Structure of a resistive touch screen

　　Resistive touchscreens have gained popularity in the market because they are cheap to produce and have excellent stylus capabilities, which has many fans, especially for applications using Asian characters. However, as multi-touch applications are becoming a trend, resistive technology does not support multi-touch. In addition, due to the need for multiple layers or "stacks" that affect optical performance, the display has poor visibility in sunlight due to reflections, and the brightness of the display is greatly reduced. Resistive touchscreens are also easily scratched, and moisture and dust enter because they need a soft outer layer to contact the stylus (or anything that touches it).

　　Projected capacitive touch technology

　　A competing technology to resistive touch uses a projected capacitive field. This technology quickly gained traction with users because it has a hard, "smooth," premium-looking exterior surface and, for practical purposes, is completely sealed to prevent the intrusion of dust and moisture. Manufacturers have responded quickly to consumer demand, and most appear to have settled on capacitive touch as the way forward. The technology works by measuring small changes in capacitance (the ability to hold an electrical charge) when an object, such as a finger, approaches or touches the surface of the screen. Not all capacitive touchscreens are created equal, though. Different capacitance-to-digital conversion (CDC) technologies and the spatial arrangement of the electrodes used to collect the charge determine the overall performance and functionality that can be achieved with a device.

　　Device manufacturers have two basic choices for how to arrange and measure changes in capacitance in a touchscreen: self-capacitance and mutual capacitance. Most early capacitive touchscreens relied on self-capacitance, which measures changes in capacitance across an entire row or column of electrodes. This approach is not a problem for single-point touch or simple two-point touch interactions, but it is very limiting for more advanced applications because of the position ambiguity that occurs when a user presses two points. The system effectively detects two (x) coordinates and two (y) coordinates, but it has no way of knowing which (x) is paired with which (y). This results in "ghost" locations when identifying touch points, reducing accuracy and performance.

　　Figure 2 – Difference between self-capacitance and mutual capacitance

　　As another solution, mutual capacitive touch screens use a set of transmitting and receiving electrodes arranged in an orthogonal matrix, so that the intersection of a row of electrodes and a column of electrodes can be measured. In this way, mutual capacitive touch screens can detect each touch operation represented by a specific pair of (x, y) coordinates. For example, a mutual capacitive system can detect two touch points (x1, y3) and (x2, y0), while a self-capacitive system can only detect (x1, x2, y0, y3). (See Figure 2) Basic CDC technology also affects performance. During the charge acquisition process, the potential of the receiving row is kept at zero, and only the charge between the transmitting X and receiving Y electrodes touched by the user is transferred. Other technologies can also do this, but the advantage of CDC is its ability to resist noise and parasitic effects. This capability can increase the flexibility of system design. For example, the sensor IC can be placed on the FPC close to the sensor or on the main circuit board farther away.

　　Sensor Design

　　An important parameter in sensor design, "electrode pitch", refers to the density of electrodes, or more precisely, the number of (x, y) "nodes" on the touch screen, which largely determines the resolution, accuracy, and multi-finger resolution of the touch screen. Although different applications have different resolution requirements, today's multi-touch applications need to identify small-scale touch actions such as stretching and pinching fingertips, so higher resolution is required to identify multiple adjacent touch actions.

　　Generally speaking, touch screens require row and column electrode pitches of about 5 mm or less (the distance between the fingertips when the thumb and index finger are folded together). This distance allows the device to correctly track fingertip movements, support stylus input, and use reasonable firmware algorithms to prevent accidental touch movements. When the electrode pitch is between 3 and 5 mm, the touch screen can support input from a stylus with a finer tip, thereby greatly improving accuracy and allowing the device to support a wider range of applications.

　　Touch screen driver chip

　　The underlying chip and software are the core of any successful touch sensing system. Like all other chip designs, touch screen driver chips should have high integration, minimum size, near-zero power consumption, and flexible support for a wide range of sensor design and implementation scenarios. Any driver chip should strike a balance between speed, power consumption, and flexibility.

　　Realize true multi-touch

　　Users of Apple iPhones and other modern devices are very familiar with today's multi-touch gestures, the most common of which is a pinch and stretch motion of two fingers. On a larger screen, we can imagine even more complex multi-touch gestures. For example, you can imagine drawing and music applications for students that involve gestures with all ten fingers, or tablet games where two or more users compete on a single screen. Regardless of how large-scale touch computing technology develops, application developers will want the flexibility to fully exploit all the new ways of interacting with touch screens. Device manufacturers don't want to be a roadblock, and they certainly don't want to build devices that can't support the next hot touch application.

　　As large-scale touch applications begin to use 4, 5, and 10-point touch, we must consider not only how new applications will take advantage of these capabilities, but also how the control chip will use this richer information to create a better user experience. For example, the ability to track occasional touches around the edge of the screen and classify them as "rejected" is more important on a large device than on a small device.

　　Just as a phone touchscreen should be able to recognize when the user is holding the phone or resting the screen against their cheek, large-scale systems must also account for the various ways a user holds and uses the device, such as resting the outside of the palm on the screen when using a stylus or resting both palms on the screen when using a virtual keyboard. It is not enough to recognize and suppress incidental touches, the device must also track them so that they are continuously suppressed even if they accidentally enter the active area. The more touches that the controller can simultaneously and accurately recognize, classify, and track, the more intuitive and accurate the user experience will be.

　　When designing a touchscreen application, engineers need to consider many factors. Only by carefully considering each of these factors can a high-performance, responsive display touch application be created. The first consideration is usually the required accuracy, that is, how accurately the touchscreen reports the location of the user's finger or stylus on the screen. An accurate touchscreen should report touch locations with an accuracy of less than +/- 1 mm. Just as important as accuracy is linearity, which is used to indicate the "straightness" of a line drawn on the screen. Linearity depends on the screen pattern design and should also be less than +/- 1 mm. Another practical consideration involves the size of the screen's active area and the number of potential touch points that the application may display. Fingers can only be brought together before being recognized as a single touch, so multi-finger resolution is also important. We also need to pay attention to the screen resolution because it determines the smallest finger or stylus movement that can be detected. There are several reasons why resolution down to a fraction of a millimeter is important, the most important of which is to support stylus-based handwriting and drawing applications.

　　From the user's perspective, the most important characteristic of a touchscreen device is its response time. Response time refers to the time it takes for the device to register a touch and respond. For basic touch gestures such as tapping, the device should register the input and provide feedback to the user within 100 milliseconds. Taking into account various system delays, this generally means that the touchscreen should report the first qualified touch position within 15 milliseconds. Applications such as handwriting recognition require even faster response times. Another factor that affects the user's screen experience but may not be obvious to the user is the signal-to-noise ratio (SNR). The signal-to-noise ratio refers to the screen's ability to distinguish between capacitance signals generated by real touch operations and capacitance signals generated by temporary noise. Capacitive touchscreen controllers measure small changes in row and column coupling capacitance, and the measurement method can greatly affect the controller's sensitivity to external noise. Large-size touchscreens face a particularly great challenge in this regard because one of the largest noise sources is the LCD itself.

　　As touchscreens get larger, support more simultaneous touches, and support more complex interactive content, top performance for all of these functions becomes increasingly important.