Multi-touch technology for large displays arrives

Multi-touch interfaces have revolutionized computer user interfaces, but we have only seen their potential in simple touch interfaces or in applications that are just beginning to be digitized, and applications that require touch interaction on large display panels are just beginning.

Multi-touch-based signs and billboards offer advertisers new ways to interact with potential customers, enabling them to rotate and zoom in on product images displayed on end-of-aisle displays in stores or on large outdoor display panels. In schools, teachers and students can move from simple electronic whiteboards to interactive displays that allow them to search for images and text online, then zoom in and annotate for others in the class.

In industrial and other real-time control applications, multi-touch makes it possible to include more control functions on the touch screen, allowing operators to highlight objects on the screen to view their details and then dial in new parameters without having to enter detailed commands or search for the relevant control function key among the numerous function keys on the nearby control panel.

In medicine, surgeons can use gestures to rotate and zoom tomographic images to better view areas of the image that need closer inspection to aid in diagnosis or to accurately mark surgical areas.

The technology could even make furniture interactive: Universities and companies including Microsoft are working on interactive surfaces for tables that can detect not only the touches of people sitting around them but also objects placed on them, a feature that could help enable augmented reality games and other forms of entertainment.

However, these applications are only possible with the right technology. Projected capacitive touchscreens have driven the multi-touch revolution in portable electronic devices such as mobile phones and tablets, but the technology is not well suited for use outside of these applications. One issue is the ability to scale to larger screens. The technology's accuracy decreases as screen size increases, and it cannot be used for displays larger than 65 inches, which is required for whiteboards and interactive signage.

Another issue is the nature of touch itself. Capacitive touch technology requires some capacitance to be present in the object being touched for the sensor to register the touch. A bare finger or a specially designed stylus will satisfy this requirement, but a gloved finger will likely not be detected unless the glove is very thin. The technology also doesn’t work well in wet environments, making it difficult to use for outdoor signage and medical, industrial and home desktop environments prone to liquid spills.

In-glass multi-touch technology provides an alternative that meets all the requirements of these applications. It uses the principle that transparent materials can reflect light inside when light hits the surface at an angle greater than the critical angle and the reflectivity of the medium on the surface is lower than the reflectivity of the material itself.

In the technology developed by FlatFrog and implemented by Dialog Semiconductor in the DA8901 Smartwave multi-touch integrated circuit (MTIC), an LED array directs infrared light at an oblique angle into a cover plate above the display. Much of this light is reflected into the glass or plastic cover plate when it reaches the opposite side. Detectors are positioned to receive the light that has been reflected multiple times inside the cover plate (Figure 1).

Figure 1: The DA8901 drives an array of infrared light-emitting diodes (LEs) in a controlled sequence, directing light into the display’s cover glass.

If an object touches these surfaces, the internal reflections are disrupted, the infrared light is scattered into the cover, and the detectors that could have received them cannot receive the signal. This light intensity change is recorded by all affected detectors, amplified, and dynamically filtered to eliminate the influence of ambient light, and then converted into a digital touch signal through a high-linearity analog-to-digital converter and a dedicated touch algorithm.

An important aspect of in-glass sensing technology is that it is pressure sensitive: the spread of a finger as it presses down harder is detected as a further change in light intensity. As a result, the technology provides smooth and responsive pressure sensing across the entire display. Because the technology relies on internal reflections, it doesn’t matter if the display is flat or curved (Figure 2). This enables OEMs to take advantage of curved display technology developed for large-screen TVs and makes it easier to touch the entire surface of the display when interacting with the system via touch.

Figure 2: TV sizes similar to tablets, with flat edge-to-edge displays and curved displays.

Because the light reflection is changed entirely by the motion of the physical object touching the screen, in-glass technology can detect anything touching it, in contrast to projected capacitive systems. Also, unlike previous matrix infrared touchscreens where the sensors and emitters are mounted on the edge of the display above the display panel and its cover, in-glass technology supports full multi-touch sensing, able to sense many objects touching or placed on the display surface.

At the same time, the control electronics can perform functions such as palm rejection to avoid registering "false" touches, such as when a user accidentally touches the display with another part of their hand while they are interacting with the system with their fingers. By adjusting the angle of some of the infrared emitters, it is possible to detect motion at close range above the display surface by reflecting light from a target into the cover. This provides another layer of support for gesture recognition. For example, a user can 'flip' through a set of images by waving a hand or finger at close range above the screen surface, as if the images were stacked in a book. More advanced motion processing can also capture a wide range of gestures, opening up many ways for users to interact with large-format displays.

Support for active stylus implementation, communicating via protocols such as Bluetooth, allows for precise use such as sketching and mapping. Based on the accuracy of this touch technology core - providing resolution up to 400dpi, the various control functions on the stylus enable users to selectively draw, edit and control points on the display.

Figure 3: Block diagram of the DA8901

In-glass technology can be used in environments where liquids may spill or collect. Because all sensing electronics are behind the cover, the touchscreen can be placed in a fully sealed housing, immune to contaminants such as dust and water, while still being able to detect touches made by physical objects.

Because the electronics are attached to a cover glass or plastic sheet placed on top of the actual display module, in-glass technology can be easily combined with standard displays, making it adopted by professional system integrators in key vertical application markets such as signage and industrial controls.

This technology has other advantages over typical touchscreen technologies. Because it uses only infrared light, which the human eye cannot distinguish, and does not rely on a thin metal mesh to provide the function of the touch electrode, light transmission is improved, as well as clarity and contrast. Likewise, the technology has excellent immunity to electrical noise, which is an important factor in industrial applications.

The core technology also has excellent resistance to shock and vibration. Combined with a sufficiently strong cover layer, in-glass technology can be used in situations where accidental damage or vandalism may occur.

Due to its rich combination of features, in-glass multi-touch technology will drive a new wave of applications - especially systems requiring large display areas, and revolutionize user interface development in many industries and markets.

About the Author:

Faisal Ahmad is the Director of Power Management Marketing for the Mobile Systems Business Unit at Dialog. He has 16 years of experience in the power management semiconductor industry, having held senior marketing positions at Transphorm, Akros Silicon, Fairchild Semiconductor, Intersil, and International Rectifier. His earlier career included working as an analog design engineer at Dallas Semiconductor and Texas Instruments. Faisal holds a BSEE from Georgia Institute of Technology and an MBA from Southern Methodist University.