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Inductive Pen Sensing vs. Resistive Input Technology

Rich Functionality Demands Better User Interfaces for Mobile Devices

(By Ian Scholey, Technical Manager, Wacom Components)

Resistive and inductive pen sensing technologies are the most widely-used touch screen technologies, but each has unique characteristics that can make it the preferred choice for certain mobile applications. Mobile device developers have to weigh the pros and cons of each to determine the right fit for their application.

Resistive technology defined

Resistive technology is currently recognized as the industry's incumbent pen/screen interface in many applications. It currently exists in four-wire, five-wire, and eight-wire configurations. Due to its mainstream availability and low cost, resistive technology is the touch technology of choice for many markets and applications, especially portable and handheld products. This technology offers a fast, reasonably accurate, and affordable technology that recognizes touch input from any stylus, finger, gloved hand, pen or tool.

Resistive technology consists of a mechanical sensor mounted on top of a display and an embedded controller. The sensor is composed of a flexible polyester top layer and a rigid glass bottom layer separated by air and/or insulating dots. The inside surface of each of the two layers is coated with a transparent metal oxide coating (ITO) that facilitates a gradient across each layer when voltage is applied. When a stylus or finger compresses the flexible membrane, it touches the lower resistive layer, activating the signal. The control electronics alternate voltage between the layers and pass the resulting X and Y touch coordinates to the touch screen controller. The touch screen controller data is then passed on to the CPU for processing. (See figure 1.)

Four-Wire and Eight-Wire resistive touch screen technologies are the lowest cost and most widely used configurations. They are also less durable and less accurate over time and so are primarily used in less demanding applications. Five-Wire technology is a more accurate and reliable touch screen technology used in demanding workplace applications, such as point-of-sale systems, industrial controls, and medical systems. Operationally tested to more than 35 million touches in one location with no performance degradation, it has the longest life. It also utilizes the bottom substrate for both X- and Y-axis measurements so that the touch screen will continue to work even with nonuniformity in the cover sheet's conductive coating. In addition, Five-Wire's simple plastic-on-glass construction offers the lowest number of layers for the best optical characteristics.

Inductive technology alternative

Inductive technology methods have been used for more than 20 years in graphic tablets and tablet PCs, and are used in 90 percent of Tablet PCs today. Recent technological advances have produced single-chip solutions that are seven to eight times smaller than existing silicon designs, sparking interest in inductive technology for use in smaller mobile devices. One of the most innovative applications is the ultra personal computer from OQO, which measures only 10.4 x8.6 x 2.3 cm, but runs on a normal Windows XP operating system and supports all existing Windows applications. The continued development of smaller form factors at lower costs will ensure the future integration of inductive technologies in Personal Digital Assistants (PDAs), mobile phones and smart phones.

Inductive technology is comprised of a printed circuit board (PCB) sensor, a mixed-signal IC controller, driver software, and a stylus pen. Unlike resistive technology, the sensor here sits underneath the LCD display and backlight. The sensor is made up of copper tracks providing a multitude of over-lapping antenna coils in both the X and Y directions. The sensor emits electro-magnetic signals that can be detected by a special electro-magnetic pen, with either active or passive circuitry. The energy of the magnetic field maintains the circuitry, taking energy from the sensor into the pen. The pen's own circuitry receives the energy and an inductor/capacitor resonates to the frequency to determine its value. The energy is then reflected back toward the sensor where it is received as an analog signal. All of this analog data is then collected and sent to the controller to be converted into digital signals that can be post-processed to give X and Y coordinates for position analysis. (See figure 2.)

Because the magnetic field permeates the LCD and extends 15 mm beyond the screen, the pen's position can be detected even when it is hovering above the surface of the panel. The ability to hover without touching the screen enables the capture of not only the static but also the dynamic parameters of signatures such as the speed of writing, pressure applied, letter shape and the rhythm of the writing process. In addition, the sensor only recognizes the special electro-magnetic pen, so anything else that might inadvertently slide across the screen won't throw the cursor off course.

Quantifiable Design Considerations, Resistive vs. Inductive

For mobile device designers trying to determine which of the two technologies will provide the best solution for their feature-rich mobile application, they should compare the technologies based on the features they deem most important, such as display clarity, durability, size, power consumption, device and screen size, ease of use, ease of design and cost.

Display Clarity

For manufacturers of tiny handheld mobile devices, trying to maximize screen performance to not only display but also enhance the latest graphics and messaging software is a real challenge. Resistive panels are popular because they offer high touch resolution and are not affected by fingerprints or smudges. However, because of the technology's inherent design, which involves a "sandwich" of resistive overlays mounted on top of the display, both light and color transmission of the underlying LCD is reduced, so the display clarity greatly suffers. As a result, up to 25 percent of the brightness, color saturation and contrast of the display is lost.

In contrast, inductive technology sensors are embedded behind the LCD. As a result, the display can achieve 100 percent luminous light transmission, thereby eliminating color washout and making the display clearer and the colors more vivid. It is a far superior choice for applications such as streaming video and picture messaging.

Durability

Inadvertently, people tend to abuse their portable devices. They drop them, throw them, and sometimes subject them to sharp implements - things you wouldn't dream of doing to your laptop or PC. So it is important that portable devices be designed with durability in mind. Resistive touch screens use a clear polyester film surface that is sensitive to gouges and scratches. The constant flexing that occurs on the flexible top layer can also cause it to stretch and deform. This is particularly troublesome with Four- and Eight-Wire technologies. The constant flexing of the conductive coversheet can actually change the electrical resistance with use (drift), degrading the linearity and accuracy of the coordinate axis so that they need to be recalibrated. Most resistive touchscreens can only withstand 100,000 activations, which is considered normal usage over just a few months' time. Five-Wire technology offers significantly better performance withstanding more than 35-million touches with no performance degradation, but at higher cost.

In an inductive solution, the PCB sensor is located directly behind the display and backlight. As such, it is well protected from the dirt, moisture, light and impact that cause high scrap rates in resistive screens.

Power Consumption

For space restricted mobile devices, power efficiency is key. Unfortunately, because resistive overlays reduce the brightness, color saturation and contrast of the display, strong backlighting is required to compensate for transmission losses. This, in turn, increases the current consumption. In contrast, the location of the inductive solutions' PCB behind the LCD allows for 100 percent luminous light transmission, so the backlight power consumption can be reduced significantly.

Size

Size restriction is probably the most difficult challenge mobile device designers face in developing complex, feature-rich mobile products. Since the screen, air gap and membrane of resistive technologies will always result in a relatively thick display, they are not exactly ideal for mobile equipment. In contrast, the overall thickness of the inductive sensor and the shield is only 0.5 mm, allowing display solutions that are both slim and lightweight. In addition, the inductive input pen receives its power from the electromagnetic field of the sensor. Hence, it does not need a power supply of its own and can be very lightweight and thin.

Ease of Use

One of the key benefits of resistive technology is its intuitive operation and tactile feel - operating only when the user actually touches the screen and recognizing touch input from a wide variety of implements to navigate the solution. However, this also means the technology will react to any kind of contact, even accidental touches to the display.

Inductive technology's special electromagnetic pen allows users to write directly on the display. In conjunction with the right software, handwriting and sketches can be entered quickly and easily. Navigating in menus is also simplified. All of this can be done with natural movements and a uniform tool thanks to the hovering flight point feature and the pen's advanced pressure sensitivity. In addition, optional side switches allow right, left, and double clicking and can also double as an eraser. The only drawback is the reliance on the pen for all input. If you lose it, you're out of luck. However, Wacom and its OEM partners have addressed this by making replacement pens easily available.

Ease of Design

Resistive technology is a proven technology used in more applications than any other touch technology. Inductive technology is also a proven technology, used in over 90 percent of tablet PCs today. But perhaps even more important is inductive technology's scalability. Sensor boards are available in all common sizes and can be easily integrated into existing display solutions. Coils used for X, Y coordinates are totally customizable to fit any screen size. The smaller the screen area, the fewer coils needed.

Cost Considerations

Resistive technology is widely known to be one of the most affordable touch screen technologies, and as such is used in a number of cost-sensitive applications. It is also available in a number of formats to meet the demands of a variety of workplace environments. Both the touch screen and its electronics are simple to integrate into embedded systems, thereby providing the most cost-competitive solution. However, because of the reliability issues and manufacturing challenges with the resistive layers, manufacturing needs to take place in a clean room environment, which adds to the overall cost of the solution. In addition, because the mechanical elements of resistive systems are exposed, they are not durable, and so typically additional costs arise after sale in terms of failure rate, rework in manufacturing, the return of units, and high scrap rates.

Existing inductive technologies are more expensive than resistive technologies, but the cost of the overall solution is expected to decline in relation to the ready adoption of the technology into high-volume portable devices. Inductive technologies also have the advantage of being assembled in a standard manufacturing environment. And because the positioning of the PCB sensor behind the LCD keeps it well protected from the dirt, moisture, light and impact that causes high scrap rates in resistive screens, the long-term replacement costs are negligible.

Future of Mobile Phone Applications

The increased bandwidth of 3G networks is giving birth to a new generation of 'smart' mobile devices. These promise to enable users access to common desktop business applications while on the move, as well as streaming video, email, web pages, m-commerce and Multimedia Messaging Service (MMS). But in order to promote the adoption of these advanced applications into smaller size consumer electronics devices, existing user interfaces need to improve to make it easier for consumers to access the complex functionality, and easily navigate new applications.

In short, if your application needs a basic, cost-effective user interface, resistive technology is the way to go. But if your high-end application demands strong color fidelity, low current consumption, and a long useful life, inductive technology makes an excellent choice.

New Wacom Solutions for Mobile Applications

Wacom has spent the past several years further advancing inductive pen technology. The first successful advance was the introduction of digitizers for use with the Tablet PC. Control electronics were redesigned to provide lower power consumption, and a reduction in components on digitizers in the 8.4" diagonal and larger sizes. The next step was more sophisticated scanning methods to improve performance, and more efficient manufacturing to help reduce cost.

Wacom's next major effort was to shrink the control electronics to a one-chip solution, down from the two-chip solution used on the Tablet PC. This provided a smaller package footprint, lower power yet, and also very low cost. The resultant digitizers that are possible using this technology can be used with very small displays, down to 2" diagonal. These digitizers provide incredible resolution, and enable superior display performance on a mobile device.

Going in the other size direction, Wacom has developed components for its digitizers that are targeted for larger monitor displays (or very large notebook PCs). Signal boosting technology will allow for lower cost LCD monitor digitizers in sizes greater than 15" diagonal.

These advances are all very exciting, and the result will be seen in the coming two years in the form of some new commercial products from Wacom's OEM customers. There soon should be products in the market such as e-books (using alternative display technologies), PDAs, and Smartphones.