Advancements propel touchscreens to HMI forefront

Recent advances in touchscreen technologies have done much to make touchscreens the method of choice for HMI system interaction. Once questionable in terms of durability, reliability, and ease of use, these devices have improved in performance, often replacing keyboard and mouse. Progress has been made on many fronts, from refinements in long-standing, widely used resistive types to totally new possibilities.

Advancements, refinements, improvements

Primary touch solutions operate fundamentally through the interruption of a mechanical field (resistive), electrical field (capacitive), acoustic waves (SAW), and infrared (IR) light. Little more than a month ago, a new method was introduced by 3M Touch Systems Inc. Dispersive signal technology recognizes touch through vibrations made to the substrate. Designated by its developers as a ‘break-through’ in the touchscreen field, this patented technology is said to offer high degrees of light transmission, stylus support, accuracy, signature capture, and all-glass durability.

It works through the use of sensors in each corner of the screen, which measure vibrational energy. Through advanced digital signal processing, dispersion adjustment algorithms are applied to analyze the signals and report an accurate touch. The approach helps eliminate issues associated with screen contaminants and surface scratches, and offers enhanced palm rejection. For example, a touch can be registered while an operator’s palm or another object is resting on the screen surface. A finger, gloved hand, or stylus can initiate a signal even though a person may be leaning on the surface or a clipboard is resting on it. The touch creates a vibration, which radiates a bending wave through the substrate from the point of contact spreading out to the edges. Other resting items are ignored.

‘This technological advancement could change the landscape of the touch business, and will be the platform for many upcoming products to be offered by 3M Touch Systems,’ according to Doug Kuller, program manager, 3M Touch Systems, who announced the development at the Society for Information Display (SID) 2004 International Symposium, Seminar, and Exhibition in May. The advancement represents the result of a joint research and development effort between 3M and NXT plc, a UK-based company involved in enabling technologies in sound and speech.

This advancement follows the introduction last year of digital ink technology by Touch International, which combines many of the features of capacitive and resistive screens into one. Thin, form fitting, unbreakable, and wear resistant, the material accepts stylus, finger, and gloved-hand inputs while ignoring accidental pressure from an operator’s hand or palm. The technology is especially suitable for small applications such as phones, PDAs, and other hand-held devices.

Resistive—still the one

Despite the promise of newly introduced technologies, traditional products likely will remain dominant in the marketplace for years to come. Although product selection is obviously determined by application and industry, resistive-type screens are still the most common choice. They are comparatively inexpensive and easy to manufacture and integrate into a display. Refinements have overcome many of the accuracy and transmissivity objections once directed at them, making them a flexible and economic solution for most situations. The availability of four, five, seven, and eight-wire models has reduced performance concerns and improved optical characteristics and product durability.

‘In any application, you reduce the brightness of the TFT [thin film transistor] behind the screen because you’re going through multiple layers of material,’ observes Al Zelasko, business development, display technolgies, Avnet Applied Computing Solutions. ‘This has led to the development of different types of film technologies internal to the touchscreens that make them more transmissive. You can achieve a better optic yield at less cost by putting money into the screen instead of the TFT. Improvements in TFTs have facilitated this. A standard TFT now is 400 nits. If you lose 2% or 3% in transmissivity, it doesn’t have nearly the impact as it would with older displays of only 250 nits.’ [A nit is a measure of luminescence; one nit equals one candela per sq m.]

All touchscreen technologies—whether resistive, capacitive, surface acoustic wave, or infrared, dispersive signal—have advantages and limitations. ‘There really aren’t any bad touchscreen technologies,’ says Joe Kirby, vice president, technology development, Dolch Computer, ‘just the misapplication of technologies. For example, resistive and traditional capacitive screens would not normally be used in vandal-prone or harsh environments and are generally more suitable for indoor than outdoor use. Enhanced IR screens are very durable and do not lose registration. However, they have mechanical restrictions and cannot be used on small devices such as PDAs. In every case, one needs to determine how, where, and why the unit is going to be used before making a decision. If cost is the driving factor and the environment in which the device is going to operate is relatively benign, resistive is often the logical choice.’

Wonderware’s Ann Ke agrees. Her company entered the touchscreen market last fall with the introduction of the Touch Panel Computer line for its InTouch HMI. ‘We’ve tried to standardize on the resistive type of touchscreen. We believe it addresses most of our market needs. Of course, it doesn’t address everything. But it does cover a majority of applications.’

Making choices

While not as widespread as resistive types, capacitive touchscreens are also a popular choice. Accurate, responsive, and offering high resolution in larger sizes, these devices tend to be more durable (scratch resistant) and are popular in gaming applications. A related technology, near-field imaging (or projected capacitive) offers improvements, limiting drift that may occur with resistive and capacitive screens.

Advanced technologies such as SAW and IR (and enhanced IR) are effective for specialty applications. SAW is highly accurate, can be used outdoors and with gloves, and is not susceptible to temperature and humidity fluctuations. Dirt may block the sound waves and cause a false input; therefore SAW touchscreens are a frequent choice for clean rooms. A costly but solid technology, they are somewhat more difficult to integrate into a display. IR, based on a light-beam interruption principle, is primarily found in heavy-duty industrial applications. It works well indoors and out, can be made with tempered anti-glare glass for scratch resistance, and can be environmentally sealed.

Touchscreen users can anticipate more modifications and improvements to existing technologies, while suppliers seek also to bring totally new methods to the field.

‘This is a mature market, but some unique things are happening,’ points out Sriram Peruvemba, general manager, Three-Five Systems Inc. ‘Some manufacturers are trying to embed the touchscreen into the display, essentially placing the touch-sensing elements inside the LCD. This structure could lead to a touchscreen that is also a scanner.’

Possibilities appear limitless. On one hand, technology has taken users to the point of effectively meeting most needs with traditional methods. On the other, it has led to a variety of exotic possibilities, including holographic touchscreens, screens made out of water particles, and screens made out of thin air. Don’t take your eye off the future.

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Reviewing common touchscreen technologies

Basic touchscreen technologies in use today fall into four major categories: resistive, capacitive, acoustic wave, and infrared.

Resistive screens consist of a glass or plastic substrate overlay with a thin metallic coating over which a second flexible layer of polyester has been placed. Insulating dots or beads keep the two surfaces apart. A hard coating applied to the external surface helps minimize damage. Current is pulsed through the overlay along the x- and y-axes. When pressure from a finger or stylus presses the two layers together and contact is made, the control electronics determines the location coordinates and transmits the results to the computer. These units come in 4-, 5-, 7-, and 8-wire configurations. They are typically low in cost, applicable to wide range of displays, and have no input restrictions (accepts stylus, gloved hand, etc.) They are subject to scratching and damage, but do wear well and can withstand heavy use without degradation.

Capacitive screens use a glass overlay coated with a thin, insulated, transparent conductive coating. Input from a grounded, conductive object draws a minute amount of current through the point of contact. Current flow from screen corners is proportional to the distance to the touch location; the ratios of the flows are measured by a controller, which locates the touch.Somewhat more expensive than other technologies, they are durable, accurate, and can be sealed, but are not particularly well suited for outdoor applications. Input must be from a grounded, conductive source (no gloves, stylus). Variations include projected capacitive and near-field imaging, which overcome some of the limitations of regular capacitive technology.

Surface acoustic wave (SAW) screens transmit acoustic waves across the surface of a glass overlay placed on the display surface. A transducer at one edge of the glass emits waves over the surface to a receiver on the opposite side. An acoustically dense object touching the glass surface blocks the waves and generates the touch event. Control electronics detect the action and determine the location. SAW screens are bright, have high resolution and no stylus limitations, and are not susceptible to external variables (humidity, temperature). However, they are expensive, harder to integrate into a monitor than other technologies, and not well suited to environments that use liquid contaminants. Embedded acoustic wave is a variation of this technology.

Infrared (IR) screens function when a beam of light from the IR grid in the front of the display screen is interrupted. The grid of invisible infrared light is formed by rows of IR-LEDs and photo transistors on opposite sides of the screen frame. Opto-electronics are concealed behind an IR-transparent bezel. A controller sequentially pulses the LEDs to create the grid. When a stylus or finger obstructs the beam, photo transistors detect the absence of light and transmit a signal that identifies the location of the touch. IR screens are expensive and have somewhat fewer touchpoints than other types. Because the touch event involves blocking a beam of light, the stylus size must be fairly large (pencil-eraser size). However, these screens are highly transmissive, not subject to drift, have no input material limitations, are durable, and can be sealed to NEMA 4 requirements.

Information for this sidebar was prepared by Control Engineering with input from Three-Five Systems Inc. and Dolch Computer Systems.

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