Technology Spurs Touchy Situations

April 18, 2008
Analog Resistive Touchscreens Allow the Use of Covered Hands, Such as Gloves and Other Devices
By Loren Shaum, Contributing Editor

Touchscreen implementation in machine control has seen a significant increase over the past several years. Much of this might have to do with the increased number of PC-based control installations on machines of all types.

By observation alone, touchscreen implementation in machine control appears to have increased. In a report from IMS Research, 47% of all industrial PC revenue comes from panel PCs, which include touchscreens.

One might get a sense that touchscreen deployment is becoming more commonplace as the technology becomes less costly and more reliable. Since its commercial introduction in the early ’80s, touchscreen’s costs have dropped so dramatically that the technology is deployed in many personal devices such as PDAs, Nintendo, cell phones and Blackberries.

Touchscreen Types

A resistive touchscreen panel consists of several layers. The most important are two, thin, metallic, electrically conductive and resistive layers separated by a thin space. When an object touches the panel, the layers are connected at that point, and the panel electrically acts similar to two voltage dividers with connected outputs. This causes a change in electrical current, which is registered as a touch event and sent to the controller for processing. A resistive touch panel output can consist of between four and eight wires. The positions of the conductive contacts in resistive layers differ depending on how many wires are used. Resistive touchscreen panels are not affected by outside elements such as dust or water.

A capacitive touchscreen panel is coated with a material, typically indium-tin oxide, that conducts a continuous electrical current across the screen. As a result, the screen exhibits a controlled static field in both the horizontal and vertical axes, hence achieving capacitive structure. When the panel sensor's capacitance field is altered by another capacitance field, such as the human body, circuits at each corner of the panel measure a resultant distortion in reference field characteristics and send the information about the event to a controller to process. Capacitive screens can be activated with a human finger or a handheld conductive device and are not affected by outside elements. The screen exhibits high clarity, but the complex signal processing electronics increase cost.

Who Uses What and Where?

Advantech uses resistive touch on all its HMI products because of price and performance benefits over all other technologies. “We see no change to any other touch technology in the near future,” says Eric Lai, HMI product manager. “Furthermore, we see no emerging technology that might impact future products. Eighty percent of all screen products shipped today are touchscreen, and that appears to be holding steady from year to year.”
An emerging market opening for Advantech is in digital signage  “The new interactive digital sign market is moving toward touchscreens, which will increase our shipments substantially more than the 80%,” says Chuck Harrell, promotion manager. With digital signage adding substantial volume to touchscreen production, even lower costs can be expected—all the more advantageous to Machine Builder Nation.

Ralph Damato, vice president, product management, Nematron concurs with the Advantech claims. Nematron uses resistive technology throughout and ships about the same ratio of touchscreens to non-touch. “Analog resistive touchscreens allow the use of covered hands, such as gloves, and other devices,” says Damato. “It also provides a polyester overlay that has good chemical resistance, and it is very cost-effective. We use both four-wire and five-wire. On our low-end operator interfaces with smaller–than-12.1-in. screens, four-wire is mostly used.”

Going forward, Nematron sees greater acceptance of touchscreen technology. “The reliability of five-wire touchscreens has improved greatly and is being accepted more readily,” says Damato. However, Nematron finds that in many legacy applications upgrading to touchscreens is not possible because the onboard programs aren’t compatible. So there appears to remain a large legacy market for non-touch types of operator interfaces.

Christensen Display sees similar trends. “Resistive touchscreens are by far the predominant technology because of stability, low-cost, ease of integration, reliability and availability from multiple sources,” says Rick Tomfohrde, vice president. Christensen depends heavily on the reliability factor as it offers a relatively complete line of true NEMA 4, IP65-rated HMI packages. “We and others constantly strive for enhancements, so we continue to meet with component suppliers” says Tomfohrde. “But from a functional standpoint, today’s touchscreens are quite appropriate for almost every application. Enhancements in physical durability provide longer life under the harshest conditions, including scratches, abrasions and direct frontal impacts.” Christensen continues to ship about 2.5 times more touchscreen products than non-touch, but still some users working in Class I, Div. 1, or ATEX Zones 0 and 1 environments are skeptical of touchscreen reliability.

Horner APG plays in the small machine control market and is moving its all-in-one controller products to touchscreen technology. “Resistive touchscreen technology really has dropped in price over the past couple of years,” says Phil Horner, president. “So much so that we now offer touchscreen capability on our controller for small machines.”

Raimund Ruf, business unit manager, B&R Industrial Automation, says his company uses mainly analog-resistive touchscreens for the same reasons. “Having all information on the display and not on push buttons makes it easy to support language switching,” he says. Ruf also reminds users that regardless of their user-friendliness, “touchscreens are not allowed for safety critical operations such as e-stop and enabling switches.”

Other Touchscreen Technologies

Surface acoustic wave: Perhaps the least robust of all touchscreen technologies
Infrared: Used in many military applications
Strain Gauge: Commonly used in public places because of high resistance to vandalism
Optical Imaging: Growing in popularity, particularly on larger screens
Dispersive Signal Technology: Perhaps the newest of touch technologies
Acoustic Pulse Recognition: A piezoelectric technology that turns mechanical energy, touch, into an electric signal
Total Internal Reflection: An optical system

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