The decision to use touchscreen systems on a human-machine interface (HMI) can be a good one.
For complex equipment, a well-conceived touchscreen display eliminates distractions and the error-producing potentials of the keyboard, mouse, and other peripheral devices in industrial environments.
"Touch reduces the chance of mistakes and provides a more efficient data-entry mechanism than do other input devices," says Frank Shen, strategic market manager for Elo TouchSystems (www.elotouch.com). "A key benefit of touch technology is its inherent flexibility that allows users to perform multiple functions from one machine."
There are four basic touch technologies available: infrared, surface-wave, capacitive, and resistive. The touch technologies differ in the way touch is detected.
With scanning infrared systems, a touch is registered when a finger or stylus encounters an array of infrared beams. When a user's finger touches a surface-wave touchscreen, the finger absorbs the acoustic waves propagating on the touch surface. The controller electronics identify a touch by the drop in acoustic signal spectrum. Capacitive technologies use the conductivity of a finger to shunt a small alternating current to ground through the operator's body.
Resistive technology-based touchscreens have good durability and resolution. Resistive touchscreens consist of a glass panel with a uniformly resistive coating, plus a polyester cover sheet tightly suspended from the top of the glass and separated from it by small, transparent insulating dots. "The cover sheet has a hard, durable coating on the outer side, and a conductive coating on the inner side," says Paul Scanlon, manager, automation equipment, GE Fanuc. "When touched, the conductive coating makes electrical contact with the coating on the glass, and a touch is registered by the analog controller."
Resistive touchscreens are most frequently used in industrial equipment, but they also can be found in handheld computers, medical equipment, and consumer electronics.
The big limitation of capacitive technology in industrial environments is the gloved hand. "Whereas most people use a bare finger to activate a touchscreen, industrial personnel are frequently required to wear gloves," says Shen. "Common capacitive touchscreens do not respond to a touch obscured by cloth, plastic, or other nonconductive materials because the material blocks the ground current through the user's body." If an operator must always remove a glove to enter data, time is wasted and accuracy could suffer as a result of the distraction.
"Unlike capacitive touch technology, surface-acoustic-wave, infrared, and resistive touchscreens can be activated by a gloved hand," adds Shen. "A technician can activate the screen immediately, and return promptly to work."
Resistive touchscreens can be activated by all kinds of input devices, including fingers, styli, and gloved hands. Because resistive touch technology allows operation with gloved hands,and by objects not part of the body,it is a good candidate for industrial applications.
Two liabilities of resistive technology: The surface can be damaged with a knife or other sharp object, and the technology inherently reduces display brightness.
Infrared (IR) technology uses infrared emitter-collector pairs to project an invisible grid of light a small distance over the surface of the screen. When one or more beams is interrupted, the absence of the signal is detected and converted to an X/Y coordinate. Since the method of determining a touch is electrical instead of mechanical, IR touchscreens are not as sensitive to damage as resistive and capacitive.
"IR technology offers a lower resolution than other touch technologies, requiring a slightly larger minimum touch target size for proper operation," advises Scanlon. "Due to this resolution limitation, IR is not recommended for Windows applications." Another disadvantage of IR for industrial applications is unsuitability for direct washdown environments.
Surface acoustical wave (SAW) technology uses a set of transducers that emit a mechanical wave across the horizontal and vertical axes via reflective arrays. Receivers on the other side pick up the flow of these waves. If the surface of the screen is touched, then a disruption in the wave occurs and the software determines the X and Y coordinates. These units offer high durability and can be used with gloves.
This technology's weak points: It's difficult to maintain a watertight seal, and it can transmit false signals during washdowns.
Scanlon would include near-field imaging (NFI) as a fifth technology option. Here the sensing circuitry generates a profile of a touch through data acquisition and image processing techniques. Made of strengthened glass and a laminated construction with no mechanically sensitive components, NFI can withstand significant vibration and shock.
"It's ideally suited for harsh environments, withstands high-pressure washdown and is unaffected by most surface contaminants found in industrial environments," says Scanlon.
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