Originally Published MEM Fall 2001
DISPLAY TECHNOLOGIES
Five-Wire Resistive Touch Screen Technology
Frank Shen, strategic market manager of medical products, Elo TouchSystems
Touch screen systems simplify complex medical systems, reduce the chance of mistakes, and provide a more efficient data-entry mechanism than do other input devices. In fact, touch technology is gaining popularity in the healthcare field, where touch-operated medical products range from $300 handheld organizers to $3 million magnetic resonance imaging systems. Touch screens help medical professionals with the management of electronic patient records, medical inventory, lab records, and patient check-in and registration. The use of touch screen technology increases the productivity and efficiency of medical personnel while requiring minimal training.
Although each technology has its strengths, five-wire resistive technology has proven to be the most popular because it delivers the accuracy, durability, and performance necessary in a medical environment.
Touch Detection Technologies
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5. Components of a five-wire resistive touch screen.
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Touch technologies differ in the way a 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-acoustic-wave touch screen, the screen absorbs the acoustic waves propagating on the touch surface. The controller electronics identify a touch by the drop in acoustic signal from the touch location. Capacitive technologies use the conductivity of a finger to shunt a small alternating current to ground through the operator's body. Resistive touch technologies are based on two layers of conductive material held apart by small, nearly invisible spacers. When the screen is touched, the two layers come in contact, and two-dimensional coordinate information is generated by the voltages produced at the touch location. Resistive touch screens are most frequently used in medical equipment, but they also can be found in handheld computers, personal digital assistants, industrial equipment, point-of-sale equipment, office automation equipment, and consumer electronics.
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Whereas most people use a bare finger to activate a touch screen, medical personnel are frequently required to wear gloves. Common capacitive touch screens do not respond to a touch obscured by cloth, plastic, or other nonconductive materials such as latex because the material blocks the ground current through the user's body. If a lab technician must always remove a glove to enter data, time is wasted and accuracy could suffer as a result of the distraction. In addition, the cost of supplies would increase, as gloves would need to be replaced more frequently.
Unlike capacitive touch technology, surface-acoustic-wave, infrared, and resistive touch screens can be activated by a gloved hand. The lab technician can activate the screen immediately, without pausing to remove a glove, and return promptly to work.
Resistive touch screens can be activated by all kinds of input devices, including fingers, fingernails, styli, gloved hands, and credit cards, to name just a few. Because resistive touch technology allows operation with gloved hands—and by objects not part of the body—it is a good candidate for medical applications. This cost-effective technology is easily integrated into embedded systems and offers an inherently stable design. These notable attributes add to its appeal for implementation in equipment designed for medical environments.
Resistive Technologies
Although resistive touch systems are available in 4-, 5-, and 8-wire variants, five-wire resistive touch screens offer a good combination of features for use in medical applications because of their construction.
Five-wire resistive technology involves a glass substrate overlaid with a suspended polyester cover sheet. Because of its simplicity, five-wire resistive technology is extremely easy to integrate and, therefore, is less expensive than alternative touch options. Resistive technology also supports signature capture, a feature that is becoming more important in health care as doctors and technicians automate their paperwork.
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| Figure 6. The sealing of the cover sheet to the glass. |
In four-wire touch screens, accuracy will drift with environmental changes such as changes in temperature and humidity. The polyester cover sheet expands and contracts with changing conditions, thereby causing long-term degradation to the coatings and drift in the touch-point location.
With five-wire resistive touch screens, however, the flexible cover sheet acts only as a voltage-measuring probe, which means that the touch screen continues working properly even with nonuniformity in the cover sheet's conductive coating. The result is accurate and reliable drift-free operation.
In comparison with four-wire resistive technology, eight-wire technology uses four additional sensing points to stabilize the system against drift, but the addition points do not improve the durability or life expectancy of the screen.
Both four- and eight-wire technologies are limited to about 10% of the tested capability of five-wire technology to accommodate finger-touch volume. Through a combination of product design and material selection, some touch screen suppliers can point to an installed base of five-wire resistive touch screen products that have endured years of heavy usage without any apparent falloff in touch performance. Reliably accurate performance over time is important to hospitals, clinics, and laboratories that may lack the funds to replace aging equipment.
Five-Wire Resistive Technologies
Five-wire resistive touch screens have a glass panel with a uniform, resistive metallic coating. A thick polyester cover sheet is tightly suspended over the glass. Small, transparent insulating dots are used to prevent the two surfaces from contacting each other until the screen is touched lightly with a finger or stylus. The cover sheet has a hard, durable coating on the outward-facing side to reduce damage from sharp styli and a conductive metallic coating on the inward-facing side (see Figure 5). A touch on the screen pushes the conductive coating on the cover sheet against the coating on the glass, making electrical contact.
To determine the coordinates of the touch location, a voltage gradient is first applied along the x-axis and then along the y-axis. When a finger or stylus presses the two layers together, the x-axis and y-axis voltages at the point of contact are measured. The voltages produced by the electrical contact are the analog representations of the position touched. The control electronics then transmit the coordinates of the position to the host computer.
Sealing the Cover Sheet
Five-wire resistive technologies on the market today differ in that some offer features designed to enhance accuracy and operational longevity. Key considerations are the technique used to seal the cover sheet, and the degree to which the technology can ensure drift-free operation. In most resistive touch screens, the cover sheet is secured to the glass substrate with a double-sided adhesive strip that runs along the edge of the screen. Although the sealant does protect against moisture, it is not completely moisture-proof. And, because the cover sheet material can expand and contract in reaction to heat and humidity, pressure-sensitive adhesives will loosen over the life of the product, which may cause the cover sheet to ripple.
Some manufacturers of five-wire resistive touch screens use alternative sealing methods to prevent moisture damage. Elo TouchSystems' (Fremont, CA) AccuTouch touch screens, for example, employ industrial-grade caulking in addition to a film carrier–based pressure-sensitive adhesive to secure the cover sheet to the glass (see Figure 6). The customized industrial-grade caulk keeps the cover sheet oriented correctly in the x and y dimensions and provides tension to help the sheet retain its shape. This multidirectional method of securing the cover sheet to the glass allows the sheet to expand and contract with changes in the environment while keeping it flat and under tension, resulting in a fit that remains tight over time.
With its improved gasket sealing, this type of touch screen stands up to long-term use, frequent cleaning, and inevitable liquid spills. When integrated into liquid crystal display touch monitors, the sealing ensures that the device sheds water and can be sterilized with hospital-grade disinfectants. Consequently, the monitor can be safely placed in patient care areas, such as the operating room, where contamination may occur.
Conclusion
The practical application of touch screens with medical equipment is varied. Touch screens act as an interface that can improve the efficiency of analytical instruments, x-ray and ultrasound machines, and cardiac management machines.
Leading touch technology developers are now introducing touch monitors in which the touch screens, displays, and control interfaces are combined into a single unit. Users benefit from full display functionality integrated with touch technology in a single piece of equipment. In the future, touch screens will appear in even more diverse applications as prices come down and more businesses recognize their advantages.
Copyright © 2001 Medical Electronics Manufacturing
