Medical Device Link.

Medical Electronics Manufacturing Spring 2000

Five-Wire Resistive Touch Technology in Medical Applications

Getting the most out of touch technology in a medical device requires a stable, reliable touch screen that accommodates the specific needs of medical professionals.

Computers are a boon to productivity in nearly every field. Especially in industries where data integrity and accuracy are crucial, computers can help companies maximize profits, divert losses, and even save lives. But computer technology advances at such a rapid pace that training employees to keep them up to date can be an expensive, sometimes futile, endeavor. One answer to this problem would be some way to make computers more accessible and easier to use, so that even untrained employees could exploit the power of computing technology in the performance of their jobs.

Enter touch technology. Touch screens offer a computer interface that is fast and easy and does not require extensive training. People see and use touch screens in shopping malls, supermarkets, banks, and casinos. And working professionals in nearly every industry are discovering the cost and performance advantages of using touch screens in daily operations.

Touch technology is gaining popularity in the healthcare field, where medical products operated via touch range from $300 handheld organizers to $3 million magnetic resonance imaging systems, from analytical instruments to cardiac management systems. Touch screens help medical professionals with the management of electronic patient records, information and data retrieval, lab record management, and patient check-in and registration. The use of touch screen technology increases the productivity and accuracy of medical personnel while requiring minimal training.

However, because so much is at stake in the use of medical technology, careful thought should be given to deciding which of the various available touch technologies is appropriate for a particular medical application. Although each technology has its strengths, five-wire resistive technology delivers the accuracy, durability, and performance necessary in a medical environment.

Why Resistive?

A touch screen typically has five elements: a touch surface; a computer; a controller card that drives the screen and converts each touch into x-y coordinates; a software driver program that communicates between the controller card and the computer's operating system; and the software application itself, which varies in accordance with the needs of the end-user. Each of the several touch technologies has characteristic strengths and weaknesses (see Table I).

Table I. This table compares the relative advantages of the various types of touch technologies. By examining the chart, it's easy to determine which technology would be appropriate for a given application. Durability, stylus flexibility, contamination resistance, and price are all key factors in choosing a touch technology for operating medical electronics equipment.

Touch technologies differ in the way each detects a touch. With scanning infrared systems, a finger is detected when it encounters an array of infrared beams. When a user's finger touches a surface-acoustic-wave touch screen, it absorbs the acoustic waves propagating on the touch surface; a touch is detected when the controller electronics identify a 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 also can be found in handheld computers, personal digital assistants, industrial equipment, point-of-sale equipment, office automation equipment, and consumer electronics.

Most people use a bare finger to activate the touch interface of screens they encounter in the world. But in the medical field, 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. Also, 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 even by a gloved hand. The lab technician can activate the screen immediately, without pausing to remove a glove, and return promptly to work. A screen that can react to a variety of touch methods is clearly better.

Resistive touch screens can be activated by all kinds of input devices, including fingers, nails, styli, gloved hands, and credit cards, to name just a few. Because resistive touch technology allows for operation with gloved hands—and by objects not part of the body—it is a good candidate for medical applications. This cost-competitive 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.

Why Five-Wire?

Resistive touch systems are available in 4-, 5-, and 8-wire variants. Because of their construction, five-wire resistive touch screens offer the best combination of features for use in medical applications.

Five-wire resistive technology involves a glass substrate overlaid with a suspended polyester cover sheet. This simple plastic-on-glass construction offers the fewest layers and the best optical characteristics of the various resistive touch screen configurations. Because of its simplicity, five-wire resistive technology is extremely easy to integrate and, therefore, less expensive than alternative touch options. It also supports signature capture, a feature becoming more important in healthcare as doctors and technicians automate their paperwork.

Four- and eight-wire resistive touch screens typically are formed from multiple layers of polyester. This is known as plastic-on-plastic construction. Such an assembly is laminated with adhesives onto a glass or plastic backer for support. These additional layers will then tend to degrade optical clarity significantly.

In four-wire touch screens, accuracy will drift with environmental changes, primarily shifts 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 monitors, 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 an accurate, durable, and reliable touch screen that offers drift-free operation.

In eight-wire resistive technology, as compared with four-wire, although the four additional sensing points help to stabilize the system against drift, they 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 sophisticated combination of product design and careful choice of materials, 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.

The Technology

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, separated from it by small, transparent insulating dots. The cover sheet has a hard, durable coating on the outer side and a conductive metallic coating on the inner side (see Figure 1). A touch on the screen pushes the conductive coating on the cover sheet against the coating on the glass, making electrical contact. The voltages produced are the analog representation of the position touched.

Figure 1. A five-wire resistive touch screen.

While the controller is "waiting" for a touch, the resistive layer of the touch screen is biased at +5 V through four drive lines. The cover sheet is grounded through a fifth, high-resistance wire. The voltage on the cover sheet is zero when the screen is not being touched. The voltage level of the sheet is continuously converted by the analog-to-digital converter (ADC) and monitored by the microprocessor on the controller.

When a user touches the touch screen, the microprocessor detects the rise in voltage and begins converting the touch coordinates into digital signals by means of the ADC. First, it finds the position of the X drive voltage on the touch screen by applying +5 V to pins H and X and grounding pins Y and L. An analog voltage proportional to the x (horizontal) position of the touch appears on the cover sheet at pin S of the touch screen connector. Once digitized, the voltage is subjected to an averaging algorithm, then stored for transmission to the host.

Next, the microprocessor locates the Y drive voltage on the touch screen by applying +5 V to pins H and Y and grounding pins X and L. An analog voltage proportional to the y (vertical) position of the touch appears on the cover sheet at pin S of the touch screen connector. This signal is converted and processed as the signal for the x position was.

The averaging algorithm applied to the digital signals reduces noise resulting from contact bounce during the making and breaking of contact with the touch screen. Successive x and y samples are tested to determine whether their values vary only within a certain range. If any samples fall outside this range, the samples are discarded and the process repeats until several successive x samples, and then y samples, fall within the range. The averages of these values are used as the x and y coordinates.

Once independent samples from both x and y axes are obtained, coordinate pairs are sampled to eliminate the effects of noise. If a sample does not fall within an internal range, all x and y coordinates are discarded and the independent x and y sequence is restarted. When acceptable coordinates have been obtained, an average coordinate is determined and communicated to the host processor.

The x and y values are similar to Cartesian coordinates, with x increasing from left to right and y increasing from bottom to top. These absolute coordinates are arbitrary and unscaled, and will vary slightly from one touch screen to another.

Performance Enhancements

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.

Sealed cover sheet. It is important that the touch screen cover sheet be secured to the glass panel in a way that does not admit moisture but does allow for flexibility, because when the screen is exposed to heat or humidity it tends to expand and contract.

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 moistureproof. Furthermore, 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's AccuTek 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 2). The company's proprietary system uses the adhesive to hold the cover sheet in the proper z-axis relationship to the substrate. The customized industrial-grade caulk applied to a 0.1-in. gap at the outer edge of the cover sheet—glass interface 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 plastic to expand and contract with changes in the environment, yet keeps it flat and under tension, resulting in a constant tight fit.

Figure 2. The cover sheet is sealed to the glass of the touch screen.

Stabilized linearity. Another important thing to consider when choosing a five-wire resistive touch screen is the stability of the cure pattern on the glass substrate. Some resistive screens use a thermal- or UV-cured silver ink that is silk-screened onto the glass or plastic substrate. Not being well bonded to the indium tin oxide (ITO)—coated substrate, it is susceptible to deterioration over time under the influence of glues and solvents, which causes the linearity of the touch screen to change. Other resistive screens have drive line traces printed on the cover sheet, on top of an insulating layer screened over the ITO layer of the sheet. This requires the trace lines to be electrically connected to the electrode pattern with conductive epoxy at the corners of the screen. Over time, these trace lines and their conductive epoxy interconnections deteriorate, which also affects linearity.

A better technique is to fire the electrode pattern and drive line traces into the glass substrate. (The ink contains glass beads that melt into the substrate surface when heated.) The linear characteristics of the glass, which establish the linearity of the touch screen, will not change for the life of the product.

Conclusion

Just as touch technology in kiosk and point-of-sale retail applications entices customers and makes consumer purchasing easier, professionals in a number of industries, particularly healthcare, are attracted to the simplicity and accuracy of computer input by touch. Touch technology can accelerate computer function because the graphical touch interface is intuitive to users. Touch eliminates the need for extensive employee training on complicated computer systems, since it requires only that the equipment operator look and touch. Hospitals and laboratories can quickly get new hires up and running, with confidence in their ability to perform the tasks easily.

Leading touch technology developers are now introducing complete touch monitor systems in which the touch interface, monitor, and PC are sold as a single unit. Users benefit from full PC functionality integrated with touch technology in a single piece of equipment occupying a small footprint. In the future, touch screens will appear in ever-more-diverse applications as prices come down and more businesses recognize their advantages. Meanwhile, more research must go toward defining the system features important for specific applications so that professionals in every field can exploit the benefits of touch.

Scott Smith is market manager of medical products for Elo TouchSystems (Fremont, CA).

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