Why you need to use capacitive touch panels in your GUI design - Embedded.com

Why you need to use capacitive touch panels in your GUI design

More developers of embedded systems are deploying projected capacitive (PCAP) touch display technology as it continues to emerge as the operator interface technology of choice for a broad range of applications. It started with mobile consumer electronics, where smartphones have shown that a PCAP interface can be intuitively easy to learn and use, efficient with regards to system resources, very engaging for the user and give the finished product a sleek and sophisticated look and feel.

Now embedded designers are discovering that the broad range of performance characteristics for PCAP touch panels makes the technology a great fit for many embedded applications. In fact, the deployment of touch displays, including PCAP units, in a wide variety of applications is exploding.

Market research firm DisplaySearch reported that the touch panel market grew by more than 29 percent in 2009 over the previous year. The future outlook for PCAP, also according to DisplaySearch, reflects PCAP becoming the number one touch technology in terms of revenue in 2010 and will pass resistive touch in unit shipments by the year 2013.

How PCAP Works

The previous generations of touch panels in embedded applications mostly involved resistive touch technology, which features a flexible outer layer of a thin plastic film. This layer must be flexible so that touching the outer layer will create contact with a rigid inner layer where the electrical contact is registered.

Unfortunately for many embedded systems, this thin plastic outer layer makes resistive touch panels less durable due to the likelihood of scratching or somehow damaging the thin plastic.

Temperature extremes or repeated usage of the same location can warp and deform the plastic layer, rendering the operator interface unreliable in most cases. In addition, the flexibility of the outer layer causes the accuracy of a resistive touch panel to drift out of calibration and, as a result, manual recalibration is required periodically.

In contrast to resistive touch, PCAP displays feature two robust layers of glass. Between the glass layers is a conductive material, Indium Tin Oxide (ITO). The entire unit is sealed and protected from the outside world.

The two ITO layers on the two sheets of glass form an X/Y grid of capacitors. The electric fields of these capacitors are projected from the top layer of glass. Placing a finger on or near the outer layer couples these electric fields and conducts a charge away from the panel.

The controller for the PCAP panel is constantly scanning through the drive electrodes along the X axes of the screen while it monitors the sensing electrodes along the Y axes (Figure 1 below ). When a change in capacitance on the screen occurs, the controller is able to filter out extraneous noise and other transients, and identify this event as a finger touch.


Figure 1. PCAP panels constantly scan through the drive electrodes along the X axes of the screen while it monitors the sensing electrodes along the Y axes

The sensing electrodes and the controller must be capable of some highly precise measurements as well as very aggressive noise filtering and complex algorithms since the change in capacitance is in the femto-farad range (10-15 F). The controller must amplify, condition and measure these signals. The grid made by the ITO traces along the X and Y axes allows the controller to determine the location of the touch on the screen.

The range of embedded systems where the operator interface might be based on PCAP display technology is certainly broad.

On one hand, the sleek and compelling look and feel of a PCAP panel is compelling for designers of systems for consumer-oriented products like navigation systems, home and automotive infotainment systems, even household white goods like washing machines and microwave ovens.

On the other hand, PCAP displays have many compelling qualities for the designer of embedded industrial systems. These include their rugged durability and the versatility of their construction which makes them adaptable to a wide variation of environmental conditions such as extreme heat and cold, intense sunlight, wear and tear from repetitive touch patterns on the screen and others.

Many medical systems provide an example of the types of embedded applications that benefit from PCAP display panels. For instance, patient monitoring units for hospital nurses could be equipped with a PCAP display.

One touch might freeze the display, while a second single swipe on the screen could quickly move back in time and a third tap on the screen could restore the display to the current conditions.

A touch panel with two-finger touch capabilities would allow two interactive cursors to be used to set alarm limits. And two-finger gesturing could implement time compression and expansion. Or, a PCAP display on an ultrasound imaging system might replace the keyboard and roller-ball that typically function as the user interface.

Of course, medical systems have stringent requirements. To combat blood-borne pathogens and other sources of disease and infection, all electronic systems including the user interface must be tightly sealed in their enclosures. Bacteria-breeding moisture must not be able to accumulate in grooves or inside enclosures.

In addition, the cleaning requirements for hospitals dictate that all surfaces must be durable enough to withstand the cleaning fluids. It is quite common for these fluids to contain as much as 10 percent bleach.

To meet these requirements, PCAP displays can be hermetically sealed to keep germs and moisture out and they can be optically bonded with highly impervious materials, such as glass, to protect against chemical solvents and cleaners. Moreover, a PCAP display can be deployed as a completely flat surface with no grooves or notches where bacteria and germs might congregate (Figure 2 below ).

Figure 2. PCAP displays are completely flat surfaces with no grooves or notches within which bacteria and germs might congregate.

Other critical requirements of medical systems are impact resistance and non-sharding, just in case breakage occurs from extreme impact. In the typical hospital room there are many projecting arms with hooks and other types of sharp-edged fasteners.

It is quite common for a piece of equipment to be accidentally bumped or suddenly pushed aside in an emergency. As a result, every display panel in the room must be able to withstand significant impact.

The hospital industry’s standard for nurse call equipment, UL 1069, specifies that a display must withstand the impact of a ball drop test to 7 Joules. Again, a PCAP display panel can be housed in an enclosure made of a protective material without suffering significant degradation of the optical quality of the image on the screen or its touch-sensing capabilities.

The non-sharding requirement is similar to that of the automotive industry. Even though the chances are remote, the glass on a display panel in a medical setting could be struck with a force great enough to break it.

If this were to happen, the glass cannot shard into tiny splinters of glass that could harm someone. PCAP display panels from some vendors already meet this requirement.

Of course, the nurses, doctors and technicians who operate medical equipment often wear latex gloves. In operating rooms it is common for doctors and nurses to wear two pairs of gloves just in case the outer glove is punctured.

This means that a PCAP panel must be able to sense a change in capacitance on the surface of the panel despite the insulating latex covering the fingers. This degree of sensitivity has been achieved in PCAP panels that are currently being deployed in medical systems.

Product designers need to appreciate the complexity of developing and tuning a PCAP panel to perform in certain environments with unique enclosures and coverplate materials.It is important for designers to understand that there are many variables that need to be addressed to ensure that a touch panel will perform satisfactorily and will meet the design requirements for a particular application and/or environment.

Over the short term, the improvements that are in store for PCAP display technology will make it even more advantageous to consumer-oriented and industrial embedded systems.

For example, larger PCAP panels are in the works. The limiting factors for the size of PCAP displays have been high node count touch panel controllers that can provide high resolution for large panel areas. Additionally material systems, including low resistance optical clear conductors, as well as high performance optical bonding materials and processes, are the primary challenges facing manufacturers.

These limitations are being overcome by new multi-chip controller solutions that boost panel node counts from hundreds of nodes up to several thousand nodes. Research and development efforts are making substantial improvements in conductive materials to reduce the optical visibility of the electrode patterns and at the same time reduce trace resistances which are needed for larger panels. Larger panels will enable new embedded applications for PCAP technology such as interactive restaurant menus, video gaming and kiosks.

Higher performing adhesive materials and optical bonding techniques are also being developed to produce high optical clarity and thin uniform dielectric layers which form the capacitive nodes of the panel. Lamination of thin dielectric layers between large sheets of glass with high production yields is critical to panel performance and controlling manufacturing costs.

Mind Your User Experience

The user experience is just as critical for embedded industrial systems as it is for consumer electronic products. Certainly the shiny sophistication and eye-popping clarity of a PCAP display might be a fashion statement for the user of a smartphone, but that same PCAP technology delivers all those qualities as well as the durability, versatility and reliability that are critical to users of industrial embedded systems. Combining all of that into a PCAP display can add up to a compelling and engaging user experience for practically any embedded application.

Larry Mozdzyn , Executive Vice President and Chief Technical Officer, Ocular, Inc., is a co-founder of Ocular. H has extensive engineering and manufacturing experience in the display industry and was responsible for the installation and start up of seven display manufacturing facilities in China between 1986 and 1996. Prior to co-founding Ocular, he was the manufacturing director for Polytronix and a process engineer at Texas Instruments. He holds a BS in Chemical Engineering from the University of Minnesota. John Groezinger is a field application engineer at Ocular.

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