Touch screens are becoming increasingly critical in more and more embedded applications, such as industrial control systems, transportation systems and others. In 2008 the market research firm iSupply forecast that the worldwide market for all touch screens would nearly double, from $3.4 billion to $6.4 billion by 2013.
Especially in embedded systems, new technologies like projected capacitive touch screens are accelerating the growth of touch screens by enhancing their durability, reliability and overall performance.
In particular, projected capacitive technology can be applied to touch screens in embedded applications where the harshness of the environment might wreak havoc on older touch technologies, such as resistive touch screens.
Inherent Issues with Resistive Touch Screens
Resistive touch screens, the most common touch panel technology today, have several well known problems. Some of these problems can be minimized by adding additional contact points, but ultimately the weaknesses of resistive touch screens are an inherent drawback of their construction.
Resistive touch screens are built around two parallel layers of conductive Indium Tin Oxide (ITO). Typically, the bottom layer is printed on a stiff material like glass. The top layer, which is the layer closest to the user, is printed onto a flexible material like a thin sheet of plastic film.
When the user touches the top layer, the plastic film bends until it contacts the bottom ITO layer. The contact between the two conductive layers alters the resistance of the ITO layers. This change in resistance is used by the touch panel's controller to determine the location where the two layers touch (Figure 1 below ).
The types of resistive touch screens are characterized by the number of connections on the ITO layers. The most basic type, a four-wire resistive touch screen, has two contact points on the top layer and two on the bottom layer.
These contacts are connected to dedicated inputs on analog-to-digital converters (ADC). Pressing the top layer changes the resistance of the two ITO layers. These changes in resistance, as measured by the ADCs, define the location of the operator's touch.
Because the top layer is a flexible film, it is susceptible to damage. The sheet can be torn by the user, it can deform over time, it can warp in high or low temperatures and it can be damaged by cleaning materials.
Basically, anything that alters the resistance of the ITO layers changes the touch screen's performance and, in some cases, can cause permanent damage. This is why resistive touch screens have to be recalibrated often and may even fail after suffering only minor damage.
Some of these problems can be mitigated by adding more contact points. For example, an eight-wire resistive touch screen includes an injected reference voltage on both ITO layers. These reference levels help the system self-calibrate, temporarily overcoming problems caused by drift and warp.
A five-wire resistive touch screen uses different connection points and drives the lines differently, allowing it to provide even better correction for drift, warp and some level of physical damage.
But no matter what scheme is used to drive a resistive touch panel, the root cause of the technology's problems is the requirement that the top layer be made of a material flexible enough to be easily depressed by the user. These flexible materials are inherently susceptible to physical damage, which affects the performance and effective life of the resistive touch screen.
Projected Capacitive Touch Screens
Projected capacitive touch screens are also built with two parallel layers of ITO, but, unlike resistive touch screens, the ITO layers in a projected capacitive touch screen are sealed between two sheets of glass.
The patterns on the two parallel ITO layers form a grid of capacitors. The electric fields of these capacitors are projected through the top layer of glass. When the user places a finger near the top surface of the touch screen, it couples with the electric fields.
This coupling changes the capacitance of several of the capacitors in the vicinity of the finger. An algorithm converts these changes in capacitance into a location along the X and Y axes of the screen (Figure 1 below ).
|Figure 1. In a resistive touch screen, when the user touches the top layer, the plastic film bends until it contacts the bottom ITO layer. In the projected capacitive scheme, when the user places a finger near the top surface of the touch screen, it changes the charge on of several of the capacitors near the finger.|
This process involves some high-precision measurements, aggressive noise filtering and complicated algorithms. The change in capacitance is typically in the femto-farad range (10-15 F). As a result, a dedicated ASIC is needed to amplify, condition and measure the signals.
Once the ASIC has determined the changes in each capacitor in the grid, it uses this information to calculate the centroid of the user's finger. Typically, the position can be resolved to one part in one or two thousand.
The projection of an electric field truly differentiates projected capacitive touch screens from resistive. Since the user is interacting with an electric field, there is no need for the two ITO layers to make physical contact or for the user to make an electrical connection with the touch screen.
As a result, the ITO, the contact ledges on the touch screen, the ASIC and all the other sensitive electronics that make up a projected capacitive touch screen can be protected by a top sheet of glass or plastic. Furthermore, the front bezel can be sealed around the edges of the touch screen, creating a completely sealed enclosure.
The touch screen can even be bonded to a clear acrylic section of the enclosure (Figure 2 below ), creating a completely smooth top surface. This is the method of construction used in the Apple iPhone, the best known application of projected capacitive touch screens.
|Figure 2. In a projected capacitive touch approach, the touch screen can be bonded to a clear acrylic section of the enclosure, creating a completely smooth top surface.|
Another advantage of projected capacitive touch screens over resistive is that because the ITO layers are printed on glass, they don't deform over time. The capacitors built into the ITO layers are always in the same place, so the touch screen never needs to be re-calibrated.
Keeping the Gloves On
As a result of environmental conditions, operators of some embedded applications may wear gloves when interacting with a touch screen.
Resistive touch screen technology works well in these types of applications since it doesn't matter what touches the screen. All that matters is whether the ITO layers contact each other. For a touch to be detected on a projected capacitive touch screen something has to alter the capacitive fields in the ITO layers.
Gloves may or may not work with projected capacitive technology, depending on the thickness of the glove, the type of material, and the sensitivity of the touch panel. For example, thinner gloves like surgical gloves usually work well, while heavy winter gloves do not.
Some of these problems can be overcome by adjusting the sensitivity of the projected capacitive touch screen. For example, if the touch screen is always going to be used in a cold environment, the sensitivity can be increased so that it can detect fingers through a thick glove. However, this can also make it difficult to operate the touch screen without gloves.
A projected capacitive touch screen can also be designed to work with an active stylus (that is, a stylus that is electrically attached to the system).
A good example of this is a credit card reader with a stylus for capturing signatures. Using a stylus greatly improves the accuracy of the touch screen. However, the stylus must be physically connected to the system to work properly.
The good news with projected capacitive touch screens is that in applications like signature capture, the stylus only touches the top glass surface. Trillions of names can be signed on the glass without damaging the touch screen in any way.
Multi-Touch and Advanced Gesturing
Because of the method used to determine the touch location in projected capacitive touch screens, it is possible to detect multiple touches simultaneously. When multiple fingers are placed on a projected capacitive screen, a two-dimensional “sheet” of changes in capacitance values is created (Figure 3 below ).
|Figure 3. When multiple fingers are placed on a projected capacitive screen, a two-dimensional “sheet” of changes in capacitance values is created.|
When the entire sheet is analyzed, the peaks in the sheet are associated with touch locations. Using this technology, multiple touch points can be identified.
It is also possible to analyze finger movements and identify specific gestures like pinching (zoom-out), stretching (zoom-in) and swiping (left, down, up, right, sideways and others).
Thanks to the tremendous popularity of the iPhone and other competitive smartphones, this type of advanced gesturing is becoming widely adopted as the standard way users interact with technology today.
While features like multi-touch and gesturing can be accomplished in software by the touch screen driver, they are very math intensive and time consuming. The newest projected capacitive touch screen controllers can interpret the touch data and generate gesture events directly, greatly simplifying the application software.
|Figure 4. The iPhone is just the first step up in screen display size for projected capacitive techniques.|
Deployment in Embedded Applications
As a result of the success of the iPhone, consumers have discovered the many benefits of projected capacitive touch screens. In particular, people are realizing that the gesturing interface used on the iPhone is a natural way of interacting with electronic devices.
Now that projected capacitive touch screens are available in larger screen sizes (Figure 4 above ), designers can bring this same experience to a variety of embedded systems where the durability, reliability and overall performance of this technology can also add substantial value to the system.
Tony Gray, principal engineer at Ocular,Inc., has a BS in Computer Engineering from Lehigh University and has spent the last 18 years developing software for a variety of embedded systems. He's currently a Principal Engineer at Ocular LCD in Richardson, TX. You can reach him at firstname.lastname@example.org.