Along with the advantages and limitations of the touchscreen technologies, you have other design considerations as you evaluate touchscreen alternatives. The additional considerations pertain more to ease of system design and manufacture than to how a given technology suits the application. These factors may be important enough in your application, however, to provide additional winnowing of the touchscreen alternatives. An apparent additional consideration is the touchscreen’s transparency. The multilayer construction of resistive touchscreens allows only 75 to 85% of the screen’s light to pass. The construction may affect image contrast if you use it with reflective LCDs. The all-glass capacitive and acoustic touchscreens fare much better, especially if you have the touchscreen bonded to the display screen surface. IR and force-sensing touchscreens don’t block the display light.
Calibration and alignment of the touchscreen to the display surface are manufacturing and field-service concerns that you might consider during system design. You need to align the screen’s and display’s coordinate systems to let the touchscreen system determine which display object the user touched. Calibration involves adjusting the sensor electronics so that the system can accurately determine the touch point. The manufacturer performs this calibration on the touchscreen and controller at the factory. If you build your own controller and integrate touchscreens with your displays, you need to account for this calibration. Touchscreens may need periodic recalibration in the field. Resistive and capacitive touchscreens are subject to drift. Acoustic and IR touchscreens, being mechanical in basis, have no drift.
Three other factors that may affect your system’s performance are the accuracy, resolution, and response speed of the touchscreen system. The first two factors are important when the display is crowded with touch options and when writing or drawing by touch or stylus forms part of the application. Response speed is important if the user is making rapid entries or is writing with a stylus.
The controller for the touchscreen determines accuracy, resolution, and response-speed characteristics more than the touchscreen technology itself. The resolution of all touchscreens except IR, for instance, depends on the resolution of the ADC that digitizes the screen’s analog signals. (The LED array spacing determines the IR touchscreen’s resolution.) Most vendors sell the controller with the touchscreen; some also sell specialized ICs for building your own controller.
The sensor’s resolution, speed, and accuracy are important in the design of touchscreen systems, but you also need to consider the human factor. For your system to be successful in the field, you must carefully design the displayed images (targets) that correspond to the touch-point choices.
One factor in screen design is the location of the targets. Target location is particularly important if the user must page through a menu comprising several screens to reach the final selection. Instead of placing targets to fit artistically each screen, place targets that repeat on several pages or are similar in function at the same location on each screen. Keeping similar options in the same location on a multiple-screen menu improves the user’s speed, comfort, and accuracy in making selections.
You should also consider target size. Larger targets produce more accurate user responses. Using a guard band around each target further improves accuracy by making the screen design more immune to touch-sensor drift and position errors in the display.
Your system could also benefit from providing user feedback about the touched point. The user already gets tactile feedback from touching the screen, although IR systems can register a touch without screen contact. You can further program the display to inform the user that the display has registered a touch. Having an option change color upon activation, making a button appear to depress, or having the system make a sound all enhance the user’s comfort and accuracy when using a touchscreen.
The other user-related factor to consider is the system’s interpretation of a touch. Your system can instantly respond to a touch, allowing no room for user error. The system can also arm an option upon touch, but activate the option only when a user lifts his or her finger, or “untouches.” By delaying activation until the “untouch,” you can offer a cancellation option. The user simply slides the finger to neutral screen territory before lifting, preventing an inadvertent selection.
Finally, don’t forget the software needed for your screen system. All touchscreen vendors supply software drivers for products, but these drivers may not suit the operating system you’re using. Some vendors go beyond drivers and offer application-programming tools, mouse emulators, and other high- level software support.