Touchscreen vs. Button Interface Design: Capacitive and Resistive Touchscreens and Haptics
Learn about some of the most important fundamentals for understanding how interface technologies have evolved.
In an All About Circuits/Moore’s Lobby podcast entitled Episode 10: NASA Astronaut Matthew Dominick on Critical Engineering in Aerospace Technology, one topic, among many, that Dave Finch and astronaut Matthew Dominick addressed was: Why is touchscreen vs. button interface design so incredibly important in the cockpit of a fighter jet?
This question will be answered in historical and technical detail in the following discussion. In this article, we'll focus on the following concepts:
- Mechanical button/keypad interfaces
- Touchscreens (resistive and capacitive)
- Haptic feedback (specifically vibrating haptics)
This will give us the basic conceptual information we need to better understand the importance of display technology in aerospace applications.
Mechanical Button/Keypad Interfaces
This type of legacy user interface provides a tactile response to the user which may be in the form of a mechanical response. These types of keypads are better suited for users wearing gloves. Physical keypads tend to be more accurate because the keys are more isolated from one another than most touchscreens; this helps to eliminate errors in activating adjacent keys.
Mechanical keyboard systems are less expensive than touchscreens and are typically lighter (usually only by a few grams) due to less technology that needs to be in the display vs. a touchscreen.
Touchscreens can provide a created mechanical feel, light up, or emit a sound when depressed, but in the process of typing or depressing these keys in succession, users may accidentally touch adjacent keys much easier than in a mechanical keypad. Touchscreens are typically flat and have no real barriers separating adjacent keys such as in a mechanical keyboard.
Their advantage over a mechanical keyboard is their greater reliability in dirty or harsh environments. Some mechanical keyboards do have a flexible membrane kind of construction protecting the keys which will prevent this kind of reliability problem by keeping dirt and debris out.
Their primary disadvantage is that they're more power-hungry; this will be detrimental in a battery-powered system. Additionally, they may have viewing problems in direct lighting.
Two of the most popular types of touchscreens are resistive and capacitive.
This architecture requires two clear conductive layers (glass or acrylic substrate and a polyester top sheet) separated by insulating dots. When a finger touches the top layer, it causes contact between the two layers. The touch is tracked by first applying a voltage gradient to the layers sequentially along an X and Y axis (with the opposite layer used as a voltage probe). A controller determines the X and Y position of the contact based on what voltage level is reported from the probed layer.
Figure 1. A resistive touchscreen construction. Image used courtesy of Wilson Hurd
This kind of design is low cost and low power is consumed. It is impervious to liquids. It may need occasional calibration, and is more prone to damage and wear.
Compare the resistive touchscreen concept above to a capacitive touchscreen. In this design, the voltage is applied to the corners of the screen. Electrodes around the edge of the screen create an electric field across the conductive surface, which allows for a finger to be tracked on the screen by measuring the capacitance change caused by the finger's conductive surface drawing current.
Figure 2. A capacitive touchscreen construction. Image used courtesy of Wilson Hurd
This type of design uses a solid glass panel with excellent optical performance, no mechanical movement with high endurance, and has multi-touch and gesture capabilities. Users may use their bare finger, gloves, or an active stylus. The architecture is able to withstand environmental extremes, is highly accurate, but susceptible to EMI.
For a more in-depth look at this concept, check out Robert Keim's intro to capacitive touch sensing.
Haptic feedback is another means of bi-directional communication between humans and computers and includes sensory feedback to enhance the user experience. Touch, sight, and sound will enhance the user interface and give confidence and confirmation to the user that the touchscreen button was depressed. Physical feedback is essential to reliability in situations such as in military fighter aircraft where the pilot needs to continuously scan their surroundings visually.
One means of giving a user confidence, that the button they are touching is actually activating the desired response, is with haptics. The haptic effect can be superimposed, via standing wave generators and pressure sensors, on a conventional touch screen; when there is a touch, there is a generated sound wave that gives the user a feeling of pushing a button and receiving a positive feedback on a conventional keyboard. This is especially critical in military fighter aircraft and can enhance spacecraft systems.
Figure 3. The basic architecture for a vibrating haptic system in a touchscreen. Image used courtesy of Catelani, Ciani, Barile, and Liberatori via IEEE Xplore
In the next article, we'll talk about how these technologies have been applied from the PalmPilot to the display in the F-18 Super Hornet, which Matthew Dominick discussed in the Moore's Lobby episode.