Designing Rugged In-Vehicle Displays
- Thursday, 01 November 2012
But what of the display itself? Glass needs to be chosen that is a suitable thickness for its area, and should then be chemically treated to ensure that it is robust against the worst of the physical abuses it will encounter in its inservice life. In addition to chemical treatment for robustness, the glass will also be treated to minimize reflections and to provide EMI integrity. All of these processes need to be of the highest standard to ensure that the LCD image is not degraded as it passes through the glass, because even the slightest hesitation in interpreting the on-screen image is intolerable.
There are differing approaches to the problem of protecting the LCD behind the glass. One of those is to leave an air gap between the various layers in the display stack, from front glass (which is susceptible to impact) and the LCD itself. The downsides of this approach are serious from the perspective of the user. First, any residual moisture in the assembly will inevitably find its way into the gap where it will condense on the colder surface, resulting in visible droplets of water that affect the image quality. Second, the thickness of the front glass, coupled with the coefficient of refraction, can give rise to a significant parallax error, which is a major problem when using a touchscreen. When eye and fingertip are perpendicular to the point on the LCD displaying the relevant symbology, parallax is not an issue, but as the eye moves from the perpendicular it becomes increasingly difficult to judge the correct point on the touchscreen to match the onscreen symbology.
There are strategies to deal with the moisture problem, including incorporating desiccants and purging the final assembly, but these are not reliable longterm solutions over the life of the unit. The parallax issue remains, however, and once again the need for instantaneous action on the battlefield means that it is unacceptable.
An alternative strategy is to bond the layers of the display stack. This is not without its challenges, which include eliminating impurities, ensuring that air-bubbles do not become trapped in the assembly, and preventing delamination over the lifetime of the product. Good component selection and bonding- compound selection are key to the solution, but a rigorous manufacturing process is vital to ensure uniform application of the bonding agent in a clean room environment.
While there is significant investment in time and equipment required to achieve the bonded solution, there can be no substitute for the resulting optical quality, yielding a bright display with no compromising visual artifacts.
It is a given, especially in light of the proliferation of such devices in day-today life, that the in-vehicle display of the future will feature touchscreen technology.
The designer has three choices: resistive, capacitive and infrared. While some of the technologies available in the commercial marketplace have attractions for the user experience — for example, the user interface revolution caused by the multi-touch capacitive touchscreen on the Apple iPhone — most have serious downsides for the vehicle market.
Capacitive touchscreens offer some tantalizing opportunities, but these are difficult to set up for use with both bareand gloved-hands. Additionally, some capacitive technologies, which offer the smoothest user experience in commercial applications can be bad EMC emitters, something that is unacceptable in land vehicle design. Similarly, infrared touchscreens offer some advantages, but these are more than outweighed by the combatant-threatening problems of the vehicle becoming an infrared emitter at nighttime.
In a moving vehicle (Figure 3), making an accurate actuation via the touchscreen can be a significant challenge. Application design can ease that challenge by ensuring adequate spacing of onscreen activated areas or buttons, but that is only part of the story. Requiring a positive pressure for activation can prevent false actuations, and this is one of the key characteristics of resistive touchscreen technology, which is preferred for in-vehicle rugged displays.
The potential applications for these highly intelligent, extremely rugged invehicle displays are wide-ranging from embedded training to terrain visualization, from digital mapping to battlefield communications and management, from signals intelligence (SIGINT) to situational awareness, including 360- degree video stitching. The important change, though, is that a new generation of rugged displays is becoming available that incorporate the same amount of processing power as would previously have been configured in a discrete, standalone box, thus saving vital space, weight and power.
Integrated With The Network
Moreover, with modern vehicles now having large numbers of electro-optic sensors, providing the display not only with processing capability but also a 10 Gigabit Ethernet interface means that it is possible to connect the display directly to the video backbone of the vehicle and subscribe directly to those channels of interest.
As with all embedded computing solutions, the design of these displays demands in-depth understanding not only of the application, but also how that application will be deployed, as well as by what kind of personnel, operating under what kind of constraints, and in what kind of environment it will be used. Few, if any, elements of an in-vehicle embedded computing system have a more vital role to play than the display, meaning the choices made in its design are nothing less than critical.