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STN LCDs Acquire New Features to Meet Rigors of Road Travel
Yoshinori Hirai
Optrex Corp.
Automobiles are becoming heavily populated with electronic devices of all kinds these days. As vehicles rely more and more on sophisticated forms of technology, the requirements for onboard information displays are becoming increasingly detailed and diverse. Super-twisted nematic (STN) LCDs already serve widely in many kinds of information terminals. Now designers are improving STN LCDs and other passive-matrix panels for the demanding environments within vehicles.
Most automotive-use electric displays consist of passive-matrix LCDs, active-matrix LCDs, and vacuum fluorescent displays (VFDs). Information displays for vehicles have grown in importance alongside the evolution of information systems. The market now issues numerous and varied demands for such displays.
One of the chief characteristics of LCDs is their low power consumption. The first commercial-scale LCDs for consumer use made their appearance early in the 1970s. Relatively soon after that, in 1980, the first automotive-use LCDs arrived, serving as clock displays. Until that point, there had been doubt as to whether LCDs, which consist or organic substances, could meet the reliability requirements of onboard systems. The LCD clocks settled that point, proving that LCDs could be reliable in-vehicle displays.
Determining if an LCD is suitable for onboard automotive use depends on the results of acceleration tests under actual driving conditions. Optrex Corp. has applied its own strict evaluation criteria, drawn from its basic research and development (R&D), to determine the reliability of LCDs. The company believes that early estimates from various acceleration tests have proved to be sufficiently correct.
Worldwide today, an average of two or three LCDs is at work in each automobile leaving the factory. From now on, onboard automotive displays will grow in importance. Demand already is riding for advanced functions and multiple installations of these displays in cars, to allow for the presentation of additional information. Displays also are taking hold in rear-seat entertainment systems. There also is rapidly rising demand for fine-definition, full dot- matrix displays, because displays now must present large vSolumes of sophisticated content, even in onboard systems. Optrex supplies a full lineup encompassing passive-matrix and active-matrix LCDs, as well as organic light-emitting diode (OLED) displays. The company has multiple solutions to meet the diverse requirements for onboard automotive displays in the future.
Fig. 1: Typical arrangement of onboard displays
Performance Requirements
Typically, the arrangement of automotive displays concentrates in the portion of the instrument panel that the driver can see most easily (Fig. 1), and in the center portion of the console, where there is ample installation space. The center area also is easily visible to those sitting in the passenger seats in front and rear. Each display, however, may have to meet different requirements, depending on its purpose.
Several different kinds of automotive displays are commercially available. Some heads-up displays integrate into the windshield, where the driver can see the information without looking away from the road. Other displays are embedded into the rear-view mirror. Large thin-film transistor (TFT) LCDs are beginning to serve in rear-seat entertainment systems. Engineers from now on will consider the best locations for in-vehicle displays, and the best kinds of information for these displays to present.
Matching Methods to Message
Besides meeting the general performance requirements for any display, onboard displays must meet certain demands unique to in-vehicle use (Fig. 2). Meeting those demands poses a big technological challenge. Among the most important points are superior visibility and reliability. The displays also must coordinate with the rest of the interior design, and must allow design flexibility. Of course, the cost performance of the electronic displays serving as a total system is of concern as well.
Figure 2: Requirements for automotive displays
Since the 1980s, electronic displays in the instrument panel, including the speedometers, have provided much of the information a driver requires. Odometers and trip meters represent the simplest forms of electronic information displays on instrument panel. Generally, these consist of small twisted-nematic (TN) LCDs.
Regional Differences
Even in the early days of vehicle-use LCDs, European markets have issued strong demands for advanced-function displays with greater density than the simple panels. European customers also emphasize the need to coordinate colors in the area near the instrument panel. For these reasons, TN or dot-matrix STN LCDs, which make the best use of the backlight's free toning ability, find widespread automotive use. These displays indicate the air temperature, the fuel level, and the fuel economy, issue warnings, and provide other data.
North American drivers also have been relying on in-vehicle information displays for quite some time. In this market, VFDs have been popular because their superb brightness ensures good visibility. Among the monochrome displays in the instrument panel (Table 1) segment-style TN LCDs find widespread adoption because of their excellent reliability, their ability to adapt to custom designs, and toning ability of the backlight. Dot-matrix, monochrome STN LCDs also serve widely, because they offer excellent display densities.
Table 1: Comparison of small monochrome displays for instrument panels.
| Characteristic | TN displays | Optrex ASTN (STN displays) |
DSTN (STN displays) |
Monochrome TFT | Unicolor OLED | VFD |
|---|---|---|---|---|---|---|
| Viewing angle | * | * | * | ** | *** | *** |
| Response speed | * | * | * | ** | *** | *** |
| Device reliability | *** | *** | *** | *** | * to ** | * |
| Reflection | ** | ** | * | ** | - | - |
| Environmental Impact | ** (excluding CFL backlight) |
** (excluding CFL backlight) |
** (excluding CFL backlight) |
** (excluding CFL backlight) |
** | ** (depends on fluorescent material) |
| Ease of tailoring | ** | ** | ** | Not | * | * |
| Backlight toning | ** | ** | ** | ** | - | - |
Note:
*= Slightly inferior
**= Good
***= No problem
Design Challenges
Arranging information displays in the mechanically structured environment of a vehicle instrument panel often taxes the ingenuity of the developers. The BMW 7 Series provides a unique example of an integrated arrangement that blends mechanical displays and LCDs. Optrex supplies STN LCDs with a hole in the panel to accommodate insertion of a meter axis.
European vehicles have been using medium-sized information displays for years. In Japan, manufacturers arrange 2-inch and 3-inch information displays strategically, for optimum driver convenience. Car manufacturers in Europe now sell vehicles with 3-inch TFT color LCDs. In the future, they will want a selection of display systems and sizes, corresponding to different grades of vehicles.
To meet such needs for medium-sized information displays, Optrex supplies its ASTN (for automotive STN) information displays. The company has designed these specifically for onboard use in automobiles, with optimum contrast levels and background colors for the vehicle setting. These displays also accommodate a wide range of operating temperatures.
Adoption of TFT LCDs in and near the instrument panel is likely to accelerate in the years ahead. Various kinds of TFT LCDs are under consideration for this task now. Engineers are also considering the possibility of using OLEDs, in hopes of obtaining wide viewing angles, strong contrast ratios, and excellent low-temperature characteristics.
Front and Center
The center console of an automobile provides ample room for installation. Manufacturers will install various information displays in this space, depending on the grade of the vehicle. Deluxe cars usually have 6-inch or 7-inch TFT LCDs with quarter-VGA to WVGA capabilities. Car navigation systems have been one of the mainstays for vehicle-use information displays in the past. Now, with central control of the audio and air conditioning systems, and the adoption of other electronics in the car, the center console is becoming an information center for the vehicle. Engineers thus are working to provide displays with improved definition, brightness, and viewing angles for use in this area. In the future, the majority of displays for this space probably will be in the 7-inch class and will offer WVGA-mode images.
In mid-class cars, manufacturers sometimes use 1-DIN, full-color TFT LCDs. Entry-level vehicles have several TN or STN passive-matrix displays that work separately or in combination to handle clock functions, air conditioner control, and audio system control. In vehicles with an arrangement of several displays, engineers also must pay attention to a unified appearance and harmonious design. Even in Japan, there are rising volumes of composite information for display. Increasingly, engineers are using STN displays that can present large volumes of information, to permit and integrated display.
Displays of Cooperation
There are three categories of vehicle-use displays. Primary displays provide information essential to the driver. Secondary displays deliver information that complements the essential information. Then there are the entertainment displays. Each displays occupies a position appropriate to the level of information it presents. However, the limits on cabin space have prompted some installers to adopt displays with greater-than-usual information density, to permit a combination of information functions. As designs continue to shift to integrated onboard information management, information display methods will change as well, offering new varieties of panels and characteristics to meet new needs.
Distinctive Twist
TN LCDs employ a one-quarter duty ratio for driving, with a corresponding restriction on optical characteristics. TN LCDs often serve as segment displays, presenting icons and text rather than video. For optimum visibility, TN LCD designers usually employ a positive format with black images on a light background. Lately, they have begun to use bright white light-emitting diodes (LEDs) as the backlights for these panels.
Negative-format models with light images on dark backgrounds also are growing in number, partly because they convey and impression of elegance. One of the problems with these LCDs, though, is that the color tone of the background changes with the viewing angles. Engineers have pursued various methods to overcome this condition. For instance, modulated TN LCDs employ find abrasion for the glass substrate (Fig. 3). This substantially decreases the dependence of the background color on the viewing angle, so the color remains steady even if the light-travel distance changes.
Figure 3: MTN operating principle and key features
Passengers as well as drivers often need to view the segment-style TN LCDs in the center console. Modulated TN LCDs with a white LED backlight make such viewing possible. Additionally, the compensation effect of the mild abrasion also helps avoid changes in the background color when the surrounding temperature changes. As a result, modulated TN LCDs deliver a constant background color across a wide range of temperatures.
Adaptions of STN
STN LCDs often require optical compensation techniques for the proper display of blacks and whites. In consumer-use equipment, designs can employ film-compensated STN displays. However, STN LCDs are also strongly temperature dependent, meaning their optical performance varies with the temperature (Table 2).
Table 2: Structures and main points of temperature-coated STN LCDs
Optical compensation systems
| Characteristic | Ordinary STN, FSTN | Onboard use: DSTN | Onboard use: ASTN |
|---|---|---|---|
| Structure of temperature-compensated layer and display cells |  |
 |
 |
| Compensation layer | Film | Liquid crystal cell | Film |
| Driving temperature range | Narrow | Broad | Broad |
| Display mode | Negative and positive | Only negative | Negative and positive |
| Thickness | Thin | Thick | Thin |
| Transmissivity | Transmissive, semi-transmissive | Only transmissive | Transmissive, semi-transmissive |
Onboard information displays therefore require temperature compensation as well as optical compensation. Double-layer STN LCDs incorporate two complementary layers of LCD panels to achieve the necessary optical and temperature performance. Optrex's ASTN devices are structurally comparable to film-compensated STN LCDs. By adding a film, Optrex has enabled its ASTN products to achieve nearly the same level of temperature compensation and optical compensation as double-layer STN displays.
Visibility is a key requirement of onboard displays: They must be easy to see, even under bright, direct light. Optrex addresses this requirement in its ASTN LCDs by installing a single-layer optical shutter (an external, semi-transparent reflective plate). This ensures superior visibility even in direct natural light.
Driving Methods
In passive-matrix LCDs, the visibility declines as the liquid crystal's response speed accelerates. Accordingly, the higher the resolution (large time-division count), the lower the contrast ratio and brilliance. This happens because of the frame response phenomenon. The viscosity of the liquid crystal determines its response speed, so as the temperature rises, the crystals lose their viscosity and begin moving more and more quickly.
It is possible to control frame response to a certain extent by increasing the frame frequency. However, this is an unsatisfactory approach, because the higher frequency gives rise to flicker, crosstalk, ghost images and other problems in display quality.
Optrex uses its own multiple-line addressing (MLA) technology to drive onboard displays. (Use of MLA depends on the quality necessary for a given display.) MLA depends on the quality necessary for a given display.) MLA driving can improve the contrast ratio when the display operates in hot environments (Figs. 4,5).
Figure 4: High-temperature contrast improvement with MLA driving
Figure 5: MLA driving and distinctive wavefore features
In recent years, displays have had to handle growing volumes of information. Increasingly, there is a need for displays with enhanced levels of gray scale. Optrex finds that a combination of MLA driving, pulse-width modulation (PWM) and frame rate control (FRC) delivers better gradations than systems using conventional driving schemes. Besides offering excellent response and a wide temperature range, MLA driving helps lower crosstalk and improves the display of gradations. It therefore enables fine-definition displays for onboard use. Under these circumstances, MLA driving is finding use in medium-sized information displays(Table 3).
Table 3: Driving methods and gradation display schemes
| Driving method | Gradation scheme | Gradations (No. of displayed colors) |
Contrast at high temperatures | Flicker | Crosstalk |
|---|---|---|---|---|---|
| IAPT | FRC | 16 (4,098) | Not sufficient | Not sufficient | Good |
| IAPT | PWM | 32 (32,768) | Not sufficient | Good | Not sufficient |
| MLA | PWM + FRC | 32 (32,768) | Good | Good | Good |
Improving Visibility
Negative-format dot-matrix displays have been sufficient as fine-density displays so far. Now, though, there is demand for improved blacks in such displays, to overcome issues like inter-pixel light leaks, and light leaks in the background, which can impair display quality. Engineers can correct such problems by intensifying the measures to block light from non-display areas of the device. In the past, one method achieving this was to print frame in black on the rear of the LCD device. However, there are limitations to the printing accuracy of this method, which also creates parallax effects. The only way to compensate for these problems is to allow a non-functioning area between one display section and the next. Thus it is a rather unsatisfactory method that can serve only in restricted uses.
Strong Blacks, Vivid Colors
As an alternative to the conventional leak-prevention method, Optrex uses thick resists of black pigment for its own high-performance color black-matrix (HPC-BM) system. This method provides the surface smoothness of ?0.1?m that STN LCDs require, and helps improve the contrast ratio. Optrex uses this system in mass-produced STN LCDs for onboard use. Together, HPC-BM and MLA driving further improve the quality of vehicle-use information displays in products like the ASTN line.
Design Freedom
Those working with onboard displays require a large degree of design freedom, to conform to the space restrictions in the cabin, and to accommodate the interior design. This requirement encompasses the shape of the panel, as well as the IC connection methods.
Optrex answers needs for freedom in panel shapes through methods like the center hole for the meter axis, and by beveling or otherwise reshaping the corners of panels. To meet demands for different IC connection methods, the company supplies diverse formats including tape automated bonding (TAB), chip-on-glass (COG), and chip-on-flexible strip (COF). These formats offer the reliability necessary in a vehicle-use display, and serve widely in other onboard electronics, not just displays. COG, for example, provides a simple, low-cost IC connection method. COF permits narrow frames and the mounting of external components. These formats will spread among onboard information displays.
Conclusion
On-board displays for automotive use are growing in diversity and importance. They contribute to improved safety, convenience and comfort in cars. Displays with enhanced visibility will continue to appear, and models conforming to new requirements will serve in many new uses. Optrex will continue developing and supplying a variety of passive-matrix LCDs and TFT LCDs for diverse purposes. The company will support the automotive industry by offering innovative solutions, including the OLEDs, to meet the changing requirements of life on the road.
About this Article
The author, Yoshinori Hirai, serves as a member f the Board and General Manager of the ACI Business General Division at Optrex Corp. (www.optrex.co.jp).