1 Introduction
With technological innovation and rapid development of communication technology, car users need more intuitive and modern terminal display devices to satisfy instant messaging, navigation and entertainment. Vehicle-mounted liquid crystal display is one of the most important solutions. With its unique characteristics of lightness, low energy consumption, and colorization, it is used in automobiles to replace old-fashioned mechanical displays and realize digital display functions.
Liquid crystal display has a wide range of applications in automobiles, mainly used in on-board instrumentation, navigation, multimedia entertainment, air conditioning and other equipment, to show users the car's speed, engine speed, fuel consumption, mileage, maps, DVD screens, radio channels, and CD tracks. , Temperature inside the car, etc. There are big differences between car LCD screens and ordinary household LCD screens in terms of cost and technical requirements. This article summarizes, analyzes and proposes feasible solutions to common faults of STN LCD screens commonly used in vehicles.
2. Basic classification of LCD
LCD can be divided into passive matrix LCD and active matrix LCD according to different control methods. Passive matrix LCDs are greatly restricted in terms of brightness and viewing angle, and their response speed is also slow. Due to picture quality issues, this display device is not conducive to the development of desktop displays, but due to low cost factors, car-mounted liquid crystal displays mostly use passive matrix LCD 4.
Passive matrix LCD can be divided into TN-LCD (Twisted Nematic-LCD, twisted nematic LCD), STN-LCD (Super TN-LCD, super twisted nematic LCD) and DSTN-LCD (Double layer STN-LCD, double Layer Super Twisted Nematic LCD) 1.
The liquid crystal display is a passive display. It cannot emit light by itself and can only use the light of the surrounding environment. The main backlight sources are LED, EL, and CCFL. Its general structure is shown in Figure 1 5. It can be seen that the main working principle of a liquid crystal display is to change the arrangement or twist of liquid crystal molecules through electrodes to transmit light to achieve the display effect.
STN-LCD display color is greatly affected by its cell thickness, and a slight change in cell thickness will cause a more obvious visual difference. Affected by the difference in the thickness of the glass material and the instability of the flattening liquid crystal process during production, bright spots will be generated at the edges. It is difficult to avoid light leakage at the edge of TN-LCD. However, it is possible to eliminate subjective unpleasant feelings by increasing the cover area for the edge of the LCD when designing the assembly.
The printing process of the internal alignment layer of STN-LCD is unstable, and the difference in the diameter of the isolation beads will cause different degrees of uneven background color. On a white-light double-display LCD, there are several white areas on the black background, which are especially obvious when viewed in the direction opposite to the viewing angle. At present, it is controlled by setting limit samples for appearance screening.
Different in-vehicle electrical appliances such as meters, radios, and air-conditioning controllers will cause differences in display effects due to their different costs, different screen specifications, and different technologies used. For example, the contrast of the display screen of the air-conditioning controller using EBTN (Extra Black Bottom) technology is much higher than that of the receiver using ordinary STN LCD. However, if a better LCD such as TFT is used in the radio and player, the cost of the radio and player will be doubled.
At present, there is no particularly good solution for batch models, and we can only try to adjust to the main and co-pilot positions to have a good visual effect, but it can be used as a key consideration in the development of new models.
3. Common LCD defect modes and rectification directions
At present, there are four common defect modes as shown in the figure below, and the reasons for these defects are mainly composed of the following three parts: defects caused by poor consideration in the design stage, defects caused by instability in the production process, and defects caused by external factors. According to different types, see the following example analysis respectively.
3.1 Display flicker caused by temperature compensation parameter setting problem
Problem description: The radio screen flickers at a certain temperature.
Reason analysis: In order to ensure the display effect under a larger temperature span, the LCD driver chip of the radio has two working modes, the critical point of switching is defined at 10 degree ; when the radio detects the ambient temperature in its mode When the switching critical point floats up and down, the working mode will be switched continuously; the radio and player program software does not perform smooth transition processing at the switching critical point, and the display will flicker at this time.
Improvement measures: modify the software of the radio and receiver, add hysteresis control at the critical point of the switching temperature of the display working mode, and smoothly process the power supply voltage of the display when switching to avoid flickering of the display. Figure 3-1a is the temperature compensation curve before modification, and 3-1b is the temperature curve after modification.
3.2 Random bright spots are displayed at high temperature due to the mismatch between the display driver chip requirements and the software programming
Problem description: LCD produces random bright spots under high temperature.
Reason analysis: LCD driver chip specifications have specific requirements for its timing and voltage, the data hold time is 450ns; the data hold time in the software design can only reach 60ns, in order to meet the 450ns requirement, the current design uses resistance The capacitance (resistance, capacitance) network is delayed; in a high temperature environment, the resistance value will decrease with the increase of temperature, which causes the data hold time sequence to be incorrect, resulting in random bright spots.
Improvement measures: The problem of random bright spots is completely eliminated by replacing the LCD driver chip, but sufficient verification tests need to be implemented.
The defects caused by the presence of foreign objects in the LCD or the poor contact/short circuit caused by the production process vary depending on the location where it is dropped or poorly connected, and the possible failure phenomena will also change accordingly.
3.3 Defects caused by foreign objects falling into the LCD
Common types of foreign objects are shown in Figure 3-2a, b:
Control scheme: It is necessary to conduct detailed analysis of the foreign matter composition, find its exact source after confirming its composition, and then adjust the process to prevent the same foreign matter from falling into it again.
3.4 Defects caused by unstable LCD production process
Common damages are shown in Figure 3-3a, b, c:
Solution: If there are other substances in the damaged position, the composition inspection is also required. Enhance the cleanliness management of the production line to prevent employee sweat from dripping onto the semi-finished products, increase equipment control to reduce the chance of human hands directly touching the semi-finished products, and use tooling to prevent errors in operation.
The above problems can be temporarily controlled by means of enhanced inspection, such as high-voltage inspection of semi-finished products, and work inspection of finished products in high-temperature and high-humidity environments. It can effectively improve the detection rate of early effective defects and reduce the outflow of defective products.
3.5 Defects caused by damage to LCD caused by external force during assembly
Common damages are shown in Figure 3-4a and b:
Control scheme: When assembling electrical equipment with LCD in the workshop, it is necessary to avoid direct contact with the surface of the LCD as much as possible, causing pressure to cause poor display or even glass damage.
3.6 Defects caused by peripheral circuit failure
Common faults are shown in Figure 3-5a, b, c:
Solution: The assembly supplier needs to strengthen its internal quality control, such as the cleanliness of the production line, the guarantee of the welding effect, and the inspection of incoming components. And through appropriate assembly factory inspection to detect hidden products early to prevent them from flowing into the hands of customers. Common methods are: high temperature work, simulated transportation environment vibration, etc.
4. Conclusion
Liquid crystal display technology is increasingly used in the automotive field, and its development direction also has the original monochrome, low resolution, and narrow viewing angle of the old LCD gradually shifting to color, high definition, and wide viewing angle. It is foreseeable that in the future, the display effect of car LCD will be basically the same as that of home computers and TVs, and the content displayed will become more and more abundant, creating a more comfortable driving environment for users. From a quality point of view, we also need to keep pace with the times, constantly understand the industry dynamics, and absorb the latest knowledge to ensure more stable product quality. There is a long way to go.
references:
1G.H.Heilmeier, L.A.Barton and L.A.Z anoni: Appl.Proc.IEEE, 1968.
2 Y.Taira, D.Nakano, H.Numata, A.Nishikai, S.Ono, F.Yamada, M.Suzuki, M.Noguchi, R.Sing, and EGColgan.Low-power LCD using a Novel Optical System, SID02 Digest. 2002: 1313-1315.
3 Henri J.B. Jagt, Hugo J. Cornelissen, Dirk J. Broer, Cees W.M. Bastiaansen, Micro-structured polymetric linearly polarized light emitting lightguide for LCD illumination, SID02 Digest. 2002: 1236-1239.
4 Xiang Huaikun. Research on key technologies of car navigation system, doctoral dissertation of Beijing University of Technology.
