Method for realizing low temperature display of liquid crystal display

　　1 Introduction

　　Liquid crystal display has the characteristics of low power consumption, high definition, long life, small size, light weight, good optical properties, etc. It is an ideal display device and is widely used in various instruments and meters.

　　Liquid crystal display is a passive display. It cannot emit light by itself and can only rely on the light of the surrounding environment to display. It only needs a small amount of energy to display patterns or characters. It is precisely because of low power consumption and miniaturization that LCD has become a better display method. The liquid crystal material used in liquid crystal display is an organic substance with both liquid and solid properties. Its rod-like structure is generally arranged in parallel in the liquid crystal box, but its arrangement direction can be changed under the action of an electric field.

　　For positive TN-LCD, when no voltage is applied to the electrode, the LCD is in the "OFF" state, and light can pass through the LCD to appear white; when a voltage is applied to the electrode, the LCD is in the "ON" state, and the long axis direction of the liquid crystal molecules is arranged along the direction of the electric field. Light cannot pass through the LCD and appears black. By selectively applying voltage to the electrodes, different patterns can be displayed.

　　TN mode can be used to make liquid crystal displays with the characteristics of low voltage, low power consumption, and long life. It is the most widely used mode among various working modes.

　　The liquid crystal display is a liquid crystal box made of two pieces of conductive glass, the upper and lower pieces, filled with liquid crystal, sealed with sealing material-plastic frame (usually epoxy resin) on all sides, and polarizers are attached to the two outer sides of the box. The interval between the upper and lower glass sheets in the liquid crystal box, which is usually called the box thickness, is generally a few microns (the accuracy of people is tens of microns in diameter). The inner side of the upper and lower glass sheets, corresponding to the part where the graphics are displayed, is coated with a transparent tin oxide-tin oxide (ITO for short) conductive film, that is, the display electrode. The main function of the electrode is to allow external electrical signals to be added to the liquid crystal through it.

　　The entire display area on the inner side of the glass sheet in the liquid crystal box is covered with an orientation layer. The function of the orientation layer is to align the liquid crystal molecules in a specific direction. This orientation layer is usually a thin layer of polymer organic matter and is treated with friction. It can also be prepared by vacuum evaporating a silicon oxide film at a certain angle on the glass surface.

　　TN type liquid crystal displays are filled with positive nematic liquid crystals. The orientation of liquid crystal molecules is to make the long rod-type liquid crystal molecules parallel to the glass surface and arranged in a fixed direction, and the direction of the long axis of the molecules is along the direction of the orientation treatment. The orientation directions of the upper and lower glass surfaces are perpendicular to each other. In this way, in the direction perpendicular to the surface of the glass sheet, the orientation of the liquid crystal molecules in the box is gradually twisted, and it is twisted 90° from the upper glass sheet to the lower glass sheet. This is the origin of the name of the twisted nematic liquid crystal display.

　　The indicators for evaluating liquid crystal displays mainly include threshold voltage, contrast and viewing angle, among which the most important is the response characteristics. Liquid crystal display is based on the change of the state of liquid crystal molecules, so it is a molecular process, and its response speed is naturally much slower than that of atomic or electronic processes. However, whether it is an ascending process or a descending process, it is a process in which the state of liquid crystal molecules changes due to power overcoming resistance. Therefore, no matter what kind of device made of liquid crystal electro-optical effect, its response time T is expressed as follows:

Figure 1. Response time and temperature relationship

　　Where is the rise time of the liquid crystal display; is the fall time of the liquid crystal display; is the anisotropic viscosity coefficient of the liquid crystal display material; are the three deformation elastic constants of the liquid crystal material; is the dielectric anisotropy of the liquid crystal material; d is the thickness of the liquid crystal layer in the display; V is the external driving voltage. E=V/d is the electric field intensity; q is called the wave number, and in the case of a nematic liquid crystal display, q＝π/d; is the vacuum dielectric constant; in the formula, it is less affected by temperature, but is exponentially related to 1/T, so it is quite affected by temperature. Although K changes greatly with temperature, the value of K decreases rapidly with the increase of temperature T, but it is approximately proportional to the square of the order parameter S. When the liquid crystal temperature rises from the crystal to the nematic phase transition temperature to below the nematic phase to the isotropic phase transition temperature, S decreases from about 0.8 to 0.3. It can be seen that K changes less with temperature than with temperature. With the addition of the effects of the two, it can still be considered to be exponentially related to 1/T. Figure 1 shows the relationship between the response time and temperature of the liquid crystal box.

　　The threshold voltage Vth of the liquid crystal display is defined as the effective value of the driving voltage when the electro-optical change of the display part of the liquid crystal display device reaches 10% of the maximum change. As the temperature decreases, the threshold voltage will increase. When the temperature drops below 0°C, the liquid crystal material will become viscous, the response speed will slow down, and the dynamic image will have a tailing phenomenon or even cannot be displayed; if the temperature is too low, the liquid crystal state will disappear and become a crystal. When the ambient temperature is below 0°C, the life of the fluorescent tube of the backlight source will be reduced, and the low temperature will reduce the brightness of the backlight source. According to the test, the brightness of the backlight source is the highest at 50°C. In order to optimize the working performance of the display, it should be operated within a certain temperature range.

　　2 Commonly used low-temperature display methods

　　2.1 Increase the driving voltage of the liquid crystal display

　　When the temperature drops, the threshold voltage of the liquid crystal display will increase. Increasing the driving voltage of the liquid crystal display can realize the display of the liquid crystal display at low temperatures. The main components of this method are temperature sensors and adjustable output voltages. The driving voltage of the liquid crystal display is changed according to the temperature of the external environment to enable the display to display at low temperatures. This method can make the operating temperature range of the liquid crystal display be -20°C to +50°C. However, the driving voltage cannot be increased infinitely. When the driving voltage is increased to a certain level, the contrast of the display will decrease significantly, and even a black screen will make the display unusable.

　　2.2 Using ITO conductive film for heating

　　The ITO heater is placed between the liquid crystal substrate and the backlight reflection cavity to directly heat the LCD substrate. This method heats the LCD substrate in a concentrated manner, with a short heating time and low heating power. However, it is necessary to disassemble and remodel the LCD display device, and the operation process is complicated. Alternatively, the shielding glass of the display window is coated with an ITO heating film, and the heating film power is turned on under low temperature conditions to heat the display device through thermal radiation. This method is relatively simple and easy, and the heating time is relatively short.

　　The interior of the liquid crystal display module has a certain degree of complexity. It is composed of multiple basic parts and is a very precise and compact structure. The collection of the three parts of the LCD screen assembly, backlight source, and drive and control circuit is called the LCD module assembly. For the LCD module assembly, its interior is composed of precision components such as LCD display screen, transmissive polarizer, flexible conductive lead-out belt, backlight source, high-density multi-channel drive integrated circuit, etc., and it is very easy to damage these precision components and optoelectronic components by disassembly and assembly again. At present, the commonly used method for heating LCD is to grow a semiconductor thin film - ITO film by vacuum evaporation or magnetron sputtering on a high-strength special glass substrate with a thickness of 0.5-0.3 mm. After special process treatment, the film becomes clear and transparent and has a certain conductivity. Using its conductivity, it can be made into an LCD internal heater. However, if this glass substrate ITO heater with a thickness of more than 0.5 mm is to be installed inside the LCD, many complex structural problems must be solved, and it is possible to destroy the original optical path, lose brightness and the sealing of the device. Even slightly stronger vibration and impact will cause the ITO glass heating substrate to break. Therefore, the module structure design and new parts manufacturing must be re-performed before the ITO film heater can be installed. This is a delicate, complex and process-demanding job with high cost, low yield, and low product reliability. Therefore, the original LCD heating technology is greatly limited in practical applications. In response to this problem, the new low-temperature reinforced LCD display has achieved a breakthrough in technology. One of its notable features is that it can solve the micro-power heating problem of the LCD display without opening the LCD module assembly and installing the heater. The heater, vacuum insulation screen and electromagnetic shielding layer are an integrated structure. This device can be embedded in the front of the LCD display window, so it will not affect the original optical path system inside the LCD display module, and can achieve the same heating effect as the built-in heater, making the structure of the entire low-temperature display simple, relatively easy to assemble, and will not damage the original structure and optical path of the LCD module assembly.

　　This method can achieve low-temperature display of the LCD display, but the process is relatively complicated, and the LCD display needs to be disassembled to varying degrees, affecting its reliability. It is important that when the ambient temperature changes, the internal temperature of the LCD display cannot be controlled, and the heating power of the heating film needs to be adjusted according to the ambient temperature. It may cause the internal temperature of the display to be too high and burn the LCD display, and its scope of use is subject to certain restrictions. The following introduces an external heating method to achieve low-temperature display of the LCD display.

　　3 External heating method

　　3.1 Heating principle

Figure 2 LCD display box

　　The system consists of a liquid crystal display, a temperature sensor and a heating resistor. The heater uses a heating resistor, which is fixed on the board on the back of the liquid crystal display. The whole system is sealed in a box and connected to the external control circuit through an interface. The liquid crystal display uses a character dot matrix liquid crystal display module MTC-C162. The MTC-C162 liquid crystal display is newly equipped for the angle sensor developed by us. The operating temperature range of the sensor is -40℃～+55℃, so the display must also work within this temperature range. MTC-C162 is a wide temperature liquid crystal display. It has the characteristics of small size, stable display and simple operation. The only disadvantage is that the operating temperature range cannot meet the requirements.

　　AD7416 is a complete temperature monitoring system installed in a chip. Compared with other digital temperature sensors, it has the advantages of small size and simple programming. It includes a bandgap temperature sensor and a 10-bit AD sensor for monitoring and digitizing the high and low temperatures, with an accuracy of up to 0.25℃, and a comparator with a programmable threshold for comparing the measured temperature. The on-chip register can be used to set high and low temperature thresholds and provide an open-drain "overtemperature indicator" (OTI) output. When the set threshold is exceeded, the OTI output is valid.

　　3.2 Temperature Control

　　The main controller of the system is a single-chip microcomputer. The single-chip microcomputer determines the ambient temperature and the temperature in the LCD display box according to the values ​​of the two temperature sensors, controls the heater to heat, and keeps the LCD display within the working temperature range. Experiments have shown that when the ambient temperature is lower than -10℃, the LCD display can work normally when the temperature in the LCD display box is maintained above 20℃. The single-chip microcomputer first determines whether the ambient temperature is lower than -10℃ according to the temperature sensor. If the ambient temperature is higher than -10℃, the relay is disconnected and no heating is performed. If the ambient temperature is lower than -10℃, the temperature of the LCD display box is determined according to the temperature sensor in the LCD display box. If the temperature is within 20-30℃, it is maintained as it is. If it exceeds 30℃, the relay is disconnected to stop heating. If it is lower than 20℃, the relay is closed to start heating. In this way, the LCD display can always display normally when the ambient temperature changes.

　　4 Conclusion

　　Using an external heater to make the LCD work at low temperatures is simple and easy compared to the built-in heating method and the method of increasing the driving voltage. It does not require the LCD to be disassembled, improves the reliability of the display, and can make the LCD work in an extremely low temperature range. When the external environment changes, the temperature in the display box can be automatically controlled without manual control, and the environmental adaptability is strong. At -40℃, when the heating voltage of the heater is +20V, the LCD can work normally after heating for 6 to 8 minutes. If +12V heating is used, the display can work normally after 15 minutes of heating. The average power of the heater is affected by the external environment temperature. The lower the temperature, the greater the average heating function power. This method is used for small LCDs in low temperature and harsh environments, but it is not good for large LCDs. The LCD can display normally at a temperature of -40℃ to +55℃.