Next-generation head-up display system: development momentum, challenges and solutions

Author: Alan Rankin, Texas Instruments DLP® Products division business development manager; Jason Thompson, Texas Instruments DLP® Products Division Applications Engineering Manager

On February 7, 2015, at the 87th Science and Technology Awards Ceremony of the Academy of Motion Picture Arts and Sciences, Dr. Larry Hornbeck, inventor of the Texas Instruments (TI) DLP® chip, won the Oscar® The DLP chipset has fundamentally changed the way movies are made, distributed and viewed, and now more than 80% of movie theaters around the world have adopted this chipset.

Similar to the impact of DLP technology on the film industry, this technology is now also preparing to transform the automotive industry through new displays and automotive lighting applications.

(Table 1)

　　

HUD2.0 development momentum

Existing head-up display (HUD) systems can often only display redundant information, that is, information that can also be obtained from other locations in the car. The new generation of this technology, HUD2.0, can display advanced driver assistance system (ADAS) information. In addition to in-vehicle sensors, cameras, and vehicle-to-vehicle / infrastructure communication (V2X), the amount of information around the vehicle is also increasing exponentially. The challenge we face is how to effectively communicate what important information is "known" by the vehicle and pass it on to the driver, but this information will only increase when we move to semi-autonomous and autonomous driving functions.

Figure 1 AR HUD orthographic projection graphics

Using HUD2.0 will convey this kind of information in a natural and intuitive way, enhance the driver's global fixed view, and display the known information of the vehicle with a regular projection graphic. Navigation instructions, lane departure warning (LDW) and adaptive cruise control (ACC) indicator functions will be displayed from the driver's perspective at a natural image distance. As shown in Figure 1, this figure shows how to 'enhance' the reality of the driver's view and provide useful information in real time. Here we can display the image in bright, lifelike colors and superimpose it on the physical object at a natural distance, so the driver can easily use the information with minimal interference.

In addition, unlike the existing HUD used as a "secondary" display system in the user interface paradigm, HUD 2.0 is located at the center of the human-machine interface (HMI) strategy and will serve as the main information display system. Similarly, the new generation HUD also expects excellent image quality and consistent readability under different sunlight conditions.

Figure 2 FOV and VID affect the perceived HUD graphic size

The challenge

HUD 2.0 requires access to many new aspects based on traditional HUD design. Although a detailed description of the HUD design is beyond the scope of this article, it is still necessary to review some key parameters.

As shown in Figure 2, the field of view (FOV) and virtual image distance (VID) play an important role in determining the perceived image size. Although the traditional HUD vision only covers one lane, HUD2.0 with a larger field of view and longer virtual image distance allows the driver to see images of traffic flow beyond one lane. These enhancements in terms of field of view and virtual image distance require higher brightness, more saturated colors, higher power efficiency, and stronger tolerance to sunlight. In addition, these new parameters need to be met while meeting the environmental conditions of all traditional cars. Table 2 below lists some parameters of HUD2.0, compared with the traditional HUD system.

Table 2 Comparison of the key parameters of the new generation HUD2.0 and the key parameters of the traditional HUD

Brightness and efficacy

The larger field of view and higher brightness level provide the driver with easy-to-view images. In order to ensure readability under various light conditions, HUD should be able to generate a virtual image between the specified 15,000cd / m2 and 30,000cd / m2. However, the absolute power required to create this image should be kept low so that not only can the capacity required for thermal management be minimized, but also the luminous flux can be maintained within the range of viable light sources (LEDs). In order to expand the field of view and increase brightness without increasing power consumption, a more efficient imager is necessary. Texas Instruments DLP 0.3 ”WVGA class A100 digital micromirror device (DMD) efficiency is higher than 66%, significantly improving the system efficiency to meet the above parameters. With the efficiency improvement, HUD systems based on DLP technology and RGB LED can achieve the required Brightness and widen the field of view. For example, a system designed with .3 ”WVGA DMD and OSRAM Q8WP RGB LED [2] uses only 6.0 W of LED power to achieve a brightness of more than 15,000 cd / m2 with a 10-degree field of view, and even The power of the smaller auxiliary HUD system is lower than the current. The efficiency of this system is 10.6 lm / W (lumens per watt).

Color saturation

Many traditional TFT / LCD HUD designs use white LEDs, which produce red, green, and blue through filters. In contrast, HUD systems based on DLP technology use three LEDs of red, green, and blue to provide more saturated colors, which enhances the readability of images on HUD displays [3]. Some key performance indicators can be used to judge the color performance of the system, including comparing the color gamut size measured by the color gamut with the NTSC color gamut, the dominant wavelength, and the hue of each color defined by color saturation.

Table 3 compares the TFT / LCD white LED architecture [1] with the HUD architecture using RGB LEDs based on DLP technology. Compared with NTSC, RGB LED has a larger color gamut and deeper red and blue saturation.

Table 3 HUD color performance

Solar heat load

As the field of vision of the HUD system expands, the solar energy collected by HUD optical instruments also increases. In addition, as the distance of the virtual image increases, the driver can browse the image at a normal angle compared to the fixed view of the real world, and the solar energy focuses more on the HUD's internal imager. The effect of collecting more sunlight and focusing this ability into a smaller spot imaged internally can be harmful. HUD systems based on DLP technology use diffuse screen materials to create internal images of the HUD system. For traditional HUD systems, the imager (generally a TFT panel) directly emits HUD images.

The diffuser screen is a passive component and it has two main advantages: 1) It does not absorb solar energy – it diffuses light 2) It is not a heat source itself. With the help of these attributes, the HUD system based on DLP technology can be more easily extended to the large field of view and longer virtual image distance required by the augmented reality HUD system.

Polarized sunglasses

In addition to the brightness enough to see in various ambient light conditions, when the driver wears polarized sunglasses, the HUD virtual image is also readable. Since DLP technology projects unpolarized light, this allows OEMs to optimize the HUD used for polarized sunglasses.

Environmental conditions

The imaging technology used in automotive HUD systems must be able to operate reliably under harsh environmental conditions, such as high humidity, extreme temperature changes, extreme temperatures, shock and vibration. DMD is a micro-electromechanical system, and its ability to cope with the temperature cycles, shocks and vibrations experienced in cars is amazing. DLP 0.3 ”WVGA Class A100 DMD meets these conditions. When the resonant frequency of the sight glass is much higher than 100 kHz, its mechanical structure is rugged under the impact and vibration in the range of 5 kHz. Table 4 lists the Some key tests successfully completed on the WVGA class A100 DMD.

in conclusion

As the application of ADAS technology in automobiles becomes more and more popular, HUD is becoming more and more important for vehicle HMI strategies. As HUD gradually transitions from small auxiliary displays to large primary displays, people's expectations for image quality, readability, and reliability are also increasing. DLP technology has been widely used in consumer electronics and commercial applications for nearly two decades, laying a solid foundation for automotive-grade chips. As shown in Figure 3, the DLP 0.3 "WVGA class A100 DMD specifically meets the environmental requirements of the new generation of automotive HUDs.

References

[1] E. Buckley, "Pixtronix DMS (tm) technology for head-up displays," in SID Vehicles and Interfaces Conference, Dearborn, MI, 2011.

[2] OSRAM Opto Semiconductor. [Online]. Available: http: //

[3] KIRSD a. BE Blankenbach, "Comparison of the Readability of Colour Head-up Displays Using LED and Laser Light Sources," in SID, Symposium Digest of Technical Papers, 2010.

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