Head-up display (HUD) technology has long become a hot topic in the automotive field, and with the development of optics and display technology, the user experience of AR-HUD has reached a new level. For example, Huawei Wenjie M9 released a few days ago, which is self-developed by Huawei, integrates intelligent driving and intelligent cockpitAR-HUD products are extremely amazing, we can take a look at the demonstration diagram first.
The HUD improves driving safety and convenience by presenting important driving information in front of the driver's line of sight, reducing the number of times the driver looks down at the dashboard, thereby reducing the risk of traffic accidents, and can also be combined with autonomous driving assistance technology.
In addition, the content that the HUD can display is becoming more and more abundant, including navigation, ** and other information, so as to improve the driving experience and enhance the convenience and comfort during driving, and enhance the sense of science and technology of the vehicle.
As HUDs become more and more popular, this article takes stock of several important HUD technologies that can help to better understand automotive electronics technology.
The type of HUD
The HUD is a transparent display technology that projects key driving information in front of the driver's line of sight. This information can include vehicle speed, navigation instructions, warning signals, and more. With the HUD, drivers can quickly get the information they need without having to look down at the dashboard.
The working principle of the HUD is mainly to project the image information onto a translucent mirror through the projector, and the mirror then reflects the image into the driver's line of sight. In this way, the driver can see the required driving information while keeping his or her gaze intact.
Figure: How the HUD works.
Depending on the projection method and position, the HUD can be divided into several types, C-HUD (combined head-up display) and W-HUD (windshield head-up display) and AR-HUD (augmented reality head-up display system). The C-HUD usually displays information on a separate transparent screen, while the W-HUD and AR-HUD project information directly onto the front windshield of the car.
In terms of experience, AR-HUD is the best, but the more difficult it is to implement, there are still many issues that need to be improved, including cost, size, display brightness, display quality, and heat dissipation.
Comparison of the effect and size of the AR HUD with other products.
Technical implementation of the HUD
The industrial chain of AR-HUD technology includes image generation unit (PGU), optical mirror, glass, software and other components. Among them, PGU is considered to be the core sector in the industrial chain, accounting for 50% of the total cost of AR-HUD, followed by optical mirrors, accounting for 20%. In AR-HUD technology, PGU is used to generate images and control brightness.
At present, AR-HUD is realized in the form of projection refraction, and the overall optical and mechanical volume is about 20L, which has high requirements for the layout of the body and electronic systems, and it is also difficult to ensure the imaging effect. Small size and high definition are also the demands of automotive OEMs. Size, cost, and effect are the three mountains that HUD needs to overcome at the moment. After all, we see that no matter which projection technology has matured in the projector market for many years, there is still a long way to go in the car.
At present, the most mature PGU technology path is Texas Instruments' DLP technology. DLP is a digital light processing technology, which is a technology that first digitizes the signal and then projects and displays it. The core of DLP technology is DMD, which stands for digital micromirror chip, which is currently the most effective and commercially available HUD technology. However, due to its high cost and patent monopoly, the industry has been looking for other HUD technologies to reduce costs.
Figure: TI's DMD, which uses MEMS technology offset to generate images.
Figure: Block diagram of TI's DLP system implementation.
Several other methods, including TFT, LCOS, LBS, and MICROLED, are considered potential technology trends for the future.
TFT is a type of LCD liquid crystal display, the principle of which is that the light emitted by the LED passes through the liquid crystal unit and projects the information on the screen. TFT is the cheapest and the least effective. Specifically, the projection distance becomes longer, and it is difficult to solve the problem of sunlight backfillingThe light is polarized and does not support sunglasses;The light efficiency is low, and the brightness of the product is lacking.
LCOS (Liquid Crystal on Silicon) is a reflective projection technology. The principle is to use semiconductor technology and aluminized coating technology to form an active lattice reflective CMOS substrate, then laminate the substrate with a glass containing ITO transparent electrodes, and finally pour a liquid crystal between the substrate and the glass to form and package it into an LCOS device. It is important that the LCOS technology PGU is not monopolized by foreign companies.
LCOS principle.
LBS stands for Laser Beam Scanning, which is an image production module (PGU) composed of laser diodes and microelectromechanical systems (MEMS). In the design process of HUD, in order to cope with different external light, weather and other influences, higher brightness is required to achieve better image quality and visual effect, and the use of LBS can effectively increase the overall brightness of the HUD system under a higher wattage laser light source.
At the same time, the LBS technology can effectively solve the problem of the volume proportion of PGUs in the AR HUD, so it is considered to be the most promising future technology to overcome the problem of too small FOV and achieve a larger FOV with the same volume. LBS is also the best choice for PGUs when facing new imaging technologies such as waveguides. Manufacturers have been experimenting with LBS technology for several years, not only for AR-HUD, but also for projectors and AR glasses. However, the commercialization of AR-HUD is still being explored. There are many representative companies, some of which are not necessarily only for the AR-HUD market, including Longma Puxin, STMicroelectronics, Infineon, Raythink, Microvision, Maradin, Mirrorcle and other companies.
Micro-LEDs are miniaturized LED arrays (1-100 micron range) structured as tiny light-emitting diodes that emit monochromatic light by applying a voltage to them, each of which can be treated as a single pixel. Due to the use of inorganic materials such as GaN and the direct emission of light from LEDs, it has the advantages of long life, wide operating temperature range, fast response time, high brightness, and low energy consumption (theoretically 90% lower than LCD). MicroLED is considered to be the next generation of display technology, but the current maturity needs to be improved, and the process stability and yield such as mass transfer in the preparation process need to be improved.
Innovations in optical modules
Regardless of the image generation method, optical reflection is used and projected onto the windshield, and the optical module is an important factor affecting the volume of the product.
In order to continue to reduce volume, the industry sees optical waveguide technology as a rising star in optical imaging. Optical waveguides themselves are not a new technology, and optical fibers are one of the specific applications. However, more commercial innovation is needed to realize its application on AR-HUD. AR optical waveguide technology is divided into geometric optical waveguide and diffractive optical waveguide, geometric optical waveguide is designed and manufactured based on the traditional principle of geometric optics, geometric optical waveguide is also known as array optical waveguide. Diffractive optical waveguide technology is divided into surface relief grating waveguide and volume holographic grating waveguide. Diffractive optical waveguides use the diffraction effect of light, and mainly use grating structure to modulate the beam. The process principle of the bulk holographic optical waveguide is relatively simple, and the grating structure can be formed by laser interference.
If a raster waveguide is to introduce light from a micro-projection system (optomechanical) into the human eye, it must go through a process of coupling in and out. That is, the light emitted by the optical machine is coupled into the grating, enters the flat plate waveguide, and propagates in it for total reflection, and finally the light is transmitted to the human eye by the coupling grating. Here, the coupling and coupling out gratings are surface embossed gratings. Since the characteristic size of the nanoscale grating is comparable to the wavelength of light, light can no longer be regarded as ordinary light, but as a kind of electromagnetic wave.
In 2020, Digilens announced the CrystalClear AR-HUD that utilizes optical waveguide technology, a display with a maximum field of view (FOV) of up to 15° x 5° and a package of only about 5 liters. Compared to any other best-in-class HUD on the market today, it's less than a third the size of its competitors.
HUD technology from military to civilian
Finally, let's take a look back at the history of HUD. The first HUD was successfully used in the 1960s on the US Navy A-5 carrier-based aircraft. This technology is called a head-up display system in an aircraft, and it is able to project flight parameters, attitude information, navigation information, etc. onto the pilot's field of view directly in front of the pilot. In this way, the pilot can see the instrument parameters and the external visual reference object at the same time without looking down, thus reducing the frequency of looking down at the instrument and improving flight safety.
By 1980, HUD technology began to be used in civil aircraft, and at that time it was mainly used to display navigation information. With the passage of time, this technology has gradually become a standard equipment on civil airliners such as Boeing and Airbus, and the information displayed is becoming more and more abundant, including airspeed, altitude, heading, vertical speed, angle of attack, flight path or speed vector, automatic driving-related instrument operation status display and warning display, etc.
Since then, the technical boundaries of the HUD have continued to expand. In the 80s, General Motors acquired Hughes Aircraft, an aerospace and defense manufacturing company, after which the technology began to appear in cars. In 1988, General Motors applied a HUD to the Oldsmobile Cutlass Supreme Indy 500 Pace Car, the world's first car to feature HUD technology. The original automotive HUD system displayed limited information and a single color, but it was already considered one of the main technological directions of the future. In 1991, Toyota installed a HUD on the Crown Majesta, in 1997, GM installed a color display HUD for the first time on the fifth generation of its "Corvette", and in 2003, BMW became the first car company in Europe to use HUD technology, and Mercedes-Benz and Audi also began to use HUD technology. In 2003, Denso developed a new head-up display system that can display both driving information and infrared images, becoming the world's first head-up display that can display both driving information and night vision imaging information.
In 2012, Pioneer introduced navigation information into in-vehicle HUDs for the first time, developing the world's first in-vehicle navigation system using HUD technology.
In 2020, the Mercedes-Benz S-Class released the AR-HUD. On Mercedes-Benz's AR-HUD system, AR (Augmented Reality) technology overlays navigation information with actual road information, making the display of navigation information more direct and easy to understand. In addition, the AR-HUD combined with the ADAS function can provide real-time road hazard warning and road condition prediction to improve driving safety.
In short, as an advanced automotive display technology, HUD has significant advantages in improving driving safety, enhancing driving experience and enhancing the sense of technology. Although there are still some challenges and limitations, with the continuous advancement of technology and the reduction of costs, as well as the need for driving comfort and convenience, it is believed that the HUD will become a major differentiator in the future of automobiles.