Detecting small or fast-moving objects – that is where lidar excels. The laser-pulse-based technology is increasingly being seen as a valuable addition to radar and camera systems.
Various sensor technologies are being used in autonomous vehicles in order to capture as broad an image of the surrounding area as possible, while minimising the risk of error. Whereas radar technology is based on high-frequency electromagnetic vibrations, lidar – as it is termed – works with laser beams to determine the distance of objects within close range of the vehicle. Lidar is primarily used to detect smaller objects on roads.
However, roads are not this sensor technology’s only area of application: “Everyone is talking about autonomous vehicles, but autonomous systems are also trending in the industry,” emphasises Dr Dirk Rothweiler, CEO at Berlin-based company First Sensor. The company produces what are known as avalanche photodiodes, which detect laser reflections in lidar systems. Lidar technology is already a technical standard in a whole host of industrial applications, as Rothweiler explains: “When it comes to industrial length measurement, our customers have been relying on lidar for years – to measure speed or to monitor safety areas for machines, for example. A growing field of application is autonomous systems. These are more readily implemented in industrial settings where surroundings and influencing factors are controllable.” Here, lidar systems make sure that mobile robots do not collide with the people working in production systems or that autonomous transport and logistic systems can take on complex tasks such as the loading of pallets, for example.
Laser diodes as a basis
Lidar stands for “light detection and ranging”. Its basic principle is time-of-flight measurement: a very brief laser pulse is emitted, makes contact with an object, is reflected, and then recorded by a detector. The laser beam’s travel time gives the distance of the object. Incredibly high-performance infrared pulse laser diodes with a short switching time form the technical basis of lidar systems. With an optical pulse power of approximately 25 watts and a spectral range of 905 nanometres, their pulses go virtually unnoticed by humans. They are completely safe for the human eye.
Scanning lidar systems use a laser beam to scan the area around the vehicle horizontally above a certain angular segment and create a high-resolution 3D map of the surroundings. Initially, the deflection of the laser beams in scanning lidar systems was implemented with mechanically driven mirrors. However, miniature laser scanning modules with integrated silicon-based MEMS micro-mirrors – which have appeared on the market in recent times – are more cost-effective, compact and robust. The micro-mirrors combine and fine-tune the laser beams. Up-to-date systems with solid-state lasers achieve ranges of more than 200 metres and a high resolution of less than 0.1 degrees.
An accompaniment to other sensor systems
The major advantage offered by lidar over other sensor systems is its extensive data acquisition: lidar collects more information from individual data points than any other system – x, y and z coordinates, period and degree of reflection to name but a few. Reflective surfaces such as road signs or even road markings provide a stronger signal, allowing lidar to detect such surfaces faster. A camera would also be capable of this. It could, however, be dazzled by backlight or sunlight and can only be used to a limited extent at night-time or in conditions of poor visibility. However, lidar systems also only deliver limited data during torrential rain. This is where radar impresses, as it is capable of “seeing through” objects. Yet to do this, radar systems do not possess the high resolution required to register objects when they are small or great in number and while moving at high speeds. Lidar, on the other hand, can also clearly distinguish small objects.
“Even the most aggressive start-ups do not expect lidar to be the silver bullet for the detection and perception of autonomous vehicles,” says James Hodgson, Senior Analyst at ABI Research. “Still, the natural characteristics of this technology are very well suited to that of radar and cameras, which until now have constituted core elements in obstacle detection.” In his view, however, the high costs in particular still prevent their wider application. Yet it is to be noted that the systems are becoming ever more affordable: Dutch company Innoluce, which is now part of Infineon, is planning to bring automotive lidar systems to the market for less than 100 dollars in the future.