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70 Focus | Lidar some problems for driverless vehicles. The mirrors can stick, which can leave a mirror ‘on’ and cause a single laser beam to shine out at an unpredictable angle. That can present a potential safety risk if a beam at a particular frequency hits a pedestrian’s eyes. There is also a question mark over their use in fully autonomous vehicles above L4. Some can show a mean time between failure of 1000 to 2000 hours, which does not meet automotive reliability standards. However, the impetus for using micro- mirrors is coming from their use in the new generation of intelligent headlights. These use a micro-mirror array to direct an LED beam with tighter control than traditional bulbs and lenses. This has improved the reliability of micro-mirror arrays from automotive component suppliers, who are experienced in the needs of car makers. That means the Lidar laser and processing can be added alongside the LED headlight without adding an extra unit. The headlight supplier then manages the supply of components, simplifying the supply chain for the car maker without having to change the design of a vehicle. One point to note here though is that each laser burst heats up the components in the sensor, so compensation and cooling are often needed, adding cost and weight and limiting the lifetime of the system. That is already addressed in systems that use high-power LED headlights. However, this is possibly only an interim issue, as fully autonomous cars may not need headlights, and forward-facing lights will be used more as a warning signal for pedestrians. As the mirrors are reflective, there are also challenges when the sun is at an angle that enters the laser’s light path. This can swamp the lasers reflecting off the mirrors and reduce the system’s detection capabilities. The mirror array also has to be accurately aligned with the bore of the sensor, which can be costly in manufacturing. However the micro-mirror does allows the cost of Lidar sensors to be reduced to a few hundred dollars Hybrid micro-mirrors Another approach is to combine a micro- mirror with a highly sensitive camera sensor that can operate in low light. This allows a ‘random access’ approach rather than a full scan. The camera sensor can detect edges of objects using the appropriate algorithms, and the laser can then be queued up to investigate a particular area of interest with a localised scan. This dramatically reduces the amount of February/March 2018 | Unmanned Systems Technology How a micro-mirror is used to provide scanning for a Lidar (Courtesy of Innoluce/Infineon) A micro-machined mirror used for intelligent headlights (Courtesy of Bosch)