MEMS mirrors vs polygon scanners for LIDAR in autonomous vehicles

LIDAR is Laser Radar
LIDAR uses a laser instead of RF energy. The laser is scanned with a mirror instead of an antenna. A polygon scanner offers high speed LIDAR scanning. LIDAR has higher resolution than RF radar. An invisible InfraRed LIDAR beam can penetrate dust & fog.  LIDAR can see in total darkness.

Polygon scanners are popular for LIDAR scanning
Polygon scanners have been used in LIDAR systems for over 30 years. The large aperture, wide scan angle (up to 120 degrees), linear scan speed, and high scan rate of polygon scanners has provided long range and high resolution for airborne, mobile and maritime LIDAR systems.  LIDAR applications include 3D ground and sea mapping from manned and unmanned aircraft. Collision avoidance for airborne and ground vehicles. In military applications, the ability to see through smoke, dust and clouds enhances the ability to identify targets, even in total darkness.

The need for smaller LIDAR scanners
A LIDAR equipped, manned aircraft can map large areas in a short time but it is expensive to fly and hard to justify for scanning a relatively small area like a farmers field. Small “quad-copter” type UAVs have become very popular for aerial photography and surveying over small areas but traditional LIDAR systems, used in large vehicles, are too heavy for small drones. The explosion of interest in autonomous passenger cars and trucks is creating great demand for smaller laser scanners for compact LIDAR systems.

The promise of MEMS mirrors for compact LIDAR scanners
MEMS (micro-electro-mechanical system) mirrors are moving mirrors on a chip. They are very compact. A lot of money has been invested in developing MEMs laser scanners and compact LIDAR systems around them. The expectations were very high for MEMS scanning technology. However, as of 2017, performance has been found to be seriously lacking for use in autonomous vehicles. MEMS scanners taken out of the laboratory and installed in a moving vehicle identified three significant issues. The vehicle movement upset the mirror motion. The wide temperature range affected the scanning. It is difficult to know where the MEMS mirror is pointing!  At speed, a MEMS scanner is a resonant device which means it scans fastest in the middle and slowest at the end of each scan. This means the center of the scan has the lowest resolution, the opposite of what is wanted for LIDAR.  Polygon mirror facets are typically flat to λ/4 @ 633nm. This is excellent for high resolution scanning to hundreds of meters. It is difficult to find a flatness spec on MEMS mirrors. The thin, fragile MEMS mirror flatness has been described as a potato chip! This limits MEMS to very short range scanning. MEMS mirrors are suspended between very fine torsion bars which will abruptly break at the end of their life. Claims that MEMS scanners are solid state are false.

Polygon scanners have none of these problems affecting MEMS scanners. Further, a polygon scanner is not a reciprocating device so it produces a constant velocity scan. A rotating device, like a polygon scanner, is among the most reliable of moving systems.

To meet the demands of the Autonomous Vehicle market, we offer polygon scanners, including the motor and controller as small as a credit card. Ball bearing motors used in polygon scanners have proved, over 30 years, to be robust in mobile applications. Ball bearing scanners are good for applications up to 10K RPM. This has been fast enough for LIDAR scanners till now. Air bearing polygon scanners are good for 10K RPM to over 50K RPM. However, until recently, air bearing polygon scanners were for static applications only. The air bearings would crash if moved while at speed. This limitation has been eliminated with a new generation of “strong” air bearing polygon scanners which are as robust as ball bearing scanners. These 10K to 50K RPM air bearing scanners can produce high definition 3D LIDAR images in moving vehicles.

With MEMS scanners found not to meet the expectations for Autonomous vehicles, LIDAR manufacturers have fallen back on the new generation of compact polygon scanners to fulfill their laser scanning needs. Polygon scanners are proven technology and produce optimum results for LIDAR scanning.

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