Uncrewed Systems Technology 044 l Xer Technolgies X12 and X8 l Lidar sensors l Stan UGV l USVs insight l AUVSI Xponential 2022 l Cobra Aero A99H l Accession Class USV l Connectors I Oceanology International 2022

42 MEMS mirror Rather than using a rotating mirror, a micro-machined MEMS mirror is becoming increasingly popular for modulating the laser beam in the sensor. However, these can struggle with ageing as the switching at megahertz speeds puts a strain on the nanoscale metal used to move the tiny mirrors. The design of the MEMS mirror array also has a key impact on the field of view. Vibration is a key issue as well for sensors using MEMS mirrors. Even tiny vibrations can have an impact on the accuracy of the laser beam at the 200- 300 m distances required for driverless cars. Researchers have evaluated a prototype of a long-range, 1D prototype sensor that can detect a tyre on the road from 130 m. Compared with the other scanning methods, 1D scan Lidar is a hybrid of the flash Lidar and 2D point-scanning Lidar, allowing a high frame rate with fewer laser pulses and less complexity in sensor read-out. However, generating a uniform laser line is not trivial with commercially available laser arrays. High laser power is still required owing to the broadening of the laser into a line along one axis. That limits the detection range since the light sources must comply with Class 1 laser specifications. The MEMS scanning system also has to be highly robust against harsh environmental conditions such as vibrations, shocks and temperature variations. To achieve this stability, an ASIC senses the timing of the MEMS mirror movements with nanosecond accuracy, and secures a stable oscillation by using a digital controller. Experimental results reveal that the MEMS control in the ASIC can successfully suppress the errors caused by vibrations. For the MEMS Lidar without the controller, the applied random vibration distorts the 3D measurements along the scan axis, leading to errors in the 3D surfaces and small, flying particle-like points around the edges of the objects. When the control of the MEMS mirror is turned on, these errors are reduced significantly, allowing reliable 3D object detection and ranging. The MEMS controller reduces the influences of innate vibrations with automotive Lidars and maintains stable 3D images, enabling reliable image processing and object detection. This is the key technique for operation under vibrations and shock. The 1D scanning MEMS Lidar exploits a multi-channel horizontal line laser to scan the scene vertically for a 10 x 11 º horizontal and vertical field of view at a frame rate of up to 29 Hz. This was tested to the LV124 automotive standard, where the vibration tests are performed in three conditions – open loop without control and two phase-locked loops (PLLs) – with default and high-gain settings. The test results demonstrate that vibration can cause wobbly distortion along the scan angle in the open-loop case, and the PLLs can effectively suppress this influence in the mean and standard deviation of the standard point to a surface error of up to 69.3% and 90.0% respectively. This verifies the benefits of the MEMS mirror control, ensuring stable point cloud measurements under vibrations in harsh automotive environments. Encapsulating MEMS mirrors One way to make the MEMS mirrors more robust against environmental issues, including vibration, is to encapsulate them in glass domes that also act as the optics. The shaping of various types of glass has proven to be particularly significant. After all, with the integration of optical functions into the world of microsystems, packaging components and systems at the wafer level is a key challenge. The process allows the fabrication of precise optical components at the wafer level for different applications at low cost. As borosilicate glass for the optics has a similar coefficient of thermal expansion to silicon, it is suitable for anodic bonding, which involves heating and applying an electric field to the substrate materials. Anodic bonding is also called field- assisted bonding or electrostatic sealing. This produces a material compound that remains stable even under large temperature changes. June/July 2022 | Uncrewed Systems Technology A substrate wafer equipped with scanner mirrors, with encapsulation of the components in a glass lid wafer (Courtesy of Fraunhofer Institute)