22 February 2018 Iterative learning control algorithm for greatly increased bandwidth and linearity of MEMS mirrors in LiDAR and related imaging applications
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Proceedings Volume 10545, MOEMS and Miniaturized Systems XVII; 1054513 (2018) https://doi.org/10.1117/12.2291428
Event: SPIE OPTO, 2018, San Francisco, California, United States
Abstract
Gimbal-less dual axis point-to-point (quasistatic) MEMS mirrors have very wide bandwidths for laser beam steering, however users are often limited to only a third of the bandwidth due to the high Q-factor and use of low-pass filters in open-loop operation to avoid overshoot and oscillation. Closed-loop driving enables the use of the full bandwidth with additional complexity in optics and electronics, which can be undesirable in some low SWaP-C systems. But for many applications which require scanning repetitive patterns, such as LiDAR and biomedical imaging, bandwidth utilization and linearity can be greatly increased without any real-time feedback control by training the device and finding an optimal driving waveform using an iterative learning algorithm.

The algorithm drives the device with a trial waveform, measures the scan on a Position Sensing Device (PSD), calculates the error between the desired waveform and the measured position, and adjusts the drive waveform for the next iteration based on an approximate linear device model. This is repeated until the error is reduced to below an acceptable specification. The waveform is then saved in the MEMS Controller and can be reliably used for extended periods of operation. Multiple such drive signals can be trained and stored on the controller to perform different types of scans.

Several MEMS mirrors, including single- and dual-axis designs, were studied and three are reported here. Overall, in all cases a high accuracy of optical scans is achieved, typically to within ±0.025° of nominal. Repeatability after training, then running in open loop is better than ±0.01° - however, this measurement was limited by the lower resolution of the position detecting sensor. Scan rates achieved vary based on mirror design, but in each case are greatly improved from those achievable with basic driving approaches. Each mirror demonstrated higher quality vector graphics content at faster refresh rates and stable linear rasters at rates below resonance where lines are scanned with uniform velocity. Additionally, each mirror could achieve stable fast rasters with the line-scanning axis rates just below resonance, giving sinusoidal scans with line rates of ~1.6fres. Finally, each mirror was also demonstrated achieving rasters with rates above resonance, giving line rates of ~2.5fres. In all of those cases the other axis could scan linear and sharp sawtooth or triangle waveforms. Based on the symmetry of the MEMS design, we demonstrated the same performance at ddegerent angles, e.g. rastering at a 45° angle.
Conference Presentation
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Veljko Milanović, Veljko Milanović, Abhishek Kasturi, Abhishek Kasturi, Hong Joo Kim, Hong Joo Kim, Frank Hu, Frank Hu, } "Iterative learning control algorithm for greatly increased bandwidth and linearity of MEMS mirrors in LiDAR and related imaging applications", Proc. SPIE 10545, MOEMS and Miniaturized Systems XVII, 1054513 (22 February 2018); doi: 10.1117/12.2291428; https://doi.org/10.1117/12.2291428
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