Novel research focuses on the use of micro scanning mirrors in mobile applications like automotive LiDAR sensors, head-mounted displays or portable micro beamer. Even under normal conditions, micro scanners are exposed to considerable environmental influences. Particularly disturbances such as shock, vibration and temperature fluctuations are relevant for miniaturized scanning systems. In this publication we show the critical environmental parameters for quasi-static micro mirrors with a staggered vertical comb drive intended for high-precision trajectory tracking control. Scanners are controlled based on a piezo-resistive position sensor feedback. Focus will be experimental shock and vibration analysis by exposure to sinusoidal and wide-band random vibration excitation as typical for automotive industry specifications. These are the most demanding requirements compared with other application fields of MEMS mirrors. The on-chip piezo-resistive sensor enables evaluation of the vibration load on the micro scanner, without any optical measurement setup. MEMS mirrors are mounted on a shaker system for characterization and are attached to a vehicle body to evaluate a real application scenario. Furthermore the performance in open-loop and closed-loop control mode is analyzed and shows very good applicability of micro scanners in an automotive environment.
Various scanning applications like LIDAR sensors, OCT systems and laser projectors require a repeated periodic linear scanning trajectory performed by a quasi-static micro mirror. Since most MOEMS systems have inherent nonlinearities like a progressive spring stiffness and the quadratic voltage-deflection-relation of electrostatic drives, a nonlinear control scheme as presented in our previous paper significantly reduces parasitic oscillations of the resonance frequency and enables a high resolution raster scan combining a quasi-static axis with a cardanic mounted resonant axis. In this paper we address a novel control scheme using a flatness-based feedback control enhanced by a plug-in repetitive controller for the linear scanning axis. We demonstrate the applicability of this feedback control for a quasi-static moving micro mirror with electrostatic staggered vertical comb drives using a microcontroller-based driver. On-chip piezoresistive sensors serve as position feedback. We compare different scan trajectories and repetition rates with respect to the linearity and repeatability showing the robustness of the proposed control regime. Furthermore we discuss the advantage of this method to reduce the individual chip characterization for ramping up mass production.
Proc. SPIE. 10116, MOEMS and Miniaturized Systems XVI
KEYWORDS: Mirrors, Sensors, Digital filtering, Error analysis, Amplifiers, Control systems, Linear filtering, Head, Microcontrollers, Micromirrors, Raster graphics, Analog electronics, Feedback control
In this paper we present a 2D raster scanning quasi-static/resonant micro mirror being controlled in both axes in closed-loop with on-chip piezo-resistive sensor feedback. While the resonant axis oscillates with a given frequency, the quasi-static axis allows static as well as dynamic deflection up to its eigenfrequency because of its staggered vertical comb (SVC) drive arrangement. Due to the high quality factor of the very low damped spring-masssystem, an adapted trajectory planning using jerk limitation is applied for the quasi-static axis . Nevertheless, inaccuracies of the applied nonlinear micro mirror model and external disturbances lead to undesired residual oscillation in open-loop control mode. To achieve high precise and fast beam positioning, we implement a flatness-based control algorithm with feedback to on-chip piezo-resistive deflection sensors. In comparison to previous work [2, 3], we developed a micro controller setup for driving the microscanner, that is equipped with an analog Bessel filter increasing the sensor signal quality significantly. In this study we demonstrate a small size and low power micro mirror driver including high-voltage generation and a microcontroller for real-time control as well as a head circuit board for high resolution sensing. We discuss experimental results of open-loop and closed-loop control for 2D raster scanning operation. Finally, the outlook is given to the intrinsic capability to compensate temperature drifts influencing the piezo-resistive sensor signal.
This paper presents the application of a real-time closed-loop control for the quasistatic axis of electrostatic micro scanning mirrors. In comparison to resonantly driven mirrors, the quasistatic comb drive allows arbitrary motion profiles with frequencies up to its eigenfrequency. A current mirror setup at Fraunhofer IPMS is manufactured with a staggered vertical comb (SVC) drive and equipped with an integrated piezo-resistive deflection sensor, which can potentially be used as position feedback sensor. The control design is accomplished based on a nonlinear mechatronic system model and the preliminary parameter characterization. In previous papers [1, 2] we have shown that jerk-limited trajectories, calculated offline, provide a suitable method for parametric trajectory design, taking into account physical limitations given by the electrostatic comb and thus decreasing the dynamic requirements. The open-loop control shows in general unfavorable residual eigenfrequency oscillations leading to considerable tracking errors for desired triangle trajectories . With real-time closed-loop control, implemented on a dSPACE system using an optical feedback, we can significantly reduce these errors and stabilize the mirror motion against external disturbances. In this paper we compare linear and different nonlinear closed-loop control strategies as well as two observer variants for state estimation. Finally, we evaluate the simulation and experimental results in terms of steady state accuracy and the concept feasibility for a low-cost realization.
Two new technological process flows for the piezoresistive position detection of resonant and quasistatic micro scanning mirrors were developed to increase sensitivities by a factor of 3:6 compared to former sensors, improve signal to noise ratio of the sensor signal and to allow controlled feedback loop operation. The sensor types use differently doped and deposited silicon. One is based on single crystal silicon with a pn-junction to isolate the active sensor area from the device layer silicon, the other one is based on a deposited and structured polysilicon. The sensor characteristics are compared including light, temperature dependence and reliability results.
This paper presents a gimbaled MEMS scanning mirror (MSM) especially developed for adaptive raster scanning in a
novel 3D ToF laser camera. Large quasi-static deflections of ±10° are provided by vertical comb drives in vertical
direction in contrast to resonant horizontal scanning of the 2.6x3.6mm elliptical mirror at 1600 Hz and 80° optical scan
range. For position feedback piezo-resistive position sensors are integrated on chip for both axes. To guarantee the full
reception aperture of effective 5 mm a synchronized driven MEMS scanner array - consisting of five hybrid assembled
MEMS devices - are used in a novel 3D ToF laser scanner enabling a distance measuring rate of 1MVoxel/s and an
uncertainty of ToF distance measurement of 3…5 mm at 7.5 m measuring range for a gray target.
This paper reports on a gimbaled MEMS scanning mirror with quasistatic resonant actuation, specially developed for adaptive raster scanning in an innovative three-dimensional (3-D) time-of-flight (ToF) laser camera with real-time foveation. Large quasistatic deflections of ±10 deg are provided by vertical comb drives in the vertical direction in contrast to resonant horizontal scanning. This mirror is 2.6×3.6 mm and operates at 1600 Hz with an 80-deg optical scan range. For position feedback, piezo-resistive position sensors are integrated on chip for both axes. To guarantee the full reception aperture of effectively 5 mm, a synchronized driven MEMS scanner array—consisting of five hybrid assembled MEMS devices—is used in an innovative 3-D ToF laser scanner. This enables a distance measuring rate of 1 MVoxel/s with an uncertainty in distance measurement of 3 to 5 mm for a 7.5-m measuring range for a gray target. Flatness-based open loop control is used for driving control of quasistatic axis in order to compensate for the dynamics of the low damped MEMS system.