Study on a Two-Dimensional Scanning Micro-Mirror and Its Application in a MOEMS Target Detector
Chi Zhang,* Zheng You, Hu Huang, and Guanhua Li
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With the rapid development of the micro-optical-electro-mechanical systems (MOEMS) technology, the micromation of the payloads in micro-satellites has become a general trend [1]. For optical scanning and target detection, the laser scanning technique is an active way to detect objects and measure both range and orientation [2]. The two-dimensional scanning micro-mirror has great advantages over the conventional scanning mechanisms, such as low power consumption, small volume and high frequency. It has a broad range of space applications for target detection in micro-satellites. At present, most of scanning micro-mirrors have been driven by electrostatic, electromagnetic or piezoelectric force. They have a spring structure and are operated on a resonant mode for high-speed scan operation with large deflection [3–7]. For target detection and location, the scanning micro-mirror requires the measurement of deflection angles with high sensitivities, which most of the current researches have not involved. A LIDAR (LIght Detection And Ranging) system with a magnetic mirror and a micro shutter array is adopted for planetary explorer [2], but the mechanism is complicated and the scanning field is narrow. A laser range finder coupled with two silicon micro-mirrors are used in the compact robotics perception system [8], but adopting a PSD sensor for position detection decreases the integration of the system. A MEMS electromagnetic optical scanner for a laser scanning microscope integrated sensing coil for deflection angle measurement [9], but the scanner is one-dimensional and the induced electromotive force is low with limited sensitivity.
With the aim of improving these deficiencies, a two-dimensional scanning micro-mirror with piezoresistor sensors for measurement of deflection angles is developed in this paper. It has a simple structure and a small volume, with a large scanning field and high sensitivities. Based on the scanning micro-mirror and the phase-shift ranging technology, a MOEMS target detector has also been developed. In the last section, the performance of the prototype and experimental results will be described in detail.
A two-dimensional scanning micro-mirror with a piezoelectric actuator and piezoresistors was designed as shown in Figure 1. The micro-mirror structure consists of a reflector, an inertia generator, a flexible beam and an excited part, which is 8 mm × 8 mm × 0.2 mm in size. The reflector and inertia generator are formed together, and linked with the excited part by the flexible beam. The excited part is connected to the piezoelectric actuator. The piezoresistors are integrated on the surface of the flexible beam for deflection angles measurement [10].
Structure of the two-dimensional scanning micro-mirror.
The piezoelectric actuator deforms along the z-axis by pulsant driving voltage and the excited part vibrates in the z-axis. As the center of gravity of the reflector and inertia generator is away from each rotational axis (x and y), the micro-mirror has two resonance vibration modes: twisting around the y-axis and bending around the x-axis, as shown in Figure 2. The two-dimensional micro-mirror is thus equivalent to a two dimensional vibration system with two different resonant frequencies. Actuating the micro-mirror at each resonant frequency can make the mirror vibrate with large deflection angles θT and θB around the y-axis and x-axis, respectively. When a resultant voltage including two different resonant frequencies is imposed to the piezoelectric actuator, both vibration modes are excited and the micro-mirror is capable of scanning a light beam two-dimensionally with large scanning angles with a single driving source.
Two resonance vibration modes. (a) twisting around the y-axis, (b) bending around the x-axis.
Deflection angle sensing is based on the piezoresistive effect, which has the advantages of favorable dynamic characteristics and high sensitivities. The surface stresses generated on the flexible beam when the micro-mirror is twisting or bending and piezoresistors are laid on the flexible beam for the deflection angles measurement of two directions. The change of the resistance in piezoresistor is related to the stresses and the piezoresistive coefficients in longitudinal, transverse and tangential directions. The piezoresistive effect in plane can be described as follows [11]:
(1)
where σl is the longitudinal stress, σt is the transverse stress and στ is the tangential stress. πl is the longitudinal coefficient, πt is the transverse coefficient and πτ is the tangential coefficient.