The two resonance frequencies of the two-dimensional scanning micro-mirror are 216.8 Hz and 464.8 Hz, respectively, which are measured by the frequency sweeping method and a laser interferometer measurement system. The relationships between each deflection angle and the actuation displacement in the resonance modes are shown in Figure 8.
Deflection angles characteristics in two directions. (a) deflection angle θT in twisting direction, (b) deflection angle θB in bending direction.
The experimental results indicate that the deflection angles become larger as the actuation displacement increases. By an actuation displacement of about 10 μm, the deflection angles twisting along the y-axis and bending on the x-axis are 13.3° × 11.8°. By reflecting the optical beam, the scanning field of the two-dimensional scanning micro-mirror is above 26° × 23°. The scan patterns of the twisting by y-axis, bending by x-axis and two-dimensional scan are shown in Figure 9.
Scan patterns of two-dimensional scanning micro-mirror. (a) Twisting by y-axis, (b) Bending by x-axis, (c)Two-dimensional scan.
The relationships between the corresponding piezoresistor output voltage and each deflection angle in the resonance modes are shown in Figure 10. There are linear relationships between each output voltage and each deflection angle. The deflection angles measurement sensitivities for two directions are 59 mV/deg and 30 mV/deg, respectively.
Piezoresistor output characteristics in two directions. (a) deflection angle measurement in twisting direction, (b) deflection angle measurement in bending direction.
Based on the two-dimensional scanning micro-mirror, the MOEMS target detector has the ability of target detection and location measurement, which is mainly composed of a laser diode, a modulator, a two-dimensional scanning micro-mirror, a beam receiver and a signal processing module as shown in Figure 11. In the MOEMS target detector, the CW laser beam is collimated and emitted from the laser diode by the modulator. The emitted beam is reflected by the two-dimensional scanning micro-mirror for a regional 2D scanning. The beam reflected from the target is received by the beam receiver and converted to the reflected signal. With the contrast between the modulated signal and the reflected signal, the relative range of the target is calculated by the phase-shift ranging method [13]. With the capture time of the reflected signal and the real-time measured deflection angles of micro-mirror, the relative orientation of the target is calculated accordingly. Therefore, the target can be located by the MOEMS target detector.
Structure of MOEMS target detector.
In order to realize the coaxial beam path in the MOEMS target detector, the optical configuration of the system is shown in Figure 12. The emitted beam from the laser diode passes through the diaphragm and the spectroscope. The through part is reflected by the micro-mirror and scanned two-dimensionally. The return beam reflected back from the target passes through the same beam path to the spectroscope and the reflected part is detected by the photosensor.
Beam path of MOEMS target detector.
When the micro-mirror is scanning two-dimensionally, the deflection angles in the two directions are measured by the two Wheatstone bridges with the decoupling measurement method. The real-time measurement results can describe the relationship between the deflection angles and the time [14]. When the reflected beam is received by the photosensor, the optical signal is converted into the reflected signal, and the relative orientation of the target can be calculated by Equation (2).
(ψB, ψT) = 2?[θB(t), θT(t)]
(2)
where ψB and ψT are the two-dimensional azimuth angles of the target, θB(t) and θT(t) are the real-time deflection angles of the micro-mirror in two directions, and t is the capture time of the reflected signal.