The target ranging is based on the phase-shift laser ranging method. The phase difference between the modulated signal and the reflected signal contains target range information. By amplification, mixing, band-pass and sampling, the phase difference is acquired by the phase meter [14], and the target range D is represented by Equation (3).
(3)
where φA is the phase of the modulated signal, φB is the phase of the reflected signal, c is the velocity of light and f is the modulated frequency of the modulator.
Therefore, based on the scan orientation and laser ranging method, the target is located in the three-dimensional space by the combination of the measurement results ψB, ψT and D.
Figure 13 shows the prototype of the MOEMS target detector. Modulated, emitted, scanning and received parts are integrated into a compact package, giving a size of 90 mm × 35mm × 50 mm. The two-dimensional scanning micro-mirror is located behind the front window from which the scanning beam and reflected beam are emitted and received. The photosensor is integrated in a PCB, which is located behind the spectroscope. The power and data signals are imported and exported through the back window of the MOEMS target detector.
Prototype of MOEMS target detector.
The experimental system is composed of MOEMS target detector, turn table, guide track and target, which are set on the optical vibration isolation platform as shown in Figure 14. The relative azimuth angles ψBand ψT of the target are varied in the scope of ±10° in the horizontal and vertical directions, by turning the turn table. The relative range D of the target is varied in the range of 0 to 3 m, by moving the target in the guide track.
Experimental system for MOEMS target detector.
The measurement results of relative orientation in the horizontal and vertical directions are shown in Figure 15 and Figure 16, respectively. The results indicate that the MOEMS target detector can receive the reflected beam from the target precisely and measure the orientation accordingly. In the scope of ±10°, the measured orientations are consistent with the actual orientations, which can verify the design and principle of the orientation measurement. The measurement precisions are 0.15° in the horizontal direction and 0.22° in the vertical direction. The orientation errors are mainly due to the measurement errors of the deflection angles for their correspondences. In the process of piezoresistors, the inaccuracy of lithography, exposure and diffusion lead to the inconsistent piezoresistors and low stabilities in the Wheatstone bridges.
Orientation measurement in horizontal direction.
Orientation measurement in vertical direction.
According to Equation (3), the maximum measurement range is limited to 75 m with f = 2 MHz. The actual measurement range in the experiment is 3 m (14.4deg phase shift). The measurement results of the relative range are shown in Figure 17. The results indicate that the MOEMS target detector can realize the measurement of the relative range by the scanning of the micro-mirror. In the range of 0 to 3 m, the measured ranges are consistent with the actual ranges, which can verify the design and principle of the range measurement. The range error is 10.2 cm, which is mainly due to the signals conversion and sampling in the signal processing module of the detector.
Relative range measurement.
Setting the target on some example locations and combining the measurement results of orientation and range, the actual location and the measured location are contrasted in Table 1. The experiment results indicate that the MOEMS target detector based on the two-dimensional scanning micro-mirror can measure the orientation and range of the target simultaneously and the target location can be exactly achieved by the scanning measurement method.
Target location measurements.
| Target location | Horizontal orientation/ Vertical orientation/ Relative range | |
|---|---|---|
| Actual location | Measured location | |
| P1 | ?3° / 2° / 0.4 m | ?3.11° / 1.82° / 0.43 m |
| P2 | 0° / 0° / 0.8 m | 0.13° / 0.19° / 0.87 m |
| P3 | 2° / 5° / 1.2 m | 2.00° / 4.84° / 1.37 m |
| P4 | 5° / 0° / 1.6 m | 4.89° / 0.19° / 1.65 m |
To replace the conventional scanning detector with optical MEMS technology, a two-dimensional scanning micro-mirror has been developed in this paper. The micro-mirror has the capabilities of regional scanning in coupled vibration modes and deflection angles measurement by the piezoresistors. The structure, piezoresistors, fabrication and characteristics of the micro-mirror are detailed. Based on the two-dimensional scanning micro-mirror and the phase-shift ranging technology, a MOEMS target detector has been developed in the size of 90 mm × 35 mm × 50 mm. The design and measurement principle are described and the experiment results show that the target can be detected in the scanning field and the relative range and orientation can be measured by the MOEMS target detector. For the target distance up to 3 m with a field of view about 20° × 20°, the measurement resolution is about 10.2 cm for range while 0.15° in the horizontal direction and 0.22° in the vertical direction for orientation. The MOEMS target detector, based on the two-dimensional scanning micro-mirror, has the great advantages of small volume, high frequency, large deflection angles and high measurement sensitivities. It is suitable for target location and has a wide foreground in the field of space detection and target identification in micro-satellites.
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