# Investigation of Electromagnetic Angle Sensor Integrated in FR4-Based Scanning Micromirror
## 3 Experimental Results and Discussion
Figure 3: **(a)** Schematic drawing and **(b)** photograph of the measurement setup. The frequency spectrum from 0 to 800 Hz (surrounding the 361.8 Hz) was measured to identify the signal-to-noise ratio (SNR) of the angle sensor. In this test, the sensor signal is recorded in the oscilloscope after going through the aforementioned amplifier and a fourth-order bandpass filter. Then, the frequency spectrum can be obtained through a fast Fourier transform of the time-domain signal. Figure 6a,b show the measured frequency spectrum at the optical scan angle of 4.6\({}^{\circ}\), and the corresponding angle sensor output in time domain, respectively. This signal shows a great signal quality of 71.9 dB SNR, which is beneficial for feedback control. Figure 7 shows the frequency characteristics of the angle sensor compared to the optical scan angle obtained from PSD measurement. In the measurement, the micromirror is actuated by the sinusoidal sweep signal from 348 Hz to 378 Hz. The frequency response functions of the micromirror are measured by the angle sensor and PSD, respectively. We can see that two frequency response functions have almost the same overall profile, which means that the angle sensor can still work, even though the micromirror is off the resonance. In addition, the phase curve in Figure 7 shows that the phase of the sensor output voltage will change when the micromirror oscillates off the resonance. Therefore, the resonance can be tracked by monitoring the phase difference between the sensor output and driving signals, i.e., a phase-lock loop [21]. Figure 5: (a) Relation between the coupling voltage in the sensing coil and the driving voltage. (b) Relation between the sensor output and the optical scan angle. Note: all the voltage values are measured as peak value. Figure 6: (a) Frequency spectrum of the sensor output at the optical scan angle of 4.6\({}^{\circ}\). (b) The corresponding signal in time domain. Based on the great angle sensor signal quality, a simple closed-loop control circuit was developed to further verify the sensor's good feasibility for feedback control. Figure 8a demonstrates the closed-loop control scheme. The micromirror is actuated to oscillate around the torsion bars by the sinusoidal driving signal. Meanwhile, the generated sinusoidal angle sensor signal, depending on the deflection angle of the micromirror, is detected by the angle sensor signal acquisition module. After going through the rectifier-filter module, the signal is converted to a DC voltage signal. To stabilize the deflection angle, the amplitude gain control module was introduced. The signal of the voltage difference between the DC voltage, representing the deflection angle and the reference voltage signal, is input to the module. Then, the amplitude gain control is mainly accomplished by multiplying the voltage difference with the sinusoidal signal generated by signal generation module. Therefore, the amplitude of the driving signal can be regulated in real time according to the feedback signal of the integrated angle sensor. Long-term stability tests on the micromirror were operated continuously for 12 h, both with and without closed-loop control. Figure 8b shows the test result. The maximum variations of the optical scan angle are 3.04% and 7.83%, respectively. Hence, the micromirror operated with closed-loop control possesses better long-term stability. This result indicates the angle sensor's good feasibility for feedback control. Finally, temperature stabilities of the micromirror, operated both with and without closed-loop control, were also tested. In this test, the micromirror is placed in a high and low temperature test chamber. The temperature is increased from 20 to 50 \({}^{\circ}\)C in increments of 5 \({}^{\circ}\)C under atmospheric pressure. The test result is shown in Figure 9. With the change of temperature, the resonant frequency of the device varies, and the micromirror will work off the resonance [21; 25]. Hence, we can see that the optical scan angle decreases with the increase of temperature.