Fig. 1. Schematic of SPMI detecting microvibration on solid surface

Fig. 2. DCM algorithm flowchart

Fig. 3. Waveforms of functions Sum1(C_n) and Sum2(C_n)

Fig. 4. Principle of phase modulation depth estimation algorithm

Fig. 5. Carrier phase delay estimation algorithm

Fig. 6. Improved PGC-DCM demodulation algorithm

Fig. 7. Distributions in time and frequency domains of simulated interference signal. (a) Distribution in time domain; (b) distribution in frequency domain

Fig. 8. Lissajous figures. (a) Before normalization；(b) after normalization

Fig. 9. Waveforms of demodulation result before filtering. (a) Distribution in time domain; (b) distribution in frequency domain

Fig. 10. Time domains of demodulation results after filtering. (a) Demodulation result of traditional PGC-DCM algorithm; (b) demodulation result of improved PGC-DCM algorithm

Fig. 11. Schematic of experimental system

Fig. 12. Experimental platform

Fig. 13. Interference detection signal of 500 Hz microvibration. (a) Time domain waveform; (b) frequency domain distribution

Fig. 14. Lissajous figures of orthogonal signal pair. (a) Before normalization; (b) after normalization

Fig. 15. Comparison of demodulation results. (a) Demodulation result of traditional PGC-DCM algorithm; (b) demodulation result of improved PGC-DCM algorithm; (c) spectrum of demodulation result of improved PGC-DCM algorithm

Fig. 16. Dynamic range of demodulation system. (a) Spectrum distribution of background noise; (b) measurable dynamic range of microvibration under different frequencies

Fig. 17. Time domains comparison between demodulation results of amplitude modulation microvibration signal and excitation signal. (a) Time domain of excitation signal after power amplification; (b) measured interference signal; (c) time domain of phase demodulation result

Fig. 18. Frequency domains comparison between demodulation results of amplitude modulation microvibration signal and excitation signal. (a) Spectrum of excitation signal; (b) spectrum of demodulation signal

Fig. 19. Demodulation results of frequency modulation microvibration signal. (a) Time domain diagram of frequency modulation microvibration signal demodulation result; (b) short-time Fourier transform result