Research on the optimization algorithm of FMCW laser ranging based on DSP

ObjectiveFrequency Modulated Continuous Wave (FMCW) laser ranging is an absolute range measurement technology based on coherent heterodyne detection. This technology has the advantages of high resolution, strong resistance to ambient light interference, no need for cooperative targets, and high Signal-to-Noise Ratio (SNR). It is especially good at weak signal detection, and is widely used in automatic driving, atmospheric science, industrial precision measurement and other fields. Compared with Time of Flight (TOF), FMCW ranging transmits frequency modulated continuous wave to the target, and calculates the range information of the target by detecting the frequency difference between the transmitted signal and the echo signal, and the frequency detection accuracy directly affects the ranging accuracy. Therefore, this technology has more complex signal processing operations, so it is of great academic significance and application value to improve its signal processing algorithm to reduce the amount of calculation.MethodsThe traditional signal processing flow first requires the Analog to Digital Converter (ADC) module to sample the beat signal until the requirement of sampling points is met, at which time the DSP is in the waiting state. In Dua Tone Multi-Frequency (DTMF) signal decoding, the Goertzel algorithm can detect the amplitude at a specific frequency. At the same time, the Goertzel algorithm supports Digital Signal Processor (DSP) sampling and calculation, which makes full use of the performance of DSP. The Goertzel algorithm is used to calculate the spectrum of the frequency index that satisfies the distance range, so as to avoid calculating the spectrum in the whole frequency range and save the waiting time. In addition, the use of Fast Fourier Transform (FFT) frequency discrimination will appear fence effect, resulting in a large error in the beat frequency rate, so the beat frequency rate needs to be corrected. Aiming at the repeated FFT in the process of all-phase FFT spectrum correction, this paper improves the algorithm: the phase value is calculated at the fixed frequency index instead of the full spectrum range, which greatly reduces the amount of calculation. Finally, the algorithm is modeled and simulated.Results and DiscussionsBy simulating the time required for frequency discrimination within a specific frequency range using the Goertzel algorithm and FFT algorithm, the results (Tab.2) show that the Goertzel algorithm is 23.34% faster than the FFT algorithm. In addition, modeling was conducted for the Goertzel and all-phase FFT algorithms (Fig.2-Fig.4), and the simulation structure of FMCW ranging system is built based on the above model (Fig.5). After the simulation is correct, the algorithm is realized on TMS320F28377 platform by using Model-Based Design (MBD) design method (Fig.7). The FMCW ranging device was used to measure the distance of large-size targets. The results (Tab.3) show that the FMCW ranging device using the proposed algorithm can achieve ranging in the range of 1.00-9.00 m, and its maximum root mean square (Fig.8) is less than 0.01m. Three different materials were selected to measure the ranging of small targets with edges and angles. The experimental results (Tab.4-Tab.6) show that the repeatability accuracy of the system is high but the ranging accuracy is poor when the ranging of small targets with 60° is 2.00 m, and the ranging error of the small target with 60° is much larger than that of other angular targets. The root mean square error of the aluminum target under the three tilt angles is larger than the other two cases.ConclusionsA FMCW laser ranging system with 1.00-9.00 m range ranging is designed, this article starts with the principle of signal processing algorithm, and finally realizes the algorithm on DSP. Subsequently, the simulation and ranging of 1.00-9.00 m range targets were carried out. The experimental results show that the simulation results and the ranging results can be well fitted, and the algorithm can effectively achieve 1.00-9.00 m range ranging.