Patent Publication Number: US-2020292688-A1

Title: Detection method by using a fmcw radar

Description:
FIELD OF THE INVENTION 
     This invention generally relates to a frequency-modulated continuous wave (FMCW) radar, and more particularly to a detection method by using the FMCW radar. 
     BACKGROUND OF THE INVENTION 
     Conventional FMCW radar can be utilized to detect object by transmitting a chirp signal to the object and receiving a reflected signal from the object. The chirp signal transmitted by the FMCW radar changes in frequency over time, thus the reflected signal reflected from the object also changes in frequency over time. The distance between the conventional FMCW radar and the object is estimated depending on the frequency difference between the chirp signal and the reflected signal at the same time. The conventional FMCW radar is employed in detection of distance and migration velocity widely because of small size, precise detection for short distance, and so on. 
     SUMMARY 
     The object of the present invention is to provide a method to detect object having tiny vibrations within a detected area by using a FMCW radar. 
     A detection method of the present invention includes following steps: obtaining a detection signal by using a FMCW radar, the FMCW radar is configured to transmit a frequency-modulated transmitted signal to an area where an object is located within, and receive a reflected signal as the detection signal from the area; dividing the detection signal into a plurality of short-time detection segments by using a processor, the detection signal is received by the processor from the FMCW radar; analyzing spectrum characteristics of the short-time segments and reconfiguring the short-time detection segments having the same frequency into a plurality of detection sub-signals by using the processor, wherein each of the detection sub-signals corresponds to a detection distance; and calculating peak-to-average ratios of the detection sub-signals by using the processor, wherein the processor is configured to define the detection distance corresponding to one of the detection sub-signals as a distance between the object and the FMCW radar according to the peak-to-average ratios. 
     In the present invention, the processor is adapted to process the detection signal received by the FMCW radar to obtain the detection sub-signals used to represent vibration levels at each of the detection distances, and calculate the distance between the object and the FMCW radar according to the PAR of each of the detection sub-signals. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart illustrating a detection method by using a FMC V radar in accordance with one embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating the FMCW radar and a processor in accordance with one embodiment of the present invention. 
         FIG. 3  is a circuit diagram illustrating the FMCW radar in accordance with one embodiment of the present invention. 
         FIG. 4  is a diagram illustrating how to divide a detection signal into short-time detection signals and how to reconfigure the short-time detection signals into detection sub-signals in accordance with one embodiment of the present invention. 
         FIG. 5  is a waveform diagram of a frequency-modulated transmitted signal and a reflected signal in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a flowchart of a detection method  10  in accordance with one embodiment of the present invention. The detection method  10  includes a step  11  of obtaining detection signal by using FMCW radar, a step  12  of dividing detection signal into short-time detection segments, a step  13  of reconfiguring short-time detection segments into detection sub-signals and a step  14  of calculating PAR of detection sub-signals. 
     With reference to  FIGS. 1 and 2 , a FMCW radar  110  in the step  11  is configured to transmit a frequency-modulated transmitted signal S T  to an area A, where an object O having tiny vibrations is located within The object O may be a life with vital signs or a machine having fixed vibration frequency. When the frequency-modulated transmitted signal S T  is sent to the object O within the area A, the object O reflects a reflected signal S R  back to the FMCW radar  110 , then the FMCW radar  110  receives the reflected signal S R  as a detection signal S d .  FIG. 5  represents the frequency variations of the frequency-modulated transmitted signal S T  and the reflected signal S R  with time. In this embodiment, the frequency-modulated transmitted signal S T  has a frequency increased linearly with time during a detection period, so that the reflected signal S R  also has a frequency increased linearly with time. 
     With reference to  FIG. 2 , the object O has a motion relative to the FMCW radar  110  because of tiny vibrations. The relative movement generates the Doppler Effect in the frequency-modulated transmitted signal S T , thus the reflected signal S R  contains the Doppler shift components caused by the relative movement. 
       FIG. 3  is a circuit diagram of the FMCW radar  110  of this embodiment. The FMCW radar  110  includes a FM signal generator  111 , a power splitter  112 , a transmitting antenna  113 , a receiving antenna  114  and a mixer  11 . 5 . The FM signal generator  111  is configured to output a frequency-modulated signal S FM  having a frequency changed with time. The power splitter  112  is electrically connected to the FM signal generator  111  and configured to divide the frequency-modulated signal S FM  into two paths. The power splitter  112  is, but not limited to, a Wilkinson power splitter. The transmitting antenna  113  is electrically connected to the power splitter  112  and configured to receive and transmit the frequency-modulated signal S FM  from one path as the frequency-modulated transmitted signal S T  to the area A. The receiving antenna  114  is configured to receive the reflected signal S R  as a received signal S r  from the object O. The mixer  115  is electrically connected to the power splitter  112  and the receiving antenna  114 , thus the mixer  115  can receive the frequency-modulated signal S FM  of the other path from the power splitter  112  and receive the received signal S r  from the receiving antenna  114 . Further, the mixer  115  is configured to mix the frequency-modulated signal S FM  and the received signal S r  to output the detection signal S d . In this embodiment, the frequency of the received signal S r  subtracted from the frequency of the frequency-modulated signal S FM  equals the frequency of the detection signal S d  from the mixer  115 . 
     With reference to  FIGS. 1 and 2 , a processor  120  is configured to receive the detection signal S d  from the FMCW radar  110  and partition the detection signal S d  into a plurality of short-time detection segments in the step  12 . The processor  120  includes a central processing unit  121  and a storage unit  122  in this embodiment. The storage unit  122  is electrically connected to the FMCW radar  110  for receiving and storing the detection signal S d . The central processing unit  121  is electrically connected to the storage unit  122  to receive the detection signal S d . The detection signal S d  is partitioned into the short-time detection segments by the central processing unit  121 . With reference to  FIG. 4 , the top one is the detection signal S d  and the blocks separated by dotted lines are the short-time detection segments. The durations T 1 , T 2 . . . and T n  of the short-time detection segments are all the same and equal to the frequency periodicity of the frequency-modulated signal S FM . 
     With reference to  FIGS. 1, 2 and 4 , the central processing unit  121  of the processor  120  is configured to analyze spectrum characteristics of the short-time detection segments and reconfigure the short-time segments having the same frequency into a plurality of detection sub-signals in the step  13 . In this embodiment, the central processing unit  121  is configured to convert the short-time detection segments from time domain to frequency domain using a Fast Fourier Transform (FFT), and then reconfigure the short-time detection segments having the same frequency into one of the detection sub-signals. Consequently, the amplitude variation of the short-time detection segments having the same frequency can be identified in each of the reconfigured detection sub-signals. In the  FIG. 4 , A 1,1 , A 1,N  of the first column represent the amplitude levels of 1 st  to N th  frequencies of the first short-time detection segment, respectively, and in the same way, A n,1 , A n,2  . . . and A n,N  of the N th  column represent the amplitude levels of 1 st  to N th  frequencies of the n th  short-time detection segment, respectively. Each rows represents one of the detection sub-signals reconfigured from the short-time detection segments having the same frequency. The first row is the first detection sub-signal reconfigured from the segments having the 1 st  frequency, the second row is the second sub-signal reconfigured from the segments having the 2 nd  frequency, and so on. Each of the detection sub-signals can be used to identify the amplitude value of the relative movement due to the detection signal S d  contains the Doppler shift components caused by the relative movement. 
     Furthermore, each of the detection sub-signals having a single frequency corresponds to a detection distance due to the relative movement is detected by the FMCW radar  110  in this embodiment and the frequency of the detection signal S d  output from the mixer  115  is the difference of the frequency of the frequency-modulated signal S FM  with respect to the frequency of the received signal S r . In this embodiment, the formula of the detection distance calculated from the detection sub-signals is given as follows: 
     
       
         
           
             R 
             = 
             
               
                 
                   c 
                   0 
                 
                 · 
                 
                    
                   
                     Δ 
                      
                     
                         
                     
                      
                     f 
                   
                    
                 
               
               
                 2 
                 · 
                 
                   ( 
                   
                     df 
                     / 
                     dt 
                   
                   ) 
                 
               
             
           
         
       
     
     where R is the detection distance corresponding to each of the detection sub-signals, c 0  is the speed of light (3·1.0 8  m/s), Δf is the frequency of each of the detection sub-signals, (df/dt) is the slope of frequency variation of the frequency-modulated transmitted signal S T . 
     With reference to  FIGS. 1 and 2 , in the step  14 , the central processing unit  121  of the processor  120  is configured to calculate a peak-to-average ratio (PAR) of each of the detection sub-signals (each rows in  FIG. 4 ), and according to the PAR, define the detection distance corresponding to one of the detection sub-signals as a distance D between the object O and the FMCW radar  110 . The higher PAR, the higher amplitude variation of the detection sub-signal, and the amplitude variation of each of the detection sub-signals can be represented as the vibration magnitude of the relative movement, so the PAR of each of the detection sub-signals is directly proportion to the vibration magnitude at the corresponding detection distance. Accordingly, an object O is regarded to be located at the detection distance corresponding to the detection sub-signal having the maximum PAR and has higher vibration intensity. The central processing unit  121  of the processor  120  is configured to define the detection distance which corresponds to the detection sub-signal having the maximum PAR as the distance D from the object O to the FMCW radar  110 . 
     If more than one objects are located within the area A, the central processing unit  121  is configured to estimate the distance D between the each objects O and the FMCW radar  110  based on not only the PAR of each of the detection sub-signals, but also a threshold value. As mentioned previously; the PAR of the detection sub-signal and the vibration magnitude of the object O at the detection distance corresponding to the detection sub-signal are in direct proportion, thus the central processing unit  121  determines the detection distances corresponding to the detection sub-signals having the PAR larger than the threshold value as the distances D of the objects O away from the FMCW radar  110 . 
     With reference to  FIG. 1 , preferably, the central processing unit  121  of the processor  120  is configured to analyze spectrum characteristics of the detection sub-signal having the maximum PAR to obtain a vital sign signal S VS  of the object O in the step  14 . The central processing unit  121  preforms a Fast Fourier Transform (HT) on the detection sub-signal to identify the vibration frequency caused by the relative movement so as to further analyze the vital sign of the object O. Additionally, when more than one objects are located within the area A, the processor  120  can analyze spectrum characteristics of the detection sub-signals having the PAR larger than the threshold value to obtain vital sign signals S VS  of the objects O. 
     If the object O is a human, a first frequency range and a second frequency range can be set in the central processing unit  121  of the processor  120  in advance. For example, the first frequency range is between 0.2 Hz and 0.35 Hz that is the frequency range of ordinary human breathing, and the second frequency range, between 1 Hz and 2.5 Hz, is the frequency range of ordinary human heartbeat. Next, the processor  120  set the frequency, within the first frequency range and having a highest amplitude value, of the vital sign signal S VS  as a breathing frequency of the object O and set the frequency; within the second frequency range and having a highest amplitude value, of the vital sign signal S VS  as a heartbeat frequency of the object O. If the object O is an animal (not human) or a non-living thing having fixed vibration frequency, one or more frequency ranges can be set in the processor  120  according to the possible vibration frequency. The range and the number of the frequency setting in the processor  120  is not limited in the present invention. 
     The processor  120  of the present invention is utilized to process the detection signal S d  detected by the FMCW radar  110  to obtain the detection sub-signals able to represent vibration levels at each of the detection distances, and estimate the distance D from the object O to the FMCW radar  110  by the PAR of each of the detection sub-signals. 
     The scope of the present invention is only limited by the following claims Any alternation and modification without departing from the scope and spirit of the present invention will become apparent to those skilled in the art.