Patent Publication Number: US-11029405-B2

Title: FMCW vehicle radar system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 U.S.C. § 371 national phase application of PCT International Application No.: PCT/EP2016/061014, filed May 17, 2016, which claims the benefit of priority under 35 U.S.C. § 119 to European Patent Application No. 15168508.8, filed May 20, 2015, the contents of which are incorporated herein by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present disclosure relates to a vehicle radar system where successive frequency ramps are generated and transmitted. When a ramp reaches a second high frequency, it is brought back to a first low frequency. 
     BACKGROUND 
     Many vehicle radar systems comprise radar transceivers that are arranged for generating so-called chirp signals that are transmitted, reflected and received by means of appropriate antennas comprised in the radar system. A chirp signal is an FMCW (Frequency Modulated Continuous Wave) signal with a certain amplitude where the frequency is continuously ramped between two values, the chirp signal thus being in the form of a continuous sinusoid where the frequency varies from a first low frequency to a second high frequency over the course of the ramp. Alternatively the ramp may be such that the frequency varies from a first high frequency to a second low frequency. The magnitude of the change in frequency from start to finish may for example be of the order of 0.5% of the starting frequency. 
     The received signals, thus constituted by reflected radar echoes, are mixed with the transmitted chirp signal in order to convert the received signals to baseband signals. These baseband signals, or IF (Intermediate Frequency) signals, are amplified and transferred in a plurality of channels to an Analog to Digital Converter (ADC) arrangement which is arranged to convert the received analog signals to digital signals. The digital signals are used for retrieving an azimuth angle of possible targets by simultaneously sampling and analyzing phase and amplitude of the received signals. The analysis is generally performed in one or more Digital Signal Processors (DSP:s) by means of Fast Fourier Transform (FFT) processing. 
     Each radar transceiver comprises its own oscillator, normally in the form of a VCO (Voltage Controlled Oscillator), which is controlled to vary the transmitted frequency from the first low frequency to the second high frequency via a phase-locked loop (PLL) which typically is achieved in a linear fashion. The frequency is controlled using a sequence of discrete frequency steps that approximate the desired frequency function. After such a frequency ramp, when a ramp reaches the second high frequency, it is brought back to the first low frequency in preparation for the next ramp in a single step. Such a step should be as short as possible, enabling fast successive ramps to be generated. 
     However, such a single step may result in an overshoot effect, where the frequency initially falls below the first low frequency. This is undesirable due to frequency restriction requirements, and may create interference with other frequency bands. This may for example be due to the inherent signal leakage from the oscillator to the radiating antennas and/or to radiation from feeding lines that distribute the oscillator signal to other devices. 
     In U.S. Pat. No. 8,638,139 this problem is solved by dividing the step into a number of equal and successively running smaller steps. However, a more versatile and efficient way to counteract an overshoot effect according to the above is desired. 
     The object of the present disclosure is thus to provide a vehicle radar system which is arranged for a more versatile and efficient way to counteract an overshoot effect when changing the frequency from the second high frequency to the first low frequency. 
     This object is achieved by a vehicle radar system having a control unit and a signal generator that is arranged to generate a least one FMCW (Frequency Modulated Continuous Wave) chirp signal. Each chirp signal forms a corresponding plurality of frequency ramps, and each frequency ramp runs between a first frequency and a second frequency. When a frequency ramp has reached the second frequency, the control unit is arranged to control the signal generator to start outputting an output signal with an output frequency for initializing a further frequency ramp by use of a frequency control signal corresponding to a desired frequency, where the desired frequency includes an initial desired frequency part and at least one further desired frequency part. The initial desired frequency part runs from the second frequency to an intermediate frequency having a magnitude between the first frequency and the second frequency. The further desired frequency part runs from the intermediate frequency to the first frequency for the further frequency ramp. The duration of the initial desired frequency part falls below the duration of the further desired frequency part. 
     This object is also achieved by means of method for a vehicle radar system, where the method includes the steps of:
         Generating a least one FMCW (Frequency Modulated Continuous Wave) chirp signal, where each chirp signal uses a corresponding plurality of frequency ramps, where each frequency ramp runs between a first frequency and a second frequency.   Controlling an output frequency for initializing a further frequency ramp when a preceding frequency ramp has reached the second frequency, using a frequency control signal corresponding to a desired frequency, where the desired frequency includes an initial desired frequency part and at least one further desired frequency part. The initial desired frequency part runs from the second frequency to an intermediate frequency having a magnitude between the first frequency and the second frequency. The further desired frequency part runs from the intermediate frequency to the first frequency for the further frequency ramp. The duration of the initial desired frequency part falls below the duration of the further desired frequency part.       

     According to an embodiment of the present invention, the signal generator it is in the form of a VCO. For example, the VCO may be a phase-locked loop type, where the VCO is arranged to output a signal with a present output frequency. 
     According to another example of the present invention, the signal generator is in the form of a reference oscillator that is arranged to output a signal of a certain frequency, which signal is fed into a frequency converting unit. The frequency converting unit is arranged to multiply and/or divide the input signal, resulting in a signal having a reference frequency that is fed into a phase frequency detector. The present output frequency is fed back to the phase frequency detector via a frequency divider. 
     According to another example of the present invention, the control signal is arranged to set the divide-by ratios in the frequency divider and/or the frequency converting unit. 
     Other examples are disclosed in this specification and accompanying drawings. 
     A number of advantages are obtained by the present disclosure. Mainly, the overshoot effect is eliminated in a versatile and straight forward manner. 
     This could for example mean that a linear PLL (phase-locked loop) architecture could be employed in such a system, despite the non-linear nature of the desired waveform trajectory. 
     In a linear PLL architecture, it is often beneficial to reduce the bandwidth of the analog filter within the feedback loop, known as the loop filter. Reducing the bandwidth can have the beneficial effect of lowering the level of noise generated by the oscillator, known as phase noise. The present disclosure allows the loop bandwidth to be reduced in order to benefit the performance of the section of the waveform used for measurement without problems of overshoot during the fly-back section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will now be described more in detail with reference to the appended drawings, where: 
         FIG. 1  shows a schematic top view of a vehicle; 
         FIG. 2  shows a simplified schematic of a vehicle radar system; 
         FIG. 3  shows a first chirp signal waveform; 
         FIG. 4  shows details of a signal generator; 
         FIG. 5  shows a graphical presentation of a desired frequency and an output frequency; and 
         FIG. 6  shows a flowchart for a method according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically shows a top view of a vehicle  1  arranged to run on a road  2  in a direction D, where the vehicle  1  includes a vehicle radar system  3  which is arranged to distinguish and/or resolve single targets from the surroundings by transmitting signals  4  and receiving reflected signals  5  and using a Doppler effect in a previously well-known manner. The vehicle radar system  3  is arranged to provide azimuth angles of possible objects  6  by simultaneously sampling and analyzing phase and amplitude of the received signals  5 . 
     With reference also to  FIG. 2 , the vehicle radar system  3  includes a transceiver arrangement  7  that is arranged for generating and transmitting sweep signals in the form of FMCW, Frequency Modulated Continuous Wave, chirp signals  4  of a previously known kind, and to receive reflected signals  5 . The transceiver arrangement  7  includes a transmitter  8 , a receiver  9 , an Analog to Digital Converter (ADC) arrangement  10 , a sampling and timing arrangement  11 , a DSP (Digital Signal Processor) arrangement  12  and a control unit  32 . 
     The transmitter  8  includes a signal generator arrangement  13  and a transmit antenna arrangement  14 , where the control unit  32  is connected to the signal generator arrangement  13 . The receiver  9  includes a receiver mixer  15  and a receiver antenna arrangement  16 . 
     With reference also to  FIG. 3 , the transmitter  8  is arranged to transmit a chirp signal  4 , and the receiver  9  is arranged to receive reflected signals  5 , where the transmitted chirp signal  4  has been reflected by an object  6 . 
     A chirp signal  4  is in the form of a continuous sinusoid where the frequency varies from a first frequency f start  to a second frequency f stop  over the course of a ramp r, where the chirp signal  4  is in the form of repeating cycles of a plurality of frequency ramps r. There the magnitude of the first frequency f start  falls below the magnitude of the second frequency f stop . The change in frequency from start to finish for each ramp r may for example be of the order of 0.5% of the first frequency f start . 
     A cycle for the chirp signal  4  lasts for a certain cycle time t c , each ramp r lasts a certain ramp time t r , having a ramp period time t T . Between two consecutive ramps of the chirp signal  4  there is a delay time t D , and during this delay time to the frequency of the signal generator  13  should be brought from the second frequency f stop  to the first frequency f start , sometimes referred to as the fly-back. 
     Referring back to  FIG. 2 , the reflected signals  5  are received by the receiver  9  via the receiver antenna arrangement  13 . The received signals  5   a ,  5   b , thus constituted by reflected radar echoes, are then mixed with the transmitted chirp signal  4  in the receiver mixer  15 . 
     In this way, IF (Intermediate Frequency) signals  17  are acquired and filtered in an IF filter  18  such that filtered IF signals  19  are acquired. 
     The difference frequency of the filtered IF signals  19  relates to the target distance and are transferred from the receiver  9  to the ADC arrangement  10 , where the filtered IF signals  19  are sampled at a certain predetermined sampling frequency f s  and converted to digital signals  20 , the sampling frequency f s  being provided in the form of a sampling and timing signal  21  produced by the sampling and timing arrangement  11 . 
     The DSP arrangement  12  that adapted for radar signal processing by means of a first FFT (Fast Fourier Transform) to convert the digital signals  20  to a range domain, and a second FFT to combine the results from successive chirp signal ramps into the Doppler domain. This results in Range-Doppler matrices  22  that are transferred for further processing, which is not further discussed here, many examples of such further processing being well-known in the art. 
     In the following, with reference to  FIG. 4  that schematically shows a typical signal generator with a PLL, the signal generator  13  will be discussed more in detail. 
     The signal generator  13  as provided in the form of a voltage controlled oscillator (VCO) arrangement  23  that in turn formed from a bias generator  24  and a VCO  25  which are connected to each other in this order. The VCO  25  outputs a signal having a frequency that is tunable over a certain frequency range by supplying an input control voltage in a previously known manner. The signal generator  13  further includes a phase frequency detector  27 , a charge pump  28 , a low pass (loop) filter  29 , a reference oscillator  37  and a frequency divider  30 . 
     The output of the VCO  25 , an actual output signal  4 , here the chirp signal  4 , with the present output frequency F out , is fed back to the phase frequency detector  27  via the frequency divider  30 . The reference oscillator  37  is arranged to output a signal of a certain frequency that is fed into a frequency converting unit  26  which is arranged to multiply and/or divide the input signal, resulting in a signal having a reference frequency F ref  that is inputted into the phase frequency detector  27 . The phase frequency detector  27  provides an up/down control signal  38  to the charge pump  28 , which in turn is connected to the VCO arrangement  23  via the low pass (loop) filter  29 . The reference oscillator  37  may for example be a high accuracy crystal oscillator with low phase noise. 
     The control unit  32  is connected to the signal generator  13  and outputs a frequency control signal  31  to the signal generator  13  via this connection as indicated on  FIG. 2  and  FIG. 4 . The frequency ramp is in practice defined by a number of discrete frequency steps (not shown). The discrete frequency steps are smoothed by the action of the loop filter. The steps may have a fixed or varying time duration using previously known techniques. The ramp is generated by varying the divider ratio in the frequency divider  30  and/or the frequency converting unit  26  over time, where the frequency control signal  31  is used to set the divide-by ratios in the frequency divider  30  and/or the frequency converting unit  26  as shown in  FIG. 4 , such that frequency steps are created. The ramp is made linear and smooth with the use of the loop filter  29 . 
       FIG. 5  shows a desired frequency  39  as a function of time, indicated with a bold, initially solid, line, where the frequency control signal  31  corresponds to the desired frequency  39 . Also shown in  FIG. 5  is the frequency F out  of the actual output signal  4  [[ 4 ]] of the signal generator  13 , indicated with a dash-dotted line. For a certain desired frequency, there is a certain corresponding frequency control signal, such that a certain corresponding frequency control signal intends to control the VCO  25  to output an actual output signal  4  having a frequency F out  that equals the desired frequency although, in reality, there are discrepancies such that the frequency F out  of the actual output signal  4  differs from the desired frequency  39 . This, for example, results in an overshoot that will be discussed later. 
     According to the present disclosure, the desired frequency  39  and the frequency F out  of the actual output signal  4  coincide until a first time t 1  where the second frequency f stop  is reached. Here, the present ramp r should end and the control unit  32  is then arranged to control the VCO arrangement  23  to start outputting a further frequency ramp r′ based on the frequency control signal  31 . The desired frequency  39  an initial desired frequency part  39   a  indicated with a bold dash-double-dotted line, and a further desired frequency part  39   b  indicated with a bold dashed line. 
     The initial desired frequency part  39   a  runs from the second frequency f stop  to an intermediate frequency f i  having a magnitude between the first frequency f start  and the second frequency f stop . The further desired frequency part  39   b  runs from the intermediate frequency f i  to the first frequency f start  for the further frequency ramp r′. 
     Here, there is a step from the second frequency f stop  to the intermediate frequency f i  for the initial desired frequency part  39   a  that in  FIG. 5  is shown to occur at the first time t 1 , and then the further desired frequency part  39   b  decreases until a second time t 2  that occurs later than the first time t 1 , where the first frequency f start  is reached. Then, the desired frequency  39  increases again, in accordance with the next frequency ramp r′. The curve slope (rate of change) of the initial desired frequency part  39   a  is greater than the slope of further desired frequency part  39   b , as shown in  FIG. 5 . Also, as illustrated, the magnitude of frequency change over the initial frequency part is greater than the magnitude of change of frequency over the further desired frequency part. 
     How steep the step of the initial desired frequency part  39   a  is may of course vary, and generally the duration of the initial desired frequency part  39   a  falls below the duration of the further desired frequency part  39   b.    
     In  FIG. 5  it is evident that the frequency F out  of the actual output signal  4  overshoots the intermediate frequency f i  between the first time t 1  and the second time t 2  before coinciding with the desired frequency  39 . However, the overshoot does not fall below a minimum frequency f min  of the present frequency band. Due to an overshoot after the second time t 2 , the minimum frequency f min  falls below the first frequency f start  in order to retain the frequency F out  of the output signal  4  within the present frequency band. 
     The intermediate frequency f i  would typically be set to be as close to the first frequency f start  as possible in order to minimize the duration of the overshoot occurring after the second time t 2 . This further allows the first frequency f start  to be close to the minimum frequency f min  which lets more of the available bandwidth be used for measurements. However if set too low, then the overshoot of the actual output signal&#39;s frequency F out  during the further desired frequency part  39   b  may extend below the minimum frequency f min  and hence create out of band interference. Generally, according to an example, the intermediate frequency f i  is closer to the first frequency f start  than the second frequency f stop . 
     With reference to  FIG. 6 , the present disclosure also relates to a method for a vehicle radar system  3 ,  3 ′, where the method includes the steps of:
         Step  33 : Generating a least one FMCW (Frequency Modulated Continuous Wave) chirp signal  4 , where each chirp signal  4  uses a corresponding plurality of frequency ramps r, where each frequency ramp r runs between a first frequency f start  and a second frequency f stop .   Step  34 : Controlling an output frequency F out  for initializing a further frequency ramp r′ when a preceding frequency ramp r has reached the second frequency f stop , using a frequency control signal  31  corresponding to a desired frequency  39 . The desired frequency  39  includes an initial desired frequency part  39   a  and at least one further desired frequency part  39   b . The initial desired frequency part  39   a  runs from the second frequency f stop  to an intermediate frequency f i  having a magnitude between the first frequency f start  and the second frequency f stop . The further desired frequency part  39   b  runs from the intermediate frequency f i  to the first frequency f start  for the further frequency ramp r′. The duration of the initial desired frequency part  39   a  falls below the duration of the further desired frequency part  39   b.          

     As indicated in  FIG. 1 , the vehicle  1  includes a safety control unit  35  and safety system  36 , for example an emergency braking system and/or an alarm signal device. The safety control unit  35  is arranged to control the safety system  36  in dependence of input from the radar system  3 . 
     The present disclosure is not limited to the examples above, but may vary freely within the scope of the appended claims. For example, all times mentioned are of course only mentioned by way of example, any suitable times and timing schedules are clearly possible in a radar system according to the above. The ramp may similarly be configured as an up-ramp as described, or a down-ramp, or some combination of both. 
     The radar system may be implemented in any type of vehicle such as cars, trucks and buses as well as boats and aircraft. 
     The schematics of vehicle radar systems are simplified, only showing parts that are considered relevant for an adequate description of the present disclosure. It is understood that the general design of radar systems of this kind is well-known in the art. For example, no devices that are arranged to use the acquired target information is shown, but many different such devices are of course conceivable; for example a warning and/or collision avoidance system. The actual output signal  4  is indicated to be the actual chirp signal  4 ; this is only an example since there may be intermediate components positioned between the VCO&#39;s output and the transmit antenna arrangement  14  that alter the actual output signal  4  before being transmitted as the chirp signal  4 . 
     The number of antenna arrangements, antennas within each antenna arrangement and IF signals may vary. 
     The ADC arrangement and the DSP arrangement should each one be interpreted as having a corresponding ADC or DSP functionality, and may each be constituted by a plurality of separate components. Alternatively, each ADC arrangement may be embodied in one ADC chip, and each DSP arrangement may be embodied in one DSP chip. 
     Each antenna arrangement  13   a ,  13   b ;  25   a ,  25   b  may for example include on or more antennas, and each antenna may be constituted by one antenna element or by an array of antenna elements. 
     Generally, the hardware used to generate the radar signal may be active only for part of the cycle period and powered down for the rest of the cycle, i.e. when it is not needed. 
     There may be more than one further desired frequency part following the initial desired frequency part  39   a  before the next ramp r′ starts, but the duration of the initial desired frequency part always falls below the duration of any of such a further desired frequency part. 
     Other types of signal generators are possible, for example a signal generator working in an open-loop mode where the control signal is fed directly to the VCO. However, having a PLL decreases a possible VCO frequency drift with time and temperature, requiring calibration of the control signal in order to ensure that the VCO ramp is linear and stays within the desired band. Using a PLL results in that the VCO mainly stays within the desired band, and mainly is linear, without the need for such a calibration. 
     Terms such as coincide should not be interpreted as being mathematically exact, but within what is practical in this field of technology and present context. Here, the term coincide might be interpreted as closely follow. 
     Generally, the present disclosure relates to a vehicle radar system  3  having a control unit  32  and a signal generator  13  that is arranged to generate a least one FMCW (Frequency Modulated Continuous Wave) chirp signal  4 . Each chirp signal  4  is formed of a corresponding plurality of frequency ramps r, and each frequency ramp r runs between a first frequency f start  and a second frequency f stop . When a frequency ramp r has reached the second frequency f stop , the control unit  32  is arranged to control the signal generator  13  to start outputting an output signal  4  with an output frequency F out  for initializing a further frequency ramp r′ using a frequency control signal  31  corresponding to a desired frequency  39 , where the desired frequency  39  is formed of an initial desired frequency part  39   a  and at least one further desired frequency part  39   b , where the initial desired frequency part  39   a  runs from the second frequency f stop  to an intermediate frequency f i  having a magnitude between the first frequency f start  and the second frequency f stop , and where the further desired frequency part  39   b  runs from the intermediate frequency f i  to the first frequency f start  for the further frequency ramp r′, where the duration of the initial desired frequency part  39   a  falls below the duration of the further desired frequency part  39   b.    
     According to an example, the radar system  3  is arranged to provide input to a safety control unit  35  that in turn is arranged to control safety system  36 , where the radar system  3 , the safety control unit  35  and the safety system  36  are provided in a vehicle  1 . 
     According to an example, the radar system  3  is arranged to:
         Process all ramps in a cycle by a first FFT, (Fast Fourier Transform), to acquire target information at different ranges in a plurality of complex vectors that corresponds to the number of ramps in a cycle;   Analyze one vector element at a time for all complex vectors by means of a second FFT and then create a two-dimensional matrix in the Doppler domain providing data regarding range and relative speed.       

     According to an example, the signal generator  13  as provided in the form of a VCO  25 . 
     According to an example, the VCO  25  is provided in the form of a phase-locked loop, where the VCO  25  is arranged to output a signal with a present output frequency F out . 
     According to an example of the present invention, the signal generator  13  provided in the form of a reference oscillator  37  that is arranged to output a signal of a certain frequency, which signal is fed into a frequency converting unit  26 , which frequency converting unit  26  is arranged to multiply and/or divide the input signal, resulting in a signal having a reference frequency F ref  that is fed into a phase frequency detector  27 , where the present output frequency F out  is fed back to the phase frequency detector  27  via a frequency divider  30 . 
     According to an example of the present invention, the frequency control signal  31  is arranged to set the divide-by ratios in the frequency divider  30  and/or the frequency converting unit  26 . 
     Generally, the present disclosure also relates to a method for a vehicle radar system  3 , where the method includes the steps of:
         Step  33 : generating a least one FMCW (Frequency Modulated Continuous Wave) chirp signal  4 , where each chirp signal  4  uses a corresponding plurality of frequency ramps r, where each frequency ramp r runs between a first frequency f start  and a second frequency f stop ; and   Step  34 : controlling an output frequency F out  for initializing a further frequency ramp r′ when a preceding frequency ramp r has reached the second frequency f stop , using a frequency control signal  31  corresponding to a desired frequency  39 , where the desired frequency  39  forms an initial desired frequency part  39   a  and at least one further desired frequency part  39   b , where the initial desired frequency part  39   a  runs from the second frequency f stop  to an intermediate frequency f i  having a magnitude between the first frequency f start  and the second frequency f stop , and where the further desired frequency part  39   b  runs from the intermediate frequency f i  to the first frequency f start  for the further frequency ramp r′, where the duration of the initial desired frequency part  39   a  falls below the duration of the further desired frequency part  39   b.          

     According to an example of the present invention, the method further includes the step of providing input to a safety control unit  35  that in turn is used to control safety system  36  in a vehicle  1 . 
     According to an example of the present invention, a VCO  25  is used for outputting a signal with a present output frequency F out , working in a phase-locked loop. 
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.