METHOD FOR DETERMINING THE DISTANCE AND SPEED OF AN OBJECT

Description

[0001] The invention relates to a method for determining a distance and a radial velocity of an object relative to a measuring location, the method comprising the following steps: a) Emitting first transmission signals which are radar radiation in the form of first frequency ramps, b) Emitting second transmission signals which are radar radiation in the form of second frequency ramps, wherein the second frequency ramps are different from the first frequency ramps, c) Receiving received signals which are first and second transmission signals reflected at the location, d) Mixing the received signals with the first or second transmission signals to form mixed signals, e) Creating a range-Doppler matrix from the mixed signals, f) Detecting two Doppler frequencies which originate from the radial velocity of the object and g) Evaluating the Doppler frequencies and / or phase information of the mixed signals so that the ambiguities when determining the radial velocity are eliminated.

[0002] The invention also relates to a device for carrying out such a method.

[0003] The first transmission signals are also called signal A and the second transmission signals are called signal B.

[0004] Such a method is known, for example, from DE 10 2012 008 350 A1.

[0005] A radar transmitter and a corresponding receiver are located at the measurement location. It is known that, using appropriately modulated radar signals, both the distance and radial velocity of an object relative to this measurement location can be determined in a single measurement. Different frequency ramps are transmitted in the form of interleaved first and second transmission signals.

[0006] Previously, it was known to modulate only a transmitted signal with short, fast, and identical ramps. During a cycle time of, say, 65 ms, 256 such ramps can be modulated, which are typically run through at increasing frequency. At the beginning of the ramp, the transmitted signal therefore has a starting frequency that increases at a fixed gradient until a final frequency is reached at the end of each ramp. Each of these 256 ramps, for example, has a length of 254 µs. If each ramp is sampled with 512 samples, this corresponds to a sampling frequency of 2.01 MHz.

[0007] A first evaluation is then performed for each ramp in the form of a first Fast Fourrier Transform (FFT). A second FFT is performed from ramp to ramp, i.e., with an effective sampling period of 254 µs, corresponding to an effective sampling frequency of 3.94 kHz. The first FFT, with a sampling frequency of 2.01 MHz, is referred to as the "Range FFT." The second FFT, with a sampling frequency of 3.94 kHz, is referred to as the "Doppler FFT." The example was idealized in that ramp recovery times between the actual ramps were neglected.

[0008] A Doppler frequency range of 3.94 kHz corresponds to a unique velocity measurement range of 24.5 m / s at a carrier frequency of 24.125 GHz. The frequency sweep f sw for each ramp, i.e., the difference between the final frequency and the starting frequency, can be assumed to be 150 MHz, for example.

[0009] As long as the radial velocity of the object to be measured is less than the unambiguous measurement range of 24.5 m / s, this velocity can be determined unambiguously. For a higher radial velocity, such as 40 m / s, a velocity of 15.5 m / s is determined.

[0010] While the first FFT (range FFT) is performed for each ramp, so that with 256 ramps, 256 frequency spectra of the first FFT are available, the second FFT (Doppler FFT) is preferably performed for each distance value ("range gate"). However, it is also possible to perform the second FFT only for selected distance values. The result of the two FFTs can be represented in a range-Doppler matrix. To increase the relatively small unambiguous range of the Doppler FFT, DE 10 2012 008 350 A1 proposes transmitting two different frequency ramps alternately. These differently designed frequency ramps are processed separately.Because the different frequency ramps are transmitted alternately, the number of identical frequency ramps during a measurement cycle is reduced by half compared to transmitting identical ramps. This also reduces the effective sampling frequency for the Doppler FFT and thus also reduces the unambiguous range. However, since the Doppler frequency to be determined depends on the carrier frequency and thus on the type of transmitted frequency ramp, two Doppler frequencies with two phase information signals can be obtained in this way, which can be used in combination to greatly increase the unambiguous range of the Doppler FFT.

[0011] The disadvantage is that for each individual ramp type emitted, the uniqueness range of the Doppler FFT is reduced and, due to the smaller number of individual ramps of a ramp type, the signal-to-noise ratio is also reduced and thus the quality of the measured values ​​is impaired.

[0012] DE 10 2006 032 539 A1 discloses an FMCW radar sensor with multiple antenna elements supplied with transmission signals with ramp-modulated frequencies. The sensor operates in a long-range mode, where the frequencies of all transmission signals are identical, and in a short-range mode, where the frequencies of the transmission signals each exhibit a specific frequency offset. The received signals are evaluated using fast Fourier transformation.

[0013] DE 10 2013 008 607 A1 relates to a method for determining the distance and relative velocity of an object from an observation point, whereby nested signals are emitted. The echo signals returned by the object are evaluated using the fast Fourier transform.

[0014] US 2014 / 197983 A1 describes a multi-frequency radar system in which radar signals are transmitted in the form of frequency ramps.

[0015] DE 2011 051 969 A1 relates to a radar device which transmits first and second signals, wherein the first and second signals have a frequency offset from one another.

[0016] The invention is therefore based on the object of improving a generic method in such a way that the unambiguousness range of the Doppler FFT is increased and at the same time the signal-to-noise ratio is improved.

[0017] The invention solves the stated problem by a method according to the preamble of claim 1, which is characterized in that the first transmission signals and the second transmission signals are transmitted simultaneously.

[0018] While in the prior art method, the two frequency ramps are always transmitted alternately, in the present invention, both frequency ramps are transmitted simultaneously, doubling the number of ramps transmitted during a measurement cycle. Since both transmission signals are transmitted simultaneously, they are both reflected by the object to be detected, so that the received signals also consist of two individual signals. However, the received signals are only mixed with one of the transmitted signals to obtain the mixed signals. These mixed signals are then subjected to a two-dimensional FFT, as in the prior art, creating a range Doppler matrix.However, since the mixed signals in the method according to the invention are generated from received signals that have two components corresponding to the two transmitted transmission signals, two frequency peaks now appear in the two-dimensional FFT for each object to be detected. These peaks arise from the different frequencies of the two transmitted transmission signals and thus of the two received reception signals. These can be easily used, given a known frequency offset of the two transmitted transmission signals, to calculate information about the distance of the object from the measurement location and a unique Doppler frequency.

[0019] As in the prior art, two Doppler frequencies are also determined, which also differ slightly due to the differing carrier frequencies of the two transmitted signals. These Doppler frequencies and / or phase information from the mixed signals are used in the usual way to resolve the ambiguity of the Doppler FFT. The advantage of the method according to the invention compared to the prior art is that the effective sampling rate of the Doppler FFT could be doubled, thus, on the one hand, increasing the unambiguousness range for each individual Doppler frequency and, on the other hand, providing a larger number of measured values ​​for each Doppler frequency, thus also improving the signal-to-noise ratio.

[0020] Since only a single range Doppler matrix still needs to be created, the computational effort has hardly increased compared to the state-of-the-art method.

[0021] Advantageously, the frequency ramps are descending frequency ramps. This means that each frequency ramp begins at the end frequency, which is the highest frequency of the frequency ramp, and ends at the start frequency, which is the lowest frequency of the frequency ramp. Of course, it is also possible to use the individual frequency ramps as ascending frequency ramps, tuning the transmitted signals starting with the lowest frequency and up to the highest frequency.

[0022] In a preferred embodiment, the first frequency ramps and the second frequency ramps are parallel frequency ramps with a frequency offset df.

[0023] This means that at any given time, the transmitted frequencies of the two transmitted signals exhibit a difference df. A constant frequency offset simplifies the calculations to be performed. The frequency offset of the two peaks after the first range FFT contains information about the frequency offset df, which, if constant and known, can be easily calculated.

[0024] In a preferred embodiment, the frequency offset df is an integer multiple of f / k, where k is the length of the range FFT and f is the sampling frequency at which these measured values ​​are recorded. Preferably, the integer multiple is at least 5, preferably at least 7, and at most 20, preferably at most 15.

[0025] When calculating the range-Doppler matrix, the range FFT is first calculated. For example, if 512 samples are sampled per ramp, a range FFT of length 512 would have to be calculated for each ramp. The Doppler FFT is then calculated for each range gate. Due to the lower uniqueness of the range FFT, especially with single-channel sampling, the Doppler FFT is generally not calculated for all range gates. Often, only slightly less than half, for example, 0.4 times the possible number, is calculated. In this case, the Doppler FFT would therefore only be calculated for 204 of the possible 512 range gates.

[0026] Each of these range gates then corresponds to a frequency of f / k, where k, as already explained, is the length of the range FFT and f is the sampling frequency with which these measured values ​​are recorded or sampled.

[0027] With a sampling frequency f of 10 MHz and a number k of 512, such a range gate frequency corresponds to 19.53 kHz. An integer multiple of 5 of this frequency therefore corresponds to a frequency offset df of 97.65 kHz, and a multiple of 15 corresponds to a frequency offset of df = 292.95 kHz. Different sampling frequencies or different numbers of measured values ​​per ramp naturally result in different frequencies.

[0028] The use of such a small frequency offset ensures that the mixed signals, which are produced by mixing the received signals with one of the two transmitted signals, are arranged in a relatively small frequency range. If a bandpass filter is used after the mixer before the signals are further processed, it can preferably be selected so that the entire mixed signals can pass through the bandpass filter and thus only a single range Doppler matrix needs to be created to detect both frequency peaks. If the frequency offset df is too large, this means that the mixed signals also have frequency components that are correspondingly far apart and which may not be able to fully pass through a single bandpass filter.

[0029] In a preferred embodiment, the first transmission signals are transmitted by a first transmission antenna and the second transmission signals are transmitted by a second transmission antenna.

[0030] Alternatively, two transmitting antennas are provided, but neither of the first transmitting signals nor the second transmitting signals is permanently assigned to them. Rather, in this case, the first transmitting antenna and the second transmitting antenna each alternately transmit the first transmitting signals and the second transmitting signals.

[0031] These embodiments are particularly advantageous when, in addition to the distance and radial velocity, the direction of the object's location is also to be determined. For this purpose, angular resolution is advantageous, which can be improved by using multiple transmitting antennas that transmit the different transmission signals.

[0032] Alternatively, it is of course also possible for the first and second transmission signals to be transmitted from a single transmitting antenna. This is particularly advantageous when an improvement in angular resolution is not required, as in this case, the equipment required can be significantly reduced.

[0033] In a preferred embodiment, the received signals are received by several, preferably at least four, particularly preferably at least eight receiving antennas. Since the various receiving antennas are spatially separated from one another, preferably equidistantly arranged, the transmitted signals emitted by the transmitting antennas, after being reflected by the object, have slightly different propagation times before reaching the various receiving antennas. From these propagation time differences and / or the resulting phase differences, angular information and thus information about the direction in which the object is located can be extracted. Furthermore, different objects can also be separated by angle and thus separated. The more receiving channels used, the better this separation capability.If at least two transmitting antennas are used simultaneously, each transmitting the first and second transmit signals or alternating the first and second transmit signals, a virtual antenna array can be created through clever selection and arrangement of the individual antennas relative to each other, which is larger than the actual number of physically present receiving antennas. This method, called the MIMO principle, is well known in the art. MIMO stands for Multiple Input Multiple Output.

[0034] An embodiment of the method according to the present invention, in which the first and second transmission signals are transmitted by different transmission antennas and the reflected reception signals are subsequently received by a plurality of different reception antennas, not only allows the improved signal-to-noise ratio and the larger unambiguousness range of the Doppler FFT already present according to the invention, but also allows a very precise angular resolution and thus a very precise determination of the direction in which the object is located.

[0035] In a preferred embodiment of the method, third and fourth transmission signals are transmitted simultaneously in the form of third and fourth frequency ramps, with first and second transmission signals and third and fourth transmission signals being transmitted alternately. Using a total of four different transmission signals, four different Doppler frequencies can be determined using the Doppler FFT, and the unambiguous range can be further increased from these and / or the phase information of the mixed signals. If third and fourth transmission signals are transmitted, the resulting reflected reception signals are received and mixed only with the third transmission signals or the fourth transmission signals. Of course, they can also be mixed with the first or second transmission signals. Preferably, the four different transmission signals are parallel frequency ramps with different frequency offset values.Of course, four transmit signals can also be transmitted simultaneously. If the frequency offset values ​​are chosen small enough, the mixed signals resulting from such transmit signals when the corresponding receive signals are received and mixed with at least one of the transmit signals can still be passed through a single bandpass filter, so that in this case, too, only a single range Doppler matrix needs to be generated and calculated. The third and fourth transmit signals can also be used to improve the angular resolution, which can be used to separate and distinguish different objects.

[0036] In a preferred embodiment, the third and fourth transmission signals are parallel frequency ramps, which are preferably decreasing frequency ramps. Of course, they can also be configured as increasing frequency ramps.

[0037] In a preferred embodiment of the method, a portion of the first or second transmission signal is mixed with the other transmission signal without being transmitted by a transmitting antenna. If a range Doppler matrix is ​​calculated from the resulting mixed signal, the mixing of the two transmission signals results in a peak whose distance in the direction of the range FFT corresponds exactly to the frequency offset, since no Doppler frequency is involved. The frequency and / or phase information of the corresponding mixed signal can be used to obtain information and control measurements about the quality of the transmission signal generators.

[0038] An embodiment of the present invention is explained in more detail with the help of the following drawings. It shows: Figure 1 - a curve and schematic evaluation signals for forming a range Doppler matrix by two-dimensional FFT according to the prior art, Figure 2 - a signal curve according to an embodiment of the present invention, Figure 3 - the schematic representation of the result in a range Doppler matrix and Figure 4 - the schematic representation of a circuit for a device according to an embodiment of the present invention.

[0039] Figure 1 shows schematically how a range Doppler matrix can be calculated. In the upper part of the Figure 1The solid line shows the frequency of the transmitted signal as a function of time. Rising ramps can be seen, which run from a starting frequency, which is the lowest frequency of the ramps, to an end frequency, which is the highest frequency of the ramps. Slightly offset from this, the dashed lines show the corresponding received signal, which is picked up by a receiver of the device after being reflected from the object whose distance and speed are to be determined.

[0040] This received signal is mixed with the transmitted signal and then subjected to a 2D FFT to create the range-Doppler matrix. Figure 1 shown, the first FFT (Range-FFT) is performed. This is performed for each ramp, so that, for example, with 2L = 256 ramps sent, the ones in the upper range of the Figure 1shown, 2L=256 range FFTs must be performed.

[0041] The ramps are not measured continuously, but rather sampled, for example, with k=512 samples. For each of these samples, which corresponds to a specific frequency value within the ramp, the second FFT (Doppler FFT) is performed. It may be useful to perform the second FFT only for the distance values ​​that correspond to the distance range of interest.

[0042] The result is the bottom right in Figure 1 The range Doppler matrix shown in Figure 1 shows frequency peaks at a beat frequency that essentially corresponds to the distance of the object and a Doppler frequency that results from the object's velocity. This method is known from the prior art.

[0043] Figure 2shows the frequency response in a method according to an embodiment of the present invention. A dash-dotted line and a solid line can be seen, which correspond to the transmission signal A (TransmitSignal A) and the transmission signal B (TransmitSignal B), and thus to the first transmission signal and the second transmission signal. They are transmitted in parallel. These are the first transmission signal and the second transmission signal, which in the present embodiment are falling frequency ramps running in parallel. In the present embodiment, the first transmission signal is the signal with the lower frequency. The frequency ramp begins at the end frequency and decreases to the start frequency. The lowest frequency of a frequency ramp is referred to as the start frequency, and the highest frequency of a frequency ramp is referred to as the end frequency, regardless of whether the ramp actually starts or ends at this frequency.Here, f sw denotes the frequency swing, i.e. the difference between the final frequency and the starting frequency.

[0044] The second transmission signal is in Figure 2 the one with the higher frequency. The second frequency ramp is a ramp parallel to the first frequency ramp, offset by a frequency offset df.

[0045] The different ramps are sampled with a sampling frequency and a sampling rate that is in the lower range of the Figure 2 is shown.

[0046] A dashed line and a dotted line represent the respective received signals, which result from the reflection of the respective transmitted signals on the object to be detected.

[0047] If these are mixed with one of the transmission signals and then subjected to the already known evaluation to calculate a range-Doppler matrix, the result is obtained which is shown in Figure 3The range Doppler matrix can be seen, which has two peaks for the object. These two peaks are located in Figure 3 in the same column of the matrix, so that they have the same, or at least nearly the same, Doppler frequency. In fact, the two Doppler frequencies differ slightly because the carrier frequencies of the different signals are slightly different. However, since the Doppler frequency is determined by the speed of the object to be detected, this difference is very small and is not represented in the range-Doppler matrix.

[0048] However, the two peaks are located in different rows of the matrix, which actually indicates two different distances. However, the distance between the two peaks corresponds exactly to the frequency offset df of the two transmitted signals. This can easily be calculated back, allowing the actual distance of the object from the measurement point to be determined.

[0049] The range Doppler matrix in Figure 3 It also has another peak, labeled the reference peak. This peak occurs when the first transmitted signal is mixed with the second transmitted signal. The line of the peak, which corresponds to the frequency difference between the two mixed signals, resulting from the time-of-flight difference caused by the distance of the object, is merely the frequency offset in this case, providing a simple means of control.

[0050] Figure 4shows the schematic arrangement of a device for carrying out such a method. A reference source, which can be designed as a quartz crystal, for example, generates reference signals that are fed into two digital signal generators. The signal generators are components known from the prior art, so a detailed description is omitted. The two signal generators generate the transmission signal A and the transmission signal B. Both are fed to the transmission antenna ("TX antenna") and transmitted by the same antenna. Alternatively, they can of course also be directed to different transmission antennas, which transmit the different signals. This can also result in the different signals being transmitted alternately by the two antennas.

[0051] The emitted signals are detected at the object to be detected, which is Figure 4not shown, is reflected and captured by the receiving antenna ("RX antenna"). In the mixer, designated as a "mixer," the received signals are mixed with the first transmitted signal and then passed through a bandpass filter (BPF) and fed for evaluation. Alternatively, the received signals can, of course, also be mixed with the second transmitted signal. In addition, a coupler is provided through which a portion of the second transmitted signal is fed to the received signal and thus also mixed with the first transmitted signal. This creates the reference peak, which is shown in the range-Doppler matrix according to Figure 3 is shown.

Claims

1. A method for determining the distance and radial speed of an object relative to a measuring point, wherein the method comprises the following steps: a) emitting first transmission signals, which are radar radiation in the form of first frequency ramps, b) emitting second transmission signals, which are radar radiation in the form of second frequency ramps, wherein the second frequency ramps are different to the first frequency ramps, c) receiving received signals, which are first and second transmission signals reflected at the object, d) mixing the received signals with the first or second transmission signals to create mixed signals, e) creating a range-Doppler matrix using the mixed signals, f) detecting two Doppler frequencies, which originate from the radial speed of the object, and g) evaluating the Doppler frequencies and / or phase information of the mixed signals, such that ambiguities are eliminated when determining the radial speed characterized by that fact that the first transmission signals and the second transmission signals are sent out at the same time.

2. The method according to claim 1, characterized by the fact that the frequency ramps are sloping frequency ramps.

3. The method according to claim 1 or 2, characterized by the fact that the first and second frequency ramps are parallel frequency ramps with a frequency offset df.

4. The method according to claim 3, characterized by the fact that df is an integer multiple of f / k, wherein k is a length of the range FFT and f the sampling frequency, with which these measured values can be recorded.

5. The method according to claim 4, characterized by the fact that the integer multiple is at least 5, preferably at least 7 and at most 20, preferably at most 15.

6. The method according to one of the above claims, characterized by the fact that the first transmission signals are emitted by a first transmitting antenna and the second transmission signals by a second transmitting antenna.

7. The method according to one of the claims 1 to 5, characterized by the fact that a first transmitting antenna and a second transmitting antenna respectively emit the first transmission signals and the second transmission signals on an alternating basis.

8. The method according to one of the claims 1 to 5, characterized by the fact that the first transmission signal and the second transmission signal are sent out by the same transmitting antenna.

9. The method according to one of the above claims, characterized by the fact that the received signals are received by several, preferably at least four, especially preferably at least eight, receiving antennae.

10. The method according to one of the above claims, characterized by the fact that third and fourth transmission signals in the form of third and fourth frequency ramps are emitted at the same time, wherein first and second transmission signals and third and fourth transmission signals are sent on an alternating basis.

11. The method according to claim 10, characterized by the fact that the third and fourth transmission signals are parallel, preferably sloping, frequency ramps.

12. A device for conducting a method according to one of the above claims.