Radar system for environmental detection with means for measuring the oscillator characteristic

Abstract

Radar system for environment detection with • Transmitting means for directed radiation of transmitting power, • Receiving means for the directed reception of transmission power reflected from objects and • Signal processing means for processing the received power, in which a) the frequency of the transmitting power is modulated by appropriate control of an oscillator, b) the oscillator is controlled with a set of discrete control signal values, c) the frequency of the oscillator is measured for at least some of these discrete control signal values, whereby an oscillator characteristic curve is created and only the control signal values ​​already measured are used for frequency modulation, whereby a set of signal control values ​​is calculated on the basis of the oscillator characteristic curve, with which a specific function for frequency modulation can be implemented, d) and this frequency measurement includes at least the following steps for each discrete control signal value: • Sampling of the oscillator signal or a signal obtained from it by frequency division, if necessary after suitable pre-processing, • Windowing of the sampled signal and • Frequency determination for the windowed signal by spectral analysis.

Description

[0001] The invention relates to a radar system for environmental detection with means for measuring an oscillator characteristic. Such an environmental monitoring system can be used, for example, in a motor vehicle in which a driver assistance or safety function is provided.

[0002] The functionality of a frequency-modulated radar system for determining the distance and relative speed of objects has been known for many years and is widely described in the literature. A frequency-modulated radar system is based on an oscillator that outputs a signal with a specified output frequency depending on an incoming control signal. The dependence of these two parameters is described by the oscillator's frequency characteristic. The frequency characteristic must be measured very precisely to avoid measurement errors when detecting surrounding objects. Errors in frequency modulation can lead to blurred images of objects, whereby objects with a small backscatter cross-section can be obscured by objects with a larger backscatter cross-section. Further effects of errors in frequency modulation can include interference lines and thus false detections, as well as increased noise.The frequency characteristic must be constantly updated in order to compensate for changes caused, for example, by temperature drift.

[0003] A well-known method for measuring frequency characteristics involves counting the number of zero crossings within a specified measurement interval for a constant oscillator drive signal. This method is either inaccurate if a short measurement interval is chosen, or very slow if a long measurement interval is chosen to increase measurement accuracy.

[0004] DE 196 32 889 A1 describes a radar system with a frequency-modulated transmission signal, in particular an FMCW radar system for determining distances, in which the actual frequency deviation is determined and used instead of the desired target frequency deviation to calculate the distance to a radar target. Furthermore, the determined actual frequency deviation can be used to control the frequency deviation of a frequency-generating, voltage-controlled oscillator. Furthermore, US 5291 081 A describes a device for detecting unmodulated signals, wherein the unmodulated signal or the head of the received signal is detected at the head of the received signal despite a fairly large frequency error in the received signal, and the frequency detection for coherent detection is established in a receiver during the detection process of the unmodulated signal.US Pat. No. 6,072,426 A discloses a modulator slope calibration circuit used to enable dynamic calibration of a radio altimeter. Furthermore, DE 43 34 079 A1 discloses a radar rangefinder for high-precision non-contact distance measurement, which is constructed according to the FMCW principle and operates with digital signal processing and a limited frequency sweep.

[0005] The task of this device is to measure the frequency characteristic of an oscillator in a radar system for environmental detection precisely and quickly.

[0006] This object is achieved according to the invention by a radar system according to independent patent claim 1. Advantageous further developments can be found in the subclaims.

[0007] The claimed radar system for environmental detection with means for measuring an oscillator characteristic curve comprises transmitting means for the directed emission of transmitted power, receiving means for the directed reception of transmitted power reflected from objects, and signal processing means for processing the received power. The frequency of the transmitted power is modulated by appropriately controlling a dedicated oscillator. The oscillator is controlled by a set of discrete control signal values. For at least some of these discrete control signal values, the frequency or output frequency of the oscillator is measured, whereby an oscillator characteristic curve is created and only the previously measured control signal values ​​are used for frequency modulation. Based on the oscillator characteristic curve, a set of signal control values ​​is calculated, with which a specific function for frequency modulation can be implemented.Frequency measurement of the oscillator output frequency involves sampling the oscillator output signal or a signal obtained from it by frequency division, possibly after suitable preprocessing, windowing the sampled signal, and determining the frequency of the windowed signal using spectral analysis. The measurement is performed for each discrete control signal value.

[0008] The radar system in question allows the oscillator characteristic to be determined precisely and quickly. These properties enable the implementation of a cost-effective radar system, as the oscillator frequency can be modulated with minimal hardware effort. The oscillator output frequency is controlled entirely by software via a control signal. Furthermore, software control makes it possible to very easily vary the signal shape of the transmitted signal, for example, by dynamically adjusting the frequency deviation and thus adapting the range resolution to the current environmental conditions, such as traffic conditions.

[0009] In a preferred embodiment of the invention, a discrete Fourier transform (DFT) is used for spectral analysis, and the oscillator's output frequency is obtained by interpolating or estimating the signal values ​​between the discrete spectral lines, taking into account the window function used. In a preferred embodiment of the invention, the discrete Fourier transform (DFT) is performed using a fast Fourier transform (FFT). In a particular embodiment of the radar system claimed here, a voltage-controlled oscillator (VCO) is provided as the oscillator for modulating the transmission power. Unmeasured control signal values, i.e., estimated or interpolated values, are not used to generate the transmission signal.

[0010] In particular, a signal path for object detection and a return measurement path for measuring the oscillator frequencies are provided. The return measurement path is arranged separately from the signal path and includes a frequency divider.

[0011] In a preferred embodiment of the invention, the return measurement path is deactivated during object detection (i.e., the signal path is active) to avoid interfering with the detection of surrounding objects. A further embodiment of the invention provides that the frequency of the output signal of the frequency divider in the return measurement path is changed in such a way that the detection of surrounding objects is not disturbed at all or only to a minimal extent. The frequency in the return measurement path is set such that the frequency ranges of the signals in the signal and return measurement paths overlap little or not at all. The same applies to the harmonics of the signals.

[0012] In a particular embodiment of the invention, the frequency divider in the return measurement path is designed so that the frequency of the sampled signal is not in the range of 1 / 2 f A , 1·f A , 3 / 2·f A , 2·f A , ..., where f A , the sampling frequency, since then an interpolation of the frequency-dependent power curve between the discrete frequency values ​​is not possible.

[0013] A special design of the radar system provides a data processing unit with an upstream analog-to-digital converter. The data processing unit is used to evaluate signals generated in the signal path and the feedback path.

[0014] In a preferred embodiment of the radar system, switching means are provided so that only one signal at a time - either a signal from the signal path or a signal from the return measurement path - is forwarded to the data processing unit.

[0015] In particular, the same DFT is used for the evaluation of measurement signals for object detection and measurement signals for measuring the oscillator frequencies.

[0016] A preferred embodiment of the radar system provides that the output frequency of the oscillator is measured at least twice for at least one discrete oscillator control and filtered using at least two measured values. This procedure increases the accuracy of the frequency determination.

[0017] In a particular embodiment of the invention, discrete signal values ​​for controlling the oscillator are generated by a digital-to-analog converter (DAC).

[0018] In an advantageous embodiment of the invention, only those voltages for driving the oscillator that correspond to the oscillator output frequencies required for ramp generation are measured. This prevents the measurement of unnecessary measured values ​​from extending the overall measurement time.

[0019] In a further embodiment of the invention, the detection of objects and the measurement of the oscillator frequencies are carried out at essentially the same repetition rate. This means that the oscillator control values ​​required to generate the ramp are measured in one or a few cycles. This ensures that a current characteristic curve is used to control the oscillator, so that rapidly changing environmental influences, e.g., temperature changes, do not negatively impact the measurement accuracy of the radar system. To reduce the measurement time, in a further embodiment of the invention, only part of the characteristic curve can be measured before an object is detected, and another part before the subsequent object detection, and so on.

[0020] The invention is explained in more detail below with reference to figures and exemplary embodiments. Fig. 1: Block diagram of a radar system with means for measuring an oscillator characteristic curve Fig. 2: Sampled oscillator signal from the return measurement path Fig. 3: Spectrum of the sampled return measurement signal Fig. 4: Frequency-dependent power spectrum of the oscillator in the return measurement path

[0021] In Fig. Figure 1 shows a block diagram of a radar system that contains all the essential elements of the invention. A voltage-controlled, modulatable oscillator (VCO) generates a frequency-modulated transmission signal, which is fed to an antenna A via a coupler structure K. Simultaneously, the oscillator signal is fed to a mixer M, where it is mixed with the received signal. The mixer's output signal is filtered in a bandpass filter. The return measurement path R comprises a frequency divider K1 and a bandpass filter BP. The return measurement path and signal path are connected to a digital signal processing unit SP via a multiplexer MUX. An analog-to-digital converter precedes the signal processing unit.

[0022] The transmitted signal uses a sequence of linear frequency ramps whose frequency deviation per unit time is so large that the difference frequency between the transmitted signal and the received signal depends almost exclusively on the propagation time and thus on the distance to the object at which the reflection occurs. The difference frequency depends only to a much lesser extent on the relative velocity. Thus, the distance information is obtained from sampling a single ramp of the mixed signal, with the signal frequency being proportional to the distance. The Doppler frequency and thus the relative velocity are determined by evaluating the phase change between the sampled ramps of the mixed signal. The distance and relative velocity are calculated using a two-dimensional Fourier transform.Errors in the frequency ramp can lead to blurred images in the distance and thus to the obscuring of smaller targets, to interference lines in the distance spectrum and thus to false detections and also to increased noise.

[0023] In addition to the described signal path, a return measurement path R is provided. Here, the oscillator signal is directly digitized and then the frequency is determined. The only additional components required for this are a frequency divider, a bandpass filter, and a multiplexer. This path is activated during transmission pauses. During the measurement of surrounding objects (transmission power is radiated and received), the return measurement signal is deactivated to prevent coupling to the signal path. During transmission pauses, the VCO drive voltage is gradually increased via a DAC (digital-to-analog converter), whereby a drive value generated by the DAC is stored for the measurement time t messis kept constant. During this time t mess The feedback signal is digitized and its frequency is measured. The sampling of the feedback signal is in Fig. 2. The line represents the analog measurement signal and the dots indicate the sample values ​​n=0, 1, 2,...N of the digitized signal.

[0024] This measurement of the oscillator output signal is performed for all relevant oscillator control values. Relevant oscillator control values ​​are the control values ​​used for environment detection. For example, the relevant oscillator control values ​​are measured before each measurement of environmental objects that uses these control signals.

[0025] A further embodiment of the invention provides for the measurement of the oscillator control values ​​to be distributed over several measurement periods, with a measurement period for determining the oscillator characteristic alternating with a measurement period for detecting surrounding objects. For example, half of the control values ​​are measured before a measurement of surrounding objects, and the other half before the subsequent measurement of surrounding objects. Accordingly, one-third or one-quarter of the control signals can be measured before each measurement period for detecting objects. This shortens the total measurement time.

[0026] In order to increase frequency measurement accuracy in the characteristic curve determination, in a further embodiment, frequencies that are measured at different times at the same oscillator control value are filtered over time.

[0027] The following describes how the frequencies are determined after sampling the oscillator output signal. The digitized signals are multiplied by a suitable window function, and then an FFT is performed. The same FFT can be used here as for the target processing, which offers a significant advantage, especially when implementing the data analysis on an FPGA (Field Programmable Gate Array).

[0028] The oscillator frequency can be read directly from the frequency-dependent power spectrum of the feedback signal. The signal shape is determined by the spectrum of the window function. Fig. 4 shows the power of a feedback signal plotted against frequency. The lines indicate the power values ​​for the discrete frequencies. The line position of the highest power value 1_m alone only leads to a very inaccurate frequency reading. Therefore, the power of the right-hand neighboring frequency value 1_r and the left-hand neighboring frequency value 1_1 are also used to interpolate the exact frequency of the signal f_0, taking the window function into account. The interpolation is performed, for example, using values ​​stored in the data evaluation unit (lookup tables) or an approximation function. Alternatively, the interpolation can first be calculated on an FPGA using a coarse grid and then calculated more precisely on a microcontroller unit (MCU) using a correction function.

[0029] By separating the signal measurement path and the return measurement path within the radar system, a good signal-to-noise ratio is achieved, which allows the interpolation of the line position to determine the oscillator frequency to be carried out very accurately.

[0030] As in Fig. As shown in Figure 1, a bandpass filter BP is arranged in the return measurement path after the frequency divider K1. The bandpass filter BP is designed to significantly attenuate the harmonics generated during frequency division. This design is advantageous because, after sampling, the harmonics may be in the same frequency range as the return measurement signal itself, which would lead to a falsification of the measured frequency.

[0031] Since the feedback signal is a real signal, it is represented symmetrically in the spectrum. This is Fig.3. Line 1 indicates the spectrum of the feedback signal shifted by the window function. The maximum of the power peak is at f_S. The line marked 2 indicates the noise during the measurement. The spectrum of a sampled signal repeats periodically with the sampling frequency. If the envelopes of the power peaks, which are given by the spectrum of the window function, are too close to each other, so that power components of one power peak can still be found at the maximum of the other power peak, this also leads to a falsification of the measured frequency. Therefore, the division factor of the frequency divider K1 and the sampling frequency must be matched to one another so that the frequency of the feedback signal is as close as possible to ƒA4+n⋅ƒA(n=0,12..) (oversampling for n>0). Secondly, the window function must be selected appropriately so that for the resulting relevant frequency ranges in which the power peaks may occur, the spectrum of the window has already decayed to such an extent that the power peaks no longer influence each other.

[0032] Based on the measured oscillator characteristic, a set of signal control values ​​can be calculated with which the desired frequency modulation function can be realized, for example a linear frequency ramp.

Claims

[1] Radar system for environment detection with • Transmitting means for directed radiation of transmitting power, • Receiving means for the directed reception of transmission power reflected from objects and • Signal processing means for processing the received power, in which a) the frequency of the transmitting power is modulated by appropriate control of an oscillator, b) the oscillator is controlled with a set of discrete control signal values, c) the frequency of the oscillator is measured for at least some of these discrete control signal values, whereby an oscillator characteristic curve is created and only the control signal values ​​already measured are used for frequency modulation, whereby a set of signal control values ​​is calculated on the basis of the oscillator characteristic curve, with which a specific function for frequency modulation can be implemented, d) and this frequency measurement includes at least the following steps for each discrete control signal value: • Sampling of the oscillator signal or a signal obtained from it by frequency division, if necessary after suitable pre-processing, • Windowing of the sampled signal and • Frequency determination for the windowed signal by spectral analysis. [2] Radar system according to claim 1, characterized by that a discrete Fourier transform (DFT) is used for spectral analysis and the frequency is obtained by interpolation between the discrete spectral lines of the corresponding spectral power peaks, whereby the window function used is taken into account for the interpolation. [3] Radar system according to one of the above claims, characterized bythat a signal path is provided for the detection of objects and a separate return measurement path with a frequency divider is used to measure the oscillator frequencies. [4] Radar system according to claim 3, characterized by that during the detection of objects the output signal of the frequency divider in the return measurement path is deactivated or its frequency is changed. [5] Radar system according to one of the above claims, characterized by that a data processing unit with an upstream analog-digital converter is provided, which is used to evaluate measurement signals occurring in the signal path and the return measurement path. [6] Radar system according to one of the above claims, characterized by that the same DFT is used for the evaluation of measurement signals for the detection of objects and of measurement signals for the measurement of the oscillator frequencies. [7] Radar system according to one of the above claims, characterized bythat the output frequency is measured at least twice for each discrete oscillator control and filtered using at least two measured values. [8] Radar system according to one of the above claims, characterized by that discrete signal values ​​for controlling the oscillator are generated by a digital-to-analog converter (DAC). [9] Radar system according to one of the preceding claims, characterized by that the detection of objects and the measurement of the oscillator frequencies are carried out at an essentially equal repetition rate.

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