WO2008040335A1 - Radar system for detecting surroundings using means for measuring the characteristic curve of an oscillator - Google Patents

Abstract

The invention relates to a radar system for detecting surroundings. According to said system, the frequency of the transmission power is modulated by the corresponding control of an oscillator, and the oscillator is controlled by a set of discreet control signal values. The radar system is designed such that the frequency of the oscillator is measured at least for some of the discreet control signal values. The frequency measurement entails scanning the oscillator signal or a signal obtained therefrom by frequency division, and determining the frequency of the scanned signal by means of a spectral analysis.

Description

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

The invention relates to a radar system for environment detection with means for measuring an oscillator characteristic. Such a system for environmental monitoring may e.g. be used in a motor vehicle, in which a driver assistance or safety function is provided.

The operation of a frequency-modulated radar system for distance and relative speed determination of objects has been known for many years and is described many times in the literature. A frequency modulated radar system is based on an oscillator which outputs a signal having a predetermined output frequency in response to an incoming control signal. The dependence of these two parameters is described with a frequency characteristic of the oscillator. The frequency characteristic must be measured very accurately in order to avoid measurement errors in the detection of environmental objects. Errors in the frequency modulation can lead to a blurred image of objects, whereby objects with a small backscatter cross section of objects with a larger backscatter cross section can be obscured. Further effects of errors in the frequency modulation may be interference lines and thus misdetections, as well as increased noise. The frequency characteristic must be constantly updated to B. to be able to compensate for a change caused by a temperature drift.

One known method of measuring frequency characteristics is to count the number of zero crossings within a given measurement interval for a constant oscillator drive signal. This procedure is either inaccurate when a short measuring interval is selected, or very slowly if a long measuring interval is selected to increase the measuring accuracy.

It is an object of the present here to measure the frequency characteristic of an oscillator in a radar system for environmental detection precisely and quickly.

This object is achieved by a radar system according to the independent claim 1. Advantageous further developments can be found in the dependent claims.

The claimed radar system for detecting the surroundings with means for measuring an oscillator characteristic comprises transmitting means for the directed emission of transmission power, receiving means for the directed reception of transmission power reflected at objects and signal processing means for processing the received power. The frequency of the transmission power is modulated by a corresponding control of a designated oscillator. The oscillator is driven by a set of discrete control signal values. For at least a portion of these discrete control signal values, the output frequency of the oscillator is measured and a control signal output frequency characteristic is generated for the oscillator. The measurement of the oscillator output frequency comprises a sampling of the oscillator output signal or a signal obtained therefrom by frequency division, optionally after suitable preprocessing, a windowing of the sampled signal and a frequency determination for the windowed signal by means of a spectral analysis. The measurement is performed for each one discrete control signal value. With the claimed radar system, the oscillator characteristic can be determined accurately and quickly. These properties allow the realization of a cost-effective radar system, since the modulation of the oscillator frequency can be realized with little hardware effort. The oscillator output frequency is controlled via a control signal only by "software." The result of the software control beyond the possibility of the waveform of the transmission signal very easy to vary, for example, to dynamically adjust the frequency deviation and thus adjust the distance resolution the current environmental conditions, for example, the traffic conditions.

In a preferred embodiment of the invention, a discrete Fourier transform (DFT) is used for spectral analysis and the output frequency of the oscillator is obtained by interpolation or estimation of 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 with a fast Fourier transform (FFT). In a particular embodiment of the radar system claimed here, a voltage-controlled oscillator (VCO) is provided as an oscillator for modulating the transmission power. In a preferred embodiment of the invention, only the already measured control signal values of the oscillator characteristic curve are used for frequency modulation. Unmeasured control signal values, ie estimated or interpolated values, are not used for the generation of the transmission signal.

In particular, a signal path for detecting objects and a return measuring path for measuring the oscillator frequencies are provided. The return measurement path is arranged separately from the signal path and comprises a frequency divider. In a preferred embodiment of the invention is during the detection of

Objects, ie the signal path is active, the feedback path is deactivated so as not to disturb the detection of environment objects. A further embodiment of the invention provides that the output signal of the frequency divider in the feedback path is changed in its frequency such that the detection of environmental objects is not disturbed or only to a small degree. The frequency in the return measurement path is set so that the frequency ranges of the signals in the signal and return measurement path have little or no overlap. The same applies to the harmonics of the signals. In a particular embodiment of the invention, the frequency divider in the return measuring path is designed so that the frequency of the sampled signal is not in the Range of 1/2-f _A , 1-fΑ, 3/2-f _A , 2-fΑ, ..., where f _A , the sampling frequency, since then an interpolation of the frequency-dependent power curve between the discrete frequency values not possible is.

A particular embodiment 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 that occur in the signal path and return path. In a preferred embodiment of the radar system switching means are provided, so that only one signal, either a signal from the signal path or a signal from the return measurement path, is forwarded to the data processing unit. In particular, the same DFT is used for the evaluation of measurement signals for the detection of objects and of measurement signals for measuring the oscillator frequencies.

A preferred embodiment of the radar system provides that the output frequency of the oscillator is measured at least twice in at least one discrete oscillator drive and filtered via at least two measured values. This procedure increases the accuracy of the frequency determination.

In a particular embodiment of the invention, discrete signal values for oscillator control are generated by a digital-to-analogue converter (DAC). In an advantageous embodiment of the invention, only the voltages for oscillator drive are measured, which required for ramp generation

Oscillator output frequencies correspond. This avoids that a measurement of unused measured values, extends the entire measurement time. In a further embodiment of the invention, the detection of objects and the measurement of the oscillator frequencies is carried out at a substantially identical repetition rate. That the oscillator drive values used for

Ramp generation needed to be measured in one or a few cycles. In this way, it is ensured that a respective current characteristic is used for controlling the oscillator, so that rapidly changing environmental influences such as temperature changes, the accuracy of the radar system do not negatively influence. In order to reduce the measuring time, only one part of the characteristic curve can be measured before a detection of objects, and another part before the subsequent detection of objects and so on.

The invention will be explained in more detail with reference to figures and embodiments.

Fig. 1: Block diagram of a radar system with means for measuring an oscillator

curve

Fig. 2: sampled oscillator signal from the Rückmesspfad

Fig. 3: Spectrum of the sampled return measurement signal

Fig. 4: frequency-dependent power spectrum of the oscillator in the return path

In Fig.1 a block diagram of a radar system is shown, which is all essential

Contains elements of the invention. With the aid of a voltage-controlled, modulated oscillator VCO, a frequency-modulated transmission signal is generated, which is supplied via a coupler structure K to an antenna A. At the same time, the oscillator signal is fed to a mixer M, where it is mixed with the received signal. The output of the mixer is filtered in a bandpass filter. The feedback path R includes a frequency divider Kl and a bandpass filter BP. Feedback path and signal path are connected via a multiplexer MUX with a digital signal processing unit SP. The signal processing unit with an analog to digital converter is connected upstream.

The transmission signal used is a sequence of linear frequency ramps whose frequency deviation per unit of time is so great that the difference frequency between the transmission signal and the reception signal depends almost exclusively on the transit time and thus on the distance to the object at which the reflection takes place. The difference frequency depends only to a much smaller extent on the relative speed. Thus one obtains from the scanning of a single ramp the composite signal, the distance information, wherein the frequency of the signal is proportional to the distance. By evaluating the phase change between the sampled ramps of the mixed signal results in the Doppler frequency and thus the relative speed. The calculation of the distance and the relative speed is done using a two-dimensional

Fourier transform. Errors in the frequency ramp can lead to blurred images in the distance and thus also to obscure smaller targets, to interference lines in the range of distances and thus to misdetections and also to increased noise. In addition to the described signal path, a feedback path R is provided. Here, the oscillator signal is digitized directly and then the frequency is determined. As additional components only a frequency divider, a bandpass filter and a multiplexer are necessary for this purpose. This path is active during the transmission pauses. During measurement of ambient objects (transmit power is emitted and received), the return measurement signal is deactivated in order to avoid coupling to the signal path. In the transmission pauses, the VCO drive voltage is gradually increased via a DAC (digital-to-analog converter), whereby a control value generated by the DAC for the measurement time t _{mess is} kept constant. In this time t _mess , the return measurement signal is digitized and its frequency measured. The sampling of the return measurement signal is shown in FIG. The line gives the analog measurement signal and the points indicate the samples n = 0, l, 2, .. N of the digitized signal.

This measurement of the oscillator output signal is performed for all relevant oscillator drive values. Relevant oscillator drive values are the control values used for the environment detection. The relevant

Oscillator drive values are measured, for example, before each measurement of surrounding objects, in which exactly these control signals are used. A further embodiment of the invention provides that the measurement of the oscillator drive values is distributed over a plurality of measurement times, a measurement time for determining the oscillator characteristic curve each having a measurement time for detecting Environment objects alternate. It is z. For example, before measuring ambient objects, they measure half of the control values and before the subsequent measurement of ambient objects, they measure the other half of the control values. Accordingly, one-third or one-fourth of the control signals can also be measured before each measuring time for detecting objects. In this way, the total measuring time is shortened.

In order to increase frequency measurement accuracy in the characteristic determination, frequencies that are measured at different times at the same oscillator drive value are filtered over time in a further exemplary embodiment. The following describes how the frequencies after sampling the

Oscillator output signal can be determined. The digitized signals are multiplied by a suitable window function and then an FFT is performed. Here, the same FFT as in the target processing can be used, which offers a great advantage, especially when implementing the data evaluation on an FPGA (Field Programmable Gate Array).

In the frequency-dependent line spectrum of the return measurement signal, the oscillator frequency can be read directly. The waveform is given by the spectrum of the window function. FIG. 4 plots the power of a return measurement signal versus frequency. The lines indicate the power values for the discrete frequencies. The line position of the highest power value l_m alone only leads to a very inaccurate frequency response. Therefore, the power of the right adjacent frequency value I_r and the left adjacent frequency value I_1 are additionally used to interpolate the exact frequency of the signal f_0 taking into account the window function. The interpolation is e.g. using values stored in the data evaluation unit (lookup tables) or a

Approximation. Alternatively, the interpolation is first computed on a FPGA in a coarse grid and then more accurately computed on a microcontroller unit (MCU) by means of a correction function. By separating the signal measurement path and the feedback path within the radar system, a good signal-to-noise ratio is achieved, whereby the Interpolation of the line position to determine the oscillator frequency can be done very accurately.

As shown in Fig. 1, a bandpass filter BP is arranged in the back measuring path after the frequency divider Kl. The bandpass filter BP is designed so that the

Harmonic, which arise in the frequency division, are strongly attenuated. This design is advantageous since the harmonics can be in the same frequency range as the return measurement signal itself after the sampling, which would lead to a falsification of the measured frequency. Since the return measurement signal is a real signal, it forms symmetrically in the spectrum. This is shown in FIG. Line 1 indicates the spectrum of the return measurement signal shifted by the window function. The maximum of the peak power is 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 likewise leads to a falsification of the measured frequency. Therefore, on the one hand the divider factor of the frequency divider Ki and the sampling frequency are tuned to one another such that the frequency of the

Feedback signal as possible at - + n - f _A (n = 0, l, 2 ..) (overload for n> 0).

On the other hand, the windowing function is suitable to choose, so that for the resulting relevant frequency ranges in which the power peaks can come to lie, the spectrum of the window has already decayed so far that the power peaks no longer influence each other.

On the basis of the measured oscillator characteristic, a set of signal control values can be calculated with which the desired function for frequency modulation can be realized, for example a linear frequency ramp.

Claims

claims 1. Radar system for environment detection with Transmission means for the directed emission of transmission power, Receiving means for the directional reception of transmitted power reflected on objects and Signal processing means for processing the received power, characterized in that a) that the frequency of the transmission power is modulated by appropriate control of an oscillator, b) that the oscillator is driven a set of discrete control signal values, c) that for at least a part of these discrete control signal values, the frequency of the oscillator, d) and that this frequency measurement for at least one discrete control signal value comprises at least the following steps: Sampling of the oscillator signal or a signal obtained therefrom by frequency division, if appropriate after suitable preprocessing, • Windowing of the sampled signal and • Frequency determination for the windowed signal through a spectral analysis. 2. Radar system according to claim 1, characterized in that for spectral analysis uses a discrete Fourier transform (DFT) and the frequency is obtained by interpolation between the discrete spectral lines of the associated spectral power peaks, wherein for the interpolation, the window function used is taken into account. 3. Radar system according to one of the above claims, characterized in that the Frequency modulation only the measured control signal values are used. 4. Radar system according to one of the above claims, characterized in that a Signal path for the detection of objects is provided and for measuring the oscillator frequencies is used for a signal path separate return path with frequency divider 5. Radar system according to claim 4, characterized in that during the Detection of objects, the output signal of the frequency divider is deactivated in the return path or changed in frequency. 6. Radar system according to one of the above claims, characterized in that a Data processing unit is provided with an upstream analog-to-digital converter, which is used to evaluate incurred in the signal path and return measurement path measurement signals. 7. Radar system according to one of the above claims, characterized in that the same DFT is used for the evaluation of measurement signals for the detection of objects and of measurement signals for measuring the oscillator frequencies. 8. Radar system according to one of the above claims, characterized in that the Output frequency measured in each case a discrete oscillator control at least twice and filtered over at least two measured values. 9. Radar system according to one of the above claims, characterized in that discrete signal values for oscillator control by a digital-to-analog converter (DAC) are generated. 10. Radar system according to one of the preceding claims, characterized in that the detection of objects and the measurement of the oscillator frequencies is performed at a substantially same repetition rate.