1. Field of the Invention
The present invention relates to a radar device, and more particularly to a vehicle-mounted radar device which is mounted on a vehicle such as an automobile and receives a transmitted electromagnetic wave which has been reflected by an object to detect a distance to the object and a relative speed of the object.
2. Description of the Related Art
In a vehicle such as an automobile, a vehicle-mounted radar device is employed in order to measure a relative speed and a relative distance with respect to a preceding vehicle or the like. As a conventional vehicle-mounted radar device, there has been known, for example, a structure shown in FIG. 11. In FIG. 11, reference numeral 41 denotes a modulator, 42 is a voltage control transmitter, 43 is a power divider, 44 is an transmit antenna, 45 is a target object, 46 is a receive antenna, 47 is a mixer, 48 is a low-pass filter (hereinafter referred to as “LPF”), 49 is an A/D converter, 50 is an FFT processing device and 51 is a signal processing device.
Subsequently, the operation of a conventional device thus structured will be described. The modulator 41 outputs a linear voltage signal for FM modulation. The voltage control transmitter 42 generates an electromagnetic wave that has been subjected to FM modulation in accordance with the FM modulation voltage signal. The electromagnetic wave is divided into two waves by the power divider 43 and one of those waves is inputted to the mixer 47. Another wave is outputted to a space from the transmit antenna 44. The electromagnetic wave that has been outputted to the space from the transmit antenna 44 is reflected by the target object 45 and then inputted to the receive antenna 46 with a delay time Td[S] with respect to the transmit electromagnetic wave. In addition, in the case where the target object 45 has a relative speed, the receive electromagnetic wave is inputted to the receive antenna 46 with a Doppler shift Fd [Hz] with respect to the transmit electromagnetic wave. The electromagnetic wave that has been received by the receive antenna 46 is mixed with the transmit electromagnetic wave that has been inputted by the power divider 43 by the mixer 47 to output a beat signal corresponding to the delay time Td and the Doppler shift Fd. The LPF 48 is so structured as to make a signal having a frequency component of the half or less of the sampling frequency Fs of the A/D converter 49 pass therethrough. The A/D converter 49 samples the beat signal that has passed through the LPF 48 at a sampling frequency Fs [Hz]. The FFT processing device 50 subjects the beat signal that has been sampled by the A/D converter 49 to high-speed Fourier transformation (FFT) to output the frequency component of the beat signal. The signal processing device 51 calculates the relative distance to the target object 45 and the relative speed of the target object 45 in accordance with the frequency component that has been outputted from the FFT processing device 50.
Subsequently, a method of calculating the relative distance and the relative speed by the signal processing device 11 will be described. FIG. 12 is an example in which the relative distance and the relative speed are calculated by using the radar device. In FIG. 12, a transmit electromagnetic wave is FM-modulated at the frequency is FM-modulated at the frequency sweep band width B of the transmit electromagnetic wave and the modulation period Tm. The receive electromagnetic wave has a delay time Td required until the transmit electromagnetic wave is reflected by the target object 45 that is apart from the transmit antenna 44 by a distance R and is then inputted to the receive antenna 46. Also, when the target object 45 has the relative speed, the receive electromagnetic wave is Doppler-shifted by Fd with respect to the transmit electromagnetic wave. Therefore, a frequency difference Fbu between the transmit signal and the receive signal is contained in the frequency components included in the beat signal that has been mixed by the mixer 47 when the frequency comes up and a frequency and a frequency difference Fbd between the transmit signal and the receive signal is contained in the frequency component when the frequency comes down. The relative distance R and the relative velocity v with respect to the target object 45 is found by the above Fbu, Fbd, Tm, B, a light velocity C (3.0×108 m/s) and the wavelength λ of a carrier wave (λ=5.0×10−3 m if a fundamental frequency of the carrier is Fo=60 GHz) from the following expression (1).R=(TmC/8B)×(Fbu+Fbd)v=(λ/4)×(Fbu−Fbd)  (1)
Subsequently, the high-speed Fourier transformation of the FFT processing device 49 will be described. Normally, the high-speed Fourier transformation inputs the sampling data of 2n FFT points and outputs 2n frequency component data. When an observation time is Tm/2, a frequency resolution is represented by the following expression (2).ΔF=2/Tm  (2)
A maximum frequency Fmax that can be detected with precision is represented by the following expression (3).Fmax=2n−1×ΔF=2n−1×2/Tm  (3)
When a frequency component that is equal to or higher than the above maximum frequency Fmax is inputted to the high-speed Fourier transformation, aliasing occurs, and a virtual frequency component caused by returning at the frequency Fmax appears as shown in FIG. 2. In order to prevent the virtual frequency component caused by the aliasing, the LPF 48 is located upstream of the input of the FFT processing device 49 so as to cut the frequency components that are equal to or higher than Fmax.
Subsequently, it is assumed that the resolutions of the relative distance R and the relative velocity v (the minimum steps of data values that are outputted discretely) are ΔR and Δv, respectively. The resolutions ΔF of the frequency differences Fbu and Fbd become the above frequency resolutions 2/Tm, and ΔR and Δv are represented by the following expressions.ΔR=(TmC/8B)×(ΔF+ΔF)=(TmC/8B)×(4/Tm)=C/2B  (4)Δv=(λ/4)×(ΔF+ΔF)=(λ/4)×(4/Tm)=λ/Tm  (5)
In the case where a radar of, for example, the distance resolution ΔR=1 [m] and the maximum detection distance Rmax=150 [m] is designed, a necessary modulation width B requires the following modulation width from the above-mentioned expression (4)B=C/2/ΔR=150 [MHz]
And the FFT points require 2n=512 points that satisfy the following condition.2n≧ΔR×Rmax×2=300
Also, assuming that the velocity resolution Δv=1 [km/h], the modulation period Tm requires the following condition.Tm=λ/Δv=5.0×10−3×3.6=18×10−3 [S]
When the object of the distance 150 m and the relative velocity 0 km/h is detected, both of the frequency differences Fbu and Fbd are represented by the following expression.Fbu=Fbd=ΔF×150/ΔR=ΔF×150
However, when the object of the distance 150 m and the relative velocity 200 km/h is detected, the frequency differences Fbu and Fbd are represented by the following expressions, respectively.Fbu=|ΔF×150/ΔR+ΔF×200/ΔR|=ΔF×350Fbd=|ΔF×150/ΔR−ΔF×200/Δv|=ΔF×50
When the FFT point is 512, the maximum frequency Fmax is represented by the following expression.Fmax=ΔF×256
And a frequency component that is equal to or higher than the maximum frequency cannot be normally detected. For that reason, the frequencies that are equal to or higher than Fmax are cut by the LPF 48. Therefore, Fbu=ΔF×350 is cut by the LPF 48 and cannot be detected.
In order to solve the above problem, there is proposed that the resolution of the distance resolution ΔR or the relative velocity resolution Δv is made coarse. For example, when ΔR=1.5 m, Δv=1.5 km/h, then Fbu=ΔF×175, and the LPF 48 allows the frequency components of Fmax′=Fmax×1.5 or lower to pass therethrough, to thereby enable the detection.
Alternatively, ΔR and Δv are not changed as they are and the FFT point increases to 1024 point, and the LPF 48 allows the frequency component of Fmax″=Fmax×2 to pass therethrough, to thereby enable the detection.
However, the coarse resolution leads to the deterioration of the radar performance, an increase in the FFT point leads to a great increase in the calculation volume and a storage device, resulting in a serious problem from the viewpoints of the costs.