JPH1039009A - Distance detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a distance detector capable of making distance detection with high precision without being influenced by noise generated by multi-reflection of signals by a method wherein reference phase differences of phase difference signals varied are acquired and a distance is calculated form variable cycles and wavelengths of these reference phase differences and phase difference signals. SOLUTION: In a distance detector for detecting phase differences of 2-frequency received signals by a phase difference detecting means M3 and detecting a distance upto a target substance, there are provided a reference value calculating means M4 for calculating reference phase differences being a reference value of phase differences from at least one cycle portion of phase difference signals, reference phase differences calculated by the reference value calculating means; and a distance calculating means M5 for calculating a distance up to a target substance from variable cycles and wavelengths of the phase difference signals. As described above, in order to calculate a distance from the reference phase differences obtained from at least one cycle portion of the phase difference signals and constant variable cycles and wavelengths of the phase difference signals, it is possible to make distance detection with high precision without being influenced by variations of the phase difference signals caused by multi- reflection of signals.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distance detecting device, which emits signals of at least two different frequencies,
The present invention relates to a distance detecting device that receives a signal reflected by an object and detects a distance to the object from a phase difference of the received signal.

[0002]

2. Description of the Related Art For example, Japanese Patent Laying-Open No. 47-7271 discloses a method of transmitting a continuous wave of two frequencies having a frequency difference and obtaining a distance from a phase difference between two Doppler frequencies of a received wave. Are described.

[0003]

When a two-frequency CW radar is applied to vehicle collision detection, it is required that a short distance measurement of about 0 m to several meters can be performed. The transmission wave radiated from the radar transmission antenna is reflected by the target object and reaches the reception antenna. However, at a very short distance, a part of the transmission wave reflected by the target object is reflected again by the transmission antenna and is irradiated on the target object. That is, the transmission wave applied to the target object is a combination of the direct wave from the transmission antenna and the indirect wave due to multiple reflection.

[0004] Therefore, the phase information of the two frequencies includes a ripple component due to a re-reflected wave in addition to the distance information. The period of the ripple corresponds to half of the wavelength of the transmission wave, and the amplitude of the ripple depends on the reflectance and the distance at the target object.
In a target object having a large reflectance, the signal strength of the indirect wave is large, so that a ripple having a large amplitude tends to occur on the short distance side of the detection range. Conversely, on the far side, the signal strength of the received wave decreases, and when the signal strength falls below the SN ratio specified by the receiving circuit, large ripples occur. That is, although the ripple becomes small near the center of the detection range, a large ripple is generated in most of the detection range, so that there is a problem that the accuracy of the distance detection is deteriorated.

By the way, the distance between the transmitting antenna and the target object is R, the direct wave is Vtp, the indirect wave is Vts, and the re-reflection coefficient as a function of the distance R of the indirect wave is γ (0 <γ <1). Then, the transmission wave Vt applied to the target object is expressed by the following equation. Vt = Vtp + Vts = cos ωt + γωcos (t−τ) Here, delay time τ: τ = 2R / c (c: speed of light) = cos ωt + γcos ωτcos ωt + γsin ωτsin ωt = cos ωt · (1 + γcos ωt ωτsine) , Acos ψ = 1 + γcos ωτ , When Asin ψ = γsin ωτ, = Acos ωtcos ψ + Asin ωtsin ψ = Acos (ωt-ψ) ^{next, a = (1 + γ 2} + 2γcos ωτ) 1/2 ψ = tan -1 [γsin ωτ / (1 + γ cos ωτ)], then: (1 + γ ² + 2γ cos ωτ) ^1/2 cos [tan ⁻¹ [ωt−γ sin ωτ / (1 + γ cos ωτ)]] When 0 <γ ≦ 1, Vt = (1 + γcos ωτ) cos (Ωt−γsinωτ), and the transmission wave becomes a signal subjected to amplitude modulation and phase modulation using the delay time τ as a variable.

Here, the influence on the distance measurement by the two-frequency CW method will be examined. Assuming that the transmission angular frequencies are ω ₁ and ω ₂ , the transmission wave is Vt ₁ = (1 + γ cos ω ₁ τ) cos (ω ₁ t−γ sin ω ₁ τ) Vt ₂ = (1 + γ cos ω ₂ τ) cos (ω ₂ t− γ sin ω ₂ τ). Vr ₁ = k (1 + γ cos ω ₁ τ) cos [ω ₁ (t−τ ′) − γ sin ω ₁ τ] Vr ₂ = k (1 + γ cos ω ₂ τ) cos [ω ₂ (t −τ ′) − γ sin ω ₂ τ]. Here, τ ′ = 2R / c, k is the reception amplitude. When the local signal is Vl ₁ = cos ω ₂ t Vl ₂ = cos ω ₂ t, the homodyne detection output is Vb ₁ = k / 2 (1 + γ cos ω ₁ τ) cos (ω ₁ τ ′ + γ sin ω ₁ τ) Vb ₂ = k / 2 (1 + γ cos ω ₂ τ) cos (ω ₂ τ ′ + γ sin ω ₂ τ) Therefore, the phase difference Δφ between Vb ₁ and Vb ₂ is expressed as follows.

Ω ₁ = ω ₂ , ω ₁ −ω ₂ = Δω, ω ₀ =
As (ω ₁ + ω ₂ ) / 2, Δφ = (ω ₁ −ω ₂ ) τ ′ + γ (sin ω ₁ τ−sin ω ₂ τ) = (ω ₁ −ω ₂ ) τ′−2γ sin [(ω ₁ − omega ₂₎ / 2] τsin _{[(ω 1 + ω 2) /} 2 ] τ = Δωτ'-2γsin (Δωτ / 2) sin ω 0 τ = 2ΔωR / c-2γsin (ΔωR / c) sin (2Rω 0/2) (1) The first term on the right side of the equation (1) represents a phase amount proportional to the distance. The second term on the right side represents a ripple component due to re-reflection of the reflected wave. The period of the ripple component is 2Rω ₀ / c
= 2π, the wavelength is の of the wavelength λ of the transmission wave.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and obtains a reference phase difference of a fluctuating phase difference signal, and calculates a distance from the reference phase difference, a fluctuation cycle of the phase difference signal, and a wavelength. It is an object of the present invention to provide a distance detection device capable of detecting a distance with high accuracy without being affected by noise caused by multiple reflections of a signal.

[0009]

According to the first aspect of the present invention, as shown in FIG. 1, a signal having at least two different frequencies is transmitted from a transmitting means M1 and a signal reflected by a target object M0 is received. The phase difference detecting means M
In a distance detecting device for detecting a phase difference of the received signal at 3 and detecting a distance from the detected phase difference signal to the target object, at least the phase difference signal fluctuating at a cycle corresponding to the cycle of the transmission signal. The reference value calculating means M4 for calculating a reference phase difference that is a reference value of the phase difference from one cycle, the reference phase difference calculated by the reference value calculating means, the fluctuation cycle and the wavelength of the phase difference signal, and the target And a distance calculating means M5 for calculating a distance to the object.

[0010] Since the distance is calculated from the reference phase difference obtained from at least one cycle of the phase difference signal and the constant fluctuation cycle and wavelength of the phase difference signal, the fluctuation of the phase difference signal caused by the multiple reflection of the signal. The distance can be detected with high accuracy without being affected by the distance.

[0011]

FIG. 2 is a block diagram showing an embodiment of the apparatus according to the present invention. In the figure, two frequencies f1 and f2 (f1−f2 = Δ) generated by a transmission circuit (transmission means) 10 are used.
The signal mixed with F) is radiated into the space from the transmission antenna 12 as a transmission wave, and is irradiated on the target object 14. The transmitted wave reflected by the surface of the target object 14 is received by the receiving antenna 16 and received by the receiving circuit (receiving means) 18 at frequencies f1 ′ and f1 ′.
The frequency is converted to a 2 'baseband signal. The band-pass filters (BPFs) 20 and 24 are composed of, for example, ceramic filters, and separate baseband signals of frequencies f1 ′ and f2 ′ and supply the signals to the phase difference detection circuit 24.

The phase difference detecting circuit (phase difference detecting means) 24 uses the local oscillation signal of frequency (f1 '+ f2') / 2 to compare the phases of two different frequencies f1 'and f2'. After one baseband signal is frequency-converted to have the same frequency (f1′−f2 ′) / 2, the phases of both baseband signals are compared. The phase difference signal thus obtained is digitized by the A / D converter 26 and supplied to the microcomputer 30, and is pulsed by the waveform shaping circuit 28 and supplied to the microcomputer 30. The above-mentioned phase difference signal is transmitted by the transmitting antenna 10
From the target object 14 to the receiving antenna 16 and a ripple component.

The microcomputer 30 calculates the distance to the target object 14 based on the digital phase difference signal by the processing described later. At the same time, the cycle of the pulsed phase difference signal is measured, and the relative speed with respect to the target object 14 is calculated. The distance and relative speed to the target object 14 obtained here are supplied to the collision prediction unit 32, where the target object 14
It is determined whether there is a risk of collision with the own vehicle. If there is a risk of collision, a collision prediction signal is generated.
The occupant protection device 34 outputs the collision prediction signal and the collision sensor 3
The collision detection signal of No. 6 is supplied, and an occupant is protected by operating an airbag or the like in accordance with the prediction or detection of the collision.

Here, as shown in FIGS. 3A and 3B, the phase difference signal output from the phase difference detection circuit 24 has a ripple having a half wavelength of the wavelength λ of the transmission wave. FIG. 3A is an example of the output of a phase difference signal from a target object (for example, a metal plate of a vehicle body) having a high radio wave reflection intensity. in this case,
Ripple with a large amplitude occurs in a region where the distance is relatively small, but the amplitude of the ripple tends to be reduced in a region where the distance is large.

FIG. 3B shows an example of the output of a phase difference signal from a target object (for example, a pole or a utility pole) having a small radio wave reflection intensity. In this case, the amplitude of the ripple increases as the distance increases. This is because the distance increases and the received power supplied to the receiving circuit 18 decreases, and the phase difference detecting circuit 2
When the required S / N ratio falls below 4, the ripple amplitude increases.

FIG. 4 shows a flowchart of the distance calculation processing executed by the microcomputer 30. In FIG. 10, in step S10, 0 is set to initialize each of the flags FLAG1 and FLAG2 in the reference phase difference correction mode and the reference phase difference setting mode. Note that the reference phase difference correction mode is represented by a flag FLAG1 = 1, and the reference phase difference setting mode is represented by a flag FLAG2 = 1. In this step, an initial value θINT which is the maximum detection distance of the radar (for example, 4 m) is set to the phase difference θ1.
Next, in step S12, the phase difference signal θ supplied from the A / D converter 26 is read by the number of data for one cycle of the ripple. Note that ripple occurs only when the distance to the target object 14 is changing, and does not occur when the distance is constant. Further, the wavelength of the ripple is １ ／ of the wavelength λ of the transmission wave, and if the sampling interval is determined, the number of data for one cycle of the ripple is determined.

Next, in step S14, the amplitude of the ripple of the phase difference signal θ is compared with a threshold value θTH2. This threshold θTH
2 is, for example, a value about the amplitude of a ripple obtained from a concrete telephone pole at the maximum detection distance Rmax. If the amplitude of the phase difference signal θ is larger than the threshold value θTH2, it is determined that the target object is outside the detection area, and the process proceeds to step S16, where 0 is set in each of the flags FLAG1 and FLAG2, and the initial value θINT is set in the phase difference θ1. To step S12. The amplitude of the phase difference signal θ is equal to the threshold value θTH.
If it is less than 2, the process proceeds to step S18.

In step S18, the amplitude of the phase difference signal θ is compared with a threshold value θTH1. The threshold value .theta.TH1 is, for example, a value on the order of the amplitude of a ripple obtained from a concrete telephone pole at a distance Rmax / 2. If the maximum amplitude of the ripple is π / 2 [rad], the following relationship is obtained.

Π>θTH2> θTH1 If the amplitude of the phase difference signal θ is equal to or larger than θTH1 in step S18, the process proceeds to step S20, where the flag FLAG2 is set to 1 to determine whether or not the mode is the reference phase difference setting mode. If FLAG2 ≠ 1, the flag FLAG2 is set to 1 in step S22 to indicate that the mode is the reference phase difference setting mode. Then, in step S24, the reference phase difference θREF is calculated and set by the following equation.

ΘREF = {(θ × dt)} / N where dt is a sampling interval, and N is the number of samplings for one cycle of the ripple (the number of data read in step S12). Next, in step S25, the reference phase difference θREF is set to the phase difference θ1, and the distance R to the target object 14 is calculated by the following equation (2) using the phase difference θ1.

R = θ1 × c / (4π × ΔF) (2) where c is the speed of light and ΔF is the difference frequency between the two frequencies f1 and f2. After performing step S25, the process proceeds to step S12. That is, when the target object 14 enters the detection area, the amplitude of the phase difference signal θ decreases to become equal to or smaller than the threshold value θTH2, and the flag FLAG2 is set from 0 to 1, the data of one cycle of the ripple of the phase difference signal θ The total phase average is taken as the reference phase difference θREF.

On the other hand, if FLAG2 = 1 in step S20, the flow advances to step S26 to determine whether or not the phase difference signal θ has changed by one ripple wavelength. here,
If it has not changed by one wavelength of the ripple, the process proceeds to step S12. If it has changed by one wavelength of the ripple, it is determined in step S28 whether or not the relative speed of the target object is positive (approaching direction). If the relative speed is positive, the process proceeds to step S30, in which the phase difference for one wavelength of the ripple is subtracted from the reference phase difference θREF by the following equation to obtain a phase difference θ1, and the phase difference θ1 is obtained.
Is used to calculate the distance to the target object 14 by the equation (2).

[0023]

(Equation 1)

Explaining the meaning of this equation, (2)
In the equation, the maximum distance when the phase difference θ1 = π is Rma
Assuming that x, Rmax = c / (4 · Δf). Therefore, the phase difference Δφ corresponding to one ripple wavelength is represented by the following equation.

[0025]

(Equation 2)

If the relative speed is not positive, step S
32, the phase difference of one wavelength of the ripple is added to the reference phase difference θREF by the following equation to obtain a phase difference θ1, and the distance to the target object 14 is calculated by the equation (2) using the phase difference θ1. .

[0027]

(Equation 3)

After the execution in step S30 or S32, the process proceeds to step S12. As described above, when the phase difference signal θ changes by one ripple wavelength, the phase difference θ1 is increased by adding or subtracting the phase difference Δφ for one ripple wavelength to the reference phase difference θREF in accordance with the polarity of the relative speed. The distance R from the phase difference θ1 to the target object 14 can be calculated accurately.

Incidentally, in step S18, the phase difference signal θ
If the amplitude of the ripple is smaller than the threshold value θTH1, the process proceeds to step S34, and if the flag FLAG1 is 1, it is determined whether or not the mode is the reference phase difference correction mode. If FLAG1 = 1, the process proceeds to step S26, and the above-described steps S26 to S32 are executed.

If FLAG1 = 0, FLAG1 is set to 1 in step S36, and a new reference phase difference θREF (NEW) is calculated by the following equation in step S38. θREF (NEW) = {(θ × dt)} / N Then, in step S40, the new reference phase difference θR is obtained by the following equation.
The phase difference θ1 is updated using EF (NEW), and the distance is calculated by the equation (2) using the phase difference θ1, and step S
Proceed to 12.

Steps S24 and S38 correspond to the reference value calculating means M4, and steps S25, S30, S3
2 and S40 correspond to the distance calculating means M5. FIG.
(A) and (B) show the waveforms of the phase difference signal θ1 and the phase difference signal θ when the amplitude of the phase difference signal θ is smaller than the threshold value θTH2. In this case, when the target object enters the detection area, the reference phase difference θREF includes an initial value θ corresponding to the maximum detection distance.
INT is set. Thereafter, the flag FLAG2 becomes 0
, The reference phase difference θREF is obtained by the total averaging of one cycle of the ripple of the phase difference signal θ, and every time there is a change in the half cycle of the ripple, λ / 2 is added to or subtracted from the reference phase difference θREF to obtain the phase difference. θ1 is obtained.

FIGS. 6A and 6B show the phase difference θ1 and the phase difference signal θ when the amplitude of the phase difference signal θ is smaller than the threshold value θTH1.
The respective waveforms are shown. In this case, since the target object 14 is stably detected and a highly accurate distance measurement is possible, when the flag FLAG1 is 0, the reference phase difference θREF is calculated by the total arithmetic mean of one cycle of the ripple of the phase difference signal θ. Is updated with the new reference phase difference θREF (NEW) obtained by In this way, the reference phase difference θREF is changed to a new reference phase difference θR with high accuracy.
By replacing with EF (NEW), the subsequent phase difference θ1 can be made highly accurate.

As the reference phase difference θREF and the new reference phase difference θREF (NEW), a total arithmetic mean of a plurality of cycles of the ripple may be used. FIG. 7 shows a flowchart of the relative speed calculation process executed by the microcomputer 30. In the figure, in step S50, the phase difference signal θ is read for one cycle of the ripple, and the gradient of the phase difference shown in FIG. 8 is calculated. FIG. 8 shows a phase difference signal waveform when the target object 14 is approaching. Next, it is determined in step S52 whether or not the gradient Δθ is 0. If Δθ ≠ 0, the gradient Δθ is determined in step S54.
Is determined whether or not exceeds 0.

If .DELTA..theta. <0 in step S54, the polarity POL of the relative speed is set to -1 in step S56, and .DELTA.
If θ> 0, the polarity POL of the relative speed is determined in step S58.
Is set to 1. Thereafter, the process proceeds to step S60, and the relative speed V is calculated by the following equation.

V = POL × c / (2 × f × Td) where c is the speed of light, f is the frequency of the transmission wave, and Td is the pulse period. After the execution of step S60, the process proceeds to step S50, and the process is repeated. Step S52
If Δθ = 0, the relative speed V is set to 0 in step S62, the process proceeds to step S50, and the process is repeated.

Incidentally, the pulse period Td is measured by the interrupt processing shown in FIG. The processing in FIG. 9 is interrupted at the rising edge or the falling edge of the pulsed phase difference signal supplied from the waveform shaping circuit 28. Step S
At 70, the period of the pulsed phase difference signal by the built-in timer of the microcomputer 30, that is, the time until the next rising edge or falling edge is measured.

[0037]

As described above, the first aspect of the present invention provides
The transmitting means transmits signals of at least two different frequencies, receives the signal reflected by the target object by the receiving means, detects the phase difference of the received signal, and detects the phase difference signal from the detected phase difference signal to the target object. A distance detection device that detects a distance; a reference value calculation unit that calculates a reference phase difference that is a reference value of a phase difference from at least one cycle of the phase difference signal that fluctuates at a cycle corresponding to a cycle of the transmission signal; And a distance calculating means for calculating a distance to the target object from the reference phase difference calculated by the reference value calculating means and a fluctuation cycle and a wavelength of the phase difference signal.

As described above, since the distance is calculated from the reference phase difference obtained from at least one cycle of the phase difference signal and the constant fluctuation cycle and wavelength of the phase difference signal, the fluctuation of the phase difference signal caused by the multiple reflection of the signal. The distance can be detected with high accuracy without being affected by the distance.

[Brief description of the drawings]

FIG. 1 is a principle diagram of the present invention.

FIG. 2 is a block diagram of the device of the present invention.

FIG. 3 is a diagram for explaining a phase difference signal.

FIG. 4 is a flowchart of a distance calculation process.

FIG. 5 is a diagram for explaining the present invention.

FIG. 6 is a diagram for explaining the present invention.

FIG. 7 is a flowchart of a relative speed calculation process.

FIG. 8 is a diagram for explaining the process of FIG. 7;

FIG. 9 is a flowchart of an interrupt routine.

[Explanation of symbols]

 Reference Signs List 10 transmission circuit 12 transmission antenna 14, M0 target object 16 reception antenna 18 reception circuit 20, 22 bandpass filter 24 phase difference detection circuit 26 A / D converter 28 waveform shaping circuit 30 microcomputer 32 collision prediction unit 34 occupant protection device 36 Collision sensor M1 Transmitting means M2 Receiving means M3 Phase difference detecting means M4 Reference value calculating means M5 Distance calculating means

Claims (1)

[Claims] 1. A signal transmitted from a transmitting unit having at least two different frequencies, a signal reflected by a target object is received by a receiving unit, a phase difference of the received signal is detected, and a phase difference signal is detected from the detected phase difference signal. In a distance detecting device for detecting a distance to the target object, a reference value for calculating a reference phase difference that is a reference value of a phase difference from at least one cycle of the phase difference signal that fluctuates at a cycle corresponding to a cycle of the transmission signal. Calculating means for calculating a distance to the target object from the reference phase difference calculated by the reference value calculating means, and a fluctuation cycle and a wavelength of the phase difference signal. Detection device.