Communications: directive radio wave systems and devices (e.g. – Determining distance
Reexamination Certificate
1999-10-15
2001-04-17
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Determining distance
C342S127000, C342S128000, C342S145000
Reexamination Certificate
active
06218982
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a distance measurement apparatus using an electromagnetic wave modulated in accordance with a pseudo random noise code.
2. Description of the Related Art
A prior-art distance measurement apparatus of a spread spectrum type which is mounted on an automotive vehicle measures the distance between the present vehicle and a preceding target object such as a preceding vehicle by using an electromagnetic wave modulated in accordance with a pseudo random noise code. Specifically, a beam of an electromagnetic wave whose amplitude is modulated in accordance with a pseudo random noise code of a predetermined bit length is emitted in a forward direction with respect to the body of the present vehicle. A moment of the transmission of the pseudo random noise code with the electromagnetic wave is memorized. The prior-art apparatus receives an echo beam caused by reflection of the forward electromagnetic-wave beam at a preceding target object. The received echo beam is converted into a binary echo electric signal. Calculation is made about the value of the correlation between the binary echo electric signal and the pseudo random noise code used for the modulation of the transmitted electromagnetic wave. A moment at which the calculated correlation value peaks is detected as a moment of the reception of the pseudo random noise code contained in the echo beam. The prior-art apparatus calculates the distance between the present vehicle and the preceding target object from the time interval between the moment of the transmission of the pseudo random noise code and the moment of the reception thereof, and also from the velocity of the electromagnetic wave.
In the prior-art apparatus, the pseudo random noise code has a sequence of bits, the number of which is predetermined. The correlation-value calculation is iterated at a period corresponding to one bit of the pseudo random noise code. Accordingly, the time interval between the moment of the transmission of the pseudo random noise code and the moment of the reception thereof is determined at a resolution corresponding to a 1-bit time interval (a 1-chip time interval). A resolution of the measured distance between the present vehicle and the preceding target depends on the resolution of the determination of the time interval. In the case where a clock frequency, the reciprocal of which corresponds to a 1-bit time interval, is equal to 20 MHz, the resolution of the measured distance between the present vehicle and the preceding target is equal to 7.5 m.
Japanese published unexamined patent application 5-312950 discloses a distance measurement apparatus which includes a maximum length code generator. A laser diode generates laser light modulated in accordance with a maximum length code produced by the maximum length code generator. A beam of the laser light is emitted toward an object. The apparatus of Japanese patent application 5-312950 includes a phase shifter which shifts the maximum length code by different values within a time interval corresponding to one bit of the maximum length code. The phase shifter outputs a plurality of shifted maximum length codes. The apparatus of Japanese patent application 5-312950 includes a photodiode which receives an echo beam caused by the reflection of the forward laser beam at the object. The photodiode converts the received echo beam into a corresponding echo electric signal. The values of the correlations between the echo electric signal and the shifted maximum length codes are calculated. The distance to the object is computed on the basis of the calculated correlation values.
Japanese published unexamined patent application 10-2963 discloses a distance measurement apparatus which uses an electromagnetic wave modulated in accordance with a pseudo random noise code. The apparatus of Japanese patent application 10-2963 includes a correlator which measures a time interval shorter than a 1-chip time interval (a 1-bit time interval) with respect to the pseudo random noise code.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a distance measurement apparatus having a high resolution.
A first aspect of this invention provides a distance measurement apparatus comprising pseudo-random-noise-code generating means for generating a pseudo random noise code in synchronism with a reference clock signal, the pseudo random noise code having a predetermined bit length; transmitting means for transmitting a first forward electromagnetic wave modulated in accordance with the pseudo random noise code generated by the pseudo-random-noise-code generating means; receiving means for receiving a first echo wave caused by reflection of the first forward electromagnetic wave at an object, and converting the received first echo wave into a binary signal; correlation calculating means for repetitively calculating a value of a correlation between the binary signal generated by the receiving means and the pseudo random noise code used for modulation of the first forward electromagnetic wave by the transmitting means at a predetermined period having a synchronous relation with the reference clock signal; first time measuring means for detecting a timing at which the correlation value calculated by the correlation calculating means peaks, and measuring a time interval taken by the first forward electromagnetic wave and the first echo wave to travel to and from the object in response to the detected timing; transmitted-pulse generating means for, after the time interval has been measured by the first time measuring means, outputting a transmitted pulse signal to the transmitting means and causing the transmitting means to transmit a second forward electromagnetic wave modulated in accordance with the transmitted pulse signal, wherein the receiving means is operative for receiving a second echo wave caused by reflection of the second forward electromagnetic wave at the object, and converting the received second echo wave into a received pulse signal; transmitted-pulse delaying means for delaying the transmitted pulse signal generated by the transmitted-pulse generating means by a delay time corresponding to the time interval measured by the first time measuring means, and thereby converting the transmitted pulse signal into a delayed transmitted pulse signal; second time measuring means for measuring a phase difference between the received pulse signal generated by the receiving means and the delayed transmitted pulse signal generated by the transmitted-pulse delaying means at a resolution higher than a resolution corresponding to the predetermined period of the correlation-value calculation by the correlation calculating means; and distance calculating means for calculating a distance to the object on the basis of the time interval measured by the first time measuring means and the phase difference measured by the second time measuring means.
A second aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the transmitted-pulse generating means operates for repetitively outputting the transmitted pulse signal a plurality of times, and the second time measuring means operates for measuring the phase difference in connection with each of the transmitted pulse signals, and wherein the distance calculating means comprises means for calculating an average of the phase differences measured by the second time measuring means, and means for calculating the distance to the object on the basis of the time interval measured by the first time measuring means and the calculated average of the phase differences.
A third aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the delay time provided by the transmitted-pulse delaying means is equal to the time interval measured by the first time measuring means minus a time interval corresponding to the predetermined period of the correlation-value calculation by the correlation calc
Hoashi Yoshiaki
Matsui Takeshi
Shirai Noriaki
Denso Corporation
Pillsbury & Winthrop LLP
Sotomayor John B.
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