Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
2002-01-14
2003-07-01
Tarcza, Thomas H. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S005030, C342S118000
Reexamination Certificate
active
06587187
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a distance measuring apparatus and method for measuring a measurement object time representing a duration from a measurement start time to an input of measurement object pulse, and also relates to a distance measuring apparatus and method for measuring a distance from the time measuring apparatus to a measurement object.
A spectrum spread type distance measuring apparatus, which measures a distance based on a pseudo-random noise code (hereinafter, abbreviated as PN code) such as a M-sequences code, is conventionally known and used in an automotive vehicle to measure a distance from this vehicle to a preceding vehicle (i.e., an object or obstacle ahead of this vehicle).
This kind of distance measuring apparatus is characterized in that an electromagnetic wave is amplitude modulated based on a PN code having a predetermined bit length and is transmitted to a measurement object. The distance measuring apparatus receives a reflection wave of the transmitted electromagnetic wave reflected by the measurement object and demodulates a binary signal corresponding to the PN code. The distance measuring apparatus obtains a correlation value between the demodulated binary signal and the PN code, and detects a specific time at which the correlation value is maximized. Then, the distance measuring apparatus detects a duration (i.e., time interval) required for the electromagnetic wave to trip (i.e., go and return) between the distance measuring apparatus and the measurement object, and finally calculates a distance based on the detected trip time and the speed of electromagnetic wave (3×10
5
km/sec).
However, according to the spectrum spread type distance measuring apparatus, the time resolution in the measurement of the trip time is substantially limited by a transmission clock (hereinafter, referred to as reference clock) used in the modulation of electromagnetic wave according to the PN code. For example, the time resolution corresponding to the clock frequency of 20 MHz is 50 nsec (=1 [sec]/20×10
6
). The corresponding measurable distance resolution is thus limited to 7.5 m (=3×10
8
[m/sec]×50×10
−9
[sec]/2).
To improve the measurable distance resolution in the spectrum spread type distance measuring apparatus, the unexamined Japanese patent publication No. 2000-121726 proposes to transmit and receive one additional pulse of electromagnetic wave and measures an error component of the trip time by using a gate delay time of a gate circuit which has a high resolution equivalent to several nsec or less, thereby correcting the trip time based on a detected error component.
However, the above-described technique takes a relatively long time to accomplish one complete time measurement operation because it is necessary to separately perform two stages of measurements, i.e., a coarse measurement based on the reference clock (having a low resolution) and a fine measurement based on the gate delay time (having a high resolution).
Furthermore, the electromagnetic wave used in the distance measuring apparatus is a laser beam which is emitted from a laser diode. So frequently actuating or driving the laser diode will lead to a large amount of heat generation and deteriorate the laser diode.
SUMMARY OF THE INVENTION
In view of the foregoing problems of the prior art, the present invention has an object to provide a time measuring apparatus and method capable of measuring a duration from a measurement start time to an input time of measurement object pulse within a short period of time by simultaneously performing a coarse measurement based on a reference clock and a fine measurement based on a shorter reference time (e.g., a gate delay time).
Furthermore, the present invention has an object to provide a distance measuring apparatus incorporating the time measuring apparatus as well as a distance measuring method incorporating the time measuring method.
To accomplish the above and other related objects, the present invention provides a time measuring apparatus which comprises first reference clock generating means for generating a first reference clock at predetermined periods and coarse measuring means for measuring an approximate measurement object time based on the first reference clock. The approximate measurement object time represents a duration from a measurement start time to an input time of measurement object pulse. This apparatus is characterized by fine measuring means which cooperates with the coarse measuring means and uses a reference time of predetermined periods shorter than those of the first reference clock, for measuring a time difference between a change point (e.g., a leading edge or a trailing edge) of the first reference clock and the input time of measurement object pulse as a correction time of the approximate measurement object time. A precise measurement objet time is obtained based on the approximate measurement object time measured by the coarse measuring means and the correction time measured by the fine measuring means.
Meanwhile, the present invention provides a time measuring method comprising the steps of generating a first reference clock at predetermined periods, and measuring an approximate measurement object time based on the first reference clock, the approximate measurement object time representing a duration from a measurement start time to an input time of measurement object pulse. This method is characterized by the steps of measuring a time difference between a change point of the first reference clock and the input time of measurement object pulse as a correction time of the approximate measurement object time by using a reference time of predetermined periods shorter than those of the first reference clock, and obtaining a precise measurement objet time based on the approximate measurement object time and the correction time.
According to the time measuring apparatus and method of this invention, it becomes possible to simultaneously perform the coarse measurement using the first reference clock and the fine measurement using the shorter reference time. Thus, an accurate time measurement using the coarse measuring means and the fine measuring means can be accomplished within a short time.
Accordingly, when the time measuring apparatus or method of this invention is incorporated into a distance measuring apparatus or method, the laser diode emitting an electromagnetic wave for distance measurement will not be activated so frequently and therefore the laser diode will not be deteriorated hardly due to generated heat.
The time difference measured by the coarse measuring means is a duration from a change point (e.g., a leading edge or a trailing edge) of the first reference clock to the input time of measurement object pulse. The change point of the first reference clock can be arbitrarily set.
If the time difference measured by the coarse measuring means exceeds one period of the first reference clock, it will be necessary to reduce one period of the first reference clock when obtaining the correction time.
Accordingly, it is preferable that the time difference is measured based on a change point of the first reference clock closest to the input time of measurement object pulse.
The measured time difference can be directly used as the correction time of the approximate measurement object time. The calculating operation can be simplified.
It is also preferable that the reference time used to measure the time difference is a gate delay time of a gate circuit (more specifically, a delay time of a signal inherently caused when passing an inverter, an OR gate, an AND gate, or any other gate circuit) or a comparable short time.
The gate delay time of a gate circuit is dependent on performance characteristics of semiconductor elements constituting the gate circuit and is a very short time in the level of several nsec or less. Thus, using the gate delay time can realize a very accurate measurement of the time difference.
Wh
Matsui Takeshi
Shirai Noriaki
Watanabe Takamoto
Andrea Brian
Denso Corporation
Tarcza Thomas H.
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