Communications: electrical – Vehicle detectors – Inductive
Reexamination Certificate
1999-04-05
2001-08-21
Pope, Daryl (Department: 2632)
Communications: electrical
Vehicle detectors
Inductive
C340S933000, C324S226000
Reexamination Certificate
active
06278380
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of sensing a vehicle which uses a sensing loop coil. Also, this invention relates to an apparatus for sensing a vehicle which uses a sensing loop coil.
2. Description of the Related Art
Some vehicle sensing apparatuses include a loop coil provided on a road or buried in the ground under a road. The loop coil forms a part of a resonance circuit in an oscillation circuit. Thus, the inductance of the loop coil determines the frequency of the output signal of the oscillation circuit. When a vehicle passes through a given region or a sensing area near the loop coil, the inductance of the loop coil varies and the frequency of the output signal of the oscillation circuit also varies. Such a variation in the output signal of the oscillation circuit is detected as an indication that a vehicle passes through the sensing area (the given region).
A prior-art vehicle sensing apparatus having a loop coil detects the deviation &Dgr;f of the frequency of the output signal of an oscillation circuit from a reference frequency. When the frequency deviation &Dgr;f exceeds a first threshold value, it is decided that a vehicle passes through a sensing area. The inductance of the loop coil, that is, the frequency of the output signal of the oscillation circuit, tends to vary in response to an environmental change such as a change in atmospheric temperature or a start of rain. In view of this fact, the prior-art vehicle sensing apparatus implements adaptive control of the reference frequency. Specifically, there is set a second threshold value smaller than the first threshold value. An allowable range for the frequency deviation &Dgr;f is provided in connection with the second threshold value. In the case where the frequency deviation &Dgr;f moves out of the allowable range, a decision is made as to whether or not the frequency deviation &Dgr;f exceeds the second threshold value during a given time interval t0 from its movement out of the allowable range. If it is decided that the frequency deviation &Dgr;f does not exceed the second threshold value until the end of the given time interval t0, the reference frequency is corrected. The given time interval t0 is referred to as a follow-up time for an environmental change. Generally, the follow-up time is set to 1 second.
The prior-art vehicle sensing apparatus has a problem as follows. Vehicles in a traffic jam tend to slowly pass through the sensing area. In some of these cases, the frequency deviation &Dgr;f remains lower than the second threshold value for longer than the follow-up time so that the reference frequency is corrected. This correction of the reference frequency is caused by the traffic jam rather than an environmental change to be considered.
SUMMARY OF THE INVENTION
It is a first object of this invention to provide a method of sensing a vehicle which is less affected by a traffic jam.
It is a second object of this invention to provide an apparatus for sensing a vehicle which is less affected by a traffic jam.
A first aspect of this invention provides a method of sensing a vehicle which comprises the steps of placing first and second loop coils at respective positions separate from each other by a predetermined distance; generating a first electric signal depending on an inductance of the first loop coil; generating a second electric signal depending on an inductance of the second loop coil; detecting a speed of a vehicle, which passes through first and second given regions determined by the respective positions of the first and second loop coils, in response to the first and second electric signals; comparing the first electric signal with a reference signal to decide whether or not a vehicle passes through the first given region determined by the position of the first loop coil; controlling the reference signal in response to conditions of the first electric signal which occur in a follow-up time; and changing the follow-up time in response to the detected vehicle speed.
A second aspect of this invention is based on the first aspect thereof, and provides a method wherein the changing step comprises increasing the follow-up time as the detected vehicle speed decreases.
A third aspect of this invention provides a method of sensing a vehicle which comprises the steps of placing first and second loop coils at respective positions separate from each other by a predetermined distance; generating a first electric signal depending on an inductance of the first loop coil; generating a second electric signal depending on an inductance of the second loop coil; comparing the first electric signal with a reference signal to decide whether or not a vehicle passes through a given region determined by the position of the first loop coil; controlling the reference signal in response to the first electric signal; deciding whether or not the first and second electric signals correlate with each other; permitting the control of the reference signal when it is decided that the first and second electric signals correlate with each other; and inhibiting the control of the reference signal when it is decided that the first and second electric signals do not correlate with each other.
A fourth aspect of this invention provides a method of sensing a vehicle which uses a loop coil. The method comprises the steps of generating an electric signal depending on an inductance of the loop coil; comparing the electric signal with a reference signal to decide whether or not the electric signal is in a range determined by the reference signal, wherein the electric signal being in the range represents that a vehicle passes through a given region determined by a position of the loop coil, and the electric signal being not in the range represents that a vehicle does not pass through the given region determined by the position of the loop coil; and changing the reference signal in cases where the electric signal continues to be in the range for a predetermined time.
A fifth aspect of this invention provides an apparatus for sensing a vehicle which comprises first and second loop coils located at respective positions separate from each other by a predetermined distance; means for generating a first electric signal depending on an inductance of the first loop coil; means for generating a second electric signal depending on an inductance of the second loop coil; means for detecting a speed of a vehicle, which passes through first and second given regions determined by the respective positions of the first and second loop coils, in response to the first and second electric signals; means for comparing the first electric signal with a reference signal to decide whether or not a vehicle passes through the first given region determined by the position of the first loop coil; means for controlling the reference signal in response to conditions of the first electric signal which occur in a follow-up time; and means for changing the follow-up time in response to the detected vehicle speed.
A sixth aspect of this invention is based on the fifth aspect thereof, and provides an apparatus wherein the changing means comprises means for increasing the follow-up time as the detected vehicle speed decreases.
A seventh aspect of this invention provides an apparatus for sensing a vehicle which comprises first and second loop coils located at respective positions separate from each other by a predetermined distance; means for generating a first electric signal depending on an inductance of the first loop coil; means for generating a second electric signal depending on an inductance of the second loop coil; means for comparing the first electric signal with a reference signal to decide whether or not a vehicle passes through a given region determined by the position of the first loop coil; means for controlling the reference signal in response to the first electric signal; means for deciding whether or not the first and second electric signals correlate with each other; means for permitting the control of t
Minamisawa Shinichi
Saitoh Yoshinobu
Matsushita Electric - Industrial Co., Ltd.
Pope Daryl
Woo Louis
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