Vehicle detection system

• US 3,875,555 A
• Filed: 05/29/1973
• Issued: 04/01/1975
• Est. Priority Date: 05/29/1973
• Status: Expired due to Term

• Current Assignee: Display Technologies, Inc. (Rhode Island)
• Original Assignee: Thomas R. Potter
• Inventors: Potter, Thomas R.
• Primary Examiner(s): Claffy, Kathleen H.
• Assistant Examiner(s): Myers, Randall P.
• Application Number:
  US05/364,492
• Time in Patent Office: 672 Days
• Field of Search: 340/38L
• US Class Current: 340/939
• CPC Class Codes: G01V 3/101 by measuring the impedance of the search coil; by measuring features of a resonant circuit comprising the search coil measuring impedance or characteristics derived therefrom G01R27/00, e.g. quality factor G01R27/26
• CPC Class Codes: G08G 1/042 using inductive or magnetic detectors

Abstract

An improved magnetic inductance vehicle detection system is provided which responds to changes in the inductance of a wire loop embedded in the roadway to sense the presence of a vehicle. The system includes a first oscillator connected to the loop whose frequency changes as the loop inductance changes, and it also includes a second oscillator whose frequency is independent of the loop inductance, and which is used as a reference. Appropriate logic circuitry establishes an output signal whenever the frequency of the loop oscillator increases with respect to the frequency of the reference osillator beyond a predetermined frequency differential, so as to indicate the fact that a vehicle has crossed in the area circumscribed by the loop. This output signal is formed into a direct current signal which actuates a relay in a vehicle indicating circuit. The system of the invention also includes a control circuit for compensating for drifts in the system. This control circuit includes a network which responds to an error signal to develop a fast-acting antidrift control whenever the frequency of the loop oscillator falls below the frequency of the reference oscillator; and which also develops a slow-acting anti-drift control to compensate for frequency drifts in the system by which the frequency of the loop oscillator rises above the frequency of the reference oscillator.

Claims

1. An inductive type of vehicle detection system including:

an inductive loop to be placed in position to sense the presence of a metallic vehicle;

first pulse generating means including a first oscillator circuit connected to the loop and utilizing the loop as a frequency determining element;

second pulse generating means including a second oscillator circuit;

logic circuit means connected to said first and second pulse generating means for developing output pulses when the frequency differential between said oscillator circuits exceeds a predetermined threshold; and

output circuit means connected to said logic circuit means for developing a control signal in response to said output pulses, in which said first and second pulse-generating means includes first and second digital counters respectively connected to the outputs of said first and second oscillator circuits for providing a predetermined frequency division to the outputs therefrom.

2. An inductive type of vehicle detection system including:

an inductive loop to be placed in position to sense the presence of a metallic vehicle;

a first oscillator circuit connected to the loop and utilizing the loop as a frequency determining element;

a second oscillator circuit;

first logic circuit means connected to said first and second oscillating circuits for developing output pulses only when the frequency of said first oscillator exceeds the frequency of said second oscillator by a predetermined threshold;

output circuit means connected to said logic circuit means for developing a control signal in response to said output signal, and which includes further logic circuit means connected to said first and second oscillator circuits for developing output pulses when the frequency of said first oscillator circuit falls below the frequency of said second oscillator circuits;

an error integrator network connected to said first logic circuit means for developing an increasing direct current output voltage in response to said output pulses from said further logic circuit means; and

a circuit connected to said integrator for applying said increasing direct current output voltage to said first oscillator to increase the frequency of said first oscillator towards the frequency of said second oscillator.

3. The vehicle detection system defined in claim 2, and which includes a source of clock pulses connected to said integrator for introducing predetermined time clock pulses to said integrator to cause said integrator to develop a direct current output voltage which decreases at a relatively slow rate as compared with the aforesaid increase of said direct current output voltage in response to said output pulses from said further logic circuit means to decrease the frequency of said first oscillator whenever said first oscillator freqUency drifts above the frequency of said second oscillator.

4. The vehicle detection system defined in claim 3, in which said error integrator includes an up-down counter responsive to said output pulses from said further logic circuit means for counting in one direction, and responsive to clock pulses from said source of clock pulses for counting in the other direction;

and a resistance network connected to said up-down counter for developing an output voltage corresponding to the count of said counter.

5. An inductive type of vehicle detection system including:

an inductive loop to be placed in position to sense the presence of a metallic vehicle;

a first oscillator circuit connected to the loop and utilizing the loop as a frequency-determining element;

a second oscillator circuit;

logic circuit means connected to said first and second oscillator circuits for developing output pulses when the frequency differential between said oscillator circuits exceeds a predetermined threshold; and

output circuit means connected to said logic circuit means for developing a control signal in response to said output pulses, and which includes a network for changing said predetermined threshold from a first level to a second level whenever said logic circuit means develops output pulses, and in response to such output pulses.

6. An inductive type vehicle detection system including:

an inductive loop to be placed in position to sense the presence of a metallic vehicle;

a first oscillator circuit connected to the loop and utilizing the loop as a frequency determining element;

a second oscillator circuit;

vehicle detection logic circuit means connected to said first and second oscillator circuits for developing output pulses upon the occurrence of a frequency differential between said first and second oscillator circuits beyond a predetermined threshold indicating a rise in the frequency of the first oscillator circuit as compared with the frequency of the second oscillator circuit;

output circuit means connected to said vehicle-detection logic circuit means for developing a control signal in response to said output pulses;

signal-drift logic circuit means connected to said first and second oscillator circuits for developing output pulses upon the occurrence of a frequency differential between said first and second oscillator circuits indicating a drop in the frequency of said first oscillator circuit as compared with the frequency of said second oscillator circuit;

an error digital integrator connected to said signal-drift logic circuit means for developing a varying direct current output voltage in response to the output pulses from said signal-drift logic circuit means; and

a circuit connected to said digital integrator for applying said direct current output voltage therefrom to said first oscillator to vary the frequency thereof in a direction to reduce said last-named frequency differential.

7. The vehicle detection system defined in claim 6, in which said digital integrator includes a up-down counter responsive to the pulses from said logic circuit means for counting in one direction;

and a resistance network connected to said up-down counter for developing an output direct current voltage having a value dependent upon the count of said counter.

8. The vehicle detection system defined in claim 7, and which includes a clock source connected to said digital integrator for introducing clock pulses to said counter to cause said counter to count in the opposite direction, said clock pulses recurring at a relatively low rate with respect to the pulses from said logic circuit means.