VEHICLE DETECTION SYSTEM

• US 3,775,742 A
• Filed: 09/18/1972
• Issued: 11/27/1973
• Est. Priority Date: 09/18/1972
• Status: Expired due to Term

• Current Assignee: John E. Higbee, Martin C. Henderson, Ralph J. Koerner
• Original Assignee: John E. Higbee, Martin C. Henderson, Ralph J. Koerner
• Inventors: Koerner, Ralph J., Henderson, Martin C., Higbee, John E.
• Primary Examiner(s): Cooper, William C.
• Application Number:
  US05/290,127
• Time in Patent Office: 435 Days
• Field of Search: 340/38L,258C
• US Class Current: 340/938
• CPC Class Codes: G08G 1/01 Detecting movement of traffic to be counted or controlled G08G1/07 - G08G1/14 take precedence
• CPC Class Codes: G08G 1/042 using inductive or magnetic detectors

Abstract

A system for detecting vehicles and suitable for use in conjunction with other equipment for traffic monitoring and control. The system includes one or more sensors capable of measuring and indicating magnetic field intensity and a central electronic unit which interprets the output signals produced by the sensors. The sensors preferably comprise flux gate magnetometers. The system can be operated in either of two modes; i.e. a '"'"''"'"''"'"''"'"'between lanes'"'"''"'"''"'"''"'"' mode (I) in which each sensor is mounted adjacent to a lane being monitored and an '"'"''"'"''"'"''"'"'in lane'"'"''"'"''"'"''"'"' mode (II) in which each sensor is mounted in a lane being monitored. In mode I the central unit produces a detect output signal in response to a magnetic field intensity reduction, as would be caused by a vehicle passing in a monitored lane, which exceeds a first threshold level. In mode II, the detect output signal is produced in response to a sensed magnetic field intensity increase, as would be caused by a vehicle passing in a monitored lane, which exceeds a second threshold level. Both the first and second threshold levels are established as some percentage of the ambient magnetic field intensity. A bias signal, developed as a function of the sensed ambient field intensity, is fed back to the sensors to null their output under quiescent conditions, thus enabling the system to automatically adapt to different ambient field conditions. Time delay means are incorporated in the feed back loop producing the bias signal to prevent the system from adapting to short term magnetic field changes.

Claims

1. Apparatus for interpreting signals appearing across first and second output terminals of a magnetic field sensor producing an output signal representative of the intensity of magnetic field sensed by said sensor, said apparatus comprising:

first and second apparatus input terminals adapted to be selectively connected to said first and second sensor output terminals in accordance with first and second opposite polarities;

circuit means connected to said apparatus input terminals for producing a DC voltage representative of the intensity of magnetic field sensed by said sensor;

servo means responsive to said DC voltage producing a bias signal at said apparatus input terminals for application to said sensor output terminals for forcing said DC voltage to a predetermined value;

threshold means for producing a first threshold signal having a value displaced from said predetermined value by an amount substantially proportional to the field intensity sensed by said sensor in the absence of said bias signal; and

comparison means for comparing the values of said DC voltage and said first threshold signal.

2. The apparatus of claim 1 wherein said servo means includes error amplifier means having a first input terminal connected to a voltage source of said predetermined value, a second input terminal connected to said DC voltage, and an error voltage output terminal.

3. The apparatus of claim 2 wherein said servo means further includes integrator means producing said bias signal and having input means connected to said error voltage output terminal and output means connected to said apparatus input terminals.

4. The apparatus of claim 3 wherein said threshold means is connected to said integrator means output means and responsive to said bias signal produced thereat.

5. The apparatus of claim 4 wherein said threshold means includes means for developing first and second threshold signals, each constituting a predetermined percentage of said bias signal.

6. The apparatus of claim 5 including a magnetic field sensor having first and second output terminals;

connector means having at least first and second contacts detachably connectable to said apparatus first and second input terminals and third and fourth contacts respectively connected to said first and second contacts; and

means connecting said sensor first and second output terminals to said third and fourth contacts.

7. The apparatus of claim 6 including means responsive to said first and second output terminals being connected respectively to said third and fourth contacts for coupling said first threshold signal to said comparison means and means responsive to second and first output terminals being connected respectively to said third and fourth contacts for coupling said second threshold signal to said comparison means.

8. The apparatus of claim 5 including a magnetic field sensor having first and second output terminals;

means responsive to said first and second output terminals being connected respectively to said first and second apparatus input terminals for coupling said first threshold signal to said comparison meAns and to said first and second output terminals being connected respectively to said second and first apparatus input terminals for coupling said second threshold signal to said comparison means.

9. The apparatus of claim 1 including a magnetic field sensor comprising a flux gate magnetometer providing an AC output signal of frequency 2f and phase phi 1, in response to a magnetic field intensity in a first direction sensed by said sensor and of frequency 2f and phase ( phi 1+180*) in response to a magnetic field intensity in an opposite direction sensed by said sensor;

means providing a reference signal of frequency 2f and phase phi 1; and

wherein said circuit means includes means for determining whether said AC output signal is in phase or out of phase with respect to said reference signal.

10. The apparatus of claim 9 wherein said circuit means for producing said DC voltage includes means responsive to the amplitude of said AC output signal for developing the magnitude of said DC voltage and responsive to the phase of said AC output signal for developing the polarity of said DC voltage.

11. The apparatus of claim 10 wherein said circuit means further includes a peak detection means for producing said DC voltage based on the peak levels of periodic signals applied to said apparatus input terminals.

12. A system for monitoring a region of a traffic lane to detect the passage of a vehicle therethrough, said system comprising:

sensor means defining a sampling area mounted adjacent to but outside of said region for producing an output signal indicative of the level of magnetic field intensity through said sampling area;

bias means responsive to said sensor means output signal for producing a magnetic bias field through said sampling area to force said sensor means output signal to a predetermined value;

means producing a threshold signal having a value displaced from said predetermined value by an amount substantially proportional to the level of magnetic field intensity through said sampling area exclusive of said magnetic bias field; and

comparison means for comparing the values of said sensor means output signal and said threshold signal.

13. The system of claim 12 wherein said sensor means comprises a flux gate magnetometer providing an AC output signal of frequency 2f and phase 1, in response to a magnetic field intensity in a first direction through said sampling area and of frequency 2f and phase ( phi 1+180*) in response to a magnetic field intensity in an opposite direction through said sampling area;

means providing a reference signal of frequency 2f and phase phi 1; and

including circuit means for developing a DC voltage whose magnitude is related to the amplitude of said AC output signal and whose polarity is dependent on whether said AC output signal is in phase or out of phase with respect to said reference signal.

14. The system of claim 13 wherein said bias means includes:

an amplifier responsive to said DC voltage and a voltage source of said predetermined value for producing an error voltage; and

integrator means responsive to said error voltage for developing a bias signal to produce said magnetic bias field.

15. The system of claim 14 wherein said means producing a threshold signal comprises voltage divider means responsive to said bias signal.

16. The system of claim 12 wherein said bias means includes time delay means for causing variations in said bias field to lag variations in said sensor output signal.

17. A system for monitoring a region of a traffic lane to detect the passage of a vehicle therethrough, said system comprising:

sensor means defining a sampling area mounted in said region for producing an output signal indicative of the level of magnetic field intensity through said sampling area;

bias means responsive to said sensor means output signal for producing a magnetic bias field through said sampling area to force said sensor means output signal to a predetermined value;

means producing a threshold signal having a value displaced from said predetermined value by an amount substantially proportional to the level of magnetic field exclusive of said magnetic bias field; and

comparison means for comparing the values of said sensor means output signal and said threshold signal.

18. The system of claim 17 wherein said sensor means comprises a flux gate magnetometer providing an AC output signal of frequency 2f and phase phi 1, in response to a magnetic field intensity in a first direction through said sampling area and of frequency 2f and phase ( phi 1+180*) in response to a magnetic field intensity in an opposite direction through said sampling area;

means providing a reference signal of frequency 2f and phase phi 1; and

including circuit means for developing a DC voltage whose magnitude is related to the amplitude of said AC output signal and whose polarity is dependent on whether said AC output signal is in phase or out of phase with respect to said reference signal.

19. The system of claim 18 wherein said bias means includes;

an amplifier responsive to said DC voltage and a voltage source of said predetermined value for producing an error voltage; and

integrator means responsive to said error voltage for developing a bias signal to produce said magnetic bias field.

20. The system of claim 19 wherein said means producing a threshold signal comprises voltage divider means responsive to said bias signal.

21. The system of claim 17 wherein said bias means includes time delay means for causing variations in said bias field to lag variations in said sensor output signal.