PRESENCE DETECTOR

• US 3,683,351 A
• Filed: 01/07/1970
• Issued: 08/08/1972
• Est. Priority Date: 01/07/1970
• Status: Expired due to Term

• Current Assignee: 33580,, Merton F. Wilcox, 3135 N. Washington Blvd.
• Original Assignee: 33580,, Merton F. Wilcox, 3135 N. Washington Blvd.
• Inventors: Merton F. Wilcox, 3135 N. Washington Blvd., 33580,
• Primary Examiner(s): John W. Caldwell
• Assistant Examiner(s): Michael Slobasky
• Application Number:
  US05/001,231
• Time in Patent Office: 944 Days
• Field of Search: 340/258C,38L,258A
• US Class Current: 340/561
• 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 electronic system for detecting the presence of an external object in the field of one of the reactors in the tank circuit of the first one of a pair of oscillators. The oscillators are interconnected for frequency synchronization. The tank circuits of the oscillators are tuned to have different natural resonant frequencies in the absence of an object whereby the oscillator signals differ in phase. The presence of an object affects the tuned resonant frequency of the tank circuit of the first oscillator in a direction or sense to decrease the phase difference between the oscillator output signals. The oscillator output signals are limited, fed to a phase detector, and a signal is developed representing the phase differential. When this signal decreases below a predetermined threshold, an output indicator is triggered to provide an indication of the presence of the external object. The threshold is established in part by a capacitor having a charge representing ambient conditions when no object is present. The charging circuit for the capacitor provides a short time constant to follow gradually changing ambient conditions but provides a long time constant to prevent change in the charge on the capacitor as the result of the relatively more rapid and large change in the impedance of the reactance element as an external object arrives within the detection field. The long time constant circuit is operative regeneratively to change the charge on the capacitor after a long delay following the detection of an arriving object in case the object has not in the interim departed from the field. The departure of the object from the field prior to the time that the charge has leaked from the memory capacitor, returns the capacitor charge to ambient condition. The system is particularly adapted to indicate presence of vehicles within the field of a loop disposed slightly below the surface of a roadway or driveway.

Claims

1. In an electronic presence detector including first and second oscillators, means maintaining said oscillators in frequency synchronization, said oscillators being tuned to slightly different frequencies thereby to provide output signals out of phase with each other, means responsive to the presence of an external object to change the phase difference between the output signals of said oscillators, means to limit the amplitudes of said output signals, a phase detector connected to receive said limited output signals, and indicator means responsive to the output of said phase detector, characterized in that said phase detector comprises a transistor having one said output signal applied to its base and the other to its emitter and having its collector connected to a load impedance, said indicator means being operatively connected to respond to the signal voltage on said collector.

2. In an electronic presence detector including first and second oscillators operating at the same predetermined frequency, means responsive to the presence of an external object to advance or retard the phase of the output signal of one of said oscillators with respect to the phase of the output signal of the other of said oscillators, means to limit the amplitudes of said output signals, a phase detector connected to receive said limited output signals, and indicator means responsive to the output of said phase detector, characterized in that said phase detector comprises a transistor having one of said oscillator output signals applied to its base and the other of said oscillator output signals applied to its emitter and having its collector connected to a load impedance, said indicator means being operatively connected to respond to the signal voltage on said collector.

3. The combination according to claim 2 wherein the maximum phase difference between said oscillator output signals is less than 90 degrees.

4. The combination according to claim 3 wherein said oscillators are tuned to different frequencies in the absence of an external object and are provided with feedback means responsive to the phase difference between the respective output signals thereof to maintain frequency synchronization of said oscillators.

5. An electronic presence detector comprising an inductance loop disposed at a location at which presence of an external object is to be detected, oscillator and detector means including said loop providing an output signal of which the voltage amplitude is responsive to the inductance of said loop and decreases in response to increase of said inductance, a memory capacitor, low impedance means to charge said capacitor to the voltage of said signal, a high resistance leak resistor for said capacitor establishing a long time constant direct current discharge circuit therefor, a second direct current discharge circuit for said capacitor comprising impedance means and a first current control device having a control electrode, the impedance of said second discharge circuit being controlled by the potential on said electrode between a maximum impedance greater than and a minimum impedance less than the impedance of said resistor, a second current control device provided with self-biasing means and having a control electrode connected through said memory capacitor to said charging means, said second current control device being driven to cut off in response to abrupt reduction of said signal voltage, means connecting said control electrode of said first current control device to said biasing meAns to cut off said first current control device in response to cut off of said second current control device, said current control devices being normally conductive, whereby said memory capacitor has a short time constant and its charge follows said voltage through changes in the inductance of said loop occurring gradually as a result of ambient temperature changes, but has a long discharge time constant and its charge does not follow said voltage through abrupt decreases thereof resulting from rapid changes in said inductance occurring upon the arrival of an object at the loop location, and whereby, following a period of slow decay of the charge on said capacitor through said resistor initiating conduction of said second current control device, said control devices regeneratively resume fully conductive condition to thereby tune out said object and condition the detector to respond to further external objects, and indicating means responsive to the condition of one of said current control devices.