Inductive loop detector system

Apparatus useful in conjunction with a plurality of wire loops each supported adjacent a different area of a roadway surface for detecting the presence of a vehicle on each of said areas, said apparatus comprising:

• oscillator circuit means;
• a source of timing pulses;
• cyclic digital counter means responsive to said timing pulses for defining a plurality of sequential phases during each cycle;
• switching means for coupling said oscillator circuit means to a different loop during each of said phases;
• period measurement means including a first counter means for counting a defined number of cycles of said oscillator circuit means during each of said phases;
• sample storage means for storing a sample count accumulated during each phase by said cyclic digital counter means during the interval said first counter means is counting said defined number of cycles of said oscillator circuit means;
• reference storage means storing a plurality of reference counts, each corresponding to a different one of said phases;
• arithmetic means for determining the difference between the sample count accumulated during a phase and the corresponding stored reference count;
• and
• first means responsive to the value of each difference determined by said arithmetic means for selectively generating a call signal.

Apparatus including oscillator circuitry adapted to be operatively connected to a remote inductive loop for monitoring the frequency of oscillation, said apparatus comprising:

• means for measuring the time duration of a defined number of cycles of said oscillator circuitry;
• digital storage means storing a reference duration;
• means for determining the difference between said measured time duration and said stored reference duration;
• means for determining whether said difference is within a range defined by first and second threshold numbers;
• means responsive to said difference being to a first side of said range for generating a call signal;
• means responsive to said difference being within said range for selectively incrementally varying said stored reference duration;
• and
• means responsive to said difference being to a second side of said range for generating a new reference duration and replacing said stored reference duration in said storage means with said new reference duration.

Apparatus including oscillator circuitry adapted to be operatively connected to a remote inductive loop for monitoring the frequency of oscillation, said apparatus comprising:

• timing means defining successive cycles;
• period measurement means for measuring during each successive cycle the time duration (S) of a certain number of oscillations;
• digital storage means storing a reference duration (H);
• means for determining the difference (S-H) between each measured time duration and said stored reference duration;
• means defining a threshold number (T);
• means responsive to (S-H) <
• for generating a call signal;
• means responsive to 0 .ltoreq. (S-H) .ltoreq. T for incrementally varying said stored reference duration;
• and
• means responsive to (S-H) >
• T for storing a new reference duration in said storage means equal to (S-T).

Apparatus useful in conjunction with a plurality of wires loops each supported adjacent a different area of roadway surface for detecting the presence of a vehicle on each of said areas, said apparatus comprising:

• oscillator circuit means;
• and
• a plurality of input circuits each connecting said oscillator circuit means to a different one of said plurality of loops;
• each of said input circuits including a transformer having primary and secondary windings, the secondary winding of each input circuit being connected to a different one of said plurality of loops;
• and
• each of said input circuits further including an electronic switch means coupling the primary winding thereof to said oscillator circuit means.

Apparatus useful in conjunction with a plurality of wire loops connected to circuitry for producing an oscillating signal therein, each loop supported adjacent a different area of a roadway surface for detecting the presence of a vehicle on each of said areas, said apparatus comprising:

• timing means defining a plurality of sequential phases;
• period measurement means for measuring during each of said phases, the period of a defined number of cycles of the oscillating signal in one of said loops;
• first digital storage means storing a plurality of reference numbers each corresponding to a different one of said loops;
• a plurality of output devices each associated with a different one of said loops;
• and
• first means for comparing the period measured for each loop with a stored reference number corresponding to that same loop for selectively supplying a call signal to the output device associated with that loop.