Inductive loop vehicle detector

Detection apparatus useful for detecting the presence of a vehicle on a specified area of a roadway comprising:

• a conductive loop mounted proximate to the surface of said roadway enclosing an area substantially coincident with said specified area;
• circuit means disposed remote from said loop and operatively connected thereto for producing an oscillator signal whose frequency is related to the inductance of said loop;
• sequence control means defining successive detect cycles and including means for generating a start signal during each detect cycle;
• a first counting means for counting cycles of said oscillatory signal occurring after said start signal and for generating a first overflow signal after M counts;
• a source of clock pulses;
• a second counting means for counting clock pulses occurring after said start signal and for generating a second overflow signal after N counts;
• preset means for loading a count number into said second counting means prior to said start signal;
• and
• means responsive to said first overflow signal occurring prior to said second overflow signal for generating a call signal indicative of the presence of a vehicle in said specified area.

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 for measuring the time duration of a fixed number of cycles of said oscillator circuitry;
• reference means defining a reference duration;
• comparison means for determining the difference between said measured time duration and said reference duration;
• feedback means responsive to said difference for varying said defined reference duration in a direction tending to reduce said difference toward zero;
• and
• call threshold means responsive to said difference exceeding a call threshold value for 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:

• loop counter means for counting a fixed number of cycles of said oscillator circuitry;
• reference means defining a reference duration;
• a source of clock pulses;
• error counter means for counting the number of clock pulses representing the difference between the duration of said fixed number of cycles and said reference duration;
• means responsive to said error counter means for varying said reference duration in a direction to reduce said error counter means count toward zero;
• and
• call means responsive to said error counter means counting above a threshold value for generating a call signal.