Inductive loop detector system

US3943339A · kind A · utility

Assignee

• Canoga Controls Corporation · US

Inventors

• Ralph J. Koerner · Ramona, US
• Milton G. Bienhoff · Calabasas, US
• Martin C. Henderson · Los Angeles, US
• John E. Higbee · Simi Valley, US
• Steve J. Koerner · Mesa, US

Key dates

• Filing date: Apr 29, 1974
• Grant date: Mar 9, 1976
• Expiry date: Apr 29, 1994

Classification

• Technology area (CPC G): Physics
• CPC primary: G08G1/042
• WIPO field: Measurement
• WIPO sector: Instruments

Abstract

Apparatus for use in combination with multiple inductive loops for detecting metal objects, e.g. vehicles, in the immediate vicinity of each of said loops. Each loop may, for example, be a coil of wire buried in a roadway in a plane parallel to the roadway surface. A common oscillator circuit is operatively connected to each loop on a time shared basis with the frequency of oscillation at any time being determined by the inductance of the connected loop. The inductance in turn is dependent on whether or not a vehicle is over the loop. Successive scan cycles are defined with multiple loop phases occurring during each scan cycle. During each loop phase, a different one of the multiple loops is connected to the oscillator circuitry and during a "period measurement" portion of each loop phase, the loop frequency is monitored by digital circuitry including a loop counter which counts loop oscillator cycles and a duration counter which measures the time duration or period of a certain number of such loop oscillator cycles. The measured time duration (period) is then compared with an historical reference duration (accumulated over prior scan cycles) during a "calculation and storage" port…