Inductive Loop |
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Detector Overview
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Basic Detector Theory
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| The inductive loop detection system is comprised of two elements, the electronic detector module and the inductive loop coil and lead-in cable. The detector oscillator circuit drives energy, 10 - 200KHz, through the loop wire creating an electro-magnetic field. The loop detector forms a tuned electrical circuit of which the loop wire is the inductive element. If a metallic mass passes through the field, eddy currents will be induced in the conducting object. Since the loop inductance is proportional to the magnetic flux, it results in a decrease in loop inductance. The detector senses the change in inductance and actuates its electronic output. |
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Basic Loop Theory
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The loop wire and lead-in cable are the inductive elements of the detection
system and possess a combination of resistance and capacitance (both interwire
and wire-to-earth). The loop wire is constructed
to form a coil (usually two to six turns depending on loop size) where the
magnetic field becomes more concentrated creating the zone of detection. All
conductors or wires carrying an electrical current produce magnetic flux caused
by the current flowing through the wire. The effect of this flux is the
electrical property called inductance
which is measured in microhenrys (µh).
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Loop Wire
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The inductive loop and lead-in wires are typically constructed using #14 to #18
AWG wire with low AC and DC resistance. The wire gauge and number of strands are
important. However, more critical are the quality, thickness and type of wire
insulation. The insulation may be rubber, thermoplastic or synthetic polymer.
Cross-link polyethylene (XLP) is the most popular insulation and is strongly
recommended. The insulation must withstand wear and abrasion from shifting slabs
or pavement, moisture and attack by solvents and oils. Stranded loop wire is
suggested over solid wire because of its mechanical characteristics. Stranded
wire is more likely to survive bending and stretching than solid wire.
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