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Inductive Loops and Related Sensors
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Motivation
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  • The Task:
    • Detect the presence of a vehicle
  • The Technology:
    • There are many ways to do this
    • We consider some elsewhere
    • Here we are concerned with a group of related technologies
Inductive Loops
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  • History:
    • Introduced in the 1960s
  • Components:
    • One or more turns of insulated wire (in a loop) "under the vehicle" (e.g., embedded in the pavement or in a rubber mat on the pavement)
    • A controller
    • Cables connecting the loop and the controller
    • A power source
Inductive Loops (cont.)
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  • The Process:
    • The controller transmits energy into the loop
    • The controller senses decreases in inductance (the electromotive force generated by a change in the current)
  • Why It Works:
    • Recall that an electrical current (through a wire) produces a magnetic flux (that encircle the wire)
    • The inductance of the coil (which is measured in henrys) is an increasing function of the coil area and the number of turns, and a decreasing function of the coil length
    • An iron mass (e.g., the engine or transmission in a car) increases the inductance (called the ferromagnetic effect)
    • The other metal in a vehicle causes eddy currents which reduce the inductance
    • The net difference (which is negative) is used to detect a vehicle
Inductive Loops (cont.)
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  • Types of Vehicles:
    • Automobiles and Trucks - The undercarriage is predominantly a horizontal target
    • Bicycles - Are predominantly vertical targets
  • Implication:
    • The eddy currents are different
Pneumatic Tubes
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  • Components:
    • A rubber tube
    • A controller
    • A power source
  • The Process:
    • The vehicle's tire compresses the tube
    • A pressure pulse is created
  • Why It Works:
    • The pressure pulse closes a switch
    • The switch closing produces an electrical signal
    • The electrical signal is processed by the controller
Piezoelectric Cables and Mats/Disks
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  • The Components:
    • A tube or mat/disk that is made of a piezoelectric material (typically a ceramic or single crystal material such as gallium phosphate, quartz or tourmaline)
    • A controller
  • The Process:
    • The vehicle rides over or stops on the tube or mat/disk
    • A voltage is generated
    • The controller measures the voltage
  • Why It Works:
    • A piezoelectric material generates a voltage when deformed
There's Always More to Learn
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