Inductive Loops and Related Sensors

Inductive Loops and Related Sensors
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

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

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.)

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.)

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

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

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