Loop Info-Q-2-16
4
SECTION 1 - HOW THE LOOP SYSTEM WORKS
A loop sets up an inductive field, called a field of sensitivity. This field of sensitivity completely surrounds the loop wires
extending above, below, and on all sides of the loop. When a vehicle (or any metallic object) enters this field of sensitivity, the
inductance of the loop changes as more of the inductive field is disrupted by the vehicle. Once the disruption of the inductive
field is high enough, the loop detector senses this change and outputs a signal, usually in the form of a dry contact relay switch
activation.
The height of the field of sensitivity, and therefore the height of detection, is determined by the size of the loop. The height
of detection is especially important when you need to detect a high bed truck, which requires more height than a passenger car.
The height of detection above (and below) the loop is determined by the shortest leg of the loop. The height of detection is
equal to 1/2 to 2/3 the length of the SHORT LEG. For example, a four-foot by eight-foot loop will give you approximately 2-1/2
feet of detection height, whereas a six-foot by eight-foot loop will give you approximately 4-feet of detection height. The short
leg of a loop used for vehicle detection should never be shorter than 18 in. The maximum “detection height” of a loop is
6-feet on the short leg. If larger loops are used, the “detectable height” of the loop will NOT increase.
Field of Sensitivity
Height of Detection
Cutaway of Underground Loop
Inductive Field of Sensitivity
Detection Height
Approx. 1 ft.
Detection Height
Approx. 3 ft.
Larger short leg needed to detect trailer height vs. car height.
Maximum Detection Height
Approx. 4 ft.
3 Turn
Saw Cut Loop
Sensitivity Field
Roadway
Short Leg
Short Leg
L
o
ng
L
eg
Short Leg
L
o
ng
L
eg
L
ong
L
eg
18”
M
in.
5’
6’
To Next Page
Loading...
Need help?
General