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Loop Detectors & Driveway Loops

Articles and Illustrations:
Overview - Introduction to Loops
Cut-in Loop Installation Diagram
General Loop Info
Trouble Shooting Loops

Technical Loop Information
Find Loop Detectors Here
1. What is an inductive loop?
An inductive loop is a wire wound in a rectangular, square, or round shape that is typically saw cut into the pavement. The ends of the wire are brought back to an enclosure, which houses an inductive loop detector module. The detector module powers the loop and causes a field to form around the loop. The loop automatically tunes to a resonant frequency. The detector module monitors this resonant frequency to determine if a vehicle is in the loop area.
Loop diagram_1

2. What is inductance?
Inductance is defined as the opposition to a change in current flow. When a current is applied to a conductor such as a wire, a magnetic field is formed around the wire. If the current source is removed, the magnetic field collapses into the wire trying to maintain the current flow. By winding several turns of the wire into a coil, the magnetic field is intensified, which increases the inductance.

There is a common misconception that an inductive loop requires a mass of metal or ferrous material for detection. Placing a single wire around the perimeter of the loop and shorting the ends together will quickly disprove this misconception. The single wire forming a shorted turn provides a current path for the magnetic field; thus causing a loading effect similar to that of a vehicle. The shorted turn effect of the single wire coil in the proximity of the loop acts much like a shorted turn secondary of a transformer.

4. What is the minimum acceptable loop inductance?
An inductive loop detector will tune to inductance values ranging from 20 to 1000 microhenries. It is preferable that the combination of the loop and lead-in inductance values has a minimum of approximately 50 microhenries for stability. As a general rule, the loop inductance should be equal to or greater than the lead-in inductance.

5. How many turns of wire should be installed in the loop?
The number of turns required in the loop is dependent on the loop size. The loop inductance can be calculated as follows:

L=P(t2 + t)/4; WHERE:
L = Inductance (Microhenries)
P = Perimeter (feet)
t = Number of turns

The formula can be simplified to: L = PK substituting a constant K for (t2 + t)/4.

Filling in the number of turns and calculating K:

Number of Turns (t) K (constant) K=(t2 + t)/4
1= 0.5
2= 1.5
3= 3.0
4=5.0
5= 7.5
6= 10.5
7= 14
Example: 4' x 8' loop with 4 turns
L = P K
P = 4' + 4' + 8' + 8' = 24'
K = 5.0
L = 24 x 5.0
L = 120 microhenries

Loop Inductance in Microhenries (µH)
    Number of Turns
1
2
3
4
5
6
7
 
P
E
R
I
M
E
T
E
R
(FT)
 10
5
15
30
50
75
115
140
  20
10
30
60
100
150
230
280
  30
15
45
90
150
225
345
420
  40
20
60
120
200
300
460
560
  50
25
75
150
250
375
575
700
  60
30
90
180
300
450
690
840
  70
35
105
210
350
525
805
980
  80
40
120
240
400
600
920
1120
  90
45
135
270
450
675
1035
1260
  100
50
150
300
500
750
1150
1400

Recommended Number of Turns per Size of Loop
P
E
R
I
M
E
T
E
R
(FT)
  
Number of Turns
10
5
20
4
30
3
40
3
50
2
60
2
70
2
80
2
90
2
100
2

Use the highlighted values listed in the table above to determine the number of turns required for a given size loop. Always use at least 2 turns.

6. Does increasing the number of turns in the loop increase the sensitivity of the loop?
NO. Increasing the turns does not increase the sensitivity of the loop. It can improve the efficiency of the loop system (loop inductance + lead-in inductance), if the lead-in length is over 400 feet. The amount of inductance change a vehicle can cause in a loop is determined by the following factors:

Amount of Change » Vehicle Size
Caused by Vehicle (Loop Size) x (Vehicle Height)

The above formula indicates the following:

1. Increasing the loop size will decrease the amount of change caused by the vehicle.
Example: If a vehicle causes a 1.0% change on a 6'x6' loop, then the same vehicle will cause a 0.5% change when over one of two 6'x6' loops connected in series.
2. A smaller vehicle will cause less change. A small motorcycle causes approximately 1% to 2% of the change caused by standard automobiles.
3. The higher the vehicle is from the road (loop) surface, the smaller the inductance change.

7. Does increasing the number of turns in the loop increase the detection height of the loop?
NO. Increasing the turns does not increase the detection height. Rule of Thumb: The reliable detection height of a loop is 2/3 of the short side of the loop.
Examples: 6'x6' loop. The sort side is 6 feet. 2/3 of 6 = 4 feet
6'x20' loop. The sort side is 6 feet. 2/3 of 6 = 4 feet.
4'x20' loop. The sort side is 4 feet. 2/3 of 4 = 2 feet 8 inches.

8. How deep should the loop wire be installed?
The deeper the wires are below the road surface the more they are protected from road surface wear and the elements. The top wire should be a minimum of 1 inch below the road surface.
Loop diagram_2

Nonconductive materials such as concrete and asphalt will not influence the loop fields. Installing the loop one inch deeper (e.g. 3" depth instead of 2" depth) would have the same result as raising the vehicle one inch above the pavement surface.

To reduce stress and abrasion of the loop wire the 90° corners should be cut at a 45° angle; core drilled (1.5" diameter); or at a minimum, the sharp inner corners should be rounded with a chisel.
Loop diagram_3

9. What type of wire should be used for the loop?
Number 16 or 20 AWG stranded wire can be used. The wire gauge is not critical to proper operation of the loop detector. The wire should maintain its integrity under the pavement stress. Since asphalt is more flexible than concrete, it is recommended that a heavier gauge wire be used for loop installations in asphalt.

The main consideration in selecting a wire for loop installations is the type of insulation. Cross-linked polyethylene (XLPE) insulation rated at 600 volts is highly recommended over PVC insulation. Under similar conditions, XLPE insulation will absorb approximately one percent of the moisture absorbed by PVC. When insulation absorbs moisture, loop drift occurs, which if great enough, can cause false detections. XLPE also has higher resistance to abrasion, heat, oils, and gasoline.

After insulation, and any time there appears to be a loop related problem, the loop should be tested. Use a MegOhm Meter to test the integrity of the loop / lead-in wire insulation. Readings of 100MO or less indicate possible insulation damage. Use a Multimeter to check the total resistance of the loop / lead-in combination. Total loop / lead-in resistance should never exceed 4 Ohms.

10. How far from a gate should the loop be installed?
As the length of the sides of the loop that parallel the gate increases, the inductance change caused by the gate also increases. The graph shows the inductance change for different distances between the gate and the loop for different sized loops.

The closer the loop is to a gate, the more influence the gate has on the loop! Hence, the detector sensitivity must be set lower to ensure the gate will not cause the detector to generate an output when the gate closes.

The following rule should be observed: The longer the loop, the greater the spacing must be between the gate & the loop!

The inductance change at two feet is one third of the change at one foot. At four feet, the effects of the gate on the loop are minimal.
Loop diagram_4