Loop basics

      Loop basics

How to lay a loop in the ground.

Who are Henry and Eddy and how does a loop work.








Author  :    Huw Jones

Are road loops old school?

The first traffic light was installed in Pariament Square in 1868. Computer controls were intriduced in the 1960's, when road loops where introduced. For the next 50 years, inductive road loops would be the technology of choice.

A road loop is a tuned aerial that resonates like a guitar string. When a car drives over the loop, the inductance changes, and so does the resonant frequency.

The precise frequency is not really important. It is the sudden change that counts in vehicle detection. That is why all loops take a measurement on power up, and use that as reference.

Loops respond best to steel and other magnetic materials. Other conductors have some reactance (thru eddy currents). They do not work on carbon based lifeforms like us, and not well on Maclaren F1's.

Joseph Henry discovered self-inductance and gave his name to the unit of measure.

Loop testing

Diablo loop modules all have a loop fail detection routine built in. The indicator light has a pattern for an open circuit loop and a short circuit loop. In fact, the module is measuring frequency, and open & short circuit represent opposite extremes of the inductance range.

The goto message here is that most failures are related to the loop in the ground, not the detector. A deteriorating loop will continue to worsen, so keeping note of loop inductance is a good early indication of a failing loop.

Loop resistance

Loop health is not a function of resistance, so an ohmmeter will only identify a broken loop. If the loop module reports the failure as 'open circuit', it may mean the frequency is too high to measure, or the loop is not resonating.

High loop resistance does impair oscillation, so very long lead in wires will risk poor performance. Loop wire should be at least 22awg or less. That may counter intuitive, but 21awg wire is actually thicker! 

Loop shape and size

Diablo modules have a wide operating range quoted in 'microhenries'. Ground loops are air core inductors that need to be at the bottom end of the operating range, around 100uH (micro Henrys).

The trick is to use 20-30 metres of cable in the loop. That doesn't include the leads up to the loop area. So a square loop with 2m sides has a circumference of 8m. Three circuits makes 24m - that works!

This calculator allows you to work out long thin loops under a barrier arm - 1.5m wide, 5m long, 2 loops makes 26m - thant works too!

The height that a loop will detect is about 65% of its shortest dimension. You will need about 600mm for the chassis of a hiigh bed truck, so a loop's short edge should be at least 1m long. 

Sensitivity and positioning

A car half on and half off a loop will have half the effect. Some cars have a bigger magnetic footprint than others. The general rule of thumb is any wheel inside the loop should trigger the relay.

Sensitivity is a ratio of the vehicle's magnetic material versus the background magnetic material. So position the loop away from iron drains and man hole cover.

Old iron pipes or tanks below ground can mess with sensivity and range. Power cables that run under the loop will interfere in other ways. Refer to figure of 8 loop patterns.

Cable type

You will need a waterproof cable resistant to abrasion and crushing. Core resistance affects performance, so stick to copper with at least 0.64sqmm cross section. Some people recommend a solid core because it is more resistant to corrosion if the insulation is pierced.

Loop cable is readily available by the reel. See also the article here on using a multicore cable in a duct,

Road surface and fillers

Loops in asphalt  are laid in a 50mm deep slot using a cut off saw. Hot pitch is always used in public roads to infill. A layer of sand can be laid between the cable and pitch to protect it against the heat.

A bitumastic roof sealer dispensed from a mastic gun is convenient but less reliable. Never use silicon filler; it comes out like one big rubber band!

Concrete cut loops can infilled with a dry mortar mix with plasticisor. Just sweep it in and spray with water. 

Gravel drives are a challenge, because they are fluid and sharp. Loops can be pre-wound in blue water pipe and at least 300mm deep. As the ground moves, they will tend to float to the surface. A ground stabilising mat will help hold the material together.

Double loop applications

Directional sensing loops

'Exit' loops open a gate or barrier to any vehicle. If vehicles are moving at speed, clients may wish to place the loops 100 yards from the barrier to allow the barrier time to open.

With a single 'exit' loop set a dustance away from the gate or barrier, a car entering (from right to left) along the same route would open the barrier for a second time unnecessarily.

Placing two loops can produce an open pulse in one direction, not in the other. The loops need to be within a car's length of each other so the car can be sensed both at the same time.  

The wiring shows loop B with a delay off, or extended presence. It is available on Diablo plug-in modules and VD955). B loop disables the trigger from A loop. When loop B senses the car first (incoming cars), it hold the presence until the car has left loop A, thus disabling the A output.

Direction sensing is a standard function of DSP21.

Presence loops placed either side of a sliding gate

2 loops on a sliding gate


Presence loops placed either side of a sliding gate

A typical application is presence loops either side of a sliding gate. It gives two centres of sensitivity, but each loop will be half as sensitive.

The two loops are connected to the same module in series. They must therefore have the same function, and be the same size, and the same number of turns. Loop module sensitivity will need to be doubled for the pair to get the same sensitivity. 

When setting the loop, mark the beginning of the loop and wind clockwise. An arrow on the drawing shows the winding direction.

When connecting to the module in series, ensure that the beginning of one loop connects to the end of the other. That ensures the magnetic field goes through in the same direction. This will cause a low sensitivity point between the loops, so help to cancel the effect of a steel sliding gate.

2 exit loops on a swing gate


In this application, a gate opens to an area like a car park, where cars approach from either side. A single thin exit loop would not cover the area, so two loops are cut.

The loops have the same function, size, and number of turns so can be connected to the same module input. Sensitivity will be shared so module sensitivity setting is doubled. If the module has a fixed sensitivity, add additional turns to both loops. 

When setting the loop, mark the beginning of the loop. In this application where the two loops are close to each other, wind A loop clockwise and B loop anti-clockwise.

When connecting to the module in series, ensure that one loop's start connects to the other loop's end. Because the loops are counter wound, the magnetic field goes through in different directions. The space between the loops will also be sensitive.

Loops placed for extra wide activation zone

Loop installation tips

When replacing a loop, cut or remove the old loop. If an old loop is short circuited it will apply a load to the new loop, similar to the principles of passive RFID tags.

Lead in wire should be twisted in the same direction once every 200mm. This prevents the lead in being sensitive to unintended ferrous objects. 

Slots cut in asphalt or concrete create a lot of dust. If it is left in the slot, it will prevent the filler adhering to the wall sides. Wash with a hose and dry with an air line before filling. 

While a loop cut is asphalt or concrete is rectangular, corners should be cut at 45 degrees to avoid sharp edges that could damage cable insulation. One pin prick and the rot starts!

The same cable should leave the detector, go around the loop, and return in one piece. Any connectors are a liability, so if absolutely necessary, it is recommended the joint be soldered. 

If more than one loop is used in a system, the lead-in wires should be run back to the control panel in separate conduits to avoid cross interference. 

If planning a loop into concrete, check for reinforcement bars (re-bar), or buried cast iron pipes. 

Two loops with different functions could cross interfere. A rule of thumb is to keep the loops at least a metre apart.

Direct burial cable needs a waterproof core insulation. An outer 'sheath' can be any wear resistant material. XPLE is tough and water proof but thin. A rubber sheath is good against abrasion.

If a loop or lead-in cut crosses an expansion joint, allow some slack in the cables for movement. Re make the expansion joint with a similar infill compound. 

Lead-in has inductance (about 0.7uh/m) so reduces sensitivity. A 30m lead-in de-sensitises the loop by a third. Add an extra turn in the loop to compensate.

It is always better for a loop to be too large than too small. The multicore loop method increases lead-in, but allows the fitter to choose the number of turns in the loop at commissioning. 

When planning two loops close to each other on the same module, mark the wire that starts each loops. This allows the loops to be wired to opposite polarities, which will help sensitivity.