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Loop Detection Basics

loop detector, loop detection

Loop Detection

There are a number of ways to detect vehicles, ranging from hose style detection to ultra-sonic, to inductive loop.  For traffic control or drive-thru, inductive loop detection technology is the most reliable,
bar none.  An inductive loop vehicle detector system consists of three components: a loop (preformed or saw-cut), loop extension cable and a detector. When installing or repairing an inductive loop system the smallest detail can mean the difference between reliable detection and an intermittent detection of vehicles. Therefore, attention to detail when installing or troubleshooting an
inductive loop vehicle detection system is absolutely critical.

loop detection, loop detectors

How it Works:

The preformed or saw-cut loop is buried in the traffic lane, the loop is a continuous run of wire that enters and exits from the same point. The two ends of the loop wire are connected to
the loop extension cable, which in turn connects to the vehicle detector. The detector powers the loop causing a magnetic field in the loop area. The loop resonates at a constant frequency
that the detector monitors. A base frequency is established when there is no vehicle over the loop. When a large metal object, such as a vehicle, moves over the loop, the resonate frequency
increases. This increase in frequency is sensed and, depending on the design of the detector, forces a normally open relay to close. The relay will remain closed until the vehicle leaves the
loop and the frequency returns to the base level. The relay can trigger any number of devices such as an audio intercom system, a gate, a traffic light, etc.

The proper installation and material is critical! In general, loop vehicle detectors will work reliably if the installation is done properly and the correct materials are used.

Loop Installation

Follow closely the manufacturers installation instructions for the saw-cut or preformed loop that you purchased. However, there are a couple of important points to make with regard to saw-cut
loop and preformed loop installation.
It is important that when the installation is complete the loop be no more than 2” below the surface of the asphalt or concrete. The deeper the loop the less sensitive the loop detection system
becomes.
It is also important that the lead-in wires from the detector to the beginning of the loop be twisted a minimum of five times per foot

 

Troubleshooting

Most detectors provide LEDs that will indicate a problem with the loop, such as a short or an open. It is possible for a problem to occur that will cause the error indicating LED to stay on
and yet the installation is OK, but simply needs a reset. Lightning can cause such a problem.
Electrical storms can cause havoc with equipment, especially vehicle detectors because the loop is outside.  If problems persist, check the connections to the extension cable and to the loop lead-in wires.  Bad connections are a very common problem with inductive loops.  If the installed detector has a communications port for diagnostics, beg, borrow or steal, a copy
of the software and cabling needed to utilise this feature. Diagnostics software is an amazingly powerful tool for diagnostics, is less expensive than the test equipment needed to do the same
job and will provide more information. Some diagnostics software will even capture the data to disk. This is especially useful if you have an intermittent problem. You can leave the computer
running for days in order to capture the problem.  In addition to using diagnostics software to capture or see a problem as it occurs it can often be
used to help locate the loop in the pavement in order to determine if it’s been properly positioned and buried at the right depth.

To place an order or discuss your loop detection application further, contact the Ellard team today.

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