Inductance Loops: Understanding and Preventing
Phantom / False Detections
Why is my gate/door/barrier arm opening for no reason?
By Brian Dickson

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“So that’s why my gate opens when I come down my driveway on my horse!” Proclaimed Mr. J, “I was told it was the metal in the horse shoes, but the gate would still open even when they were off.” This is what one my friends told me after I educated him about issues with air pockets in loop design. The problem with Mr. J.’s gate was that his horse was causing ground vibrations that resulted in a Phantom Detection. Phantom or False Detections happen when loops "detect" an object that isn't there, which causes the gate to perform a function when nothing is supposed to happen.

False Detections and detector lock ups are one of the leading reasons why installers must do a repeat service call on brand new gate systems with loops installed. Which is unfortunate because it is a very difficult issue to diagnose, especially if you aren't considering air pockets as the cause of the problem. Understanding what causes “Phantom Detections” and how they can be prevented can save you time, money, and preserve your image of professionalism and knowledge to your customer.

The best place to start would be to understand how an inductance detector and loop works. The detector will energize the loop wire with an oscillating signal current. This current will cause Electro Magnetic Field (EMF) fields around each wire and a level of inductance will result. The inductance level will change if any conductive material enters the detection field or if the wires move in relation to one another the total inductance of the circuit will change. This change in induction will cause a change in current flow. The detector has a circuit that looks for a change in current and will trip a switch when a change is detected, the detector will also change frequency.

A change in inductance can be caused by slightly moving the loop wire windings closer or further apart from each other (an example is the vibration caused when a horse comes down the driveway). This moving of the loop winding CAN TRIGGER the detection circuit or even lock up the detector. When loop windings are lying loose in a conduit or air hose, slight ground vibrations from a vehicle or gate movement could cause loop winding to move ever so slightly that end up setting off the detector. To see this effect for yourself, hook up a coiled up induction loop to a detector and slightly disturb the coil. You will experience a detector trip with the slightest movement. This happens because each loop wire has its own EMF field that will interact with the adjacent wire’s EMF field. Depending on the field relationship, either the fields will double in value or cancel each other out. This change will either increase or decrease the inductance that will result in a change in loop current, this can also occur in a saw-cut loop that has been installed in a 1/8" groove.

 Depending on which loop in the loop system is being affected by ground vibration will cause different results. A free exit loop (Automatic exit loop) the gate will open on its own. A reverse loop (Obstruction detection loop) the gate will being to close then reopen. In the case of a shadow/center loop the gate will not open or close. By understanding which loops are being affected gates can be serviced more efficiently.

 Sadly it is still common practice that installers will continue to make their own direct burial loops by running wire through PVC, an air hose, or use a 1/8" saw blade. Another problem is the air pocket in the loop design can weaken a concrete structure, in some cases as much as 40%. If a loop is being installed in a parking garage or where the concrete is vital to structural integrity, an air pocket shouldn’t be allowed at all. Loops wrapped through PVC also have a history of water getting inside of the air pocket causing the loop to short to ground causing the loop to not work at all. In the case of a saw-cut loop, the groove can fill with water eventually push the loop out of the groove. Problems can often occur after rain or sprinklers then magically disappear when the area dries. This is because an air pocket is present. To prevent an air pocket in the loop, try using a preformed loop without an air pocket and a fully independent outer jacket that completely eliminates movement of the wires or a loop filled with a sealant throughout the entire loop.


Examples of air pockets in 1/8" grooves.

For more information on the air pocket problems installers face when they wrap their own loops in 1/8" saw-cut grooves view our report on saw-cut loops in 1/8" grooves.

Brian Dickson is the General Manager of BD Loops, an assembler of preformed direct burial and saw-cut inductance loops for the gate, door, and parking industries. With over 10 years in business the quality of our loops is unparalleled. BD Loops products are available through over 220 distributors nationally. BD Loops offers 45 standard preformed loop sizes, all standard and custom loop sizes are ready to be shipped the same day. The company has several letters of recommendation testifying their professionalism and design, and is a member of the following associations: AFA, IDA, NOMMA, IPI, CODA and IMSA. Visit www.bdloops.com and use the distributor locator to find a distributor near you. If you would like to speak to Brian Dickson please call BD Loops at 714-890-1604.

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