Importance of Driveway Loops

Written by jonah on February 20th, 2011. Posted in Home Page Posts, Uncategorized

Driveway loops are an integral part of most automatic gate systems. They are used to command the gate to open or prevent it from closing on a vehicle in the path of the gate. They can also be used for peripheral activities such as turning on driveway lights or signaling a residence that a vehicle has arrived.

Gate loops have the following functions:

  • Exit Loop – Typically placed 60 feet or more away from a gate, but can be placed as little as 4 feet away from the gate. When tripped, they cause the gate to open.
  • Safety Loops (also called Reverse Loops) – Typically placed close to the gate (4 or 5 feet away) on the inside and outside. If tripped when gate is closing, will cause the gate to re-open. For instance, if a gate is closing and a car passes over a safety loop.
  • Shadow Loops (also called Center Loops) – Used for large swinging gates when the arc of a gate is longer than a typical car. Placed under the swinging portion of the gate. The shadow loop acts like a safety loop but is deactivated when the gate begins to close to prevent the metal in the gate from tripping a re-open occurrence.
  • Entry Loops – Placed on the outside of the gate, entry loops are used to command the gate to open in situations where security is not a concern. For instance, when a gate is only used as a barrier to keep deer out. Entry loops can also be used to turn on driveway lights or send a signal to a house that a car has arrived.
Gate-Loops-Sensors

Loops can be installed using two types of loop – preform or saw cut. Preform loops come with the wire already cut to length, twisted, and inside of flexible conduit or a protective sheath for the sensing portion of the loop. Preform loops are used for roads that are not paved or for roads prior to paving or concrete pours. A trench is dug 1″-6″ below the finish grade of the road in the case of asphalt paving with the loop inserted and then covered and run back to the gate operator. In the case of concrete, the loop is tied to the rebar structure before the pour.

Saw cut loops are used for existing paved roads. A good rule of thumb for loop installation is that the read height of the loop (i.e. the distance off the ground that a vehicle will be detected) is about 2/3 the distance of the shortest leg of the loop. For a 5′ x 5′ loop, this will give a read height of 3-1/3 feet. For detecting high bed vehicles, one should increase the loop size, depending on the height of the typical vehicle.A 6′ x 8′ loop is adequate in many instances. Adding loop turns, for a saw cut loop, will not increase loop sensitivity, but it will make the loop more stable so that false detects will be less likely, provided the rest of the installation of the loop was done properly.

preform-driveway-loop

So, how do driveway loops (a.k.a inductance loops or sensing loops) work? When direct current (DC) passes through a wire, an electromagnetic field (EMF) is created around the wire. When the DC current is removed, the electromagnetic field similarly disappears. With driveway sensing loops, a loop detector is used. The loop detector is a small device that plugs into the main circuit board of the gate operator. The loop detector creates an alternating current (AC), which changes electrical current direction at a predefined frequency. When the current changes direction in the loop, the magnetic field that is collapsing pushes against the new field that is being created. This is a form of resistance called inductance, which is measured in henrys. Many different factors affect the inductance of a loop including the wire size, number of loop windings, distance of loop from loop detector.

driveway-loop-detector-bx-lp The loop detector, the brains of the loop sensing device, sets the amount of AC current flowing through the loop, as well as detects when a metal object – over the loops electromagnetic field – absorbs some of the EMF that is collapsing. When a metal object absorbs some of the collapsing EMF, it lowers the resistance in the wire of the loop which causes in increase in the electrical current flow in the wire which is detected by the loop detector. The detector registers this and opens or closes a relay to activate the gate (or any peripheral device being controlled by the loop detector). For controlling the gate, an exit loop detector will cause the gate to open while a safety (or reverse) loop will cause the gate to reopen.

A common misconception with regards to sensing loops is that the metal that is detected must contain magnetic properties, such as steel that contains iron. This is untrue, as aluminum is also detected by sensing loops. The reason is that aluminum is electrically conductive and as such has and effect on the inductance in the loop. Another misconception is that the loops are pressure sensors, sensing the weight of a vehicle. This also is untrue.