Loop Detectors and Where They Go

Saw cuts in the driveway with a coil of wire in them are the whole of a vehicle detection system, and where those cuts go decides whether the gate behaves sensibly for the next decade. The layout diagrams kept at Pearland Sliding Gates Freesizer show three loops rather than one, because the two people leave out are the ones stopping a sliding leaf closing on a car in Pearland.

The Three Jobs

An exit loop asks the gate to open for a vehicle leaving. A safety loop under the path of the leaf holds it open while anything stands on that ground. A shadow loop just inside the closing edge stops the gate starting to close while a vehicle is still crossing. Those are different jobs and one loop cannot do all three.

Depth and Shape

A cut about an inch and a half deep, a rectangle set back roughly a foot from the edges of the drive, with corners cut at an angle so the wire is never kinked. Three or four turns for a typical size. The lead in pair is twisted the whole way back to the loop detector, because untwisted lead in is where most false calls come from.

Sealing the Cut

The slot is dried, the wire pushed down with a blunt tool that will not nick the insulation, and the cut filled with a flexible sealant rather than a rigid grout. On clay that moves, a rigid filler cracks in the first season and water reaches the wire and the conduit running back to the operator. A loop reading erratically after rain is nearly always a sealing failure.

Steel and Interference

Reinforcing bar under a loop lowers its inductance and can make a small vehicle invisible. Where a drive is heavily reinforced, more turns and a slightly larger loop restore sensitivity. A loop set too close to the steel frame of the gate will also see the gate, which shows up as a detector latching on the moment the leaf passes over it.

Testing After Installation

Drive a small car onto each loop and watch the indicator, then reverse over it slowly. Test a motorcycle if one is available, since that is the load case exposing a loop that is too deep or has too few turns. Every one of these checks is easier before the sealant has cured than after the gate is handed over.