Vehicle Detection Loops

Inductive loops are coils of wire buried in pavement. A detector sends electrical current through the loop, creating a magnetic field. When a vehicle (which contains large amounts of metal) enters the field, the vehicle's metal changes the loop's electrical properties,specifically, its inductance. The detector senses this change and signals that a vehicle is present.

This technology is simple, reliable, and proven over decades of use. Loops detect virtually all vehicles regardless of speed, size, or approach angle. They work in all weather conditions,rain, snow, extreme temperatures. They do not require line of sight and are not affected by lighting conditions.

The limitations are physical: loops require pavement to be cut for installation, and they only detect vehicles over the loop itself, not approaching vehicles. They can be damaged by pavement deterioration, utility work, or resurfacing. Despite these limitations, inductive loops remain the standard for vehicle detection at gates.

Loop size and shape affect detection characteristics. Standard configurations serve different purposes:

The loop detector is the electronic unit that senses inductance changes and provides output signals. Detector quality and proper tuning affect detection reliability.

Detectors vary in features: basic units provide simple presence detection; advanced units offer adjustable sensitivity, multiple detection modes, diagnostic outputs, and self-tuning capabilities. Commercial and industrial applications benefit from detectors with diagnostic features that aid troubleshooting.

Sensitivity adjustment balances detection reliability against false triggers. Too sensitive, and the loop may trigger on adjacent traffic, rebar in concrete, or other metal. Too insensitive, and small vehicles or high-clearance vehicles may not be detected. Proper tuning requires testing with actual vehicles.

Most modern detectors are self-tuning,they automatically adjust to the installed loop characteristics. However, initial setup and periodic verification remain important. Environmental changes (temperature extremes, pavement movement) can drift sensitivity over time.

Vehicle detection serves critical safety functions under UL 325. The primary safety application is preventing the gate from closing on a vehicle.

Shadow loops in the gate path detect vehicles that would be struck by a closing gate. When a vehicle is detected, the gate stops or reverses. This is an entrapment protection function,required by UL 325 for vehicular gates.

Proper shadow loop positioning covers the entire danger zone,the area where a vehicle could be struck by the gate. For slide gates, this is the gate path. For swing gates, it includes the swing arc. Loop size and position must be carefully planned.

Shadow loops work in conjunction with other safety devices (photo eyes, edge sensors) to provide layered protection. No single device covers all scenarios; the combination provides comprehensive protection.

Properly installed loops last 10-20 years or more. Poorly installed loops may fail in months. The difference is installation quality and pavement conditions.

Routine maintenance includes visual inspection of loop sealant for cracking or deterioration, verification of detection function, and detector check (indicator lights, diagnostic outputs). Problems are easier to address before complete failure.

Pavement work is the main threat to existing loops. Resurfacing can damage loops if not coordinated properly,loops can often survive overlay if the contractor is aware and careful. Major pavement work usually requires loop replacement.

When loops fail, accurate diagnosis determines whether repair is possible. Sometimes lead-in wire repair or detector replacement solves the problem. Complete loop failure requires saw-cutting and re-installation.