Cable-shaped Axle Sensors

SPT / SPZ Fiber Optic Load Sensors for Axle Detection

SENSOR LINE's axle sensors are designed for permanent embedding in the road surface and have – besides the general benefits of all kinds of fiber optic sensors – some additional unique features which are of particular importance for detection of vehicles in road traffic:

They only respond to vertical, not to lateral pressure
They are insensitive to vibrations (no microphones)
In practice noise is not perceptible (although there must be some because of physical reasons)
They are exceptionally fast
They deliver static output signals (signal is present as long as pressure is applied)
They still deliver a signal when no pressure is applied (self-test capability!)
They are flexible and elastic
Their signals are load-dependent (more load - more signal)

Their vertical sensitivity in conjunction with their lack of microphonic behaviour along with their low noise and high speed gives fiber optic axle sensors an outstanding accuracy of detection: They report the pressure of a tire on the surface of the installation slot and really nothing else, as soon as it appears and – almost more important – as long as it persists. Since the load signal does not vanish with time (as piezo sensor signals tend to do) fiber optic axle sensors are well suited for ultra low speed vehicle detection which is especially important for road pricing applications.

For the majority of road traffic applications SENSOR LINE's SPZ/SPT series sensors in conjunction with SL MA-110, MA-310 or MD-220 opto-electronic interfaces are the right choice to set up sensor subsystems.

SPZ-Series Fiber Optic Traffic Sensors

Cable-shaped sensors with lengths up to 6m. Sensitivity can be varied from few grams up to many tons by selecting the mode of installation. The completely non-metallic structure ensures highest protection against any kind of electromagnetic interference. The SPZ-series sensors have a fiber optic structure fitted into a special conduit and are designed for permanent embedding into the road surface. The materials used allow for a range of in-ground operating temperature from -40°C to +80°C (-40°F to 176°F). The feeder cables are spliced to the sensor fiber, so the feeder type and length must be specified in advance. They can be obtained with or without connectors.

Installed SPZ series axle sensors (CLASSAX system including TKV speed processing) at Schrobenhausen. The front part of the slots is filled with a standard sealant normally used for piezo axle sensors, the rear part with SL-Cast encapsulation material from SENSOR LINE.

Typical signals as produced by a passenger car with front motor at about 60 km/h (37 mph). Test equipment: SPZ fiber optic axle sensor with SL MA-110 optical transmittance analyzer. Upper: Load dependent analog voltage, lower: digital trigger signal.

SPT-Series Fiber Optic Traffic Sensors

These sensors are essentially the same as SPZ sensors except for the feeder cable. If its length does not exceed 3 m (10 ft) the sensor waveguide can be extended to form a feeder. Therefore no splice process is needed to connect the feeder to the sensor. SPT sensors are especially used at toll plazas where the processing unit is close to the sensors. They can be obtained with lengths up to 3 m with or without connectors.

Treadle Strips

SENSOR LINE fiber optic traffic sensors are also available as treadle frame inserts (treadle strips) for any common treadle strip shape. These sensors combine the benefits of the SPZ/SPT sensors with the possibility of an easy and quick sensor replacement which makes them the number one choice for all kind of toll applications where minimum down time is required.

AIRPORT SENSORS

FPL-Series Fiber Optic Axle Sensors

These sensors look similar to SPZ-series sensors but are designed for applications where extremely long sensors and feeder cables are required, e.g. for aircraft detection on airports. For these purposes special materials and manufacturing processes are used which are not necessary for road traffic applications. Also the FPL-series sensors have different feeder connections considering the special installation requirements on airports. They are available with lengths up to 6m.