Dual tire detection combined with counting the number of axles on a vehicle provides additional vehicle classification capabilities.

Traditionally, toll collection systems use the number of axles to determine the vehicle class. Further descrimination can be obtained by including the number of dual tires as well as the number of axles on a vehicle, This results in more meaningful toll collection as a 2-axle vehicle with dual tires can be charged a different toll than a 2-axle vehicle without dual tires.

Sensors installed in the lane provide signals that are processed by the controller to determine the number of axles and the presense of dual tires on the vehicle. The image on the left has the sensors highlighed. The two "red" sensors installed perpendicular to the lane are used to count axles and determine vehicle direction. The two sets of "orange" sensors installed on angles are used to detect dual tires. It is possible to use only one set of dual tire sensors. However two sets provide reduncancy. If a dual tire is detected on either set of angled sensors, the axle is considered to have dual tires on the axle. The two grey housings on the side of the lane are light curtains used for vehicle separation.

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The sketch on the left illustrates the sensors as they are installed in the lane.

A light curtain is used for vehicle separation. Other separators such as lasers can also be used. The light curtain is placed directly over the axle counting sensors (C and D shown in red).

Two sets of dual tire sensors are installed before the axle counting sensors. The dual tire sensors are shown in orange and are labled A1, B1, A2, and B2.

Cables from each sensor are installed in conduit and are connected to the controller which is typically installed in the tollbooth or in the tunnel under the booth.

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Distinguishing dual tires from single tires is based on the geometry of installing the dual tire sensors on angles to the curb and direction of vehicle travel.

When an axle with single tires crosses the dual tire sensors, only one of the sensors is activated at a time. This concept is shown in the illustration to the left.

In this sketch, the front axle of a vehicle is crossing the two sets of dual tire sensors. Since the tires are single, only one sensor strip of each pair of dual tire sensors is activated at a time.

This is because a single tire is not wide enough to span both sensors and activate them both at the same time.

The animation on the left illustrates the concept described above for a single tire crossing two of the angled dual tire sensors.

As the "footprint" of the tire activates the first sensor (A) the signal for sensor (A) turns ON. "Footprint" is the area of rubber on a tire that actually touches the pavement.

As the tire clears the first sensor, (A) turns OFF.

Since a single tire is not wide enough to activate both sensors at the same time, sensor (A) turns OFF before the second sensor (B) turns ON.

This indicates a single tire, therefore a dual tire would not be counted.

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The sketch on the left illustrates an axle with dual tires crossing the dual tire sensors.

Dual tires cause both sets of each dual tire sensor to be activated at the same time. This is due to the total width of the pair of tires. The extra width causes the sensors to be ON at the same time.

The animation on the left illustrates the concept described above for a set of dual tires crossing a pair of the angled dual tire sensors.

Due to the extra width of the set of dual tires, there is a point when both the (A) and (B) sensor are ON at the same time.

A dual tire is counted when both sensors(A) and (B) are ON at the same time.

Light curtains allow all axles on a vehicle to be associated with that vehicle. Other devices such as loops sometimes cause axles for a single vehicle to be falsely "split" into two vehicles. As shown in the image on the left, tractor trailers are often high off the ground. However, the light curtain still detects the trailer portion. If a loop detector were used as the vehicle separator, the high profile of the trailer would likely cause the loop to "drop-out". Meaning, this tractor trailer would be treated as two separate vehicles. Light curtains improve vehicle separation.

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All of the Tolltex software used to count axles, detect dual tires, and interface with the light curtains is written in the"C" programming language.

If a standalone version of this sub-system is needed, the software can be operated by our RoadMate embedded processor (shown on the left). Using the RoadMate controller allows the dual tire sub-system to be integrated into an existing lane controller via a n RS-232 serial communications link.

If dual tire detection is needed as part of a complete system, the software can be integrated with our Lane Controller software using either the NEMA-4 Lane Controller or Rackmount Lane Controller.