The induction loop vehicle detector is a type of vehicle detector based on the principle of electromagnetic induction. It is typically installed in the road base under the same lane as a circular loop, powered by a certain working current, serving as a sensor. When a vehicle passes through or stops on the loop, the metal on the vehicle will change the magnetic flux within the loop, causing a change in the inductance of the loop circuit. The detector determines the status of passing vehicles by detecting the change in inductance.

The vehicle detection system includes induction loops and detectors. The loop serves as data collection, while the detector is used for data judgment and outputs the corresponding logic signals. The detector generally consists of a frame, a central processing unit, a detection card, and terminal blocks.

The central processing unit is the module that calculates the collected signals, typically a single board computer with an embedded operating system, possessing strong digital computing, storage capabilities, and communication interfaces. By scanning the ports, it captures the timing of level changes to calculate the corresponding traffic data.

When a vehicle passes through or remains stationary in the detection area of the induction loop, the inductance of the loop will decrease. The function of the detection card is to detect this change and accurately output the corresponding level. When detecting vehicles passing at high speeds, there may be inaccuracies in detecting vehicle length and speed, necessitating proper adjustment of the detector’s sensitivity. Currently, most vehicle detectors have multi-level sensitivity adjustment functions.

In the parking lot system, the “induction loop” is essentially an oscillation circuit. It is constructed by first creating a circular trench on the ground, approximately 1 meter in diameter, or an area equivalent rectangular trench. Then, two to three turns of wire are buried in this trench, forming an inductive loop buried beneath the surface. This loop is part of an oscillation circuit, consisting of it and a capacitor, with the principle being that the oscillation is stable and reliable. This oscillation signal is transformed and sent to a frequency measurement circuit composed of a microcontroller, which can measure the frequency of this oscillator.

1. Loop Materials

During the design process, typically only the perimeter and turns of the induction loop are considered, while the material used is often overlooked by designers. When the project reaches the implementation stage, it is necessary to consider the mechanical strength of the wire and its resistance to high and low temperatures and aging. In some harsh environments, acid and alkali corrosion resistance must also be considered. However, conventional wires in actual use may age or lack tensile strength, leading to wire damage, which would cause the detector to malfunction. Therefore, it is recommended to use high-temperature fluoroplastic multi-stranded soft wire with a diameter of more than 1.0mm.

2. Loop Shapes

1. Rectangular Installation

Typically, the detection loop should be rectangular. The two long sides should be perpendicular to the direction of metal movement, with a recommended spacing of 1 meter between them. The length of the long sides depends on the width of the road, usually narrower by 0.3 to 1 meter at both ends compared to the road spacing.

2. 45° Inclined Installation

In some cases where it is necessary to detect bicycles or motorcycles, consider installing the loop at a 45° angle to the direction of travel.

3. “8” Shape Installation

In some cases, when the road is wide (over six meters) and the vehicle’s chassis is too high, this installation form can be used to disperse detection points and improve sensitivity. This installation form can also be used for sliding door detection, but the loop must be close to the sliding door.

3. Number of Turns of the Loop

To ensure that the detector operates at its best, the inductance of the loop should be maintained between 100uH and 300uH. With the inductance of the loop held constant, the number of turns of the loop has significant relation to its perimeter. The smaller the perimeter, the more turns there are. Generally, it can be referenced:

Due to various cables, steel bars, and manhole covers buried under the road, these can greatly affect the actual inductance of the loop. Thus, the above data is for reference only. During actual construction, users should use an inductance tester to test the inductance value of the induction loop to determine the actual number of turns, ensuring that the final inductance value of the loop is within a reasonable working range (e.g., between 100uH and 300uH).

4. Output Leads

When winding the loop, sufficient length of wire must be reserved to connect to the loop sensor while ensuring that there are no joints in between. After winding the loop cable, the output cable must be tightly twisted together, requiring at least 20 twists per meter. Otherwise, the untwisted output leads will introduce interference, making the inductance value of the loop unstable. The output lead length should generally not exceed 5 meters. As the sensitivity of the detection loop decreases with increasing lead length, the length of the lead cable should be kept as short as possible.

5. Installation Methods

To install the loop, first use a cutting machine to cut a groove in the road surface. Chamfer the corners at a 45-degree angle to prevent sharp corners from damaging the loop cable. The groove width is generally 4mm to 8mm, and the depth is 30mm to 50mm. A groove must also be cut for the loop leads to reach the roadside.

However, note that the groove must be clean and free of water or other liquids. When winding the loop, it must be kept straight, but not too tight and pressed against the bottom of the groove. After winding the loop, the twisted output leads should be pulled out through the lead groove.

Note:

The sensitivity of the vehicle detector loop decreases with increasing lead length, so the lead cable length should be kept as short as possible (generally not exceeding 5 meters). Untwisted output leads will inevitably cause crosstalk, making the vehicle detector loop inductance unstable, resulting in detector errors.

During the winding of the loop, an inductance tester should be used to test the inductance value of the induction loop and ensure that it is between 100uH and 300uH. Otherwise, adjustments should be made to the number of turns. After the loop is buried, to enhance protection, a nylon rope can be wound around the loop. Finally, seal the groove with asphalt or flexible resin.

1. Loop materials: Standard 1.0 square high-temperature tin-plated wire.

2. There should not be a large amount of metal within a 1-meter radius, such as manhole covers, rainwater ditch covers, etc.

3. There should not be any power lines exceeding 220V within a 1-meter radius.

4. When making multiple loops, the distance between loops must be greater than 2 meters, otherwise, they will interfere with each other.

5. For a standard 3-meter-wide road, the dimensions of the vehicle detector loop are 2 meters long and 1 meter wide, with 45° and 10-centimeter long chamfers at the corners.

6. The distance of the loop from the roadside should be around 50 centimeters, with the loop wound vertically for 4 to 8 turns, and the total length should be between 40 to 80 meters (including the lead wire).

7. Groove cutting parameters for installation: Width 4mm, Depth 50-80mm. The depth and width should be uniform, avoiding inconsistencies.

8. The loop should be at the same balanced position as the barrier or control device.

9. The two wires leading out from the loop should be twisted together, with a density of at least 20 twists per meter. Untwisted output leads will cause interference. The recommended output lead length should not exceed 5 meters. The length of the lead cable should be kept as short as possible due to the decrease in sensitivity of the detection loop with increasing lead length.

10. After installation, use cement, asphalt, or epoxy resin to seal and cure the groove. For cement pavements, any of the three materials can be used. For asphalt stone pavements, use asphalt or epoxy resin, taking care with the asphalt pouring.

11. The groove must be free of debris after cutting, especially hard objects, and must be cleaned thoroughly.

12. The lead groove of the vehicle detector loop should be cut to the safe island range to avoid exposing the leads on the road surface.

13. When winding the loop, it must be kept straight, but not too tight and pressed against the bottom of the groove, avoiding overlapping layers. After winding the loop, the twisted output leads should be pulled out through the lead groove.

Latest low-voltage data update—low-voltage construction organization plan and various ppt plans March 18, 2020