Principle of Vehicle Detectors and Ground Loops in Parking Lots
1. Working Principle
Ground loop vehicle detectors are based on the principle of electromagnetic induction. They are typically embedded under the roadbed of the same lane in a circular loop, powered by a working current, acting as a sensor. When a vehicle passes over or stops on this loop, the iron content of the vehicle alters the magnetic flux within the loop, causing a change in the inductance of the loop circuit. The detector determines the status of the passing vehicle by detecting this change in inductance. There are generally two methods to detect the change in inductance: one uses a phase-locked loop and phase comparator to detect phase changes; the other uses a coupling circuit formed by the loop to detect its oscillation frequency.
2. System Composition
Ground vehicle detectors consist of ground loops and detectors. The loop serves as data collection, while the detector is used for data judgment and outputs corresponding logical 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 sensing loop, the inductance of the loop decreases; the function of the detection card is to detect this change and accurately output the corresponding level. When detecting high-speed passing vehicles, there may be inaccuracies in vehicle length and speed detection, necessitating proper adjustment of the detector’s sensitivity. Currently, most vehicle detectors have multi-level adjustable sensitivity functions.
Role of Ground Loops in Parking Systems
In parking systems, to determine the role of ground loops, we must first understand where ground loops are installed. Ground loops are generally placed in four locations: at the entrance ticket box (entrance control machine), one at each entrance and exit gate, and at the exit ticket box (exit control machine). The ground loops used in the industry typically consist of copper wire. Anyone who has completed middle school should know that when a metal object passes through the loop, it generates a current, which is the principle utilized in parking systems.This is why we refer to it as a loop; the method of making the loop is relatively simple, just wrapping a certain length of copper wire several times.
The ground sensing function at the entrance control machine can be divided into two aspects: first, preventing card loss. As we all know, in a standard one-in-one-out system, temporary vehicles enter the parking lot by automatically taking a card. They simply need to press a button, and at this point, the function of the ground sensor is to allow card retrieval only when a vehicle is sensed at that location, not when a person stands there and presses the button; second, ground sensing card reading. In long-distance reading systems (Bluetooth systems, license plate recognition systems), this ground sensing is used to identify vehicle direction.
Considerations for Installing Ground Loops
In parking systems, the “ground loop” is an oscillating circuit. It is constructed by first creating a circular trench on the ground, approximately 1 meter in diameter, or an equivalent rectangular trench, and then embedding two to three turns of wire in this trench. This forms an inductive loop buried at ground level, which is part of an oscillating circuit composed of it and capacitors. The principle is oscillation stability and reliability, and this oscillation signal is sent to a frequency measurement circuit composed of a microcontroller, allowing the microcontroller to measure the frequency of this oscillator.
When a large metal object, such as a car, passes by, the change in the spatial medium causes a change in the oscillation frequency (the oscillation frequency increases when a metal object is present), and this change serves as the confirmation signal for the car passing the “ground loop”. The time interval between the start and end of this signal can also be used to measure the speed of the vehicle. This is the “ground loop”; the technical key is to design an oscillator that is stable and reliable with a noticeable frequency change when a vehicle passes.
1. Loop Material
During the design process, the perimeter and number of turns of the ground loop are usually considered, while the material used is often overlooked by designers. When the project reaches the implementation stage, it is essential to consider the mechanical strength of the wire and its resistance to high and low temperatures, and in some harsh environments, resistance to acid and alkali corrosion must also be considered. However, conventional wires can become aged or lack tensile strength, leading to wire breakage, which will prevent the detector from functioning properly. Therefore, it is recommended to use high-temperature multi-strand soft wire with a diameter of over 1.0mm.
2. Loop Shape
1. Rectangular Installation
Typically, the detection loop should be rectangular. The two long sides should be perpendicular to the direction of metal object movement, and the recommended distance between them is 1 meter. 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, when detecting bicycles or motorcycles, consider installing the loop at a 45° angle to the direction of travel.
3. “8” Shape Installation
In certain situations, when the road surface is wide (over six meters) and the vehicle’s chassis is too high, this installation method can be used to disperse detection points and improve sensitivity. This installation method can also be used for sliding door detection, but the loop must be close to the sliding door.
4. Number of Turns in the Loop
To ensure the detector operates optimally, the inductance of the loop should be maintained between 100uH-300uH. With the inductance of the loop remaining unchanged, the number of turns is significantly related to the perimeter. A smaller perimeter results in more turns. Generally, refer to:
Due to various cable pipes, steel bars, and sewer covers potentially buried under the road, these can significantly affect the actual inductance value of the loop, so the data in the table is for reference only. During actual construction, users should use an inductance tester to measure the inductance value of the ground loop to determine the actual number of turns needed, ensuring that the final inductance value of the loop falls within a reasonable operating range (e.g., between 100uH-300uH).
5. Output Leads
When winding the loop, leave a sufficient length of wire to connect to the loop sensor, ensuring there are no joints in between. After winding the loop cable, the output cable must be tightly twisted, with a minimum of 20 twists per meter. Otherwise, non-twisted output leads will introduce interference, making the inductance value of the loop unstable. The output lead length generally should not exceed 5 meters. Since the sensitivity of the detection loop decreases with increasing lead length, the length of the lead cable should be kept as short as possible.
6. Installation Method
The loop installation begins with using a cutting machine to create a trench on the road surface. Chamfer the corners at 45 degrees to prevent sharp corners from damaging the loop cable. The trench width is generally 4mm to 8mm, and depth is 30mm to 50mm. Additionally, a trench must be cut for the loop leads to reach the roadside. However, it is important to note that the trench must be clean and free of water or other liquids. When winding the loop, it must be pulled straight, but not too tight and should not stick to the bottom of the trench. After winding the loop, the twisted output leads should be pulled through the lead-out trench.
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). Non-twisted output leads will definitely cause crosstalk, making the vehicle detector loop inductance unstable, leading to errors in detection.
During the winding of the loop, an inductance tester should be used to measure the inductance value of the ground loop, ensuring the inductance value is between 100uH-300uH. If not, adjustments to the number of turns of the loop should be made. After the loop is buried, to enhance protection, a nylon rope can be wound around the loop. Finally, seal the trench with asphalt or flexible resin.
7. Important Considerations
1. Loop Material: 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 or storm drain covers.
3. There should not be power lines exceeding 220V within a 1-meter radius.
4. When making multiple loops, the distance between loops should be greater than 2 meters to avoid mutual interference.
5. For a standard 3-meter wide road, the dimensions of the vehicle detector loop are 2 meters long and 1 meter wide, with a 45° chamfer of 10 centimeters at the corners. As shown below:
6. The distance between the loop and the roadside should be about 50 centimeters. The loop should be vertically stacked with 4-8 turns, with a total length of 40-80 meters (including the lead).
7. Trench cutting parameters for loop installation: width 4mm, depth 50-80mm. The depth and width should be uniform, avoiding variations in depth and width. As shown below:
8. The loop should be at the same balanced position as the barrier or control machine.
9. The two wires leading out of the loop should be twisted, with a density of no less than 20 twists per meter. Non-twisted output leads will cause interference. The recommended length of output leads should not exceed 5 meters. Since the sensitivity of the detection loop decreases with increasing lead length, the length of the lead cable should be kept as short as possible.
10. After installation, use materials such as cement, asphalt, or epoxy resin to seal and cure the trench. For concrete surfaces, any of the three materials can be used, while for asphalt gravel surfaces, use asphalt or epoxy resin. When using asphalt for sealing, care must be taken with the loop.
11. The trench must be free of debris after cutting, especially hard objects, and should be cleaned thoroughly.
12. The lead trench for the vehicle detector loop should be cut within the safety island range to avoid exposing the leads on the road surface.
13. When winding the loop, it must be pulled straight, but not too tight, and should not be pressed against the bottom of the trench, avoiding crossing layers. After winding the loop, pull the twisted output leads through the lead-out trench.
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