Introduction:
The installation of inductive loops in parking systems is crucial for the stability of the entire system. This article starts with the principle of inductive loops and comprehensively introduces the construction of these loops.
Main Body:
1. Principle of Vehicle Detectors and Inductive Loops in Parking Systems
1. Working Principle
Inductive loop vehicle detectors are based on the principle of electromagnetic induction. They are typically embedded in a circular loop under the roadbed of the same lane and powered by a working current, acting as sensors. When a vehicle passes over or stops on the loop, the iron in the vehicle changes 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 this change in inductance. There are generally two methods for detecting inductance changes: one utilizes 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
The inductive vehicle detector consists of the inductive loop and detector. The loop serves as data collection, while the detector performs data judgment and outputs the corresponding logic signals. The detector generally comprises a frame, central processing unit (CPU), detection card, and terminal block.
The CPU calculates the collected signals and is 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 inductive 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 vehicles, there may be inaccuracies in detecting vehicle length and speed, necessitating proper adjustment of the detector’s sensitivity. Current vehicle detectors generally have adjustable multi-level sensitivity.
2. Role of Inductive Loops in Parking Systems
To determine the role of inductive loops in parking systems, we must first understand where inductive loops are installed. Inductive loops are generally installed in the following four locations:
1. At the entrance ticket machine (entrance control unit);
2. At the entrance and exit barriers;
3. At the exit ticket machine (exit control unit);
Inductive loops used in the industry are generally made of copper wire. Those who have studied the subject should know that when a metal object passes through the loop, it generates current. The parking system utilizes this principle;
This is also why we refer to it as a loop. The method of making the loop is relatively simple; it involves winding a certain length of copper wire into several turns. This explanation should be familiar to everyone;
3. The Function of Inductive Loops at the Entrance Control Unit Can Be Divided into Two Aspects:
1. Preventing Card Loss
As we all know, in a standard one-in-one-out system, temporary vehicles enter by automatically taking a card. They only need to press a button, and the role of the inductive loop here is to ensure that a vehicle must be present at that location to take the card, rather than a person standing there pressing the button.
2. Reading Cards with Ground Induction
In long-distance card reading systems (Bluetooth systems, license plate recognition systems), the inductive loop is used to determine the direction of the vehicle;
4. The Role of Inductive Loops at Entrance and Exit Barriers Also Has Two Aspects
1. Preventing Vehicle Damage; the barrier will not drop when a vehicle is present at the inductive loop;
2. Vehicle Passage with the Barrier
The inductive loop at the exit control unit is similar to that at the entrance control unit; in long-distance card reading systems (Bluetooth systems, license plate recognition systems), this inductive loop is used to determine vehicle direction.
5. Construction Considerations for Inductive Loops
The inductive loop in the parking system is an oscillation circuit. It is constructed by first creating a circular trench on the ground, approximately 1 meter in diameter, or a rectangular trench of equivalent area, and then embedding two to three turns of wire in this trench. This forms an inductive loop buried beneath the surface, which is part of an oscillation circuit formed by it and a capacitor. The principle is to ensure stable and reliable oscillation, and this oscillation signal is transformed and sent to a frequency measurement circuit composed of a microcontroller, which can measure the frequency of the 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 frequency increases with the presence of metal), and this change serves as confirmation that a vehicle has passed over the “inductive loop.” The time interval between the start and end of this signal can also be used to measure the vehicle’s speed. This is the “inductive loop.” The technical key is to design a stable and reliable oscillator with a significant frequency change when a vehicle passes by.
1. Loop Material
During the design process, the circumference and turns of the inductive loop are often considered, but the material used is frequently 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. In harsh environments, resistance to acid and alkali corrosion is also necessary. However, conventional wires can deteriorate during actual use, and if the wire ages or lacks tensile strength, it can lead to wire damage, causing the detector to malfunction. Therefore, it is recommended to use high-temperature, multi-strand soft wire with a fluoropolymer coating, at least 1.0mm in diameter.
2. Loop Shape
1. Rectangular Installation
Typically, detection loops 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 narrowing by 0.3 to 1 meter at both ends compared to the road spacing.
2. 45° Inclined Installation
In certain cases, such as when detecting bicycles or motorcycles, it may be necessary to install the loop at a 45° angle to the direction of travel.
3. “8” Shaped Installation
In some cases, 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 placed close to the sliding door.
3. Number of Turns in the Loop
To ensure optimal operation of the detector, the inductance of the loop should be maintained between 100uH-300uH. The number of turns in the loop is significantly related to the circumference when the inductance remains unchanged. The smaller the circumference, the more turns there will be. Generally, the following can be referenced:
Due to various cables, steel bars, and manhole covers potentially being buried under the road, these can greatly affect the actual inductance value of the loop. Therefore, the above data is for user reference only. During actual construction, users should use an inductance tester to measure the inductance value of the inductive loop to determine the actual number of turns to be constructed, ensuring that the final inductance value of the loop is within a reasonable working range (e.g., between 100uH-300uH).
4. Output Leads
When winding the loop, sufficient lead length should be left for connection to the loop sensor, ensuring there are no joints in between. After winding the loop cable, the output leads must be tightly twisted together, requiring at least 20 twists per meter. Otherwise, untwisted output leads will introduce interference, causing the inductance value of the loop to become unstable. The length of the output leads 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 Method
The loop installation begins by using a cutting machine to create a trench on the surface. Chamfering at 45 degrees at the corners prevents sharp angles from damaging the loop cable. The trench width is generally 4mm-8mm and the depth is 30mm-50mm. Additionally, a trench for the loop leads should be cut to the roadside. However, care must be taken to ensure the trench is clean and free of water or other liquids. When winding the loop, it must be kept straight but not overly tight against the trench bottom, and 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). Untwisted output leads will certainly cause interference, making the inductive loop’s inductance value unstable, resulting in detector errors.
During the winding process, the inductance tester should be used to measure the inductance value of the inductive loop, ensuring it remains between 100uH-300uH. If not, the number of turns in the loop should be adjusted. 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.
6. Considerations
6.1 Loop Material: Standard 1.0 square high-temperature tinned wire.
6.2 There must be no significant metal presence (such as manhole covers, drainage covers, etc.) within a 1-meter radius.
6.3 There must be no power lines exceeding 220V within a 1-meter radius.
6.4 When multiple loops are used, the distance between loops must be greater than 2 meters; otherwise, they will interfere with each other.
6.5 For a standard 3-meter wide road, the dimensions of the vehicle detector loop should be 2 meters long and 1 meter wide, with 45° chamfers of 10 centimeters at the corners. As shown below:
6.6 The distance from the loop to the road edge should be about 50 centimeters, and the loop should be wound vertically in 4-8 turns, with a total length of 40-80 meters (including lead-out wires).
6.7 Trench cutting parameters for installation: width 4mm, depth 50-80mm. The depth and width should be uniform to avoid inconsistencies.
6.8 The loop should be at the same balanced position as the barrier or control unit.
6.9 The two wires leading out from the loop should be twisted together, with a density of no less than 20 twists per meter; untwisted output leads will cause interference. The output lead length is recommended to not exceed 5 meters. As the sensitivity of the detection loop decreases with increasing lead length, the lead cable length should be kept as short as possible.
6.10 After installation, seal the trench with materials such as cement, asphalt, or epoxy resin. All three materials can be used for cement surfaces, while asphalt or epoxy resin can be used for asphalt surfaces. Care should be taken when using asphalt to seal the loop.
6.11 The trench must be free of debris, especially hard objects, and should be cleaned thoroughly.
6.12 The lead-out trench for the vehicle detector loop should be cut to the safety island range to avoid exposing the leads on the road surface.
6.13 When winding the loop, it must be kept straight but not overly tight against the trench bottom, and care should be taken to avoid overlapping layers. After winding the loop, the twisted output leads should be pulled through the lead-out trench.
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