1. Principles of Vehicle Detectors and Ground Loops in Parking Systems
1. Working Principle
The ground loop vehicle detector is a type of vehicle detector based on the principle of electromagnetic induction. It is usually installed under the roadbed of the same lane, with a circular coil buried and powered by a certain working current, serving as a sensor. When a vehicle passes over or stops on the coil, the iron material of the vehicle will change the magnetic flux within the coil, causing a change in the inductance of the coil circuit. The detector judges the vehicle’s status by detecting this change in inductance. There are generally two methods to detect the change in inductance: one uses phase-locked loops and phase comparators to detect phase changes; the other uses a coupling circuit formed by the circular coil to detect its oscillation frequency.
2. System Composition
The ground vehicle detector consists of the ground loop and the detector. The loop serves as data collection, while the detector is used to make data judgments and output corresponding logic signals. The detector generally consists of a rack, a central processing unit, a detection card, and terminal blocks.
The central processing unit is the module that calculates the collected signals, usually a single-board computer with an embedded operating system, which has 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 is 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, requiring proper adjustment of the detector’s sensitivity. Currently, most vehicle detectors generally have the function of adjustable multi-level sensitivity.
2. The Role of Ground Loops in Parking Systems
To determine the role of ground loops in parking systems, we must first know where the ground loops are installed. Ground loops are generally installed in the following four locations:
1. At the entrance ticket machine (entrance control machine);
2. One at each entrance and exit barrier;
3. At the exit ticket machine (exit control machine);
The ground loops used in the industry are generally made of copper wire. As anyone who has attended middle school should know, when a metal object passes through the loop, it generates current, which is the principle utilized in parking systems;
This is also why we refer to it as a loop. The manufacturing method of the loop is relatively simple; it involves winding a certain length of copper wire a few times around, making it a familiar concept to everyone;
3. The Ground Loop’s Role at the Entrance Control Machine Can Be Divided into Two Aspects:
1. Preventing Card Loss
Everyone should know that in a standard one-in-one-out system, temporary vehicles enter by automatically taking a card; they just need to press a button, and the role of the ground loop here is to ensure that a card can only be taken when a vehicle is present, rather than when a person simply stands there and presses the button.
2. Ground Loop Card Reading
In long-distance card reading systems (Bluetooth systems, license plate recognition systems), the ground loop is used to determine the direction of the vehicle;
4. The Role of Entry and Exit Barriers Also Has Two Aspects
1. Preventing Vehicle Damage; the barrier rod will not fall when a vehicle is present at the ground loop;
2. Vehicle Passing Under the Rod
The ground loop at the exit control machine is similar to that at the entrance controller; in long-distance card reading systems (Bluetooth systems, license plate recognition systems), the ground loop is used to determine the direction of the vehicle.
5. Precautions 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 burying two to three turns of wire in this trench, forming an inductive loop buried beneath the surface. This loop is part of an oscillating circuit, consisting of itself and a capacitor to form an oscillating circuit, 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. When a large metal object, such as a car, passes by, the change in spatial media causes a change in the oscillation frequency (the oscillation frequency increases when there is a metal object), and this change serves as the confirmation signal for the vehicle passing through the “ground 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 “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 focus is usually only on the perimeter and turns of the ground loop, 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, as well as aging issues. In some harsh environments, acid and alkali corrosion resistance must also be considered. However, conventional wires, when aged or insufficient in tensile strength, 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, 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 width.
2. 45° Inclined Installation
In certain cases, when detecting bicycles or motorcycles, consider installing the loop at a 45° angle to the direction of travel.
3. Figure “8” Shape Installation
In some cases, when the road is wide (more than 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 detecting sliding doors, but the loop must be close to the sliding door.
3. 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 constant, the number of turns is significantly related to the perimeter. The smaller the perimeter, the more turns there are. Generally, refer to:
Due to the possibility of various cabling, steel bars, and sewer covers buried under the road, these can significantly affect the actual inductance value of the loop. Therefore, the data in the above table is for reference only. During actual construction, users should use an inductance tester to measure the actual inductance value of the ground loop to determine the actual number of turns required for construction, ensuring that the final inductance value of the loop is within a reasonable operating range (e.g., between 100uH-300uH).
4. Output Leads
When winding the loop, leave sufficient length of wire for connection to the loop detector, ensuring 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 un-twisted output leads will introduce interference, making the loop’s inductance value unstable. The length of the output leads should generally 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.
5. Installation Method
The loop installation begins with cutting a groove in the road surface using a cutting machine. Make 45-degree bevels at the four corners 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, it is essential to ensure that the groove is clean and free from water or other liquids. The loop must be pulled straight during winding but not too tight and must not touch the bottom of the groove. After winding the loop, the twisted output leads should be pulled through the lead groove.
Note: The sensitivity of the vehicle detector loop decreases with increasing lead length, so the lead cable length should be as short as possible (generally not exceeding 5 meters). Un-twisted output leads will cause interference, making the vehicle detector loop’s inductance value unstable, leading to errors in the vehicle detector.
During the winding process of the loop, an inductance tester should be used to measure the inductance value of the ground loop, ensuring that the inductance value is between 100uH-300uH. Otherwise, adjustments to the number of turns in 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 groove with asphalt or flexible resin.
6. Precautions
6.1 Loop Material: Standard 1.0 square high-temperature tin-plated wire.
6.2 There should not be a large amount of metal within a 1-meter radius, such as manhole covers, rainwater drainage covers, etc.
6.3 There should not be 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 are 2 meters long and 1 meter wide, with 45-degree, 10-centimeter long bevels at the corners. As shown below:
6.6 The distance from the loop to the roadside should be about 50 centimeters, with the loop wound vertically for 4 to 8 turns, with a total length of 40 to 80 meters (including the leads).
6.7 Installation groove cutting parameters: width 4mm, depth 50-80mm; the depth and width should be uniform and consistent, avoiding situations where it is sometimes deep and sometimes shallow, or wide and narrow. As shown below:
6.8 The loop should be in a balanced position with the barrier or control machine.
6.9 The two wires leading out from the loop should be twisted together, with a density of at least 20 twists per meter. Un-twisted output leads will cause interference. The recommended length of the output leads should not exceed 5 meters. Since 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 and cure the groove with materials such as cement, asphalt, or epoxy resin. For cement roads, any of the three materials can be used. For asphalt-stone roads, use asphalt or epoxy resin, taking care to avoid damaging the loop when using asphalt.
6.11 The groove must be free of debris, especially hard objects, and must be cleaned thoroughly.
6.12 The lead groove for the vehicle detector loop must be cut to the safety island’s range to avoid exposing the leads on the road surface.
6.13 When winding the loop, ensure the loop is straight but not overly tight and not touching the bottom of the groove, avoiding cross layers. After winding the loop, pull the twisted output leads through the lead groove.
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