Many friends in the weak current VIP group have repeatedly asked about parking management systems. When it comes to parking management systems, the first application should be in 2001, using the SYRIS controller, equipped with a long-range card reader from Sains and a barrier gate from Youchuang Enterprise, realizing the first no-stop vehicle entry and exit recognition system, 17 years ago.

Now looking back, recognition technology, recognition software, and recognition hardware have all undergone super development. But no matter how it changes, it is still a certification of identity recognition information.

Nowadays, many parking systems use license plate recognition, long-range ID card recognition, Bluetooth recognition, and other technologies. There has been a significant improvement in recognition accuracy and the convenience of parking management. I remember when we designed the first system, the client had one condition: we are a high-end community, and the owners do not want to swipe cards to enter and exit; we need a long-range solution. Back then, you can imagine. Jieshun had just started not long ago, but we got it done!

Having talked so much about the development of recognition technology and feelings, there is one thing that has not changed much: vehicle detection and ground loop detectors. Today, let’s discuss this.

First, let’s talk about the working principle of parking lot vehicle detectors and ground loop detectors, and then discuss their composition and applications!

The ground loop vehicle detector is a type of vehicle detector based on the principle of electromagnetic induction. It is usually installed under the pavement of the same lane, with a circular loop buried and powered by a certain working current, acting as a sensor. When a vehicle passes through or stops on this loop, the iron material on 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 the passing vehicle by detecting this change in inductance.

There are generally two methods for detecting changes in inductance:

One is to use a phase-locked loop and phase comparator to detect phase changes;

The other is to use a coupling circuit formed by the loop to detect its oscillation frequency.

The ground vehicle detector consists of a ground loop and a detector. The loop serves as data collection, and the detector is used to make data judgments and output corresponding logic signals. The detector generally consists of a rack, central processor, detection card, and terminal block.

The central processor is the module that calculates the collected signals, usually 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 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, and 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 vehicle length and speed detection, requiring proper adjustment of the detector’s sensitivity. Currently, vehicle detectors generally have multi-level sensitivity adjustment functions.

In parking systems, to determine the role of ground loops, we must first know the installation locations of the ground loops.

Ground loops are generally installed in the following four locations: at the entrance ticket machine (entrance control machine), one at the entrance and exit barrier, and at the exit ticket machine (exit control machine).

The ground loops used in the industry are generally made of copper wire. Anyone who has completed junior high school should know that when a metallic object passes through the loop, it generates a current; the parking system utilizes this principle. This is why we call it a loop; the method of making the loop is relatively simple, just wrapping a certain length of copper wire several times.

The ground loop’s function at the entrance control machine can be summarized in two aspects: first, preventing card loss; everyone should know that in a standard one-in-one-out system, temporary vehicles enter by automatically taking a card, and they only need to press a button. The function of the ground loop here is to ensure that a vehicle is present at that location before a card can be taken, rather than allowing a person to stand there and press the button to take a card; second, it can suppress the ground loop reading card in long-range reading systems (Bluetooth systems, license plate recognition systems), where the ground loop is used to determine vehicle direction.

In a parking system, a ground loop is an oscillation circuit. It is constructed by first creating a circular groove on the ground, approximately 1 meter in diameter, or an equivalent rectangular groove, and then burying two to three turns of wire in this groove, forming an inductive loop buried beneath the surface. This loop is part of an oscillation circuit, composed of it and a capacitor, with the principle being stable and reliable oscillation. This oscillation signal is converted and sent to a frequency measurement circuit composed of a microcontroller, which can measure the frequency of this oscillator.

When a large metal object like a car passes by, the change in spatial medium 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 that a car has passed through 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 key technical aspect is the stability and reliability of the designed oscillator and the significant frequency change when a car passes.

During the design process, it is often overlooked what materials to use for the ground loop, focusing only on the circumference and turns. 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. In some harsh environments, resistance to acid and alkali corrosion must also be taken into account. However, conventional wires may age or lack tensile strength, leading to wire damage, causing the detector to malfunction. Therefore, it is recommended to use multi-stranded soft wires with a fluoropolymer coating, with a diameter of more than 1.0mm.

To ensure the detector operates at its best, the inductance of the loop should be maintained between 100uH-300uH. With the inductance of the loop unchanged, the number of turns is closely related to the circumference. A smaller circumference means more turns. Generally, it can be referenced:

Due to various cables, pipes, rebar, and manhole covers that may be buried under the road, these can greatly 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 for construction, ensuring that the final inductance value of the loop is within a reasonable working range (e.g., between 100uH-300uH).

When winding the loop, leave enough 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 in pairs, requiring at least 20 twists per meter. Otherwise, the untwisted output leads will introduce noise, making the loop’s inductance value unstable. The length of the output lead should generally not exceed 5 meters. Since the sensitivity of the detection loop decreases with the increase in lead length, the length of the lead cable should be kept as short as possible.

To install the loop, first use a road cutter to create a groove on the surface. Chamfer 45 degrees 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. At the same time, a groove for the loop leads must be cut to the roadside. However, be careful: the groove must be clean and free of water or other liquids. When winding the loop, it must be straight but not too tight and should not stick to the bottom of the groove. After winding the loop, lead the twisted output cable through the output lead groove.

Note: The sensitivity of the vehicle detector loop decreases with the increase in lead length, so the lead cable length should be kept as short as possible (generally not exceeding 5 meters). Untwisted output leads will introduce interference, making the vehicle detector loop’s inductance value unstable, causing the vehicle detector to malfunction.

During the winding of the loop, the inductance tester should be used to measure the inductance value of the ground loop and ensure that the inductance value is maintained between 100uH-300uH. Otherwise, adjustments to the number of turns of the loop should be made. After the loop is buried, for added protection, a nylon rope can be wound around the loop. Finally, seal the groove with asphalt or soft resin.

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