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 Enterprises, achieving the first non-stop vehicle identification system, 16 years ago.
Looking back now, recognition technology, recognition software, and recognition hardware have all seen tremendous development. But the core remains the same, which is the authentication of identity information. Haha.
Nowadays, many parking systems use license plate recognition, long-range ID card recognition, Bluetooth recognition, and other technologies, significantly improving recognition accuracy and the convenience of parking management. I remember when we designed the first system, the client had only one condition: we are a high-end community, and the owners don’t want to swipe cards to enter and exit; give us a long-distance solution. You can imagine how that was. Jieshun had just started not long before.
After talking so much about the development of recognition technology, there is one thing that hasn’t changed much: vehicle detection and inductive loops. Today, let’s talk about that.
First, let’s discuss: the principle of parking lot vehicle detectors and inductive loops, and then their composition and application!
1. Working Principle
Inductive loop vehicle detectors are a type of vehicle detector based on the principle of electromagnetic induction. They are usually buried under the roadbed of the same lane in a circular loop, energized with a certain working current, serving as a sensor. When a vehicle passes over or stops on this loop, the iron content in the vehicle will change the magnetic flux within the loop, causing changes in the loop’s inductance, which the detector uses to determine the vehicle’s status.
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.
2. System Composition
The inductive vehicle detector consists of an inductive loop and a detector, where the loop serves as data collection, and the detector is used for data judgment and outputs corresponding logic signals. The detector generally consists of a frame, central processing unit, 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, possessing strong digital computation, storage capability, 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 stops in the detection area of the inductive loop, the inductance of the loop will decrease, and the function of the detection card is to detect this change and accurately output the corresponding level. When detecting vehicles passing at high speed, there may be inaccuracies in detecting vehicle length and speed, requiring proper adjustment of the detector’s sensitivity. Currently, vehicle detectors generally have multi-level sensitivity adjustment capabilities.
The Role of Inductive Loops in Parking Systems
To determine the role of inductive loops in parking systems, we must first know where the inductive loops are installed.
Inductive loops are generally installed in the following four locations: at the entrance ticket machine (entrance control machine), at the entrance and exit barriers, and at the exit ticket machine (exit control machine).
The inductive loops used in the industry are generally made of copper wire. Anyone who has completed junior high school should know that when a metal object passes through the loop, it generates 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 winding a certain length of copper wire several times.
Inductive Loop Installation Precautions
In the parking system, the inductive loop is an oscillating circuit. It is structured by first creating a circular trench on the ground, with a diameter of about 1 meter, or an equivalent rectangular trench, and then burying two to three turns of wire in this trench. This creates an inductive loop buried beneath the surface, which is part of an oscillating circuit formed with a capacitor, following the principle of stable and reliable oscillation. 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 the spatial medium causes a change in the oscillation frequency (the oscillation frequency increases when a metal object is present). This change serves as the confirmation signal for a vehicle passing through the inductive loop, and 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 key technology is to design an oscillator that is stable and reliable, with a significant frequency change when a vehicle passes.
1. Loop Material
During the design process, only the circumference and turns of the inductive 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, as well as aging issues. In some harsh environments, resistance to acid and alkali corrosion is also crucial. However, conventional wires, once aged or lacking tensile strength, can lead to wire damage, causing the detector to malfunction. Therefore, it is recommended to use soft multi-stranded Teflon wire with a diameter of 1.0mm or more.
2. Loop Shape
1. Rectangular Installation
The detection loop should generally be rectangular. The two long sides should be perpendicular to the direction of metal movement, with a recommended spacing of 1 meter. The length of the long sides depends on the width of the road, usually narrower than the road spacing by 0.3 to 1 meter at both ends.
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 some cases, when the road is wide (over six meters) and the vehicle’s chassis is too high, this installation method can be used to disperse the 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.
3. Number of Turns
To ensure the detector operates at its best, the inductance of the loop should be maintained between 100uH and 300uH. Given a constant inductance, the number of turns is closely related to the circumference. The smaller the circumference, the more turns there are. Generally, refer to:
4. Output Leads
When winding the loop, leave sufficient length of wire to connect to the loop sensor, ensuring there are no joints in between. After winding the loop cable, it must be tightly twisted, requiring at least 20 twists per meter. Otherwise, un-twisted output leads will introduce interference, making the loop’s inductance unstable. The output lead length should generally 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.
5. Installation Method
Loop installation begins with cutting a groove in the pavement using a cutting machine. Bevel the corners at 45 degrees to prevent sharp angles from damaging the loop cable. The groove width should generally be 4mm to 8mm, with a depth of 30mm to 50mm. Also, cut a groove for the loop leads to the roadside. However, it is essential to ensure that the groove is clean and free of water or other liquids. The loop must be pulled straight during winding, but not too tightly or pressed against the bottom of the groove. After winding the loop, lead the twisted output leads through the lead-out groove.
6. Precautions
1. Loop Material: Standard 1.0 square high-temperature tinned wire.
2. There should be no large amounts of metal, such as manhole covers or drainage covers, within a 1-meter radius.
3. No power supply lines exceeding 220V should be 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 should be 2 meters long and 1 meter wide, with a 45°, 10-centimeter bevel at the corners.
6. The distance between the loop and the road edge should be about 50 centimeters, with the loop vertically stacked and wound 4 to 8 turns, with a total length of 40 to 80 meters (including the leads).
7. Cut groove parameters: width 4mm, depth 50-80mm; depth and width should be uniform and consistent, avoiding variations.
8. The loop should be in the same balanced position as the barrier or control machine.
9. The two leads of the loop should be twisted together, with a density of at least 20 twists per meter; untwisted output leads will cause interference. The output lead length is recommended not to 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.
10. After installation, seal the groove with materials such as cement, asphalt, or epoxy resin. For cement roads, any of the three materials can be used. For asphalt roads, asphalt or epoxy resin can be used; caution is needed when sealing with asphalt.
11. The groove must be free of debris, especially hard objects, and should be cleaned thoroughly.
12. The lead groove for the vehicle detector loop should be cut within the safe island range to avoid exposing the leads on the road.
13. The loop must be pulled straight during winding, but not too tightly or pressed against the bottom of the groove, avoiding overlapping layers. After winding the loop, lead the twisted output leads through the lead-out groove.
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