Practical Implementation: Installation of Inductive Loops in Parking Systems

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When it comes to parking management systems, the first application should be in 2001, using SYRIS (希瑞) controllers, equipped with long-range card readers from Sains and barrier gates from Youchuang, achieving the first non-stop vehicle entry and exit recognition system, 16 years ago.

Looking back now, recognition technology, software, and hardware have all developed tremendously. But the essence remains the same, which is the authentication of identity recognition information. Haha.

Nowadays, many parking systems use license plate recognition, long-range ID card recognition, Bluetooth recognition, and so on. 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 owners do not want to swipe cards to enter; they wanted a long-range system. At that time, it was easy to imagine. Jieshun had just started not long ago. We did it!

Having talked so much about the development of recognition technology and my feelings, there is one thing that hasn’t changed much: vehicle detection and inductive loops. Today, let’s talk about it.

First, let’s discuss: the principles of parking vehicle detectors and inductive loops, and then their composition and applications!

1. Working Principle

The inductive loop vehicle detector is a type of vehicle detector based on electromagnetic induction principles. It is usually buried under the roadbed of the same lane with a circular loop, powered by a certain working current, serving as a sensor. When a vehicle passes through or stops on the loop, the iron material of the vehicle will change the magnetic flux inside the loop, causing a change in the inductance of the loop circuit. The detector determines the vehicle status by detecting this change in inductance.

There are generally two methods for detecting the change 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 circular loop to detect its oscillation frequency.

2. System Composition

The inductive vehicle detector includes the inductive loop and the detector, where the loop serves as data collection, and the detector is used for data judgment and outputs corresponding logical 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, typically a single-board computer with an embedded operating system, capable of strong digital computation, storage, 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 inductive 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 fast-moving vehicles, there may be inaccuracies in detecting vehicle length and speed, requiring correct adjustment of the detector’s sensitivity. Currently, vehicle detectors generally have multi-level sensitivity adjustment functions.

The Role of Inductive Loops in Parking Systems

In parking systems, to determine the role of inductive loops, 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), one at the entrance and exit barrier, 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 studied physics should know that when a metal object passes through a loop, it generates current, which is the principle utilized in parking systems. This is why we call it a loop. The method of making a loop is relatively simple; it involves winding a certain length of copper wire several times.

The role of the inductive loop at the entrance control machine can be summarized in two aspects: first, preventing card loss. Everyone should know that in a standard one-entry-one-exit system, temporary vehicles enter the parking lot by automatically taking a card; they only need to press a button. The role of the inductive 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 the card; second, pressure-sensitive reading at the inductive loop. In long-range reading systems (Bluetooth systems, license plate recognition systems), the inductive loop is used to identify vehicle direction.

Considerations for Installing Inductive Loops

In parking systems, the inductive loop is an oscillating circuit. It consists of creating a circular trench on the ground, about 1 meter in diameter, or a rectangular trench of equivalent area, and then burying two to three turns of wire in this trench. This forms an inductive loop buried beneath the surface, which is part of an oscillating circuit formed with a capacitor. The principle is that the oscillation is stable and reliable; this oscillation signal is converted and sent to a frequency measurement circuit consisting 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 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 that a car has passed through the “inductive loop”, and the time interval between the start and end of this signal can also be used to measure the speed of the car. This is the “inductive loop”. The technical key is to design an oscillator that is stable and reliable, with a significant frequency change when a car passes.

1. Loop Material

During the design process, usually only the perimeter and turns of the inductive loop are considered, while the material used is often overlooked by designers. When the project reaches the implementation stage, mechanical strength and high/low temperature aging resistance of the wire must be considered. In some harsh environments, acid and alkali corrosion resistance must also be considered. However, conventional wires, when aged or lacking tensile strength, can lead to wire damage, causing the detector to fail to work normally. Therefore, it is recommended to use high-temperature multi-stranded soft wire with a diameter of 1.0mm or more.

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 distance of 1 meter apart. 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° Tilt Installation

In some cases where bicycles or motorcycles need to be detected, consider installing the loop tilted at 45° to the direction of travel.

3. “8” Shape Installation

In some cases, when the road surface is wide (over six meters) and the vehicle 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

To ensure the detector operates under optimal conditions, the inductance of the loop should be maintained between 100uH and 300uH. With the inductance of the loop unchanged, the number of turns has a significant relationship with the perimeter. The smaller the perimeter, the more turns there are. Generally, it can be referred to:

Due to various cables, steel bars, and sewer covers buried under the road, these will greatly affect the actual inductance value of the loop, so the above data is for user reference only. During actual construction, users should use an inductance tester to test the inductance value of the inductive 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 (such as between 100uH and 300uH).

4. Output Leads

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, the output cable must be tightly twisted in pairs, with a minimum of 20 twists per meter. Otherwise, un-twisted output leads will introduce noise, making the inductance value of the loop unstable. The length of the output leads should generally not exceed 5 meters. Since the sensitivity of the detection loop decreases as the length of the leads increases, the length of the lead wires should be kept as short as possible.

5. Installation Method

The first step in burying the loop is to cut a trench in the road surface using a cutting machine. Chamfer the corners at a 45-degree angle to prevent sharp corners from damaging the loop cable. The width of the trench is generally 4mm to 8mm, and the depth is 30mm to 50mm. At the same time, a trench should be cut for the loop leads to reach the roadside. However, it is important to ensure that the trench is clean and free of water or other liquids. When winding the loop, it must be straightened but not too tight against 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 as the length of the lead wires increases, so the length of the lead cable should be kept as short as possible (generally not exceeding 5 meters). Un-twisted output leads will definitely cause crosstalk, making the inductive 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 test the inductance value of the inductive loop, ensuring that the inductance value is between 100uH and 300uH. Otherwise, adjustments should be made to the number of turns of the loop. 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. Precautions

1. Loop Material: Standard 1.0 square high-temperature tinned wire.

2. There should not be a large amount of metal within a 1-meter radius, such as manhole covers, rainwater ditch covers, etc.

3. There should not be any power supply lines exceeding 220V within a 1-meter radius.

4. When making multiple loops, the distance between loops should be greater than 2 meters; otherwise, they will interfere with each other.

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° chamfering of 10 cm at the corners.

6. The distance between the loop and the roadside should be about 50 cm, and the loop should be wound vertically for 4 to 8 turns, with a total length of 40 to 80 meters (including lead wires).

7. Trench cutting parameters for installation: width 4mm, depth 50-80mm. The depth and width should be uniform and consistent, avoiding variations in depth and width.

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. Un-twisted output leads will introduce interference. The recommended length of output leads should not exceed 5 meters. Since the sensitivity of the detection loop decreases as the length of the leads increases, 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 solidify the trench. For cement surfaces, any of the three materials can be used; for asphalt gravel surfaces, asphalt or epoxy resin can be used. When using asphalt for sealing, care should be taken with the loop.

11. The trench must be clear of debris after cutting, especially hard objects; it should be cleaned thoroughly.

12. The leads of the vehicle detector loop should be cut to the safe island range to avoid exposing the leads on the road surface.

13. When winding the loop, it must be straightened but not pulled too tightly against the bottom of the trench, avoiding crossing layers. After winding the loop, the twisted output leads should be pulled through the lead-out trench.

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