“The vehicle detection technology implemented by phase-locked loop technology is more reliable than the current vehicle detection technology that uses single-chip counting, and by detecting the loss of phase-locked loop to determine whether there is a vehicle passing by, it is a relative frequency change rather than an absolute frequency Therefore, it has the ability of self-adjustment and will not fail due to the shift of the center frequency caused by the change of the surrounding electromagnetic environment. Therefore, the phase-locked loop technology proposed in this paper has obvious effect in improving the sensitivity, and the cost is low, and it has a wide application prospect.
“
Authors: Cheng Jing, Sun Wensheng
1 Introduction
At intersections, in order to avoid traffic jams and ensure traffic safety, it is stipulated that traffic vehicles must drive on the road and not cross the road. Therefore, corresponding sensing devices must be used to monitor the violations of vehicles. At present, most of the recording and processing of running a red light rely on the installation of corresponding sensing devices under the waiting line. When a vehicle passes by, a specific signal will be sent immediately, and the system will judge and process according to the signal, and then take pictures of the illegal vehicles through the intersection camera. Record. A similar approach can also be used to detect vehicle off-road behavior, ie using a sensor device buried in the ground. However, since the size and shape of the sensing device are limited by the width of the lane-dividing marking line, in order to effectively detect the vehicle’s off-lane driving behavior, higher requirements are placed on the measurement sensitivity. The current scheme generally uses a single-chip microcomputer to measure the frequency, which has low sensitivity and poor anti-interference ability. This is because the single-chip computer measures the frequency to count the number of pulse signals within a certain time t, and reflects the frequency change according to the change of the pulse number. Assuming that the frequency of the sensing oscillation circuit is 100 kHz (this value is close to the practical application limit, if the oscillation frequency is increased, energy will be emitted outward and electromagnetic interference will be generated to the surroundings), if the statistical time t=l ms, it can be When 100 pulses are recorded, the maximum theoretical value of sensitivity can reach 1%, but in fact, excluding the influence of unstable factors, the sensitivity cannot be too high; if the statistical time is extended to 10 ms, the maximum theoretical value of sensitivity can reach 0.1 %, which not only affects the reflection speed, but also discounts the actual value, so the sensitivity still cannot meet the practical application requirements.
Although there have been some improvement schemes using single chip microcomputer to detect vehicles, it is difficult to fundamentally improve the detection sensitivity. To this end, a design scheme for improving the sensitivity and anti-interference ability of the sensing device by utilizing the phase-locked loop technology is proposed. Experiments show that the design scheme can meet the requirements of practical application.
2 Principle of Phase Locked Loop
“Phase lock” is the automatic control of phase synchronization. An automatic control closed-loop system that can complete the phase synchronization of two electrical signals is called a phase-locked loop. Phase-locked loops are widely used in technical fields such as broadcast communication, frequency synthesis, automatic control and clock synchronization.
Phase-Locked Loop (PLL) is mainly composed of three parts: Phase Detector (PD), Voltage Controlled Osillator (VCO) and Loop Filter (LF). composition. PD is a phase comparison device, which is used to compare the phases of the input signal Ui
The working principle of the PLL is to compare the phase difference between Ui
3 System Design
3.1 Sensing circuit
Figure 1 shows the sine wave oscillator circuit. It is a capacitor three-point circuit, that is, a Colpitts oscillation circuit. Sensing circuits are used to sense the presence of metallic objects. In the design, the eddy current sensing method that has been applied in practice is adopted, and the buried detection coil is directly connected to the sinusoidal oscillation circuit, that is, the frequency output by the oscillation circuit has a sensing signal.
In the circuit of Figure 1: Rbl=Rb2=62 kΩ; Rc=1 kΩ; Re=2.2 kΩ; Ce=Cb=10μF; C1=C2=0.1μF; Enlarge the configuration and work in the enlarged state.
In the normal state of the system, that is, when no vehicle passes by, the output signal us
It can be seen from formula (1) that f is related to the inductance L, capacitance C1 and C2 in the circuit (C1 and C2 are fixed values), so f is a single-valued function of L. When L changes, the oscillation frequency of the oscillator circuit is also change in the opposite direction. The ground coil is used as L, which is directly connected to the circuit, and the change of L is reflected by detecting the change of the oscillation frequency of the circuit, so as to realize the detection of metal objects.
Assuming that the output signal frequency is f0 in the normal state, that is, when no vehicle passes by, and the oscillation frequency when there is a vehicle passing by is f’, it is easy to know: f’>f0. Since f’ is not easy to be calculated directly by a fixed formula, it can only be estimated by experimental method, and it is not necessary to know the specific value of f’ in practical application, just estimate the range of △f=f’-f0.
3.2 Detection circuit
3.2.1 PLL circuit design
Determine whether there is a vehicle passing by by detecting the phase-locked loop. When the vehicle moves to the coil buried in the ground, the output frequency of the sensing circuit changes, and the PLL loses lock. By detecting the lock loss signal, it can be known whether there is a vehicle crossing the lane. Because the detection circuit uses relative frequency changes instead of absolute frequency changes to determine whether there is a vehicle passing by, it has the ability to self-regulate and will not fail due to changes in the surrounding electromagnetic environment.
In this design, the PLL is used to track the frequency change of the output signal of the sensing circuit to generate the local oscillator signal u1
(1) Within the frequency variation range (f0~fmax) of us
(2) The locking time established by the PLL cannot be too long or too short. If the established locking time is too short, the PLL locks quickly and cannot see the difference frequency signal. If the established lock time is too long, it is difficult for the PLL to recover from the loss-of-lock state to the locked state, resulting in a slow response. Therefore, it is necessary to choose the appropriate integrating resistors R and C, and choose the appropriate locking time.
The design uses an integrated phase-locked loop CD4046 to achieve. Figure 2 is its connection circuit. According to the working principle of CD4046, its center frequency is determined by C1 between pins 6 and 7 and the grounding resistance R1 of pin 11, so it is necessary to select appropriate R1 and C1 to make the center frequency of the PLL meet the requirements. In order to increase the tunability of the center frequency of the PLL, a variable resistor R2 is connected in series with R1. Because the PLL here does not require the width of the input frequency, so pin 12 is left floating.
As shown in Figure 2, adjust the variable resistor R3, so that the voltage of pin 9 U9=Ucc/2=4.5 V, the output frequency of pin 4 is the center frequency of the PLL at this time. Select C1 = 1 000 pF, R1 = 5.1 kΩ, R2 = 10 kΩ, at this time the center frequency f0 = 23.7 kHz, just meet the system requirements.
3.2.2 Determining the establishment lock time
According to the working principle of the phase-locked loop CD4046, the establishment of the lock time is determined by the integral resistor-capacitor between pin 13 (or pin 2) and pin 9. The output of pin 13 is charged and discharged through the integrating capacitor C2, which makes U9 change, thereby adjusting the output of the VCO, that is, the output frequency of pin 4.
4 Experiment and analysis of results
In order to verify the feasibility of the design, a verification test was done in the laboratory. Both the vehicle and the coil are scaled down the same. A common wire with a diameter of 5 mm is wound with a coil of 100 cmx2 cm (the number of turns is n=16), which is used to simulate the ground coil of practical applications; and a rectangular metal plate of 20 cmxl0 cm is used to simulate the actual vehicle. Experiments show that the output frequency f0 = 23.7 kHz in normal state, the maximum output frequency when a car passes by (the value measured when the metal plate is completely placed on the coil) is fmax = 24.5 kHz, so △fmax = 0.8 kHz.
Change the vertical distance of the metal plate to the coil and measure the output frequency of the PLL after the metal plate is moved over the coil. The experimental results show that the closer the metal plate is to the coil, the higher the output frequency of the PLL. Using an oscilloscope to observe the state of the PLL from loss of lock to lock, it is found that when the metal plate is 2 to 15 cm away from the coil, an obvious change from loss of lock to lock can be observed. When the metal plate is stationary on the coil for a long time, the PLL can also recover the locked state. The vertical distance from the metal plate to the coil corresponds to the distance from the vehicle chassis to the underground coil in practice. According to the corresponding proportion, the range that can be measured in the experiment can fully meet the practical needs. Because the experiment is to observe the oscilloscope to determine the loss of PLL lock, in order to achieve obvious effect, the set lock time is longer. In practice, other methods can be used to detect the loss of PLL lock, so as to determine that there is a car passing by, and the corresponding circuit parameters can be adjusted to make the locking time shorter and the detection sensitivity correspondingly improved. Experiments show that the detection sensitivity achieved by PLL fully meets the system requirements.
5 Conclusion
The vehicle detection technology implemented by phase-locked loop technology is more reliable than the current vehicle detection technology that uses single-chip counting, and by detecting the loss of phase-locked loop to determine whether there is a vehicle passing by, it is a relative frequency change rather than an absolute frequency Therefore, it has the ability of self-adjustment and will not fail due to the shift of the center frequency caused by the change of the surrounding electromagnetic environment. Therefore, the phase-locked loop technology proposed in this paper has obvious effect in improving the sensitivity, and the cost is low, and it has a wide application prospect.
The sensing circuit is designed using the existing technology for the preliminary detection of the vehicle. The detection circuit converts the output signal of the sensing circuit into an electrical signal that can be directly displayed or measured for further processing by the system. This part of the design is implemented using phase-locked loop technology.
The Links: CLAA170EA02Y EL4836LP GET PARTS