1 Introduction
According to incomplete statistics, at present, the traffic system of urban intersections in China mostly adopts timing control (it is not excluded that the traffic lights are replaced by traffic police in busy sections or rush hours), which will inevitably lead to the following disadvantages: when the traffic volume of a certain section is large, you have to wait for the red light, while the other lane is empty or the traffic volume is much less, but it turns green for a long time. This embarrassing phenomenon of waiting less and less is caused by not monitoring the actual situation in real time, which not only makes drivers and passengers complain.
Intelligent control of traffic system is the current research direction, and many achievements have been made. In a few developed countries, intelligent control of traffic signals has been adopted, among which GPS is mainly used. In order to be convenient and effective, we can use the following scheme to control traffic conditions: let sensors detect the number of vehicles to control the duration of traffic lights. The details are as follows: the induction coil is buried underground near the entrance in all directions as required. When the car passes, it will produce eddy current loss, and the inductance of the ring-shaped insulated wire will start to decrease, so that the car can be detected, and this signal can be converted into a standard pulse signal as the control input of the programmable controller, and the time of the traffic lights can be automatically adjusted according to certain control rules through PLC counting.
Compared with the traditional timed traffic light control and intelligent traffic light control, it can be seen that the biggest advantage of the latter is to slow down the stagnation phenomenon, avoid the time-consuming situation of empty lanes, improve the traffic throughput of expressways, and the cost is lower than that of global positioning system.
2 Detecting the presence and passing of vehicles
(1) induction coil (induction sensor)
The main component of the inductive sensor is a ring-shaped insulated wire buried at a depth of more than ten centimeters under the highway (especially suitable for newly paved roads, which can be directly buried with concrete, and the old road needs to be dug and buried again). When the high-frequency current passes through the inductor, the high-frequency magnetic field formed by the dotted line in figure 1(a) will be formed on the road surface. When the car enters this high-frequency magnetic field, the car will produce eddy current loss, and the inductance of the annular insulated wire will begin to decrease. When the car is just above the induction coil, the inductance of the induction coil is the smallest. When the car leaves this high-frequency magnetic field, the inductance of the induction coil gradually returns to the initial state. Due to the change of inductance, the amplitude (detection method involved in this paper) and phase of high-frequency current flowing in the induction coil change. Therefore, by connecting a detector to detect the phase or amplitude change at the beginning of the loop, the electric signal passing by the car can be obtained. If the ring-shaped insulated wire is used as a part of the oscillation circuit, the existence and passing of the car can be known only by detecting the change of the oscillation frequency.
The high frequency current frequency of the inductive sensor is 60kHz, the size is 2×3m, and the inductance is about100μ h. The detectable inductance change rate of the sensor is above 0.3% [1 2].
The inductive sensor is installed under the expressway, which is an ideal sensor from the perspective of traffic safety and aesthetics. It is best to choose raw materials with good moisture-proof performance for sensors.
(2) Circuit
The concrete realization of detecting the existence of automobile is to connect the detector to detect the change of inductance current at the beginning of induction coil and convert it into standard pulse voltage output. Its specific circuit diagram consists of three parts: signal source part, detection part and comparison and identification part. The principle block diagram is shown in Figure 2, and the output pulse waveform is shown in Figure 1(b).
(3) laying of sensors
Vehicle counting is the key of intelligent control. In order to prevent vehicles from missing inspection, we set the same sensor at the exit (parking line) of each roadway and the entrance at a certain distance from the exit. The scheme is shown in Figure 3 (taking a typical intersection as an example), and the distance between two sensors in the same lane is the longest parking queue allowed during normal operation of the lane.
3 using PLC to realize intelligent traffic light control
3. 1 control system composition
The programmable controller (PLC) is used to realize the flow counting of vehicles and the duration control of traffic lights. Of course, other kinds of computers can also be selected as controllers. In this case, PLC is chosen as the control device, because the core of the programmable controller is the computer, which is specially designed and manufactured for industrial environment applications. High reliability, rich input and output interfaces and strong driving ability; It uses a programmable memory to store programs, perform logical operations, sequence control, timing, counting and arithmetic operations and other user-oriented instructions, and controls various types of machines or production processes through digital or analog input/output; It adopts modular structure and is simple to program, install and maintain.
Using PLC, the above sensors and signal lights at each intersection can be directly connected with it, which is very convenient and reliable.
In this design example, PLC adopts FX2N-64, whose input receives the output standard electric pulse measured by the vehicle detector at each intersection, and outputs the red and green signal traffic lights at the intersection. Selection of signal lamp: In this example, red, yellow and green LEDs are selected as signal lamps (arrow direction type).
3.2 Measurement of traffic flow
There are many ways to measure traffic flow:
(1) The traffic volume of each lane is separately counted by PLC. When the vehicle enters the intersection and passes the first sensor 1 (see Figure 3), the statistics increase by 1, and decrease by 1 when the vehicle passes the second sensor 2. The difference is that the vehicle retention (dynamic value) of this lane can be compared with the values of other lanes as the basis for adjusting the length of traffic lights.
(2) First, the number of vehicles stranded in each lane is counted, and then the statistics are accumulated according to the general direction principle. For example, add up the traffic jams in the left, straight and right lanes in the east-west direction (see Figure 3), and then compare them with the traffic flows in the other three directions, which can be used as the basis for adjusting the duration of traffic lights.
(3) After counting the number of vehicles stranded in each lane, make cumulative statistics according to the principle of traffic maximization (multi-lane driving in opposite directions will not affect traffic safety). For example, the vehicle ownership in the left, straight and right lanes in the east and west directions are all added up, and then compared with the total traffic flow in the north-south direction as the basis for adjusting the length of traffic lights (as in the following example).
The above calculation and discrimination are all completed by PLC. The above different measurement discrimination methods can be compiled into different subroutines for easy calling.
3.3 program flow chart
In this case, according to the above traffic flow statistics method, the program flow chart of PLC automatically adjusting the length of intersection traffic lights in Figure 3 is shown in Figure 5. The driving sequence is the same as in real life [4], but the time is different.
(1) When the number of vehicles stranded at each intersection reaches a certain value and overflows (equivalent to serious traffic jam), the existing conventional timing control method is adopted to switch the traffic lights;
(2) When the vehicle detention at the east and west intersections is greater than that at the south and north intersections (and vice versa), the transit time in this direction = minimum transit time+delay time increased by adaptive delay (variable), but not greater than the maximum allowable transit time. The shortest timing time is to avoid the disadvantage of switching traffic lights too fast; The maximum transit time is to ensure fairness, and other cars or pedestrians should not be kept waiting for too long. Further explanation is given in the following notes.
(3) Determination of adaptive lag comparison increase time (the core control law in this example) If the number of stranded vehicles in the east-west direction is greater than or equal to a deviation σ in the north-south direction (such as 30 vehicles or other values), let the left-turning vehicles in the east-west direction turn left first 15s (timing control, this value can be changed), and then let the straight-ahead traffic flow go straight for 30s (the minimum straight-ahead time), and the traffic flow in the east-west direction is less than the traffic flow in the north-south direction by a deviation σ. The hysteresis characteristics are shown in Figure 6. In practical application, it is necessary to set the value of σ. Too small will lead to overclocking of traffic lights, and too large will make it impossible to control them in time.
3.4 flow chart notes
(1) The times of 15s, 30s and 75s in the flow chart are respectively the left turn time, the shortest straight-ahead time and the longest allowed passage time set by the traffic control department; σ is the deviation of traffic flow. The above values and the traffic flow overflow values of four intersections can be changed at will when the program is initialized.
(2) Vehicle turning left is an important aspect of traffic congestion, which should be properly restricted. Therefore, the minimum timing control is always adopted when the vehicle turns left to reduce the complexity of the system and improve the reliability.
(3) The vehicle passing time includes the flashing time of green and yellow lights, and the switching of red, yellow and green lights is the same as the current mode, so the details are not repeated.
(4) The connection of traffic lights on the sidewalk is the same as the current mode, and the time when the green light turns on is synchronous with the time when the green light of the straight vehicle in this direction turns off, which is earlier than the time when the green light of the straight vehicle turns off, and the green light is turned on at regular intervals, such as 18s. Its purpose is to prevent right-turn vehicles from being excessively restricted by sidewalk lights. If people and vehicles are diverted, right-turn vehicles are not restricted. Simple, omitted in the flow chart.
(5) The measurement of traffic flow is uninterrupted and parallel to the control. The system is multi-tasking, so special attention should be paid to programming.
4 conclusion
Compared with the traditional timed traffic light control and intelligent traffic light control, it can be seen that the biggest advantage of the latter is to slow down the stagnation phenomenon, avoid the time-consuming situation of empty lanes, improve the highway traffic rate, and the cost is lower than that of the global positioning system, especially suitable for busy intersections, more suitable for more than four intersections, and convenient for networking.
refer to
[1] Huang Jichang et al. Working principle and application examples of sensors [M]. Beijing: People's Posts and Telecommunications Publishing House, 1998.
Zhang. Application technology of programmable controller [M]. Beijing: Chemical Industry Press, 200 1.
[3] British R.J. salter. Road traffic analysis and design [M]. Translated by Zhang Zuozhou and others. Beijing: China Building Industry Press, 1982.
Incomplete, you can refer to it.
I hope it helps you.