Abstract: Trunk line control systems have the advantages of low cost and short construction period. Using PLC to build them greatly reduces the hardware development cycle and makes them easy to promote and apply.
Keywords: Mainline control unit; Intersection signal controller; Programmable logic controller (PLC); Drive-by-wire optimization algorithm
Urban traffic control systems can effectively improve traffic, alleviate traffic congestion, enhance the service level of the road network, increase system traffic flow, reduce delays and parking frequency, reduce fuel consumption, reduce environmental pollution from traffic noise and exhaust fumes, and improve traffic safety, thereby promoting the further development of urban economic construction.
However, implementing a centralized urban traffic control system is expensive and time-consuming, making it unaffordable for some small and medium-sized cities. Furthermore, traffic signal control in these cities is often concentrated at intersections on a limited number of main roads, making arterial control a more ideal and practical choice. Therefore, developing a small-to-medium-scale arterial control system better suits the traffic control needs of small and medium-sized cities. This system is also suitable for coordinated traffic signal control at intersections on arterial roads in large cities that are not controlled by a central traffic control system. Compared to a centralized adaptive urban traffic control system, the arterial control system has advantages such as lower cost and shorter construction time, making it easier to promote and apply.
The trunk line control system mainly consists of signal controllers at each intersection of the trunk road and a trunk line control computer (intersection line control computer) located at a certain intersection (generally defined as a key intersection), as shown in the figure below.
Composition of intersection drive-by-wire system
Programmable Logic Controller (PLC) is a new type of industrial control device based on microprocessors, integrating computer technology, automatic control technology, and communication technology. It boasts advantages such as simple structure, convenient programming, and high reliability, and is widely used in the automatic control of industrial processes and positions. Statistics show that PLCs are the most widely used equipment in industrial automation. Experts believe that PLCs will become one of the main means and important basic equipment for future industrial control, and that PLCs, robots, and CAD/CAM will become the three pillars of industrial production. PLCs are based on relay control logic and have been continuously developed and improved by combining with 3C technologies (Computer, Control, Communication). Currently, they have evolved from small-scale single-machine sequential control to all control fields, including process control and position control.
1. Signal control machine
Functions and performance indicators
It conforms to the People's Republic of China Public Industry Standard GA47-2002 "Road Traffic Signal Controllers";
The operating modes include: lights off, all red, yellow flashing, multi-time period timer control, sensor control, cableless coordination, and area coordination control (including trunk line coordination mode).
It features both manual hardware control and manual software control by the host computer user.
It can communicate with the host computer (trunk control computer) for relevant data.
It can connect to at least 32 detectors and drive 48 signal lights;
It can execute up to 16 phase signal controls and can be set to run 32 time periods, 32 schemes, and 16 special day time period schemes;
Relevant parameters can be easily set via handheld devices or buttons on the control panel;
It has a display screen that intuitively displays the relevant working status and parameters of the signal;
It can work around the clock.
2. Trunk line control computer functions
It communicates with the signal controller, receives information from the signal controller, and outputs relevant commands to the signal controller;
It has a user-friendly interface that displays the current operating and configuration information of the mainline control system and receives and processes user input.
The control parameters of each signal are calculated based on the optimization algorithm and relevant information.
structure
In terms of hardware, it can be composed of a mature and stable industrial control computer and additional serial port expansion cards, or it can be composed of a PC104 embedded microcomputer and serial port expansion board. It is mainly responsible for communication control of the signal machines under the trunk line, and at the same time, it reserves a communication expansion interface with the upper-level central control computer. The main equipment must meet the requirements of operation in an industrial environment.
The software primarily acquires relevant information from each signal, calculates control parameters (cycle, green light ratio, phase difference) using a drive-by-wire optimization algorithm, and sends these parameters to the relevant signals for execution. It also acquires user input, analyzes user commands, modifies system configurations, or sends them to the relevant signals. Furthermore, it displays the execution status of each signal on the user interface. The structure is shown in the diagram below.
Schematic diagram of trunk line control computer software structure
The main line control unit and the intersection signal controller can also use a PLC programmable controller as the main controller. In principle, the two can be combined into one. The main consideration for selection is:
1) The number of input/output points must be at least 120;
2) Equipped with a real-time clock;
3) Equipped with an RS232 or 422 communication interface;
4) It can build point-to-point communication or serial bus communication;
5) Possesses register data management capabilities;
6) Data processing speed: 0.7µs
7) The module has a self-diagnostic function.
3. Communication between intersection signal controllers and mainline controllers
Communication structure
Communication between the signal controller and the trunk control computer currently still uses the RS232C serial port, with a point-to-point communication structure (as shown in the diagram below). The equipment can be a modem with a telephone line, an optical transceiver with an optical cable, or a dedicated serial port device with a dedicated line. A multi-serial port expansion device is used at one end of the trunk control computer.
Intersection drive-by-wire system communication structure
Communication interface content
The handshake protocol and related connection procedures between the signal controller and the host computer (mainline control computer);
The information transmitted by the signal:
Date and time;
Current control methods, time periods, and plans;
Phase switching notification;
Fault status of each component;
The detector's status and real-time raw data;
Traffic and market share data;
Notification of changes to configuration parameters;
Information regarding configuration parameters.
Commands sent by the host computer (trunk controller):
Set the date and time for the signal;
Commands for querying various information about traffic signals, such as date and time, control mode, time period scheme, phase execution status, and fault status of various components of the traffic signal;
The various configuration parameters of the read/write signal controller;
Configure the control mode of the signal, such as downgrading the signal operation mode to single point and manual intervention, etc.
The signal controller is set to software manual operation mode, allowing for remote manual control of phase execution.
4. Implementation of a PLC-based signal controller
In PLC control, the KOYO S series mid-sized PLC SU-6M, which offers high performance and price, is selected. Its performance can fully meet the control functions, and it can perform complex calculations using ASCII-BASIC modules and use DIRECTSOFT programming software for complex programming, thereby improving speed and reducing costs.
The SU-6M CPU module includes an RS-232/422 communication interface, which can be used to connect to the GC-53LM3 touch-operated programmable display. All working data, operating information, and auxiliary operating instructions for the PLC can be set/displayed on this display. Thanks to the use of this display, all human-machine interface operations are very intuitive and convenient.
If the trunk control unit is also built using a PLC, then communication ports need to be expanded, which can be achieved using a DM module. DM is a dedicated data communication interface module used for the networked operation of the entire trunk or system, and the command center. On this network, the decision to use a management PLC depends on the number of stations in the network. When there are many stations, a dedicated PLC is used to collect various data from lower-level stations to reduce the burden on the central computer; when there are few stations, data can be collected directly by the host computer.
To meet the demanding real-time computation requirements of signal controllers, an ABM module can be used. ABM is an ASCII/BASIC co-processor module used on SU series CPUs. Complex data calculations are performed within the ABM module using BASIC programs. Compared to calculations within a PLC, it is not only simpler to program and faster, but more importantly, it can perform necessary but not feasible calculations that the SU-6M PLC cannot, such as floating-point arithmetic, trigonometric functions, and string processing. The module's communication port can be connected to external communication displays, computers, printers, etc.
ASII-BASIC modules and languages:
The ABM module, programmed in BASIC, can access the PLC's I/O points, intermediate relays, bit function memories, and data registers. The state of the bit function memories and the contents of the data registers can also be controlled by the ABM.
The ABM module for the SU-6M CPU can be installed in any location without occupying an I/O point. (The ABM module for the SR series is slightly different.)
When the PLC system is powered on, the ABM module can enter either RUN or COMMAND mode according to its settings. In RUN mode, it executes the contents of the BASIC program, and in COMMAND mode, it executes commands entered via the keyboard. The operation of the ABM is independent of the operation of the PLC CPU.
The ABM BASIC language and syntax in RUN mode are similar to those of regular BASIC, especially QBASIC. ABM programs can be almost completely modified to run on QBASIC systems. However, accesses to PLC function memory in ABM programs are treated as arrays in QBASIC. For example, SU6-R(1400) and SU-6M(1000) access data register R1400 and intermediate relay M1000 in ABM programs, while the same program would be treated as a large array in QBASIC.
Commands in COMMAND mode include menu operations such as transferring programs, setting parameters, and printing programs, as well as direct command input, such as deleting, saving, listing programs, selecting programs, running programs, and changing running modes.
5) Wire-controlled optimization algorithm
The basic parameters of signal control are period, green ratio, and phase difference. The algorithm for drive-by-wire can draw on the sub-zone optimization algorithm in adaptive traffic control systems. There is a key intersection in each drive-by-wire intersection, and the period of the key intersection serves as the common period for all intersections. The green ratio is adjusted individually for each intersection, and the phase difference is optimized for all intersections.
Detector data preprocessing
The traffic flow and occupancy data for each traffic flow cycle are obtained from the raw detection data. Due to the random fluctuations in traffic flow, the detector data should be smoothed to reflect the trend of real-time traffic changes and avoid frequent changes in the control scheme. The smoothing method is to take a weighted average of the current cycle's data with the data from the previous few cycles.
Determining saturation
The saturation of a phase is defined as the ratio of the green light time occupied by vehicles in that phase to the effective green light time for vehicles to pass through.
Optimal signal period
The cycle length is determined by the key intersection. The drive-by-wire algorithm collects traffic data from the intersection over three cycles. If the cycle length needs to be increased or decreased in two of these three cycles, the direction of the cycle change is determined. The magnitude of the cycle change is determined by the intersection's saturation level and a cycle length-related factor, ranging from ±(1-6 seconds). When drive-by-wire is activated, the current cycle length of the key intersection is taken as the initial cycle length.
Adjustment of green credit ratio
The green light ratio is adjusted separately for each intersection, and the green light time of each phase is allocated according to the "equal saturation principle", so that the change value of the green light time of each phase is within ± (1-4 seconds).
Preferred phase difference
Phase difference reflects the coordination between intersections. First, the route of the line-controlled system is determined. Then, the phase difference between intersections is calculated based on factors such as signal cycle, green light duration, phase color step sequence, intersection spacing, and average vehicle speed. The goal is to maximize the width of the green wave band in both directions of the line-controlled route. The phase difference varies within ±(1-4 seconds).
Using a PLC as the main control unit for the signal controller significantly reduces the hardware development cycle. Its powerful communication and computing capabilities fully meet the real-time control requirements of the signal controller.
