1. Overview Asphalt mixing plants are complex, integrated systems. Due to their field operations, harsh working environments, large temperature fluctuations, and numerous sources of interference, the design of their control systems is significantly more challenging. This is especially true for equipment used in high-grade highways, which requires high precision in the metering of asphalt, aggregates, powders, and temperature. Therefore, the hardware and software design of the control system for this equipment also has relatively high requirements. All of the above factors must be comprehensively considered, and necessary design measures must be taken to meet the requirements of high-grade highway construction. 2. Mechanical Structure and Process Flow Taking an intermittent asphalt mixing plant as an example, it mainly consists of the following parts: a cold aggregate batching device; an aggregate drying and heating system; a hot aggregate lifting device; a hot aggregate screening and weighing device; a powder supply system; an asphalt supply system; a mixer; a finished product elevator; a finished product storage silo; and a dust removal device. Its mechanical structure is shown in Figure 1. Figure 1 Schematic diagram of the intermittent asphalt mixture mixing equipment The process flow of the equipment is as follows: (1) Cold aggregate and sand of different specifications → quantitative feeding and coarse mixing → cold aggregate conveyor → drying drum for drying and heating → hot material elevator → hot material screening machine for screening and storage in each hot material bin according to different specifications → hot material grouping and cumulative weighing → mixer mixing; (2) mineral material → powder bin → conveying and weighing device → mixer mixing; (3) asphalt → asphalt tank → conveying and weighing device → mixer mixing; (4) finished material mixed by the mixer → finished material elevator → finished material storage bin → transported to the construction site. 3 Design task and index requirements of the control system Asphalt mixture mixing equipment is a comprehensive supporting equipment with many independent operating parts and a complex process flow. The task of its control system is to start, run and mix and shut down each part according to the predetermined program, simplify the operation as much as possible and improve the automation control level of the whole machine under the premise that the overall performance meets the requirements. Principles of Control System Design: (1) Improve the reliability of the control system, so that the reliability index of the system meets the requirements of the national standard, and strive to reach or approach the international advanced level. (2) The hardware part should use high-quality mature products or advanced foreign products as much as possible. Under the premise of meeting various performance indicators, try to reduce the production cost of the control system. (3) Adopt international standards, implement the current national and enterprise standards, and strive to improve the "three-in-one" level. The indicators of the control system have different requirements according to different models and grades. According to the Chinese transportation industry standard JT/270-1995 "General Technical Conditions for Asphalt Mixture Mixing Equipment", see Table 1. At present, the technical indicators of the control system of advanced equipment at home and abroad should meet the requirements of the superior products in Table 1. 4. Selection of Control Mode The current popular working modes of the control system are as follows: (1) Fully automatic working mode; (2) Semi-automatic working mode (automatic + manual); (3) Manual working mode. Different models have different requirements for the working mode of the control system. The control system selects the control mode according to the working mode and user requirements. The current control methods can be roughly divided into three categories: relay contact control; programmable logic controller (PLC) control; and computer control. Relay contact control is simple to operate, reliable, and has low manufacturing cost, but has low automation and poor control accuracy. Programmable logic controller (PLC) control can realize time-sequence control, and has good standardization, generalization, and reliability, with high control accuracy. Computer control has high accuracy, large information processing capacity, and can realize fully automatic control and production process management automation, but the system is complex and costly, and is suitable for equipment with high control accuracy requirements. 5 Hardware Design The hardware design of the control system is related to the control method. The hardware design should consider using existing mature products and standardized and generalized products as much as possible to reduce the manufacturing and commissioning cycle and improve the reliability and maintainability of the system. The hardware design of various control methods is illustrated below with examples. (1) The hardware of the relay contact control system mainly consists of relays, contactors, timers, voltmeters, ammeters, and some buttons. Figure 2 is an example of the control of the mixing motor of the asphalt mixture mixing equipment. When switch A is pressed, the motor rotates forward; when switch B is pressed, the motor rotates in reverse. This circuit ensures an interlock relationship between the forward and reverse rotation of the motor. Figure 2 Example of a mixing motor for asphalt mixture mixing equipment (2) The hardware of the programmable logic controller (PLC) system mainly consists of a PLC module, intermediate relays, contactors, digital meters, and several buttons. Figure 3 is an example of PLC control for the finished material elevator of an asphalt mixture mixing equipment. The system's operation is as follows: the elevator bucket starts at the bottom loading position A. When the PLC detects that the agitator is in the open state, the elevator bucket starts to lift after a 10s delay. It is lifted to the high-end unloading position B. After unloading for 10s, the winch is switched on and the elevator bucket descends. When it descends to the bottom proximity switch C, the winch is de-energized after a 1s delay, and the elevator bucket slides down to the bottom loading position A to wait. When the agitator is detected to be in the open state, the next cycle starts again. Figure 3 Example of PLC control for the finished material elevator of an asphalt mixture mixing equipment (3) The hardware of the computer control system mainly consists of an industrial control computer system, a management computer, a printer, a power supply UPS, intermediate relays, contactors, digital meters, and several buttons. An industrial control computer system mainly consists of a CPU, data acquisition and amplification section, A/D converter, analog amplifier, communication, I/O input/output switching and bus section, etc. Figure 4 shows the computer control system of an asphalt mixture mixing plant. The main control objective is to control the accuracy of asphalt, aggregate, powder, and temperature metering. The control input and output signals mainly come from the mixing plant, finished product elevator, main burner, drum, etc., and its front-end control core is the industrial control computer. The control system operates as follows: First, start the drying drum, hot stripper, vibrating screen, induced draft fan, and blower. Then, simultaneously ignite and start the oil pump, preheating the drum with a low flame for 3-5 minutes. Next, the cold aggregate grading machine begins feeding, gradually increasing the throttle to raise the temperature. Once the material temperature reaches the set temperature, automatic temperature control is activated to maintain the temperature within ±5℃ of the set value. Observe the material level in the hot aggregate bin. Once the hot aggregate reaches a certain level, the automatic mixing button is activated. The feeding, weighing, unloading, and mixing of various materials are all performed automatically. The management computer can set and display the weight and proportion of various materials, aggregate temperature, control the production process and issue fault alarms, and work with the printer to print production process parameters in real time. Figure 4: Computer Control System of Asphalt Mixture Mixing Equipment. Figure 5: Flowchart of Computer Control Powder Metering Program for Intermittent Asphalt Mixture Mixing Equipment. Software Design The control system of intermittent asphalt mixing equipment can have different control software depending on the control method. Relay contact control does not have dedicated control software, but sequential control can be achieved through combinations of various relays and contactors. Programmable Logic Controller (PLC) control has dedicated control instructions, with fewer instructions, simpler program design, and reliable system operation. Computer control software design can combine high-level languages ​​with machine languages, resulting in flexible program design, powerful functions, dynamic process simulation, and a more user-friendly human-machine interface. Figure 5 is a flowchart of the computer-controlled powder metering and control program. After successful program debugging, a suitable EPROM chip can be selected to solidify the program, then inserted into the CPU for debugging. After confirming that all actions and communications are working properly, it can be installed and used in the machine.