[ Abstract ] This paper introduces the application of VIPA's 300S and 500S PLCs and 200V slave station system in an asphalt mixing plant. A comparison of VIPA's PLCs with similar PLCs is made, highlighting the outstanding advantages of VIPA's PLCs. Design descriptions of the PLC, PC, and HMI units are provided. After the system was put into application, it not only reduced costs but also improved the performance of the entire control system. The system operates more safely, reliably, and with superior performance, achieving the design objectives.
[ Keywords ] Asphalt mixing plant, VIPA PLC, 300S, 500S, 200V slave system
1. Overview
Asphalt mixing plants are mainly used in the construction and maintenance of highways. They are complex, integrated equipment with many independently operating components and a complex process flow. Especially for equipment used on high-grade highways, the requirements for the metering accuracy and temperature control of asphalt, aggregates, and powder are high. Because the equipment operates in harsh outdoor environments, the performance requirements for its control system are also relatively high. Only by comprehensively considering these factors and taking necessary design measures can the requirements of high-grade highway construction be met.
Asphalt mixture mixing equipment mainly includes material feeding control, temperature control, metering control, dust removal control, and finished product silo control. Its control system's task is to control each piece of equipment according to the process flow sequence and automatically meter and mix materials according to the production formula. Currently, the control system basically adopts a PC+HMI+PLC control mode. The upper-level PC and HMI are mainly used to issue control commands for production monitoring and data management; the lower-level PLC is responsible for all process control.
2. Application Characteristics of Programmable Logic Controllers (PLCs) in the Control of Mixing Plants
2.1 General Features
As the core control component of asphalt concrete mixing equipment, the performance of the PLC directly affects the performance of the entire machine. Therefore, the following points should be considered when selecting a PLC:
- Dynamic metering requires a fast PLC processing speed, with a scan cycle of less than 10ms.
- The equipment has many control points, requiring a large PLC memory with sufficient margin.
- The communication speed should be fast; the refresh rate of the host computer screen should be greater than 5 times per second.
- Modular PLCs are selected to facilitate system expansion.
- While ensuring the overall performance meets the requirements, simplify the system as much as possible, reduce costs, and improve the overall performance-price ratio.
2.2 Comparison of Features
Currently, most control systems use Siemens PLCs. Among them, the S7-400 (S7-412) is a large PLC with a fast CPU, large memory, and excellent performance, but it is more expensive. The S7-300 (S7-314C, S7-315) has a lower cost, but its performance is inferior.
VIPA's 300S series PLC is a high-performance PLC fully compatible with Siemens S7-300/400 PLCs. Its features include: fast CPU operation speed, large memory (2MB), multiple communication interfaces (including PROFIBUS-DP master station, MPI, built-in Ethernet interface, etc., and integrated high-speed analog input, high-speed counter and other ports), full programming compatibility with STEP7, and programs can be run directly without modification. Its I/O modules and accessories are directly interchangeable with S7-300.
To improve equipment performance and reduce costs, we selected VIPA's programmable logic controllers and designed both a centralized control system and a distributed bus control system.
2.3 Centralized Control System
In 2006, we selected the VIA-300S PLC for use on a 1300-type asphalt mixing plant, and the system adopted a centralized control mode.
The system consists of:
- 300S-CPU: VIPA 314-6CF02
- 300V Digital Output Module: VIPA 322-1BL00
- 300V Digital Input Template: VIPA 321-1BL00
- 300V Analog Input Template: VIPA 331-1KF01
- 300V Analog Output Module: VIPA 332-5HD01
2.4 Distributed Control System
To meet the needs of export products and facilitate rapid equipment installation and relocation, we developed and designed a distributed bus control system in early 2008. The system is currently being used on a 2000-type asphalt mixing plant. The control system is divided into eight independent units based on the equipment structure, installed at the corresponding equipment locations on-site. The unit wiring is completed in the factory, requiring only power and communication cables to be laid on-site, enabling rapid installation. Each unit control cabinet is equipped with an external main power switch and an emergency stop button for easy on-site operation and maintenance, meeting the needs of both export and high-end domestic customers.
The system uses a VIPA-515S board-type CPU as the master station, installed in the industrial computer in the control room; 200V I/O modules are installed in each control box in the field as slave stations to collect and control the equipment in their respective units. The system uses PROFIBUS-DP fieldbus communication to realize process control of the asphalt mixing plant.
The system consists of:
- 500S-CPU: VIPA 515-2AJ00
- 200V-DP template: VIPA 253-1DP01
- 200V Digital Output Module: VIPA 221-1BF00
- 200V Digital Input Template: VIPA 222-1HF00
- 200V Analog Input Template: VIPA 231-1BD53
- 200V Analog Output Module: VIPA 232-1BD51
3. PLC Programming
The VIPA PLC is programmed using Siemens STEP7 programming software, with programs written using ladder diagrams and statement lists. STEP 7 programming software employs a modular programming structure, which can be divided into organization blocks (OB), system function blocks or system functions (SFB or SFC), function blocks (FB), functions (FC), background data blocks (background DB), and data blocks (DB). OB1 is mandatory among the OB blocks; other blocks are added or removed as needed. This system's PLC program utilizes organization blocks such as the main program loop OB1, loop interrupt OB32, and warm start OB100. It also uses custom function blocks FC, data blocks DB, and system function blocks SFB. The program includes metering control function block FB100, material feeding control FB20, temperature control FC51, dust removal control FC60, and finished product silo control FB300. A brief introduction to the centralized control system using the VIPA 300S is given below.
3.1 Measurement Control
Metering control is the most critical part of the mixing plant control, requiring high accuracy, rapid response, and precise weighing control from the PLC A/D converter. The VIPA CPU-314ST integrates a 4-channel AD unit with a data acquisition speed of up to 170 times/second and an accuracy of 12 bits, which fully meets the process requirements. We use it to dynamically acquire the weighing values of aggregates, powders, and asphalt. After the PLC processes the weighing signals according to the production formula, the output module automatically controls the start and stop of the aggregate weighing gate, powder screw conveyor, asphalt pump, etc., for metering control and mixing process control.
The FB100 function block is programmed using a ladder diagram and includes simulation data acquisition and transformation, production formula processing, cumulative metering of 6 types of aggregates, cumulative metering of 3 types of powders, metering of 1 type of asphalt, and metering of 1 additive; mixing control; and report and data upload processing.
The stone metering system employs a dual-gate cylinder control. The metering program must adjust the mixing parameters promptly according to different conditions based on the mixing process. For example, the mixing parameters differ between a full and empty hot aggregate bin, resulting in significant differences in the mass of material falling from the air—hundreds of kilograms when the bin is full, but only a few kilograms when empty. This requires automatic correction and adjustment within the program. An automatic drop correction function has been incorporated into the metering program, providing both static and dynamic correction control to ensure more accurate gradation.
The accuracy of the asphalt-aggregate ratio and its automatic tracking are crucial indicators of asphalt mixture quality. Insufficient or excessive asphalt content reduces the strength of asphalt concrete, affecting compaction and stability. Excessive asphalt content can also create free asphalt between aggregate particles, ultimately leading to "bleeding" on the pavement. Therefore, the asphalt-aggregate ratio must be strictly controlled to maintain an error within ±0.3%. To improve the accuracy of the asphalt-aggregate ratio, a two-stage metering method is used. First, 120% of the set weight is weighed. After the aggregate and powder are weighed, a second calculation is performed before spraying according to the formula, improving the quality of the output.
Powder metering is controlled by a metering screw. Depending on the mixing process, the powder setting varies greatly. When the set weight is small, large errors occur; when the set weight is large, the metering time is long, affecting production speed. To improve metering accuracy and production efficiency, a frequency converter is used to control the screw speed. Based on the set powder weight, the PLC analog output is automatically adjusted to regulate the frequency converter speed and control powder metering.
3.2 Temperature Control
The discharge temperature of asphalt mixtures is a crucial factor affecting construction quality. Too low a temperature negatively impacts the mixture quality, resulting in uneven mixing and a "blooming" appearance, and also hinders paving and compaction. Conversely, excessively high temperatures lead to significant fuel waste and, in severe cases, asphalt aging and coking. Therefore, strict control of the mixture temperature is essential; asphalt mixture temperature control is a critical production process.
The temperature control program consists of functions FC51 (burner control) and FC60 (dust removal control). The stone is baked and heated by an oil burner. The temperature is adjusted by the size of the damper. A fast-response non-contact infrared thermometer is used to measure the temperature. This signal is fed back to the PLC. The program calls the PLC's built-in temperature regulation function block FB59 to control the temperature and automatically adjusts the air-oil ratio to track the set hot aggregate temperature.
The negative pressure of the dust collector is one of the key factors for the combustion efficiency of the burner. Measuring the negative pressure of the drum and adjusting the induced draft damper in time to stabilize it provides conditions for complete combustion. The program calls the PLC's built-in PID function block FB42 to automatically control the negative pressure.
3.3 Finished Goods Warehouse Control
The finished product bins are an important component of an asphalt mixing plant. Material transport trolleys deliver the metered and mixed finished material to different bins within a specified time frame, according to instructions. The trolleys are driven by Schneider ATV71 frequency converters, utilizing a 4-channel high-speed counting unit integrated in the CPU-314ST to count the encoder mounted on the motor, achieving precise positioning of the trolleys.
The program consists of function blocks FB300 and FC310, including high-speed counting settings, silo selection control, silo position measurement and positioning, and frequency converter control.
3.4 Functional Expansion and Compatibility
The batching plant control system also includes: a material feeding control function block FB20, a motor control function FC150, etc. With the continuous improvement and updating of road construction technology, the number of peripheral devices in asphalt batching plants will continue to increase, requiring the PLC to have strong scalability for integrated control. The expandable capacity of the CPU-314ST single-rack 32-module system provides convenient operation and reliable assurance for the addition of peripheral devices to the batching plant.
The demand for intelligent and user-friendly control of asphalt mixing equipment is becoming increasingly prominent. In the past, mixing plants faced the problem of control system transformation. For the old control system using Siemens PLC, the transformation using VIPA PLC is quick and convenient, reducing hardware and software modifications.
4. Host Computer Software Design
The host computer uses a combination of an industrial computer and a human-machine interface to form a dual operating system.
4.1 Computer configuration software
As customers demand higher standards for batching plant processes, supervisory control and data acquisition (SCADA) systems generally require sophisticated graphical interfaces, powerful data processing capabilities, real-time interactive recipe reports, and alarm functions. The functionality of SCADA systems relies on seamless and rapid communication with the PLC. VIPA PLCs' Ethernet communication mode provides a perfect solution for achieving these functions.
The host computer uses FameView industrial configuration software, communicating with the VIPA-PLC via Ethernet (100MHz), and runs on a Windows 2000/XP environment. It features a user-friendly, fully Chinese interface, novel, simple yet powerful configuration functions, and can quickly and easily complete production tasks. The configuration software supports a 3000-point variable system, accurately controlling every detail of the production process. The 3000-pixel screen can update within one second, resulting in smoother and more aesthetically pleasing visuals. Utilizing the powerful multi-screen functionality of the configuration software, the computer system is configured with dual monitors: one for displaying equipment monitoring screens, and the other for displaying data and performing production operations. The configuration operation interface uses menus, sub-menus, and button hyperlinks to switch between screens. Screens include:
a) Main screen: Production process monitoring and metering monitoring;
Multiple operation tasks can be completed through drop-down menus and button clicks; the system can switch to other control screens during the production process. To prevent accidental operation, interlocking conditions and operation permissions have been added to the relevant buttons.
b) Control sub-screen:
Material supply control, equipment control, heating control, etc.
c) Production settings:
The system allows for setting production mixing parameters, storing recipes, and recalling recipes. It includes 200 recipes that can be saved as files on hard drives or floppy disks and easily converted to Excel files.
d) PLC status monitoring:
It accurately reflects the I/O status on site, which is beneficial for fault inspection and troubleshooting;
e) Data Management:
It records weighing data for each cycle in real time, and stores the data in a large SQL Server 2000 database for easy summarization, querying, and printing. Based on the content of the data report, the printer can be controlled to print line by line or page by page in real time; and it can be easily converted to an Excel file.
f) Data Management:
It mainly contains various alarm prompts and maintenance-related information.
g) Configuration supports GPRS/CDMA/dial-up communication, enabling remote monitoring of production status.
The system monitors the status of the equipment and the PLC's operation. A remote computer can access on-site production data and monitor equipment operation.
h) The configuration supports SMS functionality, allowing users to receive production status updates from the construction site via mobile phone. Additionally, video acquisition equipment is available, supporting CDMA and LAN transmission, enabling more intuitive monitoring of on-site conditions through video servers and monitoring stations.
4.2 Human-Machine Interface System
The human-machine interface system uses a SIMATIC TP 270 touch panel and communicates with the VIPA-300S via MPI/DP, enabling production process control and monitoring. Based on the Windows CE operating system, the TP270 offers innovative operator control and monitoring functions, along with the inherent advantages of operator panels: robust and durable, stable and reliable, and easy to use.
The configuration software for the human-machine interface is Simatic Protool CS. Protool is for Win98/nt/xp and, based on a complete graphical user interface, allows users to easily create object-oriented, symbol-based projects without special programming knowledge.
The HMI (Human Machine Interface) configuration screen is divided into four functional sections: Motor Control, Cold Material Control, Combustion System, Parameter Setting, Operating Status, and Alarm Information. The layout is clear and logical, with fast screen switching and refresh rates. Operation of the HMI is touch-based, simple and direct. This HMI system replaces traditional manual and automatic operation, representing a completely new control concept. It retains the unique operational characteristics of traditional control methods while integrating button and switch controls, simplifying the control system. The Motor Control page centralizes all field equipment motors, featuring touch-sensitive start/stop buttons and dynamic motor status text indicators. The Cold Material Control page primarily controls the start/stop and speed adjustment of the cold storage motor, with digital speed adjustment for more precise control. The Combustion System page mainly controls the induced draft damper, forced draft damper, and burner ignition. The operating status of the induced draft damper, forced draft damper, and burner is clearly displayed. The parameter setting page displays all necessary parameters for the production process, including settings for each batch of aggregate, powder, and asphalt, production task assignment, mixing time, trolley unloading time, mixing tank door opening time, powder addition delay, and asphalt addition delay. Numerical settings are all completed using a pop-up numeric keypad, which is convenient and quick. The operating status page displays important information under normal equipment operation, mainly including weighing status, finished product bin, asphalt, sand bin, dust collector inlet and outlet temperatures, sand vibration, powder bin vulcanization status, and trolley operation status. Digital displays with highlighted colors provide operators with a clear overview of the equipment's operating status. The alarm information page displays detection points and fault points summarized from years of engineering experience by technical personnel. These include alarms for motor operation status, overloading, material shortage, overheating, and bin/trolley position, hydraulic oil status, etc., in the finished product bin system. Alarm prompts are displayed in bright red flashing light, making them highly visible. When an alarm occurs while other screens are displayed, a small exclamation mark will pop up on the screen, accompanied by a bright flash.
The human-machine interface (HMI) can function as an independent system, capable of handling daily production tasks. In electrical control systems, the HMI primarily works in conjunction with the computer system, providing redundancy and enhancing the reliability of the host computer. The HMI can also intervene simultaneously during computer-controlled production, increasing operational flexibility, expanding the user interface, and demonstrating exceptional superiority in emergency response.
5. Conclusion
Beijing Deji Machinery Co., Ltd. is a professional manufacturer of asphalt mixing plants. Originally, our control system used Siemens S7-400 series PLCs. Since 2006, we have adopted VIPA-300S PLCs for asphalt mixing plant control, and dozens of VIPA PLCs are now in operation. Our products are exported to countries and regions such as Australia, India, Russia, and the Middle East. Since its implementation, the system has exhibited a low failure rate and stable equipment performance. We also compared the operating speeds of the two systems: the original S7-400 system had a scan cycle of 7 milliseconds, while the VIPA-314ST CPU system had a scan cycle of 2 milliseconds. Furthermore, the communication method was changed from MPI communication (187.5KHz) to Ethernet communication (100MHz), which improved communication speed and reduced costs, resulting in smoother dynamic monitoring on the host computer.
Because this system is fully compatible with S7-300 series products, its I/O modules have a high market share in China and are technologically mature, even if the PLC control system hardware fails during production, the cause can be identified immediately and the corresponding components replaced, minimizing online maintenance time. Using VIPA-300S PLC not only reduces costs but also improves the performance of the entire control system, making the entire system safer, more reliable, and with superior performance.
About the Author
Wei Jianqiang, born in 1963, male, Han nationality, native of Beijing, Bachelor's degree, Engineer, Department of Electrical Control
Mailing Address: No. 20, Wansheng South Street, Liyuan, Tongzhou District, Beijing
Postal code: 101101
Telephone: 010-81515493
E-mail: [email protected]
