Abstract: This paper mainly describes a control system that utilizes fieldbus and servo control technology to achieve high positioning and cutting accuracy. The control system adopts a single-master linear network topology with the highest transmission efficiency of the PROFIBUS-DP fieldbus. Based on the requirements of the entire production line, a Siemens S7-315-2PN/DP was selected as the master PLC, serving as the DP master. Servo controllers for two drive motors and two absolute displacement encoders are connected to the position controller. The position controller itself has a DP interface and can be directly connected to the PROFIBUS bus as a slave station of the fieldbus control system. Remote parameter configuration can be completed through the master station, thus developing a tire tread length cutting system. Practice has proven that the control system is safe and reliable, with a low failure rate, a high level of production and management automation, improved production efficiency, and ease of maintenance.
Keywords: servo controller, bus communication, network topology, fixed-length trimming
0 Introduction
The cord arrangement of radial tires differs from that of bias-ply tires. In radial tires, the cords are not intersecting but rather nearly parallel to the tire's cross-section, resembling the meridians of the Earth. The cord angle is small, typically 0°, and there are no intersecting points between the cords. When the tire is in motion, the stress around the crown increases, causing circumferential stretching and radial cracks in the tire body. Therefore, the buffer layer of radial tires uses a near-circumferentially arranged intersecting cord layer, intersecting the tire body cords at a 90° angle (typically 70° to 78°), forming a nearly inextensible rigid annular belt that holds the entire tire in place and restricts circumferential deformation. This buffer layer bears 60% to 70% of the tire's internal stress, becoming the main load-bearing component of the radial tire, hence it is called the belt layer of a radial tire. In contrast, the main load-bearing component of bias-ply tires is not in the buffer layer; 80% to 90% of its internal stress is borne by the carcass plies. Therefore, the design of the belt layer of radial tires is very important. It must have good rigidity and can use multi-layer, large-angle, high-strength, and non-stretchable fiber materials, such as steel wire or aramid fiber [1-2].
The control process of tread cutting length detection is a process of cutting to a fixed length first and then weighing each piece [3]. Developing a tread cutting device with high cutting accuracy (including the inclination and smoothness of the processing end face and the accuracy of the fixed length cutting) and adaptability to high cutting speed is extremely important in terms of increasing output, reducing scrap rate and improving the utilization rate of raw materials.
1. Overview of the composition and functions of the cutting equipment
The cutting system consists of several parts, including a tread storage device, a fixed-length cutting conveyor belt, a cutting device, and auxiliary devices.
1.1 Tread storage device
The tread material transmitted from the extrusion line is stored between the downhill belt and the cutting device, ensuring that a sufficient length of tread is provided to the cutting device, mainly serving a buffering function.
1.2 Fixed-length cutting conveyor belt
The tread conveyor belt is used to transport the tread stored in the tread storage device. It is driven by a servo motor controlled by a position-type AC servo controller, and a position controller is installed inside to achieve precise position control of the tread.
1.3 Cutter device
The cutting device consists of two parts. One part uses a cylinder that moves up and down to drive the cutting blade holder to rise and fall. The cutting blade motor is a 2-pole motor that drives the blade to rotate. The other part uses a servo motor controlled by a position-type AC servo controller to drive a high-precision linear motion module, which converts the rotational motion of the motor into linear motion to propel the cutting blade back and forth on the slide to cut the tire tread.
1.4 Auxiliary devices
The auxiliary device mainly includes a pressure brush that is lifted and lowered by a cylinder that moves up and down. A steam valve controls the flow of steam to lubricate the blade. Limit switches are installed on both sides of the linear motion module as the positioning origin and limit protection switches for the tool holder controller.
2. Control section development
The control system adopts a single-master linear network topology with the highest transmission efficiency of PROFIBUS-DP fieldbus, as shown in Figure 2. Based on the requirements of the entire production line, a Siemens S7-315-2PN/DP was selected as the master PLC, serving as the DP master station. The servo controllers of two drive motors and two absolute displacement encoders are connected to the position controller. The position controller itself has a DP interface and can be directly connected to the PROFIBUS bus as a slave station of the fieldbus control system. Remote parameter configuration can also be completed through the master station. The control console has numerous operation and display requirements. Setting and displaying the cutting length, the stroke and speed of the cutter head's left and right movement, manual control signals, and modifications to certain system parameters are all accomplished through the operation terminal. The TP270 operation terminal is a powerful Siemens Windows-based operation terminal. The host computer is used to monitor and display the working status and alarm information of each controller [4-5]. The control principle diagram of the cutting system is shown in Figure 1.
Figure 1 Control principle diagram of the cutting system
The detection device of the cutting system includes various non-contact limit switches, ultrasonic sensors and absolute displacement encoders, which are used to measure mechanical displacement and running speed, ensuring the orderly, safe and reliable operation of the cutting servo control system. The status detection signal is connected to the CLM switch input port (E1-E16 in terminal X3) of the Rexroth position controller, including the lifting and lowering signals of the cutter and pressure brush, the left and right limit signals of the tool holder, the tool holder positioning origin, etc. The action of the cutter, pressure brush and steam valve is controlled by the output of the switch output port (A1-A16 in terminal X4). The ultrasonic sensor is installed in the storage tank at the front end of the fixed length conveyor belt and the rear end of the front tire tread conveyor belt. The high and low signals (0-10V) of the tire tread at the storage tank are detected by the ultrasonic sensor and input to the PLC through the analog input port. The two displacement encoders that detect the position of the conveyor belt and the tool holder are connected to the CLM terminal of the position controller [6].
3 Software Design
This control system uses Siemens S7-315-2PN/DP as the Profibus fieldbus master station to provide high-speed cyclic communication service directly with the position controller. It has high communication speed, good control timeliness, strong anti-interference ability and is easy to program. Import the position controller CLM device GSD file into the PLC programming software STEP7, complete the hardware network configuration, assign a network address to the position controller, which must be the same as the one set in the controller parameters, and select the SFC14 and SFC15 “DPWR_DAT” system function blocks in the organization block OB35 to receive/send process data to the position controller [7].
Set the bus communication rate between the position controller and the master station, set the slave station network address in the parameters, and select the parameter process data object (PPO) type. In this way, the field devices of the system and the PLC can complete the reading and writing of data and the transmission of control data through the Profibus-DP bus. In addition to process data, Profibus-DP also transmits the parameter settings and diagnostic signals of the drive system [8].
The PLC coordinates and controls the belts and cutter holders based on the operating speed of the conveyor line, operating commands, and the status of the cutting device. The amount of tire tread stored between the two conveyor belts causes the sensor to generate a corresponding analog output signal, which, combined with the speed of the preceding conveyor belt, determines the operating speed of the cutting belt according to a certain algorithm. The PLC then adjusts the speed accordingly to ensure the smooth and coordinated operation of the conveyor belts. The control program flowchart is shown in Figure 2.
Figure 2 Control Program Flowchart
The cutting control HMI uses a Siemens TP270 touchscreen, connected to the PLC host via a PROFIBUS-DP bus. The touchscreen is programmed using Siemens' WinCC Flexible configuration software. WinCC Flexible's complete graphical user interface, along with its built-in project configuration wizard, allows users to easily create various object-oriented and symbol-based projects. In WinCC Flexible, communication between the operating units and actuators on the interface is achieved through variables stored on the PLC, which can be directly read or written by the HMI.
Based on the completion of basic human-machine interaction functions, a software system was designed to enable operators to learn operating procedures independently. Through the human-machine interface, operators can set certain basic parameters, such as fixed length values, error adjustments, and cutting counts; monitor the system's alarm status; learn the system's operating procedures; and set passwords. During automatic operation, following the designed motion flow of the equipment, parameters that need adjustment, such as speed and position, can be easily adjusted and modified on the touchscreen. In case of abnormal stop, the servo motor should immediately stop operating and generate an error code displayed on the touchscreen, allowing maintenance personnel to promptly understand the problem.
The system uses a PLC as the primary master station and an industrial PC as the secondary master station. The industrial PC acts as the host computer, providing a user-friendly human-machine interface to manage and monitor the entire production line and connect to the workshop-level Ethernet network. The main control PLC is the core of the entire tire tread production line control system, handling production process data acquisition and processing, as well as sending control signals and communicating with the industrial PC to facilitate operator monitoring of the equipment. The system's operation, working status, and measurement analysis results are graphically displayed and monitored on the industrial PC. Relevant data is uploaded to the PLC via the fieldbus, system alarms are processed, historical data is stored, various reports are generated, and graphical displays and human-machine dialogue are provided. Control commands are also sent to the PLC, thus enabling information management between the monitoring computer and the field equipment.
4. Positioning accuracy control simulation
The design employs an AC position servo controller with a built-in position controller accuracy of one ten-thousandth. Combined with the control performance of the servo motor, it can output rated torque even at zero speed, allowing for controller overload. Therefore, the inherent error of the electrical control is essentially negligible. The influence of environmental, human, and product factors can be overcome using specific parameter factors and a control strategy that adds shrinkage increments to the nominal length of the cut.
The controller adopts an incremental operation mode. To effectively eliminate the mechanical errors caused by the belt starting and stopping during the cutting cycle, the S-curve function of the position controller is used to smooth the acceleration and deceleration profile of the speed. See Figure 3 below. VTVEL is the positioning speed, VTACC is the acceleration, and tr is the acceleration transition time.
Figure 3. S-shaped profile curve of acceleration and deceleration of position controller
5. Conclusion
This control system fully utilizes advanced technologies such as PLC, Profibus-DP fieldbus technology, WinCC Flexible, and servo control. The system adopts a distributed open architecture, featuring fast response, flexible configuration, comprehensive control functions, and simple and standardized operation. The fixed-length cutting system has been designed and implemented in a radial tire production line, achieving a control accuracy of ±1.3mm. Practice has proven that this fieldbus-based control system is safe, reliable, and has a low failure rate. The product fully meets the high standards required by the next process, demonstrating a high level of production and management automation, improving production efficiency, and generating significant economic benefits. Furthermore, it features a compact and simple hardware structure, stable and reliable operation, and ease of maintenance.
6 References
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[2] Li Hao; Lan Xinliang; Li Gang; Lai Jianmin. Development of a new high-precision tire tread length cutting control system. Rubber & Plastics Technology and Equipment. 2004. 42-45
[3] Deng Yanni et al. Design of Servo Control System for Tire Tread Fixed Length Cutting Based on Profibus Fieldbus [J]. Automation Technology and Application, 2005, 24, 9: 58-60
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[7] Hans Berger, translated by Zhang Tongzhuang et al., Siemens S7-300/400 PLC Programming: Ladder Diagram and Function Block Diagram Description
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[8] Lenz GmbH, Germany, 9500EP Position Servo Controller Operation Manual, 2001
