Foreword
With the development of industries such as packaging, printing, and building materials, the demand for raw materials of fixed lengths is increasing. Equipment used to produce these products includes cross-cutting machines for corrugated cardboard, sheet cutting and creasing units in printing presses, and flying saws for cutting profiles of different sizes. Besides the diverse requirements in terms of size specifications, these products generally require strict dimensional tolerances.
In 1946, Michelin, a French tire company, produced the world's first radial tire. The invention of the radial tire was a revolution in the tire industry and has become a new direction for automobile tire development.
Radial tires have experienced rapid development due to their advantages such as wear resistance, fuel efficiency, ride comfort, traction, stability, and high-speed performance. Currently, radial tires account for 80% of the international market, with passenger car radial tires accounting for 90% and truck radial tires for 63%. my country's radial tire industry started later. China's huge automobile demand has spurred the rapid growth of the tire industry. From January to August 2007, China's tire production reached 338.36 million units, a year-on-year increase of 23%, with radial tire production reaching 153 million units, a year-on-year increase of 36.7%. Radial tires accounted for 45% of the total tire production, indicating that my country's tire structure is shifting towards pursuing high quality and high added value. The cord arrangement of radial tires differs from that of bias-ply tires. The cords of radial tires are not arranged in a crisscross pattern, but rather are nearly parallel to the cross-section of the tire, like the meridians of the Earth. The cord angle is small, generally 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.
Tread cutting to length is one of the key processes in tire production. After the rubber compound is fed into the extruder, it is pressed out, stretched, cooled, and cut to length to obtain the tread. The control process for tread cutting to length is a process of first cutting to a fixed length and then weighing each tread individually. The weight index is one of the final inspection standards in tire design, while the length index serves as the actual operational process index for the next process. Therefore, the accuracy of the length cutting is crucial.
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. The equipment composition of the system is shown in Figure 1.
Figure 1. Equipment composition of the fixed-length cutting system
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
This 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 also be performed 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 performed through the operator terminal, a powerful Siemens Windows-based operator terminal. The host computer is used to monitor and display the working status and alarm information of each controller. The control principle diagram of the cutting system is shown in Figure 2.
Figure 2 Control principle diagram of the cutting system
The cutting system's detection devices include various non-contact limit switches, ultrasonic sensors, and absolute displacement encoders, used to measure mechanical displacement and operating speed, ensuring the orderly, safe, and reliable operation of the cutting servo control system. Status detection signals are connected to the Rexroth position controller CLM switch input ports (terminals X3, E1-E16), including signals for the cutter and pressure brush to raise and lower to their designated positions, left and right limit signals for the cutter holder, and the cutter holder's positioning origin. The actions of the cutter, pressure brush, and steam valve are controlled by the switch output ports (terminals X4, A1-A16). Ultrasonic sensors are installed in the storage tanks at the front end of the fixed-length conveyor belt and the rear end of the preceding tire tread conveyor belt. The high/low signals (0-10V) of the tire tread position detected by these sensors are input to the PLC through analog input ports. Two displacement encoders, respectively detecting the positions of the conveyor belt and the cutter holder, are connected to the position controller CLM terminals.
3. Software Design
This control system uses a Siemens S7-315-2PN/DP as the Profibus fieldbus master to provide high-speed cyclic communication services directly with the position controller. It features high communication speed, good real-time control, strong anti-interference capability, and ease of programming. The position controller's CLM device GSD file is imported into the STEP7 PLC programming software to complete the hardware network configuration. A network address is assigned to the position controller, which must be the same as the one set in the controller parameters. In organization block OB35, the SFC14 and SFC15 "DPWR_DAT" system function blocks are selected to receive/send process data to the position controller.
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 3.
Figure 3 Control Program Flowchart
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 4 below. VTVEL is the positioning running speed, VTACC is the acceleration, and tr is the acceleration transition time.
Figure 4. S-shaped profile curve of acceleration and deceleration of position controller
5. Conclusion
The fixed-length cutting system has been designed and implemented in a radial tire production line. Practice has proven that this fieldbus-based control system is safe, reliable, and has a low failure rate. The products fully meet the high standards of 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. Additionally, the system adopts a distributed and open architecture, offering fast response, flexible configuration, comprehensive control functions, and simple and standardized operation.
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