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.
0 Introduction
In recent years, foreign countries have placed great emphasis on improving tire tread cutting equipment, resulting in numerous patents. The focus of these improvements is on enhancing cutting accuracy, increasing cutting speed, and extending tool life. Foreign research primarily focuses on laser cutting machines, for which patents are held. The application of laser technology eliminates the need for specialized and complex cutting mechanisms, allowing the cutting device to be directly integrated into the feeding system of radial tire forming machines, thereby improving cutting accuracy and tool life. Currently, Chinese tire manufacturers use a wide variety of cutting machines, and there is no unified standard among manufacturers. The accuracy of these machines still lags behind advanced foreign products, and a continuous process of improvement is needed to fully replace imported equipment.
Many tire manufacturers in my country use ultrasonic systems from Emerson's Presence, a subsidiary of Emerson, for their semi-steel radial tire one-step forming machines and tread/composite component cutting devices. These systems are modified from ultrasonic welding systems and were not specifically designed for tire production, resulting in high costs and energy consumption. The biggest problem with ultrasonic cutters is their high price; a set costs around 120,000 yuan, and replacing the blades costs 50,000 to 60,000 yuan. This high price and maintenance cost make them unacceptable to most domestic tire manufacturers.
Ultrasonic cutters are not only expensive, but also have many problems in actual production: 1. They are prone to instability when cutting tire treads; 2. The cutting angle is prone to change, and in severe cases, the blade may hit the lower cutting plate, damaging the blade; 3. The cutting angle is difficult to adjust, and maintenance and repair are inconvenient; 4. The program design is unreasonable. The lower cutting plate moves up and down at the beginning of the tire tread length setting, after the length setting is completed, and after the cutting is completed. The entire cutting process is divided into three levels: high speed, medium speed, and low speed. Sometimes there are stains on the conveyor belt or abnormalities on the tire tread surface, which can cause the blade position detection sensor to generate false signals, resulting in chaotic cutting speed; 5. The energy consumption is relatively high.
The design employs a new type of blade with adjustable cutting angle and unidirectional cutting, ensuring consistent tread cut direction. A silicone electric heating plate heats the blade, and a platinum resistance temperature sensor monitors the blade temperature in real time. The feedback signal controls the heating plate, resulting in smooth and rapid cutting with a stable cut shape, which is beneficial for jointing and improves the quality of semi-finished products. The cutter's advance and retreat are controlled by a travel motor via a synchronous belt. Synchronous belts offer advantages such as simple structure, smooth transmission, shock absorption, and the ability to achieve large-distance transmission between two shafts. Compared to cylinder-driven operation, they provide more accurate positioning, shorter running time, and easier maintenance. The travel motor is a DC servo motor. The start and stop of the cutter are limited by an inductive proximity switch. Tread length cutting is a key process in radial tire production. After the rubber compound is fed into the extruder, it is pressed out, stretched, cooled, and cut to length to obtain the tire blank. The control process for tread length detection involves first cutting to the specified length, then weighing each blank individually. Developing a tire tread cutting device with high cutting precision (including the inclination and smoothness of the processing end face and the accuracy of fixed-length shearing) and the ability to adapt to high cutting speeds is extremely important in terms of increasing output, reducing scrap rate, and improving the utilization rate of raw materials.
1. Brief introduction to the equipment composition and functions of the cutting system
1.1. Mechanical Components
Figure 1 shows the mechanical composition of the cutting system: 1-conveyor belt servo motor; 2-cutting blade motor; 3-blade holder servo motor; 4-blade holder linear guide rail; 5-conveyor belt; 6-ultrasonic sensor.
Figure 1. Mechanical composition diagram of the cutting system
The cutting device consists of a fixed-length conveyor, a cutting blade device, a glue-pressing device, and a blade holder rotation device. The fixed-length conveyor has a low-platform structure, and the conveyor belt is driven by an AC servo motor, meeting the requirements for high-precision fixed-length cutting. The speed can be infinitely adjusted within a certain range. The cutting blade device consists of a cutting blade holder and a blade holder transmission device. The cutting blade, a miniature cylinder, and a motor are mounted on the cutting blade holder. The cutting blade rotates at high speed driven by the motor and is raised and lowered by the miniature cylinder. It is lowered during cutting and raised during the return stroke. The cutting blade holder moves back and forth on a Rexroth high-precision linear motion guide rail driven by a servo motor to cut the tire tread. The glue-pressing device consists of a sponge roller and a pressure brush driven by a cylinder that can be raised and lowered to press down on the tire tread during the cutting process to prevent slippage. The entire cutting blade transmission device can rotate around a vertical axis to adjust the cutting angle.
After cooling, the tire tread enters the fixed-length cutting conveyor belt through the storage tank. An ultrasonic sensor is installed above the storage tank to detect the degree of tire tread storage, causing the conveyor belt to run at different speeds. The conveyor belt servo motor starts running, and at the same time, the rotary encoder directly mounted on the shaft starts pulse counting. The motor drives the conveyor belt forward. When the tire tread length reaches the preset value, the conveyor belt decelerates and stops running. At this time, the pressing brush of the pressing device is driven by the cylinder to press the tire tread downward. The blade holder servo motor drives the blade holder to move quickly from the initial end along the linear guide to the other end and cut the tire tread. During the tire tread cutting process, the water spray solenoid valve is activated and sprays water onto the cutter. After cutting is completed, the tire pressing device rises, the cutter is raised, the cutting blade holder on the linear guide returns to the initial position and stops running, the water spray solenoid valve stops spraying water, and the cutter is lowered. Everything returns to the initial state for restarting.
1.2 Components of the Electrical Control System
This control system adopts a single-master linear network topology with the highest transmission efficiency of the PROFIBUS fieldbus, as shown in Figure 2. Based on the requirements of the entire production line, a Siemens S7-315-2DP was selected as the master PLC, serving as the DP master. Two Rexroth DKC servo controllers for the drive motors and two absolute displacement encoders are connected to a Rexroth CLM1.4-LAP 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, controlling the left and right movement and speed of the cutter head, handling manual control signals, and modifying certain system parameters are all done through the TP270, a powerful Siemens Windows-based operating terminal.
Figure 2 Control system network topology
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's digital 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 digital 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 X1 and X2).
2 PLC control program design
The system uses a Siemens S7-315-2DP as the Profibus fieldbus master to provide direct and convenient high-speed cyclic communication with the Rexroth position controller CLM. It features high communication speed, good control real-time performance, strong anti-interference capability, and simple programming. The position controller CLM device database file (IN2_04eb.gsd) 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 the organization block OB, the SFC14 "DPRD_DAT" and SFC15 "DPWR_DAT" system function blocks are selected to receive/send process data to the position controller.
In position controller parameter B007, set the bus communication rate with the master station; in parameter B008, set the slave station network address and select the Process Data Object (PPO) type. This allows the system's field devices and the PLC to read and write data and transmit control data, such as control words, status words, setpoints, and actual values, via the PROFIBUS-DP bus. In addition to process data, PROFIBUS-DP also transmits drive system parameter settings and diagnostic signals.
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 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 smooth and coordinated operation of the conveyor belts. Users can set parameters such as the tread cutting length according to product production needs, and data transmission between the PLC and the position controller is completed via a bus.
3. Servo Program Design for Position Controller (CLM)
The Rexroth Position Controller (CLM) is a compact, modular two/four-axis CNC system that directly drives Rexroth DKC servo drives to achieve precise positioning of AC servo motors. In this system, two Rexroth DKC servo drives respectively control the fixed-length conveyor belt transport and the lateral movement of the cutting tool holder. The position controller has a rich instruction set, and control programs can be written on its operator panel or on a computer with programming software (MotionManager).
The flowchart of the cutting servo control program is shown in Figure 3. The program mainly consists of three parts: bus communication, conveyor belt control, and cutting blade holder control. The transmission of control and status information between the controller and the PLC is completed by the bus communication program. The controller receives control information such as speed value, length value, and operation command from the PLC, and at the same time transmits the running status information to the PLC for analysis and display. The conveyor belt control program completes the speed and position control of the conveyor belt servo motor, enabling precise length determination and smooth and fast operation of the tire tread cutting. The cutting blade holder control program completes the lateral movement and cutting of the cutting blade holder and the control of its auxiliary devices, ensuring the normal execution of the cutting blade action and obtaining a better cutting end face.
Figure 3 Servo control program flowchart
4. Monitoring System Design
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 INTRANET network. The primary master station (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.
The cutting control HMI uses a TP270 touchscreen, connected to the PLC host via a PROFIBUS bus. Touchscreen programming is performed using Siemens' ProTool/ProCS configuration software. ProTool/ProCS'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 ProTool/ProCS, communication between the operating units and actuators on the interface and the PLC is achieved through variables, specifically memory addresses on the PLC that the HMI can directly read or write. The software design philosophy, based on basic HMI interaction functions, includes a system that allows operators to learn operating procedures independently. Through the HMI, operators can set basic parameters such as length, error adjustment, and cutting count, monitor system alarm status, learn system operating procedures, and set passwords. During automatic operation, parameters such as speed and position can be easily adjusted and modified on the touchscreen, following the designed motion flow of the equipment. Abnormal Stop: When the positioning module, servo driver, limit switch, or machine malfunctions, the servo motor should stop immediately and generate an error code displayed on the touchscreen so that maintenance personnel can understand the problem in a timely manner.
5. Conclusion
This control system fully utilizes advanced technologies such as PLC, Profibus fieldbus technology, and servo control. The system adopts a distributed open architecture, featuring fast response, flexible configuration, comprehensive control functions, and simple and standardized operation. After completion, the fixed-length cutting system has been put into use in several all-steel radial tire production lines, achieving a control accuracy of ±1mm. Practical experience has proven that this Profibus-DP fieldbus-based control system is safe, reliable, and has a low failure rate. The product fully meets the high standards required by subsequent processes, exhibiting a high level of production and management automation, improving production efficiency, and generating significant economic benefits.
