[ Abstract ] Three way catalytic converter is an important component to reduce exhaust emissions. Aiming at the problems of low efficiency and unstable operation of automatic assembly of three-way catalytic converter in an automobile parts company, a design scheme of automatic control system for three-way catalytic converter assembly line based on PLC is proposed. Mitsubishi Q series PLC and CC Link bus technology are used to control the assembly line automatically. After the design is completed, through the installation and commissioning of mechanical and electrical equipment, the final production rhythm is about 32s, and the product qualification rate reaches 98%, meeting the production requirements.
[ Key words ] Assembly line; PLC ； robot; Control system
The three-way catalytic converter of automobile is the key device of automobile environmental protection, which is generally composed of five parts: shell, gasket, carrier, catalyst and coating. It can change the three toxic gases produced by automobile fuel consumption into harmless CO2, H2O and N2, which can be discharged as tail gas. Assembly line is a serial oriented process system engineering [1]. As an organization form of large-scale and efficient production, assembly line is usually composed of several stations and transmission devices connecting these stations. Assembly is the process in which the products to be assembled pass through the assembly station according to a certain production rhythm and the corresponding parts are assembled on the objects to be assembled, and finally the finished products are output at the end of the production line [2]. Based on the research of PLC control technology and CC Link fieldbus technology, according to the assembly process of three-way catalytic converter, a control system for this product is designed. After on-site debugging and trial production, the control system can more efficiently complete the assembly of three-way catalytic converters of different models. The interaction between PLC and CC Link can achieve the high efficiency of data transmission, ensuring the reliability of the three-way catalytic converter assembly line.
1 Overall plan
Fig. 1 Layout of high-efficiency three-way catalyst assembly line
Aiming at the problems of low efficiency and unstable operation of automatic assembly of three-way catalytic converter in an auto parts company, an automatic production line was independently developed and designed. This paper does not study the whole production line, but only the processes behind the printing station. The handling robot is responsible for clamping the products on the printing station to the locking slide, and transporting the cover to the products. After the above actions are completed, the slide enters the locking station; The locking robot takes the screws from the screw feeding positioning mechanism, locks the product and the cover, and after the locking is completed, the sliding table exits the locking station; The handling robot clamps the locked product to the printing station, and the printing mechanism prints the product model, manufacturer and other information on the cover. Finally, the product is transported to the production line by the conveyor belt.
The layout of efficient three-way catalytic converter assembly line is shown in Figure 1. The three-way catalytic converter automatic assembly control system in this paper includes handling robot, locking robot, sliding table, inkjet printer, conveyor belt, various sensors and other equipment, integrating the executive control module, status monitoring module and data acquisition module [3]. The executive control module realizes the joint control of all institutions and equipment of the production line; The status monitoring module and the data acquisition module monitor and collect the data of various sensors in the production line and the status of each station, and transmit them to the PLC for data processing. PLC regulates the following executive control module according to the transmitted data to achieve automatic and intelligent control of the actuator. The overall structure of the control system is shown in Figure 2.
Fig. 2 Overall Structure of Control System
2. Hardware design of control system
2.1 PLC selection
The basic principle of PLC selection [4] is to select the model with the highest cost performance, which is easy to maintain and operate in the later period, on the premise that PLC meets the control requirements of the production line system. Compared with Siemens PLC, Mitsubishi PLC's programming language is easier to understand and master, and its performance in motion control is better than that of other brands at the same level. At the same time, Mitsubishi PLC has a unique positioning control command, which is more convenient in controlling servo motor and stepping motor. To sum up, the system adopts Mitsubishi PLC. According to the production requirements of the three-way catalytic converter assembly line, the performance of signal acquisition and data processing, it was decided to use Mitsubishi Q series PLC to complete this design [5].
For the selection of PLC key modules, Mitsubishi general-purpose CPU module (Q06UDVCPU) is determined to be used according to the requirements of the assembly line and in combination with Mitsubishi PLC model selection manual. Mitsubishi general-purpose CPU module has:
(1) Higher production efficiency. Ultra high speed processing can shorten the production time. As the application becomes larger and more complex, it is necessary to shorten the running cycle time of the system. The operation cycle can be shortened by the ultra-high basic operation processing speed of 1.9n (s LD instruction).
(2) Better storage performance. The capacity of the built-in soft component memory of the CPU is increased to 60K words at most, and the increased control and quality management data can also be processed at high speed.
(3) Convenient fault handling. When a fault occurs, it can also respond quickly, just extract the data related to the problem, do not spend time filtering a large number of diagnostic data, can quickly determine the cause of the fault, and develop solutions.
(4) Simplify program debugging. The software component test function of the conditions to be executed can be used to change the software component value to the user specified value at any step of the program.
The Mitsubishi simple motion module (QD77MS16) has powerful performance and can control the motion of 16 axes at the same time; It has rich positioning control functions: through linear interpolation control, 2-axis arc interpolation control, constant length feeding control, continuous track control and other rich control methods, it can meet various purposes; The positioning address, speed, etc. can be set through PLC program to easily realize automatic operation; The auxiliary functions such as code, skip function, single step operation and target location change function can meet different needs of users; Any data monitoring function; The information of the servo driver and the servo motor can be monitored through the simple motion control module, and can also be displayed on the screen made by the user. Monitorable data include: payload rate, regenerative load rate, peak load rate, load inertia ratio, servo motor rotation speed, position feedback, 1-cycle position of absolute position detector, power consumption, etc. The graphical configuration of the module is simple. Use GX Works2 software to set the simple motion control module. With simple configuration and parameter setting, you can complete positioning data setting, synchronization control, cam control and other settings. You can also set and adjust the parameters of the servo driver in linkage with MR Configurator 2, which can improve the efficiency of servo operation and make the system easy to maintain. Configuration parameters, positioning data, block startup data, etc. can be saved using the built-in flash ROM, and data can be saved without batteries.
CC Link network bus module (QJ61BT11N) is a communication port used to link Q series PLC in CC link network system. In the network system, the master station exchanges information with each slave station through the QJ61BT11N interface module. The master station and slave station use the link scanning method to exchange data between stations. The method used is to automatically map through the buffer registers in the module. There are special communication instructions between registers for link scanning of data. The module has the following characteristics:
(1) High speed processing. The maximum transmission distance can reach 100m, and 64 sites can be connected. The link scanning time is only 1~5ms.
(2) Simple connection. CC Link system can connect various industrial control equipment (limit switch, photoelectric sensor, frequency converter, touch screen, etc.) into a unified equipment network layer through a simple bus. There are also serial communication modules (QJ71C24N-R2), input modules (QX42) and output modules (QY42P) as important components. Figure 3 shows the physical pictures of the connected PLC modules.
Fig. 3 PLC module connection diagram
2.2 I/O port address assignment of PLC
Since the focus of this design is on the research of the last two robots and their related stations of the whole assembly line, only part of the I/O address distribution of the operation box and relay boxes 3~5 are shown here.
(1) Address assignment of I/O ports of input and output modules of the operation box. The input/output module of the operation box is mainly used to control the switching, pause, emergency stop and flashing of various indicators of the assembly line operation mode through the digital input of the operation panel and the data processing by the PLC. Mitsubishi AJ65SBTB1-32DT mixed input/output module is used, and 8 PLC inputs and 9 PLC outputs are used. See Table 1 for partial address distribution of its I/O ports.
Table 1 I/O Port Address Distribution of Operation Box I/O Module
(2) Address distribution of I/O ports of input and output modules of No. 3 relay box. The input module of No. 3 relay box is mainly composed of the limit position of the sliding platform, the status of each cylinder and the status of the sensor. The output module is mainly used to control the on-off of the solenoid valve of each position cylinder and the start and stop of the servo motor. Two Mitsubishi AJ65SBTB1-32D input modules, one AJ65SBTB1-32D input and output module and one AJ65SBTB1-32T output module are used, and 69 PLC inputs and 28 PLC outputs are used.
(3) Address distribution of I/O ports of input and output modules of No. 4 relay box. The signal of the input module of No. 4 relay box is mainly composed of the I/O of the printing mechanism servo motor, printing station cylinder and related sensors. The output module mainly controls the actions of each mechanism in the printing process, including two input modules: AJ65SBTB1-32D and AJ65SBTB1-16D, and one output module: AJ65SBTB1-32T, which uses 38 PLC inputs and 20 PLC outputs.
(4) Address distribution of I/O ports of input and output modules of No. 5 relay box. The input module of No. 5 relay box is mainly composed of the output signals of the handling robot and the locking robot, as well as the cylinder status and sensor status related to the screw positioning mechanism. The output module is composed of the input signals of the two robots and the signals that control the cylinder of the related mechanism to act, thus controlling the robot to complete a series of tasks. Three input modules are used: AJ65SBTB1-32D × 3. Three output modules: AJ65SBTB1-32TE1 × 2. AJ65SBTB1-32T uses 96 PLC inputs and 82 PLC outputs.
2.3 Selection of touch screen
The touch screen is a human machine interface, which can monitor the operation status of the production line, control the action of the actuator in the production line, and store and share the data in the production line. Since the PLC module used in this design is Mitsubishi series, in order to facilitate the communication connection between them and ensure the stability and real-time data transmission, GS2110-WTBD-N touch screen with Ethernet interface is used. As shown in Figure 4, after the touch screen interface is designed and connected with the production line system, I/O monitoring, alarm processing, viewing production information and other operations can be carried out on the assembly line. After entering the password and obtaining the permission, the parameters of the production line station and some processes can be changed.
Fig. 4 Touch screen interface of production line
3 Application of robots
As one of the most important actuators of the assembly line, the robot is responsible for product handling and screw locking. Guangzhou CNC robots are selected for this assembly line. The models are GSK-RB35 and GSK-RB50. See Table 2 for specific parameters.
Table 2 Robot Parameters
3.1 Definition of robot I/O
Most actuators in the assembly line are controlled by PLC, and the robot communicates with PLC through physical I/O. Therefore, it is necessary to make a signal agreement on the I/O and PLC sides of the robot at the early stage of commissioning. In this way, PLC can control the action of the robot by analyzing and processing the signals of external sensors and then outputting them as signals. There are 32 input and output signals in GSK-RB35 robot, and the address of each I/O signal corresponds to the input and output instructions of a robot. For example, the robot input port X0.8 corresponds to IN8 in the robot command. When the PLC side sends a high level signal to X0.8 on the robot side, the corresponding IN8 will also be connected to control the robot's action. Since the robot program is loaded normally, IN0 ～ IN7 (OT0 ～ OT7) are system defined signals and IN8 ～ IN32 (OT8 ～ OT32) are user-defined signals among the 32 I/O ports of the robot. The signals defined by the system are mainly the system emergency stop signal, pause signal, running signal, servo preparation signal and other signals indicating the robot's state. The user-defined signal is the I/O defined by the user to achieve some required functions. See Table 3 for customized I/O of the handling robot.
Table 3 Customized I/O of handling robot
Each customized signal in Table 3 needs to be debugged constantly, otherwise it will cause the robot to make wrong actions and consume manpower and material resources.
3.2 Relationship between robot, PLC and touch screen
The robot acts as an actuator to complete the handling and locking of the whole production line; As the main controller, PLC coordinates the cooperation of various mechanisms and actuators on site; As the monitoring equipment of the production line, the touch screen allows operators to operate the production line more conveniently, and view the status and production information of the production line more conveniently; Fieldbus is used as the medium of information transmission between various control signals, sensors and actuators in the production line. The relationship between the three is shown in Fig 5.
Fig. 5 Relationship between robot, PLC and touch screen
3.3 Design of robot action logic
This design mainly describes the work stations of the last two robots of the whole production line, which are named after the handling robot and the locking robot respectively. As shown in Fig 6, the handling robot is structurally designed with two end effectors, namely a gripper controlled by an air cylinder and a suction cup. The task of the handling robot is to take the product completed in the previous station to the locking slide table with clamping jaws, and then transport the cover to the product through the suction cups. The qualified/unqualified products of the locking robot also need to be transported to the printing station or NG belt with clamping jaws by the handling robot, and transmitted to the PLC control system through the real-time signal of the servo tightening mechanism of the locking robot, which will be processed by the system to judge whether they are qualified.
Fig 6 Effect Picture of the End Actuator of the Handling Robot
The task of the locking robot is to take out the screw from the screw feeding mechanism and lock the cover with the product. The action logic of the locking robot is shown in Fig 7. In order to reduce the number of screws taken out by the robot and improve the locking efficiency, the locking robot is installed with three servo tightening mechanisms. The whole process requires two screws taken out, as shown in Fig 8.
Fig 7 Action logic of locking robot
4 Field commissioning
The on-site commissioning of the control system of the three-way catalytic converter assembly line involves the coordination of various mechanisms, such as robots, engraving machines, locking fixtures, servo tightening mechanisms, vibrating feeding devices, etc. It takes a lot of time for these mechanisms to achieve the production status required by the manufacturer. Through constant debugging of PLC program and robot program and revision of robot action on site, the assembly line control system has been able to run stably on site, and achieve a good beat and production qualification rate. The beat in the actual production process can reach 32s, meeting customer requirements, and the production qualification rate reaches 98%.
5 Conclusion
Based on the production requirements of the enterprise, this research has completed the control system design of the three-way catalytic converter automatic assembly line, designed a set of control scheme using PLC master control technology and CCLink bus technology, and developed the control system according to this scheme. According to the scheme, each module of PLC and hardware such as touch screen are selected and configured, and the robot program flow is designed. Through on-site debugging and trial operation, the safety factor, beat and qualification rate meet the production requirements. Compared with the actual use effect, the I/O module in this paper takes up less space, and the saved space of the branching module can be used to install other components. The actual layout of the electrical cabinet using Siemens I/O module PP72/48 is shown in Figure 7, and the layout of the electrical cabinet using domestic I/O modules in this paper is shown in Fig 8.
Fig 8 Servo tightening mechanism of locking robot
