introduction
Because sanitary ceramics such as toilets, water tanks, and washbasins have complex surface shapes, the glazing process for both the inner and outer surfaces, as well as the planning of the spray gun trajectory, are correspondingly complex. According to surveys, the domestic sanitary ceramics industry still predominantly uses manual glazing methods, resulting in low productivity and difficulty in guaranteeing quality. Some companies have used general-purpose articulated robots, along with related peripheral equipment, to create robotic glazing production lines. However, these production lines typically use industrial control computers or microcontrollers as the main controller, leading to poor stability in high-humidity and high-temperature glazing environments. Furthermore, the use of general-purpose articulated robots presents challenges. Firstly, the limited workspace creates blind spots when spraying inner cavities, requiring manual touch-ups in subsequent processes. Secondly, the robot and the external turntable use different control systems, making it difficult to achieve联动 (interlocking) between the robot and the workpiece turntable; the control process still mimics manual glazing actions.
Taking into account the above issues, this paper proposes a glazing robot system for sanitary ceramics. The robot control system hardware platform is built around a PLC CPU, a multi-axis motion control module, and an industrial touch screen. It can realize the linkage between the robot and the workpiece turntable, ensuring that the system has high stability and adaptability to the working environment. It also has strong versatility and openness, adapting to product updates and networked management.
Overall structure of robot system
The glazing robot workstation consists of a spraying robot body, a multi-station turntable, a workpiece turntable, a robot and production line control system, a glaze supply system, and a glaze cabinet, as shown in Figure 1. The control system includes a system management unit, a motion control unit, an external servo sensor unit, and a power management unit. The multi-station turntable has four stations: a loading/unloading station, a ready station, a spraying station, and a drying station. The glaze supply system consists of spray gun control air, return air, and a glaze supply circuit. The robot body structure is shown in Figure 2. The sanitary ware glazing robot has six degrees of freedom, with five degrees of freedom in the robot body, including three degrees of freedom for determining the end effector's spatial position. Two wrist rotation degrees of freedom are also present, with the workpiece turntable acting as an external axis that rotates at varying speeds following the spray gun. An orthogonal two-degree-of-freedom wrist structure is used to improve flexibility within the glazing workspace and simplify the transmission mechanism of a general-purpose three-degree-of-freedom wrist. The motor and reducer are mounted at the tail of the upper arm to reduce motion mass and inertia.
Control system hardware structure
The control system is required to control the aforementioned five-DOF glazing robot and workpiece turntable. The workpiece is transferred to the workpiece turntable via a multi-station turntable, which rotates the workpiece while the robot, carrying a spray gun, applies glaze. Therefore, the workpiece turntable, as an external axis of the robot, forms a linked system with the articulated robot. AC servo motors drive the robot's joints to perform variable-speed movements. The position and velocity data of each point are obtained beforehand through a teaching method, i.e., a human operator holds the teaching pendant and performs the teaching operation. The control system records the position and velocity of each intermediate point and stores them in memory. After teaching is complete, the operator can adjust the teaching according to the actual situation. During the automatic spraying process, the robot reproduces the movements based on the memorized teaching data.
Based on the process flow, the control system is required to store a large amount of teach data, which can be easily accessed and modified through a program and displayed on a touchscreen. This data can also be stored in a computer or memory card for data preservation or sharing among multiple machines. In general, the control system should have the following control functions:
(1) Calculation of position and velocity using forward and inverse kinematics:
(2) Robot joint servo motion control;
(3) Teaching programming for the glazing process:
(4) Demonstration and reproduction;
(5) Manual operation;
(6) Sequence control of glazing process;
(7) Process monitoring and emergency handling;
(8) Recording and statistics. Hardware structure composition of the control system.
Because the glazing site is a high-temperature and high-humidity environment, the stability of the robot control system is a necessary prerequisite for ensuring the normal operation of the robot. Mitsubishi series PLCs have rich programming instructions, a good software design environment, and flexible programming capabilities. Therefore, this paper adopts a hardware platform for the control system built around Mitsubishi Q-series PLCs, multi-axis motion control modules, and industrial touchscreens. The PLC CPU module is responsible for system management, workspace trajectory planning, interpolation calculation, forward and inverse kinematics calculation of position and velocity, logic control, and communication between units. The motion control module is responsible for interpolation of joint variables and motor motion control. The touchscreen is the human-machine interface, used for data input/output operations and display. The hardware structure of the control system is shown in Figure 3. The PLC is modularly connected to the base unit Q38, including a power supply module Q61P-A2, a high-performance axis motion control module QUN, a servo external signal input module QLX, a digital input module QX, and a digital output module QY.
The industrial touchscreen G OT SB features a clear 8-color display and is easy to operate and maintain. The first five servo motors drive the waist, lower arm, upper arm, and orthogonal wrist of the glazing robot body, respectively, while the sixth servo motor drives the workpiece turntable to rotate at variable speed.
For communication between the industrial touchscreen and the PLC CPU, the A9GT-BUSS bus connection module is selected. It is a communication module that saves cable connection space, and the connection is direct, fast and convenient. It also supports high-speed display and micro-motion fast response, and is the fastest connection method for communication.
Figure 4 shows the shared memory settings control program transmitted to the PLC CPU module and motion control module via USB interface. The PLC CPU also has an SRAM card to expand its internal memory capacity. The PLC CPU and motion CPU share their memory. The shared memory settings are shown in Figure 4.
Communication between the motion control module and the servo amplifiers, as well as between multiple servo amplifiers, uses SSCNET, a serial servo control system proposed by Mitsubishi. Serial control has its own communication protocol. According to this protocol, the controller and the controlled device exchange data for motion control or to obtain relevant servo information to send back to the controller. This communication is based on a fixed clock for data exchange and updates. Programmed motion control technology is employed to eliminate transmission time errors, thereby achieving synchronous motion control.
Conclusion
The glazing robot developed using the aforementioned control system hardware configuration has been applied to the glazing site of Tangshan Huida Ceramics (Group) Co., Ltd., demonstrating excellent performance. The glaze thickness on the inner and outer surfaces of the sprayed workpieces is uniform and does not run, thus saving glaze. Compared to manual spraying, it reduces the workload of operators, significantly improves production efficiency, and is highly welcomed by operators. Practice has proven that building a control system hardware platform based on a PLC CPU, motion control module, and industrial touchscreen ensures system stability and fully meets production needs.
References
Gong Jianhui and Zhang Zhendong designed the PLC control system for a robotic glazing production line.
[J]1 Electrical Engineering Technology Magazine, 2003 (2): 30-30. Jiang Xinsong 1 Robotics and Industrial Automation
[M] Shijiazhuang: Hebei Education Press. Click here to download the document: Control System for Glazing Robots in Sanitary Ceramics.
Edited by: He Shiping
