Abstract: Complex working conditions significantly impact the accuracy and safety of high-speed CNC machine tools. Therefore, it is necessary to set detection parameter thresholds and determine the safety of the machining conditions by comparing the detected data with expected values. This paper designs an integrated monitoring system for high-speed CNC machine tools based on multi-sensor technology. The system's process design, hardware design, and PLC software development are described. Sensor selection is explained, and seamless integration of the multi-sensor system with the 840D/828D CNC system is achieved. Finally, practical experiments demonstrate that the monitoring system improves the safety and controllability of high-speed CNC machine tools under complex working conditions, while ensuring product quality and efficiency.
Reliable operating conditions are crucial for ensuring the production efficiency, product quality, and equipment safety of high-speed CNC machine tools. Because the machining process of high-speed CNC machine tools is flexible and dynamic, monitoring the machining process and its operating status involves significant complexity [1-3]. Although the positioning and repetitive positioning accuracy of domestically developed machine tools in my country is already very high, the complexity of the machining process, coupled with factors such as vibration, tool wear, and thermo-coupling deformation, leads to a decrease in production accuracy and stability. Therefore, the stability and reliability of the production process, product quality, and production efficiency still lag far behind those of advanced countries [4-5]. In actual production, due to factors such as the rational selection of process parameters, cutting vibration, tool wear, and thermal deformation, errors exist between the finished product and the theoretical model. The performance and lifespan of the equipment, machining efficiency, and cost are all factors that need to be considered, all of which rely heavily on machining process monitoring technology. Monitoring the machining equipment and process can effectively ensure the final machining accuracy of the product and comprehensively grasp the operating status of each unit during machine tool operation, improving machine tool performance, eliminating waste, and reducing costs. Monitoring technology can improve the reliability and controllability of the machining process. To improve machining stability and reliability, machining quality and efficiency, it is necessary not only to strengthen the basic research on machine tool design and technology, but also to make good use of modern computer technology, information technology and monitoring methods to enhance the perception and control functions of machining equipment and machining process.
Machine tool and machining process monitoring technology is the means to acquire information, discover problems, and solve problems. It is the prerequisite and foundation for realizing digital manufacturing and intelligent manufacturing, and a favorable guarantee for efficient, high-quality, safe, and reliable machining. Only by monitoring machine tools, cutting tools, and machining processes can we obtain equipment and process status information, thereby enabling intelligent optimization and control. Therefore, advanced monitoring technology is the foundation and the only feasible way to achieve intelligent manufacturing and make machine tools smarter. This paper designs a high-speed CNC machine tool integrated monitoring system based on multi-sensor technology to monitor complex working conditions in real time and control and evaluate machining quality throughout the entire process.
1. Process design of integrated monitoring system
The process flow of the integrated monitoring system for high-speed CNC machine tools under complex working conditions is shown in Figure 1. Before the machining begins, a virtual testing process is used to detect each representative working condition and save its characteristics to the database, which serves as a reference for identifying working conditions and optimizing their parameters.
Multi-sensor monitoring will cover the entire production process, enabling real-time detection and identification of sudden accidents such as collisions, tool breakage, chipping, overload, and chatter. It will then dynamically detect the degree of tool wear and the vibration state of cutting. When tool wear is detected, the machine vision process will be activated to determine the degree of wear. The online inspection process will complete the inspection of key nodes and the end of processing to ensure production quality.
This integrated monitoring system utilizes a CNC system to control complex working conditions, treating the probe and CCD as tools and placing them in corresponding positions. It uses the CNC program and the machine tool's own operation to achieve detection. Therefore, this integrated monitoring system not only integrates hardware but also software.
2. Monitoring System Design
The detected spindle motor power signal and cutting vibration signal are transmitted in real time to the 840D/828D CNC system and host computer. The detection data stored in the range of 200 to 203 can be viewed in real time through the 840D/828D MHIR variable mode. At the same time, the CNC system processes the detection data in real time and implements the corresponding control steps. Then, the host computer monitoring interface realizes the real-time display of the detection data.
Figure 1. Process Design of Integrated Monitoring System
2.1 Sensor Selection
The selection of sensors includes accelerometers and power sensors, as shown in Table 1.
2.2 Hardware Design
An embedded monitoring system is used to acquire and transmit data from various hardware interfaces, such as acceleration, motor current and temperature signals [6]. The structure and principle of the monitoring system are shown in Figure 2.
Table 1 Sensor parameters and installation locations
The signal output of the accelerometer is connected to the front-end dedicated signal acquisition module CS2ACCSF/CS2ACCPW using a 4-wire connection to convert it into an Ethernet signal output and then connected to the Ethernet interface of the embedded monitoring system [7] to complete the detection data acquisition. The acquisition frequency is 10Hz. Similarly, the signal output of the power sensor is connected to the front-end dedicated signal acquisition module CS2ACCPW using a 5-wire connection to convert it into an Ethernet signal output and then connected to the Ethernet interface of the embedded monitoring system to complete the detection data acquisition. The acquisition frequency is 10Hz.
2.3 Monitoring System Software Design
The integrated monitoring system prioritizes abnormal operating conditions based on their urgency. It scans these conditions sequentially from highest to lowest priority and automatically employs appropriate countermeasures. The highest priority conditions are overload, exceeding limits, and other situations that could lead to serious accidents or equipment damage. If vibration or power exceeds the set maximum threshold and the duration exceeds the set maximum time, the system will sound an alarm and illuminate warning lights, while simultaneously recording and archiving the alarm.
Each processing step and its specified duration are set according to its maximum threshold and duration. The duration for emergency events such as tool damage or collisions is typically set to be less than or equal to 15 ms, ensuring that the alarm system is activated and appropriate measures are implemented before equipment damage occurs. For non-emergency events such as tool wear, the duration is appropriately extended to 60–90 ms.
Figure 2. Schematic diagram of the monitoring system.
2.4 PLC Development Programming
Figure 3 shows a portion of the PLC development program used in the PLC automatic control integrated monitoring system.
Figure 3 PLC Development Program
2.5 Seamless integration design with CNC
Seamless integration with CNC is achieved by using the 840D/828D expansion interface to edit the integrated monitoring HMI window. First, two methods for launching the integrated monitoring system interface are designed: automatic and programmatic. The corresponding configuration files are MA_AUTO.COM and PROG.COM, stored on the PC50's hard drive in the path: \DH\CUS. DIR\; Secondly, create the interface display text: set the interface text save file name to ALUC_XX, the storage path to: \OEM\, and the text code range to 85000~89899; Thirdly, design and program 8 horizontal and 8 vertical soft keys in each interface to realize interface jumps; Fourthly, the designed and developed integrated monitoring system is integrated into the HEBUT soft key of the HMI interface. Clicking the HEBUT soft key enters the high-speed CNC machine tool integrated monitoring system interface, and communication between them is realized through the PROFIBUS bus. Through hardware and software integration, the seamless connection between the monitoring system and the CNC is completed.
3. Test Results
The HTC2550HS high-speed CNC turning center, using a Siemens 828D series CNC system, was used to process 45 steel bars with a diameter of 600 mm. The spindle speed, motor power, and maximum torque were 0-6000 rpm, 20.5/15 kW, and 162 N·m, respectively. The X-axis and Z-axis traverse speeds and feed motor power were 60 m/min and 4.71 kW, respectively. The maximum feed resistance of the Z-axis (spindle) and X-axis were 1290 N and 1720 N, respectively. Six different operating conditions were manually set, and the test results are shown in Table 2. For the pre-set abnormal operating conditions of overload, collision, chatter, and severe tool wear, different levels of alarms were triggered, warning lights were illuminated, and the maximum values and durations were recorded.
When multiple sensors detect tool wear, they invoke the machine vision-based tool condition diagnostic function. Because environmental noise, defects in the blank or material can cause false alarms from sensors, leading to incorrect control decisions, a combination of real-time multi-sensor monitoring and time-sharing CCD monitoring is employed to ensure timely and accurate identification and control of tool conditions. Once wear is confirmed, tool change information and tool compensation values are fed back as PLC variables.
The test results show that the integrated monitoring system can identify various complex working conditions of high-speed CNC machine tools in a timely and accurate manner, and take corresponding control measures based on the working condition classification results.
Table 2. Six operating conditions and corresponding identification results and countermeasures.
4. Conclusion
The high-speed CNC integrated monitoring system based on multiple sensors not only improves the reliability and controllability of the processing, but also enhances its stability, processing quality, and efficiency. Furthermore, users can easily, conveniently, and intuitively utilize the system, significantly improving its predictive and monitoring capabilities for complex operating conditions.
