Abstract: This paper addresses the positioning inaccuracy fault of the CNC system of a W200HD boring and milling machine in a certain factory. An in-depth analysis was conducted based on the position control principle, the root cause of the fault was found, and a solution was proposed. Timely maintenance was carried out. Finally, some reasonable suggestions are put forward based on this fault handling experience. Keywords : CNC system, position control, positioning error 1 Fault Phenomenon The W200HD is a CNC boring and milling machine imported from the Czech Republic by Wuhan Iron and Steel Company in 1994. During one use, it was found that no matter how far the machine traveled, there was always a certain error between the distance displayed on the coordinate system and the actual measured distance, sometimes reaching several millimeters or even tens of millimeters. This large positioning error prevented the CNC machine from machining normally. 2 Fault Analysis and Handling 2.1 Fault Analysis The CNC system equipped on the W200HD boring and milling machine is the SINUMERIK 810 GA3 system from Siemens, Germany. It is a mid-range CNC system with three-axis linkage function and a resolution of 1µm. The aforementioned error is abnormal for this type of system. The system itself has a self-diagnostic function; however, the aforementioned fault was not displayed on the monitor and required manual judgment and troubleshooting. Positioning error, as a crucial indicator in position control, can comprehensively determine whether position control is functioning correctly. The fault symptoms indicate a problem with the position control section of the machine tool's CNC system, causing excessive positioning error in the entire closed-loop system and affecting normal machine tool processing. Based on the principle block diagram of closed-loop position control shown in Figure 1, the principle of feed position control is briefly analyzed. The position sensor (here, an optical grating) mounted on the machine tool's worktable converts mechanical displacement into digital pulses. The pulse is sent to the position measurement interface of the CNC system. A counter counts the computer sampling the feedback value at fixed time intervals. The sampled value is compared with the output of the interpolation program (i.e., the current command position) to obtain the position error. This error is amplified by software gain (K[sub]c[/sub]) and output to the digital-to-analog converter (D/A) to provide control voltage to the servo unit, driving the worktable to move in the direction of reducing the error. If the interpolation program continuously generates feed amounts, the worktable continuously follows these feed amounts. Only when the position error is zero does the worktable stop at the required position. Therefore, the position control function is implemented by both software and hardware. The software is responsible for calculating the position error and the speed loop setpoint, while the hardware receives the setpoint digital value, performs D/A conversion, provides control voltage to the speed loop, drives the coordinate axes, and feeds back the machine tool worktable position to the CNC system. Based on the fault symptoms, the source of the problem can be analyzed. This machine tool is a fully closed-loop control system. The position sensor is installed on the worktable, and theoretically, the positioning error is not affected by the lead screw backlash and pitch error. The fact that the machine tool can move normally indicates that the machine tool servo unit is working properly. In the machine tool feedback loop, if the feedback line is broken, the system effectively becomes an open loop. No position feedback increment is subtracted from the command increment, and the following error accumulates until it overflows, causing the drive to malfunction. At this point, the CNC system's self-diagnostic function kicks in, issuing an alarm message indicating a broken feedback loop. Based on the above analysis, the hardware of the position control is not the problem; therefore, the fault likely lies in the software. The machine tool's servo unit can control the current and speed loops. The position loop (i.e., position control) is handled by the CNC system software. The CNC system software has hundreds of parameters that can be set by the user to facilitate secondary development for the CNC system and machine tool. These parameters are stored in the CMOS memory. The parameters are not lost after the machine tool loses power. However, insufficient battery power (below 85% of the rated value) or external interference can cause parameter loss and confusion. The absence of a low battery alarm indicates that the battery has sufficient power. If all the control parameters of the CNC system were lost, the machine tool would be impossible to operate; therefore, some form of external interference is highly probable. This causes some parameters of the position control to become disordered, resulting in significant errors in the software calculations for position control and causing the positioning error to exceed the allowable range. As shown in Figure 2, the accuracy of the machine tool's position feedback measurement system and the position loop gain K[sub]c[/sub] affect the output position error. The position loop gain reflects the sensitivity of the entire servo system. Under stable system conditions, a higher gain results in a smaller positioning error, but the machine tool is prone to oscillation and is difficult to adjust; conversely, a lower gain results in a larger positioning error, but the machine tool is easier to adjust and less prone to oscillation. The system parameters of the CNC system were checked to see if the position loop gain was too low, causing a large positioning error. After calibration, the position loop gain was found to be consistent with the gain after the machine tool was properly adjusted. Therefore, the problem with this machine tool may lie in the mismatch between the accuracy of the machine tool's position feedback measurement system and the position adjustment accuracy of the control device. Since this machine tool uses a fully closed-loop position control system, it directly obtains the actual position movement value from the machine tool's moving parts (worktable), so its detection accuracy is not affected by the mechanical transmission accuracy. For the position loop to function properly, a correct position adjustment closed-loop circuit is required. Therefore, the pulses of the position measurement system must be matched with the position adjustment accuracy of the control device. In this system, there is a configurable parameter MD5002 (8 bits) representing the feedback input resolution and position control resolution. Only when the settings of the feedback input precision and the position control precision match can the position loop operate correctly, enabling the machine tool to move normally and accurately reach the specified position within the allowable error range. In the CNC system, the positioning error is calculated in units of servo resolution. If the precision of the position feedback input and the precision of the position control are inconsistent, the position error is calculated using non-uniform units, and the result is not a true error between the commanded position and the actual position. Therefore, the worktable cannot reach the position required by the command. 2.2 Troubleshooting Based on the above fault analysis, the system parameters were retrieved, and the contents of parameter unit MD5002 were checked. It was found that there was a discrepancy with the previously debugged parameters, possibly due to some external interference causing the parameters in RAM to become corrupted. After correctly modifying the parameters, the CNC system of the machine tool was reset, and the machine tool was restarted. After testing, the positioning accuracy of the machine tool reached the normal requirements, and the machine tool resumed normal production. 3 Summary Through this fault maintenance, it can be seen that in order to give full play to the advantages of CNC machine tools, such as multiple functions, high flexibility, high processing accuracy, and high degree of automation, and to produce high-quality products to meet the needs of market competition, it is necessary to pay attention to strengthening the maintenance of CNC machine tools. Specifically, the following points should be done: (1) Ensure that the operating environment of CNC equipment is good and meets the requirements of the product specifications. (2) Pay attention to the maintenance of the power supply used by CNC equipment. In particular, when adding or removing large-capacity equipment that shares AC power with the equipment, it is necessary to observe whether instantaneous interference signals are generated to prevent them from damaging the internal program or parameters, or even causing the machine to crash and affecting the normal operation of the machine tool. (3) CNC equipment should be operated according to the procedures. In particular, prevent machine tool operators from arbitrarily modifying internal parameters. (4) There should be maintenance records. Generally, machine tool faults have regularity (such as frequently occurring faults), and maintenance records are of guiding significance for future work. (5) It is necessary to strengthen the training of relevant CNC equipment technicians and operators, and improve their comprehensive professional quality by combining theory and practice. References: 1 Yan Aizhen, Li Hongsheng, Wang Mulanwan. Machine Tool Numerical Control Principles and Systems. Beijing: Machinery Industry Press, 1999. First author: Wang Mulan, Department of Industrial Automation, Nanjing Mechanical Engineering College (149, Postal Code: 210013, Tel: (025) 3499194. Click here to download the original text.