Qu Haibo, from the Industrial Technology Department of Weihai Vocational and Technical College, notes that the detection devices in the servo unit are a common source of failure in CNC machine tool equipment. This article, based on maintenance practice, points out common faults and repair methods for detection devices, as well as precautions for daily maintenance. For CNC systems, the servo unit is the most prone to failure. This is because the movement of each axis is achieved by the servo unit controlling the servo motor to drive the ball screw. The detection device is a crucial component of the CNC machine tool servo system, responsible for detecting the displacement and speed of each control axis. It feeds back the detected signals, forming a closed-loop system. Normal measurement methods can be divided into indirect and direct measurements. Indirect measurement uses rotary detection devices to measure the linear displacement of the machine tool. Commonly used indirect detection devices include pulse encoders, rotary transformers, circular induction synchronizers, circular gratings, and circular magnetic gratings. Direct measurement uses linear detection devices to measure the linear displacement of the machine tool. Commonly used detection devices include linear induction synchronizers, magnetic rulers, laser interferometers, and metrological gratings. In actual use, wear and contamination frequently cause detection element failures, preventing the servo system from driving the machine tool normally. 1 Common Faults and Repairs of Detection Devices When the machine tool exhibits the following fault phenomena, the first consideration should be whether it is caused by a fault in the detection device, and the fault location should be correctly analyzed and found. 1.1 Mechanical Oscillation (During Acceleration/Deceleration) Common causes of this type of fault include: (1) The pulse encoder is faulty. At this time, the focus should be on checking whether the voltage on the feedback line terminal of the speed detection unit drops at certain points. If there is a drop, it indicates that the pulse encoder is faulty and the encoder should be replaced. (2) The pulse encoder cross coupling may be damaged, causing the shaft speed to be out of sync with the detected speed. Replace the coupling. (3) The tachogenerator is faulty. Repair and replace the tachogenerator. In maintenance practice, there are many faults such as brush wear and jamming in the tachogenerator. The tachogenerator should be disassembled, the brushes should be carefully removed, polished a few times on fine sandpaper, and the dirt on the commutator should be cleaned at the same time before reassembling. 1.2 Abnormally fast mechanical movement (runaway) When troubleshooting this type of fault, in addition to checking the operation of the position control unit and speed control unit, the following should also be checked: (1) Whether the pulse encoder wiring is incorrect, check whether the encoder wiring is positive feedback, and whether phase A and phase B are reversed. (2) Whether the pulse encoder coupling is damaged, and replace the coupling if damaged. (3) Check whether the tachogenerator terminals are reversed and whether the excitation signal line is connected incorrectly. 1.3 The spindle cannot move in a directional direction or the directional movement is not in place When troubleshooting this type of fault, in addition to checking the setting and adjustment of the directional control circuit, checking the directional board, and adjusting the spindle control printed circuit board, the position detector (encoder) should be checked for malfunction. At this time, the output waveform of the encoder should generally be measured, and the quality of the encoder can be judged by judging whether the output waveform is normal. (Maintenance personnel should pay attention to recording the normal output waveform of the encoder when the equipment is normal, so as to check it when there is a fault.) 1.4 Vibration during coordinate axis feed When checking for vibration, check whether the motor coil is short-circuited, whether the mechanical feed screw is connected to the motor well, and whether the entire servo system is stable. Check whether the pulse encoder is good, whether the coupling connection is smooth and reliable, and whether the tachometer is reliable. 1.5 NC error alarm NC alarm is an alarm caused by program error or operation error. For example, NC alarm 090.091 of FAUNUC 6ME system. The occurrence of NC alarm may be caused by main circuit failure and too low feed speed. At the same time, it may also be caused by: (1) faulty pulse encoder. (2) pulse encoder power supply voltage is too low (at this time, adjust the power supply voltage to 15V so that the voltage value on the +5V terminal of the main circuit board is within 4.95-5.10V). (3) no input pulse encoder one-turn signal and therefore cannot perform reference point return normally. 1.6 Servo System Alarms When a servo system malfunctions, the following alarm numbers often appear: For example, servo alarms of the FAUNUC 6ME system: 416, 426, 436, 446, 456. Servo alarm of the SIEMENS 880 system: 1364. Servo alarm of the SIEMENS 8 system: 114, 104, etc. At this time, pay attention to checking: (1) The feedback signal of the shaft pulse encoder is disconnected, short-circuited, or lost. Use an oscilloscope to measure the A-phase and B-phase one-turn signals to see if they are normal. (2) The encoder has an internal fault, which causes the signal to be unable to be received correctly. Check if it is contaminated, too dirty, deformed, etc. 2 Analysis of Common Fault Repair Examples of Detection Devices Our institute currently has more than 30 CNC equipment. From the faults that occur during use, the faults of detection devices account for a large proportion. The following is an analysis of several typical faults. Fault 1: Damage to the photoelectric disk of the pulse encoder leads to a large error in the processing dimensions of the workpiece. Fault Phenomenon: When the CNC 862 CNC 20 lathe is cutting parts in the X direction, the size error is large, reaching 0.32mm/250mm, and there is no alarm display on the CRT. Fault Analysis: The X and Z axes of this machine tool are driven by DC servo motors controlled by servo units, and photoelectric pulse encoders are used for position detection. According to the analysis, the reasons for the machining size error are generally: (1) There is a large gap between the ball screw and the nut in the X direction or the bearing connecting the motor and the screw is damaged, resulting in an error between the actual stroke and the detected size; (2) The measurement circuit is faulty. Fault Solution: According to the above analysis, after inspection, it was found that the gap between the screw and the nut was normal, the bearing was also fine, the cable connection and connector of the measurement circuit were good, and finally we used an oscilloscope to check the encoder detection signal, and the waveform was abnormal. After removing the encoder, it was found that three transparent points appeared in the opaque part of the photoelectric disk for some unknown reason, causing the detection signal error. The encoder was replaced and the problem was solved. Because the self-diagnostic function of the CNC 862 system is not particularly strong, when such a fault occurs, the machine tool will not stop and there will be no NC alarm display: Fault 2: The A phase signal of the pulse encoder is incorrect, causing vibration of the axis movement. Fault phenomenon: The X-axis of the FAUNUC 6ME system double-sided machining center vibrates during movement and NC416 alarm appears on the CRT. Fault analysis: Based on the fault phenomenon, we analyze that the causes of the fault may be as follows: (1) The speed control unit is faulty; (2) The position detection circuit is faulty; (3) The connection of the pulse encoder feedback cable is poor; (4) The pulse encoder is faulty; (5) The servo motor and tachometer are faulty; (6) The machine tool data is not correct. Troubleshooting: Based on the analysis above, the connections and wiring of the speed control unit, main circuit board, and pulse encoder feedback cable were checked and found to be normal. The machine tool data was normal. Then, the motor was disconnected from the mechanical parts, and the motor was rotated by hand to observe the status of diagnostic code 713. The diagnostic code 713 is as follows: 713.3 is the X-axis pulse encoder feedback signal; if there is a disconnection, this bit will be 1. 713.2 is the X-axis encoder feedback signal for one revolution. 713.1 is the X-axis pulse encoder B-phase feedback signal. 713.0 is the X-axis pulse encoder A-phase feedback signal. When 713.2, 713.1, and 713.0 are normal, the motor rotation should show a continuous change between "0" and "1". When the motor was rotated, it was found that the 713.0 signal was only "0" and did not change to "1". We then used an oscilloscope to check the A-phase, B-phase, and one-revolution signals of the pulse encoder and found that the A-phase signal was abnormal. Therefore, based on the above checks, it was determined that the shaft adjustment pulse encoder was faulty. After replacing it with a new encoder, the fault was resolved. Fault 3: Pulse encoder contamination causing axis positioning failure. Fault symptom: A SIEMENS 880 horizontal machining center's worktable malfunctioned during rotary positioning, causing an interruption in operation. The CRT displayed alarm number 1364 with the message "1364 ORD 4B2 measuring System Dirty," indicating contamination of the measuring system. Troubleshooting: Based on the alarm message, we first removed the detection circuit board and feedback cable connector, cleaning them with alcohol to remove dust and oil. Starting the worktable did not resolve the issue. We then removed the pulse encoder from the worktable and found it filled with a large amount of machine oil. It turned out there was a compressed air path leading to the encoder, which blows out dust and cleans the encoder. This machine oil was brought in by the unclean compressed air. We cleaned the oil with gasoline and improved the compressed air quality. After reinstalling the encoder, starting the worktable resolved the fault. Fault 4: Broken closed-loop circuit detection signal line causing control axis malfunction. Fault Phenomenon: During operation, a SIEMENS 8 system horizontal machining center suddenly stopped running, and the CRT displayed NC alarm 104. Powering off and restarting cleared the alarm, and the machine returned to normal. However, the same fault recurred shortly afterward. Fault Analysis: Checking the NC 104 alarm revealed the following: X-axis measurement closed-loop cable breakage and short circuit, signal loss, incorrect threshold signal, incorrect frequency signal. The X, Y, and Z axes of this machine tool use linear encoders to detect machine displacement and form a feedback control closed-loop system. Troubleshooting: Based on the fault symptoms and alarm, we first checked the reading head and grating ruler. The grating ruler was well sealed and clean inside. The reading head and grating ruler were not contaminated by oil or dust and were normal. Then we checked the differential amplifier and measuring circuit board. No problems were found after the inspection. After these tasks, we focused on the feedback cable and measured the feedback terminal. We found that the voltage of line 13 was unstable. After powering off, we measured line 13 and found that there was a large resistance. After careful inspection, we found that a section of the line was about to break in the X direction as it moved with the guide rail. It was partially connected, causing the feedback value to be unstable and deviating from its actual value, resulting in the motor losing steps. After reconnecting the broken wire, the machine tool was started and the fault was eliminated. 3 Daily maintenance of detection devices Detection devices are extremely precise and easily damaged devices. It is necessary to use and maintain them correctly in a timely manner. The following aspects should be noted when maintaining them. (1) The rated power supply voltage must be the rated value and the working environment temperature must not exceed the standard to ensure the normal operation of the integrated circuits and electronic components of the system. (2) Avoid strong vibration and friction to prevent damage to the code board, and avoid dust and oil contamination to avoid affecting the output of normal signals. (3) Avoid external power supply and noise interference, and ensure good shielding to avoid affecting the feedback signal. (4) Ensure that the resistance and capacitance of the feedback connection line are normal to ensure the transmission of normal signals. (5) The installation method of each component must be correct. For example, the encoder connection shaft must be concentric and aligned to prevent the shaft from exceeding the allowable load to ensure its normal performance. 4 Conclusion In the failure of CNC equipment, the failure of detection device accounts for a considerable proportion. Maintenance experience must be continuously accumulated. When a failure occurs, as long as the problem is correctly and thoroughly analyzed, the failure can be solved in time. If timely and correct maintenance is carried out on a daily basis, the failure rate of the equipment can be reduced and the normal operation of the equipment can be guaranteed. Proceedings of the Second Servo and Motion Control Forum Proceedings of the Third Servo and Motion Control Forum