Example 1. Troubleshooting a blown fast-acting fuse on an incoming line.
Fault phenomenon: A horizontal machining center equipped with SIEMENS 8MC failed to start after a sudden power outage.
Analysis and troubleshooting process: Upon inspection, the fast-acting fuse of the X-axis servo drive of the machine tool was found to have blown. The feed system of this machine tool uses a SIEMENS 6RA series DC servo drive. Inspection of the servo motor and drive unit against the drive revealed no damage or short circuits in any components.
The machine tool's mechanical components were found to be functioning normally. After replacing the fuse, the machine tool was started and resumed normal operation. The cause was determined to be an intermittent fault caused by a sudden power outage.
Example 2. Repairing a fault in the SIEMENS 8MC measurement system
Fault Description: A horizontal machining center equipped with a SIEMENS 8MC automatically disconnects its hydraulic motor and displays an alarm message: Y-axis measurement system malfunction when the X-axis reaches a certain position. After power is turned off and then on again, the machine tool resumes normal operation, but the same fault may occur whenever the X-axis moves to a certain position.
Analysis and troubleshooting process: This machine tool is an imported horizontal machining center, equipped with a SIEMENS 8MC CNC system and a SIEMENS 6RA series DC servo drive. Because a Y-axis alarm occurred during X-axis movement, the X-axis measurement feedback cable was disconnected for testing to verify the system's correctness. The system then displayed an X-axis measurement system fault alarm, thus ruling out the possibility of a false alarm.
Upon inspection of the X-axis at and near the location where the alarm occurred, it was found that the alarm did not interfere with or affect the Y-axis measurement system (grating), and moving only the Y-axis did not trigger an alarm, indicating that the Y-axis was functioning normally. Further inspection of the Y-axis motor cable connector, grating reading head, and grating ruler revealed no abnormalities.
Considering that this equipment is a large machining center with numerous cables, and the cable length between the electrical cabinet and the machine tool is quite long, and all cables are fixed on cable racks and move back and forth with the machine tool, based on the above analysis, it is preliminarily determined that the partial cable breakage is most likely due to cable bending.
During maintenance, the X-axis was intentionally moved to the location of the fault, and the cable was manually moved. The connection of each feedback signal line on the Y-axis was carefully measured. It was eventually discovered that one of the signal lines occasionally became open-circuited during the continuous movement of the cable. After replacing the broken line with the spare line in the cable, the machine tool returned to normal.
Examples 3-4. Repairing a tracking error exceeding tolerance alarm caused by a driver malfunction.
Fault phenomenon: A CNC gear hobbing machine equipped with a SIEMENS PRIMOS system and a 6RA26** series DC servo drive system, after powering on, moves the Z-axis of the machine tool and the system generates an "ERR22 Follower error out of tolerance" alarm.
Analysis and troubleshooting process: When a CNC machine tool experiences a following error exceeding the alarm, the essence is that the actual machine tool cannot reach the commanded position. This fault is usually caused by a servo system malfunction or a fault in the machine tool's mechanical transmission system.
Because the machine tool's servo feed system is a fully closed-loop structure, it is impossible to conduct tests by disconnecting the motor from the mechanical parts. To identify the fault location, during maintenance, the Z-axis lead screw was manually rotated with the machine tool powered off and the clamping mechanism released. No abnormalities were found in the mechanical transmission system, leading to the preliminary conclusion that the fault was caused by a malfunction in the servo system or CNC device.
To further pinpoint the fault location, during repair, with the system connected, the Z-axis was moved slightly using the handwheel (the movement distance should be controlled within the system's maximum allowable following error to prevent following error alarms). The speed command voltage of the Z-axis DC driver was measured. Inspection revealed a voltage input to the speed command, the value of which was related to the distance and direction of the handwheel movement. This confirmed that the CNC device was functioning normally, and the fault was caused by a malfunction in the servo driver.
Inspection of the driver revealed that the driver's status indicator light had no alarm, which essentially ruled out a fault in the driver's main circuit. Considering that the X and Z axis drivers of this machine tool are of the same model, the fault was confirmed to be on board A2 of the 6RA26** DC driver by swapping the control boards of the drivers one by one.
Based on the schematic diagram of the SIEMENS 6RA26** series DC servo driver, each level of signal was checked and measured one by one. Finally, it was confirmed that the fault was caused by a malfunction of the integrated voltage comparator N7 (model: LM348) on the A2 board. After replacement, the machine tool returned to normal.
Example 4. Fault phenomenon: An imported horizontal machining center equipped with a SIEMENS 850 system and a 6RA26** series DC servo drive system, after being powered on, when the X-axis is manually moved, the X-axis worktable of the machine tool does not move, and the CNC displays an X-axis following error error exceeding the tolerance alarm.
Analysis and troubleshooting process: Since other axes of the machine tool were working normally, the X-axis driver had no alarms, and all status indicator lights showed no faults, in order to determine the fault location, considering that the speed/current adjustment board A2 of the 6RA26** series DC servo driver is the same, the A2 board of the X-axis driver and the A2 board of the Y-axis driver were swapped during the repair test. The test revealed that the X-axis could work normally, but the Y-axis exhibited a follow-tolerance alarm.
Based on this phenomenon, it can be concluded that the speed/current regulator board of the X-axis driver is faulty. According to the schematic diagram of the SIEMENS 6RA26** series DC servo driver, measurement and inspection revealed that when the X-axis is moved slightly, there is an analog input between the speed setpoint input terminals 57 and 69 of the driver. The analog voltage of the speed is correct when the driver's detection terminal B1 is measured, but the output of pin 6 of the speed proportional regulator N4 (LM301) is always 0V.
Check each component against the schematic diagram: feedback resistors R25, R27, and R21 of the speed regulator LM301; offset adjustment resistors R10, R12, R13, R15, R14, and R12; input protection diodes V1 and V2 of the LM301; input filter components R1, C1, R20, and V14; and peripheral components R27, R28, R8, R3, C5, and R4 of the speed feedback filter. Confirm that all components are fault-free.
Therefore, the fault was confirmed to be caused by a malfunction in the LM301 integrated operational amplifier; after replacing the LM301, the machine tool returned to normal operation and the fault was eliminated.
Example 5. Repairing a CNC malfunction causing an out-of-tolerance alarm for following error.
Fault phenomenon: A CNC gear hobbing machine equipped with a SIEMENS PRIMOS system and a 6RA26** series DC servo drive system, after powering on, moves the Z-axis of the machine tool and the system generates an "ERR22 Follower error out of tolerance" alarm.
Analysis and troubleshooting process: The fault analysis process is the same as the previous example, but in this example, when the Z-axis is moved slightly by handwheel, the speed set voltage of the Z-axis DC driver is always 0. Therefore, it can be preliminarily determined that the fault is in the CNC device or the connection cable between the CNC and the driver.
The cable connection between the CNC unit and the driver was checked and found to be normal, confirming that the fault was caused by the CNC unit. Upon opening the CNC unit for inspection, it was discovered that the digital input of the D/A converter for the Z-axis speed setpoint output was correct, but there was no analog output, thus confirming that the fault was caused by a malfunctioning D/A converter.
After replacing the 12-bit D/A converter DAC0800 with the Z-axis speed setpoint output, the machine tool recovered.
Example 6. Fault phenomenon: A CNC gear hobbing machine equipped with a SIEMENS PRIMOS system and a 6RA26** series DC servo drive system will trigger an "ERR21, Y-axis measurement system error" alarm after powering on.
Analysis and processing: The reasons for measurement system alarms in CNC systems are generally as follows:
1) The position feedback signal interface circuit of the CNC device is faulty.
2) The connecting cable between the CNC device and the position detection component is faulty.
3) The position measurement system itself is faulty.
Because this machine tool's servo drive system uses a fully closed-loop structure, and the detection system uses a HEIDENHAIN grating, to determine the fault location, the X and Y axis speeds output by the CNC device were first set during repair. The drive enable and X and Y axis position feedback were then swapped, so that the CNC's X-axis output controlled the Y-axis, and vice versa. After this swap, operating the CNC system and manually moving the Y-axis resulted in movement of the machine tool's X-axis, and the machine functioned normally, proving that the CNC device's position feedback signal interface circuit was fault-free.
However, when operating the CNC system and manually moving the X-axis, the machine tool's Y-axis did not move, and the CNC displayed an "ERR21, X-axis measurement system error" alarm. This confirmed that the alarm was caused by a malfunction in the position measurement system and was unrelated to the CNC device's interface circuit. Checking that the measurement system cable connections were correct and reliable ruled out cable connection issues.
Using an oscilloscope to inspect the output waveforms of Ua1 and Ua2, and *Ua1 and Ua2 of the preamplifier EXE601/5-F in the position measurement system, it was found that there was no output on phase Ua1. Further inspection of the grating output (input of the preamplifier EXE601/5-F) signal waveform revealed no signal input on Ie1. After verifying that the grating was installed correctly, it was confirmed that the fault was caused by a faulty grating. Replacing the grating LS903 restored normal operation of the machine tool.
Example 7. Fault phenomenon: A CNC gear hobbing machine equipped with a SIEMENS PRIMOS system and a 6RA26** series DC servo drive system will trigger an "ERR21, X-axis measurement system error" alarm after powering on.
Analysis and troubleshooting process: The fault analysis process is the same as the previous example. However, in this case, the output waveforms of Ual and Ua2, *Ual and *Ua2 of the preamplifier EXE601/5-F in the position measurement system were checked using an oscilloscope. It was found that Ual had no output. Further inspection of the grating output (input of the preamplifier EXE601/5-F) signal waveform revealed Ie1, indicating that the signal input was correct. This confirmed that the fault was caused by a malfunction in the preamplifier EXE601/5-F.
Based on the principle of EXE601/5-F (details below), the signals of the EXE601/5-F preamplifier were measured step by step, and it was found that one of the LM339 integrated voltage comparators was faulty; after replacement, the machine tool returned to normal operation.
Example 8. Troubleshooting a drive that is not ready
Fault phenomenon: A horizontal machining center equipped with a SIEMENS 850 system and a 6RA26** series DC servo drive system suddenly stopped during machining. The "drive fault" indicator light on the rear panel lit up after powering on, and the machine tool could not start normally.
Analysis and troubleshooting process: Based on the phenomenon that the "drive fault" indicator light on the panel is on, and combined with the analysis of the machine tool's electrical schematic diagram and the system PLC program, it was confirmed that the cause of the machine tool's fault was that the Y-axis driver was not ready.
Upon inspecting the drive unit inside the electrical cabinet, the power input of the 6RA26** drive's main circuit was measured. Only phase V had voltage. Further inspection against the machine tool's electrical schematic revealed that phases U and W of the 6RA26** drive's input fast-acting fuse were blown. Using a multimeter to measure the 1U and 1W input terminals of the drive's main circuit confirmed an internal short circuit in the drive's main circuit.
Since the main circuit input of the 6RA26** AC driver is directly connected to the thyristor, it can be confirmed that the fault is caused by damage to the thyristor.
The main circuit thyristors V1-V6 were measured one by one, and it was confirmed that V1 and V2 were faulty (short-circuited); after replacing them with spare parts of the same specification, the machine tool returned to normal.
Since there were no other faults in the driver, the machine tool returned to normal operation after the thyristor module was replaced. The cause of the fault was likely an occasional failure caused by instantaneous voltage fluctuations or load fluctuations.
Example 9. Troubleshooting an external fault that causes the motor to stop turning.
Fault phenomenon: During tool changing, the tool magazine of an imported vertical machining center equipped with a SIEMENS 6M system failed to rotate normally.
Analysis and processing: Through analysis of the machine tool's electrical schematic diagram, it was found that the tool magazine rotation control of this machine tool adopts the 6RA** series DC servo drive, and the tool magazine speed is controlled by the "tool magazine setpoint conversion/positioning control" board manufactured by the machine tool manufacturer.
On-site analysis and observation of the tool magazine's rotation revealed that while the PLC's rotation signal was input and the tool magazine's mechanical latch was pulled out, the analog input to the 6RA26** driver was not being input. Since this analog output originated from the "Tool Magazine Setpoint Conversion/Positioning Control" board, step-by-step measurements were taken using the schematic provided by the machine tool manufacturer. Ultimately, it was discovered that the analog switch (model DG201) on this board was damaged. After replacing it with a spare of the same model, the machine tool returned to normal operation.
Example 10. Troubleshooting a motor that rotates at high speed immediately upon startup.
Fault phenomenon: A machine tool of the same model as Example 268, during startup and debugging, caused the tool magazine to rotate at high speed after the tool magazine rotation button was manually pressed, resulting in a machine tool alarm.
Analysis and troubleshooting process: Based on the fault symptoms, it can be preliminarily determined that the fault is caused by incorrect polarity of the speed feedback of the tool magazine DC driver or a loose speed feedback wire, resulting in positive feedback or open loop in the speed loop. Measurement confirmed that the servo motor speed feedback wire was connected, but the polarity was incorrect; after swapping the speed feedback polarity, the tool magazine operation returned to normal.
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