Analysis and maintenance of six common faults of CNC machine tool plc

1. Electromagnetic interference

Electromagnetic interference faults often occur during the commissioning stage of new machine tools. The machine tool stops frequently, but it can work. Therefore, it can be ruled out that the parameters are confused and the internal causes of components are caused. Instability, the external cause is the frequency conversion inductive interference source. This is because the capacity of the selected components is too small, and the excessive power grid interference pulse causes magnetic saturation of the inductance element inside the filter, and cannot filter out the high-frequency interference pulse.

Connect a 2.2mF capacitor in parallel between the system power supply input lines, that is, add a absorption network, and the fault is eliminated.

2. The PLC does not work when the power grid fluctuates too much

The performance is that the PLC has no output. First check the input signal (power signal, interference signal, command signal and feedback signal). For example, the built-in PLC of the CNC lathe using SINUMERIK 3G-4B system cannot work. Using the observation method, check the voltage waveform of the power grid with an oscilloscope first, and find that the power grid fluctuates too much, and the duration of the undervoltage noise jump is greater than 1s (external cause). Since the machine tool is in the debugging stage, component failures in the power system should be excluded. From the internal anti-grid interference measures (filtering, isolation and voltage stabilization), it can be seen that the conventional power system can no longer isolate or filter out the grid undervoltage that lasts too long. Noise, which is caused by insufficient anti-grid measures (internal cause), causes the PLC to fail to obtain normal power input and fail to work.

Add an AC voltage regulator to the input of the system power supply, and the PLC works normally.

3. PLC device failure

This failure often prevents the indexing plate of the upper table from falling back on the machining center. For example, the table of the Hungarian MKC500 horizontal machining center using the SIMENS 820 system does not fall back, the CRT7035 alarms, and the program is interrupted, which is a hardware failure.

The failure of the index plate of the workbench to fall is related to the proximity switches SQ28 and SQ25 for detecting the rotation of the index plate of the lower workbench and the fall of the workbench. The corresponding PLC input interfaces are E10.6 and E10.0 respectively. Observed from the PLC STATUS (status), E10.6 is “1”, indicating that the index plate of the workbench is rotated in place, and E10.0 is “0”, indicating that the index plate of the workbench has not fallen, and the fall of the workbench is caused by The output interface Q4.7 drives the solenoid valve YS06 through the relay KA32 to complete the action, and then observe that Q4.7 is “0”, the relay KA32 is not powered, the solenoid valve YS06 does not act, so the table index plate does not fall back, and an alarm is generated.

Manual solenoid valve YS06, force it to reset, the index plate of the workbench falls back, and the fault is eliminated.

4. PLC-MD parameter fault

This fault often occurs in the debugging stage. During the zero return operation, it can only move along the negative direction of the coordinate axis, and an overtravel alarm will appear when moving in the positive direction. For example: a machining center of FANUC 0M system, Return to the reference point after power-on (zero return operation), the overtravel alarm occurs after the feed axis moves forward for a certain distance, but the travel switch is not actually touched. The “reset method” cannot eliminate the alarm. After the power failure, the power is turned on again, and the fault remains the same, indicating that the essence of the alarm is soft overtravel. Since the machine tool is in the debugging stage, the possibility of false overtravel caused by hardware failure can be ruled out.

First check whether the parameter setting table is confused (may be due to environmental electromagnetic interference), and then use the parameter modification method, method ① Turn off (OFF) the alarm soft key, and restore the alarm soft key to ON after performing zero return operation; method ② Temporarily modify the soft limit parameter (143) to the maximum value +999999, and restore the original parameter value after the zero return operation. Two processing methods, after rebooting, can troubleshoot.

Attention: In the actual zero return operation, the travel switch is hit and an overtravel alarm is generated. At this time, the “reset method” is not allowed to prevent the machine tool from being damaged by another collision.

5. Crossover address error

This error often occurs in CNC machine tools after maintenance. For example, the SIMENS 8M system CNC machine tool after maintenance has a shutdown failure, and the red light on the built-in PLC template lights up, indicating that “PLC stops working”. It can be determined that the PLC module is faulty, which is a hardware fault. Check the maintenance records and know that the sub-module on the PLC has been replaced in the last maintenance, so it is suspected that it may be caused by installation and wiring. Find the installation address of the sub-module according to the model and technical manual of the sub-module to be replaced. However, the board requires a “Chuan”-shaped jumper code to be inserted into the specified socket position (corresponding address), and it was found from the scene that the jumper code was inserted into the socket in the wrong position. The wrong jumper address should be the cause of the alarm shutdown failure.

Correct the bridge address, troubleshooting.

It should be noted that if the fault occurs during the debugging stage of the new machine tool, it is difficult to locate the fault immediately. And because the PLC itself does not work, it is impossible to use the PLC program method. At this point, an automatic programmer (such as PG675) can be used to check the interrupt stack of the sub-module to find out the program module (number) and error address on which the PLC stops.

6. PLC input board failure

PLC input board failure often occurs during automatic processing, an alarm is displayed on the CRT, and the machine tool cannot work. For example: when the FANUC 3T-A system CK7815/1 CNC lathe is automatically processed, “NOT READY” is displayed on the CRT, and the machine tool cannot work normally. The alarm display shows that the main body of the main control board, whether CNC or PLC, is good, and there is no specific content of the alarm. Suspect a malfunction in the servo amplifier or PLC. However, if the servo amplifier is faulty, it should report to the police when self-diagnosis is started. Therefore, the fault can be located on the I/O interface board of the PLC, which is a hardware fault. If it is the power failure of the PLC, it is impossible to have the PLC alarm text display. Therefore, the main cause should be the hardware failure of the front link circuit corresponding to the output “NOT READY”.
Replacement, troubleshooting.

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