Analysis and Troubleshooting of Several Common PLC Faults in CNC Machine Tools
Abstract: Several common faults occur in CNC machine tool PLCs during operation. The following corresponding solutions are provided to ensure normal equipment operation. I. Excessive Power Grid Fluctuations Cause PLC Malfunction: This manifests as no output from the PLC. First, check the input signals (power signals, interference signals, command signals, and feedback signals). For example, in a CNC lathe using the SINUMERIK 3G-4B system, the built-in PLC may not function. Using observation, first check the power grid voltage waveform with an oscilloscope. Excessive voltage fluctuations and undervoltage noise jumps lasting longer than 1 second are observed. Since the machine tool is in the commissioning phase, faults within the unit system should be ruled out. Based on internal power grid interference measures (filtering, isolation, voltage stabilization), conventional power supply systems cannot disconnect or filter out...
The following are some common faults that may occur during the operation of CNC machine tool PLCs. In the course of work, the following corresponding solutions should be taken to ensure that the equipment operates normally.
1. PLC malfunctions due to excessive power grid fluctuations.
The problem manifests as no PLC output. First, check the input signals (power signals, interference signals, command signals, and feedback signals). For example, in a CNC lathe using the SINUMERIK 3G-4B system, the built-in PLC may not function. Using observation, first check the mains voltage waveform with an oscilloscope. It was found that the mains voltage fluctuations were too large, and the undervoltage noise transition duration was greater than 1 second. Since the machine tool was in the commissioning phase, component failures within the unit system should be ruled out. Based on the internal power grid interference measures (filtering, isolation, voltage stabilization), it was determined that the conventional power supply system could not cut off or filter out the prolonged undervoltage noise. This was due to insufficient anti-power grid measures (internal factor), causing the PLC to malfunction.
Adding an AC voltage regulator to the system power input terminal resulted in normal PLC operation.
II. Electromagnetic Interference
Electromagnetic interference faults often occur during the commissioning phase of new machine tools, resulting in frequent machine shutdowns despite the machine still being operational. This rule out parameter inconsistencies and internal component issues. The likely cause is electromagnetic interference from the power grid or environment, leading to system instability. External causes include inductive interference from the frequency converter. This can be due to undersized components or excessively large power grid interference pulses causing magnetic saturation in the filter's internal inductors, rendering them unable to filter out high-frequency interference pulses.
Connecting a 2.2mF capacitor in parallel between the system power input lines adds an absorption network, thus eliminating the fault.
III. PLC-MD parameter faults
This fault occurred during the commissioning phase. During the zero-return operation, the machine could only move in the negative direction of the coordinate axis; normal movement triggered an overtravel alarm. For example, on a machining center with a FANUC 0M system, after powering on, when performing a return-to-reference-point (zero-return) operation, the feed axis moved a certain distance in the positive direction before triggering an overtravel alarm, even though the limit switch was not actually triggered. The alarm could not be cleared by the "reset method." Powering off and then on again did not resolve the fault, indicating that the alarm was essentially a soft overtravel. Since the machine tool was in the commissioning phase, the possibility of a false overtravel caused by a hardware malfunction could be ruled out.
First, check if the parameter setting table is disordered, then use the parameter modification method. Method 1: Turn off the alarm soft key, perform a zeroing operation, and then turn the alarm soft key ON again; Method 2: Temporarily modify the soft limit parameter (143) to +999999, perform a zeroing operation, and then restore the original parameter value. Both methods can eliminate the fault after restarting.
Note: If an overtravel alarm is triggered by hitting the limit switch during actual zeroing operation, the "reset method" is not allowed to prevent further impact and damage to the machine tool's accuracy.
IV. PLC input board malfunction
PLC input board malfunctions often occur during automatic machining, resulting in an alarm displayed on the CRT and the machine tool failing to operate. For example, on a FANUC 3T-A system CNC machine tool during automatic machining, the CRT displays "NOT READY," the machine tool cannot function properly, and the alarm message indicates that the main control board (whether CNC or PLC) is functioning correctly, without any specific alarm content. A fault in the servo amplifier or PLC is suspected. However, if the servo amplifier is faulty, it should trigger an alarm during self-diagnosis. Therefore, the fault can be located in the PLC's I/O interface board. This type of fault is a hardware fault, primarily caused by a hardware failure in the circuitry preceding the "NOT READY" output.
The fault was eliminated after the parts were replaced.
