Diagnostic methods for CNC machine tools

The electrical fault diagnosis of CNC machine tools includes three stages: fault detection, fault diagnosis and isolation, and fault localization. The first stage of fault detection is to test the CNC machine tool to determine whether there is a fault; The second stage is to determine the nature of the fault and isolate the faulty components or modules; The third stage is to locate the fault to a replaceable module or printed circuit board to shorten repair time. In order to timely detect system faults, quickly determine the location of the fault and eliminate it in a timely manner, it is required that fault diagnosis should be as few and simple as possible, and the time required for fault diagnosis should be as short as possible. To achieve this, the following diagnostic methods can be used:
1. Intuitive method
Using sensory organs, pay attention to various phenomena when a malfunction occurs, such as whether there are sparks or bright lights during the malfunction, whether there is abnormal noise, where there is abnormal heat, and whether there is a burnt odor. Carefully observe the surface condition of each printed circuit board that may have malfunctioned, whether there are signs of burning or damage, to further narrow the inspection scope. This is the most basic and commonly used method.
2. Self diagnosis function of CNC system
Relying on the ability of the CNC system to quickly process data, multiple and fast signal acquisition and processing are carried out on the error location, and then the diagnostic program conducts logical analysis and judgment to determine whether the system has a fault and locate the fault in a timely manner. The self diagnosis function of modern CNC systems can be divided into the following two categories:
1) Boot self-diagnosis refers to the automatic execution of functional tests on modules such as CPU, memory, bus, I/O unit, printed circuit board, CRT unit, photoelectric reader, and floppy disk drive by the internal diagnostic program of the system from the start of each power on until it enters the normal operation preparation state, to confirm whether the main hardware of the system can work normally.
2) Fault information prompts that when a fault occurs during machine operation, the number and content will be displayed on the CRT display. According to the prompts, consult the relevant maintenance manual to confirm the cause of the malfunction and the troubleshooting method. Generally speaking, the richer the fault information provided by the diagnostic function of CNC machine tools, the more convenient it can be for fault diagnosis. However, it should be noted that some faults can be directly confirmed based on the fault content prompts and manual reference; However, the true cause of some faults does not match the fault content prompt, or there are multiple fault causes displayed on a fault display, which requires maintenance personnel to identify the internal connection between them and indirectly confirm the cause of the fault.
3. Data and Status Check
The self diagnosis of the CNC system can not only display fault alarm information on the CRT display, but also provide machine tool parameters and status information in the form of multiple pages of "diagnostic address" and "diagnostic data". Common data and status checks include parameter checks and interface checks.
1) Parameter inspection: The machine data of a CNC machine tool is an important parameter obtained through a series of experiments and adjustments, which ensures the normal operation of the machine tool. These data include gain, acceleration, contour monitoring tolerance, reverse clearance compensation value, and lead screw pitch compensation value. When subjected to external interference, data loss or confusion may occur, and the machine tool may not function properly.
2) Interface inspection: The input/output interface signals between the CNC system and the machine tool include the interface input/output signals between the CNC system and the PLC, as well as between the PLC and the machine tool. The input/output interface diagnosis of the CNC system can display the status of all switch signals on the CRT display, with "1" or "0" indicating the presence or absence of the signal. By using the status display, it can check whether the CNC system has output the signal to the machine side, and whether the switch signals on the machine side have been input to the CNC system, so as to locate the fault on the machine side or in the CNC system.
4. Alarm indicator light display malfunction
In the CNC system of modern CNC machine tools, in addition to the self diagnosis function and status display and other "software" alarms mentioned above, there are also many "hardware" alarm indicators distributed on devices such as power supply, servo drive, and input/output. Based on the indications of these alarm lights, the cause of the fault can be determined.
5. Board replacement method
Using spare circuit boards to replace templates with suspected faults is a fast and simple method to determine the cause of faults, commonly used in functional modules of CNC systems, such as CRT modules and memory modules. It should be noted that before replacing the backup board, the relevant circuits should be checked to avoid damage to the good board due to short circuits. At the same time, the selection switch and jumper wire on the test board should also be checked to ensure they are consistent with the original template. Some templates also need to pay attention to the adjustment of the potentiometer on the template. After replacing the memory board, the memory should be initialized according to the system requirements, otherwise the system will still not function properly.
6. Exchange method
In CNC machine tools, there are often modules or units with the same function. By exchanging the same modules or units with each other and observing the fault transfer, the fault location can be quickly determined. This method is commonly used for fault detection of servo feed drive devices, and can also be used for exchanging the same modules in CNC systems.
7. Percussion method
The CNC system is composed of various circuit boards, each with many solder joints, and any faulty soldering or poor contact may cause faults. When gently tapping a suspected circuit board, connector, or electrical component with insulation, if a fault occurs, it is likely that the fault is at the location of the tapping.
8. Measurement comparison method
For the convenience of detection, modules or units are equipped with detection terminals. By using instruments such as a multimeter and oscilloscope, the level or waveform detected by these terminals can be compared with the normal value at the time of the fault, and the cause and location of the fault can be analyzed. Due to the comprehensive and complex nature of CNC machine tools, there are multiple factors that can cause malfunctions. The above fault diagnosis methods sometimes require several simultaneous applications to comprehensively analyze the fault, quickly diagnose the fault location, and thus eliminate the fault. At the same time, some fault phenomena are electrical, but the cause is mechanical; On the contrary, it is also possible that the fault phenomenon is mechanical, but the cause is electrical; Or both. Therefore, its fault diagnosis often cannot be solely attributed to electrical or mechanical aspects, but must be comprehensively and comprehensively considered.