In CNC machine tools, most of the faults are available for investigation, but there are also some faults. The alarm information provided is vague or even no alarm at all, or the period of occurrence is long, irregular, and irregular, which brings to the search and analysis Many difficulties. For such machine tool failures, it is necessary to analyze the specific conditions and perform patient search. In addition, comprehensive knowledge of machinery, electricity, hydraulics, etc. is particularly required during the inspection, otherwise it is difficult to quickly and correctly find the real cause of the failure.
Abnormal machining accuracy failures: system parameters change or change, mechanical failures, machine tool electrical parameters are not optimized, abnormal motor operation, abnormal machine tool position loops or incorrect control logic are common causes of abnormal machining accuracy failures of CNC machine tools in production. Find out the relevant If the fault point is dealt with, the machine tool can return to normal. In production, we often encounter faults with abnormal machining accuracy of CNC machine tools. Such faults are highly concealed and difficult to diagnose.
There are five main reasons for this type of failure:
1. The feed unit of the machine tool is changed or changed;
2. The zero offset (NULLOFFSET) of each axis of the machine tool is abnormal;
3. The axial backlash (BACKLASH) is abnormal;
4. The motor running status is abnormal, that is, the electrical and control parts are faulty;
5. Mechanical failure, such as screw rod, bearing, shaft coupling and other parts.
In addition, the preparation of the processing program, the selection of tools and human factors may also cause abnormal processing accuracy.
If the machining accuracy is abnormal due to mechanical failure, the following aspects should be checked one by one.
1. Check the machining program segment that is running when the accuracy of the machine tool is abnormal, especially the tool length compensation, the proofreading and calculation of the machining coordinate system (G54～G59).
2. In the jog mode, move the Z axis repeatedly, and diagnose the motion state by sight, touch, and listen. It is found that the sound of Z-direction movement is abnormal, especially when the jog is fast, the noise is more obvious. Judging from this, there may be hidden dangers in machinery [1].
Troubleshooting
1. Initialization reset method: Under normal circumstances, system alarms caused by instantaneous faults can be cleared by hardware reset or switch system power in turn. If the system working storage area is lost due to power failure, unplugging the circuit board or battery undervoltage, it will cause confusion , The system must be initialized and cleared. Before clearing, you should make a record of data copy. If the fault cannot be eliminated after initialization, perform hardware diagnosis.
2. Parameter modification and program correction method: System parameters are the basis for determining system functions, and parameter setting errors may cause system failures or invalid functions. Sometimes due to user program errors can also cause failures to stop, this can be checked by the system's block search function to correct all errors to ensure its normal operation.
3. Adjustment and optimization adjustment method: Adjustment is the simplest and most feasible method. Correct the system failure by adjusting the potentiometer. For example, during maintenance in a factory, the system display screen is chaotic, and it is normal after adjustment. For example, in a factory, belt slippage occurs when the main shaft starts and brakes. The reason is that the main shaft load torque is large, and the ramp-up time of the drive device is set too small, which is normal after adjustment.
Optimal adjustment is a comprehensive adjustment method to systematically achieve the best match between the servo drive system and the mechanical system being dragged. The method is very simple. Use a multi-line recorder or a dual-track oscilloscope with storage function, respectively Observe the response relationship between the command and the speed feedback or current feedback. By adjusting the proportional coefficient and integral time of the speed regulator, the servo system can achieve the best working condition with high dynamic response characteristics without oscillation. In the absence of an oscilloscope or recorder on site, based on experience, adjust to make the motor vibrate, and then slowly adjust in the reverse direction until the vibration is eliminated.
4. Spare parts replacement method: replace the faulty circuit board with a good spare part, and do the corresponding initial startup, so that the machine tool can be quickly put into normal operation, and then the broken board is repaired or repaired. This is the most commonly used troubleshooting method.
5. Method of improving power quality: Regulated power supply is generally used to improve power supply fluctuations. Capacitor filtering method can be used for high frequency interference, through these preventive measures to reduce the failure of the power board.
6. Maintenance information tracking method: Some large manufacturing companies constantly modify and improve system software or hardware based on accidental failures caused by design defects in actual work. These modifications are continuously provided to maintenance personnel in the form of maintenance information. Using this as a basis for troubleshooting, the fault can be eliminated correctly and thoroughly.
diagnosis method
The electrical fault diagnosis of CNC machine tools has three stages: fault detection, fault judgment, isolation and fault location. 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 component or module; the third stage is to locate the fault to a replaceable module or print Circuit board to shorten repair time. In order to find the fault in the system in time, quickly determine the location of the fault and eliminate it in time, 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 this end, the following diagnostic methods can be used:
1. The intuitive method
Use the sensory organs to pay attention to various phenomena when the malfunction occurs, such as whether there is spark or bright light during the malfunction, whether there is abnormal sound, where is abnormal heating, and whether there is burning smell, etc. Carefully observe the surface condition of each printed circuit board that may fail, whether there are burnt and damage marks, to further narrow the scope of inspection, this is one of the most basic and most commonly used methods.
2. Self-diagnosis function of CNC system
Relying on the ability of the CNC system to quickly process data, multi-channel and rapid signal acquisition and processing of the error location, and then logical analysis and judgment by the diagnostic program, to determine whether the system is faulty, and to locate the fault in time. The self-diagnosis function of modern CNC system can be divided into the following two categories:
1) Power-on self-diagnosis Power-on self-diagnosis means that from the start of each power-on to the normal operation preparation state, the internal diagnostic program of the system is automatically executed for the CPU, memory, bus, I/O unit and other modules, printed circuit boards, CRT unit, photoelectric reader and floppy disk drive and other equipment before operating the functional test to confirm whether the main hardware of the system can work normally.
2) Failure message prompt When a failure occurs during the operation of the machine tool, 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 failure and the troubleshooting method. Generally speaking, the richer the fault information prompted by the CNC machine tool diagnostic function, the more convenient it will be for fault diagnosis. However, it should be noted that some faults can directly confirm the cause of the fault according to the fault content prompt and refer to the manual; while the real cause of some faults does not match the fault content prompt, or a fault shows multiple fault causes, which requires maintenance personnel to Find out the internal connection between them and indirectly confirm the cause of the failure.
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 parameter and status information in the form of multiple pages of "diagnostic address" and "diagnostic data". Common data and status checks include parameter checking and Two kinds of interface checks.
1) Parameter check The machine data of CNC machine tools is an important parameter obtained after a series of tests and adjustments, and it is a guarantee for the normal operation of the machine tool. These data include gain, acceleration, contour monitoring tolerance, backlash compensation value and screw pitch compensation value. When subjected to external interference, data will be lost or chaotic, and the machine tool will not work normally.
2) Interface check The input/output interface signals between the CNC system and the machine tool include the input/output signals between the CNC system and the PLC, and between the PLC and the machine tool. The input/output interface diagnosis of the CNC system can display the status of all digital signals on the CRT display. Use "1" or "0" to indicate the presence or absence of the signal. Use the status display to check whether the CNC system has output the signal to the machine tool. Whether the switch value and other signals on the machine tool side have been input to the CNC system, so that the fault can be located on the machine tool side or in the CNC system.
4. The alarm indicator shows the fault
In the CNC system of modern CNC machine tools, in addition to the above-mentioned self-diagnosis function and status display and other "software" alarms, there are also many "hardware" alarm indicators, which are distributed on the power supply, servo drive and input/output devices. The indications of these warning lights can determine the cause of the failure.
5. Spare plate replacement method
Using spare circuit boards to replace modules with suspected faults is a quick and easy way to determine the cause of faults. It is often used in the functional modules of CNC systems, such as CRT modules, memory modules, and so on. It should be noted that before replacement of the spare board, the relevant circuit should be checked to avoid damage to the good board due to a short circuit. At the same time, it should be checked whether the selector switch and jumper on the test board are consistent with the original template. Some templates should also pay attention to the template. Adjustment of the upper potentiometer. After replacing the memory board, the memory should be initialized according to the requirements of the system, otherwise the system still cannot work normally.
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 failure transfer situation, the fault location can be quickly determined. This method is often used for fault checking of servo feed drives, and it can also be used for interchange of the same modules in CNC systems.
7. Percussion
The CNC system is composed of various circuit boards, and each circuit board has many solder joints. Any false soldering or poor contact may cause malfunctions. When using an insulator to gently tap the circuit board, connector or electrical component with the suspected fault, if a fault occurs, the fault is likely to be at the knocked part.
8. Measurement comparison method
For the convenience of detection, the module or unit is equipped with detection terminals. Using multimeters, oscilloscopes and other instruments and meters, the level or waveform detected by these terminals can be compared with the normal value and the value at the time of the failure to analyze the cause of the failure and The location of the fault. Due to the comprehensiveness and complexity of CNC machine tools, there are many factors that cause failures. The above-mentioned fault diagnosis methods sometimes require several simultaneous applications to conduct a comprehensive analysis of the fault, and quickly diagnose the faulty part, so as to eliminate the fault. At the same time, some failure phenomena are electrical, but the cause is mechanical; conversely, it is also possible that the failure phenomenon is mechanical but the cause is electrical; or both. Therefore, its fault diagnosis can not be attributed solely to electrical or mechanical aspects, but must be integrated and considered in an all-round way.
