Crawling and Vibration Fault Diagnosis Analysis of CNC Machine Tool

CNC machine tools are machine tools controlled by digital programs, that is, the specific application of CNC technology in machine tool control. The CNC machine tool not only has an automatic tool changer, but also has various additional devices such as automatic workpiece feeding, loading and unloading, tool life detection system, chip removal, etc., which can be used for long-term unmanned operation processing. The comprehensiveness and complexity of CNC machine tools determine that the faults of CNC machine tools are comprehensive and have their own characteristics. Crawling and vibration of CNC machine tools are typical examples.

Creeping means that when the moving parts driven by the feed servo system of the index control machine tool are running at low speed, the moving parts cannot start at first, and then suddenly accelerate after starting, then stop, and then accelerate again, and so on. The moving parts stop and jump suddenly, fast and slow. Crawling and vibration of machine tools is a typical failure example. When the machine tool is running at a low speed, the machine tool table is moving forward; when the machine tool is running at a high speed, vibration occurs.

The reason for the crawling and vibration of the machine tool is generally considered to be due to poor lubrication, which increases the frictional resistance when the machine tool table moves. When the motor is driven, the table does not move forward, so that the ball screw produces elastic deformation, and the energy of the motor is stored in the deformation. The motor continues to drive, and when the elastic force produced by the stored energy is greater than the static friction force, the machine tool table will creep forward and move like this repeatedly, resulting in the phenomenon of crawling. But if you take a closer look at the lubrication of the guide rail surface, you will find that the crawling and vibration are not necessarily due to lubrication.

The causes of machine tool crawling and vibration failures can be found from the following aspects:

Diagnosis and Analysis of Speed ​​Regulator of Machine Tool

Machine tool crawling and vibration problems are speed problems. Since it is a problem of speed, we need to find the speed ring. We know that the entire adjustment process of the speed of the machine tool is completed by the speed regulator. In particular, it should be pointed out that the time constant of the speed regulator, that is, the integral time constant of the speed regulator is measured in milliseconds. Therefore, the servo movement of the entire machine tool is a transition process and an adjustment process.

All speed-related problems can only be found in the speed regulator. Therefore, the vibration problem of the machine tool should also look for the speed regulator. The faults of the speed regulator can be found from the following places: one is the given signal, one is the feedback signal, and the other is the speed regulator itself.

The first one is the simulation sent from the position deviation counter to the speed regulator through D/A conversion.VCMD, whether this signal has a vibration component, you can use the pin on the servo board (the servo board of the FANUC6 system is X18 pin) to see if it vibrates there. If it is a periodic vibration signal, there is no doubt that the vibration of the machine tool is correct. There is no problem with the speed regulator, but there is a problem with the previous stage. Find the problem with the D/A converter or the deviation counter. If the measurement results in a periodic waveform without any vibration. Then the problem must be in the other two parts.

Diagnostic analysis of motor or tachogenerator

Observe the waveform of the tachogenerator. Since the machine tool is vibrating, it means that the speed of the machine tool is in violent oscillation. Of course, the waveform fed back by the tachogenerator must also be turbulent. It can be seen from it that whether there are regular ups and downs in the waveform fed back by the tachogenerator is very chaotic.

Because the vibration frequency has a certain ratio to the motor speed, it is necessary to check whether the motor is faulty first, check its carbon brushes, the surface condition of the commutator, and the mechanical vibration, and check the lubrication of the ball bearings. The entire inspection does not need to be disassembled completely, it can be observed by the inspector, and the bearing can be inspected by listening to the sound with the ear. If there is no problem, check the tachometer generator.

A problem that often occurs in the tachogenerator is that the carbon powder ground by the carbon brush accumulates in the groove between the commutator segments, causing a short circuit between the tachogenerator segments. Once such a problem occurs, the vibration cannot be avoided question. This is because the short-circuited component will be in the upper branch circuit for a while, the lower branch circuit for a while, and just in the commutation state for a while. In these three cases, there will be three different speed measurement feedback voltages. In the upper branch, the voltage of the upper branch is bound to be small due to the lack of one component, and when the component is transferred to the lower branch, the lower voltage is also small. At this time, whether it is in the upper branch or the lower In the branch circuit, the terminal voltage of the two branch circuits must be reduced, and a balanced current flows through the two parallel branch circuits, causing a certain voltage drop. When this element is in commutation, it happens to be in short circuit. At this time, there is no short circuit element in the upper and lower branches, and the voltage is restored without circulating current. In this way, it is the same as the normal tachogenerator state. For this reason, the voltage changes periodically in three different situations. When this voltage is fed back to the regulator, it will inevitably cause the output of the regulator to change accordingly and periodically. The feedback signal and the reference signal are exactly the same for the regulator. Therefore, the fluctuation of the feedback signal will inevitably cause the speed regulator to adjust in the opposite direction, which will cause the vibration of the machine tool.

When this happens, it is very easy to handle, as long asRemove the back cover of the motor to expose the commutator of the tachometer generator. At this time, there is no need to do any disassembly, just use a sharp hook to carefully hook each slot, then use fine sandpaper to lighten the raised burrs, wipe the surface of the rectifier with absolute alcohol, and then put charcoal on it. Just brush it. Special attention should be paid here that when using a sharp hook to hook the notch between the commutation plates, do not touch the winding, because the winding wire is very thin, once it is broken, it cannot be repaired, and the winding must be replaced. One more thing, don’t use water-containing alcohol to wipe, so that the insulation resistance will drop and you can’t dry it, which will delay the repair period.

Diagnostic analysis of other causes

In addition to these causes of vibration, it may also be caused by the parameters of the system itself. As we all know; a closed-loop system may also cause system oscillation due to poor parameter setting, but the best way to eliminate this oscillation is to reduce its magnification, adjust RV1 in the FANUC system, and rotate counterclockwise, then you can It can be seen that it will be better immediately, but because the range of the RV1 adjustment potentiometer is relatively small, sometimes it cannot be adjusted, and the short-circuit rod can only be changed, that is, the value of the feedback resistance is cut off, and the magnification of the entire regulator is reduced.

After adopting these methods, the vibration cannot be completely eliminated, or even ineffective. It is necessary to consider replacing or thoroughly checking the waveforms of the speed regulator board after replacement.

When crawling occurs, the motor is at a low speed, and once the speed is increased, it will vibrate, and an overcurrent alarm may occur at this time. The reason for this kind of alarm is that in order to quickly change the working surface of the machine tool with the change of the limit feedback signal, there must be a large acceleration. This acceleration is given by the torque of the motor. The change of the motor torque responds to the change of the given speed signal (actually the feedback signal). Torque is the current signal. A large torque is caused by a large current signal. There is a drastic change in the current in the current loop, resulting in an overcurrent phenomenon. There is no alarm when vibrating, but when the vibration increases, an over-current alarm occurs.

Since the CNC machine tool is a mechatronic product, there are many factors that affect the normal operation of the machine tool. It is necessary to comprehensively use all knowledge to analyze and judge the cause of the fault, and to have a comprehensive understanding of the CNC machine tool before diagnosis and maintenance, including the structure and characteristics of the machine tool, the ladder diagram of the machine tool, the working principle and block diagram of the CNC system, and their cable connections.

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