Machine crawling and vibration

There are many obvious abnormalities in CNC machine tools, but in some economical CNC systems, there are no alarms. Even if alarms occur in some cases, the alarm information indicates that it is not an alarm that you see as abnormal. The crawling and vibration of the machine tool is an obvious example. When the machine tool is running at a low speed, the machine table is creeping and moving forward; when the machine tool is running at a high speed, vibration occurs.

A book about crawling on a machine tool reads: Due to poor lubrication, frictional resistance increases when the machine table moves. When the motor is driven, the table does not move forward, so that the ball screw elastically deforms, and the motor energy is stored in the deformation. When the electric motor continues to drive, and the elastic force produced by the stored energy is greater than the static friction force, the table of the machine tool creeps forward and moves in a cyclic manner, resulting in a crawling phenomenon. However, this is not the case. If you look closely at the lubrication of the rail surface, you can conclude that it is not the problem.

Machine crawling and vibration problems are speed problems. Since it is the speed problem, we must find the speed loop. We know that the whole adjustment process of the speed of the machine tool is done by the speed regulator. In particular, it should be emphasized that the time constant of the speed regulator, that is, the integral time constant of the speed regulator, is in milliseconds. Therefore, the servo motion of the entire machine tool is a transitional process and is an adjustment process.

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

The first one is the VCMD that is sent from the position deviation counter through D/A conversion to the speed regulator. Whether this signal has vibration component can be through the pin on the servo board (FANUC6 system servo board is X18) Look to see if it vibrates there. If it is a vibration signal with one cycle, then there is no doubt that the machine vibration is correct. There is no problem in this part of the speed regulator, but there is a problem in the previous stage, and the D/A converter or deviation counter is used to find the problem. If we measure the result without any periodic waveform of vibration. Then the problem is definitely in the other two parts.

We can observe the waveform of the tachometer generator. Since the machine tool is vibrating, it indicates that the speed of the machine tool is in the fierce oscillation. Of course, the waveform of the tachometer generator feedback must be turbulent. However, we can see whether the regular fluctuations in the waveform of the tachogenerator feedback are very chaotic. At this time, we should be able to test whether there is an accurate ratio relationship between the vibration frequency of the machine tool and the speed of the motor rotation. For example, the vibration frequency is four times the motor speed. At this time, we must consider the fault of the motor or tachometer generator.

Because the vibration frequency and the motor speed are in a certain ratio, first check whether the motor is faulty, check its carbon brush, the surface condition of the commutator, and the condition of the mechanical vibration, and check the lubrication condition of the ball bearing. , It does not have to be completely dismantled, it can be observed by the inspector, and the bearing can be checked with the ear to hear the sound. If there is no problem, check the tachogenerator. Speed ​​measuring generators are generally direct current.

The tachogenerator is a small permanent magnet DC generator whose output voltage is proportional to the speed, that is, the output voltage is linear with the speed. As long as the rotation speed is constant, the output voltage waveform should be a straight line, but due to the effect of the cogging and the effect of the commutator commutation, a small crossover variable is attached to this straight line. For this reason, a filter circuit is added to the speed feedback circuit. This filter circuit attenuates the AC component attached to the voltage.

A common problem in tachogenerators is that charcoal milled charcoal accumulates in the slots between the commutator segments, causing a short circuit between the tachogenerators. Once this problem arises, this vibration problem cannot be avoided.

This is because this short-circuited component will branch on the top, and will branch to the bottom one at a time, and it will be in a commutation state immediately. In these three cases, there will be three different speed feedback feedback voltages. In the above branch, the upper branch is routed with one less component and the voltage must be small. When it is transferred to the lower branch, the voltage below is also small, whether it is on the branch above or below. In the branch, it is inevitable that the terminal voltages of the two branches will drop, and a balanced current flows through these two parallel branches, causing a certain voltage drop. When this component is in commutation, it is also short-circuited. At this time, the upper and lower branches have no short-circuit components, the voltage is restored, and there is no circulation. In this way, it is the same as the normal tachometer generator. For this reason, in three different cases, the voltage is changed periodically. When this voltage is fed back to the regulator, the output of the regulator is bound to change correspondingly and periodically. This is just saying that one element is short-circuited. In particular, when the commutator segments were completely filled with toner, all of them were short-circuited, which caused even more severe voltage fluctuations.

The feedback signal and the given 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 as the rear cover of the motor is removed, the commutator of the tachometer generator is exposed. Then do not have to do any dismantling, just use a sharp hook, carefully check each groove, and then use a fine sandpaper to light up the burrs, wipe the surface of the rectifier with a dry alcohol and then put the charcoal Brush it. Special attention here is to use a sharp hook to switch to the slot between the plates, do not touch the winding, because the winding line is very thin, once broken, it can not be repaired, only to replace the winding. Another one should never be rubbed with water-based alcohol. This will not dry the insulated resistors, which will delay the repair period.

In addition to the above-mentioned causes of vibration, we may also have oscillations caused by parameters of the system itself. It is well known that 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 amplification factor. In the FANUC system, adjust RV1 and rotate it against the clockwise direction. Obviously, it will obviously become better, but because the range of the RV1 regulating potentiometer is relatively small, sometimes it cannot be adjusted. Only the shorting bar can be changed, that is, the value of the feedback resistance is cut off, and the whole regulator is reduced in magnification.

After using these methods, it is still impossible to completely eliminate the vibration, or even invalid, it is necessary to consider thoroughly checking the waveforms after replacing or replacing the speed regulator board.

In this example, when a crawl occurs, the motor is at a low speed, and once the speed is increased, it will vibrate. At this time, the current may have an overcurrent alarm. The reason for this alarm is that in order to quickly change the feedback signal of the machine tool table, there must be a large acceleration, this acceleration is given by the torque of the motor. The change in motor torque responds to changes in this speed reference signal (actually a feedback signal). Torque is the current signal. The large torque is caused by a large current signal, and a drastic change in the current in the current loop causes an over-current phenomenon. When the vibration is not alarmed, an overcurrent alarm occurs when the vibration increases.

From this example, we can sum up this way: the location problem to find the location loop, and the speed problem to find the speed loop. The so-called position loop is to study the size of parts processing problems, the accuracy of the size of the parts to study the position of the ring. Of course, the repeatability of the part size is also related to the reference point. We will also discuss the reference point return later. But generally speaking, the size problem, location problem, the object to be considered is the position loop, or the part related to the position loop should be the main object of consideration. The problem of speed is to study the speed loop and the part related to the speed loop.

There is a problem with the shape of the machined part, which is apparently caused by interpolation of several axes. This is the pulse distribution of the NC to the axis. If we think that the pulse distribution of the NC to the axis is correct (usually this is the case, it is rarely encountered as something wrong with the NC, or the interpolation software is faulty and the shape is incorrect. Phenomenon), then there must be problems with the axes in faithfully executing the NC instructions. We can check the problems of each axis servo unit. If we want to process a straight line with a certain slope, the speeds of these two axes are given in terms of the ratio of slopes.

As CNC machine tools are mechatronics products, there are many factors that affect the normal operation of the machine tools. For example, we discussed the causes of the machining shape errors mentioned above. In addition to electrical problems, we used the CNC machine tool acceptance section. Discussing the measurement of the amount of kinetic energy, which is also an important issue affecting the geometry of the machining, this mechanical problem is also mixed with the electrical problem. This situation is very difficult to tell in the end what factor in the proportion of the problem.

These related factors are important factors that restrict us from quickly detecting the failure.

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