There are many obvious abnormalities in CNC machine tools, but in some economic CNC systems, there is no alarm. Even if an alarm occurs sometimes, the alarm information indicates that it is not an alarm that you see abnormal phenomena. The crawling and vibration of machine tools is an obvious example. When the machine is running at low speed, the machine table is moving forward with creep; when the machine is running at high speed, vibration occurs.
The book about machine tool crawling says: Because the lubrication is not good, the frictional resistance increases when the machine table moves. When the motor is driven, the table does not move forward, causing the ball screw to elastically deform and store the energy of the motor in the deformation. When the motor continues to drive, and the stored energy produces more elastic force than the static friction force, the machine table moves forward and moves like this, resulting in creeping phenomenon. However, this is not the case. If you look closely at the lubrication of the rail surface, you can conclude that this is not the problem. Machine crawling and vibration problems are speed issues. Since it is a speed problem, we must find the speed loop. We know that the entire adjustment process of the speed of the machine 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 measured in milliseconds. Therefore, the servo motion of the entire machine tool is a transition process and is an adjustment process.
For speed-related problems, you can only find the speed regulator. Therefore, the problem of machine vibration is also to find the speed regulator. You can find speed regulator faults from one of these: a given signal, one is a feedback signal, and the other is the speed regulator itself.
The first one is the VCMD from the position deviation counter and the D/A conversion to the speed regulator. The signal has vibration component, which can be passed through the pin on the servo board (the servo board of the FANUC6 system is the X18 pin). Take a look at whether it vibrates there. If it has a periodic vibration signal, 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 front stage. Look for the problem to the D/A converter or the deviation counter. If we measure the results without any periodic waveforms of vibration. Then the problem must be in the other two parts.
We can observe the waveform of the tachogenerator. Because the machine is vibrating, the speed of the machine is in a strong oscillation. Of course, the waveform returned by the tachogen generator must be turbulent. However, we can see whether there is a regular ups and downs in the waveform of the tachometer generator feedback, which is very chaotic. At this time, we should measure whether there is an accurate ratio of the vibration frequency of the machine to the speed of the motor rotation. For example, the frequency of vibration is four times the speed of the motor. At this time we have to consider the problem of motor or tachogen generator failure.
Because the vibration frequency is proportional to the motor speed, first check whether the motor is faulty, check its carbon brush, the surface condition of the commutator, and the mechanical vibration, and check the lubrication of the ball bearing. It is not necessary to dismantle it all, 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 tachogenerator. The tachogenerator is generally DC.
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 speed is constant, its output voltage waveform should be a straight line, but due to the influence of the cogging and the commutation of the commutator, a small cross variable is attached to the line. To this end, a filter circuit is added to the speed feedback circuit, and this filter circuit weakens the AC component attached to the voltage.
One of the common problems in tachogenerators is that the carbon powder milled in the carbon brush accumulates in the groove between the commutator segments, causing a short circuit between the tachometer generators. Once such a problem occurs, the vibration problem cannot be avoided.
This is because the short-circuited component will be on the upper branch, and will be in the lower branch, and will be in the commutation state. In these three cases, three different speed feedback voltages will appear. In the above branch, the upper branch is routed with one component missing, the voltage must be small, and when the component is turned to the lower branch, the voltage below is also small, regardless of whether it is on the branch or below. In the branch, the terminal voltages of the two branches are inevitably lowered, and a balanced current flows through the two parallel branches, causing a certain voltage drop. When this component is in the reversing direction, it is also in a short circuit. At this time, the upper and lower branches have no short-circuiting elements, the voltage is restored, and there is no circulation. This 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 is bound to cause the output of the regulator to change accordingly. This is just to say that one component is shorted. In a particularly serious case, the commutator segments are all filled with toner, and all are short-circuited, which will cause more serious voltage fluctuations.
The feedback signal and the given signal are identical for the regulator. Therefore, the fluctuation of the feedback signal occurs, which inevitably causes the reverse adjustment of the speed regulator, which causes the vibration of the machine tool.
When this happens, it is very easy to handle. As long as the motor back cover is removed, the commutator of the tachogenerator is exposed. At this time, you don't have to do any disassembly. Just use a sharp hook, carefully hook each slot, then use a fine sandpaper to light the burrs, rub the surface of the rectifier with anhydrous alcohol, and then put on the charcoal. Brush it. Special attention should be paid here when using a sharp hook to change the slot between the pieces, do not touch the winding, because the winding wire is very thin, once it is broken, it cannot be repaired, only the winding is replaced. Do not use a water-based alcohol to wipe, so that the insulation resistance can not be dried, which will delay the repair period.
In addition to the reasons for the vibrations we discussed above, it may be the oscillations caused by the 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. In the FANUC system, RV1 is adjusted and rotated in the counterclockwise direction. It can be seen that it will become better immediately, but because the range of the RV1 adjustment potentiometer is relatively small, sometimes it can not be adjusted, only the shorting bar can be changed, that is, the feedback resistance value is cut off, and the magnification of the entire regulator is reduced.
After using these methods, it is not possible 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 the 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 the current. The reason for this kind of alarm is that the machine table is changed in order to quickly follow the change of the feedback signal. There must be a large acceleration, which is given by the torque of the motor. The change in motor torque responds to changes in this speed given signal (actually the feedback signal). Torque is the current signal. The large torque is caused by a large current signal, and a sharp change in current occurs in the current loop, resulting in an overcurrent phenomenon. There is no alarm when vibrating, and an overcurrent alarm occurs when the vibration is increased.
From this example, we can sum up this: position problem to find the position loop, and speed problem to find the speed loop. The so-called position loop is to study the size of the part processing, the accuracy of the size of the part to study the position loop. Of course, the repeatability of the part size is also related to the reference point, we will discuss the reference point return problem later. But in general, the size problem, the 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 parts related to the speed loop.
There is a problem with the shape of the machined part, which is obviously caused by interpolation of several axes. This is the pulse distribution of the NC to the axis, then if we think that the pulse distribution of the NC to the axis is correct (often this is the case, it is rarely encountered that the NC is out of order, or the interpolation software is out of order and the shape is wrong. Phenomenon), then each axis must have problems in faithfully executing the NC instructions. We can check the problems of each axis servo unit. If we want to machine a straight line with a certain slope, then the speed of the two axes should be given by the ratio of the slope.
Since CNC machine tools are mechatronic products, there are many factors that affect the normal operation of machine tools. For example, the reason for the shape error that we discussed above, in addition to the electrical problems, we used to be in the acceptance section of CNC machine tools. The determination of the amount of loss is also an important issue affecting the geometry of the process. This mechanical problem is also mixed with the electrical problem. It is very difficult to tell which factor accounts for the proportion of the problem.
These related factors are important factors that restrict our ability to quickly detect faults.
The book about machine tool crawling says: Because the lubrication is not good, the frictional resistance increases when the machine table moves. When the motor is driven, the table does not move forward, causing the ball screw to elastically deform and store the energy of the motor in the deformation. When the motor continues to drive, and the stored energy produces more elastic force than the static friction force, the machine table moves forward and moves like this, resulting in creeping phenomenon. However, this is not the case. If you look closely at the lubrication of the rail surface, you can conclude that this is not the problem. Machine crawling and vibration problems are speed issues. Since it is a speed problem, we must find the speed loop. We know that the entire adjustment process of the speed of the machine 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 measured in milliseconds. Therefore, the servo motion of the entire machine tool is a transition process and is an adjustment process.
For speed-related problems, you can only find the speed regulator. Therefore, the problem of machine vibration is also to find the speed regulator. You can find speed regulator faults from one of these: a given signal, one is a feedback signal, and the other is the speed regulator itself.
The first one is the VCMD from the position deviation counter and the D/A conversion to the speed regulator. The signal has vibration component, which can be passed through the pin on the servo board (the servo board of the FANUC6 system is the X18 pin). Take a look at whether it vibrates there. If it has a periodic vibration signal, 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 front stage. Look for the problem to the D/A converter or the deviation counter. If we measure the results without any periodic waveforms of vibration. Then the problem must be in the other two parts.
We can observe the waveform of the tachogenerator. Because the machine is vibrating, the speed of the machine is in a strong oscillation. Of course, the waveform returned by the tachogen generator must be turbulent. However, we can see whether there is a regular ups and downs in the waveform of the tachometer generator feedback, which is very chaotic. At this time, we should measure whether there is an accurate ratio of the vibration frequency of the machine to the speed of the motor rotation. For example, the frequency of vibration is four times the speed of the motor. At this time we have to consider the problem of motor or tachogen generator failure.
Because the vibration frequency is proportional to the motor speed, first check whether the motor is faulty, check its carbon brush, the surface condition of the commutator, and the mechanical vibration, and check the lubrication of the ball bearing. It is not necessary to dismantle it all, 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 tachogenerator. The tachogenerator is generally DC.
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 speed is constant, its output voltage waveform should be a straight line, but due to the influence of the cogging and the commutation of the commutator, a small cross variable is attached to the line. To this end, a filter circuit is added to the speed feedback circuit, and this filter circuit weakens the AC component attached to the voltage.
One of the common problems in tachogenerators is that the carbon powder milled in the carbon brush accumulates in the groove between the commutator segments, causing a short circuit between the tachometer generators. Once such a problem occurs, the vibration problem cannot be avoided.
This is because the short-circuited component will be on the upper branch, and will be in the lower branch, and will be in the commutation state. In these three cases, three different speed feedback voltages will appear. In the above branch, the upper branch is routed with one component missing, the voltage must be small, and when the component is turned to the lower branch, the voltage below is also small, regardless of whether it is on the branch or below. In the branch, the terminal voltages of the two branches are inevitably lowered, and a balanced current flows through the two parallel branches, causing a certain voltage drop. When this component is in the reversing direction, it is also in a short circuit. At this time, the upper and lower branches have no short-circuiting elements, the voltage is restored, and there is no circulation. This 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 is bound to cause the output of the regulator to change accordingly. This is just to say that one component is shorted. In a particularly serious case, the commutator segments are all filled with toner, and all are short-circuited, which will cause more serious voltage fluctuations.
The feedback signal and the given signal are identical for the regulator. Therefore, the fluctuation of the feedback signal occurs, which inevitably causes the reverse adjustment of the speed regulator, which causes the vibration of the machine tool.
When this happens, it is very easy to handle. As long as the motor back cover is removed, the commutator of the tachogenerator is exposed. At this time, you don't have to do any disassembly. Just use a sharp hook, carefully hook each slot, then use a fine sandpaper to light the burrs, rub the surface of the rectifier with anhydrous alcohol, and then put on the charcoal. Brush it. Special attention should be paid here when using a sharp hook to change the slot between the pieces, do not touch the winding, because the winding wire is very thin, once it is broken, it cannot be repaired, only the winding is replaced. Do not use a water-based alcohol to wipe, so that the insulation resistance can not be dried, which will delay the repair period.
In addition to the reasons for the vibrations we discussed above, it may be the oscillations caused by the 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. In the FANUC system, RV1 is adjusted and rotated in the counterclockwise direction. It can be seen that it will become better immediately, but because the range of the RV1 adjustment potentiometer is relatively small, sometimes it can not be adjusted, only the shorting bar can be changed, that is, the feedback resistance value is cut off, and the magnification of the entire regulator is reduced.
After using these methods, it is not possible 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 the 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 the current. The reason for this kind of alarm is that the machine table is changed in order to quickly follow the change of the feedback signal. There must be a large acceleration, which is given by the torque of the motor. The change in motor torque responds to changes in this speed given signal (actually the feedback signal). Torque is the current signal. The large torque is caused by a large current signal, and a sharp change in current occurs in the current loop, resulting in an overcurrent phenomenon. There is no alarm when vibrating, and an overcurrent alarm occurs when the vibration is increased.
From this example, we can sum up this: position problem to find the position loop, and speed problem to find the speed loop. The so-called position loop is to study the size of the part processing, the accuracy of the size of the part to study the position loop. Of course, the repeatability of the part size is also related to the reference point, we will discuss the reference point return problem later. But in general, the size problem, the 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 parts related to the speed loop.
There is a problem with the shape of the machined part, which is obviously caused by interpolation of several axes. This is the pulse distribution of the NC to the axis, then if we think that the pulse distribution of the NC to the axis is correct (often this is the case, it is rarely encountered that the NC is out of order, or the interpolation software is out of order and the shape is wrong. Phenomenon), then each axis must have problems in faithfully executing the NC instructions. We can check the problems of each axis servo unit. If we want to machine a straight line with a certain slope, then the speed of the two axes should be given by the ratio of the slope.
Since CNC machine tools are mechatronic products, there are many factors that affect the normal operation of machine tools. For example, the reason for the shape error that we discussed above, in addition to the electrical problems, we used to be in the acceptance section of CNC machine tools. The determination of the amount of loss is also an important issue affecting the geometry of the process. This mechanical problem is also mixed with the electrical problem. It is very difficult to tell which factor accounts for the proportion of the problem.
These related factors are important factors that restrict our ability to quickly detect faults.
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