Crawling and vibration fault diagnosis analysis of CNC machine tools - News - Global IC Trade Starts Here.

CNC machine tools are machine tools controlled by digital programs, that is, the specific application of numerical control 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 perform long-time unmanned operation. The comprehensiveness and complexity of CNC machine tools determine the fault of CNC machine tools and have their own characteristics. The crawling and vibration of CNC machine tools is a typical example.

Crawling is the moving part driven by the index-controlled machine feed servo system. At low speed, the moving parts can not start, and then suddenly start the acceleration movement after the start, and then pause, and then accelerate the movement, so that the moving parts are suddenly repeated. Stop and jump, slow and slow movements. Crawling and vibration of the machine tool is a typical example of failure. 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 reason why the machine tool crawls and vibrates is generally considered to be due to poor lubrication, which increases the frictional resistance 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 in such a manner, resulting in a creep phenomenon. But if you look closely at the lubrication of the rail surface, you will find that it is not necessarily due to lubrication and creep and vibration.

The cause of the machine's creep and vibration can be found in the following aspects:

Diagnostic analysis of speed regulators for machine tools

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 the measurement results without any periodic waveform of vibration. Then the problem must be in the other two parts.

Diagnostic analysis of motor or tachogenerator

Observing the waveform of the tachogenerator, the vibration of the machine tool indicates that the speed of the machine tool is in a strong oscillation. Of course, the waveform returned by the tachometer generator must also be turbulent. It can be seen from the above that whether there is a regular ups and downs in the waveform of the tachometer generator feedback is very chaotic.

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.

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. 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.

Other causes of diagnostic analysis

In addition to these causes of vibration, it may be the oscillation 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.

When the crawling occurs, the motor is at a low speed. 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.

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