Fault Diagnosis and Maintenance of Spindle Servo System of CNC Machine Tools

The rapid development of the electronics industry has led to an endless stream of variable-speed drives with high levels of integration and advanced performance. This has provided favorable conditions for the upgrading of CNC machine tools. However, the current situation in which large and medium-sized enterprises are unable to retrofit all old CNC machine tools has been favorable. The old drive system is still a daunting task on the maintenance front. Let's talk about the faults and processing methods encountered by the author in the maintenance of the main circuit using the misplaced touchless reversible speed control drive system in the CNC horizontal and vertical lathes, for the reference of their counterparts.

1 fault 1

Symptoms: The 1.8m CNC sleeper makes a huge noise while parking, and the workshop's main power supply trips.
Inspection: (1) The workshop electrician inspects the power supply system, where the tripped automatic air circuit breaker is located, and the link mechanism for automatic trip in the switch box is corrosive, and the one-phase contact in the three-phase contact is only a small one. Some contact. (2) Power transformers in the workshop are small in capacity and overloaded. Its normal phase voltage is only 340V. (3) A thyristor has been burned out. Look at the driving circuit. The B phase trigger pulse is short and only one quarter of the normal trigger pulse amplitude. Further investigation is due to the poor performance of the amplifier tube T3 in the B phase trigger circuit. .
The thyristor trigger pulse circuit is as shown in FIG. 1 , and the principle of the diagram is simple and will not be described here.


Analysis: The thyristors are different in phase loss in the rectified state and in phase loss in the inverted state. In the rectified state, a thyristor with a higher potential, such as SCR1, is always triggered, and at the same time, the previous-phase thyristor SCR3 is subjected to the reverse voltage and turned off. During the OFF period of SCR3, the inverting blocking state dominates. Even if the latter thyristor is not triggered, SCR3 will automatically turn off due to zero crossing at a certain time. However, if it is at the time of parking down, that is, in the case of an inversion (it is also the turn-on of the thyristor with a higher trigger potential, and the previous thyristor is subjected to back pressure and turned off), the thyristor at this time is turned off. For a long time in a positive blocking state. In this way, if the latter thyristor does not conduct, the thyristor continues to conduct for one cycle and enters the positive half cycle due to the discharge action of the inductor L, and the thyristor will continue to conduct, while hindering the conduction of the following thyristor. Thus, the forward voltage output from the thyristor superimposed on the motor potential generates a large amount of current, which in turn generates an inverter subversion, which in turn causes the fuse to burn out and the thyristor to burn out. If the shop's voltage supply system is normal and there are no major fluctuations, the thyristor may not be burned. AC power grid voltage fluctuations, small capacity transformer workshop, overload operation, coupled with the B-phase positive group trigger pulse amplitude is small, and the overall switch box power supply system damage and other comprehensive causes caused the failure.
Remedy: (1) Replace the automatic air circuit breaker. (2) Replace the new thyristor.

2 Fault 2

Symptom: When the 1.8m sleeper moves, the faceplate swings back and forth.
Check: The ripple of the ±20V DC regulated power supply in the measurement drive control system is 4V peak-peak, which greatly exceeds the specified range.
Analysis: In the control system's amplifier circuit, high and low-pass filters can be filtered out, such as: tachometer feedback, current feedback, voltage feedback in the harmonic interference signal, but can not filter the system itself in the DC power circuit The harmonic component, because it exists in the entire system, these harmonics will cause the amplifier to block into the amplifier, causing the system to produce various abnormal phenomena. In the jogging state, due to the low speed of the motor, these harmonics have exceeded the voltage value at the time of jogging, causing oscillation of the system, causing the spindle disk to swing back and forth, and once the harmonic signal is removed, the fault disappears immediately.
Treatment: Replace the 100MF and 1000MF filter capacitors in the voltage board with a new capacitor, measure the ripple only a few millivolts, and then install the power board. Start the test and eliminate the fault.

3 Fault 3

Symptoms: After the 5m vertical brake rattles during operation, it burns.
Inspection: It was found that 5FC5FG and 5RG5RQ positive and negative groups have no pulse output (see Figure 2 for circuit). The measurement result shows that the IC7 inverter is damaged. It is found that the output waveform of 1FG1FC is much lower than that of other waveforms.

Analysis: The drive voltage of the 5m vertical main drive DC motor is provided by the thyristor full-control bridge anti-parallel rectification circuit. In the 12-way trigger pulse, two paths disappear and the amplitude of the other path trigger pulse is shorter than that of other normal trigger pulses by one-third. When a squeaky gear collision sound occurs, the hydraulic motor coupling is mistakenly seen. The problem, but after a while the two fuses burned out. In fact, two insurances had been burned in the past some time before this failure. At that time, only accidental grid instability was caused. After the fuse was replaced, the fault occurred. eliminated. Due to the low speed of the 5m vertical turning machine, although the thyristor rectifier circuit is a bridge rectifier, when the trigger pulse loss and the small amplitude exist in the line, the current will be discontinuous and the output voltage will be unstable. As a result, the rotational speed of the motor is unstable. The initial buzzing sound is actually a manifestation of unstable speed. The burn-in fault caused by the interruption of the current can occur at any time after the stop and normal operation of the operation.

Processing: Replace the amplification tube T1 (amplification tube in another trigger circuit, function T7 in Figure 2) and inverter IC7, and eliminate the fault.

4 fault four

Fault phenomenon: When the 1.8m sleeper spindle is parked, it makes a loud noise, but the insurance is not burned out.
Check: Use the diagnostic board to readjust the system parameters according to the instructions in the manual. During the adjustment process, the line time should be 15s, but it has been adjusted to 10s during the fault. In addition, the position of the gain potentiometer is adjusted more than required. High, even if the gain increases.
Analysis: The so-called line time refers to the rise and fall time of the system. For the control system of a 56 kW motor, the line time was changed from 15 s to 10 s, then the mechanical inertia generated by the motor would make a huge noise on the gear when it was stopped, and the gear would be damaged.
Increasing the gain can reduce the dead zone and reduce the speed error. However, if the gain is too large, the overshoot will be serious, which will cause trouble to the transient response of the system. In addition, the decrease of the line time will increase the mechanical noise.
The various parameters in the drive system must be strictly calculated and determined on the basis of the actual conditions of each motor and machine. Although the machine tool has been running for many years, some of the parameters have also changed, but this change It is extremely small. Under normal circumstances, the position of each potentiometer arrow should not be arbitrarily changed.
Processing: (1) Adjust the line time to 15s. (2) Reduce the gain.

5 Fault 5

Phenomenon: 46′′ Suddenly stopped when the vertical axis of the vehicle is running. After a moment, it continues to run. This phenomenon is sometimes absent. There is no law to follow. When there is a parking failure, the computer does not show any fault signal. After a few days of operation, The spindle will not operate.
Check: The DFP signal in the main control board disappears. When using a multimeter to measure the C pole of the T104 transistor, the DFP signal appears again, but it disappears after a long time. The voltage between be and bc of the T104 tube is no longer normal.
Analysis: As shown in Figure 3, the DFP signal is in the machine tool needs to run (the computer and the drive system are not faulty, a -4V PRE start voltage sent by the interface board, the signal is sent to the T98 emitter so that T105 Cut off, T104 turns on, sends out the +20V signal of DFP, enters the pulse amplifying circuit, controls the launch of SCR trigger pulse.If the computer and drive system have the trouble, then PRE is 0V, T104 is cut off, T105 is turned on, make DFP be 0V, trigger pulse cannot be issued.