Many years of maintenance of CNC machine tools have confirmed that in the total number of faults, faults caused by the power supply account for a considerable proportion. According to the China Machine Tool Network (Machine35.com), many power supply failures of CNC machine tools are machine tool users who have the ability to self-exclude device damage, and its field has already belonged to chip-level repairs.
The power supply of the CNC machine tool divides the power supply used by the CNC machine tool into three levels. From the primary power source to the tertiary power source, the power supply is derived in turn, and the frequency and difficulty of failure caused by the numerical control machine tool are increased in turn. The specific rating is as follows:
(1) Primary power supply. The primary power supply is the three-phase 380 V power supplied by the workshop power grid, which is the total energy supply for the CNC machine tool work. It is required that the power supply should be stable, the general voltage fluctuation range should be controlled at 5% to 10%, and there should be no high-frequency interference.
(2) Secondary power supply. Three-phase power is derived from the primary power supply via a transformer. Its use mainly includes:
1) Derived single-phase AC 220 V, AC 110 V, power supply to CNC unit and display unit, as the power supply for heat exchanger, machine tool control loop and switching power supply.
2) Some three-phase low voltage derived from the CNC machine tool is used as the power source of the DC 24 V rectifier bridge block. Some CNC machine tools generate three-phase AC 220 V from a three-phase transformer and supply servo amplifier power supply components as their operating power.
(3) Three power supplies. The three power supplies are various DC power supplies used by CNC machine tools, which are converted from secondary power supplies. There are mainly such kinds:
1) The DC voltage provided by the servo amplifier power supply assembly, the three-phase AC power, which is reversed from the servo amplifier assembly into a variable frequency and voltage amplitude, controls the speed of the AC servo motor.
2) The AC 24 V provided by the bridge rectifier is used as the "ready" and "controller enable" signal source for the solenoid valves of the hydraulic system, the power supply for the motor of the motor and the servo amplifier unit.
3) Low-voltage DC voltage supplied by the switching power supply or DC/DC power supply module, these voltages are: +5 V, ±12 V, ±15 V, which are used as the power supply for measuring gratings, numerical control units and servo unit electrical boards, respectively.
3 Devices used in the power circuit of numerically-controlled machine tools The devices used in numerical control machine tools from primary to tertiary power supplies are:
(1) The power distribution equipment in a workshop generally includes: a three-phase AC voltage regulator and a circuit breaker (also called an air switch, or a knife switch) connected to a power grid in a workshop.
(2) Machine tool components, including: filters, reactors, three-phase AC transformers, circuit breakers, rectifiers, fuses, servo power components, DC/DC modules, and switching power supplies.
4 Power Failure Example Analysis (1) If the power grid fluctuates too much, the PLC will not work. It shows no output from the PLC. First check the input signal (power signal, interference signal, command signal and feedback signal). For example, a CNC lathe using the SINUMERIK 3G-4B system does not work with its built-in PLC. Using the observation method, first use an oscilloscope to check the voltage waveform of the power grid, and find that the power grid fluctuates too much and the duration of the undervoltage noise jump is >1s (external factor). As the machine tool is in the debugging stage, the component failures in the power system should be excluded. It can be known from the internal anti-grid interference measures (filtering, isolation and regulation) that the conventional power system has no way to cut off or filter out the undervoltage that is too long. Noise, which is caused by insufficient anti-grid measures (internal cause), causes PLC to fail to obtain normal power input and cannot work. An AC voltage regulator is added to the system power input, and the PLC is working properly.
(2) Power failure. For a two-position CNC lathe, each station is controlled by a separate NC system. The NC system uses a SINUMERIK 810/T system from Siemens. The NC system of the right station often interrupts the electrical shutdown during the automatic machining of the parts. After restarting the system, the NC system can still work automatically. Check the 24 V power supply load and there is no short circuit problem. The pattern is analyzed and two NC systems share a 24 V rectified power supply. There are two possible causes for this failure:
1) The power supply quality is not high, the power supply fluctuates, and the failed NC system is more sensitive to the power supply requirements.
2) Problems with the NC system itself, the system is unstable. According to this judgment, the 24V power supply voltage is first monitored, and the voltage amplitude is found to be as low as 21V. It was observed that at the moment of failure, the voltage floated downwards, and after the NC system was powered off, the voltage immediately rose back to about 22V. Faults usually occur when the spindle is started. The reason may be that the 24V rectifier transformer is faulty, the capacity is not enough, or the short circuit between turns, so that the rectified voltage is low and the voltage of the power grid fluctuates, which affects the normal operation of the NC system. To determine the cause of this failure, the AC 380V supply voltage was increased to 400 V with an AC regulated power supply, and this failure did not recur. To replace this 24V rectifier transformer, the problem is completely resolved.
(3) A PNE480L CNC lathe manufactured by VDF.BOEHRINGER (Germany). When the main switch is turned on to start the CNC system, all the indicators are on except the READY lamp on the display panel. The machine CNC system is Siemens SYSTEM5T system. Because the fault occurred at the moment of starting, you should check whether the reset signal RESET is abnormal. Since the DP6 lamp on the main board is on and the DP6 is monitoring the DC power supply, it is necessary to check the relevant circuits that drive the DP6 and the related DC power supply. The procedure is as follows: Because the DP6 lamp is an alarm display, the relevant circuit of the DP6 is first checked. Upon inspection, it was confirmed that the logic state of the bi-stable flip-flop LA10 driving DP6 was incorrect and was damaged. After replacing with a new one, although the DP6 indicator light is off, the fault phenomenon still exists and the CNC box still cannot be started. Check the connection of the *RESET signal and each connector in the CNC box is good, but the *RESET signal is not normal and it is found that the logical relationship between the LA01 and NOT gates on the A38 position is not correct. So check each DC current. Check ±15V, ±5V, ±12V, +24V. The voltage was found to be -5V to 4.0V with an error of more than ±5%. After further inspection, it was found that after the circuit rectifier bridge there was a break in the printed circuit board copper foil at the foot of the filter bulk capacitor C19. After welding it, the voltage is normal, the logical relationship of LA01 circuit and the *RESET signal are correct, the fault is eliminated, and the NC box can start normally.
(4) Return to reference point abnormality. This is due to the fact that when the reference point is returned, the condition "must be returned to the reference point direction, and must not be too close to the reference point (more than 128 pulses) and the progress of the return reference point cannot be too low" is satisfied. The processing steps for such failures are [2,3]:
1) From the reference point position > 128 pulses, return to the reference point during the process.
1 If the motor does not turn 1 turn (that is, no 1-turn signal is received), change the starting position at the time of return. At a position deviation of more than 128 pulses, perform 1 turn or more in the direction of returning to the reference point. The rapid feed detects if a 1-turn signal has been input.
2 The motor has been rotated more than 1 revolution. This is a separate type of pulse encoder. At this time, check whether the pulse encoder's 1-turn signal is input to the shaft card when the position is returned. If it is, the shaft card is defective; if not, check whether the power supply voltage for the encoder is low (allowing voltage fluctuations Within 0.2V), otherwise bad pulse encoder.
2) <128 pulses from the reference point.
1 Check the feedrate command value, rapid feedrate override signal, and return the reference point deceleration signal and external deceleration signal.
2 Change the starting position when returning so that the position deviation exceeds 128 pulses.
3 The reference point speed is too low. The speed must be the speed at which the positional deviation exceeds 128 pulses. If the speed is too low, the signal of the motor 1 is scattered and it is impossible to perform correct position detection.
(5) A machining center, equipped with an F-0M system, suddenly saw the magazine and the worktable rotate at the same time during automatic operation. After resetting, adjusting the magazine, workbench, it works normally. However, the servo alarm No. 410 appeared on the CRT when the power was turned off and the machine was restarted. Check the L/M axis servo PRDY, VRDY two lights are on; feed axis servo power supply AC100V, AC18V normal; x, y, z servo unit PRDY indicator are not bright, three MCC did not suck; measurement An abnormality of 24V and ±15V was found on the voltage, and the resistance of the power supply fuse on the shaft servo unit was too large. After the replacement, the DC voltage returned to normal and the machine was restarted. The alarm No. 401 disappeared.
(6) Failure phenomenon: A company produces VF2 type vertical milling center. After one year and seven months of operation of the machine tool, an x-axis over current or drive fault occurred during machining, and the machine tool stopped operating. Alarms can be cleared using the "RESET" key and the machine can resume operation. Occurred occasionally, this failure phenomenon occurred two years after the machine was running sick, the frequency of failures increased, and the phenomenon of failure occurred: When the 161 alarm was cleared using the reset button, the alarm information was reported to No. 162 (Y-axis over current or drive fault ), if cleared again, the z-axis is reported again, and so on. The machine has failed to maintain operation.
Failure analysis and inspection: According to the fault alarm information transfer between several servo axes, it is not difficult to see that the fault occurs in the common link with each servo axis, that is, the "position control board" or servo unit of the numerical control unit. The power supply component has failed. The position control board is one of the components of the numerical control unit. According to empirical analysis, the probability of the failure of the numerical control unit electrical board is very low. Therefore, it is more feasible to check and check the servo power supply components. It was found that the servo power supply of this machine tool was divided into two parts, in which the output low voltage direct current ±12 V was the switching power supply. The measurement results are: +11.73 V, -11.98 V. Analyzing this result, the positive voltage output is 0.27 V lower and the voltage reduction is 2.3%. Due to the lack of a quantitative concept, it is assumed that the switching power supply is faulty in the absence of other fault sources. Troubleshooting: In order to verify that the output voltage deviation is the root cause of the machine tool failure, replace the original power supply with a WYJ dual-transistor DC voltage regulator, adjust the two output voltages symmetrically, and adjust the amplitude to 12V. After the power is turned on, the machine alarms. disappear. In the next 20 working days of test operation, the fault no longer repeats. It is fully confirmed that the failure was caused by the damage of this servo power supply unit. Theoretical analysis [4]: Operational amplifiers and comparators, some with a single power supply, some with dual power supply, the use of dual power supply op amp requires positive and negative power supply symmetry, the difference is generally not greater than 0.2 V (with regulation function of the transport Except when placed, otherwise it will not work. With this faulty power supply, the two output voltages differ by 0.25 V, which is beyond the tolerance range. This is the root cause of the fault.
5 Conclusion Through the above fault instance, we can easily see that the abnormality of the power supply of CNC machine tools is sometimes very inconspicuous, and sometimes only a deviation of less than 0.3 V, but it caused the failure of the machine. Therefore, in the maintenance, we must sum up the maintenance experience in this area and improve the efficiency of machine tool failure maintenance.
