1, the power circuit
During the inspection, you should first check whether the power circuit is normal and then check other parts. Power failures have the following conditions:
(1) No power source voltage or power source voltage is low.
Numerical control systems often use ±5V, ±12V, ±15V, ±24V, and a few +3.3V. The abnormal power supply voltage will cause the system to work abnormally.
(2) Use a voltmeter to check that the power supply voltage is normal, but use an oscilloscope to detect that the voltage waveform is large. Usually, the power filter capacitor is open circuited, and the rectifier diode is defective or caused by false soldering. Sometimes the breakdown of a component causes the power supply to be overloaded.
(3) The system is normal when it is turned on, and the power supply voltage drops after working for a period of time. This is usually a poor voltage regulator circuit or high power transistor. When the temperature rises, it causes the power supply voltage to drop. It may also be that one of the components is cold-welded, and the contact is poor after the temperature rises.
When there is no power supply voltage or the power supply voltage is seriously reduced, it will cause the system to stop or stop working. This kind of failure is easy to detect and discover. It is difficult to judge when the power supply has a drop in load capacity or the filter circuit fails. It will suddenly stop the system, cause equipment accidents, cause equipment and personnel injuries, and must be highly valued.
For example, a machining center consisting of FANUC 3MA does not display any CRT after power on. Using the multimeter to detect the system power board A14B - 0067 - B002-01, the +5V supply voltage was found to be 3.75V, removing the load detection and the +5V supply was +4.9V. Description The power supply has a low load capacity or the load has a short circuit. After checking the load is normal. The power supply has a reduced load capacity. After checking and replacing the C36 filter capacitor, the system works normally.
2, the clock circuit
The clock circuit is mainly on the system main board, which is the basic condition for large-scale integrated circuits to work. It is based on a crystal oscillator (commonly known as a crystal) and produces a constant square wave signal in the circuit. The crystal stops, just as the human heart stops beating, leaving the system paralyzed. After the crystal oscillator works normally, the system circuit can work according to the beat of the crystal clock under the command of the CPU. The number and frequency of crystal oscillators vary with different control systems, but generally there is at least one, and the different clock frequencies required for the rest of the circuit are solved by the frequency divider circuit or another crystal oscillator.
The damage rate of the crystal oscillator is relatively high, and its faults are commonly found in the following types:
(1) Crystal leakage damage. You can use the multimeter P × 10 K to measure, if the resistance is infinite, it is normal; if there is resistance, it is leakage.
(2) Crystal internal open circuit. Using a multimeter to measure its resistance is infinite, but in the circuit can not produce oscillation pulse.
(3) The crystal is degenerated to change its parameters. Only oscilloscope and frequency meter can detect. Although the crystal oscillator can oscillate, its clock frequency deviates from its nominal value. Although there are oscillation pulses at this time, the system circuit cannot work due to the wrong number of pulses. Only the frequency meter can accurately measure the deviation at this time.
(4) In the actual clock circuit, a crystal capacitor of a few picofarads to several tens of picofarads is connected to both ends of the crystal to the ground. The failure of the clock circuit caused by leakage and deterioration of the capacitor is also common. The quality of the crystal is best measured with an oscilloscope and a frequency meter, and it is difficult for the multimeter to judge whether it is good or bad.
For example, a machining center controlled by FANUC 6M, after working for a period of time, suddenly appeared CRT black screen and the machine tool did not move. Turn off the power supply and send it back to the power supply. The machine can work for a while. Check that everything is normal. The fault may be on the system board. After overhauling the main board A16B-1000- 0220/04A, it was found that one of the two crystals had a poor internal contact with the 16. 3840MHz crystal oscillator, and the same type of failure did not occur again after replacement.
3, reset circuit
The reset circuit is also a circuit existing on the system main board, which is a circuit unique to a large-scale digital integrated circuit. Microprocessors, interface circuits, etc. all have reset terminals.
The reset pulse generated by the reset circuit clears the program counter to enable the CPU to recall the initialization file from the memory and initialize each control chip port. If the reset circuit is defective, the system will malfunction, crash, and other failures.
When an oscilloscope is used to observe the reset pulse, the power supply should be turned on and off repeatedly, and the reset pulse should be observed every time the switch is turned on. The reset pulse should be an ideal rectangular square wave. If there is no reset pulse, check the resistors, capacitors, transistors, etc. in the reset circuit. The reset terminal of the integrated circuit should be a regular low or high level, otherwise it should be a reset circuit failure or an integrated circuit damage.
If a large machining center using PLASMA CNC system is used, the system cannot be started and the CRT has no alarm display. After checking that the ±5V, ±12V, ±24V power supply voltage is normal, the clock circuit is normal. Suspected to be a problem with the system board, when checking the reset circuit, it was found that there was no reset pulse on the reset side of the CPU. Further inspection revealed that a 3. 3k/ 0. 5W resistor was opened at the reset end and the system started normally after the replacement.
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