1. Diagnosing Power Supply Short-Circuit Fault Using Resistance Comparison Method

Barrier example: FANUC-BESK servo drive board ten 15V load soft-break burning fuse.

In our maintenance, the primary cause of the fault was a partial short circuit in the load, and a ten-thousand-volt resistance to ground was measured with a digital meter. The normal board was 1.3KΩ and the fault board was 300Ω. Because the power is good to burn the fuse, it cannot be checked at all. Therefore, it can only do resistance measurement or demolition inspection.

However, since the printed circuit of the servo 15V power supply and its load (24 integrated components) is in a radial structure, it is impossible to perform circuit cutting and separation during resistance measurement, and it is also impossible to perform direct current soldering due to a large number of components. Dismantle inspection one by one. The actual operation of the repair is very difficult, even if the fault is solved, it often results in scars on the circuit board. To deal with this kind of failure that can neither be done in the circuit cutting or disassembling, nor can it be disassembled and checked, we use the resistance comparison method to check it easily. During the diagnostic inspection, the circuit does not cut off or remove the components. Instead, it directly measures the resistance value of the 10V terminal and each integrated component, and compares the fault board with the normal board to find the fault. . When dealing with the above faults, taking into account the multiple pins of the device, first analyze the internal circuit of the thick film block (marked in the figure) and the functional block diagram of the integrated block pin, and then select several major test points from it to do resistance measurement. When Q7 is measured, it is found that the pin 3 (+15V) and pin 14 (output) resistance is 150Ω (normally 6KΩ. Q7 (LM339) is suspected to be a problem. After Q7 is replaced, the servoboard returns to normal, indicating the Q7 pin Abnormal resistance is internal soft breakdown, which causes a short circuit in the power supply.

2. Step-by-step simulation of fast process

Some control processes, such as the stepping motor’s automatic speed-up and down process, and the DC governor’s parking brake process, have only a fraction of a second of instantaneous time. Looking for circuit faults in this fast process, obviously the general instrument cannot be used for fault tracking detection, so fault diagnosis is difficult. The following example describes a special step-by-step simulation method that we adopted for faults in which the 5V thyristor main drive has a long downtime.

After a preliminary inspection of the fault board, determine the cause of the fault in the V5 main drive braking circuit. The brake control logic is complex, involving too many circuits, and diagnosis failures are by no means an issue. Because the braking process is short and cannot be measured, we use a step-by-step simulation method to perform diagnostic tests. According to the circuit principle, the braking process is as follows: (1) The bridge inverts and releases energy; (2) automatically changes the bridge and regeneratively brakes; (3) it replaces the bridge again, and the circuit recovers.

For the needs of step-by-step measurement, with the speed command, speed feedback and current feedback as the set amount, the above process is subdivided into eight steps (listed into a table), and then gradually change the corresponding set amount, detect the relevant circuit signal, Compare the circuit logic to detect the fault. When we do step-by-step testing to the second step (that is, the speed command changes from 1 to 0), we find that “a backward movement” and “integral stop” are both high level, and according to the circuit logic, it should be low level. Check the circuit and quickly find out that there is a problem with the NAND gate Dl06 (model: FZHI01) in the A2 board. After the replacement, troubleshooting is done.

3. A special case of CT4 to OS3 frequency detector

CT4-os3 inverters are commonly used for the magazine drive of YBM90 and MK5oo machining centers. In maintenance, we encountered many times the phase-failure fault of the inverter, and measured the missing phase voltage is only 60 to 2ooV (normal 400v). Since this is a soft failure, diagnostic search is difficult.

However, we have found that most of the causes of this type of fault in the frequency converter are the problem of the pulse isolation stage – unstable oscillation. This failure phenomenon, using an oscilloscope to check, it is difficult to find “waveform loss”, but generally there are three groups of pulse amplitude is not equal, even the phenomenon of soft big difference. In fact, careful analysis of the characteristics of the isolation stage circuit will be able to see the problem, this is a more special intermittent oscillator, with only two three tubes, respectively, for the oscillation tube and oscillator power switch. Due to the use of single-tube oscillation, and the oscillating circuit is connected in series with the current-limiting resistor and the two transistors, plus the output load of the transformer, the oscillating circuit has large losses and low gain, which easily leads to accidental suspension of the circuit and insufficient pulse amplitude. Produce a good and bad motor phase failure. From the above analysis we can see that this kind of circuit has strict requirements on pulse transformer Q value and triode β value. When users are repairing, they can use the following measures to make up for it: (1) Select high β (120 to 180) oscillator tubes; (2) Appropriately reduce the current-limiting resistor value, that is, connect a 100-270Ω resistor across the 51Ω resistor.

4. Diagnose multiple failure syndromes

The following is a typical example of a CVT035 transistor DC drive to illustrate the diagnostic methods for multiple fault syndromes. The faulty servo board, after preliminary inspection to see that the appearance of the circuit board is very dirty, serious burn-out of the output level, showing that the user’s maintenance is relatively lacking, to deal with this kind of failure, you should first remove dirt, repair the output level, should not rush to power, Otherwise it may cause a short circuit and increase the fault surface. For example, iron powder dust is conductively short-circuited, and the output-stage switch tube breaks down to the short circuit between the front stage and the power supply. After the above-mentioned processing, the power-on check also finds the following faults: (1) “undervoltage” red light sometimes flashes (“READY” green light flashes); (2) motor does not turn; (3) switching power supply (± 15V) transformer Tl and power switch V69 are abnormally hot.

This is a typical syndrome, and there may be some kind of causal relationship between failures. Therefore, it is necessary to deal with failures in sequence. Otherwise, it may be less effective, or even cause the failure to expand. Through analysis, we make the following maintenance order: Switching power supply -> Undervoltage light -> Motor operation. First check the power board, after measuring the main circuit 150V DC voltage and disconnect the ± 15V load inspection, the fault is found inside the switching power supply board, check the power supply board found 10V regulator V32 voltage is only 9.5V by This check goes on and finds the cause of the fault: the resistance value of the current limiting resistor Rl85 of V32 becomes large. After replacing the Rl85, the ±15V power supply board and the “undervoltage” lamp all returned to normal, but the motor still did not turn. It can be seen that the above-mentioned lamp flashing and component hot-burning are all caused by the Rl85 variable value, and there is another reason for the non-rotation of the motor. According to the usual inspection methods, it can be detected step by step, but due to experience, we only do a simple conversion steering test. As a result, it is found that the reverse operation is normal, so the cause of the fault is quickly found out: the integrated block N5 of the commutation circuit (TL084 ) Failure, after replacing N5, everything is normal.

5. PC interface method

Because each unit of the CNC machine tool (except the drive) and the numerical control system is through the PC interface (1/O) to achieve signal transmission and control, many faults will be reflected through the PC interface signal. We can consult the PC machine tool. The 1/O signal on the side diagnoses various complex machine faults or determines whether the fault is in the CNC or in the machine tool. The method is very simple. It is required to be familiar with the current status and normal status of all PC (machine side) interface signals (or to make a table). During diagnosis, the current status and normal status of all PC (machine side) interface signals are passed. Check one by one to find the faulty interface signal, and then find the cause of the fault based on the external logic of the signal. When you are familiar with the PC interface signal, the application of this PC interface alignment method is very simple and quick, and avoids the complicated ladder diagram program of the board.

6. Recovery of data abnormality in Siemens 3GG system

Swiss STUDER s45-6 grinding machine is equipped with Siemens 3GG system. It is a dual NC dual PLC structure. The system has a strong self-diagnostic function. When a fault occurs, it can be quickly diagnosed and repaired with the help of screen prompts. However, if the system fails to start and the PLC is stopped and the screen does not light, the system’s self-diagnostic function will not work, making it difficult to diagnose. There are many reasons for this failure. If the battery voltage is lower than 2.7V, the battery must be replaced; if the NC or PLC hardware is damaged, the circuit board needs to be replaced; if the 24V power supply of the machine tool is lower than 21V, the power circuit and load need to be checked.

However, the reason we encountered more failures was not a hardware failure but a soft failure such as an abnormal machine data. The reasons for this are relatively complex. For example, power grid interference, electromagnetic wave interference, battery failure, and operation errors all may cause the loss or confusion of machine data, and the system cannot start.

Like this type of soft failure, we can use the full recovery method to restore the system to operation. The clearing steps of the 3GG system are as follows:
(1) Clearing of machine data, user program, setting data and background memory;
(2) Initialization of the 3GG system;
(3) PLc is cleared;
(4) Restore all data and programs that have been cleared. Generally need to set the baud rate, call out 128KB memory, then, through the disk and other media input data, programs.
(5) Test and check the entire KV factor of the servo system.
(6) After completing these steps, the system returns to normal.

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