Several special methods and examples of diagnosing

2022-06-10
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Several special methods and examples of diagnosing difficult faults of NC machine tools

1. The resistance comparison method is used to diagnose the short-circuit fault of power load

fault example: fanuc-besk servo drive board ten 15V load soft breakdown and fuse burning

during maintenance, we determined that the fault was caused by local short circuit of load through preliminary inspection, and measured the resistance of ten 15V to "ground" with a digital meter, which was 1.3k Ω for normal board and 300 Ω for fault board. Because it is easy to burn the fuse when powered on, it is impossible to check when powered on, so we can only measure the resistance or dismantle the components for inspection

however, since the ten 15V power supply of the servo board and the printed circuit of its load (24 integrated elements) form a radial structure, it is impossible to cut and separate the circuit during resistance measurement, and because there are many components and they are directly welded, it is impossible to disassemble and check them one by one. The actual operation of maintenance is very difficult. Even if the fault is solved, the circuit board is often scarred. It is very convenient for us to use the resistance comparison method to deal with this kind of fault, which can not be cut and separated, disassembled or energized. During diagnosis and inspection, the circuit is not cut or the components are not welded off, but the resistance value between the ten 15V terminal and the relevant pins of each integrated component is directly measured, and the fault can be found by comparing the corresponding value between the fault board and the normal board. When dealing with the above faults, considering the large number of component pins, first analyze the internal circuit of thick film block (marked in the figure) and the pin function diagram of integrated block, and then select several main test points for resistance measurement. When Q7 is measured, it is found that the resistance between pin 3 (+ 15V) and pin 14 (output) is 150 Ω (6K Ω normally). It is suspected that there is a problem with Q7 (LM339). After Q7 is replaced, the servo board returns to normal, indicating that the abnormal resistance between pins of Q7 is due to internal soft breakdown, resulting in short circuit of power supply

2. Step by step simulation method of fast process

some control processes, such as the automatic speed rise and fall process of stepping motor and the parking braking process of DC governor, have only a few tenths of a second. Looking for the circuit fault in this fast process, it is obviously impossible to use general instruments for fault tracking and detection, so fault diagnosis is more difficult. Next, through the fault example - the fault that the shutdown time of 5V DC thyristor main drive is too long, we introduce our special method - step-by-step simulation method

through the preliminary inspection of the fault board, it is judged that the fault is caused by the braking circuit of V5 main driver. The braking control logic is complex and involves many circuits. It is by no means easy to diagnose the fault. Moreover, because the braking process is short and cannot be measured, we use the step-by-step simulation method for diagnosis and inspection. According to the circuit principle, the braking process is as follows:

(1) the bridge inverts to release energy; (2) Automatic axle changing and regenerative braking; (3) Replace the bridge again and restore the circuit

in order to meet the needs of step-by-step measurement, take speed command, speed feedback and current feedback as the set quantity, subdivide the above process into eight steps (listed in a table), then gradually change the corresponding set quantity, detect the relevant circuit signals, compare the circuit logic and find out the fault. When we did the step-by-step test and went to the second step (i.e. the speed command changed from 1 to 0), we found that "a backward" and "integral stop" were both high levels. According to the circuit logic, they should be low levels. Based on this, we checked the circuit and quickly found out that there was a problem with NAND gate dl06 (model: fzhi01) in board A2. After replacement, the fault was eliminated

3. A special fault of ct4-os3 frequency finder

ct4-os3 frequency converter is often used to drive the tool magazine of ybm90 and mk5oo machining centers. During the maintenance, we have encountered the phase failure of the frequency converter many times, and the measured phase failure voltage is only 60 to 2oov (normal 400V). Because this is a kind of soft fault with good and bad times, it is difficult to diagnose and search

however, we found that most of the faults of the frequency converter are caused by the problem of pulse isolation stage - unstable oscillation. This fault phenomenon is difficult to find "waveform loss" when checked with oscilloscope, but generally there are three groups of pulse amplitudes that are not equal or even soft

in fact, the problem can be seen by carefully analyzing the characteristics of the isolation stage circuit. This is a special intermittent oscillator. Only two three-stage transistors are used as the power switch of the oscillator and oscillator respectively. Due to the use of single tube oscillation, and the oscillation circuit is connected in series with current limiting resistance and two triodes, plus the output load of the transformer, the oscillation circuit has large loss and low gain, which is easy to cause the problems of occasional circuit vibration stop and insufficient pulse amplitude, that is, the motor phase failure from good to bad. From the above analysis, it can be seen that this circuit is sensitive to the Q value of pulse transformer and triode β The value requirements are strict. When users repair, the following measures can be taken to make up for it: (1) select high β (120 to 180) oscillating tube; (2) Appropriately reduce the current limiting resistance, that is, connect 100-270 Ω on the 51 Ω resistance in parallel

4. Diagnosis of multiple fault syndromes

the following illustrates the diagnosis methods of multiple fault syndromes through a typical example of cvt035 transistor DC driver. The preliminary inspection of the faulty servo board shows that the appearance of the circuit board is very dirty and the output stage is seriously burned. It can be seen that the maintenance of the user is relatively lacking. To deal with this kind of fault, we should first remove the dirt and repair the output stage. Do not turn on the power rashly, otherwise it may cause a short circuit and expand the fault surface. For example, the conductive short circuit of iron dust, the breakdown of output stage switch, the short circuit to the front stage and power supply, and so on. After the above treatment, the power on inspection found the following faults: (1) the "undervoltage" red light sometimes flashes ("ready" green light flashes); (2) The motor does not rotate; (3) Switching power supply (± 15V) transformer TL and power switch tube v69 are abnormally hot

this is a typical syndrome, and there may be some causal relationship between faults, so fault handling needs to be carried out in sequence, otherwise it may get twice the result with half the effort, or even lead to the expansion of fault area. Through analysis, we make the following maintenance sequence: switching power supply I> "undervoltage" lamp>> motor operation. First, check the power board. After measuring the 150V DC voltage of the main circuit and disconnecting the ± 15V load, it is known that the fault is inside the switching power board. In the inspection of the power board, it is found that the voltage of 10V regulator V32 is only 9.5v. Therefore, check it and find the cause of the fault: the resistance of current limiting resistor rl85 of V32 becomes larger. After replacing rl85, ± 15V power supply board and "undervoltage" lamp return to normal, but the motor still does not rotate. It can be seen that the flashing of the above lights and the scalding of components are caused by the change of rl85 value, and there are other reasons for the non rotation of the motor. According to the usual inspection method, it can be detected step by step, but due to experience, we only did a simple transformation and steering test, and it was found that the reverse operation was normal, so we quickly found out the cause of the fault: the integrated block N5 (tl084) of the commutation circuit failed, and everything was normal after N5 was replaced

5. PC interface method

because the signal transmission and control between each unit of NC machine tool (except driver) and NC system are realized through PC interface (1/O), many faults will be reflected through PC interface signals. We can diagnose various complex machine tool faults or judge whether the fault is in NC system or in machine tool electrical system by consulting the 1/O signal on the side of PC machine tool. The method is very simple, that is, it is required to be familiar with the current state and normal state of all PC (machine tool side) interface signals (or make a table). During diagnosis, check and compare the current state and normal state of all PC (machine tool side) interface signals one by one to find out the faulty interface signal, and then find out the cause of the fault according to the external logical relationship of the signal. When you are familiar with the PC interface signal, the application of this PC interface comparison method is very simple and fast, and avoids the complex ladder program of board division

6. Recovery of abnormal data of gate 3GG system of Western wet molding technology

Swiss student s45-6 grinder is equipped with Siemens 3GG system, which is a dual NC dual PLC structure. The system has strong self diagnosis function. In case of fault, it can quickly diagnose and repair the fault with the help of the screen without assuming that class a materials are not used. However, if the system cannot be started, and the PLC is stopped and the screen is not lit, the self diagnosis function of the system will not work, resulting in difficult diagnosis. 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, check the power supply circuit and load

however, the cause of more failures is not hardware failure, but software failure such as abnormal machine tool data. The reasons are complex, such as electrical interference, electromagnetic wave interference, battery failure, operation error and so on, which may cause the loss or confusion of machine tool data, so that the system cannot be started

for such soft faults, we can use the full clear recovery method to restore the system to operation. The full clearing steps of 3GG system are as follows:

(1) clearing machine tool data, user program, setting data and background memory

(2) initialization of 3GG system

(3) PLC reset

(4) recover all data and programs that have been cleared. Generally, you need to set the baud rate, call out 128KB memory, and then input data and programs through disk and other media

(5) test and check all kV coefficients of the servo system

(6) after completing these steps, the system returns to normal. (EN and timely remove d)

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