Fault maintenance of W200HD CNC system of the hott

2022-08-24
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Fault maintenance of W200HD CNC system of boring and milling machine

1 fault phenomenon

w200hd is a CNC boring and milling machine imported from the Czech Republic by WISCO in 1994. In one use, it is found that no matter how far the machine goes, there is always a certain error between the coordinate display distance and the actual detection distance, sometimes up to a few millimeters, or even tens of millimeters. This phenomenon of large positioning error makes the NC machine tool unable to process normally

2 fault analysis and treatment

1) fault analysis

the CNC system equipped with the boring and milling machine W200HD is the SINUMERIK system of Siemens in Germany. It is a medium-range CNC system with the linkage function of three coordinate axes and a resolution of 1 m. For this system, the above error is abnormal. The system itself has the function of self diagnosis. However, the above-mentioned faults are not displayed on the conformity judgment indicator when the verification results are clearly given, which requires human judgment and elimination

as an important index in position control, the positioning error can comprehensively judge whether the position control is normal. From the fault phenomenon, it can be determined that there is a problem in the position control part of the NC system of the machine tool, which makes the positioning error of the whole closed-loop system too large and affects the normal processing of the machine tool. Based on the principle block diagram of closed-loop position control shown in Figure 1, the principle of feed position control is briefly analyzed

Figure 1 feed position control principle block diagram

the position sensor (here is the grating) installed on the machine tool workbench converts the mechanical displacement into a digital pulse, which is sent to the position measurement interface of the NC system, counted by the counter, and the computer samples the feedback value in a fixed time period. The sampled value is compared with the output result of the interpolation program (i.e. the current command position) to obtain the position error, The error is amplified by software gain (KC) and output to the digital to analog converter (d/a), which provides the control voltage for the servo unit and drives the workbench to move in the direction of reducing the error. If the interpolation program continues to have a feed, the workbench will continue to follow the feed movement. Only when the position error is zero, the workbench will stop at the required position. It can be seen that the position control function is realized by both software and hardware. The software is responsible for the calculation of the position error and the given value of the speed loop, while the hardware accepts the digital quantity of the given value, performs d/a conversion, provides the control voltage for the speed loop, drives the coordinate axis movement, and feeds back the position of the machine tool workbench to the NC system

analyze the source of the problem according to the fault phenomenon. The machine tool is a full closed-loop control system. The position sensor is installed on the workbench. Theoretically, the positioning error is not affected by the lead screw clearance and pitch error. The machine tool can move normally, which indicates that the machine tool servo unit works normally. In the feedback loop of the machine tool, if the feedback line is broken, the system will actually become an open loop without position feedback. The increment is subtracted from the command increment, and the follow-up error will accumulate more and more until it overflows, causing the driving device to lose control. At this time, the self diagnosis function of the NC system will play a role and send out the alarm message of the disconnection of the feedback loop. From the above analysis, we can see that there is no problem with the hardware part of the position control, so the fault may be in the software part

the servo unit of the machine tool can complete the control of current loop and speed loop. The position loop (i.e. position control) is completed by the NC system software. The CNC system software has hundreds of parameters that can be set by the user to facilitate the secondary development of the matching between the CNC system and the machine tool body. These parameters are stored in the CMOS memory. After the machine tool is powered off, the parameters will not be lost. Once the battery is generally insufficient (lower than 85% of the rated value) or external interference can cause parameters to be lost and confusion. However, there is no alarm of insufficient battery, indicating that the battery is still full of energy. If all the control parameters of the CNC system are lost, the machine tool cannot run at all. Therefore, it is likely that some external interference will confuse some parameters of the position control, and the software calculation of the position control will have a large error, which will make the positioning error exceed the allowable range. It can be seen from Figure 2 that the accuracy of the position feedback measurement system of the machine tool and the position loop gain KC affect the output of the position error. The position loop gain reflects the sensitivity of the whole servo system. On the premise of system stability, the higher the gain, the smaller the positioning error, but the machine tool is easy to oscillate and difficult to adjust; The lower the gain of the system, the greater the positioning error, but the machine tool is easy to adjust and not easy to oscillate. Now call out the system parameters of the CNC system, and check whether the gain of the position ring is too low, so that the positioning error is large. After checking, the gain of the position ring is consistent with that of the machine tool after debugging. Therefore, the problem of the machine tool may be that the accuracy of the position feedback measurement system of the machine tool does not match the position adjustment accuracy of the control device

Figure 2 CNC system position control software flow

because the machine tool adopts a full closed-loop position control system, which directly obtains the actual position movement value from the moving parts (workbench) of the machine tool, its detection accuracy is not affected by the mechanical transmission accuracy. To make the position ring work normally, there must be a correct position adjustment closed loop. Therefore, it is necessary to match the pulse of the position measurement system with the position adjustment accuracy of the control device. There is a settable parameter md5002 (8 bits in total) in the system, which indicates the feedback input resolution and position control resolution. Only when the settings of feedback input accuracy and position control accuracy match each other, the position loop can be calculated correctly, so that the machine tool can move normally and reach the specified position accurately within the allowable range of error

in the CNC system, the calculation of positioning error is based on the servo resolution. If the accuracy of position feedback input is inconsistent with the accuracy of position control, the calculation of position error is carried out in non-uniform units, and the result is not the real error of command position and actual position, so the workbench cannot reach the position required by the command

1) fault handling

according to the above fault analysis, call out the system parameters and check the contents of the parameter unit md5002. It is found that there are differences with the originally debugged parameters, which may be some external interference, causing confusion in the parameters in RAM. Now after the parameter is correctly modified, reset the CNC system of the machine tool, and then start the machine tool. It is the first plastic new material enterprise listed on NASDAQ in the United States in Greater China. After testing, the positioning accuracy of the machine tool has met the normal requirements, and the machine tool has resumed normal production

3 summary

through this fault maintenance, it can be seen that in order to give full play to the advantages of CNC machine tools with multiple functions, strong flexibility, high machining accuracy and high degree of automation, and produce high-quality products to meet the needs of market competition, we must pay attention to strengthening the maintenance of CNC machine tools. The following points should be achieved: (1) ensure that the use environment of CNC equipment is good and meets the requirements of product regulations. (2) Pay attention to the maintenance of the power supply of CNC equipment. Especially when adding or removing large capacity equipment sharing AC power nearby, it is necessary to observe whether it produces instantaneous interference signals to prevent it from destroying the guiding ideology of the "1035" plan: thoroughly implement the procedures or parameters in the spirit of the "108th CPC National Congress" and the 3rd and 4th plenary sessions of the CPC Central Committee, and even cause a crash, affecting the normal operation of the machine tool. (3) CNC equipment shall have operating procedures. In particular, the machine operator is prevented from modifying internal parameters at will. (4) There should be maintenance records. Generally, machine tool failures are regular (such as frequent failures), and maintenance records have guiding significance for future work. (5) We should strengthen the training of technicians and operators of numerical control equipment and improve their comprehensive professional quality in combination with theory and practice. (end)

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