Fault phenomenon W200HD is a CNC boring and milling machine imported by WISCO from the Czech Republic in 1994. In one use, it is found that the distance between the coordinate display and the actual detection distance always has a certain error, no matter how far the machine tool is. A few millimeters, or even a dozen millimeters. This phenomenon of large positioning error makes the CNC machine tool unable to process normally.
â—†Fault analysis
The CNC system of the boring and milling machine W200HD is the SINUMERIK system of Siemens AG of Germany. It is a mid-range CNC system with three coordinate axes linkage function with a resolution of 1μm. For such a system, the above error is not normal. The system itself has a self-diagnostic function, however the above faults are not displayed on the display and require human judgment and exclusion.
As an important indicator in position control, the positioning error can more 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 machine numerical control system, so that the positioning error of the entire closed-loop system is too large, which affects the normal processing of the machine tool.
A position sensor (here a grating) mounted on the machine table converts the mechanical displacement into a digital pulse, which is sent to the position measurement interface of the CNC system, counted by the counter, and the computer samples the feedback value for a fixed period of time, sampling The value is compared with the result of the interpolation program output (ie, the current command position) to obtain a position error, which is amplified by software gain (Kc) and output to a digital-to-analog converter (D/A) to provide a control voltage for the servo unit. The drive table moves in the direction of reducing the error. If the interpolation program continues to have a feed rate, the table continues to follow the feed motion, and the table stops at the desired position only when the position error is zero. 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 speed loop reference value. The hardware accepts the given value digital quantity and performs D/A conversion to provide the speed loop. Control the voltage, drive the coordinate axis motion, and feed back the position of the machine table to the CNC system. String 3
Analyze the source of the problem based on the failure phenomenon. The machine tool is a full-closed control system, and the position sensor is mounted on the workbench. In theory, the positioning error is not affected by the screw gap and pitch error. The machine tool can move normally, indicating that the machine servo unit is working properly. In the machine feedback loop, if the feedback line is broken, the system actually becomes open loop, there is no position feedback, the increment is subtracted from the command increment, and the following error will accumulate more and more until it overflows, causing the drive to run out of control. At this time, the self-diagnosis function of the numerical control system will play a role, and issue an alarm message that the feedback loop is disconnected. Based on the above analysis, it can be seen that there is no problem in the hardware part of the position control, and thus the fault may be in the software part.
The servo unit of the machine can control the current loop and speed loop. The position loop (ie position control) is done by the CNC system software. There are hundreds of parameters of the CNC system software that can be set by the user to facilitate the secondary development of the matching between the CNC system and the machine body. These parameters are stored in the CMOS memory. After the machine is powered off, the parameters will not be lost. (less than 85% of the rated value) or external interference can cause confusion due to missing parameters. However, there is no alarm of insufficient battery, indicating that the battery is still full of energy. If the control parameters of the CNC system are all lost, the machine tool is impossible to run at all, so it is likely to be some kind of external disturbance, which causes some parameters of the position control to be confusing, and the software calculation of the position control has a large error, so that the positioning error exceeds the position. Allowable range.
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. Under 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 larger the positioning error, but the machine tool is easy to adjust. Not easy to oscillate. Now adjust the system parameters of the CNC system, check whether the gain of the position loop is selected too low, so that the positioning error is large. After proofreading, the gain of the position loop is consistent with the gain of the machine after commissioning. Therefore, the problem of the machine tool may be in the mismatch between the accuracy of the position feedback measurement system of the machine tool and the position adjustment accuracy of the control device.
Since the machine adopts a full-closed position control system, the actual movement value of the position is obtained directly from the moving part (workbench) of the machine tool, so the detection accuracy is not affected by the mechanical transmission accuracy. For the position loop to work properly, it must have the correct position adjustment closed loop. For this purpose, the pulse of the position measuring system must be matched to the position adjustment accuracy of the control device. In this system, there is a parameter MD5002 (8 bits in total), which indicates the feedback input resolution and position control resolution. The position loop can be correctly calculated only when the feedback input precision setting and the position control precision setting match each other. , so that the machine tool moves normally and reaches the specified position within the allowable range of error.
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