1 Maintenance of position detection elements 1. 1 The grating ruler itself has certain protective measures. Some people need to introduce clean air into the ruler box to keep the air pressure inside the ruler greater than the outside air pressure and prevent moisture from entering. However, it is limited by the field production environment and the processing conditions of the machine tool itself (such as high-pressure cutting fluid, etc.). ), but also to do a good job of pollution prevention and vibration prevention maintenance. 1. 1. 1 The anti-fouling grating ruler is easily polluted by cooling liquid because it is directly installed on the workbench and machine tool, resulting in signal loss and affecting the position control accuracy. The cooling liquid will produce slight crystallization during use, and a thin film will be formed on the scanning head, which has poor light transmittance and is not easy to remove, so be careful when choosing the cooling liquid. In the process of machining, the pressure and flow of coolant are too large, which is easy to form a large number of water mist and pollute the grating ruler. It is best to introduce low-pressure compressed air into the grating ruler to avoid the negative pressure formed when the scanning head moves to suck dirt into the grating. Compressed air must be purified, and the filter element should be kept clean and replaced regularly. 1.65438+ Maintenance of photoelectric pulse encoder 1.2 The photoelectric pulse encoder has a gap with equal spacing on the edge of a disk, and the light source and photosensitive element are installed on both sides of the disk respectively. When the optical disc rotates, the light intensity changes, which is detected by the photosensitive element and becomes an electrical signal, thus obtaining a pulse signal. The output signals of the encoder are: two signals with a phase difference of 90 degrees, which are used for direction discrimination; The zero signal (also called rotation signal) is used to control the machine tool to return to the reference point; There are also +5 V power supply and ground signals. The maintenance of encoder mainly pays attention to the following two problems. 1.2. 1 The anti-vibration and anti-pollution encoder is a precise measuring element with good sealing. When using and disassembling, pay attention to shock prevention and pollution prevention like a grating ruler. Pollution is easy to occur at the connection between the lead-out section and the plug, and protective measures should be taken for these parts. Vibration easily causes internal fasteners to loosen and fall off, resulting in internal short circuit. 1.2.2 Connection problems are divided into loose connection and improper connection adjustment. There are built-in and external connections for encoders. Built-in coaxial with servo motors, such as ROD320 encoders on Siemens 1FT5 and 1FT6 servo motors. Externally installed at the end of the transmission chain, when the transmission chain is long, this installation method can reduce the influence of the accumulated error of the transmission chain on the position detection accuracy. Because of loose connection, the position control accuracy is often affected. In addition, some built-in encoders of AC servo motors not only detect the position, but also have the functions of measuring the speed and detecting the rotor position of AC servo motors. Therefore, the loose connection of encoder will also cause the instability of feed movement, affect the commutation control of AC servo motor, and thus cause the vibration of machine tool. In addition, the encoder is driven by a belt. If the drive belt is adjusted too tightly, the encoder bearing is easily damaged. Maintenance example 1: The feed shaft of a CNC machine tool is out of control. The servo system of this machine tool is Siemens 6SC6 10 drive device and 1FT5 AC servo motor with ROD320 encoder. After eliminating the factors such as numerical control system, driving device and speed feedback, the fault is located in the position detection control device. After inspection, the encoder output cable and connection are normal. When disassembling ROD320 encoder, it was found that a fastening screw fell off and was placed between +5 V and the grounding terminal, resulting in short circuit of power supply, no signal output from the encoder, and the numerical control system was in an open loop state, thus causing runaway failure. Maintenance example 2: When the tool of the spindle was replaced in a machining center, the spindle position was inaccurate and deviated from the test position after reset. The spindle position of machine tool is detected by pulse encoder, and the spindle and encoder are driven by belt 1: 1. Because the system has the function of C-axis position display, manually rotate the spindle once and find that the position change is less than 360. Suspect there is something wrong with the encoder. When it was removed, it was found that some areas of the circular grating were worn. After analysis, it is considered that the spindle and encoder drive belt are too tightly adjusted, and the encoder bearing is damaged after long-term operation, which makes the circular grating and the reading head partially rub. After replacing the new encoder and properly adjusting the belt tightness, no similar failure occurred. Example of fault diagnosis and maintenance of position detection element When position ring alarm occurs, disconnect J2 connector, connect the +5 V line on J2 connector with the alarm line ALM on the CNC system side, and turn on the power supply of the CNC system. According to whether the alarm appears again, it can be quickly judged whether the fault part is on the measuring equipment or on the system interface board. If the problem lies in the measuring device, you can measure whether there is a signal input on the J 1 connector, so that you can locate the fault in the grating ruler or EXE pulse shaping circuit. Maintenance example 1: A horizontal machining center adopts SIN8 system with EXE grating measuring device, and the alarm number 1 14 appears during operation, accompanied by the alarm number 1 13. From the alarm reason, the alarm number 1 14 led to the alarm number 1 13, so the fault was located in the position detection device. There are two possibilities for alarming 1 14: first, the cable is disconnected or grounded; The second is signal loss. The former can be diagnosed by visual inspection and measurement, while the latter is mainly signal missing reading. If the amplitude of the sinusoidal signal output by the grating ruler decreases for some reason, it will affect the zero-crossing position of the processed signal in the signal processing process, and in serious cases, the output pulses will be crowded together, resulting in loss. Because the signal generated by photocell is proportional to the light intensity, the decrease of signal amplitude is nothing more than the decrease of light source brightness or the dirty optical system. When the scanning unit is pulled out from the scale, it can be seen that the surface of the lens under the bulb is ground glass, indicating that there is a layer of fog on the grating surface, and this kind of dirt also exists on the bulb and photovoltaic cell, which leads to the decrease of the luminous efficiency of the light source and the weakening of the output signal. Cleaning the grating can eliminate the fault. Maintenance example 2: A CNC vertical milling machine is equipped with FANUC 3M CNC system, and the position detection device is an encoder coaxially connected with the servo motor. During operation, the Z axis generates 3 1 alarm. Check the maintenance manual, and the alarm number 3 1 indicates that the contents of the error register are greater than the specified value. According to the alarm prompt No.31,the limit value set by the error register is enlarged, that is, the corresponding parameter is changed from 2 000 to 5 000, and then the manual pulse generator sends a move command to the Z axis, and the No.32 alarm appears again. Alarm number 32 indicates that the contents of the error register exceed 32 767, or the value of the digital-to-analog conversion command exceeds. This kind of fault needs to check the position deviation diagnosis of the system. The error register is used to store the difference between the control value and the feedback value. When the position detection device or position control unit fails, the error register will be out of tolerance. Therefore, the fault lies in the position control device. Diagnostic numbers 800(X axis), 80 1(Y axis) and 802(Z axis) can be used to diagnose the position control signal. Call three diagnostic numbers, give instructions to each axis with manual pulse generator, observe its changes, and give instructions to X axis and Y axis. The process of position deviation change is consistent with the movement of machine tool. The deviation of the Z-axis instruction does not disappear. Further locate whether the fault is in the Z-axis control unit or encoder, and exchange the Z-axis and Y-axis driving devices and feedback signals at the same time by exchange method. It is found that the same fault phenomenon occurs in Y axis, which shows that there is no problem with the Z axis control unit, and the fault occurs in the encoder coaxially connected with the Z axis servo motor. Maintenance example 3: DECKEL system is installed in a CNC milling machine, the position detection device adopts HEIDENHAIN LS907 grating ruler, and the fault alarm is that the Z-axis detection system is dirty. After the system is started, when moving the Z axis, it is relatively stable at low speed. When the following error exceeds 60, the machine tool will overshoot and give an alarm, and it will not give an alarm when it rises and give an alarm when it falls. According to the alarm content, first confirm whether the grating ruler needs cleaning. After disassembly and inspection, it was found that there was more lubricating oil on the grating ruler shell. This is because the machine tool protection measures of grating ruler are not in place. After long-term use, the lubricating oil of the machine tool guide rail flows along the bed to the grating ruler. Clean the grating ruler, try again when installing, and the grating ruler will give an alarm. At this time, analyze whether the grating ruler itself is faulty. It happens that the specifications and models of the Y-axis grating ruler of the machine tool are the same as those of the Z-axis, and the two grating rulers are exchanged by replacement method. As a result, the Y-axis measurement system failed, so it can be determined that it is the grating ruler itself. Further identify the grating ruler, confirm that the reading head is faulty, and the machine tool returns to normal. Position detection device is a key component of CNC machine tools, which often appears in CNC machine tools. In the process of maintenance, it must be carefully studied in order to quickly find out the fault and ensure the normal operation of the machine tool.